Using observations from the GPS and TOPEX satellites to investigate night-time TEC enhancements at mid-latitudes in the southern hemisphere during a low sunspot number period

The state of the ionization of the upper atmosphere at low and mid latitudes in the Australian region has been studied by investigating the total electron content (TEC) obtained by a dual-frequency group path and phase path GPS technique. For the low sunspot number time period of March 1995–February 1996, one week of data centred on the Priority Regular World Day for each month have been used to investigate night-time mid-latitude peaks occurring around midnight in the Australian region. TEC from TOPEX provided additional information related to the formation of the night-time peaks. Although night-time TEC enhancements have been observed previously, there is no general agreement on their origin. From the results of the present study, the development of midnight TEC enhancements coincided with the low latitude processes occurring at around the time of vertical E×B drift velocity reversal. The TOPEX results confirmed that the upward E×B drift velocity reversal and the downward plasma flow from greater heights producing the night-time peaks at mid latitudes are triggered from a common source: the westward electric field.