Using Particle Tracing Codes to Understand Acceleration of Relativistic Electrons via Electrostatic and Electromagnetic Waves


Using Particle Tracing Codes to Understand Acceleration of Relativistic Electrons via Electrostatic and Electromagnetic Waves

A key longstanding problem in space and astrophysical plasmas is determining the mechanism by which electrons can be accelerated to relativistic energies. This group's work has focused on the Van Allen radiation belts, where trapped energetic electrons may damage spacecraft systems. Their studies have shown that the usual theoretical treatment of electron energization and scattering is inadequate for a complete understanding of radiation belt dynamics. Most current theoretical studies of whistler acceleration take a quasi-linear approach and assume whistler amplitudes on the order of 1 mV/m. These researchers discovered narrow-band whistler-mode waves in the outer Van Allen belt with electric field amplitudes an order of magnitude larger than previously observed and have very recently discovered similar large wave associated with intense ground transmitters. Their MSI particle tracing results have shown that these large waves (>100 mV/m) result in nonlinear coherent effects that can produce energization to several MeV on subsecond time-scales. They have also shown large angle scattering consistent with observations by the low altitude SAMPEX satellite of relativistic electron microbursts, bursts of energetic particles lost from the radiation belts to the atmosphere.

The the twin Radiation Belt Storm Probe (RBSP) satellites, for which the University of Minnesota is the electric field instrument PI, were launched in August 2012 and continue to provide unprecedented measurements of radiation belt wave activity and the associated particle energization and loss. RBSP measurements of high time resolution electric and magnetic field waveforms are being used with filter bank data (continuous records of peak and average wave amplitudes) to develop more detailed spatial maps and occurrence statistics for large amplitude waves in the radiation belts. These data will be used with this group's simulation results to model rapid changes in trapped radiation belt particle populations in response to magnetic storms. The RBSP mission also includes a coordinated Balloon Array for RBSP Relativistic Electron Losses (BARREL) to allow multi-point measurements of relativistic electron scattering and loss to the atmosphere. The first 20-balloon campaign was completed in early 2013 and provides detailed measurements of energetic particle precipitation to the atmosphere in conjunction with the twin RBSP satellites. A second campaign in 2014 expanded the dataset and showed that a range of wave modes may dramatically alter radiation belt particle populations throughout the radiation belts. Previous supercomputing allocations were used to run simulations across a wide range of input particle and wave amplitude parameters. The 2014 allocation was used to run the simulations across a more fine-grained spectrum of wave propagation directions. In 2015, the group will continue this work, exploring the effect of a more realistic wave packet structure on the interaction. 

The reseachers have also begun work on a 3D test particle code that will be used to model wave-particle interactions in the magnetosphere, solar wind, and magnetotail. They are continuing to develop their 3D test particle code. This code will be used to examine acceleration of ions during magnetic "superstorms" for comparison to satellite observations of long-duration energy-banded ions. The researchers have worked closely with MSI staff on the development of their simulations, and will continue to do so as they fill in their simulation models and move towards more detailed analysis and visualization of their simulation output

A bibliography of this group’s publications is attached.

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