The dynamics of the proton aurora in auroral breakup events

Proton auroral dynamics during auroral breakup events are examined compared to electron auroral dynamics using monochromatic imaging and tilting‐filter photometer data obtained at Syowa station, Antarctica (corrected geomagnetic latitude: −66.23°, L = 6.15). During the period from May to September 1992, 35 typical breakup events were observed, in which 23 events showed a signature of poleward expansion of the auroral bulge, while 12 events showed a signature of the westward traveling surge. Careful analysis revealed that when an electron aurora arc is enhanced and moves poleward after substorm onset, the region of proton aurora also expands poleward, forming a sharp boundary, and that this poleward boundary always coincides with the leading edge of the electron auroral bulge. At the poleward boundary the Doppler shift of proton aurora is quite large (∼0.8 nm), while the maximum intensity of proton aurora is observed somewhat equatorward of the leading edge. These facts imply that precipitating protons with higher energies and highly field aligned pitch angles are produced at the poleward boundary through the plasma injection process in the magnetosphere. In 6 out of 35 breakup events, multiple N–S aligned electron aurora arcs (N–S auroras) appeared. An important finding is that the appearance of prominent N–S auroras in the auroral bulge results in the rapid poleward contraction of the proton aurora belt when the activity of electron aurora becomes low. It is therefore inferred that in the case accompanied by the N–S auroras the complete dipolarization of the magnetic field occurs after the recovery phase.