Temporal fine structure of nighttime spike events in auroral radio absorption, studied by a wavelet method

The auroral absorption spike event, occurring generally at the onset of a s ubstorm. is distinguished by its high intensity and short duration. This pa per reports the presence of a Rile structure within the spike event. Analys is of selected examples using the Morlet wavelet shows that within the 1 - 2 min duration. of the spike are significant modulations with periodicities in the bands 15 - 60 s (67 - 16 mHz), and 5 - 10 s (200 - 100 mHz), the fo rmer being the stronger. The slower fluctuations can amount to more than 10 % of the absorption. and they were observed in every example (seven out of nine) in which the spike was moving poleward. They were absent in the other two cases, when the spike moved equatorward. In the examples studied, the 15 - 60 s absorption pulsations were accompanied by magnetic micropulsation s of impulsive type (Pi) having a periodity that was similar or harmonicall y related. The connection is only close while the spike event is moving. Co nsideration of the details suggests that both the magnetic and the absorpti on pulsations are related to the acceleration process at substorm onset, th e flux of energetic particles into the auroral zone producing the radio abs orption being modulated with, though not by, the geomagnetic field variatio ns. The 5 - 10 s pulsations, which are considerably weaker. appeared in bot h the absorption and the magnetic records, but in this case with no obvious connection between them.