Study of small-scale features observed in solar atmosphere

Abstract

The solar atmosphere is populated with various small-scale features which are observed to be very dynamic and highly structured. The advent of modern instruments with high spatial, spectral and temporal resolution, have aided us to study the evolution and different dynamical properties of the fine-scale structures. This thesis is a compilation of detailed analysis of different small-scale features and events as observed in the solar chromosphere and transition region. The chromosphere and transition region act as an interface between highly dense but cool photospheric plasma and hot low dense coronal plasma. Together, known as interface region, chromosphere and transition region play a key role in mass and energy supply from photosphere to corona. These layers are home of various small-scale features and events which are being proposed to play a vital role in coronal heating and acceleration of solar wind. The statistical study of various properties of the some of the small-scale features is the aim of the thesis. The effect of the magnetic field in generation and evolution of the features is also investigated. We have explored the possible association of polar network bright points, observed in chromopshere, with the photospheric magnetic fields using special HINODE campaigns devoted to observe polar regions of the Sun. The intensity of such bright points is found to be correlated well with the underneath photospheric magnetic field strength with a linear relation existing between them. These chromospheric Ca H bright points seems to be co-spatial with groups of G-band bright points in the photosphere. This indicates that these different features are directly coupled with each other. Though, present in different layers of the solar atmosphere they happen to be manifestations of the magnetic field concentrations present in the lower photosphere. We have performed a statistical study of network jets and estimated their dynamical properties using IRIS C II 1330 ̊A images. We have reported that the coronal hole jets appear to be faster and longer than those in the quiet sun. In spite of different magnetic

structures in the coronal hole and quiet sun in transition region, there appears to be no substantial difference for the increase in foot-point brightness of the jets. This has led us to infer that the generation mechanism of these network jets is likely the same in both regions. Whereas, the larger speeds and extents of the jets in coronal holes can be attributed to the presence of open magnetic field lines in coronal holes in comparison to the quiet-sun. Using joint spectral and imaging observations of IRIS, we have investigated the sources responsible for origin of the two-components of the double Gaussian profiles deduced from Si IV 1403 ̊A transition region emission line. A very close spatial association is observed between the raster images of the spectral properties with intensity in IRIS images and magnetic field in magnetograms. By the statistical analysis, we have showed that the double Gaussian fitting model is preferred over the single Gaussian model in bright network regions and neighbouring locations. On comparison with IRIS images, these locations can be seen to be associated with the complex jet structures and thus are claimed as signatures of transition region transients. We have studied the dynamics of high-frequency oscillations of an active region moss as recorded by simultaneous imaging and spectral data of IRIS. Power maps generated from IRIS images in Si IV 1400 ̊A passband and sit-and-stare spectroscopic observations of Si IV 1403 ̊A spectral line reveal the presence of high-frequency oscillations with periodsof 1–2 minutes. The presence of such short periods is further confirmed by intrinsic mode functions (IMFs) as obtained by empirical mode decomposition (EMD) technique. We find evidence of presence MHD waves as well as repetitive reconnection events that can be held responsible for the observed short-period variability in the bright moss regions.