Characteristics of night-time absorption spike events

Abstract. Sudden increases in cosmic radio noise absorp-tion, known as spike events, have been identified as sig-natures 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 struc-ture and the background magnetic activity as indicated by a local westward electrojet (IL index) derived from the IM-AGE (International Monitor for Auroral Geomagnetic Ef-fects) magnetometer network as well as Pi2 magnetic pul-sations from SAMNET (The UK Sub-Auroral Magnetome-ter Network). Classifying the types of absorption spikes al-lows 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 de-cidedly 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 tempo-ral 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 au