Author name code: joshi-jayant
ADS astronomy entries on 2022-09-14
=author:"Joshi, Jayant" 
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Title: Properties of ubiquitous magnetic reconnection events in the
    lower solar atmosphere
Authors: Joshi, Jayant; Rouppe van der Voort, Luc H. M.
Bibcode: 2022A&A...664A..72J
Altcode: 2022arXiv220308172J
  Context. Magnetic reconnection in the deep solar atmosphere can give
  rise to enhanced emission in the Balmer hydrogen lines, a phenomenon
  referred to as Ellerman bombs. Recent high-quality Hβ observations
  indicate that Ellerman bombs are more common than previously thought,
  and it was estimated that at any time, about half a million Ellerman
  bombs are present in the quiet Sun. <BR /> Aims: We performed an
  extensive statistical characterization of the quiet-Sun Ellerman
  bombs (QSEBs) in these new Hβ observations. <BR /> Methods: We
  analyzed a 1 h dataset of the quiet Sun observed with the Swedish
  1-m Solar Telescope that consists of spectral imaging in the Hβ
  and Hα lines as well as spectropolarimetric imaging in Fe I 6173
  Å. We used the k-means clustering and the 3D connected component
  labeling techniques to automatically detect QSEBs. <BR /> Results:
  We detected a total of 2809 QSEBs. The lifetimes vary between 9 s
  and 20.5 min, with a median of 1.14 min. The maximum area ranges
  between 0.0016 and 0.2603 Mm<SUP>2</SUP>, with a median of 0.018
  Mm<SUP>2</SUP>. The maximum brightness in the Hβ wing varies between
  1.06 and 2.76 with respect to the average wing intensity. A subset
  (14%) of the QSEBs displays enhancement of the Hβ line core. On
  average, the line core brightening appears 0.88 min after the
  onset of brightening in the wings, and the distance between these
  brightenings is 243 km. This gives rise to an apparent propagation
  speed ranging between −14.3 and +23.5 km s<SUP>−1</SUP>, with an
  average that is upward propagating at +4.4 km s<SUP>−1</SUP>. The
  average orientation is nearly parallel to the limbward direction. QSEBs
  are nearly uniformly distributed over the field of view, but we find
  empty areas with the size of mesogranulation. QSEBs are located more
  frequently near the magnetic network, where they are often larger,
  live longer, and are brighter. <BR /> Conclusions: We conclude
  that QSEBs are ubiquitous in the quiet Sun and appear everywhere,
  except in areas of mesogranular size with the weakest magnetic
  fields (B<SUB>LOS</SUB> ≲ 50 G). Our observations support the
  interpretation of reconnection along vertically extended current
  sheets. <P />Movies associated to Figs. 1 and 3 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202243051/olm">https://www.aanda.org</A>

Title: Evidence of the multi-thermal nature of spicular
    downflows. Impact on solar atmospheric heating
Authors: Bose, Souvik; Rouppe van der Voort, Luc; Joshi, Jayant;
   Henriques, Vasco M. J.; Nóbrega-Siverio, Daniel; Martínez-Sykora,
   Juan; De Pontieu, Bart
Bibcode: 2021A&A...654A..51B
Altcode: 2021arXiv210802153B
  Context. Spectroscopic observations of the emission lines formed in the
  solar transition region commonly show persistent downflows on the order
  of 10−15 km s<SUP>−1</SUP>. The cause of such downflows, however, is
  still not fully clear and has remained a matter of debate. <BR /> Aims:
  We aim to understand the cause of such downflows by studying the coronal
  and transition region responses to the recently reported chromospheric
  downflowing rapid redshifted excursions (RREs) and their impact on the
  heating of the solar atmosphere. <BR /> Methods: We have used two sets
  of coordinated data from the Swedish 1 m Solar Telescope, the Interface
  Region Imaging Spectrograph, and the Solar Dynamics Observatory for
  analyzing the response of the downflowing RREs in the transition
  region and corona. To provide theoretical support, we use an already
  existing 2.5D magnetohydrodynamic simulation of spicules performed
  with the Bifrost code. <BR /> Results: We find ample occurrences of
  downflowing RREs and show several examples of their spatio-temporal
  evolution, sampling multiple wavelength channels ranging from the cooler
  chromospheric to the hotter coronal channels. These downflowing features
  are thought to be likely associated with the returning components of
  the previously heated spicular plasma. Furthermore, the transition
  region Doppler shifts associated with them are close to the average
  redshifts observed in this region, which further implies that these
  flows could (partly) be responsible for the persistent downflows
  observed in the transition region. We also propose two mechanisms -
  (i) a typical upflow followed by a downflow and (ii) downflows along a
  loop -from the perspective of a numerical simulation that could explain
  the ubiquitous occurrence of such downflows. A detailed comparison
  between the synthetic and observed spectral characteristics reveals a
  distinctive match and further suggests an impact on the heating of the
  solar atmosphere. <BR /> Conclusions: We present evidence that suggests
  that at least some of the downflowing RREs are the chromospheric
  counterparts of the transition region and lower coronal downflows. <P
  />Movies associated to Figs. 1-3, 8, and 10 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141404/olm">https://www.aanda.org</A>

Title: Line formation of He I D<SUB>3</SUB> and He I 10 830 Å in
    a small-scale reconnection event
Authors: Libbrecht, Tine; Bjørgen, Johan P.; Leenaarts, Jorrit;
   de la Cruz Rodríguez, Jaime; Hansteen, Viggo; Joshi, Jayant
Bibcode: 2021A&A...652A.146L
Altcode: 2020arXiv201015946L
  Context. Ellerman bombs (EBs) and UV bursts are small-scale reconnection
  events that occur in the region of the upper photosphere to the
  chromosphere. It has recently been discovered that these events can
  have emission signatures in the He I D<SUB>3</SUB> and He I 10 830 Å
  lines, suggesting that their temperatures are higher than previously
  expected. <BR /> Aims: We aim to explain the line formation of He I
  D<SUB>3</SUB> and He I 10 830 Å in small-scale reconnection events. <BR
  /> Methods: We used a simulated EB in a Bifrost-generated radiative
  magnetohydrodynamics snapshot. The resulting He I D<SUB>3</SUB> and He
  I 10 830 Å line intensities were synthesized in 3D using the non-local
  thermal equilibrium (non-LTE) Multi3D code. The presence of coronal
  extreme UV (EUV) radiation was included self-consistently. We compared
  the synthetic helium spectra with observed raster scans of EBs in He I
  10 830 Å and He I D<SUB>3</SUB> obtained at the Swedish Solar Telescope
  with the TRI-Port Polarimetric Echelle-Littrow Spectrograph. <BR />
  Results: Emission in He I D<SUB>3</SUB> and He I 10 830 Å is formed
  in a thin shell around the EB at a height of ∼0.8 Mm, while the He I
  D<SUB>3</SUB> absorption is formed above the EB at ∼4 Mm. The height
  at which the emission is formed corresponds to the lower boundary of the
  EB, where the temperature increases rapidly from 6 × 10<SUP>3</SUP> K
  to 10<SUP>6</SUP> K. The synthetic line profiles at a heliocentric angle
  of μ = 0.27 are qualitatively similar to the observed profiles at the
  same μ-angle in dynamics, broadening, and line shape: emission in the
  wing and absorption in the line core. The opacity in He I D<SUB>3</SUB>
  and He I 10 830 Å is generated through photoionization-recombination
  driven by EUV radiation that is locally generated in the EB at
  temperatures in the range of 2 × 10<SUP>4</SUP> − 2 × 10<SUP>6</SUP>
  K and electron densities between 10<SUP>11</SUP> and 10<SUP>13</SUP>
  cm<SUP>−3</SUP>. The synthetic emission signals are a result of
  coupling to local conditions in a thin shell around the EB, with
  temperatures between 7 × 10<SUP>3</SUP> and 10<SUP>4</SUP> K and
  electron densities ranging from ∼10<SUP>12</SUP> to 10<SUP>13</SUP>
  cm<SUP>−3</SUP>. This shows that both strong non-LTE and thermal
  processes play a role in the formation of He I D<SUB>3</SUB> and
  He I 10 830 Å in the synthetic EB/UV burst that we studied. <BR />
  Conclusions: In conclusion, the synthetic He I D<SUB>3</SUB> and He I 10
  830 Å emission signatures are an indicator of temperatures of at least
  2 × 10<SUP>4</SUP> K; in this case, as high as ∼10<SUP>6</SUP> K.

Title: Signatures of ubiquitous magnetic reconnection in the deep
    atmosphere of sunspot penumbrae
Authors: Rouppe van der Voort, Luc H. M.; Joshi, Jayant; Henriques,
   Vasco M. J.; Bose, Souvik
Bibcode: 2021A&A...648A..54R
Altcode: 2021arXiv210111321R
  Context. Ellerman bombs are regions with enhanced Balmer line wing
  emission and mark magnetic reconnection in the deep solar atmosphere
  in active regions and the quiet Sun. They are often found in regions
  where opposite magnetic polarities are in close proximity. Recent
  high-resolution observations suggest that Ellerman bombs are more
  prevalent than previously thought. <BR /> Aims: We aim to determine
  the occurrence of Ellerman bombs in the penumbra of sunspots. <BR />
  Methods: We analyzed high spatial resolution observations of sunspots
  in the Balmer Hα and Hβ lines as well as auxiliary continuum channels
  obtained with the Swedish 1-m Solar Telescope and applied the k-means
  clustering technique to systematically detect and characterize Ellerman
  Bombs. <BR /> Results: Features with all the defining characteristics of
  Ellerman bombs are found in large numbers over the entire penumbra. The
  true prevalence of these events is only fully appreciated in the Hβ
  line due to the highest spatial resolution and lower chromospheric
  opacity. We find that the penumbra hosts some of the highest Ellerman
  bomb densities, surpassed only by the moat in the immediate surroundings
  of the sunspot. Some penumbral Ellerman bombs show flame morphology
  and rapid dynamical evolution. Many penumbral Ellerman bombs are fast
  moving with typical speed of 3.7 km s<SUP>−1</SUP> and sometimes more
  than 10 km s<SUP>−1</SUP>. Many penumbral Ellerman bombs migrate from
  the inner to the outer penumbra over hundreds of km, and some continue
  moving beyond the outer penumbral boundary into the moat. Many penumbral
  Ellerman bombs are found in the vicinity of regions with opposite
  magnetic polarity. <BR /> Conclusions: We conclude that reconnection
  is a near continuous process in the low atmosphere of the penumbra of
  sunspots that manifest in the form of penumbral Ellerman bombs. These
  are so prevalent that they may be a major sink of sunspot magnetic
  energy. <P />Movies associated to Figs. 1 and 6 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202040171/olm">https://www.aanda.org</A>

Title: Spicules and downflows in the solar chromosphere
Authors: Bose, Souvik; Joshi, Jayant; Henriques, Vasco M. J.; Rouppe
   van der Voort, Luc
Bibcode: 2021A&A...647A.147B
Altcode: 2021arXiv210107829B
  Context. High-speed downflows have been observed in the solar
  transition region (TR) and lower corona for many decades. Despite
  their abundance, it has been hard to find signatures of such downflows
  in the solar chromosphere. <BR /> Aims: In this work, we target
  an enhanced network region which shows ample occurrences of rapid
  spicular downflows in the Hα spectral line, which could potentially
  be linked to high-speed TR downflowing counterparts. <BR /> Methods:
  We used the k-means algorithm to classify the spectral profiles of
  on-disk spicules in Hα and Ca II K data observed from the Swedish
  1 m Solar Telescope and employed an automated detection method based
  on advanced morphological image processing operations to detect such
  downflowing features, in conjunction with rapid blue-shifted and
  red-shifted excursions (RBEs and RREs). <BR /> Results: We report
  the existence of a new category of RREs (termed as downflowing RRE)
  for the first time that, contrary to earlier interpretation, are
  associated with chromospheric field aligned downflows moving toward
  the strong magnetic field regions. Statistical analysis performed
  on nearly 20 000 RBEs and 15 000 RREs (including the downflowing
  counterparts), which were detected in our 97 min long dataset, shows
  that the downflowing RREs are very similar to RBEs and RREs except
  for their oppositely directed plane-of-sky motion. Furthermore, we
  also find that RBEs, RREs, and downflowing RREs can be represented
  by a wide range of spectral profiles with varying Doppler offsets,
  and Hα line core widths, both along and perpendicular to the spicule
  axis, that causes them to be associated with multiple substructures
  which evolve together. <BR /> Conclusions: We speculate that these
  rapid plasma downflows could well be the chromospheric counterparts
  of the commonly observed TR downflows. <P />Movies are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202040014/olm">https://www.aanda.org</A>

Title: Signatures of ubiquitous magnetic reconnection in the lower
    solar atmosphere
Authors: Joshi, Jayant; Rouppe van der Voort, Luc H. M.; de la Cruz
   Rodríguez, Jaime
Bibcode: 2020A&A...641L...5J
Altcode: 2020arXiv200614975J
  Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in
  the quiet Sun, also known as quiet Sun Ellerman bombs (QSEBs), are a
  signature of the fundamental process of magnetic reconnection at the
  smallest observable scale in the lower solar atmosphere. We analyze
  high spatial resolution observations (0<SUB>.</SUB><SUP>″</SUP>1)
  obtained with the Swedish 1-m Solar Telescope to explore signatures of
  QSEBs in the Hβ line. We find that QSEBs are ubiquitous and uniformly
  distributed throughout the quiet Sun, predominantly occurring in
  intergranular lanes. We find up to 120 QSEBs in the field of view for a
  single moment in time; this is more than an order of magnitude higher
  than the number of QSEBs found in earlier Hα observations. This
  suggests that about half a million QSEBs could be present in the
  lower solar atmosphere at any given time. The QSEB brightenings
  found in the Hβ line wings also persist in the line core with a
  temporal delay and spatial offset toward the nearest solar limb. Our
  results suggest that QSEBs emanate through magnetic reconnection along
  vertically extended current sheets in the lower solar atmosphere. The
  apparent omnipresence of small-scale magnetic reconnection may play
  an important role in the energy balance of the solar chromosphere. <P
  />Movies associated to Figs. 1-3, B1, and B2 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202038769/olm">https://www.aanda.org</A>

Title: Characterization and formation of on-disk spicules in the Ca
    II K and Mg II k spectral lines (Corrigendum)
Authors: Bose, Souvik; Henriques, Vasco M. J.; Joshi, Jayant; Rouppe
   van der Voort, Luc
Bibcode: 2020A&A...637C...1B
Altcode:
  No abstract at ADS

Title: Characterization and formation of on-disk spicules in the Ca
    II K and Mg II k spectral lines
Authors: Bose, Souvik; Henriques, Vasco M. J.; Joshi, Jayant; Rouppe
   van der Voort, Luc
Bibcode: 2019A&A...631L...5B
Altcode: 2019arXiv191005533B
  We characterize, for the first time, type-II spicules in Ca II K 3934
  Å using the CHROMIS instrument at the Swedish 1 m Solar Telescope. We
  find that their line formation is dominated by opacity shifts with
  the K<SUB>3</SUB> minimum best representing the velocity of the
  spicules. The K<SUB>2</SUB> features are either suppressed by the
  Doppler-shifted K<SUB>3</SUB> or enhanced via increased contribution
  from the lower layers, leading to strongly enhanced but unshifted
  K<SUB>2</SUB> peaks, with widening towards the line core as consistent
  with upper-layer opacity removal via Doppler-shift. We identify spicule
  spectra in concurrent IRIS Mg II k 2796Å observations with very
  similar properties. Using our interpretation of spicule chromospheric
  line formation, we produce synthetic profiles that match observations.

Title: Magnetic field variations associated with umbral flashes and
    penumbral waves
Authors: Joshi, Jayant; de la Cruz Rodríguez, Jaime
Bibcode: 2018A&A...619A..63J
Altcode: 2018arXiv180301737J
  Context. Umbral flashes (UF) and running penumbral waves (RPWs)
  in sunspot chromospheres leave a dramatic imprint in the intensity
  profile of the Ca II 8542 Å line. Recent studies have focussed on also
  explaining the observed polarization profiles, which show even more
  dramatic variations during the passage of these shock fronts. While most
  of these variations can be explained with an almost constant magnetic
  field as a function of time, several studies have reported changes in
  the inferred magnetic field strength during UF phases. These changes
  could be explained by opacity effects or by intrinsic changes in the
  magnetic field strength. <BR /> Aims: In this study we investigate the
  origin of these periodic variations of the magnetic field strength by
  analyzing a time-series of high-temporal-cadence observations acquired
  in the Ca II 8542 Å line with the CRISP instrument at the Swedish 1-m
  Solar Telescope. In particular, we analyze how the inferred geometrical
  height scale changes between quiescent and UF phases, and whether those
  changes are enough to explain the observed changes in the magnetic
  field, B. <BR /> Methods: We have performed non local thermodynamical
  equilibrium (non-LTE) data inversions with the NICOLE code of a
  time-series of very high spatio-temporal-resolution observations in
  the Ca II 8542 Å, Fe I 6301.5, and Fe I 6302.5 Å lines. We analyze
  in detail the variations of the different physical parameters of the
  model as a function of time. <BR /> Results: Our results indicate
  that the Ca II 8542 Å line in sunspots is greatly sensitive to
  magnetic fields at log τ<SUB>500</SUB> = -5 (hereafter log τ = -5)
  during UFs and quiescence. However this optical depth value does not
  correspond to the same geometrical height during the two phases. Our
  results indicate that during UFs and RPWs the log τ = -5 is located
  at a higher geometrical height than during quiescence. Additionally,
  the inferred magnetic field values are higher in UFs (up to ∼270 G)
  and in RPWs (∼100 G). <BR /> Conclusions: Our results suggest that
  opacity changes caused by UFs and RPWs cannot explain the observed
  temporal variations in the magnetic field, as the line seems to form
  at higher geometrical heights where the field is expected to be lower.

Title: Three-dimensional magnetic structure of a sunspot: Comparison
    of the photosphere and upper chromosphere
Authors: Joshi, Jayant; Lagg, Andreas; Hirzberger, Johann; Solanki,
   Sami K.
Bibcode: 2017A&A...604A..98J
Altcode: 2017arXiv170508404J
  <BR /> Aims: We investigate the magnetic field of a sunspot in the
  upper chromosphere and compare it to the photospheric properties
  of the field. <BR /> Methods: We observed the main leading sunspot
  of the active region NOAA 11124 during two days with the Tenerife
  Infrared Polarimeter-2 (TIP-2) mounted at the German Vacuum Tower
  Telescope (VTT). Through inversion of Stokes spectra of the He I
  triplet at 10 830 Å, we obtained the magnetic field vector of the
  upper chromosphere. For comparison with the photosphere, we applied
  height-dependent inversions of the Si I 10 827.1 Å and Ca I 10 833.4 Å
  lines. <BR /> Results: We found that the umbral magnetic field strength
  in the upper chromosphere is lower by a factor of 1.30-1.65 compared
  to the photosphere. The magnetic field strength of the umbra decreases
  from the photosphere toward the upper chromosphere by an average rate of
  0.5-0.9 G km<SUP>-1</SUP>. The difference in the magnetic field strength
  between both atmospheric layers steadily decreases from the sunspot
  center to the outer boundary of the sunspot; the field, in particular
  its horizontal component, is stronger in the chromopshere outside the
  spot and this is suggestive of a magnetic canopy. The sunspot displays
  a twist that on average is similar in the two layers. However, the
  differential twist between the photosphere and chromosphere increases
  rapidly toward the outer penumbral boundary. The magnetic field
  vector is more horizontal with respect to the solar surface by roughly
  5-20° in the photosphere compared to the upper chromosphere. Above
  a lightbridge, the chromospheric magnetic field is equally strong as
  that in the umbra, whereas the field of the lightbridge is weaker than
  its surroundings in the photosphere by roughly 1 kG. This suggests a
  cusp-like magnetic field structure above the lightbridge.

Title: Vertical magnetic field gradient in the photospheric layers
    of sunspots
Authors: Joshi, Jayant; Lagg, Andreas; Hirzberger, Johann; Solanki,
   Sami K.; Tiwari, Sanjiv K.
Bibcode: 2017A&A...599A..35J
Altcode: 2016arXiv161000500J
  <BR /> Aims: We investigate the vertical gradient of the magnetic
  field of sunspots in the photospheric layer. <BR /> Methods:
  Independent observations were obtained with the Solar Optical
  Telescope/Spectropolarimeter (SOT/SP) on board the Hinode spacecraft and
  with the Tenrife Infrared Polarimeter-2 (TIP-2) mounted at the German
  Vacuum Tower Telescope (VTT). We apply state-of-the-art inversion
  techniques to both data sets to retrieve the magnetic field and the
  corresponding vertical gradient along with other atmospheric parameters
  in the solar photosphere. <BR /> Results: In the sunspot penumbrae we
  detected patches of negative vertical gradients of the magnetic field
  strength, I.e., the magnetic field strength decreases with optical depth
  in the photosphere. The negative gradient patches are located in the
  inner and partly in the middle penumbrae in both data sets. From the
  SOT/SP observations we found that the negative gradient patches are
  restricted mainly to the deep photospheric layers and are concentrated
  near the edges of the penumbral filaments. Magnetohydrodynamic (MHD)
  simulations also show negative gradients in the inner penumbrae, also
  at the locations of filaments. In the observations and the simulation
  negative gradients of the magnetic field vs. optical depth dominate
  at some radial distances in the penumbra. The negative gradient with
  respect to optical depth in the inner penumbrae persists even after
  averaging in the azimuthal direction in the observations and, to a
  lesser extent, in the MHD simulations. If the gradients in the MHD
  simulations are determined with respect to geometrical height, then
  the azimuthal averages are always positive within the sunspot (above
  log τ = 0), corresponding to magnetic field increasing with depth,
  as generally expected. <BR /> Conclusions: We interpret the observed
  localized presence of negative vertical gradient of the magnetic
  field strength in the observations as a consequence of stronger field
  from spines expanding with height and closing above the weaker field
  inter-spines. The presence of the negative gradients with respect
  to optical depth after azimuthal averaging can be explained by two
  different mechanisms: the high corrugation of equal optical depth
  surfaces and the cancellation of polarized signal due to the presence
  of unresolved opposite polarity patches in the deeper layers of the
  penumbra.

Title: Observations of Ellerman bomb emission features in He I
    D<SUB>3</SUB> and He I 10 830 Å
Authors: Libbrecht, Tine; Joshi, Jayant; de la Cruz Rodríguez, Jaime;
   Leenaarts, Jorrit; Ramos, Andrés Asensio
Bibcode: 2017A&A...598A..33L
Altcode: 2016arXiv161001321L
  Context. Ellerman bombs (EBs) are short-lived emission features,
  characterised by extended wing emission in hydrogen Balmer lines. Until
  now, no distinct signature of EBs has been found in the He I 10 830 Å
  line, and conclusive observations of EBs in He I D<SUB>3</SUB> have
  never been reported. <BR /> Aims: We aim to study the signature of
  EBs in neutral helium triplet lines. <BR /> Methods: The observations
  consisted of ten consecutive SST/TRIPPEL raster scans close to the
  limb, featuring the Hβ, He I D<SUB>3</SUB> and He I 10 830 Å spectral
  regions. We also obtained raster scans with IRIS and made use of the
  SDO/AIA 1700 Å channel. We used Hazel to invert the neutral helium
  triplet lines. <BR /> Results: Three EBs in our data show distinct
  emission signatures in neutral helium triplet lines, most prominently
  visible in the He I D<SUB>3</SUB> line. The helium lines have two
  components: a broad and blueshifted emission component associated with
  the EB, and a narrower absorption component formed in the overlying
  chromosphere. One of the EBs in our data shows evidence of strong
  velocity gradients in its emission component. The emission component of
  the other two EBs could be fitted using a constant slab. Our analysis
  hints towards thermal Doppler motions having a large contribution to
  the broadening for helium and IRIS lines. We conclude that the EBs
  must have high temperatures to exhibit emission signals in neutral
  helium triplet lines. An order of magnitude estimate places our
  observed EBs in the range of T 2 × 10<SUP>4</SUP>-10<SUP>5</SUP>
  K. <P />Movies associated to Figs. 3-5 are available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201629266/olm">http://www.aanda.org</A>

Title: The cause of spatial structure in solar He I 1083 nm multiplet
    images
Authors: Leenaarts, Jorrit; Golding, Thomas; Carlsson, Mats; Libbrecht,
   Tine; Joshi, Jayant
Bibcode: 2016A&A...594A.104L
Altcode: 2016arXiv160800838L
  Context. The He I 1083 nm is a powerful diagnostic for inferring
  properties of the upper solar chromosphere, in particular for the
  magnetic field. The basic formation of the line in one-dimensional
  models is well understood, but the influence of the complex
  three-dimensional structure of the chromosphere and corona has however
  never been investigated. This structure must play an essential role
  because images taken in He I 1083 nm show structures with widths
  down to 100 km. <BR /> Aims: We aim to understand the effect of
  the three-dimensional temperature and density structure in the
  solar atmosphere on the formation of the He I 1083 nm line. <BR />
  Methods: We solved the non-LTE radiative transfer problem assuming
  statistical equilibrium for a simple nine-level helium atom that
  nevertheless captures all essential physics. As a model atmosphere we
  used a snapshot from a 3D radiation-MHD simulation computed with the
  Bifrost code. Ionising radiation from the corona was self-consistently
  taken into account. <BR /> Results: The emergent intensity in the He
  I 1083 nm is set by the source function and the opacity in the upper
  chromosphere. The former is dominated by scattering of photospheric
  radiation and does not vary much with spatial location. The latter
  is determined by the photonionisation rate in the He I ground state
  continuum, as well as the electron density in the chromosphere. The
  spatial variation of the flux of ionising radiation is caused
  by the spatially-structured emissivity of the ionising photons
  from material at T ≈ 100 kK in the transition region. The hotter
  coronal material produces more ionising photons, but the resulting
  radiation field is smooth and does not lead to small-scale variation
  of the UV flux. The corrugation of the transition region further
  increases the spatial variation of the amount of UV radiation in the
  chromosphere. Finally we find that variations in the chromospheric
  electron density also cause strong variation in He I 1083 nm
  opacity. We compare our findings to observations using SST, IRIS and
  SDO/AIA data. <P />A movie associated to Fig. 4 is available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201628490/olm">http://www.aanda.org</A>

Title: Magnetic and Velocity Field of Sunspots in the Photosphere
    and Upper Chromosphere
Authors: Joshi, Jayant
Bibcode: 2014PhDT.......127J
Altcode:
  Sunspots, the dark blemishes on the solar surface have been widely
  studied for the past 400 years. Sunspots are the most readily
  identifiable manifestation of magnetic field concentrations on the solar
  surface. Interaction of the sunspot magnetic field with the plasma
  makes them one of the most interesting objects for research in solar
  physics. <P />This thesis presents a study of the photospheric and
  upper chromospheric velocity and magnetic field structure of sunspots
  by analyzing spectro-polarimetric observations. These observations
  comprise different spectral lines obtained with two ground based
  telescopes and a space borne telescope. <P />The lower brightness
  of sunspots on the solar surface is due to the presence of strong
  magnetic fields (up to 4 kG in the umbra), which makes the overturning
  convection inefficient. Convection is the main heat transport mechanism
  in the quiet Sun. The Penumbra, the annular part around the umbra
  has a brightness of about 75% of that in the quiet Sun. At the same
  time it has an average magnetic field strength of around 1.5 kG. The
  brightness of penumbrae has been an enigma for solar physicists for
  a long time. Theoretical models like the gappy penumbra model and the
  convective roll model as well as magnetohydrodynamic (MHD) simulations
  suggest that the heat transport in penumbrae is based on the presence of
  overturning convection. Direct observational evidence for the presence
  of convective flows in penumbral filaments was missing so far. In
  Chapter 3 we present observations of a penumbra in the C i 5380 Å
  spectral line formed in the deep photosphere. These high spatial
  resolution observations (0. '' 14) are obtained with the Swedish
  1-m Solar Telescope (SST). Doppler map clearly shows the presence of
  several dark downflow lanes at the edges of the penumbral filaments
  which surround the bright upflows at the center of the filaments,
  supporting overturning convection as a mechanism of heat transport
  in penumbrae. <P />Chapter 4 analyses the vertical gradient of the
  magnetic field strength in sunspots in the photosphere. This study
  includes two different sets of observations: (1) Observations with
  the Solar Optical Telescope (SOT) onboard the Hinode satellite. (2)
  Observations with the Tenerife Infrared Polarimeter-2 (TIP-2) mounted
  at the German Vacuum Tower Telescope (VTT). The spatially coupled
  inversions (using the SPINOR code) of the Zeeman sensitive Fe i
  6301.5 Å and Fe i 6302.5 Å spectral lines observed with the SOT show
  local patches in the penumbra where the magnetic field decreases with
  optical depth. Such detection of local magnetic "canopies" have not
  been reported earlier in the literature. These patches coincide with
  the bright inner parts of penumbral filaments and indicates that the
  stronger fields from the dark spines expand and cover i.e. overlie
  the weaker and more horizontal fields. The radial profile of the
  vertical gradient obtained with azimuthal averages shows that in
  the inner penumbra the magnetic field decreases with depth in the
  lower photosphere whereas in the outer penumbra it has the opposite
  trend. This result is consistent with low spatial resolution (1. ''
  0) spectro-polarimetric observations obtained with VTT/TIP-2. These
  particular observations include the photospheric Si i 10827.1 Å and Ca
  i 10833.4 Å spectral lines. These observational results are compared
  with MHD simulations of a sunspot. These comparisons show that the
  decreasing magnetic field strength with optical depth can be attributed
  to highly corrugated iso-optical depth surfaces. <P />The 3D structure
  of a sunspot's magnetic field is studied in Chapter 5 by comparing
  the measurements of photospheric and the upper chromospheric magnetic
  field. Here the photospheric magnetic field is obtained by inversion
  (using the SPINOR code) of the Si i 10827.1 Å and Ca i 10833.4
  Å spectral lines together, whereas inversion of the He i triplet
  at 10830 Å using the HeLIx + code provided the upper chromospheric
  magnetic field. These observations were recorded with VTT/TIP-2. The
  umbral magnetic field strength in the upper chromosphere is found to
  be lower by a factor 1.4-1.6 compared to the photosphere. The vertical
  gradient of the magnetic field strength between the photosphere and the
  upper chromosphere is in a range between 0.5 G km<SUP>-1 </SUP>and 1.0
  G km<SUP>-1 </SUP>in the umbra. The difference of the magnetic field
  strength between both atmospheric layers steadily decreases from the
  sunspot center to the outer boundary of the sunspot. The magnetic
  canopy at the upper chromospheric heights is found just outside the
  visible boundary of the sunspot. The most striking result found is that
  the magnetic field vector is less inclined in the upper chromosphere
  compared to the photosphere. A higher magnetic field strength is
  observed in a light-bridge in the upper chromosphere compared to that
  in the photosphere.

Title: Erratum:"Convective Nature of Sunspot Penumbral
Filaments: Discovery of Downflows in the Deep Photosphere" <A
    href="/abs/2011ApJ...734L..18J">(2011, ApJ, 734, L18)</A>
Authors: Joshi, Jayant; Pietarila, A.; Hirzberger, J.; Solanki, S. K.;
   Aznar Cuadrado, R.; Merenda, L.
Bibcode: 2011ApJ...740L..55J
Altcode:
  No abstract at ADS

Title: Convective Nature of Sunspot Penumbral Filaments: Discovery
    of Downflows in the Deep Photosphere
Authors: Joshi, Jayant; Pietarila, A.; Hirzberger, J.; Solanki, S. K.;
   Aznar Cuadrado, R.; Merenda, L.
Bibcode: 2011ApJ...734L..18J
Altcode: 2011arXiv1105.1877J
  We study the velocity structure of penumbral filaments in the deep
  photosphere to obtain direct evidence for the convective nature of
  sunspot penumbrae. A sunspot was observed at high spatial resolution
  with the 1 m Swedish Solar Telescope in the deep photospheric C I 5380
  Å absorption line. The Multi-Object Multi-Frame Blind Deconvolution
  method is used for image restoration and straylight is filtered out. We
  report here the discovery of clear redshifts in the C I 5380 Å line
  at multiple locations in sunspot penumbral filaments. For example,
  bright head of filaments show larger concentrated blueshift and are
  surrounded by darker, redshifted regions, suggestive of overturning
  convection. Elongated downflow lanes are also located beside bright
  penumbral fibrils. Our results provide the strongest evidence yet
  for the presence of overturning convection in penumbral filaments and
  highlight the need to observe the deepest layers of the penumbra in
  order to uncover the energy transport processes taking place there.

Title: Effect of Polarimetric Noise on the Estimation of Twist and
    Magnetic Energy of Force-Free Fields
Authors: Tiwari, Sanjiv Kumar; Venkatakrishnan, P.; Gosain, Sanjay;
   Joshi, Jayant
Bibcode: 2009ApJ...700..199T
Altcode: 2009arXiv0904.4594T
  The force-free parameter α, also known as helicity parameter or twist
  parameter, bears the same sign as the magnetic helicity under some
  restrictive conditions. The single global value of α for a whole active
  region gives the degree of twist per unit axial length. We investigate
  the effect of polarimetric noise on the calculation of global α value
  and magnetic energy of an analytical bipole. The analytical bipole
  has been generated using the force-free field approximation with a
  known value of constant α and magnetic energy. The magnetic parameters
  obtained from the analytical bipole are used to generate Stokes profiles
  from the Unno-Rachkovsky solutions for polarized radiative transfer
  equations. Then we add random noise of the order of 10<SUP>-3</SUP>
  of the continuum intensity (I <SUB> c </SUB>) in these profiles to
  simulate the real profiles obtained by modern spectropolarimeters such
  as Hinode (SOT/SP), SVM (USO), ASP, DLSP, POLIS, and SOLIS etc. These
  noisy profiles are then inverted using a Milne-Eddington inversion
  code to retrieve the magnetic parameters. Hundred realizations of this
  process of adding random noise and polarimetric inversion is repeated
  to study the distribution of error in global α and magnetic energy
  values. The results show that (1) the sign of α is not influenced
  by polarimetric noise and very accurate values of global twist can
  be calculated, and (2) accurate estimation of magnetic energy with
  uncertainty as low as 0.5% is possible under the force-free condition.

Title: Software for interactively visualizing solar vector
    magnetograms of udaipur solar observatory
Authors: Gosain, Sanjay; Tiwari, Sanjiv; Joshi, Jayant;
   Venkatakrishnan, P.
Bibcode: 2008JApA...29..107G
Altcode:
  No abstract at ADS

Title: Evolution of Magnetic Helicity in NOAA 10923 Over Three
    Consecutive Solar Rotations
Authors: Tiwari, Sanjiv Kumar; Joshi, Jayant; Gosain, Sanjay;
   Venkatakrishnan, P.
Bibcode: 2008ASSP...12..329T
Altcode: 2009arXiv0904.4024T; 2008tdad.conf..329T
  We have studied the evolution of magnetic helicity and chirality
  in an active region over three consecutive solar rotations. The
  region where it first appeared was named NOAA10923 and in subsequent
  rotations it was numbered NOAA 10930, 10935 and 10941. We compare the
  chirality of these regions at photospheric, chromospheric and coronal
  heights. The observations used for photospheric and chromospheric
  heights are taken from Solar Vector Magnetograph (SVM) and H-α imaging
  telescope of Udaipur Solar Observatory (USO), respectively. We discuss
  the chirality of the sunspots and associated H-α filaments in these
  regions. We find that the twistedness of superpenumbral filaments is
  maintained in the photospheric transverse field vectors also. We also
  compare the chirality at photospheric and chromospheric heights with
  the chirality of the associated coronal loops, as observed from the
  HINODE X-Ray Telescope.

