Spread F occurrence over a southern anomaly crest location in Brazil during June solstice of solar minimum activity

Abstract

[1] We present a study of Spread F occurrence over a location under the southern crest of the equatorial ionization anomaly (Cachoeira Paulista 22.7°S, 45.0°W, mag. Lat.: 16°S, dip angle: −32.3°, Brazil) during the last solar cycle, which presented an extended solar minimum activity. After analyzing hundreds of ionograms obtained with a digital ionosonde DGS 256, between 2001 and 2010, we verified high Spread F occurrence around midnight-postmidnight during June solstice (Southern Hemisphere winter) with a peak occurrence between 2006 and 2009, when the solar flux has reached very low values (<70 SFU). At the Brazilian sector the occurrence of Spread F is known to be associated with equatorial plasma bubbles (EPBs) and is observed mainly between September and March. On the other hand, EPBs are rarely observed over Cachoeira Paulista (hereafter referred as CP) during June solstice because of the weak magnitudes of the equatorial electric field prereversal enhancement, an essential condition to the development of the large-scale irregularities at the equatorial region. Despite the rarity of EPBs, we observed several strong Spread F events occurring under low plasma densities conditions which can extend for several hours. We have found evidences that the Spread F events over CP during June solstices of low solar activity can be caused by ionospheric disturbances that are unrelated to equatorial processes. Finally we present a statistical analysis of the events and we suggest that traveling ionospheric disturbances could be one of the main sources of these nonequatorial Spread F events.