Multi-wavelength Investigations of Solar Eruptive Phenomena

Solar eruptive phenomena correspond to various kind of transient activities occurring in the solar atmosphere in the form of flares, prominence eruptions and coronal mass ejections. They mostly originate from solar active regions which consist of complex magnetic structures extending from the deeper sub-photospheric layers, crossing through the photosphere to the coronal heights. An eruptive flare typically spreads across all the atmospheric layers of the Sun and involves substantial mass motions and particle acceleration. Multi-wavelength observations are thus crucial to probe the underlying physical processes occurring at different layers and regions at and above the photosphere. In this thesis, I have studied some key aspects of solar eruptive phenomena such as solar flare, prominence eruption, coronal implosion, failed eruption, and sigmoid-to-arcade evolution. These investigations have employed contemporary multi-wavelength solar observations with superior resolution. The imaging and spectroscopic capabilities of Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have been extensively utilized to investigate the thermal and nonthermal energy release processes associated with different stages of the eruptive phenomena. Complementary to RHESSI X-ray measurements, we have combined solar observations at Extreme Ultraviolet (EUV), Ultraviolet (UV), Microwave (MW), optical, and radio wavelengths to investigate the complex physical processes occurring at different atmospheric layers of the Sun during the eruptive events. We study two spectacular prominence eruptions and associated flare activities which occurred in active regions NOAA 10656 on 2004 August 18 (event I) and NOAA 11548 on 2012 August 18 (event II) with the motivation to identify the prominence destabilization through the observations of pre-eruption phase and pre-flare activity. The violent eruption of the prominences is accompanied by major flares. We also explore the signatures of magnetic reconnection in the corona and discuss the thermal and non-thermal effects driven by prominence eruption during their eruptive phase. During event I, three localized episodes of energy release were observed in the vicinity of the filament before the eruption that produced intense heating along with non-thermal emission. The prominence eruption was accompanied with an X1.8 flare during which multiple HXR bursts were observed up to 100–300 keV energies. We have noted striking HXR coronal source at 50–100 keV energy band that was formed at the time of detachment of the prominence from the source region. From the location, timing, strength, and spectrum of HXR emission, we conclude that the prominence eruption was driven by distinct events of magnetic reconnection occurring in the current sheet below the erupting prominence. During event II, we observed multitude of coronal activities in the form of a blowout jet, rapid evolution of a flux rope above the prominence, and events of episodic energy release. Out of these activities, the flux rope exhibited the most dramatic evolution, characterized by splitting and rotation along with localized brightenings during its outward expansion. The prominence underwent catastrophic loss of equilibrium with the onset of an M1.8 flare, suggesting large-scale energy release by magnetic reconnection. During the impulsive phase of the flare, a plasmoid eruption was observed below the apex f erupting prominence at multiple EUV channels. The temporal, spatial and kinematic correlations between erupting prominence and plasmoid imply that the magnetic reconnection supported the fast ejection of prominence in the lower corona.\ Flare research has been dominated by the study of eruptive flares because of their large-scale structure and long duration. On the other hand, it is rather challenging to investigate energy release processes in confined flares due to their rapid evolution in a compact region that impose severe observational constraints. Now with the availability of observations at unprecedented temporal, spatial, and spectral resolutions from RHESSI and SDO, we have made an effort to investigate the triggering mechanism and magnetic reconnection scenario in a confined M4.0 flare in AR NOAA 11302 on 2011 September 26. From this case study, we infer some important conclusions about the evolution of flare loops and thermal/nonthermal emissions within the confined environment of active region corona. This event was associated with a magnetic transient which was observed ∼1 minute prior to the flare onset at the early flare location within the inner core region. The spectral, temporal, and spatial properties of magnetic transients suggest that the sudden changes in the small-scale magnetic field have likely triggered the flare by destabilizing the highly sheared pre-flare magnetic configuration. We have studied a confined eruption (or failed eruption) of a flux rope which occurred in AR NOAA 10646 on 2004 July 14. After striking pre-flare phase, we observed a major M6.2 flare during which a large flux rope and associated prominence material underwent confined eruption. A major highlight of the preflare phase lies in the observation of large-scale contraction in overlying coronal loops for a span of ∼30 minutes during which the overlying loops underwent an altitude decrease of ∼20 Mm (40% of the initial height) during pre-flare phase. The impulsive phase of the flare is characterized by multiple non-thermal peaks during which very high plasma temperature (T ∼30 MK) and substantial nonthermal characteristics were observed. The time-evolution of thermal energy exhibits a good correspondence with the variations in cumulative non-thermal energy which suggest that the energy of accelerated particles efficiently converted to hot flare plasma implying validation of the Neupert effect in terms of flare energetics. Finally, we have presented a multi-wavelength investigation of a sigmoid-toarcade development in AR 11719 on 2013 April 11. The study aims to explore several crucial aspects involved during the process of solar eruption right from the formation stage of coronal EUV sigmoid to the post-eruption phase of the source region. The evolution of sigmoid was observed at 94 ˚A images which implies that the structure comprised of very high temperature plasma. The sigmoid eruption was accompanied with a large M6.5 two-ribbon flare which is characterized by a prolonged rise phase of ∼21 minutes. We have especially emphasized morphological and spatial evolution of flare sources during the prolonged rise phase and discussed their observational disparities with the standard flare model.by Upendra Kumar Singh KushwahaPh.D