Introduction: to cover up the dynamic range of coronal

Introduction: 

The first
impressions of solar corona are dated back to the late 1800s, when there were
no cameras and pictures were only hand-made. The coronal structures are visible
only during the total solar eclipse. The rays can be seen coming out most
efficiently from the polar regions of the sun. These rays are present for
almost full year but can only be captured during the eclipse. The pinkish
coloured prominences of corona are very attractive to the eyes. Consequently,
scientists from all over the world try to catch and capture the glimpse of
these coronal structures.

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The emission from the photosphere
can only be perceived due to presence of free electrons in corona. This is the
fact behind observed white light emission of solar eclipse. Therefore, the
shapes captured in eclipse images outline the coronal magnetic fields and are
directly connected with them as well. Plus, they are also the tracers of
distribution of coronal electron density. The coronal magnetic models are build
up n the basis of these structures. As far as we do not get any tools to
measure the exact magnetic field of corona. Till then, these coronal structures
can be used to make assumptions for coronal magnetic models and to find
origin  of solar wind.  

In this research, eclipse
observations will be used to yield novel insights into structures defining the
coronal plasma and magnetic field despite the proliferation of space-based
white light coronagraph observations of the extended corona and
extreme-ultraviolet (EUV) observations very close to the Sun. A sequence of 10
or more exposures is needed during the totality to cover up the dynamic range
of coronal white light emission out to a few solar radii. The creation of
single composite image with a resolution of 2″-3″ is enabled with the
application of mainly two techniques. First one is sub-pixel alignment of
images taken with different exposure time and other one is Adaptive Circular
High-pass Filter (ACHF). The total time needed for effective exposure of each
image is between 20 to 50 s. The resulting composite expose the solar
atmosphere starting from the upper chromosphere out to a distance of several
solar radii all around the Sun.  

Background:

Let’s start with the sun!
Sun is really important part of our solar system as it is nearest and only star
existing in it (solar system). Also, it provides all of the energy of the
planetary system. Since, it is our nearest star, it is easy to study its to
study its structure, atmosphere and physical characteristics. The information
so obtained can be used to test the theories of stellar structure and
evolution. And it will also help us improve the theories related to stars and
have a better understanding of other stars too.

We know that on sun it is
easy to resolve features (due to closeness) and study other physical processes
also. Whereas, it is not possible to do so with distant stars and other objects
of the space. Corona being the mysterious part should be given even more
attention. Studies conducted on the sun helped us to understand many important
characteristics. For e.g., it is now well-known fact that that other stars also
have spots and hot coronae. Corona is found to be the most powerful accelerator
in the universe. It has the power to accelerate ions uo to tens of
Giga-electron volts and electrons up to hundreds of mega electron volts with
the help of flares and coronal mass ejections (CMEs).  In this way, solar wind arises that affects
the whole planetary system.

Sun’s atmosphere:

All the visible radiation of sun comes from its
surface known as photosphere. Photosphere is thin layer of the sun’s surface.
It can be observed in white light. The irradiance spectrum shows maximum at
visible wavelengths, which can be fitted with black body spectrum with a
temperature of 6400 K and wavelength 2000 Å i.e. the temperature of solar
surface. It has a granular structure. Most of the knowledge that we have gained
till today of solar magnetic field is deduced from the observations of
photospheric field. We have information regarding Zeeman effect of photosphere.
This information is related to spectral lines in visible wavelengths. The
three-dimensional conoral magnetic field is interpreted from two- dimensional
maps of photospheric magnetic field or by tracing subphotospheric origin from
emerging magnetic flux element. Above photosphere, there exists atmosphere of
sun. There are two different layers named as chromosphere and corona.
Chromosphere constitutes of h ~2000 Km above the photosphere. It rises to 10000
K in the chromosphere h~2000 Km. The solar Corona is uppermost part of Sun’s
atmosphere, extending millions of kilometers into space Solar corona is made up
of dynamic changing structures. These arise in response to activities on Sun’s
surface such as CMEs. Corona and chromosphere can only be observed during total
solar eclipse. The fact is that density of matter in both corona and
chromosphere is very low. Consequently, they emit very little light and appear
faint. In bright light of photosphere, they are not visible.

Corona’s temperature,
structure and spectrum: 

The solar Corona is divided
into three zones, which all vary their size during the solar cycle: (1) active
regions, (2) quiet-Sun regions, and (3) coronal holes. (1) Active regions are
located in areas of strong magnetic field concentrations, visible as sunspot
groups in optical wavelengths or magnetograms. Sunspot groups typically exhibit
a strongly concentrated leading magnetic polarity, followed by a more
fragmented trailing group of opposite polarity. Because of this bipolar nature,
active regions are mainly made up of closed magnetic field lines. Due to the
permanent magnetic activity in terms of magnetic flux emergence, flux
cancellation, magnetic reconfigurations, and magnetic reconnection processes, a
number of dynamic processes such as plasma heating, flares, and CMEs occur in
active regions. Consequences of plasma heating in the chromosphere are upflows
into coronal loops, which give active regions the familiar appearance of
numerous filled loops, which are hotter and denser than the background corona,
producing bright emission in soft X-rays and EUV wavelengths. (2) historically,
the remaining areas outside of active regions were dubbed quiet-Sun regions.
Today, however, many dynamic processes have been discovered all over the solar
surface, so that the term quiet Sun range from smallscale phenomena such as
network heating events, nanoflares, explosive events, and soft X-rays jets, to
large scale structures, such as transquatorial loops or coronal arches. The
distinction between active regions and quiet-Sun regions becomes more and more
blurred because most of the large-scale structures that overarch quiet-Sun
regions are rooted in active regions. A good working definition is that
quiet-Sun regions encompass all closed magnetic field regions (excluding active
regions), which demarcates the quiet-Sun territory from coronal holes (that
encompass open magnetic field regions). (3) the northern and southern polar
zones of the solar globe have generally been found to be darker than the
equatorial zones during solar eclipses. Max Waldmeier thus dubbed those zones
as coronal holes (i.e. Koronale Locher in German). Today is fairly clear that
these zones are dominated by open magnetic field lines, which act as efficient
conduits for flushing heated plasma from the corona into the solar wind,
whenever they are fed by chromospheric upflows at their footprints. Because of this efficient
transport mechanism, coronal holes are empty of plasma most of the time, and
thus appear much darker than the quiet-Sun, where heated plasma flowing upward
from the chromosphere remains trapped, until cools down and precipitates back
to the chromosphere.  

To our eyes, sun appears to
be lifeless and changing, except the monotonic rotation which can be traced
down by observing sunspot cycle. But in reality, sun is actually made up of
vibrant dynamic plasma processes which are observed in solar corona. The
processes can be detected with the help of EUV light and soft X-rays. However,
there is currently a shift in the pattern and realized that most of the
structures which are apparent in corona can be controlled by plasma flows and
intermittent heating. 

It is, however, not easy to
measure and track these flows with our remote sensing methods, like the
apparently motionless rivers seen from an airplane. For slow flow speeds, the
so-called laminar flows, there is no feature to to track, while the turbulent
flows may be easier to detect because they produce whirls and vortices that can
be tracked. A similar situation happens in the solar corona. occasionally, a
moving blob is detected in a coronal loop; it can be used as a tracer. Most of
the flows in coronal loops seem to be subsonic (like laminar flows) and thus
featureless. Occasionally, we observe turbulent flows, which clearly reveal
motion, especially when cool and hot plasma mixes by turbulence and thus yields
contrast by emission and absorption in a particular temperature filter. Motion
can also be detected with Doppler shift measurements, but this yields only the
flow component along the line of sight. There is increasing evidence that flows
are ubiquitous in the solar corona.    The spectrum of Corona
consists of bright lines superimposed on a continuous spectrum. The observed
emission lines of highly ionized atoms of iron, nickel, neon, calcium in
spectrum of corona clearly indicate that temperature prevailing in Corona is
very high (more than million degrees kelvin). Due to high temperature,
electrons in Corona region have very high energies. These electrons interact
with ionized atoms and give rise to emission of X-rays. The coronal X-ray
emission is much larger than that of photosphere. The temperature of
photosphere is only 6000 K. So, it emits very little energy in X-ray region. As
one goes further up in Corona, temperature rises to million-degree kelvin. The
second law of thermodynamics precludes such a scenario as heat cannot flow from
a cooler region to a hotter region on its own. We also know that the radiation
from photosphere passes through Corona almost freely because of its (Corona’s)
low density. Since hardly any absorption of radiation takes place in Corona,
existence of such high temperature in Corona presents paradoxical situation.
Studying Corona is therefore important for understanding what drives its
structure and how energy is released from sun. 

Existing imaging
techniques:  

There are numerous solar
imaging techniques present in today’s world, but they are often restricted by
their fields of view. For example, the solar dynamics observatory AIA has
really good resolution but it only images out one third of solar radius above Sun’s
limb. And on the other hand, LASCO C2 provides broad view of outer region of
corona but picturises only 2.2 solar radii above sun’s limb. Even if we try to
gather information from both these observatories, then also a continuous view
of corona cannot be obtained. So, in order to image down a broader view of
Corona, my research project can be helpful. 

Although eclipse
observations have frequently captured the imprint of the passage of CMEs
through the corona, as they disrupt its large-scale structures for several
hours, very rarely have they captured a CME in eclipse images. One of the first
such records was illustrated in a hand drawn image made by G. temple during the
1860 July 18 eclipse that traversed Spain. In Tempel’s remarkable image, an
unusual helical structure was drawn very close to the Sun. At the time, it was
considered an oddity, and some were skeptical about the reliability of Tempel’s
account. With the discovery of Space-based coronagraphs in the early 1970s, it
became plausible the Tempel had actually captured one. However, to date, there
have been no reports of spatially uninterrupted detection of tethered
prominence-CME systems starting from the Sun, and expanding into interplanetary
space.   

Objectives of research: 

1.  
Corona to be viewed during
solar eclipse with help of 10-cm telescope mounted with a DSLR. 

2.    
  A full view of
Corona will be taken. 

3.    
A broad view with
continuous information will be gathered. 

4.    
Information gathered will
be assessed to understand transfer, storage and dissipation of energy from
photosphere to corona. 

5.    
Assessed data is used to
understand working of magnetic field lines in solar corona.

Methodology: 

The eclipse images will be
processed using method developed by Druckmiller, known as noise adaptive fuzzy
equalization (NAFE) method. It is a sequence of images at different exposure
times are aligned using a modified phase correlation technique (a technique
that does not require reference points). The alignment of images will be
carried out for a sequence of 10 or more exposures. The Adaptive Circular
High-pass Filter (ACHF) will then be applied to enhance coronal structures in
order to create an image with a resolution of 2-3 arcsec. The resulting images
will hopefully yield better coronal structures spanning over solar
radii.   

The next total solar
eclipse will appear on 2nd July 2019. It can be
viewed from Argentina. So, a team of two PhD students (including me) and Professor
John C. Brown will be taken to Buenos Aires for viewing the eclipse. A
redesigned German equatorial mount will provide a sturdy platform for the
instruments and also a quick relocation wherever needed, if there were any
sudden changes in the weather. It will be a 5-day travel for the team so
that setting up cameras and telescopes can be done. The data will be recorded
by preprogrammed Python scripts and the cameras will be fully controlled by
laptop computers. After collecting the data, the simulations gathered will be assessed
by the astronomy and astrophysics students for two years. The collected data
will be used to understand the ” coronal heating problem”.  The research will start in January 2018 and will
be completed by December 2019. In the first year, continuous information of
solar eclipse will be collected. In second and final year, the collected
information will be studied to understand magnetic field of corona. 

The team:

Professor John C. Brown: His main
research interests are plasma physics, solar physics, stellar wind
astrophysics, signal analysis methods, solar-sailed spacecraft, comets and
meteors. He was awarded an IOP award 2002 for promotion of public awareness of
physics, particularly in recognition of planetarium work and use of magic in
science. He has been involved in many space projects such as ISRO TD1A, Skylab,
Solar Max Mission, Yohkoh, SOHO; SMM workshop team leader, Spacelab WUPPE, UK
co-investigator on NASA RHESSI mission. He is the 10th astronomer
royal for Scotland and honorary Senior Research Fellow in University of
Glasgow. He is also an Honorary Professor in University of Edinburgh and
University of Aberdeen. Over 250 papers have been published, mainly in solar
physics, stellar polarimetry, plasma astrophysics, and inverse problems.

Dr lain G. Hannah (a
collaborative professor): He is a Royal Society University Research Fellow
working in University of Glasgow. His research is focused on physical processes
behind emission observed in solar flares.  

Funding grants:

The study of sun should be
conducted thoroughly. It is source of the solar wind and its proximity provides
heat and light to maintain light on earth, as well as being a unique laboratory
to test our theories of the evolution of other stars and formation of galaxies.
Moreover, This will be one of cost efficient researches in the history of
astrophysics field. It is because the grants will only be basically used for
instruments and the travel. Other than that, there are no extra costs.

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