dc.description.abstract |
The solar atmosphere is structured by the evolving magnetic fields. These
structures occur on different spatial scales. On small scale - bright points with
thin boundaries have been observed. On large spatial scale - plages, sunspots
and filaments/prominences have been observed. In the large scale structures,
the magnetic field dominates and convection is inhibited. In each of these
structures the magnetic field varies spatially and temporally. The spatial
variation depends on the structures and the temporal variation depends on
the flux emergence, cancellation and shear motions of its foot points. One such
magnetic structure is solar filament which contain dense and cool (∼ 104 K)
plasma embedded in the tenuous and hot corona. They normally form in
active regions, though not very uncommon in quiet regions. Filaments appear
above a boundary between the opposite polarity of magnetic fields on the
Sun, normally called as polarity inversion line. When viewed in Hα, filaments
appear to be aligned along the PIL and the field lines are mostly non-potential
in nature. Their presence is visible in the chromosphere and lower part of the
corona. The quiet filament structures are highly stable and most of them will
survive for weeks to months. On the other hand, the active region filaments
evolve rapidly and they can survive over a period varying from few hours to
days. Most often, the filament ends up with eruption associated with solar
flares and CMEs. In this thesis, I have studied a few active region filament
eruption using high resolution coronal data from AIA/SDO and ground based
Hα data from BBSO and GONG.
Filaments/prominences exhibit a variety of dynamical processes during
their formation, evolution and prior to eruption. The observation made in
multiple wavelengths around a chromospheric spectral line are used to infer
dynamical processes such as converging motions and rotational motions in
the ends of the filaments during the eruption. In this thesis, another work
is to develop a narrow band imager using Fabry-Perot interferometer, which
can provide images at the chromospheric height in the solar atmosphere and
also it is capable of producing dopplergrams to study the line-of-sight motions
during the filament eruptions. This thesis is organized as follows:
The first chapter highlights the different trigger and instability mecha-
nisms behind filament eruptions studied in the past. I have discussed differ-
ent dynamical processes often seen during filament eruptions. This chapter
also discussed the motivation to build a narrow band imager to study the
rotational motion at the legs of the filaments/prominence during eruption.
Finally, a brief content of each chapters are presented.
In the second chapter, I have discussed about different data set used for
the observational study of the filament eruption events. A two-stage filament
eruptions associated with the active region NOAA 11444 have been stud-
ied to understand sequence of events occurred during eruptions. A detailed
study about converging motion and flux cancellation in connection with pre-
eruption brightening and bright flow at the coronal images during activation
of first phase of eruption are presented here. During the second phase of
eruption, the flux cancellations, expansion of loops, brightened cusp shaped
loops and the contraction of overlying coronal loops near the filament have
been observed. The importance of the magnetic flux cancellations, expansion
of loops, reconnection and contraction of overlying coronal loops towards the
second phase of filament eruption are also discussed in this chapter.
In the third chapter, the study of an another active region filament erup-
tion event is presented in EUV and visible wavelength regime. The filament
eruption occurred in the southern hemisphere of the Sun on 08 July 2011.
The study suggests that the emerging flux, converging motion and injection
of opposite magnetic helicity could be responsible for destabilization of the
western footpoint of the filament leading to eruption. As an interesting phe-
nomena, an anti-clockwise rotational motion in both the footpoints just after
the onset of filament eruption is observed during the eruption. In this chap-
ter, a plausible reason for the eruption in the context of observed rotational
motion has been discussed.
Primary objective of the fourth chapter is to understand the relation be-
tween the occurrence of the rotational/vortical motion observed near the ends
of the filament and eruption of the filaments. Several active region filament
eruptions are presented in details. For all the reported events, the rota-
tional/vortical motion in the photosphere near the ends of all active region
filaments during their initial phase of eruption or at the onset phase of the fast
rise of the filament are observed. The importance of this rotational/vortical
motions at the endpoints of the filaments during their activations towards
eruptions are discussed in this chapter.
Fifth chapter describes the development of the narrow band imager (NBI)
using Fabry-Perot (FP) interferometer and an order sorting filter. The FP
and filter have been tested in a series of experiments carried out in the labo-
ratory. The NBI is capable to imaging the solar atmosphere at 6563 ̊A chro-
mospheric Hα line. The observed data are used to produce dopplergrams at
the chromospheric height to study the rotational motion near the legs of the
filaments during its activation. At the end, I have presented several observed
filaments using NBI and made few dopplergrams of the filament region at
chromospheric height.
The last chapter provides the summary and conclusions of this thesis work.
The thesis ends with a brief descriptions of the planned future work in which
research can be carried out further. |
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