The asymmetrical features of plasma density during extreme solar minimum in 2008–2009

The variations of plasma density in topside ionosphere during 23^rd/24^th solar cycle minimum attract more attentions in recently years. In this analysis, we use the data of electron density (Ne) from DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions) satellite at the altitude of 660-710 km to investigate the solstitial and equinoctial asymmetry at LT(local time) 1030 and 2230 during 2005-2010, especially in solar minimum years of 2008-2009. The results reveal that □Ne (Dec.-Jun.) is always positive over southern hemisphere and negative over northern part whatever at LT 1030 or 2230, and it becomes smaller with the declining of solar flux from 2005 to 2009. The □Ne between September and March is completely negative during 2005-2008, but in 2009, it turns to be positive at latitudes of 20° S-40° N at LT 1030 and 10° S-20° N at LT 2230. Furthermore, the solstitial and equinoctial asymmetry index (AI) are calculated and studied respectively, which all depends on local time, latitude and longitude. The notable differences occur at higher latitudes in solar minimum year of 2009 with those in 2005-2008. The equinoctial AI at LT 2230 is quite consistent with the variational trend of solar flux with the lowest absolute AI occurring in 2009, the extreme solar minimum, but the solstitial AI exhibites abnormal enhancement during 2008 and 2009 with bigger AI than those in 2005-2007.