Observational study of decayless waves across different scales in the solar corona

Abstract

The long-standing problem of coronal heating focuses on probing sources that can continuously supply energy to sustain million-degree temperatures. Magnetohydrodynamic (MHD) waves play a crucial role because their dissipation in the solar corona can supply the necessary energy to compensate for radiative losses. Moreover, MHD waves offer an effective tool for diagnosing local physical conditions through coronal seismology. Elucidating the characteristics of MHD waves across various coronal regions and at different spatial scales is crucial in the context of coronal heating and its driving mechanisms.

This thesis examines the properties of decayless kink waves, a type of MHD wave mode in coronal loops with no significant decay in the wave amplitude over multiple cycles. Numerical simulations and modeling suggest that the dissipation of decayless waves can be compensated by continuous energy supplied by footpoint drivers, highlighting the importance of these waves in coronal heating. Using simultaneous observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) and the Slit-Jaw Imager (SJI) onboard the Interface Region Imaging Spectrograph (IRIS), we explore the role of coronal rain in altering the properties of already oscillating two large active region coronal loops with loop lengths of the order of 100 Mm. The increase in amplitude observed during rain suggests that decayless oscillations can be sustained by both footpoint driver and coronal rain acting simultaneously. Furthermore, the detection of rain blob oscillations in emission provides a valuable constraint on applying coronal seismology using the coronal passbands of AIA. In the second part, the thesis explores the presence of decayless kink waves in short loops (< 50 Mm). Using observations with a pixel scale of 200 km from the Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter, we identified 42 decayless waves in short coronal loops rooted in the quiet Sun and coronal holes. Unlike previously studied large active region loops, we discover no correlation between loop length and the periods of decayless waves in these shorter loops. The wave modes of decayless waves in short loops remain uncertain, with the standing waves as one possibility. The energy flux of these waves is insufficient to counterbalance the energy losses in the quiet Sun and coronal holes.

Finally, using the EUI data of two active regions, we analyze the 105 decayless oscillations in active region short loops. Similar to findings in loops within the quiet Sun and coronal holes, no significant correlation is observed between loop length and period in active region small-scale loops, and the wave modes can have multiple possibilities. The comparison of periods in short loops rooted in different coronal regions indicates that the excitation mechanism of short-period decayless oscillations in active regions may be different compared to those in the quiet Sun and coronal holes. The P-mode is suspected to be one of the possible drivers of these oscillations. Magnetic field estimates derived using MHD seismology based on standing kink modes reveal lower values for multiple oscillations compared to earlier estimates for long loops in active regions. In summary, this thesis explores the cross-scale characteristics of decayless oscillations across different coronal regions, examining their energy content, driving mechanisms, and the feasibility of using coronal seismology.