Statistics
Scientific paper
Feb 1974
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1974p%26ss...22..255b&link_type=abstract
Planetary and Space Science, Volume 22, Issue 2, p. 255-307.
Statistics
12
Scientific paper
Sixty auroral absorption substorms (30 in IQSY and 30 in IASY) have been analysed on the basis of riometer-recordings taken at some 40 stations distributed over auroral, subauroral and polar cap latitudes. Synoptic maps showing isoabsorption curves have been produced every 15 min (sometimes every 5 min) of the 60 substorms; 705 maps altogether. Some of the results of the analysis are as follows. Initiation of a substorm most frequently occurs near midnight but may occur anywhere between early evening and late morning. The time of onset becomes earlier and the latitude of onset moves equatorward as the level of magnetic activity increases. The longitude expansion velocities are contained in the range 0.7-7 km/sec except for a few extreme values which exceed 20 km/sec. The auroral absorption eastward expansion velocity is smaller than the corresponding velocity of the boundary of the region of activation of the visual aurora after break up by a factor 14/-12. The expansion velocity corresponds, in general, to drift velocities of electrons of energies in the range 50-300 keV but, for the extreme speeds, electron energies around 1 MeV are needed. Expansion of the absorption in the westward direction was seen in about half of the substorms studied. In about half of these, expansion along the auroral oval could be indentified, but in almost all of these cases some expansion in the auroral zone latitudes was also seen. In about an equal number of events, expansion was confined primarily to the auroral zone. The velocity of the westward expansion was about 1 km/sec along the auroral oval (i.e. approximately equal with the speed of the westward travelling surge) but about 2 km/sec along the auroral zone. The meridional expansion velocities found agree well with those measured for visual aurora (<= 1 km/sec). The variability of the behaviour of different substorms is very large. To illuminate this the following may be mentioned, in addition to what has been stated above about the statistics. Although the absorption maximum practically always moves eastward from the initiation region, exceptions have been seen in which the maximum started moving west and in a later phase went eastward. Sometimes the absorption maximum stays in the injection area or very close to it, although in most cases it moves eastward into the dayside. In extreme eases it has been found to move more than 270° in the eastward direction. There are auroral absorption substorms in which injection seems to take place in more than one area simultaneously. The observations cannot all be understood in terms of gradient and curvature drift of electrons from a small area of injection only. A broad intrusion of hot plasma from the tail into the inner magnetosphere seems to be needed. No strong dependence of particle precipitation on the illumination of the upper ionosphere by sunlight was seen. The results do, therefore, not support the hypothesis of Brice and Lucas (1971) that cold plasma density increases, originating in the ionosphere, significantly increase the precipitation rate of energetic trapped particles.
Berkey Tom F.
Driatskiy V. M.
Henriksen Kim
Hultqvist Bengt
Jelly Doris H.
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