explanation      blue bibcodes open ADS page with paths to full text
Author name code: bose
ADS astronomy entries on 2022-09-14
=author:"Bose, Souvik"

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Title: ML pipeline for Solar Dynamics Observatory (SDO) data
Authors: Salvatelli, Valentina; Neuberg, Brad; Dos Santos, Luiz F. G.;
   Bose, Souvik; Cheung, Mark C. M; Janvier, Miho; Jin, Meng; Gal, Yarin;
   Güneş Baydın, Atılım
2022zndo...6954828S    Altcode:
  This software has been developed from the [FDL SDO
  Team](https://frontierdevelopmentlab.org/2019-sdo). The
  package contains: a configurable pipeline to train and
  test ML models on data from the Solar Dynamics Observatory
  some notebooks for data exploration and results analysis. It
  contains all the code supporting the publications: [Multi-Channel
  Auto-Calibration for the Atmospheric Imaging Assembly using Machine
  Learning](https://arxiv.org/abs/2012.14023) "Exploring the Limits of
  Synthetic Creation of Solar EUV Images via Image-to-Image Translation"
  Accepted for publication on ApJ (July 2022)

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Title: Exploring the Limits of Synthetic Creation of Solar EUV Images
    via Image-to-Image Translation
Authors: Salvatelli, Valentina; dos Santos, Luiz F. G.; Bose, Souvik;
   Neuberg, Brad; Cheung, Mark C. M.; Janvier, Miho; Jin, Meng; Gal,
   Yarin; Gunes Baydin, Atilim
2022arXiv220809512S    Altcode:
  The Solar Dynamics Observatory (SDO), a NASA multi-spectral decade-long
  mission that has been daily producing terabytes of observational data
  from the Sun, has been recently used as a use-case to demonstrate the
  potential of machine learning methodologies and to pave the way for
  future deep-space mission planning. In particular, the idea of using
  image-to-image translation to virtually produce extreme ultra-violet
  channels has been proposed in several recent studies, as a way to
  both enhance missions with less available channels and to alleviate
  the challenges due to the low downlink rate in deep space. This
  paper investigates the potential and the limitations of such a deep
  learning approach by focusing on the permutation of four channels and
  an encoder--decoder based architecture, with particular attention to
  how morphological traits and brightness of the solar surface affect the
  neural network predictions. In this work we want to answer the question:
  can synthetic images of the solar corona produced via image-to-image
  translation be used for scientific studies of the Sun? The analysis
  highlights that the neural network produces high-quality images
  over three orders of magnitude in count rate (pixel intensity)
  and can generally reproduce the covariance across channels within
  a 1% error. However the model performance drastically diminishes in
  correspondence of extremely high energetic events like flares, and we
  argue that the reason is related to the rareness of such events posing
  a challenge to model training.

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Title: A textbook example of magnetic flux emergence leading to EBs,
    UV bursts, surges and EUV signatures
Authors: Cabello, Iballa; Moreno-Insertis, . Fernando, , Prof; Rouppe
   van der Voort, Luc; Bose, Souvik; Nóbrega Siverio, Daniel
2022cosp...44.2531C    Altcode:
  Small-scale eruptive phenomena (like Ellerman bombs (EBs), UV bursts,
  surges) constitute both a true challenge and an opportunity for
  progress in understanding the solar atmosphere since they involve very
  different layers from the photosphere to the low corona. In our work,
  we are aiming to characterize small-scale eruptive phenomena related to
  emerging flux regions. In particular, we use coordinated observations
  from the Swedish $1-$m Solar Telescope (SST), the Interface Region
  Imaging Spectrograph (IRIS) and the Solar Dynamics Observatory (SDO,
  both HMI and AIA) to analyze an episode of magnetic flux emergence
  in an enhanced network that leads to an EB, a UV burst, a cool surge,
  and coronal signatures in the EUV. Through Milne-Eddington inversions
  of the {\ion{Fe}{I}} 6302 \AA\ line observed with SST/CRISP we
  obtain high-resolution (0.057"/pixel) magnetograms that allow us to
  reliably measure the magnetic field at the photosphere. A comparison
  with the corresponding SDO/HMI magnetograms reveals that this type
  of small-scale events are barely discernible in low-resolution (1")
  observations. During the emergence, a roundish dark bubble is visible
  in {\ion{Ca}{II} K} 3933 \AA\ at the location where the two opposite
  polarities of the emerging dipole are splitting apart. Several minutes
  later, indirect evidence of reconnection is found above the positive
  polarity of the dipole through the appearance of an EB in the wings
  of the {H$\alpha$} 6563 \AA\ and {\ion{Ca}{II} K} 3933 \AA\ lines
  from SST, and also in the SDO/AIA 1600 and 1700 \AA~data. Later,
  a surge shows up as an elongated structure visible in absorption in
  {H$\alpha$} and {\ion{Ca}{II} K}, extending over 12 Mm projected size
  on the disk. The shape of the surge is also apparent as an absorption
  feature in the SDO/AIA channels. Simultaneously with the surge (and
  at the location where the EB had appeared earlier on) a UV burst
  is clearly discernible as a strong and bright emission feature both
  in IRIS/SJI 1400 and 2796 \AA. Interestingly, this UV burst also has
  counterpart in SDO/AIA 94, 171, 193, 211, 304, and 335 \AA, meaning that
  we can find multi-thermal plasma up to a few MK in the reconnection
  site. This observation clearly shows the impact of the emergence of
  new magnetic field from the photosphere through the chromosphere and
  transition region and up into the corona. In addition, it provides an
  illustrative case to test new realistic simulations.

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Title: On the relationship between spicules and coronal bright points
Authors: Bose, Souvik; De Pontieu, Bart; Rouppe van der Voort, Luc;
   Nóbrega Siverio, Daniel
2022cosp...44.2522B    Altcode:
  Coronal bright points (CBPs) are a set of small-scale, lower coronal
  loop systems connecting opposite magnetic polarities and are primarily
  characterized by enhanced emission in the extreme ultraviolet (EUV)
  wavelengths and X-rays. Being ubiquitous they are thought to play a
  definite role in heating the solar corona. This study aims to explore
  the chromospheric components associated with a CBP by focusing on
  spicules and small-scaled flux emergence. We used high-resolution
  observations in H$\beta$ and Fe I 617.3 nm spectral lines obtained
  from the Swedish 1-m Solar Telescope (SST) in coordination with the
  images acquired from the Atmospheric Imaging Assembly (AIA) instrument
  on-board the Solar Dynamics Observatory (SDO). On-disk spicules were
  automatically detected by employing advanced image processing techniques
  on the Dopplergrams derived from H$\beta$, and Mile-Eddington inversions
  of the Fe I 617.3 nm line provided the photospheric vector magnetic
  field. The AIA co-observations were co-aligned to SST with the latter
  serving as a reference. We find abundant occurrences of chromospheric
  spicules close to the "footpoints" of the CBP. The orientation of the
  spicules is predominantly aligned along with CBP loops which further
  indicates that they form a fundamental part of the same magnetic
  structure. Several examples of the spatio-temporal evolution indicate
  that much of the chromospheric plasma is heated to coronal temperatures
  implying that spicules potentially supply mass and energy to the CBP
  loops. Furthermore, we study chromospheric and corresponding coronal
  responses to two magnetic flux emergence events and their impact on the
  dynamics of the CBP. This study presents unique and unambiguous evidence
  that connects chromospheric spicular dynamics and flux emergence with
  a CBP for the very first time using high-resolution observations.

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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: On the dynamics of spicules and mass flows in the solar
    atmosphere
Authors: Bose, Souvik
2021arXiv211010656B    Altcode:
  Popular scientific summary -- The atmosphere of the Sun is envisioned as
  composed of inherently complex, non-homogeneous, and dynamic layers. A
  detailed understanding of the physical processes involved in these
  layers is still lacking. For example, it is largely unknown why the
  outermost layer of the Sun's atmosphere (the solar corona) is so much
  hotter than the photosphere by millions of degrees. Astrophysicists
  think that the layer sandwiched between the photosphere and the
  corona, known as the interface region, may hold the key to a better
  understanding of the nature of this enigma. With the help of coordinated
  high-resolution, ground- and space-based observations from the Swedish
  1-m Solar Telescope (SST) on La Palma, Spain, and NASA's Interface
  Region Imaging Spectrograph (IRIS) and Solar Dynamics Observatory (SDO),
  along with the support from an advanced numerical simulation, I aim to
  unlock some of the mysteries surrounding the dynamics of the interface
  region with a focus on small-scale jets, known as "spicules". Spicules
  are found almost everywhere on the Sun's surface and at any given
  moment there can be as many as 10 million of them rapidly shooting
  outwards. They are often found to be heated beyond chromospheric
  temperatures and appear in the transition region and (even) coronal
  passbands. Because of their "omnipresence", it is suggested that they
  play a major role in energizing the outer atmospheric layers of the
  Sun. This thesis focuses on the physical characteristics and dynamics
  of spicules, along with their role in mass-balance and heating of the
  solar atmosphere.

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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: Multichannel autocalibration for the Atmospheric Imaging
    Assembly using machine learning
Authors: Dos Santos, Luiz F. G.; Bose, Souvik; Salvatelli, Valentina;
   Neuberg, Brad; Cheung, Mark C. M.; Janvier, Miho; Jin, Meng; Gal,
   Yarin; Boerner, Paul; Baydin, Atılım Güneş
2021A&A...648A..53D    Altcode: 2020arXiv201214023D
  Context. Solar activity plays a quintessential role in affecting the
  interplanetary medium and space weather around Earth. Remote-sensing
  instruments on board heliophysics space missions provide a pool of
  information about solar activity by measuring the solar magnetic
  field and the emission of light from the multilayered, multithermal,
  and dynamic solar atmosphere. Extreme-UV (EUV) wavelength observations
  from space help in understanding the subtleties of the outer layers
  of the Sun, that is, the chromosphere and the corona. Unfortunately,
  instruments such as the Atmospheric Imaging Assembly (AIA) on board
  the NASA Solar Dynamics Observatory (SDO), suffer from time-dependent
  degradation that reduces their sensitivity. The current best calibration
  techniques rely on flights of sounding rockets to maintain absolute
  calibration. These flights are infrequent, complex, and limited to
  a single vantage point, however. <BR /> Aims: We aim to develop a
  novel method based on machine learning (ML) that exploits spatial
  patterns on the solar surface across multiwavelength observations to
  autocalibrate the instrument degradation. <BR /> Methods: We established
  two convolutional neural network (CNN) architectures that take either
  single-channel or multichannel input and trained the models using the
  SDOML dataset. The dataset was further augmented by randomly degrading
  images at each epoch, with the training dataset spanning nonoverlapping
  months with the test dataset. We also developed a non-ML baseline model
  to assess the gain of the CNN models. With the best trained models,
  we reconstructed the AIA multichannel degradation curves of 2010-2020
  and compared them with the degradation curves based on sounding-rocket
  data. <BR /> Results: Our results indicate that the CNN-based models
  significantly outperform the non-ML baseline model in calibrating
  instrument degradation. Moreover, multichannel CNN outperforms
  the single-channel CNN, which suggests that cross-channel relations
  between different EUV channels are important to recover the degradation
  profiles. The CNN-based models reproduce the degradation corrections
  derived from the sounding-rocket cross-calibration measurements
  within the experimental measurement uncertainty, indicating that
  it performs equally well as current techniques. <BR /> Conclusions:
  Our approach establishes the framework for a novel technique based
  on CNNs to calibrate EUV instruments. We envision that this technique
  can be adapted to other imaging or spectral instruments operating at
  other wavelengths.

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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: 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: Auto-Calibration of Remote Sensing Solar Telescopes with
    Deep Learning
Authors: Neuberg, Brad; Bose, Souvik; Salvatelli, Valentina; dos
   Santos, Luiz F. G.; Cheung, Mark; Janvier, Miho; Gunes Baydin, Atilim;
   Gal, Yarin; Jin, Meng
2019arXiv191104008N    Altcode:
  As a part of NASA's Heliophysics System Observatory (HSO) fleet of
  satellites,the Solar Dynamics Observatory (SDO) has continuously
  monitored the Sun since2010. Ultraviolet (UV) and Extreme UV (EUV)
  instruments in orbit, such asSDO's Atmospheric Imaging Assembly
  (AIA) instrument, suffer time-dependent degradation which reduces
  instrument sensitivity. Accurate calibration for (E)UV instruments
  currently depends on periodic sounding rockets, which are infrequent
  and not practical for heliophysics missions in deep space. In the
  present work, we develop a Convolutional Neural Network (CNN) that
  auto-calibrates SDO/AIA channels and corrects sensitivity degradation
  by exploiting spatial patterns in multi-wavelength observations to
  arrive at a self-calibration of (E)UV imaging instruments. Our results
  remove a major impediment to developing future HSOmissions of the
  same scientific caliber as SDO but in deep space, able to observe the
  Sun from more vantage points than just SDO's current geosynchronous
  orbit.This approach can be adopted to perform autocalibration of other
  imaging systems exhibiting similar forms of degradation

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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: Using U-Nets to Create High-Fidelity Virtual Observations of
    the Solar Corona
Authors: Salvatelli, Valentina; Bose, Souvik; Neuberg, Brad; dos
   Santos, Luiz F. G.; Cheung, Mark; Janvier, Miho; Gunes Baydin, Atilim;
   Gal, Yarin; Jin, Meng
2019arXiv191104006S    Altcode:
  Understanding and monitoring the complex and dynamic processes of
  the Sun is important for a number of human activities on Earth and
  in space. For this reason, NASA's Solar Dynamics Observatory (SDO)
  has been continuously monitoring the multi-layered Sun's atmosphere
  in high-resolution since its launch in 2010, generating terabytes of
  observational data every day. The synergy between machine learning
  and this enormous amount of data has the potential, still largely
  unexploited, to advance our understanding of the Sun and extend the
  capabilities of heliophysics missions. In the present work, we show that
  deep learning applied to SDO data can be successfully used to create a
  high-fidelity virtual telescope that generates synthetic observations of
  the solar corona by image translation. Towards this end we developed
  a deep neural network, structured as an encoder-decoder with skip
  connections (U-Net), that reconstructs the Sun's image of one instrument
  channel given temporally aligned images in three other channels. The
  approach we present has the potential to reduce the telemetry needs
  of SDO, enhance the capabilities of missions that have less observing
  channels, and transform the concept development of future missions.

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Title: Semi-empirical model atmospheres for the chromosphere of the
    sunspot penumbra and umbral flashes
Authors: Bose, Souvik; Henriques, Vasco M. J.; Rouppe van der Voort,
   Luc; Pereira, Tiago M. D.
2019A&A...627A..46B    Altcode: 2019arXiv190508264B
  Context. The solar chromosphere and the lower transition region
  are believed to play a crucial role in the heating of the solar
  corona. Models that describe the chromosphere (and the lower transition
  region), accounting for its highly dynamic and structured character
  are, so far, found to be lacking. This is partly due to the breakdown
  of complete frequency redistribution (CRD) in the chromospheric
  layers and also because of the difficulty in obtaining complete sets
  of observations that adequately constrain the solar atmosphere at
  all relevant heights. <BR /> Aims: We aim to obtain semi-empirical
  model atmospheres that reproduce the features of the Mg II h&amp;k
  line profiles that sample the middle chromosphere with focus on a
  sunspot. <BR /> Methods: We used spectropolarimetric observations
  of the Ca II 8542 Å spectra obtained with the Swedish 1 m Solar
  Telescope and used NICOLE inversions to obtain semi-empirical model
  atmospheres for different features in and around a sunspot. These
  were used to synthesize Mg II h&amp;k spectra using the RH1.5D
  code, which we compared with observations taken with the Interface
  Region Imaging Spectrograph (IRIS). <BR /> Results: Comparison of
  the synthetic profiles with IRIS observations reveals that there
  are several areas, especially in the penumbra of the sunspot,
  where most of the observed Mg II h&amp;k profiles are very well
  reproduced. In addition, we find that supersonic hot down-flows,
  present in our collection of models in the umbra, lead to synthetic
  profiles that agree well with the IRIS Mg II h&amp;k profiles, with
  the exception of the line core. <BR /> Conclusions: We put forward
  and make available four semi-empirical model atmospheres. Two for
  the penumbra, reflecting the range of temperatures obtained for the
  chromosphere, one for umbral flashes, and a model representative of
  the quiet surroundings of a sunspot. <P />Data of semi-empirical model
  atmospheres are only available at the CDS via anonymous ftp to <A
  href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/627/A46">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/627/A46</A>.

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Title: On the Variability of the Solar Mean Magnetic Field:
    Contributions from Various Magnetic Features on the Surface of the Sun
Authors: Bose, Souvik; Nagaraju, K.
2018ApJ...862...35B    Altcode: 2018arXiv180605291B
  The solar mean magnetic field (SMMF) is referred to as the disk-averaged
  line-of-sight (LOS) magnetic field that also reflects the polarity
  imbalance of the magnetic field on the Sun. The origin of the SMMF has
  been debated over the past few decades, with one school of thought
  suggesting that the contribution to the SMMF is mostly due to the
  large-scale magnetic field structure, also called the background
  magnetic field, whereas other and more recent studies have indicated
  that active regions have a major contribution to the observed SMMF. In
  this paper, we re-investigate the issue of the origin of the SMMF
  by decomposing the solar disk into plages, networks, sunspots, and
  background regions, thereby calculating the variation in the observed
  SMMF due to each of these features. We have used full-disk images
  from Solar Dynamics Observatory (SDO)/AIA recorded at 1600 Å for
  earmarking plages, networks, and background regions and 4500 Å images
  for separating the sunspots. The LOS fields corresponding to each of
  these regions are estimated from the co-temporal SDO/Helioseismic
  and Magnetic Imager full-disk magnetograms. The temporal variation
  of the SMMF shows a near one-to-one correspondence with that of the
  background field regions, suggesting that they constitute the major
  component of the observed SMMF. A linear regression analysis based
  on the coefficient of determination shows that the background field
  dominates and accounts for 89% of the variation in the SMMF, whereas
  the magnetic field from the other features accounts for the rest 11%.

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Title: Role of the background regimes towards the Solar Mean Magnetic
    Field (SMMF)
Authors: Bose, Souvik; Nagaraju, K.
2018IAUS..340...85B    Altcode: 2018arXiv180511151B
  The Solar Mean Magnetic Field (SMMF) is generally defined as the
  disc-averaged line-of-sight (LOS) magnetic field on the sun. The role
  of the active regions and the large-scale magnetic field structures
  (also called the background) has been debated over the past few decades
  to understand whether the origin of the SMMF is either due to the
  active regions or the background. We, in this paper have investigated
  contribution of sunspots, plages, networks and the background towards
  the variability of the SMMF using the datasets from the SDO-AIA &amp;
  HMI, and found that 89% of the SMMF is due to the background whereas
  the remaining 11% originates from the active regions and the networks.

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Title: High Precision Full Stokes Spectropolarimetry of the Sun as
    a star-Instrument design aspects
Authors: Bose, Souvik
2016arXiv161001581B    Altcode:
  The magnetic field plays a major role in governing the dynamics of the
  sun. Many interesting features like sunspots, flares, prominences,
  and Coronal Mass Ejections (CMEs) occur on its surface due to the
  dynamics associated with the magnetic fields. The magnetic activity
  exhibits spatial scales ranging from very fine scale (below the
  resolution limit of the current largest telescope) to large scale
  such as sunspots, active regions and the spatial scales as large as
  the sun itself. While the major efforts in building large telescopes
  is going on towards the goal of resolving smallest structure possible
  we propose here to measure the magnetic field on the global scale. For
  this purpose we propose an instrument to carryout high precision and
  high accurate spectropolarimetry of sun-as-a-star. In this thesis,
  we explore various instrumental design aspects that are necessary to
  make such observations. As part of the design consideration we have
  analysed a major noise source i.e. seeing induced cross-talk through
  simulation as well as using the measured scintillation data. Further,
  we have analysed full disk Stokes images from SOLIS/VSM and SDO/HMI
  for the purpose of getting a rough idea on the disk averaged Stokes
  signal level. The instrument design aspects are mainly constrained
  by the need to modulate the light before it enters the telescope and
  image the full sun as a point source. Modulating the light before
  it enters the telescope has the advantage of completely avoiding
  the instrumental polarization which will enable us to carry out high
  accurate polarimetric measurements. We have explored in this thesis
  various concepts of polarization modulator and spectral discriminator
  and worked out their suitability for the purpose of carrying out high
  precision and high accurate spectropolarimetry of sun-as-a-star at
  high spectral resolution.