On phase space density radial gradients of Earth's outer‐belt electrons prior to sudden solar wind pressure enhancements: Results from distinctive events and a superposed epoch analysis

Here, we present the results of a study of phase space density radial gradients for outer‐belt electrons at and beyond geosynchronous orbit prior to 86 sudden solar wind pressure enhancements from 1993 through 2007. All of the events are classified and analyzed based on the results for equatorial electrons with first adiabatic invariants of 50, 200, 750, and 2000 MeV/G. Examples of three distinctive events are compared, and the results from a superposed epoch analysis are presented. We find that the radial gradients are dependent on the first adiabatic invariant (i.e., energy), and that for the majority of cases, the gradient is negative for electrons with energies above a couple of hundred keV, while it is either positive or relatively flat for electrons with energies lower than this, which is evidence of two distinct populations. In the cases where a positive gradient is observed for 2000 MeV/G electrons, the solar wind and geomagnetic conditions are very quiet for at least two days prior to the event, but for the events when the gradient for the same electrons is negative, there is a consistent evidence of enhanced substorm activity and/or convection in the days leading up to the events. Overall, this study puts previous observations of phase space density (PSD) gradients into a broader context of solar wind and geomagnetic conditions, while encompassing a broad range of energies, from the source population of tens to hundreds of keV electrons to relativistic electrons with energies exceeding 1 MeV. We discuss how 41 of the 86 events are consistent with and can be explained by local heating by wave‐particle interactions, and we provide evidence of the solar wind and geomagnetic conditions that are important to different types of sources of outer‐belt electron PSD.