Structural Characterization and Temporal Evolution of the Heavy Ion Afternoon Bulge

We present a survey of the plasma composition of the 5-20 keV heavy ion "afternoon bulge" population measured by the Van Allen Probes HOPE mass spectrometers. Previous studies of this population show a 10 keV Oxygen- and Helium-rich plasma in the afternoon and evening sectors from L-shell 2-4 during quiet geomagnetic times. These studies postulate that the presence of this afternoon bulge results from the magnetotail plasma access to low L-shell during enhanced geomagnetic activity, which then becomes trapped on closed "banana" orbits when geomagnetic activity decreases.

We extend these studies by exploring the coupling between energy, L-shell, MLT location, and the intensity for both He+ and O+ bulge populations. Specifically, we study the relationship between the afternoon bulge and the time history of Kp, including bulge formation relative to a drop in Kp, to the maximum Kp intensity, to the duration of a Kp increase, and to the rate at which Kp decreases following an event. The bulge has been observed to endure for weeks­ during extended periods of low Kp. We investigate the behavior of the He+ and O+ bulge populations during such periods, including intensity variations with time for each species and decay of the bulge during long-lasting quiet intervals.