Night time absorption spike events (NASE) are common signatures of magnetospheric substorms. Their occurrence in the ionosphere can be easily detected by riometers located at ground based stations. This unique feature is used to achieve a comprehensive study based on 500 NASE occurred during the period 1994-2003 in the IRIS (imaging riometer for ionospheric studies) field of view at Kilpisjarvi, Finland centred at 69.05° N, 20.79° E (L-shell 6.1). NASE generally had similar behaviour which has been mentioned for substorms in the literature. Occurrence of NASE dominates around magnetic local midnight (MLM) with majority of events in the pre-midnight sector. NASE seem to occur more often during high geomagnetic activity according to Kp index variation. Their occurrence during geomagnetic equinoxes is slightly more than that of s~lstices with the peak in the autumn and minimum in the summer time. They also tend to be solar cycle dependent as their appear3?ces during solar minimum dominate in agreement with occurrence of substorms. Our study confIrms most results of previous NASE studies. . . North/westward motion of spike events was dominant with speed in a range of few hundreds to few kms per second. Pi2 pulsations and auroral breakup (found from PIXIE, polar ionospheric X-ray imaging experiment images) are a common feature of NASE. The signifIcant frequency modulation of NASE is in the order of 50 mHz -200 mHz based on wavelet analysis. Apart from these new findings are also discussed in this thesis: Considering the temporal structure and variation of IT.- index, NASE are categorized into 4 classes. Classifications of spike events allow the identification of phenomena such as pseudobreakups from substorms. Another important finding of this study is the location of mapped points of NASE which is in the range of near Earth magnetotail rather than midtail region in favour of current disruption (CD) substorm model as opposed to near Earth neutral line (NENL) model. This yields using geomagnetic field model T-96 and NASE of IRIS and SGO (SodankyHi geophysical observatory) riometers where together covers auroral zone latitudes between 62.42° Nand 77.00 N (L- shell between 3.8-13.6).
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:485176 |
| Date | January 2007 |
| Creators | Aminaei, Amin |
| Publisher | Lancaster University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://eprints.lancs.ac.uk/6764/ |
Page generated in 0.0021 seconds