Ionospheric Disturbances During the September 2017 Geomagnetic Storm

The geomagnetic storm of September 7-9, 2017 occurred during an interval of enhanced solar activity resulting in multiple flares, several coronal mass ejections (CMEs), and a number of space weather-related phenomena. Although the minimum value of the Dst index reached only -150 nT, the storm had significant effects on the geomagnetic field. This paper uses multi-instrument observations of the ionospheric F-region densities and electric fields to characterize the storm-time response to the magnetospheric disturbance. We use in situ F-region ( 400 km altitude) ion densities measured by the Floating Potential Measurement Unit (FPMU) instrument suite on board the International Space Station (ISS), global maps of the total electron content (TEC) derived from ground-based GPS receivers, and incoherent radar measurements from the Jicamarca Radio Observatory (near the magnetic equator) and from the Millstone Hill incoherent scatter data. Our analysis shows that the ionospheric response near local midnight (between 20 and 02 local time) is the most pronounced in the American longitudinal sector, where both in situ density data and TEC maps indicate significant enhancements. We associate these density peaks with the storm-time Appleton anomaly that extends to late local times. We will also present observations of significant plasma density depletions and irregularities in the FPMU data near the equator in the post sunset region that are interpreted as arising from spread-F type instabilities. The dependence of these irregularities will be analyzed in terms of their local time and storm phase. Finally, we will use incoherent scatter measurements (including ExB ion drift velocities) to place these results in the context of storm-time electrodynamics.