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Particle Acceleration in Solar Flares and Associated CME Shocks  

Vahé Petrosian   Submitted: 2016-05-17 17:17

Observations relating the characteristics of electrons seen near Earth (SEPs) and those producing flare radiation show that in certain (prompt) events the origin of both population appears to be the flare site, which show strong correlation between the number and spectral index of SEP and hard X-ray radiating electrons, but in others(delayed), which are associated with fast CMEs, this relation is complex and SEPs tend to be harder. Prompt event spectral relation disagrees with that expected in thick or thin target models. We show that using a a more accurate treatment of the transport of the accelerated electrons to the footpoints and to the Earth can account for this discrepancy. Our results are consistent with those found by Chen & Petrosian (2013) for two flares using non-parametric inversion methods, according to which we have weak diffusion conditions, and trapping mediated by magnetic field convergence. The weaker correlations and harder spectra of delayed events can come about by re-acceleration of electrons in the CME shock environment. We describe under what conditions such a hardening can be achieved. Using this (acceleration at the flare and re-acceleration in the CME) scenario we show that we can describe the similar dichotomy that exists between the so called impulsive, highly enriched (3He and heavy ions) and softer SEP events, and stronger more gradual SEP events with near normal ionic abundances and harder spectra. These methods can be used to distinguish the acceleration mechanisms and to constrain their characteristics.

Authors: Vahé Petrosian
Projects: Wind

Publication Status: ApJ accepted for publication
Last Modified: 2016-05-17 20:49
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Stochastic Acceleration by Turbulence  

Vahé Petrosian   Submitted: 2012-08-01 14:06

The subject of this paper is stochastic acceleration by plasma turbulence, a process akin to the original model proposed by Fermi. We review the relative merits of different acceleration models, in particular the so called first order Fermi acceleration by shocks and second order Fermi by stochastic processes, and point out that plasma waves or turbulence play an important role in all mechanisms of acceleration. Thus, stochastic acceleration by turbulence is active in most situations. We also show that it is the most efficient mechanism of acceleration of relatively cool non relativistic thermal background magnetized plasma particles. In addition, it can preferentially accelerate electrons relative to protons as is needed in many astrophysical radiating sources, where usually there are no indications of presence of shocks. We also point out that a hybrid acceleration mechanism consisting of initial acceleration by turbulence of background particles followed by a second stage acceleration by a shock has many attractive features. It is demonstrated that the above scenarios can account for many signatures of the accelerated electrons, protons and other ions, in particular 3He and 4He, seen directly as Solar Energetic Particles and through the radiation they produce in solar flares.

Authors: Vahé Petrosian
Projects: None

Publication Status: Space Science Reviews 2012, DOI: 10.1007/s11214-012-9900-6
Last Modified: 2012-08-02 12:56
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Particle Acceleration in Solar Flares and Associated CME Shocks
Stochastic Acceleration by Turbulence

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