Professor Cynthia Cattell

A key longstanding problem in space and astrophysical plasmas is determining the mechanism by which electrons and ions can be accelerated to relativistic energies, and the heating of the solar corona and solar wind. Theoretical studies of wave-particle interactions often employ quasi-linear theory on the assumption that wave amplitudes are small. This group's observational studies indicate that this assumption may not be valid in the solar wind, and previous simulations on MSI resources have shown that the interaction of electrons with large amplitude waves may lead to nonlinear coherent effects that can result in scattering and energization of electrons on subsecond time-scales.

In the last few years, the researchers have completely re-written the code to be more efficient and more accurately model non-linear interactions. In 2021, they published the results of the particle tracing code modeling both single waves and wave packets, based on observations close to the Sun at .3 au and near the Earth at 1 au, with more recent results published in 2022.

The researched added a new project in 2021 that uses the particle tracing code, modeling whistler interactions with electrons in the auroral zone of Jupiter. Sub-projects in this area include research into:

• Intense whistler waves seen in the NASA Juno satellite data, and associated energization of electrons to MeV energies
• More broadband waves seen in the Juno data, to see if the waves can explain the observed energization of electrons, using an adaptation of the code that uses a model magnetic field based on Jupiter's field; the code has also been run on a large set of initial particle energies and pitch angle, and researchers are modeling the un-accelerated population

In 2022, a new project began based on the PI’s discovery that the whistler-mode waves disappear close to the Sun, and some other mode must regulate the heat flux. Close to the Sun, we see a variety of electrostatic modes, which might be able to interact with the electrons that carry the heat flux. The group will implement models of the new range of wave modes into the code. The results of this work are extremely significant in a range of astrophysical settings including for the understanding and modeling of the evolution of flare-accelerated electrons, and the regulation of heat flux, including other stellar winds, the interstellar medium, accretion disks, and the intra-galaxy cluster medium. 

The results of these simulations are critical to recently launched NASA Parker Solar Probe mission for which the U of M supplied the high time resolution waveform instrument and on which the PI is a co-investigator. The work on energization and scattering of electrons in the solar wind is important for theoretical understanding of these new solar wind observations.

The group will also use the new code and diagnostics to revisit diagnostics and visualization tools to revisit the original problem of the interaction of relativistic electrons and whistlers in the earth's radiation belts. Another project will automatically identify small solar flares in extreme ultraviolet images of the sun once the approach has been tested on a small dataset.