Temporal and spatial variations of GNSS-TEC from the high-latitude to equatorial regions during a geomagnetic storm on 27 and 28 September 2017

Temporal and spatial variations of total electron content (TEC) and electron density in the ionosphere from the high-latitude to equatorial regions during a geomagnetic storm that occurred on 27 and 28 September 2017 have been investigated using global TEC data obtained from many global navigation satellite system (GNSS) stations together with the ionosonde, geomagnetic field, Jicamarca incoherent scatter (IS) and SuperDARN radar data. Our analysis results show that a clear enhancement of the ratio of the TEC difference (rTEC) first occurs from noon to afternoon at high latitudes within 1 hour after sudden increase and expansion of the high-latitude convection and prompt penetration of the electric field to the equator associated with southward turning of the interplanetary magnetic field (IMF). Approximately 1-2 hours after the onset of the hmF2 increase in the mid-latitude and low-latitude regions associated with the high-latitude convection enhancement, the rTEC and foF2 values begin to increase and the enhanced rTEC region expands to low latitudes within 1-2 hours. The enhanced rTEC region corresponds to the mid-latitude broad storm-enhanced density (SED) phenomenon. This signature suggests that the ionospheric plasmas in the F2-region move at a higher altitude due to local electric field drift, where the recombination rate is smaller, and that the electron density increases due to additional production at the lower altitude in the sunlit region. Later, another rTEC enhancement related to the equatorial ionization anomaly (EIA) appears in the equatorial region approximately 1 hour after the prompt penetration of the electric field to the equator and expands to higher latitudes within 3-4 hours. This structure finally merges with the mid-latitude broad SED one.