Precipitation of auroral protons in detached arcs

Abstract

Recent global‐scale observations by the IMAGE‐FUV instrument demonstrate the existence of regions of particle precipitation at sub‐auroral latitudes on the dayside. The signature of this precipitation is seen infrequently, but when so, it is clear in all 3 channels of the FUV instrument. A conjugate hemisphere conjunction with the FAST satellite demonstrates the presence of precipitating protons and the notable absence of precipitating electrons in these arcs. With this knowledge, one can determine the mean energy and energy flux of the precipitating protons by intercomparison of the response in the three FUV channels. Assuming that the protons have a kappa energy distribution, the mean energy is found to be ∼20 keV, with a peak in total energy flux of ∼1 mW/m 2 /sec, consistent with fits to the FAST ion measurements. These phenomena are observed mainly during times of high solar wind dynamic pressure and variable interplanetary magnetic field, and are associated with earlier nightside enhancements in the brightness and latitudinal extent of the proton aurora.