Investigating small scale particle acceleration events at the sun and its implications for coronal heating

Abstract

Particle acceleration in the Solar atmosphere is an ill-understood problem. This lack of understanding creates impediments in our knowledge about how energy is generated and transported in the solar atmosphere. Studies show that the majority of energy released in transient events are used in the acceleration of particles. A proper understanding of this process is then vital to understand how the solar corona is heated. Since most of the coronal heating is carried out by small transient events, in this thesis, we lay particular emphasis on understanding the particle acceleration dynamics in them. With the current sensitivity levels of space instruments, it is challenging to directly observe and quantify extremely small energy release events on the scale of nanoflares. Hence we turned our attention on very small flaring events which had both HXR and a SEP correspondence, but which did not have any big flares or CMEs associated with it. The simultaneous occurrence of HXR radiation and SEP particles makes sure that the selected events for the study had significant amounts of particle acceleration. From our study, we found that these small events accelerations almost similar amounts of electrons at the HXR, producing electrons and SEP electrons. This number for bigger events are much smaller, i.e. the number of SEP electrons that escape towards Earth as SEP is only a small fraction of the HXR producing electron. We observe that the ratio of the number of escaping electrons to that of HXR electrons ranges from 6% to over 100%. By comparison, this ratio is only ∼ 0.2% for large flares. The total energy in the HXR producing electrons is ∼10​ 24 –10​ 26 while the same for electron population that escaped into the interplanetary medium is ∼10​ 23​ to 10​ 25​ erg (James et al., 2017).

In another study, we used high temporal resolution type I noise storm data from the NRH to investigate the energetics. Type I noise storm bursts represents some of the smallest examples of particle acceleration. We employed a model-independent particle acceleration framework developed by Subramanian et al to calculate the energy in a representative burst and the amount of non-thermal energy contained in them. We find that non-thermal electrons are only a small fraction of the thermal population. The power input by a representative burst was in the range of 10​ 20 to 10​ 23 ergs/s. Thus these bursts are likely contributors to active region coronal heating (James and Subramanian, 2018).

Lastly, we tried to decouple the effects of source acceleration effects and transport effects on the SEP particles. SEP particles are the only means by which we can directly sample particles accelerated in the Sun. Hence to properly understand the actual source information carried by these particles, it is essential first to know how transport through interplanetary space changes their characteristics. We observe that SEPs can be divided into symmetric and non-symmetric based on their time-intensity profile shape. We use this distinction to find any possible correlation between other physical quantities such as spectral indices, flare intensities, low energy electrons etc.

Understanding transport effects is also essential to understand why certain SEP events show a delay in arrival eventhough simultaneous injection happened at the flare site. We also show initial results from a focussed transport model based Monte-Carlo simulations. The power law of the magnetic field fluctuations in the interplanetary magnetic field line seems to play a significant role in deciding the symmetricity of the SEP events.