Author name code: vandenoord
ADS astronomy entries on 2022-09-14
author:"van den Oord, G.H.J." 
------------------------------------------------------------------------  

Title: Towards Open and FAIR Hydrological Modelling with eWaterCycle
Authors: Drost, Niels; Aerts, Jerom P. M.; Alidoost, Fakhereh; Andela,
   Bouwe; Camphuijsen, Jaro; van de Giesen, Nick; Hut, Rolf; Hutton,
   Eric; Kalverla, Peter; van den Oord, Gijs; Pelupessy, Inti; Smeets,
   Stef; Verhoeven, Stefan; van Werkhoven, Ben
Bibcode: 2021EGUGA..23.7797D
Altcode:
  The eWaterCycle platform (https://www.ewatercycle.org/) is a fully
  Open Source system designed explicitly to advance the state of Open and
  FAIR Hydrological modelling. While working with Hydrologists to create
  a fully Open and FAIR comparison study, we noticed that many ad-hoc
  tools and scripts are used to create input (forcing, parameters) for a
  hydrological model from the source datasets such as climate reanalysis
  and land-use data. To make this part of the modelling process better
  reproducible and more transparent we have created a common forcing
  input processing pipeline based on an existing climate model analysis
  tool: ESMValTool (https://www.esmvaltool.org/). Using ESMValTool,
  the eWaterCycle platform can perform commonly required preprocessing
  steps such as cropping, re-gridding, and variable derivation in a
  standardized manner. If needed, it also allows for custom steps for
  a hydrological model. Our pre-processing pipeline directly supports
  commonly used datasets such as ERA-5, ERA-Interim, and CMIP climate
  model data, and creates ready-to-run forcing data for a number of
  Hydrological models.Besides creating forcing data, the eWaterCycle
  platform allows scientists to run Hydrological models in a standardized
  way using Jupyter notebooks, wrapping the models inside a container
  environment, and interfacing to these using BMI, the Basic Model
  Interface (https://bmi.readthedocs.io/). The container environment
  (based on Docker) stores the entire software stack, including the
  operating system and libraries, in such a way that a model run can
  be reproduced using an identical software environment on any other
  computer.The reproducible processing of forcing and a reproducible
  software environment are important steps towards our goal of fully
  reproducible, Open, and FAIR Hydrological modelling. Ultimately, we hope
  to make it possible to fully reproduce a hydrological model experiment
  from data pre-processing to analysis, using only a few clicks.

Title: XIOS-GRIB compare
Authors: van den Oord, Gijs
Bibcode: 2021zndo...4906175V
Altcode:
  Python scripts to compare IFS GRIB files and XIOS NetCDF files.

Title: IFS CY43R3 and XIOS 2.0 integration
Authors: Yepes-Arbós, Xavier; van den Oord, Gijs
Bibcode: 2021zndo...4905832Y
Altcode:
  This project implements the integration between the IFS CY43R3 model
  and the XIOS 2.0 I/O server.

Title: Towards Reproducible Hydrological Modelling with eWaterCycle
Authors: Drost, N.; Aerts, J. P. M.; Alidoost, F.; Andela, B.;
   Camphuijsen, J.; Dzigan, Y.; Van De Giesen, N.; Hut, R.; Hutton, E.;
   Kalverla, P.; van Meersbergen, M.; van den Oord, G.; Pelupessy, I.;
   Verhoeven, S.; Weel, B.; van Werkhoven, B.
Bibcode: 2020AGUFMIN016..15D
Altcode:
  The eWaterCycle platform( <A
  href="https://www.ewatercycle.org/">https://www.ewatercycle.org/
  </A>) is a fully Open Source system designed explicitly to advance the
  state of Open and FAIR Hydrological modelling. Reproducibility is a key
  ingredient of FAIR, and one of the driving principles of eWaterCycle. <P
  />While working with Hydrologists to create a fully Open and FAIR
  comparison study, we noticed that many ad-hoc tools and scripts are
  used to create input (forcing, parameters) for a hydrological model
  from the source datasets such as climate reanalysis and land-use
  data. To make this part of the modelling process better reproducible
  and more transparent we have created a common forcing input processing
  pipeline based on an existing climate model analysis tool: ESMValTool
  ( <A href="https://www.esmvaltool.org/">https://www.esmvaltool.org/
  </A>). <P />Using ESMValTool the eWaterCycle platform can perform
  commonly required pre-processing steps such as cropping, re-gridding,
  and variable derivation in a standardized manner. If needed, it also
  allows for custom steps for a Hydrological model. Our pre-processing
  pipeline directly supports commonly used datasets such as ERA-5,
  ERA-Interim, and CMIP climate model data, and creates ready-to-run
  forcing data for a number of Hydrological models. <P />Besides
  creating forcing data, the eWaterCycle platform allows scientists
  to run Hydrological models in a standardized way using Jupyter
  notebooks, wrapping the models inside a container environment,
  and interfacing to these using BMI, the Basic Model Interface (
  <A href="https://bmi.readthedocs.io/">https://bmi.readthedocs.io/
  </A>). The container environment (based on Docker) stores the entire
  software stack, including the operating system and libraries, in such
  a way that a model run can be reproduced using an identical software
  environment on any other computer. <P />The reproducible processing of
  forcing and a reproducible software environment are important steps
  towards our goal of fully reproducible, Open, and FAIR Hydrological
  modelling. Ultimately, we hope to make it possible to fully reproduce
  a Hydrological model experiment from data pre-processing to analysis,
  using only a few clicks.

Title: HyMUSE: a multi-model framework for Hydrology.
Authors: Pelupessy, Inti; Drost, Niels; Hut, Rolf; Aerts, Jerome;
   Portegies Zwart, Simon; van Elteren, Arjen; van den Oord, Gijs;
   van Werkhoven, Ben; Verhoeven, Stefan; Weel, Berend
Bibcode: 2019EGUGA..2114271P
Altcode:
  We present the Hydrological Multipurpose Software Environment (HyMUSE),
  a Python framework for Hydrological model simulations, that will
  form part of the computational core of the eWaterCycle project. The
  eWaterCycle II project aims to develop a platform for researchers
  to easily develop and use hydrological models and deploy them on
  HPC resources. HyMUSE is being developed at the Netherlands eScience
  Center using technology developed in the AMUSE and OMUSE projects, which
  where developed for the astrophysical and oceanographic domains. HyMUSE
  presents the user with a homogeneous interface to different hydrological
  simulation codes. For this it provides a number of services such
  as unit conversion, encapsulation of the internal model data to a
  common object oriented representation and maintaining the simulation
  in a consistent state. HyMUSE can be accessed from within the online
  notebook environment of the eWaterCycle toolset, where researchers can
  explore, adapt and collaborate on simulations models. The use cases
  for HyMUSE range from running simple numerical experiments with single
  codes and the addition of data analysis tools in model runs, to running
  large model run ensembles or setting up coupled solvers for problems
  where different types of physics interact. In addition to AMUSE-type
  low level interfaces, it is possible in HyMUSE, to interface directly
  with the CSDMS Basic Model interface (BMI). For this we have developed
  an interoperability layer for the BMI, that can optionally route its
  low level communication through the newly developed GRPC4BMI library,
  which enables models to be run on remote HPC resources. We discuss the
  current status of the project, and the medium and long term development
  goals, as well as giving example applications.

Title: Ece2Cmor3
Authors: van den Oord, Gijs
Bibcode: 2017zndo...1051094V
Altcode:
  ece2cmor3 processes your EC-Earth output data to be CMIP6-compliant. It
  postprocesses the atmosphere output and adds the correct metadata to
  all netcdf files for publishing on your egsf data node.

Title: Ozone monitoring instrument flight-model on-ground and
    inflight calibration
Authors: Dobber, Marcel; Dirksen, Ruud; Levelt, Pieternel F.; van den
   Oord, Gijsbertus; Jaross, Glen; Kowalewski, Matt; Mount, George H.;
   Heath, Donald; Hilsenrath, Ernest; Cebula, Richard
Bibcode: 2017SPIE10568E..0CD
Altcode:
  The Ozone Monitoring Instrument (OMI) is an ultravioletvisible imaging
  spectrograph that uses two-dimensional CCD detectors to register both
  the spectrum and the swath perpendicular to the flight direction. This
  allows having a 114 degrees wide swath combined with an unprecedented
  small ground pixel (nominally 13 x 24 km<SUP>2</SUP>), which in turn
  enables global daily ground coverage with high spatial resolution. The
  OMI instrument is part of NASA's EOSAURA satellite, which will be
  launched in the second half of 2004. The on-ground calibration of the
  instrument was performed in 2002. This paper presents and discusses
  results for a number of selected topics from the on-ground calibration:
  the radiometric calibration, the spectral calibration and spectral
  slit function calibration. A new method for accurately calibrating
  spectral slit functions, based on an echelle grating optical stimulus,
  is discussed. The in-flight calibration and trend monitoring approach
  and facilities are discussed.

Title: In-flight calibration of the ozone monitoring instrument
Authors: Dirksen, Ruud; Dobber, Marcel; Voors, Robert; Kleipool,
   Quintus; van den Oord, Gijsbertus; Levelt, Pieternel
Bibcode: 2017SPIE10567E..24D
Altcode:
  This paper discusses various aspects of the on-ground and in-flight
  calibration of the OMI instrument.

Title: Sensitivity analysis of a new SWIR-channel measuring
    tropospheric CH <SUB>4</SUB> and CO from space
Authors: Jongma, Rienk T.; Gloudemans, Annemieke M. S.; Hoogeveen,
   Ruud W. M.; Aben, Ilse; de Vries, Johan; Escudero-Sanz, Isabel;
   van den Oord, Gijsbertus; Levelt, Pieternel F.
Bibcode: 2006SPIE.6302E..14J
Altcode: 2006SPIE.6302E..29J
  In preparation for future atmospheric space missions a consortium
  of Dutch organizations is performing design studies on a nadir
  viewing grating-based imaging spectrometer using OMI and SCIAMACHY
  heritage. The spectrometer measures selected species (O <SUB>3</SUB>,
  NO <SUB>II</SUB>, HCHO, H <SUB>II</SUB>O, SO <SUB>II</SUB>, aerosols
  (optical depth, type and absorption index), CO and CH4) with sensitivity
  down to the Earth's surface, thus addressing science issues on air
  quality and climate. It includes 3 UV-VIS channels continuously covering
  the 270-490 nm range, a NIR-channel covering the 710-775 nm range,
  and a SWIR-channel covering the 2305-2385 nm range. This instrument
  concept is, named TROPOMI, part of the TRAQ-mission proposal to ESA in
  response to the Call for Earth Explorer Ideas 2005, and, named TROPI,
  part of the CAMEO-proposal prepared for the US NRC decadal study-call
  on Earth science and applications from space. The SWIR-channel is
  optional in the TROPOMI/TRAQ instrument and included as baseline in
  the TROPI/CAMEO instrument. This paper focuses on derivation of the
  instrument requirements of the SWIR-channel by presenting the results
  of retrieval studies. Synthetic detector spectra are generated by
  the combination of a forward model and an instrument simulator that
  includes the properties of state-of-the-art detector technology. The
  synthetic spectra are input to the CO and CH <SUB>4</SUB> IMLM
  retrieval algorithm originally developed for SCIAMACHY. The required
  accuracy of the Level-2 SWIR data products defines the main instrument
  parameters like spectral resolution and sampling, telescope aperture,
  detector temperature, and optical bench temperature. The impact of
  selected calibration and retrieval errors on the Level-2 products has
  been characterized. The current status of the SWIR-channel optical
  design with its demanding requirements on ground-pixel size, spectral
  resolution, and signal-to-noise ratio will be presented.

Title: In-Flight Calibration of the Ozone Monitoring Instrument
Authors: Dirksen, R.; Dobber, M.; Voors, R.; Kleipool, Q.; van den
   Oord, G.; Levelt, P.
Bibcode: 2006ESASP.621E..84D
Altcode: 2006spop.confE..84D
  No abstract at ADS

Title: Ozone Monitoring Instrument flight-model on-ground and
    in-flight calibration
Authors: Dobber, Marcel; Dirksen, Ruud; Levelt, Pieternel F.; van den
   Oord, Gijsbertus; Jaross, Glen; Kowalewski, Matt; Mount, George H.;
   Heath, Donald; Hilsenrath, Ernest; Cebula, Richard
Bibcode: 2004ESASP.554...89D
Altcode: 2004icso.conf...89D
  The Ozone Monitoring Instrument (OMI) is an ultraviolet-visible imaging
  spectrograph that uses two-dimensional CCD detectors to register both
  the spectrum and the swath perpendicular to the flight direction. This
  allows having a 114 degrees wide swath combined with an unprecedented
  small ground pixel (nominally 13×24 km<SUP>2</SUP>), which in turn
  enables global daily ground coverage with high spatial resolution. The
  OMI instrument is part of NASA's EOS-AURA satellite, which will be
  launched in the second half of 2004. The on-ground calibration of the
  instrument was performed in 2002. This paper presents and discusses
  results for a number of selected topics from the on-ground calibration:
  the radiometric calibration, the spectral calibration and spectral
  slit function calibration. A new method for accurately calibrating
  spectral slit functions, based on an echelle grating optical stimulus,
  is discussed. The in-flight calibration and trend monitoring approach
  and facilities are discussed.

Title: EOS-Aura's Ozone Monitoring Instrument (OMI): Performance
    and science goals
Authors: Levelt, P. F.; van den Oord, G. H. J.; Veefkind, J. P.;
   de Haan, J. F.; Hilsenrath, E.; Bhartia, P. K.; Leppelmeier, G. W.;
   Maelkki, A.
Bibcode: 2004cosp...35.2822L
Altcode: 2004cosp.meet.2822L
  The Ozone Monitoring Instrument (OMI) will fly on NASA's EOS-AURA
  satellite, now scheduled for launch in June 2004. OMI is a UV/VIS,
  nadir viewing spectrometer that will provide near global coverage of
  solar backscatter radiances in one day. OMI has several technological
  advances, but has heritage from the TOMS, SBUV, GOME, GOMOS and
  SCIAMACHY. OMI employs a wide field telescope to image a 2600 km wide
  Earth swath onto two CCDs, which detect light from two wavelength
  bands (UV and VIS). Using the wavelength range 270 to 500 nm with
  a 0.5 nm resolution, OMI will measure several key parameters for
  stratospheric and tropospheric chemistry and for climate research,
  including O<SUB>3</SUB>, NO<SUB>2</SUB>, SO<SUB>2</SUB>, OClO, HCHO,
  BrO, UVB, aerosols, and cloud heights and fraction. Combining OMI data
  with the other Aura instruments will allow derivation of tropospheric
  gases important for air quality studies. OMI's high spatial resolution
  (13 x 24 km<SUP>2</SUP>) will allow observation between clouds, thus
  giving better penetration into the troposphere than any other UV/VIS
  backscatter instrument flown to date. Proven TOMS and DOAS retrieval
  algorithms for trace gases will be employed as well as new ones,
  which will take advantage of experience gained by both European and
  US scientists. OMI ozone data will be of the same quality as TOMS to
  insure continuity of ozone trends detected to date. Tropospheric ozone
  will be retrieved using two algorithms developed for TOMS but tailored
  for OMI and using ozone profiles from all Aura instruments. Clouds
  heights will be retrieved using two different algorithms. Some of
  these algorithms are discussed in other EGS sessions, while all OMI
  algorithms are available at http://eospso.gsfc.nasa.gov/. In this
  paper performance characteristics with respect to meeting the science
  requirements will be presented.

Title: Ozone Monitoring Instrument: system description and test
    results
Authors: te Plate, Maurice B.; Draaisma, F.; de Vries, Johan; van
   den Oord, Gijsbertus H.
Bibcode: 2001SPIE.4540..122T
Altcode:
  The Ozone Monitoring Instrument (OMI) is a nadir viewing wide field
  imaging spectrometer for ozone monitoring. The instrument is the
  Dutch/Finnish contribution to the NASA EOS-AURA mission. OMI observes
  earth's back scattered radiation in two spectral channels: the UV
  channel (270 nm - 350 nm) and the VIS channel (350 nm - 500 nm). Each
  channel employs a CCD detector (576 X 780 px). The extreme wide field
  of view of 114 degrees, equal to a swath wide of 2600 km, is obtained
  by an all reflective telecentric telescope and enables global ozone
  coverage in one day. Other key features are the spectral range (270
  nm - 500 nm) and resolution (spectral sampling distance 0.15 - 0.32
  nm/px), the application of a polarization scrambler and its compact
  design (400 X 300 X 500 mm). Excellent stray light performance in the
  UV channel is obtained by an elegant opto-mechanical design of the UV
  optics where the UV wavelength range is split in two parts with separate
  optical paths and the separate spectra are imaged on one CCD. Onboard
  calibration includes a white light source, LEDs, and multi-surface
  solar-calibration diffuser. The OMI-EOS project follows a Proto-Flight
  approach, supported by breadboards and engineering qualification models
  on parts and sub-system level. In order to increase confidence in the
  design, the instrument development model was built. During intensive
  testing critical performance parameters were checked , e.g. UV stray
  light behavior, polarization sensitivity, distortion, spatial and
  spectral ranges and resolutions.

Title: Exploring the dynamical nature of the lower solar chromosphere
Authors: Doyle, J. G.; van den Oord, G. H. J.; O'Shea, E.; Banerjee, D.
Bibcode: 1999A&A...347..335D
Altcode:
  We examine spectral time-series of two lower-chromospheric lines (N i
  1319 Ä and C ii 1335 Ä) observed with the SUMER instrument on the SOHO
  spacecraft. We point out differences between (intensity and velocity)
  power spectra of network and internetwork regions and argue that the
  behaviour resembles that of Ca ii power spectra. No significant phase
  differences are found between the intensities of both lines. However,
  when phase spectra are averaged along the slit there is some evidence
  that the C ii intensity lags that of N i by 16 sec near 3 mHz. Intensity
  power spectra of C ii are affected at higher frequencies by streams
  of emitting structures. Using contrast-enhanced time slices we show
  that 1) there exists a grain-like pattern which is found in both
  network and internetwork regions; 2) streams of supersonically moving
  structures probably outline a wave interference pattern; 3) the sizes
  of structures observed in N i are smaller than when observed in C ii. At
  various points our findings disagree with earlier results from SUMER. A
  cookbook formalism is presented to derive confidence levels for power,
  phase, gain and coherency spectra.

Title: Stellar Flares: Theory and Observations
Authors: van den Oord, G. H. J.
Bibcode: 1999ASPC..158..189V
Altcode: 1999ssa..conf..189V
  No abstract at ADS

Title: Dynamical Nature of the Quiet Solar Outer Atmosphere
Authors: Banerjee, D.; Doyle, J.; O'Shea, E.; van den Oord, G.
Bibcode: 1999ASPC..183..389B
Altcode: 1999hrsp.conf..389B
  No abstract at ADS

Title: The oscillatory nature of the upper solar atmosphere.
Authors: O'Shea, E.; Doyle, J. G.; Banerjee, D.; van den Oord, G. H. J.
Bibcode: 1999joso.proc..128O
Altcode:
  The authors examine time series spectral data obtained by the SUMER
  instrument onboard SOHO. Lines of N V 1242 Å, Si II 1533 Å, C IV 1548
  Å, C II 1037 Å and O VI 1037.6 Å were measured and found to show
  intensity oscillations. The oscillations were usually observed to occur
  in small regions along the slit, limited to a few arcsec. N V 1242 Å
  typically showed frequencies of between 1 - 2 mHz. The Si II and C IV
  lines observed simultaneously were found to have differing oscillation
  frequency ranges of 2 - 8 mHz and 2 - 4 mHz respectively. The C II
  and O VI lines also measured simultaneously were both found to have
  oscillation frequency ranges of 2 - 4 mHz. In addition a time lag
  of ≡50 seconds was measured between the intensity oscillations in
  these two lines, perhaps signifying a propagating wave from the low
  temperature C II line region to the higher temperature O VI region of
  the atmosphere.

Title: The effect of delays on filament oscillations and stability
Authors: van den Oord, G. H. J.; Schutgens, N. A. J.; Kuperus, M.
Bibcode: 1998A&A...339..225V
Altcode:
  We discuss the linear response of a filament to perturbations, taking
  the finite communication time between the filament and the photosphere
  into account. The finite communication time introduces delays in the
  system. Recently Schutgens (1997ab) investigated the solutions of the
  delay equation for vertical perturbations. In this paper we expand his
  analysis by considering also horizontal and coupled oscillations. The
  latter occur in asymmetric coronal fields. We also discuss the effect
  of Alfven wave emission on filament oscillations and show that wave
  emission is important for stabilizing filaments. We introduce a fairly
  straightforward method to study the solutions of delay equations
  as a function of the filament-photosphere communication time. A
  solution can be described by a linear combination of damped harmonic
  oscillations each characterized by a frequency, a damping/growth
  time and, accordingly, a quality factor. As a secondary result of
  our analysis we show that, within the context of line current models,
  Kippenhahn/Schlüter-type filament equilibria can never be stable in
  the horizontal and the vertical direction at the same time but we also
  demonstrate that Kuperus/Raadu-type equilibria can account for both
  an inverse or a normal polarity signature. The diagnostic value of our
  analysis for determining, e.g., the filament current from observations
  of oscillating filaments is discussed.

Title: Large-scale coronal heating by the small-scale magnetic field
    of the Sun
Authors: Schrijver, C. J.; Title, A. M.; Harvey, K. L.; Sheeley,
   N. R.; Wang, Y. -M.; van den Oord, G. H. J.; Shine, R. A.; Tarbell,
   T. D.; Hurlburt, N. E.
Bibcode: 1998Natur.394..152S
Altcode:
  Magnetic fields play a crucial role in heating the outer atmospheres
  of the Sun and Sun-like stars, but the mechanisms by which magnetic
  energy in the photosphere is converted to thermal energy in the corona
  remain unclear. Observations show that magnetic fields emerge onto
  the solar surface as bipolar regions with a broad range of length
  scales. On large scales, the bipolar regions survive for months before
  dispersing diffusively. On the smaller scales, individual bipolar
  regions disappear within days but are continuously replenished by new
  small flux concentrations, resulting in a sustained state of mixed
  polarity. Here we determine the rate of emergence of these small
  bipolar regions and we argue that the frequent magnetic reconnections
  associated with these regions (an unavoidable consequence of continued
  flux replacement) will heat the solar atmosphere. The model that
  describes the details of these mixed-polarity regions is complementary
  to the traditional diffusion model for large-scale flux dispersal and
  a combination of the two should lead to a more complete understanding
  of the role of magnetic fields in stellar atmospheres.

Title: Waves in the solar transition region
Authors: Doyle, J. G.; van den Oord, G. H. J.; O'Shea, E.; Banerjee, D.
Bibcode: 1998SoPh..181...51D
Altcode:
  The power spectra for line intensities of several lines formed in the
  upper transition region around 100000 to 250000 K are presented. A
  period of ∼5 min is clearly present in lines due to Oiii, Oiv,
  and Ov. In one dataset a period approaching 10 min is present for
  ∼40 min. The size of the emitting features is limited to ∼7 arc
  sec squared. In all datasets examined, there is excess power below 4
  mHz everywhere along the slit, although the observed periods do not
  always come from the most intense regions. In ∼40% of instances clear
  periods are observable in the 2-5 mHz range with the largest power peak
  at 3.0 mHz. In all regions, the 5.0 mHz power peak is smaller. For the
  frequencies investigated there are no significant time delays in any of
  the datasets examined. This finding may not be entirely unexpected as
  the formation temperatures of Oiii (∼100000 K) and Ov (∼250000 K)
  may be too close in order to result in an observable phase shift.

Title: Evidence of Prominences on Cool Late-Type Stars (Review)
Authors: Byrne, P. B.; Eibe, M. T.; van den Oord, G. H. J.
Bibcode: 1998ASPC..150..227B
Altcode: 1998IAUCo.167..227B; 1998npsp.conf..227B
  No abstract at ADS

Title: Prominence Oscillations and the Influence of the Distant
    Photosphere
Authors: Schutgens, N. A. J.; Kuperus, M.; van den Oord, G. H. J.
Bibcode: 1998ASPC..150..147S
Altcode: 1998IAUCo.167..147S; 1998npsp.conf..147S
  No abstract at ADS

Title: Prominence-like Clouds Near HK AQR
Authors: van den Oord, G. H. J.; Bryne, P. B.; Elbe, M. T.
Bibcode: 1998ASPC..150..251V
Altcode: 1998IAUCo.167..251V; 1998npsp.conf..251V
  No abstract at ADS

Title: Randomly sampling the chromospheric peak power distribution.
Authors: Doyle, J. G.; van den Oord, G. H. J.; O'Shea, E.
Bibcode: 1997A&A...327..365D
Altcode:
  We have analyzed the UV continuum light curves of 2535 pixels
  obtained during fifteen separate observations of active regions
  with the Ultraviolet Spectrometer and Polarimeter (UVSP) on board
  the solar maximum mission in 1989. Specifically we have looked for
  periodicities. In the power spectra of 738 light curves (i.e., 29%),
  evidence was found for periodicities at multiple frequencies. For
  each power spectrum, containing significant power, we determined the
  frequency at which the maximum power is found. The distribution N(ν),
  which describes the number of pixels having maximum power at frequency
  ν, is strongly concentrated in the 2-5mHz band with a distinct maximum
  at 3-3.5mHz. No pixels had their maximum power above 10mHz and only
  a few had their maximum power in the 5-10mHz range. The oscillations
  in the 2-5mHz range are probably related to evanescent acoustic waves
  driven by the photospheric five minute oscillations. In the 2-5mHz band,
  the distribution N(ν) resembles very much the power spectra as have
  been observed near the temperature minimum. This suggests that the
  photospheric power distribution can be interpreted as a probability
  distribution for finding a given frequency in the UV continuum light
  curves. In the 2-5mHz band the spread of the maximum powers and the
  spread of the count rates, at any frequency, is much larger than above
  5mHz where both are relatively constant. No clear correlation is found
  between the maximum power and the count rate in a pixel. Furthermore,
  no evidence is found for emission from the shocks which have recently
  been invoked to explain the formation of CaII grains. Whether this is
  due to temperature effects is unclear. Our general procedure adopted to
  estimate the confidence level in the power spectrum of a light curve
  containing photon noise can be equally applied to SUMER and CDS data
  from SOHO.

Title: EUV spectroscopy of cool stars. III. Interpretation of EUVE
    spectra in terms of quasi-static loops.
Authors: van den Oord, G. H. J.; Schrijver, C. J.; Camphens, M.;
   Mewe, R.; Kaastra, J. S.
Bibcode: 1997A&A...326.1090V
Altcode:
  We discuss the limitations of coronal spectroscopy to derive physical
  parameters of stellar magnetic loops. We distinguish between the
  intrinsic non-uniqueness of emitted spectra for models of quasi-static
  coronal loops, and the supplemental ambiguity introduced by both
  instrumental effects and spectral line formation. We demonstrate
  that the spectrum emitted by loops with constant cross-sections is
  the same for a large range of values of the conductive flux at the
  base when the apex temperature is fixed. Because it is impossible to
  estimate the conductive flux at the base from observations, it is also
  impossible to determine the volume heating rate and the loop length
  uniquely. For geometrically expanding (tapered) loops, the emitted
  spectrum depends on the expansion and on the conductive flux at the
  base, and there is a trade off between them without significant changes
  in the spectrum. We show that loop length and heating rate can only be
  derived if the density is known, but that even then a large intrinsic
  uncertainty remains for these loop parameters. We conclude that there
  is no unambiguous relationship between loop parameters and emitted
  spectra: modeling the spectra as the sum of spectra from discrete loops
  cannot result in a unique determination of coronal structure. Based
  on spectra observed with the Extreme Ultra Violet Explorer (EUVE) we
  find that quasi-static loop models allow adequate modeling of stellar
  coronal spectra. We show that coronal loops on active cool stars must
  expand with height. The minimum required areal expansion between base
  and apex is not very large, lying between 2 and 5. For three stars
  (α Cen, Capella and ξ UMa) the observations suggest the presence of
  two distinct, dominant loop populations, while for χ^1^ Ori a single
  population, characterized by a single apex temperature, suffices. The
  high electron densities (10^12^-10^13^cm^-3^) for coronal components
  on Capella and ξ UMa require abnormally large heating rates. It is
  likely that these high densities are related to a multitude of small
  volumes that are temporarily excited.

Title: ASCA and EUVE observations of II Pegasi: flaring and quiescent
    coronal emission.
Authors: Mewe, R.; Kaastra, J. S.; van den Oord, G. H. J.; Vink, J.;
   Tawara, Y.
Bibcode: 1997A&A...320..147M
Altcode:
  We have analyzed X-ray and EUV spectra of both the quiescent and
  flaring state of II Peg, obtained from observations with ASCA and
  EUVE. Coronal temperature structure and abundances have been derived
  from multi-temperature and differential emission measure (DEM) analyses
  of the spectra. The abundances are non-solar; in the case of ASCA for
  most elements (O, Ne, Mg, Si, S, Ar, Ca, Ni) we obtain abundances that
  are consistent with about 1/2-1/5 of the solar photospheric abundances
  of Anders and Grevesse (1989, Geochim. Cosmochim. Acta 53, 197),
  but the Fe abundance is even lower, i.e. 0.1xSolar. The multi-T and
  DEM fitting analysis shows that the quiescent EUVE and ASCA spectra
  can be described by two temperature components: 4 and 10MK (EUVE),
  10 and 20MK (ASCA). The two flares detected by EUVE and ASCA show
  peak temperatures of 20 and &gt;35MK, respectively. The latter flare
  has a total energy (0.1-10keV) of 2.7x10^34^erg, a peak luminosity of
  2.6x10^30^erg/s. There is evidence for an increase of a factor ~4 of
  the iron abundance during the rise phase of the flare. Application of
  a cooling model yields a loop height of about 8x10^10^cm and a plasma
  density of 8x10^10^cm^-3^.

Title: Constraints on mass loss from dMe stars: theory and
    observations.
Authors: van den Oord, G. H. J.; Doyle, J. G.
Bibcode: 1997A&A...319..578V
Altcode:
  We show that the flux distribution of a wind from a cool star
  differs considerably from what is predicted by the theory for mass
  loss from hot stars. The differences are caused by the facts that 1)
  the mass loss rates are lower, resulting in smaller optical depths
  in the wind, and 2) for winds from cool stars the temperature of the
  wind is higher than the temperature of the star while for winds from
  hot stars the reverse holds. These differences result in substantial
  modifications of the flux distribution and imply that care must be
  exercised when applying the flux predictions by e.g. Wright and Barlow
  (1975MNRAS.170...41W) to winds from cool stars. By using observational
  constraints we show that the mass loss from cool dwarf stars equals at
  most 10^-12^M<SUB>sun</SUB>_/yr. This is a factor hundred lower than
  previous estimates. At this rate the mass loss from dMe stars is of
  little importance for the enrichment of the interstellar medium. By
  solving the radiative transfer equations for stellar winds from dMe
  stars, we show that the inferred power-law flux distributions, based
  on radio, JCMT and IRAS data, cannot be reconciled with the flux
  distributions from a stellar wind of 10^-10^M<SUB>sun</SUB>_/yr as
  was previously assumed. The maximum allowable mass loss rate is at
  most a few times 10^-12^M<SUB>sun</SUB>_/yr which implies that the
  fluxes observed with JCMT, IRAS, and in the future with ISO, require
  a different interpretation than free-free emission from a stellar wind.

Title: Book reviews
Authors: Howard, R. F.; van den Oord, G. H. J.; Švestka, Z.
Bibcode: 1996SoPh..169..225H
Altcode:
  No abstract at ADS

Title: Flare energetics: analysis of a large flare on YZ Canis
    Minoris observed simultaneously in the ultraviolet, optical and radio.
Authors: van den Oord, G. H. J.; Doyle, J. G.; Rodono, M.; Gary,
   D. E.; Henry, G. W.; Byrne, P. B.; Linsky, J. L.; Haisch, B. M.;
   Pagano, I.; Leto, G.
Bibcode: 1996A&A...310..908V
Altcode:
  The results of coordinated observations of the dMe star YZ CMi
  at optical, UV and radio wavelengths during 3-7 February 1983 are
  presented. YZ CMi showed repeated optical flaring with the largest
  flare having a magnitude of 3.8 in the U-band. This flare coincided
  with an IUE exposure which permits a comparison of the emission measure
  curves of YZ CMi in its flaring and quiescent state. During the flare a
  downward shift of the transition zone is observed while the radiative
  losses in the range 10^4^-10^7^K strongly increase. The optical flare
  is accompanied with a radio flare at 6cm, while at 20cm no emission
  is detected. The flare is interpreted in terms of optically thick
  synchrotron emission. We present a combined interpretation of the
  optical/radio flare and show that the flare can be interpreted within
  the context of solar two-ribbon/white-light flares. Special attention
  is paid to the bombardment of dMe atmospheres by particle beams. We
  show that the characteristic temperature of the heated atmosphere is
  almost independent of the beam flux and lies within the range of solar
  white-light flare temperatures. We also show that it is unlikely that
  stellar flares emit black-body spectra. The fraction of accelerated
  particles, as follows from our combined optical/radio interpretation
  is in good agreement with the fraction determined by two-ribbon flare
  reconnection models.

Title: DEM Analyses with the Utrecht Codes
Authors: Mewe, R.; van den Oord, G. H. J.; Schrijver, C. J.; Kaastra,
   J. S.
Bibcode: 1996aeu..conf..553M
Altcode: 1996IAUCo.152..553M
  No abstract at ADS

Title: EUV spectroscopy and coronal loop models
Authors: van den Oord, G. H. J.; Schrijver, C. J.; Mewe, R.; Kaastra,
   J. S.
Bibcode: 1996ASPC..109..231V
Altcode: 1996csss....9..231V
  No abstract at ADS

Title: Are Some Stellar Coronae Optically Thick?
Authors: Schrijver, C. J.; van den Oord, G. H. J.; Mewe, R.; Kaastra,
   J. S.
Bibcode: 1996aeu..conf..121S
Altcode: 1996IAUCo.152..121S
  No abstract at ADS

Title: Non-thermal Eimission Mechanisms in Stellar Corona
Authors: van den Oord, G. H. J.
Bibcode: 1996ASPC...93..263V
Altcode: 1996ress.conf..263V
  No abstract at ADS

Title: Is the optical depth of stellar coronae really negligible?
Authors: Schrijver, C. J.; Mewe, R.; Kaastra, J. S.; van den Oord,
   G. H. J.; Bruls, J. H. M. J.
Bibcode: 1996ASPC..109..289S
Altcode: 1996csss....9..289S
  No abstract at ADS

Title: Coronal loops and their modeling (review)
Authors: van den Oord, G. H. J.; Zuccarello, F.
Bibcode: 1996IAUS..176..433V
Altcode:
  No abstract at ADS

Title: EUV spectroscopy of cool stars. II. Coronal structure of
    selected cool stars observed with the EUVE.
Authors: Schrijver, C. J.; Mewe, R.; van den Oord, G. H. J.; Kaastra,
   J. S.
Bibcode: 1995A&A...302..438S
Altcode:
  We analyze the coronal EUV spectra of seven cool stars, solar-like
  single stars and components of RS CVn-like binaries, as observed with
  the Spectrometers of the Extreme Ultraviolet Explorer (EUVE). The
  observations cover the wavelength range of 60A up to 800A with a
  resolution of λ/{DELTA}λ=~160-320. The data constrain the coronal
  temperature structure between several hundred thousand Kelvin up
  to roughly 10 million Kelvin through a differential emission measure
  analysis. The resulting differential emission measure distributions show
  distinct features from source to source, but the common properties are
  a) a relatively weak emission from coronal plasma below about 1MK, b)
  a dominant component somewhere between 2MK and about 10MK, often peaking
  at solar-like coronal temperatures of 2 to 4MK, and c) in all cases but
  χ^1^ Ori a very hot component in the formal solution with a temperature
  exceeding several tens of million of Kelvin. This hot tail in the
  differential emission measure distribution may reflect, as discussed
  in this paper, one or several of the following sources or processes:
  a real hot component, a reduced coronal abundance of heavy elements,
  or scattering in some of the strongest coronal lines with subsequent
  photon destruction upon impact on the lower, dense atmosphere. Coronal
  electron densities of brightly emitting regions are constrained by an
  analysis of ratios of density-sensitive iron lines. Strengths of Fe
  XIX-Fe XXII lines (corresponding to a temperature range of T=6-11MK) for
  α Aur, AU Mic (of which the spectrum is dominated by a large flare),
  ξ UMa, and σ Gem suggest typical electron densities in the range
  n_e_~10^12^-10^13^cm^-3^. Cooler Fe X and Fe XII-Fe XIV lines (T=1-2MK)
  in the case of α CMi suggest n_e_~10^9^-10^10^cm^-3^. In general, the
  electron densities of the hot 5-15MK components are some three orders
  of magnitude larger than typical of the solar-like component around
  2MK; the volume filling factors of the hot components are therefore
  expected to be substantially smaller than those of the cooler component.

Title: The electrodynamics of beam/return current systems. The effect
    on the accelerator.
Authors: van Oss, R. F.; van den Oord, G. H. J.
Bibcode: 1995A&A...299..297V
Altcode:
  The initial value problem of the injection of an electron beam into a
  plasma cylinder is investigated. Emphasis is put on the effect of the
  return current on the acceleration region. The full set of Maxwell's
  equations is solved in combination with Ohm's law. By splitting all
  vector fields into their solenoidal and irrotational components,
  the inductive and electrostatic response of the plasma electrons to
  the beam injection can be described separately. It is found that the
  effect of the return current on the beam acceleration region, located
  somewhere in the plasma cylinder, is negligible on time scales shorter
  than the time scale for magnetic diffusion. In realistic circumstances
  the duration of the acceleration process is much shorter than this time
  scale so that the accelerator does not experience any influence of the
  return current. Several cylinder radii from the accelerator site our
  solutions approach those obtained in earlier work where a self-similar
  approach was used. The energy partitioning in the beam/return current
  system is discussed.

Title: Book-Review - Fragmented Energy Release in Sun and Stars -
    the Interface Between Magnetohydrodynamics and Plasma Physics
Authors: van den Oord, G. H. J.; Rijnbeek, R. P.
Bibcode: 1995Obs...115...94V
Altcode:
  No abstract at ADS

Title: EUV spectroscopy of cool stars. I. The corona of α Centauri
    observed with EUVE. R
Authors: Mewe, R.; Kaastra, J. S.; Schrijver, C. J.; van den Oord,
   G. H. J.; Alkemade, F. J. M.
Bibcode: 1995A&A...296..477M
Altcode:
  We perform an emission measure analysis of spectra of the coronae of
  the cool star binary α Cen (A: HD 128620 (G2 V) + B: HD 128621 (K1 V))
  as observed with the Spectrometers of the Extreme UltraViolet Explorer
  (EUVE). These observations, covering the wavelength range of 60A up
  to 800A with a resolution of λ/{DELTA}λ=~160-320, constrain the
  coronal temperature structure between about 10^5^K up to roughly 10MK,
  while some additional line and continuum information is available to
  extend that coverage from several tens of thousands of Kelvin up to
  several tens of millions of Kelvin, although the solution is poorly
  constrained in the extended range. We performed a re-calibration of
  the wavelength scale for each of the three instrumental pass bands by
  comparing the model list of emission lines with the spectra of α Cen
  and eight other cool stars. The high signal-to-noise ratio of the α
  Cen spectrum yielded indications for a Lorentzian rather than a Gaussian
  line profile for the MW passband. The observed spectrum was decomposed
  into a linear combination of spectra emitted by isothermal plasmas in
  thermal equilibrium, using the SPEX plasma emission code developed in
  Utrecht. The resulting differential emission measure distribution shows:
  a) emission from plasma below ~5MK with a broad peak around ~3MK and
  with a tail extending down to about 0.5MK, b) very little emission from
  plasma between 0.1MK and 0.5MK, c) emission from plasma below 0.1MK,
  and d) possibly a hot component exceeding several tens of million of
  Kelvin which, however, may (in part) have another origin than a hot
  coronal plasma component. We propose that the hot component may be
  an artifact of the corona being close to unit optical thickness for
  resonant scattering in the strongest spectral lines, so that part
  of the line photons can be destroyed by being scattered towards the
  stellar surface while the plasma remains optically thin for continuum
  photons. Diagnostics for the electron density n_e_, using Fe X, XII,
  XIII, and XIV lines, yields values in the range 2-20x10^8^cm^-3^
  at T=1-2MK.

Title: Books-Received - Fragmented Energy Release in Sun and Stars -
    the Interface Between MHD and Plasma Physics
Authors: van den Oord, G. H. J.
Bibcode: 1994Sci...266..310V
Altcode:
  No abstract at ADS

Title: The optical thickness of stellar coronae in the EUV lines.
Authors: Schrijver, C. J.; van den Oord, G. H. J.; Mewe, R.
Bibcode: 1994A&A...289L..23S
Altcode:
  Stellar coronae are commonly assumed to be optically thin. Recent
  spectroscopic observations in the EUV, however, lead us to question
  the validity of this approximation for strong lines. We argue that
  scattering may significantly affect the strongest coronal lines in
  coronae composed of magnetic loops, possibly embedded in a hot stellar
  wind. Even if the average number of scatterings per photon in some
  coronal lines is only of order unity, the relative line strenghts
  and the line-to-continuum ratio can be significantly affected in
  non-symmetric inhomogeneous atmospheres: photons in weak lines and
  in the optically thin continuum escape without any scattering, but
  strong lines can be weakened or enhanced depending on the balance
  between outward traveling line photons that are scattered back toward
  the stellar surface (if not lost by branching), there destroyed by
  absorption, and downward traveling line photons that are scattered
  upward and escape. We draw attention to the fact that line scattering
  due to the non-negligible optical thickness in strong coronal lines
  can have severe implications for differential emission measure models
  and for abundance and density determinations, while it may serve as
  a diagnostic for the existence of tenuous hot winds.

Title: Book Review: Whistler-mode waves in a hot plasma / Cambridge
    U Press, 1993
Authors: van den Oord, G. H. J.
Bibcode: 1994SSRv...69..414V
Altcode: 1994SSRv...69..414S
  No abstract at ADS

Title: Unusual flares at 360 and 609 MHz in the RS Canum
Venaticorum-system II Pegasi observed with the WSRT: coherent emission
    from II Pegasi
Authors: van den Oord, G. H. J.; de Bruyn, A. G.
Bibcode: 1994A&A...286..181V
Altcode:
  We report on the detection of flare and quiescent emission from the
  RS CVn system II Peg at 360 and 609 MHz. The radio flare at 360 MHz
  lasted for half an hour and showed strong, spikelike variability on
  time scales probably shorter than the instrumental time resolution
  of one minute. The emission is coherent and must occur at or near the
  local plasma frequency in the source. Electron cyclotron maser emission
  can be excluded because of the low magnetic field strengths required
  at this observing frequency. Plasma emission by a moving exciter
  (beam or shock) can be excluded on base of the available spectral
  information. Plasma maser emission originating in a collection of
  double layers is consistent with the available spectral information. We
  discuss the role of free-free opacity which can dramatically lower
  the observed brightness temperature and influence the shape of the
  observed spectrum. The flare at 609 MHz was only observed during
  the decay phase. This flare has the typical characteristics of the
  incoherent radio flares previously observed in RS CVn systems except
  for an unusual high percentage of circular polarization (90%) which on
  a timescale of a few hours decreases to approximately 50%. We discuss
  the most likely emission mechanisms but are not able to provide a
  definite conclusion concerning the nature of the emission at 609 MHz.

Title: Fragmented Energy Release in Sun and Stars
Authors: van den Oord, G. H. J.
Bibcode: 1994SSRv...68....1V
Altcode:
  No abstract at ADS

Title: Foreword
Authors: van den Oord, Bert; Kuijpers, Jan; Kuperus, Max; Benz, A. O.;
   Brown, J. C.; Einaudi, G.; Kuperus, M.; Raadu, M. A.; Trottet, G.;
   van den Oord, G. H. J.; Vlahos, L.; Zheleznyakov, V. V.; Wijburg,
   Marion; Fletcher, Lyndsay; Volwerk, Martin
Bibcode: 1994SSRv...68D..17V
Altcode:
  No abstract at ADS

Title: Book Review: Physics of solar and stellar coronae: G. S. Vaiana
    memorial symposium / Kluwer, 1993
Authors: van den Oord, G. I. J.; van den Oord, G. H. J.
Bibcode: 1994SoPh..151..397V
Altcode: 1994SoPh..151..397L
  No abstract at ADS

Title: The role of current sheets in the flare development
Authors: van den Oord, G. H. J.
Bibcode: 1993AdSpR..13i.143V
Altcode: 1993AdSpR..13..143V
  A review is presented on the physics of energy release in current
  sheets during flares. The process of energy storage is discussed and
  it is shown how the evolution of the magnetic field may give rise to
  the presence of current sheets. The presently known types of magnetic
  energy release are discussed and attention is paid to the results of
  analytical/numerical models and laboratory experiments. Recent results
  on three-dimensional reconnection are summarized. The observational
  evidence that flare energy release is a highly time-dependent process
  puts severe constraints on the models and indicates that nonlinear
  processes are important. The large scale magnetic structure in which
  sheets are embedded is discussed for both compact and two-ribbon
  flares. Apart from the MHD-picture of current sheets some plasma
  physical aspects are discussed together with the energy partitioning
  in sheets (acceleration/heating).

Title: Accretion disk flares in energetic radiation fields. A model
    for hard X-rays from black hole candidates.
Authors: van Oss, R. F.; van den Oord, G. H. J.; Kuperus, M.
Bibcode: 1993A&A...270..275V
Altcode:
  An investigation is conducted of the magnetic flares in the energetic
  radiation field of an accretion-disk corona, whose X-ray emission
  may be the source of the hard power-law component in the X-ray
  spectra of galactic black-hole candidates in their 'high' spectral
  state. The concept of the low plasma-beta accretion-disk corona (ADC),
  which produces the hard X-rays via inverse Compton scattering of
  lower-energy photons from the disk on enegetic electrons in the corona,
  is elaborated. A spectrum is derived for the up-scattered radiation
  from current sheets in the ADC; this spectrum mimics a power-law above
  a critical photon energy.

Title: Accretion Disk Flares in Energetic Radiation Fields
Authors: van Oss, R. F.; van den Oord, G. H. J.; Kuperus, M.
Bibcode: 1993IAUS..157..217V
Altcode:
  No abstract at ADS

Title: Emission up to 18 ke V in the Quiescent X-ray Spectrum of
    II Peg
Authors: Doyle, J. G.; van den Oord, G. H. J.; Kellett, B. J.
Bibcode: 1993ASSL..183..311D
Altcode: 1993pssc.symp..311D
  No abstract at ADS

Title: The Effect of Retardation on the Stability of Current Filaments
Authors: van den Oord, G. H. J.; Kuperus, M.
Bibcode: 1992SoPh..142..113V
Altcode:
  We investigate the influence of the finite Alfvén velocity on the
  evolution of an active region filament. In general, variations of
  a current result in variations of the magnetic fields which spread
  around with the Alfvén velocity. As a consequence of the fact that a
  magnetic field can only change with the Alfvén velocity, a filament
  will experience the photospheric boundary conditions as these were at
  an Alfvén travel time back in time. The inclusion of this retardation
  effect in the momentum equation of a filament leads effectively to
  an extra force term. This force contribution acts in the direction in
  which the filament moves and has therefore a destabilizing effect on
  the filament. Because a moving filament acts as an antenna of Alfvén
  waves, the filament loses energy by the emission process. This leads to
  a radiative damping term in the equation of motion of the filament. In
  general, the radiative damping will be sufficiently strong to counteract
  the retardation instability. Numerical simulations show that during
  the energy build-up phase a filament follows the van Tend-Kuperus
  equilibrium curve. After the van Tend-Kuperus equilibrium has
  disappeared the filament goes through a transient phase moving with a
  sub-Alfvénic velocity upward. At greater heights the repulsive Lorentz
  force of the photospheric surface current magnetic field is balanced
  by the radiative damping, resulting in a decreasing filament velocity.

Title: Book Review: Basic plasma processes on the sun (IAU symposium
    no. 142) / Kluwer, 1990
Authors: van den Oord, G. H. J.
Bibcode: 1992SSRv...61..424V
Altcode: 1992SSRv...61..424P
  No abstract at ADS

Title: Retardation effects and the origin of filament oscillations.
Authors: Kuperus, M.; van den Oord, G. H. J.
Bibcode: 1992ESASP.346...39K
Altcode: 1992ssts.rept...39K
  A filament current subject to a perturbation experiences the
  photospheric boundary an Alfvén bounce time later. This retardation
  effect gives rise to an extra force which causes the filament to
  oscillate around an equilibrium position, emitting Alfvén waves which
  damp the filament oscillations. The onset of filament oscillations is a
  natural consequence of the presence of the inert photospheric boundary
  and the finite Alfvén speed. Numerical simulations are presented that
  show the evolution of a filament in a bipolar magnetic field as the
  filament current slowly increases. The emission of waves during this
  evolution is an important energy source for the coronal heating.

Title: Dynamic phenomena on the RS Canum Venaticorum binary II Pegasi
    in August 1989. II. an interpretation.
Authors: Doyle, J. G.; van den Oord, G. H. J.; Kellett, B. J.
Bibcode: 1992A&A...262..533D
Altcode:
  An analysis of the quiescent X-ray and radio observations and of the
  X-ray observations of a major flare of II Pegasi is presented. For both
  the flare and the quiescent emission, photometric, and spectroscopic
  optical data are also discussed. The quiescent X-ray emission shows
  a power-law tail up to 18 keV. It is shown that an interpretation
  in terms of free-free emission by nonthernal particles requires
  unrealistically large values for the number of particles involved
  and for the energy input required to maintain the population. Basic
  flare parameters like volume, loop length, and typical magnetic field
  strength are derived. The observed flare decay on II Peg is compared
  with the observed flare decay on Sigma CrB and Algol. It is shown
  that the behavior of the temperature and emission measure during the
  II Peg flare differs significantly from the observed behavior during
  the Algol/sigma CrB flares.

Title: Small scale processes in solar and stellar plasmas.
Authors: van den Oord, G. H. J.
Bibcode: 1992ESASP.344...97V
Altcode: 1992spai.rept...97V
  The author discusses the small scale processes which occur in the
  magnetically dominated outer layers of the Sun. He discusses the
  transport coefficients for the thermal and the electrical conductivity
  in the classical limit and in the limit of anomalous processes. It
  is demonstrated that the corona and the chromosphere must have a
  filamentary structure. Various heating mechanisms which are based
  on small scale dissipation of magnetic energy are discussed. The
  observational requirements for improved modelling of the transition
  region and the chromosphere are formulated and the consequences of
  high resolution observations for flare models are pointed out.

Title: Electrodynamics of Electron Beams in Solar Flares
Authors: van den Oord, G. H. J.
Bibcode: 1992IrAJ...20..188V
Altcode:
  The electrodynamical aspects of beam/return current systems during the
  impulsive phase of solar flares are discussed. The relevant equations
  for the electrostatic and the inductive response are given and the
  solutions are briefly discussed. The conditions for beam propagation are
  derived and it is shown that under certain conditions beam propagation
  is inhibited leading to bulk plasma heating.

Title: The Quiescent X-ray Spectrum of the RS CVn Binary II Peg in
    August 1989
Authors: Doyle, J. G.; Byrne, P. B.; van den Oord, G. H. J.; Kellett,
   B. J.; Bromage, G. E.
Bibcode: 1992ASPC...26..356D
Altcode: 1992csss....7..356D
  No abstract at ADS

Title: Coronal and transition region heating (review)
Authors: van den Oord, G. H. J.
Bibcode: 1992sccw.conf...11V
Altcode:
  No abstract at ADS

Title: Inference of 'bumps' in electron energy spectra from high
    resolution X-ray bremsstrahlung spectra
Authors: Brown, J. C.; MacKinnon, A. L.; van den Oord, G. H. J.;
   Trottet, G.
Bibcode: 1991A&A...242L..13B
Altcode:
  Regions of positive slope in an electron speed distribution function
  fs(v) are shown to result in sharp downward 'knees' (spectral
  index increases) in the emitted bremsstrahlung spectra. For Kramers'
  cross-section, a criterion is established for the detectability of such
  a 'bump' in the source electron spectrum from the photon spectrum,
  realizable from state of the art X-ray spectrometry, and applying a
  fortiori for more exact cross-sections. Inference of bumps in the thick
  target injection spectrum, however, demands spectral resolution far
  beyond present capabilities. Physical implications of bumps in fs(v)
  are discussed.

Title: Effect of Electron Beams during Solar Flares
Authors: Aboudarham, J.; Henoux, J. C.; Brown, J. C.; van den Oord,
   G. H. J.; van Driel-Gesztelyi, L.; Gerlei, O.
Bibcode: 1990SoPh..130..243A
Altcode:
  Electron bombardment of the solar atmosphere has two effects: one is
  to enhance hydrogen recombination emission, the other is to increase
  the opacity via an increase of H<SUP>−</SUP> population. The first
  effect is the most important in the upper part of the atmosphere and
  the second in the lower part. We predict that, when enhanced absorption
  dominates in the part of the atmosphere where radiation originates,
  there will be a decrease in the white-light emission, leading to a
  `negative flare', or what we call a `Black-Light Flare'. This phenomenon
  occurs only for a short duration, not more than ∼ 20 s. `Black-Light
  Flares' have already been observed in the case of flare stars and we
  suggest here that they could also be present on the Sun, just prior
  to a White-Light Flare.

Title: Observations of Beam Propagation
Authors: Pick, M.; van den Oord, G. H. J.
Bibcode: 1990SoPh..130...83P
Altcode:
  We discuss the observational evidence for propagating electron
  distributions in the solar corona as obtained from type III
  observations. The location of the beam acceleration region together with
  the effects of the ambient medium on beam propagation are discussed in
  the framework of recent observational and theoretical developments. The
  results of combined X-ray and radio observations obtained during and
  after the Solar Maximum Year are summarized. Special attention is
  paid to the effect of the small-scale structure of the corona on the
  observed radio emission. We outline various desirable developments
  which are necessary to improve the understanding of the behaviour of
  charged particle beams in magnetized plasmas.

Title: The electrodynamics of beam/return current systems in the
    solar corona
Authors: van den Oord, G. H. J.
Bibcode: 1990A&A...234..496V
Altcode:
  The response of a uniform plasma to the propagation of a prescribed
  electron beam in the approximation of decoupled solenoidal and
  irrotational fields is studied. It is shown that the electrostatic and
  inductive response of a plasma are governed by two independent sets of
  equations and that both are important for the current neutralization
  process. It is also found that the electrostatic response accounts for
  neutralization of the irrotational part of the beam current while the
  inductive response leads to neutralization of the solenoidal part of
  the beam current.

Title: Black and white flares?
Authors: Henoux, J. -C.; Aboudarham, J.; Brown, J. C.; van den Oord,
   G. H. J.; van Driel-Gesztelyi, L.
Bibcode: 1990A&A...233..577H
Altcode:
  It is shown that the nonthermal ionization of a solar flare atmosphere
  by beam bombardment can result, in the initial stages of a flare, in
  dimunitions in the continuum solar flare brightness (termed 'black
  light flares'). The most favorable conditions for observing this
  diagnostic phenomenon are investigated in terms of flare location and
  wavelength. In terms of bombarding beam parameters, electron beams
  of large peak flux are shown to be the most favorable as are target
  atmospheres with minimal preheating.

Title: A periodicity in the flaring rate on the eclipsing binary
    YY Geminorum.
Authors: Doyle, J. G.; Butler, C. J.; van den Oord, G. H. J.; Kiang, T.
Bibcode: 1990A&A...232...83D
Altcode:
  Five flares were observed on the late-type binary YY Gem in March 1988
  during a total monitoring time of 408 min. The flares were unusual
  in that there is a periodicity in the occurrence of four of them,
  these being separated by 48 + or - 3 min. Considering the flares to
  be formed as a stochastic process, it is found that the probability
  of these events occurring by chance is 0.5 percent. Modeling indicates
  that for quite reasonable input parameters, the flare periodicity can
  be explained in terms of filament oscillations. The only requirement
  is that there should be a filament at these heights where the magnetic
  field drops inversely proportional to the height.

Title: Beam Heating in Solar Flares: Electrons or Protons?
Authors: Brown, J. C.; Karlicky, M.; MacKinnon, A. L.; van den Oord,
   G. H. J.
Bibcode: 1990ApJS...73..343B
Altcode:
  The current status of electron and proton beam models as candidates for
  the impulsive phase heating of solar flares is discussed in relation to
  observational constants and theoretical difficulties. It is concluded
  that, while the electron beam model for flare heating still faces
  theoretical and observational problems, the problems faced by low
  and high energy proton beam models are no less serious, and there are
  facets of proton models which have not yet been studied. At the present,
  the electron beam model remains the most viable and best developed of
  heating model candidates.

Title: Negative Flares on the Sun
Authors: van Driel-Gesztelyi, L.; Brown, J. C.; Hénoux, J. -C.;
   Aboudarham, J.; van den Oord, G. H. J.; Gerlei, O.; Fárnik, F.
Bibcode: 1990PDHO....7..202V
Altcode: 1990dysu.conf..202V; 1990ESPM....6..202V
  The authors showed that under certain conditions at the beginning
  of a solar white-light flare (WLF) event an electron beam can cause
  a transient darkening before the WLF emission starts. They propose
  optimal conditions for possible observation of solar negative flares and
  present an observation, which may be the first identified observation
  of such events.

Title: Beam/Return Current Systems in the Solar Corona.
Authors: van den Oord, G. H. J.
Bibcode: 1990ppsa.conf..265V
Altcode:
  No abstract at ADS

Title: A Model for the Observed Periodicity in the Flaring Rate
    on Yy-Geminorum
Authors: Doyle, J. G.; van den Oord, G. H. J.; Butler, C. J.; Kiang, T.
Bibcode: 1990IAUS..137..325D
Altcode:
  No abstract at ADS

Title: Ultraviolet flares on II Pegasi.
Authors: Doyle, J. G.; Byrne, P. B.; van den Oord, G. H. J.
Bibcode: 1989A&A...224..153D
Altcode:
  UV spectroscopic observations of the RS CVn star, II Peg, were obtained
  in February 1983. The observations show evidence for flare activity
  in greatly enhanced chromospheric and transition region emission
  lines. The flare radiation is isolated and the electron pressures of
  the mean quiescent atmosphere and the two flares are derived from the
  intersystem line flux ratios. Estimates are given of the total flare
  power output over the whole atmosphere and the surface filling factor
  for the flares. The implications of the observations for studies of
  flares on RS CVn stars are considered.

Title: The X-ray flare and the quiescent emission from Algol as
    detected by EXOSAT.
Authors: van den Oord, G. H. J.; Mewe, R.
Bibcode: 1989A&A...213..245V
Altcode:
  The Algol system was observed on August 18/19,1983 with different
  combinations of three instruments aboard EXOSAT: the low-energy
  channel multiplier array, the medium-energy proportional counters,
  and the transmission grating spectrometer. The X-ray flare observed on
  August 19 was analyzed and the decay phase was studied. It is shown
  that during this phase the flare cools quasistatically according to
  the scaling laws and analytical expressions for the time-evolution of
  the relevant physical quantities are presented.

Title: A study of the decay phase of an X-ray flare on Algol.
Authors: Mewe, R.; van den Oord, G. H. J.; Jakimiec, J.
Bibcode: 1989sasf.confP.123M
Altcode: 1988sasf.conf..123M; 1989IAUCo.104P.123M
  The authors have re-analyzed the X-ray flare on Algol which was observed
  with EXOSAT (White et al., 1986). The common practice of estimating
  loop volume and length from the decay time of the flare is discussed
  extensively. The authors show that during the decay phase of the flare
  both scaling laws for coronal loops are valid. This implies a unique
  determination of loop volume and length and allows a check whether
  additional heating occurs in the decay phase of a flare.

Title: Optical flares from the dwarf M star V577 MON (Gliese 234 AB =
    Ross 614).
Authors: Doyle, J. G.; van den Oord, G. H. J.; Butler, C. J.
Bibcode: 1989A&A...208..208D
Altcode:
  The paper describes a search for flaring on the Balmer emission line
  star, Gliese 234 AB. Ten flares were detected in a total monitoring time
  of 4.6 h, implying a very high level of activity. The time-averaged
  flare energy in the Johnson U-band is slightly greater than measured
  previously but due to the small number of hours of observations it
  is best to use the mean of these two measurements. Relative energies
  are given for the U, B, V, R, and I bands for a 3.8 mag U-band flare
  which had a 45 s rise time and 20 min decay time. Of the various models
  fitted to the flare data, bound-free emission seems the most promising.

Title: An analysis of the continuum light in the 3500 - 8500 Å region
    from a flare observed on the dwarf M star Gliese 234AB (= V577 Mon).
Authors: Doyle, J. G.; van den Oord, G. H. J.; Butler, C. J.
Bibcode: 1989sasf.confP..91D
Altcode: 1989IAUCo.104P..91D; 1988sasf.conf...91D
  Relative energies are given for the U,B,V,R and I bands for a 3.8
  magnitude U-band flare observed on the dwarf dMe star Gl 234 AB on 28
  Feb. 1985. This flare had a 45 second rise time and 20 minute decay
  time. The total flare energy from all five bands during the flare
  was 7×10<SUP>31</SUP>erg, 34% of this total was from the U-band
  and 20% from the two near infrared R and I bands. The energy density
  (per frequency interval) implied a rising continuum towards the red,
  however this only lasted for approximately 20 - 40 seconds, i.e. during
  the impulsive phase, afterwhich the excess flare emission could not
  be detected in the near infrared bands. Of the various models fitted
  to the flare data (i.e. optical synchrotron, bound-free emission and
  free-free emission), bound-free emission seems the most promising.

Title: A combined radio and X-ray observation of Algol.
Authors: van den Oord, G. H. J.; Kuijpers, J.; White, N. E.; van der
   Hulst, J. M.; Culhane, J. L.
Bibcode: 1989A&A...209..296V
Altcode:
  The detection of Algol (Beta Per) at 1.4 GHz is reported. The eclipsing
  phase of Algol has been observed in radio and at X-ray energies. The
  radio emission is explained as synchrotron radiation from electrons
  with energies of about 1 MeV. The magnetic field strength is of the
  order of tens of Gauss. The radio and X-ray emission are cospatial but
  the energy in the radio component is 1,000,000 times smaller than that
  of the X-ray component. The simultaneous observation indicates that
  an energetic particle component can be present in a quiescent X-ray
  emitting corona.

Title: Energetic flares on RS CVn stars.
Authors: Doyle, J. G.; Byrne, P. B.; van den Oord, G. H. J.
Bibcode: 1989sasf.confP.115D
Altcode: 1988sasf.conf..115D; 1989IAUCo.104P.115D
  Ultraviolet spectroscopic observations of the RS CVn star II Peg in
  February 1983 show evidence for flare activity in greatly enhanced
  chromospheric and transition region emission lines. The total
  radiative losses from the chromosphere and transition region during
  the flare is 3.1×10<SUP>35</SUP>erg. Over the whole atmosphere the
  authors estimate total radiative losses of 2.4×10<SUP>36</SUP>erg,
  (excluding hydrogen line radiation). At flare peak, the flare radiated
  1.5×10<SUP>32</SUP>erg s<SUP>-1</SUP>. Adopting a two-ribbon flare
  model, where the filament is located between the two stars of the
  system, one can have 10<SUP>39</SUP>(l/R<SUB>sun</SUB>) erg of magnetic
  energy available, where l is the filament length. Therefore, only a
  small fraction of this magnetic energy need be converted into heating
  of the flare plasma.

Title: Time-dependent heating phenomena in coronal loops.
Authors: van den Oord, G. H. J.; Barstow, M. A.
Bibcode: 1988A&A...207...89V
Altcode:
  The authors have investigated the behaviour of coronal loops subject
  to flare-like heating. They pay attention to the conflicting results
  obtained by Kuin and Martens (1982) and Craig and Schulkes (1985). They
  show that the description of Kuin and Martens is correct and extend
  it for a larger range of loop temperatures. Further, they show that
  the evolution of the loop is governed by the history of the heating
  function. Under certain conditions individual heat depositions are
  masked. Quasi-periodic variations observed with EXOSAT in the light
  curves of the RS CVn system HR 1099 (Barstow, 1985) are explained in
  terms of time-dependent heating. This observation serves as a case study
  for the theoretical results obtained in this paper and represents the
  first detection of flare-like heating in the corona of an RS CVn system.

Title: The possible role of proton beams in flares on dwarf stars.
Authors: van den Oord, G. H. J.
Bibcode: 1988A&A...207..101V
Altcode:
  The author has investigated the possible role of proton beams during
  flares on dwarf stars. Simultaneous observations of flares at optical
  and X-ray wavelengths indicate that a gradual phase is sometimes
  absent. This can be explained when ionization due to the beam energy
  deposition plays an important role. Two well observed optical/X-ray
  flares on BY Dra and on YZ CMi are discussed. It is shown that optical
  flares on dMe stars require energy deposition in the chromosphere by
  beams instead of by a conductive flux, in order to be consistent with
  the observations. The absence of correlation between optical flares
  and microwave emission can be explained if during flares the energy
  for the optical flare is carried to the chromosphere by proton beams.

Title: Filament support and flares in binaries
Authors: van den Oord, G. H. J.
Bibcode: 1988A&A...205..167V
Altcode:
  The possibilities for filament support in a binary are investigated. A
  description in terms of currents, the sources of the magnetic field is
  employed. Several advantages of this method are discussed. An extended
  discussion is presented on the stability of a filament in a RS Canum
  Venaticorum system. Various locations for stable filament configurations
  are identified, and analytical expressions for the amount of stored
  energy are presented. It is shown that sufficient energy can be stored
  to explain the largest flares in RS CVn and Algol systems.

Title: An EXOSAT observation of an X-ray flare and quiescent emission
    from the RS CVn binary sigma2 CrB.
Authors: van den Oord, G. H. J.; Mewe, R.; Brinkman, A. C.
Bibcode: 1988A&A...205..181V
Altcode:
  The RS CVn binary sigma2 Coronae Borealis was observed with
  medium-energy and low-energy detectors and the transmission grating
  spectrometer aboard EXOSAT, and a strong X-ray flare rising within
  about five minutes and lasting more than two hours was detected. The
  temperature rises steeply within three minutes from 19 MK to a value
  well above 70 MK, while the emission measure attains a maximum of
  6 x 10 to the 53rd/cu cm five minutes later. The e-folding time is
  about half an hour. The flare volume and density are estimated and,
  on the basis of the short rise time of the flare, can be classified
  as a compact flare. The flare energy requirements are discussed,
  and it is found that current-driven turbulence can produce such fast
  heating. An equivalent-circuit description is presented.

Title: Rotational modulation and flares on RS CVn and BY DRA
    systems. VII. simultaneous X-ray, radio and optical data for the
    dMe star YZ CMi on 4/5 March 1985.
Authors: Doyle, J. G.; Butler, C. J.; Bryne, P. B.; van den Oord,
   G. H. J.
Bibcode: 1988A&A...193..229D
Altcode:
  Broad-band photometric observations of YZ CMi show a 1.2mag U-band flare
  at 19:55 UT on 4 March 1985, however, simultaneous X-ray observations
  from EXOSAT show no detectable increase. Two possible explanations
  for the lack of an X-ray flare may be i) a flare occurring low in the
  chromosphere, where the chromosphere may not have been coupled to
  the transition region and therefore the corona via magnetic fields
  or ii) proton beam heating as opposed to electron beams. During the
  flare, optical spectra show excess emission in the wings of all the
  hydrogen Balmer lines. Interpreted in terms of mass flows would imply
  material moving at ∼300 km s<SUP>-1</SUP> simultaneously to the
  blue and red or alternatively random mass motions with a velocity
  of a similar magnitude. At flare maximum, all the Balmer lines show
  excess emission in the wings with Hγ and Hσ showing symmetrically
  broadened lines while higher members of the series such as Hζ and
  Hη show predominately red shifted material. Assuming a single loop
  flare, an interpretation in terms of directed mass flows would imply a
  loop of length ∼2-3 10<SUP>9</SUP> cm, however this would place the
  material in the corona where we should have observed it in X-rays. An
  alternative explanation, also assuming directed mass flows could
  involve several small flare kernels, which brighten successively,
  thus producing a broadened profile. An explanation similar to this
  has been suggested to explain the excess emission seen in the wings
  of Hα during solar flares. The total optical flare energy was ∼6
  10<SUP>31</SUP> erg in this event, with the Balmer lines contributing
  ∼10% to the energy in the U-band during the flare. Many short-lived
  bursts or micro-flares were detected in both the Johnson U-band and the
  Balmer emission lines. The probability for a correlation between these
  two data-sets to have occurred by chance was less than 10<SUP>-5</SUP>
  . The coronal X-ray flux show only a moderate dependence on Balmer
  line emission and none on the U-band variations. Thus for YZ CMi not
  only does the large optical flare at 19:55 UT produce no significant
  X-ray emission but also the low level optical flares, which are seen
  almost continuously, are similarly absent in soft X-rays. Several X-ray
  flares/micro-flares were observed, however the majority of these did not
  show an optical counter-part. The largest of these X-ray events were at
  18:00 UT and 23:30 UT, lasting an hour and 1 1/2 h respectively. Their
  energy in the 0.02-2 keV range were 9.1 10<SUP>31</SUP> erg and 2.1
  10<SUP>32</SUP> erg respectively. No optical data was available for
  the first large X-ray flare, while the second event had only limited
  coverage. The photometric data implied an optical energy of at least
  2.6 10<SUP>31</SUP> erg. The Balmer line contribute at least 10%
  to the U-band energy in both the 19:55 UT and 23:30 UT flares. Radio
  observations at 326 MHz give an upper limit of 4 mJy for the flux.

Title: Stellar flares
Authors: van den Oord, Gijsbertus Hendrikus Jozef Bert
Bibcode: 1987PhDT........47V
Altcode:
  No abstract at ADS

Title: Stellar flares
Authors: van den Oord, Gijsbertus H. J.
Bibcode: 1987stfl.book.....V
Altcode:
  No abstract at ADS

Title: A Large X-ray Flare on σ<SUP>2</SUP> Coronae Borealis Observed
    with EXOSAT
Authors: van den Oord, G. H. J.; Mewe, R.; Brinkman, A. C.
Bibcode: 1987LNP...291..494V
Altcode: 1987LNP87.291..494V; 1987csss....5..494V
  We report the observation of a compact flare on σ<SUP>2</SUP>
  Coronae Borealis. The energy requirements for the flare are discussed
  and we find that current-driven turbulence can produce heating fast
  enough. During the initial phase of the flare classical transport
  theory is not applicable.

Title: Calculated X-radiation from optically thin plasmas. VI -
    Improved calculations for continuum emission and approximation
    formulae for nonrelativistic average Gaunt actors.
Authors: Mewe, R.; Lemen, J. R.; van den Oord, G. H. J.
Bibcode: 1986A&AS...65..511M
Altcode:
  The continuum spectrum of an optically thin plasma whose electrons
  have a Maxwellian energy distribution is calculated for the wavelength
  range 1 - 1000 Å and temperature range 0.01 - 100 MK. The effect of
  recombination to excited states on the free-bound emission is taken
  into account. Simple analytic approximations to the total free-free,
  free-bound and two-photon Gaunt factors are presented for various
  wavelength and temperature intervals covering the whole considered
  range. A simple computer routine for calculating the continuum emission
  with the aid of this approximation is given.

Title: Simultaneous optical and X-ray observations of a flare on
    BY Draconis.
Authors: de Jager, C.; Heise, J.; Avgoloupis, S.; Cutispoto, G.;
   Kieboom, K.; Herr, R. B.; Landini, M.; Langerwerf, A. F.; Mavridis,
   L. N.; Melkonian, A. S.; Molenaar, R.; Monsignori-Fossi, B. C.;
   Nations, H. L.; Pallavicini, R.; Piirola, V.; Rodono, M.; Seeds,
   M. A.; van den Oord, G. H. J.; Vilhu, O.; Waelkens, C.
Bibcode: 1986A&A...156...95D
Altcode:
  The authors present a first report on a campaign for simultaneous
  observations in the visual, radio and X-ray ranges of stellar flares
  on the binary BY Dra. During two nights of observations they observed
  one significant flare, simultaneously in soft X-rays and visible
  wavelengths, and one or two smaller bursts. The main flare impulsively
  heated an area of ⪉2×10<SUP>7</SUP>km<SUP>2</SUP> of the stellar
  photosphere to a brightness temperature ⪆25000K, during about 5
  min, and generated hot plasma, emitting soft X-rays. This emission
  reached maximum after the impulsive burst and lasted for about an
  hour; it covered a much larger area on the star than the optical
  emission. There was no significant hard X-ray component. The authors
  ascribe the optical continuous emission to the flare's impulsive phase,
  and the soft X-ray emission to the gradual phase.

Title: An EXOSAT Observation of Quiescent and Flare Coronal X-Ray
    Emission from Algol
Authors: White, N. E.; Culhane, J. L.; Parmar, A. N.; Kellett, B. J.;
   Kahn, S.; van den Oord, G. H. J.; Kuijpers, J.
Bibcode: 1986ApJ...301..262W
Altcode:
  X-ray emission from the Algol system is believed to originate in a
  corona associated with the K star. The authors have used the EXOSAT
  Observatory to make a 35 hr continuous observation centered on the
  occultation of the K star by the B star primary. The spectrum of
  the quiescent emission in the 1 - 10 keV band gives a temperature of
  2.5×10<SUP>7</SUP>K. This spectrum, extrapolated to lower energies,
  can account for more than 80% of the observed count rate. No obvious
  X-ray eclipse was seen. An X-ray flare was detected with a rise time
  of ≡1700 s and an exponential decay of ≡7000 s. The 0.1 - 10 keV
  peak luminosity was 1.4×10<SUP>31</SUP>ergs s<SUP>-1</SUP>. The peak
  temperature was 6×10<SUP>7</SUP>K, with an iron K line confirming
  the thermal character of the emission.

Title: Flares: solar and stellar. Proceedings of an RAL workshop on
    astronomy and astrophysics, held at Abingdon, UK, 19 - 21 May 1986.
Authors: Gondhalekar, P. M.; van den Oord, G. H. J.
Bibcode: 1986fssp.book.....G
Altcode:
  No abstract at ADS

Title: The Modelling of Solar Two-Ribbon Flares
Authors: van den Oord, G. H. J.; Kaastra, J. S.
Bibcode: 1986mrt..conf..165V
Altcode:
  No abstract at ADS

Title: Calculated X-Radiation from Optically Thin Plasmas - Part Five
Authors: Mewe, R.; Gronenschild, E. H. B. M.; van den Oord, G. H. J.
Bibcode: 1985A&AS...62..197M
Altcode: 2009A&A...500..301M
  The spectral line list in a previously developed computer code has
  been updated and extended with many new lines up to 300 A. These lines
  comprise: H- and He-like n = 1-2 lines, together with their satellites
  which are produced by innershell transitions: 2l-nl-prime (n = 3, 4)
  transitions in Fe XVII-XXIV (10-17 A); 2s-2p lines in Fe XVIII-XXIII and
  Ni XX-XXV (90-140 A); 3l-nl-prime (n = 3, 4) transitions in Fe VIII-XVI
  and Ni X-XIV (50-200 A), and many lines from Li to Ne isoelectronic
  sequences of elements C to Ca (20-300 A). Intensities of 2131 spectral
  lines are tabulated over the temperature range 3 x 10 to the 4th -
  10 to the 9th K.

Title: Variations in the Radio Flux of the Hypergiant P Cygni
    (B1 Ia<SUP>+</SUP>)
Authors: van den Oord, G. H. J.; Waters, L. B. F. M.; Lamers,
   H. J. G. L. M.; Abbott, D. C.; Bieging, J. H.; Churchwell, E.
Bibcode: 1985ASSL..116..111V
Altcode: 1985rst..conf..111V
  The radio flux of P Cygni is variable on a time scale of months or
  shorter. The variations are interpreted as variations in the degree
  of ionization of the wind due to the ejection of shells which shield
  the wind from the ionizing radiation of the star. The observed time
  scale agrees with the recombination time scale of the wind.

Title: Observations of Steady Anomalous Magnetic Heating in Thin
    Current Sheets
Authors: Martens, P. C. H.; van den Oord, G. H. J.; Hoyng, P.
Bibcode: 1985SoPh...96..253M
Altcode:
  A faint steadily emitting loop-like structure has been observed
  by HXIS in its low energy channels (3.5-8.0 keV) on November 5/6,
  1980. These HXIS observations have permitted us to follow the thermal
  evolution of this loop for a period of about 15 hr and from this study
  we conclude that only a fraction of 0.1% of the volume of the loop is
  steadily heated at the rather large rate of 0.6 erg cm<SUP>-3</SUP>
  s<SUP>-1</SUP>. We interpret this heating as the dissipation of magnetic
  fields in thin current sheets and we find that the dissipation with
  classical resistivity is very unlikely, while ion-kinetic tearing, as
  proposed by Galeev et al. (1981), suits the observations very well. The
  enhancement of the resistivity over the classical resistivity then
  turns out to be a factor 4 × 10<SUP>4</SUP>. Dissipation in extremely
  thin sheets via the ion-acoustic instability (Duijveman et al., 1981)
  cannot be completely excluded when the cross-field heat conductivity
  is anomalously enhanced by a factor 400.

Title: An EXOSAT Observation of the Morphology of the Coronal X-Ray
    Emission from Algol
Authors: White, N. E.; Culhane, J. L.; Parmar, A. N.; Kellett, B.;
   Kahn, S.; van den Oord, G. H. J.; Kuijpers, J.
Bibcode: 1985SSRv...40...25W
Altcode:
  The X-ray emission from Algol is thought to originate in a corona
  associated with the K star in this system. We report the results of a 35
  hr continuous EXOSAT observation through secondary optical eclipse that
  was designed to measure the structure of the corona. No obvious X-ray
  eclipse was seen. The spectrum measured by the ME gives a temperature of
  2.5 × 10<SUP>7</SUP> K, consistent with the hard component previously
  seen by the Einstein SSS. The soft component previously reported by
  the SSS would only contribute at most 25% to the count rate seen in
  the LE (used with Al/P). The lack of a hard X-ray eclipse indicates the
  dimensions of the higher temperature emission region to be comparable
  to or greater than the size of the K star. An X-ray flare was detected
  with a peak luminosity of 1.4 × 10<SUP>31</SUP> erg s<SUP>-1</SUP>
  and a total duration of 8 hours. The peak temperature was 5.0 keV with
  an emission measure of 9.4 × 10<SUP>53</SUP> cm<SUP>-3</SUP>. The
  thermal nature of the flare is confirmed by the detection of an iron
  line with an EW of ∼2 keV. By equating the observed decay time of
  the flare to a known cooling law gives a dimension for the flaring
  loop of ∼0.3 stellar radii. This is much smaller than the dimensions
  of the hard component inferred from the lack of an eclipse. It seems
  probable that the flare occurred in one of the loops responsible for
  the lower temperature component seen by the SSS.

Title: A simultaneous X-ray and radio observation of a flare from
    Algol
Authors: Parmar, A. N.; Culhane, J. L.; White, N. E.; van den Oord,
   G. H. J.
Bibcode: 1985AdSpR...5c..69P
Altcode: 1985AdSpR...5...69P
  An X-ray flare was observed from Algol using the low and medium energy
  detectors on the European Space Agency's EXOSAT observatory. Spectra
  obtained during the flare are well fitted by thermal continua while
  an Fe XXV emission feature was also detected. The strength of this
  feature indicates a cosmic abundance for iron. The data indicate that
  the flare occurred in a loop of height approximately 0.25 of the K
  star radius &amp; with a magnetic field &gt;300 Gauss.

Title: Shock-driven mass loss of stars.
Authors: de Jager, C.; van den Oord, G. H. J.
Bibcode: 1984mlao.conf...59D
Altcode:
  The authors suggest that shocks, fed by multi-mode pulsational motion
  of the photospheres of supergiants are the basic mechanism for the
  mass loss of cool and medium type supergiants. They review the present
  theoretical situation, and describe the implementations of the observed
  transient mass loss component of the pulsating Beta Cephei stars BW
  Vul and σ Sco.

Title: The pulsation of the outer layers of the BET CEP star SIG Sco.
Authors: Burger, M.; de Jager, C.; van den Oord, G. H. J.
Bibcode: 1982A&A...109..289B
Altcode:
  The pulsation of the outer layers of the Beta Cephei-type variable σ
  Sco is investigated using 17 ultraviolet spectrograms (1200-2000 A)
  obtained during seven hours of observations with the International
  Ultraviolet Explorer. A pulsational model established earlier for
  BW Vul is confirmed. According to this model, after a period of
  photospheric rest the outer layers expand suddenly. The acceleration
  for the outermost layers, investigated with the resonance lines of C
  IV and Si IV, is found to be larger than for the photosphere. During
  the phase of deceleration that follows, no falling down of the C
  IV absorbing layers is observed; their velocity component remains
  directed outward. A transient stellar wind component sets in just
  before the star has its minimum radius (phase, approximately 0.9);
  it is strongest at a phase of approximately 0.2. The observations are
  interpreted as suggesting the mechanism of pulsational driven mass loss.

Title: The pulsation of the outer layers of the BET Cep-type variable
    BW Vul.
Authors: Burger, M.; de Jager, C.; van den Oord, G. H. J.; Sato, N.
Bibcode: 1982A&A...107..320B
Altcode:
  Eleven high-resolution ultraviolet spectrograms of BW Vul were obtained
  by means of the IUE over 1.6 periods to compare the pulsation of this
  star as observed in the strong far-UV resonance lines with observations
  of the UV lines of photospheric origin. The radial velocity curve of
  the C IV lines is different from the photospheric one, showing that
  both the photosphere and the C IV layers are accelerated upwards
  impulsively. After about one hour, the acceleration decreases to
  zero, and then the atmosphere falls with a nearly constant downward
  acceleration of 2100 cm/sq s. The varying asymmetry indicates mass loss
  due to the pulsation of the atmosphere, superimposed on the continuous
  mass loss of the star.

Title: Shock-driven Mass Loss of Stars
Authors: de Jager, C.; van den Oord, G. H. J.
Bibcode: 1982mlao.conf...59D
Altcode:
  No abstract at ADS

Title: The Outer Layers of the Beta-Cephei Type Variables
    Bw-Vulpeculae and Omicron-Scorpii
Authors: Burger, M.; de Jager, C.; van den Oord, G. H. J.; Groupe
   Etoiles Variables de L'Observatoire de Nice
Bibcode: 1981pbs..work..181B
Altcode:
  No abstract at ADS

