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.
The research concerned the importance of waves in accelerating electrons to cause strong aurora. This type of aurora is also called "broadband acceleration" because it accelerates electrons over a wide range of energies. The results from this research grant were both of scientific and practical value. The practical aspect concerned a better ability to forecast aurora. The ability to better forecast aurora from solar wind conditions was folded into the "OVATION Prime" model which is currently run by the NOAA Space Weather Prediction Center: www.ngdc.noaa.gov/stp/ovation_prime/ (Parts of this model, which includes also diffuse and other types of aurora, were funded by two other NSF grants). The attached image is from the NOAA forecast using OVATION Prime at the moment this was written. The work also had significant value for basic research. In an article published in the leading appropriate journal (Journal of Geophysical Research- Space Physics, Newell et al., 2009), we quantified the contribution of wave aurora to total power into the Earth's upper atmosphere. It turns out that only 6% to 10% of all aurora comes from wave acceleration, depending on conditions (with the higher value for disturbed conditions). However another research paper supported by this grant (same journal, but 2010) showed that wave aurora has a strong connection with the explosive increase in auroral activity called "substorms." In fact, we showed that in the few minutes that follow substorm onset, wave aurora can be comparable to other kinds of aurora. The second attached figure shows a "superposed epoch study" which combines about four thousand auroral substorms, aligned so that they have the substorm onset time at time "0". It can be seen in this figure that wave aurora rises very sharply at onset. In fact, the funded research shows that wave aurora rises at double the rate of other kinds of aurora. This close connection between wave aurora and onset is an important finding in understanding the explosive increase in auroral intensity at substorm onset.
