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Ann. Geophys., 22, 1675-1686, 2004
www.ann-geophys.net/22/1675/2004/
© European Geosciences Union 2004
Cosmic radio noise absorption events associated with equatorward drifting arcs during a substorm growth phase
J. R. T. Jussila1, A. T. Aikio1, S. Shalimov2, and S. R. Marple3
1Department of Physical Sciences, P.O. Box 3000, FIN-90014 University of Oulu, Finland
2Institute of physics of the Earth, Moscow, Russia
3Department of Communication systems, University of Lancaster, UK
Abstract. Cosmic radio noise absorption (CNA) events associated with equatorward
drifting arcs during a substorm growth phase are studied by using
simultaneous optical auroral, IRIS imaging riometer and EISCAT incoherent
scatter radar measurements. The CNA is generally attributed to energetic
particle precipitation in the D-region. However, it has been argued that
plasma irregularities or enhanced electron temperature (Te) in the
E-region could also produce CNA. Both of the latter mechanisms are related to
intense electric fields in the ionosphere. We present two events which occur
during a substorm growth phase in the evening MLT sector. In both of the
events, an auroral arc is drifting equatorward, together with a region of CNA
(auroral absorption bay) located on the equatorward side and outside of the
arc. Both of the events are associated with enhanced D-region electron
density on the equatorward side of the auroral arc, but in the second event,
a region of intense electric field and enhanced electron temperature in the
E-region is also located on the equatorward side of the arc. We show that
in the studied events neither plasma instabilities nor enhanced Te play a
significant role in producing the measured CNA, but the CNA in the vicinity of
the equatorward drifting arcs is produced by D-region energetic electron
precipitation.
Key words. Ionosphere (auroral ionosphere; particle precipitation;
electric fields and currents)
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