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Ann. Geophys., 14, 1305-1316, 1996
www.ann-geophys.net/14/1305/1996/
© European Geosciences Union 1996
A localised co-rotating auroral absorption event observed near noon using imaging riometer and EISCAT
P. N. Collis1,2, J. K. Hargreaves1, and G. P. White1
1Engineering Dept., SECAMS, University of Lancaster, Bailrigg, Lancaster LA1 4YR, UK
2On leave from EISCAT, Box 812, S-981 28 Kiruna, Sweden
Abstract. An isolated region of energetic electron
precipitation observed near local noon in the auroral zone has been investigated
using imaging riometer (IRIS) and incoherent-scatter radar (EISCAT) techniques.
IRIS revealed that the absorption event was essentially co-rotating with the
Earth for about 2 h. The spatial and temporal variations in D-region electron
density seen by EISCAT were able to be interpreted within a proper context when
compared with the IRIS data. EISCAT detected significant increases in electron
density at altitudes as low as 65 km as the event drifted through the radar
beam. The altitude distribution of incremental radio absorption revealed that
more than half of the absorption occurred below 75 km, with a maximum of 67 km.
The energy spectrum of the precipitating electrons was highly uniform throughout
the event, and could be described analytically by the sum of three exponential
distributions with characteristic energies of 6, 70 and 250 keV. A profile of
effective recombination coefficient that resulted in self-consistent agreement
between observed electron desities and those inferred from an inversion
procedure has been deduced. The observations suggest a co-rotating
magnetospheric source region on closed dayside field lines. However, a mechanism
is required that can sustain such hard precipitation for the relatively long
duration of the event.
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