Low‐latitude aurorae and storm time current systems

An analysis is made of the vibrational development shown in N 2 + 1 N (first negative) emissions in low‐latitude aurorae during the last half century. The data imply collisional excitation by energetic heavy particles (oxygen, helium, or hydrogen ions or neutrals) of velocity of the order of 10 7 cm s −1 , and there may be additional pumping in some cases due to solar photons and low‐energy electrons. When low‐energy electrons are present as large fluxes, they produce bright red (O I 6300 Å) emission, with weaker emission from N I 5200 Å and other features of excitation potential only a few electron volts. From about 40° dip latitude to the auroral zone the decrease of N 2 + 1 N vibrational development with increasing latitude is consistent with satellite observations of the increase with increasing latitude of the mean energy of precipitating O + fluxes and the increasing fraction of H + in the ion precipitation. From the equator to about 40° dip latitude the intensity and the latitudinal gradients of the emissions are consistent with energetic neutral precipitation as the primary source. The intensity variations of low‐latitude displays show large‐amplitude changes on a time scale of 0.1 hour, which are closely related to the magnetic signatures of the storm time current system. The brightest emissions occur when | Dst | is large and specifically when positive H excursions take place, with first positive then negative D excursions, as recorded on magnetograms from nearby observatories. The positive Δ H at low latitudes is accompanied by the negative Δ H of large substorms as recorded at higher latitude observatories. The 0.1‐hour time scale of these fluctuations implies that the ring current is incompletely shielded, allowing ionospheric currents and the injection of the ring current to unusually low latitudes.