| Sudden increases in cosmic radio noise absorption, known as spike events,
have been identified as signatures of substorms in the previous studies.
Using data from the IRIS (Imaging Riometer for Ionospheric Studies) at
Kilpisjärvi, Finland (L~6) more than 450 night-time spike events
between 1994 and 2003 have been identified. Spike events fall into four
distinct categories based on their structure and the background magnetic
activity as indicated by a local westward electrojet (IL index) derived from
the IMAGE (International Monitor for Auroral Geomagnetic Effects)
magnetometer network as well as Pi2 magnetic pulsations from SAMNET (The UK
Sub-Auroral Magnetometer Network). Classifying the types of absorption
spikes allows for identification of phenomena such as multiple onsets and
pseudobreakups from riometer data. In addition we have studied the
statistical variation of absorption spikes and their sub-classes. This
includes examining the magnetic local time (MLT) distribution and the
seasonal and solar-cycle variation in spike occurrence.
Those that seem to represent substorm onsets show a decidedly different MLT
variation to those isolated spikes that represent pseudobreakups. The
occurrence of spikes during different levels of geomagnetic activity is
examined using the Kp index.
Wavelet analysis has been used for studying the temporal structure of
spikes; also the direction of motion of spike events and localisation of
spikes are presented for all events and each sub-class and results are
compared with previous studies.
Statistical studies are supported with X-ray images of aurora from PIXIE
(The Polar Ionospheric X-ray Imaging Experiment) when available. |