On the equatorward motion and fading of proton aurora during substorm growth phase

Using 50 high‐quality events from the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) Gillam Meridian Scanning Photometer over 10 a (1989–1998), we show that proton aurora in the substorm growth phase exhibits a systematic tendency to fade. The average pattern of fading consists of a period of relatively stable proton aurora brightness and then a period of 15–20 min before onset during which the brightness decreases by an average of 15%. We interpret the observed proton aurora brightness variation in terms of the magnetic field stretching in the near‐Earth magnetosphere; in particular, the fading is interpreted as a result of the central plasma sheet magnetic field lines having stretched to such a degree that the loss cone closing effect dominates precipitation due to nonadiabatic proton motions.