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Ann. Geophys., 16, 1144-1158, 1998
www.ann-geophys.net/16/1144/1998/
© European Geosciences Union 1998
A statistical study of diurnal, seasonal and solar cycle variations of F-region and topside auroral upflows observed by EISCAT between 1984 and 1996
C. Foster, M. Lester, and J. A. Davies
1Radio and Space Plasma Physics Group, Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
Abstract. A statistical analysis of F-region and
topside auroral ion upflow events is presented. The study is based on
observations from EISCAT Common Programmes (CP) 1 and 2 made between 1984 and
1996, and Common Programme 7 observations taken between 1990 and 1995. The
occurrence frequency of ion upflow events (IUEs) is examined over the altitude
range 200 to 500 km, using field-aligned observations from CP-1 and CP-2. The
study is extended in altitude with vertical measurements from CP-7. Ion upflow
events were identified by consideration of both velocity and flux, with
threshold values of 100 m s–1 and 1013 m–2 s–1,
respectively. The frequency of occurrence of IUEs is seen to increase with
increasing altitude. Further analysis of the field-aligned observations reveals
that the number and nature of ion upflow events vary diurnally and with season
and solar activity. In particular, the diurnal distribution of upflows is
strongly dependent on solar cycle. Furthermore, events identified by the
velocity selection criterion dominate at solar minimum, whilst events identified
by the upward field-aligned flux criterion dominated at solar maximum. The study
also provides a quantitative estimate of the proportion of upflows that are
associated with enhanced plasma temperature. Between 50 and 60% of upflows are
simultaneous with enhanced ion temperature, and approximately 80% of events are
associated with either increased F-region ion or electron temperatures.
Key words. Ionosphere (auroral ionosphere; particle
acceleration)
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