Why Are There so Few Reports of High-Energy Electron Drift Resonances? Role of Radial Phase Space Density Gradients

Models of monochromatic Pc5 (2-7 mHz) ultralow frequency (ULF) wave interactions with high energy (greater than similar to 1 MeV) electrons predict drift resonant interactions that can cause rapid radial transport and acceleration. There are few reports of electron drift resonance at energies greater than similar to 1 MeV, in contrast to lower energies; moreover, all previous reports occur in the aftermath of interplanetary shocks. These two facts are difficult to reconcile with theory and numerical simulations predicting that greater than similar to 1 MeV drift resonances should occur more often and in a wider variety of driving conditions. In this study, we show that a combination of observational sampling biases and nominal radial phase space density gradients is one explanation for this discrepancy between theory and observations. In particular, we examine electron dynamics in two case studies with very similar satellite coverage, solar wind conditions, and Pc5 wave properties, yet with different radial phase space density profiles. Using global wave and particle observations, we show that the events have vastly different particle responses despite having similar wave properties. Placing these results in context with past studies, we further show that nominal radial PSD gradients near geostationary orbit can mask the expected drift resonance particle response and explain (1) the small number of past greater than similar to 1 MeV drift resonance reports and (2) the restriction of these reports to interplanetary shock events. We argue that future observational studies characterizing radial transport via drift resonance should examine global particle dynamics, including observations of the radial phase space density profile. Plain Language Summary The Earth's radiation belts are dynamic, with variations in radiation intensity depending on many factors. During certain conditions radiation in this region can damage satellite electronics. A variety of plasma waves can interact with this radiation and affect its overall intensity; thus, a characterization of these waves and their related interactions is needed to predict radiation belt dynamics. In this study, we examine a resonant interaction between large-scale plasma waves-wavelengths of 10,000 km or more-and high-energy electrons near geostationary orbit (GEO). We find that one of the predicted observational signatures for the resonant interaction can be masked by background radiation conditions if observations are only collected near GEO. We also find that past reports of these resonances favor a particular set of solar wind driving conditions because of this masking effect and that these resonances may in fact occur in a broader range of conditions and spatial regions. We propose that future studies seeking to characterize how these resonances affect radiation belt dynamics should use observations from a broader spatial region whenever possible and/or incorporate background radiation conditions into their analysis.