A statistical study of diurnal, seasonal and solar cycle variations of F-region and topside auroral up ̄ows observed by EISCAT

A statistical analysis of F-region and topside auroral ion up ̄ow 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 up ̄ow events (IUEs) is examined over the altitude range 200 to 500 km, using ®eld-aligned observations from CP-1 and CP-2. The study is extended in altitude with vertical measurements from CP-7. Ion up ̄ow events were identi®ed by consideration of both velocity and ̄ux, with threshold values of 100 m s and 10 m s, respectively. The frequency of occurrence of IUEs is seen to increase with increasing altitude. Further analysis of the ®eld-aligned observations reveals that the number and nature of ion up ̄ow events vary diurnally and with season and solar activity. In particular, the diurnal distribution of up ̄ows is strongly dependent on solar cycle. Furthermore, events identi®ed by the velocity selection criterion dominate at solar minimum, whilst events identi®ed by the upward ®eld-aligned ̄ux criterion dominated at solar maximum. The study also provides a quantitative estimate of the proportion of up ̄ows that are associated with enhanced plasma temperature. Between 50 and 60% of up ̄ows are simultaneous with enhanced ion temperature, and approximately 80% of events are associated with either increased F-region ion or electron temperatures.