Broadband Electron Acceleration: Outstanding Physics Questions

Johns Hopkins University, Baltimore MD

Investigators

Abstract

This project is motivated by the recent realization that, after nearly three decades of research supporting the idea that auroral particle precipitation is dominated by processes that accelerate electrons and these electrons peak at a single energy, this theory may require revision. Broadband electron acceleration, which is believed to be associated with dispersive Alfven waves, is now recognized to be of roughly comparable significance, at least at some times. To investigate this idea, the largest data set of auroral precipitation, obtained by the DMSP satellite, will be explored. Identification algorithms and pattern recognition technique will be developed and employed to conduct a broad statistical study of the relative occurrences of broadband versus monoenergetic acceleration. Diffuse electron and diffuse ion precipitation will also be included in the study. Several issues with important physical consequences involving the relationship between broadband and monoenergetic acceleration will be addressed. These include: (1) a determination of whether broadband acceleration is affected by UV insolation, as is true for monoenergetic aurora; (2) what are the acceleration energies and energy fluxes associated with broadband acceleration? The numbers have been estimated to be tens of keV and several ergs per square centimeter per second, respectively; (3) do polar cap arcs contain broadband acceleration? (4) does broadband acceleration occur at lower latitudes, or is it as rare as anecdotal reports suggest. These specific topics are chosen to provide the clearest physical insights into comparing the acceleration mechanisms. For example, broadband acceleration occurs mainly in the poleward portion of the oval. This may occur because of a relationship to highly stretched magnetic field lines, or because of processes particular to the poleward edge of the oval (such as the boundary layer). The answers provided by this study will aid theorists in constructing unified models of auroral acceleration and in deciding when which process is likely to be active. This work will also help establish the relative proportions of diffuse electron, diffuse ion, monoenergetic accelerated electron, and broadband accelerated electron precipitation. The broader impacts of the project include the continuing development and maintenance of an extensive auroral particle database based on the observations from the DMSP satellite. The database, at http://sd-www.jhuapl.edu/Aurora, is freely available via the web and includes a variety of data products, from raw data to spectrograms to various high-level data abstractions. The database was begun under an earlier grant and has been used extensively by the space science community, including graduate students and postdoctoral fellows, but also provides information and articles geared to the general public.