Author name code: joshi-jayant ADS astronomy entries on 2022-09-14 =author:"Joshi, Jayant" ------------------------------------------------------------------------ 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.
Aims: We performed an extensive statistical characterization of the quiet-Sun Ellerman bombs (QSEBs) in these new Hβ observations.
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.
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 Mm2, with a median of 0.018 Mm2. 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−1, with an average that is upward propagating at +4.4 km s−1. 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.
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 (BLOS ≲ 50 G). Our observations support the interpretation of reconnection along vertically extended current sheets.

Movies associated to Figs. 1 and 3 are available at https://www.aanda.org 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−1. The cause of such downflows, however, is still not fully clear and has remained a matter of debate.
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.
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.
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.
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.

Movies associated to Figs. 1-3, 8, and 10 are available at https://www.aanda.org Title: Line formation of He I D3 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 D3 and He I 10 830 Å lines, suggesting that their temperatures are higher than previously expected.
Aims: We aim to explain the line formation of He I D3 and He I 10 830 Å in small-scale reconnection events.
Methods: We used a simulated EB in a Bifrost-generated radiative magnetohydrodynamics snapshot. The resulting He I D3 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 D3 obtained at the Swedish Solar Telescope with the TRI-Port Polarimetric Echelle-Littrow Spectrograph.
Results: Emission in He I D3 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 D3 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 × 103 K to 106 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 D3 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 × 104 − 2 × 106 K and electron densities between 1011 and 1013 cm−3. The synthetic emission signals are a result of coupling to local conditions in a thin shell around the EB, with temperatures between 7 × 103 and 104 K and electron densities ranging from ∼1012 to 1013 cm−3. This shows that both strong non-LTE and thermal processes play a role in the formation of He I D3 and He I 10 830 Å in the synthetic EB/UV burst that we studied.
Conclusions: In conclusion, the synthetic He I D3 and He I 10 830 Å emission signatures are an indicator of temperatures of at least 2 × 104 K; in this case, as high as ∼106 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.
Aims: We aim to determine the occurrence of Ellerman bombs in the penumbra of sunspots.
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.
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−1 and sometimes more than 10 km s−1. 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.
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.

Movies associated to Figs. 1 and 6 are available at https://www.aanda.org 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.
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.
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).
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.
Conclusions: We speculate that these rapid plasma downflows could well be the chromospheric counterparts of the commonly observed TR downflows.

Movies are available at https://www.aanda.org 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.″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.

Movies associated to Figs. 1-3, B1, and B2 are available at https://www.aanda.org 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 K3 minimum best representing the velocity of the spicules. The K2 features are either suppressed by the Doppler-shifted K3 or enhanced via increased contribution from the lower layers, leading to strongly enhanced but unshifted K2 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.
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.
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.
Results: Our results indicate that the Ca II 8542 Å line in sunspots is greatly sensitive to magnetic fields at log τ500 = -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).
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
Aims: We investigate the magnetic field of a sunspot in the upper chromosphere and compare it to the photospheric properties of the field.
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.
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-1. 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
Aims: We investigate the vertical gradient of the magnetic field of sunspots in the photospheric layer.
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.
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.
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 D3 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 D3 have never been reported.
Aims: We aim to study the signature of EBs in neutral helium triplet lines.
Methods: The observations consisted of ten consecutive SST/TRIPPEL raster scans close to the limb, featuring the Hβ, He I D3 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.
Results: Three EBs in our data show distinct emission signatures in neutral helium triplet lines, most prominently visible in the He I D3 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 × 104-105 K.

Movies associated to Figs. 3-5 are available at http://www.aanda.org 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.
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.
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.
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.

A movie associated to Fig. 4 is available at http://www.aanda.org 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.

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.

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.

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.

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-1 and 1.0 G km-1 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" (2011, ApJ, 734, L18) 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 10-3 of the continuum intensity (I c ) 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.