Abstract
Abstract. 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.
Highlights
Absorption spike events have been studied since 1965 when they were first described as fast onsets or “F events” (Parthasarathy and Berkey, 1965)
We used the maximum absorption observed over the whole imaging riometer array in our analysis in the same manner conducted by Hargreaves et al (2001); following event identification, narrow-beam data were used to identify the direction of motion of spikes across the Ionospheric Studies (IRIS) field of view
Comprehensive study of absorption spike events based on large amount of data (450 events during years 1994–2003) and using multiple instruments has been performed
Summary
Absorption spike events have been studied since 1965 when they were first described as fast onsets or “F events” (Parthasarathy and Berkey, 1965). The spike event is identified as a sudden high intensity increase in absorption followed by a rapid decrease within few minutes They were thought to be east-west extended strips of precipitation moving through riometer beams (Hargreaves et al, 1979) with north-south extents of the order of tens of km (Nielsen and Axford, 1977). Later studies have shown that spikes are elliptical regions of precipitation-induced ionisation, with typical dimension in the order of 170–190 km by 70–80 km Such an impulsive precipitation of electrons has been observed during spike events using balloon measurements of X-rays. A variety of spike events occur in the dayside ionosphere These have been related to magnetic impulse events (MIEs) (Terkildsen et al, 2001) and are generally of much weaker intensity, usually of the order of 0.2–0.3 dB with durations of 1–2 min (Stauning and Rosenberg, 1996).
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