Episodic occurrence of field-aligned energetic ions on the dayside

Abstract

<p>The tens of keV ion populations observed in the ring current region at L~ 3- 7, generally have pancake-shaped pitch angle distributions (PADs), that is, peaked at 90 degrees. These pancake PADs are formed due to a combination of betatron and Fermi acceleration when they are transported from the tail plasma sheet, where the major ring current plasma originates. However, in this study, by using the Van Allen Probe observations from 2012 to 2018 on the dayside, unexpectedly we have found that about 5% of the time, protons with energies of ~30 to 50 keV show two distinct populations according to their PADs, having an additional population of field-aligned ions overlapping with the original pancake population. The newly appearing field-aligned populations have higher occurrence rates at ~12-16 MLT during geomagnetically active times. In particular, we have studied eight such events in detail and traced back these ions to their source regions according to the energy-dependent dispersion signatures caused by the differences in drift velocities. We found that the source regions are located around 12 to 18 MLT which coincides with the high occurrence rate region of 12-16 MLT. Based on the ionospheric and LANL geosynchronous observations of these eight events, it is suggested that these energetic ions with field-aligned PADs most probably are accelerated in the post-noon sector in association with ionospheric disturbances that are triggered by tail injection. These results provide evidence of another important source of the ring current ions.</p>