explanation blue bibcodes open ADS page with paths to full text
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.
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>
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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
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>
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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
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.
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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
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>
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Title: Spicules and downflows in the solar chromosphere
Authors: Bose, Souvik; Joshi, Jayant; Henriques, Vasco M. J.; Rouppe
van der Voort, Luc
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>
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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
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>
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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
2020A&A...637C...1B Altcode:
No abstract at ADS
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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
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.
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Title: Magnetic field variations associated with umbral flashes and
penumbral waves
Authors: Joshi, Jayant; de la Cruz Rodríguez, Jaime
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.
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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.
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.
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Title: Vertical magnetic field gradient in the photospheric layers
of sunspots
Authors: Joshi, Jayant; Lagg, Andreas; Hirzberger, Johann; Solanki,
Sami K.; Tiwari, Sanjiv K.
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.
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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
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>
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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
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>
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Title: Magnetic and Velocity Field of Sunspots in the Photosphere
and Upper Chromosphere
Authors: Joshi, Jayant
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.
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.
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
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.
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.
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.
