Peaks in Phase Space Density: A Survey of the Van Allen Probes Era

Abstract:

While significant advancement has been made in understanding the acceleration of outer radiation belt electrons, the relative importance of local acceleration vs. radial transport processes remains an open question. Differentiating between these processes is often done through careful examination of phase space density profiles in terms of adiabatic coordinates. In particular, local acceleration processes produce growing peaks in phase space density. Many previous studies have shown clear observations of these features for individual events. However, it remains unclear how often and where these growing peaks are observed over a long time period. With the availability of several years of high quality observations from multiple spacecraft, we now have an opportunity to quantify phase space density profiles not only for multiple events, but also across a wide range of energies.

Using more than four years of high quality phase space density data from the Van Allen Probes and THEMIS we determine the statistical properties of the observed peaks in phase space density. First, we determine how often growing peaks are observed and where peaks are located in terms of the adiabatic invariants mu, K and L*. Second, for events where peaks are observed, we determine how these locations relate to solar wind conditions, plasmapause location and observed wave activity. Using these datasets, we examine what conditions can lead to the formation of a peak and quantify for each event whether the observations are characteristic of local acceleration, radial transport, or some combination.