Unexpected field-aligned PADs of energetic ions in the dayside ring current region

In general, the tens of keV ions predominantly observed in the ring current region at L~ 3- 7, have pancake pitch angle distributions (PADs). These pancake PADs are formed due to a combination of betatron and Fermi acceleration when they 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, we have unexpectedly found that about 5% of the time, protons with energies of ~30 to 50 keV show two distinct populations according to their PADs: the higher fluxes of field-aligned populations overlap with the original pancake populations. The field-aligned PADs have higher occurrence rates at ~12-16 MLT during geomagnetic active times and are usually accompanied by energetic oxygen field-aligned PADs. In particular, we have selected four such events and traced back the source regions according to the energy dispersion signatures of the field-aligned PADs caused by the difference in drift velocities of the different energy protons. We found that the source regions are around 12 to 18 MLT with magnetic latitudes of 64-69 degrees. Based on the ionospheric and LANL geosynchronous observations of these four events, it appears that these energetic ions with field-aligned PADs probably come directly from ionosphere when the total electron content is enhanced and the aurora oval expands to the source region. These results provide evidence of another important source of the ring current ions.