Stochastic particle acceleration in solar flares

ROSES ID: NNH06ZDA001N      Selection Year: 2007      

Program Element: Focused Science Topic

Principal Investigator: Benjamin Chandran

Affiliation(s): University of New Hampshire

Project Member(s):
Ng, Chung-Sang Co-I University of Alaska Fairbanks
Mason, Glenn M Collaborator JHU / APL

Summary:

The acceleration of particles to energies exceeding 50 MeV in solar

flares is a long-standing problem and one of the Focused Science

Topics of NASA's 2006 Living-With-a-Star (LWS) program. This proposal

addresses one of the leading theories for particle energization in

flares, namely stochastic particle acceleration. In this theory,

turbulent waves are excited in the flaring region by magnetic

reconnection and the ideal fluid motions that arise as the magnetic

field relaxes to a new configuration. The energy in these waves

undergoes a turbulent cascade to small scales, resulting in

high-frequency waves that accelerate particles to high energies

through resonant wave-particle interactions. If the power spectra of

these waves have the right properties, stochastic acceleration can

explain many of the observed features of flares, including the

acceleration time scales, the energy spectra of different particle

species, and the highly enhanced abundance of helium-3 relative to

helium-4 in the accelerated particle population. However, there is still

no predictive theory for the turbulent power spectra of waves on the

Alfven/ion-cyclotron and fast-magnetosonic/whistler branches of the

dispersion relation in flaring regions. The determination of these

power spectra is the most important unsolved problem for the

stochastic-acceleration model, and is the focus of this

proposal. Building on previous work by the PI and co-I, we will carry

out weak turbulence calculations and direct numerical simulations to

determine the power spectra of the above-mentioned waves for

solar-flare conditions. A notable feature of this turbulence research

is its extensive use of analytic theory in addition to numerical

simulations, which will lead to highly detailed quantitative results,

a theoretical framework that can be applied by other investigators,

and a clear physical picture of the energy cascade mechanisms. We

will use these power spectra to determine the acceleration times and

energy spectra of different particle species as a function of the

overall turbulence amplitude (i.e., the rms velocity at the largest scales)

using quasilinear theory to treat the particle-acceleration

process. We will then compare our results to x-ray, gamma-ray, and

in-situ particle measurements. By determining the power spectra of

the different wave modes in solar-flare conditions, our work will

provide key missing information that is essential for assessing the

importance of stochastic acceleration in solar flares, and for

determining the contribution of flare-accelerated particles

to large solar-energetic-particle (SEP) events observed at

Earth.

Publications:

Performance Year	Reference	Investigation Type	Actions
1	Chandran, B., "Strong Anisotropic MHD Turbulence with Cross ...	not set
1	Pongkitiwanichakul, Peera; Chandran, Benjamin D. G.; K...	not set

Presentations:

Performance Year	Reference	Actions
1	Pongkitiwanichakul, P.; Chandran, B. D.; DeVore, C. R...
1	Pongkitiwanichakul, P.; Chandran, B. D.; (2012), Stochas...

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