Daytime Low and Middle Latitudes Plasma Density Enhancements Observed by Swarm Constellation during the 7-8 September 2017 Geomagnetic Storm

The ionospheric local plasma density enhancements at low latitude usually take place at night. Although there have been some reports on daytime middle latitude TEC enhancements during geomagnetic storms previously, few studies on daytime low and middle latitudes plasma density enhancements have been reported by utilizing in situ space-based measurements during geomagnetic storms. The geomagnetic storm occurring during 7-8 September 2017 was an intense one for the solar activity cycle 24. In the present study, we have analyzed the in-situ electron density (Ne) and electron temperature (Te) measurements probed by the Swarm constellation during 1-20 September 2017. We found that daytime low and middle latitudes plasma density enhancements were detected simultaneously at an altitude of about 450 km by both Swarm A and Swarm C satellites on 8 September 2017. Moreover, daytime middle latitude plasma density enhancements were detected at a higher altitude of -514 km by Swarm B (i.e., the third satellite of the Swarm constellation) on 8 and 9 September 2017. These daytime plasma density enhancements were detected in this geomagnetic storm's recovery phase, and they were located at the approximately same geomagnetic latitudes in the north and south hemispheres. Furthermore, we found that Te reduced at the locations of these daytime plasma density enhancements. The occurrence of these anomalous daytime plasma density enhancements is very likely to be associated with the electric field originated from the magnetosphere during this geomagnetic storm.