The spectral hardening associated with the westward travelling surge

Photometer recordings of auroral emissions from N⁺₂ and OI have demonstrated that the ratios between the emissions show a characteristic plateau just prior to the passage of a westward travelling surge. The plateau may be interpreted as a temporary (approx. 1 min) cessation in the spectral hardening of the precipitating particles associated with the surge. The characteristic energy of the plateau is about one keV above the level prior to the beginning of the surge. The results have been interpreted in terms of an instability that is switched on during the time of the plateau and then turned off when the electron density is increased above a certain limit.