Waves in solar atmosphere and their role in dynamics of the corona

Abstract

Solar atmosphere is stratified and exhibits dynamic structures with confined hot plasmas. The corona consists of dynamic magnetic structures, in the form of closed and open loops with an enhanced plasma density. Moreover, these elastic and compressible structures support different types of waves, playing the role of the wave guide. The waves can be described using magnetohydrodynamics (MHD). In this thesis, we detect and study propagating disturbances (PDs) along active region loops using simultaneous imaging and spectroscopy. An image sequence recorded in Fe ix/Fe x 171Å channel, from Transition Region and Coronal Explorer (TRACE) and spectral data in the different extreme ultraviolet (EUV) lines obtained from Coronal Diagnostic Spectrometer (CDS) are analysed. Space-time map constructed from the TRACE image sequence shows the presence of PDs close to the loop foot point propagating with an apparent speed of 39 km s-1 along an active region loop in an o_-limb position. Wavelet analysis confirms the periodicity is equal to 5:4 min. The corresponding spectroscopic data from CDS, at a location away from the foot point, show oscillations in all three line parameters roughly at the same period. At locations farther from the foot point, the line width oscillation seems to disappear while the Doppler velocity oscillation becomes prominent. We attribute this to the signature of propagating slow waves that get affected by flows/other events close to the foot point. We detect PDs in other loops from simultaneous observations by TRACE and CDS/- SOHO. The spectroscopic observations confirm the presence of PDs in the transition region, as well as in the corona. From the phase relationship of oscillations in intensity and Doppler velocity, we identify these PDs as magneto-acoustic waves. PDs dissipate as they travel along the supporting structure. Different mechanisms are responsible for such dissipation, like thermal conduction, compressive viscosity, area stratifiction, optically thin radiation and gravitational stratification. Dissipations due to thermal conduction and compressive viscosity depend on the frequency of the PDs. We identify PDs along polar plumes and interplumes. We find the damping lengths of PDs of various periodicities and perform a statistical study of frequency dependency of damping length and compare the observed variations with the theoretical predictions. We found that thermal conduction may not be the major contributor in damping. The other well accepted mechanism of coronal heating is impulsive heating by nanoares ((Parker 1988)). Nanoares are defined as localized impulsive bursts of 5 - 100 s duration. The random movements of footpoints of bipolar fields create tangential discontinuities, which are destroyed by rapid reconnection of magnetic fields. Hence, the bipolar fields above the surface of the sun are filled with nanoares. We study the high frequency dynamics in the braided magnetic structure of an active region (AR 11520) moss as observed by High-Resolution Coronal Imager (Hi-C). We detect quasi periodic ows and waves in these structures. We search for high frequency dynamics while looking at power maps of the observed region. We find that shorter periodicities (30 - 60 s) are associated with small spatial scales which can be resolved by Hi-C only. We detect quasi periodic ows with a wide range of velocities from 13 - 185 km s-1 associated with the braided regions. This can be interpreted as plasma outows from reconnection sites. We also find presence of short period and large amplitude transverse oscillations associated with the braided magnetic region. Such oscillations could be triggered by reconnection or such oscillation may trigger reconnection. Prominences and/or filaments are ubiquitous in the solar corona. They manifest as cool plasma structure embedded in the hot corona. Prominences have different properties which are primarily based on the magnetic structure of the region in which they are formed. Prominences can remain stable for a long time. Depending on different parameters like a twist in the magnetic field, mass loading and ambient magnetic field they become unstable and erupt. Prominence eruptions are usually associated with Coronal Mass Ejections (CMEs), which are huge eruptions of plasma and magnetic field into heliosphere and are the main cause of geomagnetic disturbances (see Parenti (2014) and references therein). We report and analyse twisting and untwisting motions of different parts of a prominence during and after eruption. We find that small scale twists of prominence legs propagate along the spine and initiate roll motion. We also follow the evolution of background loops and interpret its role in prominence eruption. We find that the prominence threads untwist and the twist propagates upward with the speed of ~ 83 kms-1. We also estimate the lower value of twist to be ~ 4π which is enough to destabilize the prominence. We analyse the CME associated with this prominence and find the signatures of the twist being carried away.