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"
---------------------------------------------------------
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)
---------------------------------------------------------
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.
---------------------------------------------------------
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.
---------------------------------------------------------
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.
---------------------------------------------------------
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>
---------------------------------------------------------
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.
---------------------------------------------------------
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>
---------------------------------------------------------
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.
---------------------------------------------------------
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>
---------------------------------------------------------
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
---------------------------------------------------------
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
---------------------------------------------------------
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.
---------------------------------------------------------
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.
---------------------------------------------------------
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&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&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&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&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>.
---------------------------------------------------------
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%.
---------------------------------------------------------
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 &
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.
---------------------------------------------------------
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.