Study of evolution of magnetic inhomogeneity on the sun using narrow band Imaging

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.