Local time and longitudinal variability of ionospheric response during extreme geomagnetic events

Abstract

We analyzed Total Electron Content (TEC) data from 22 GNSS (Global Navigation Satellite System) stations across Africa, Europe, and Asia, spanning equatorial, low, and mid-latitudes, to characterize daytime and nighttime ionospheric storm responses during the geomagnetic events of February 2014 and March 2015. Using a 5-min interval TEC dataset, we employed the threshold |rTEC ≤−30| or |rTEC ≥ 30| to identify positive and negative storm phases relative to a quiet-time defined by the median of the 10 quietest days. To investigate the underlying drivers of these phases, we examined variations in the ΔH-component from collocated magnetometers and evaluated spatial gradients to assess local time evolution and longitudinal variability. Our results show that ionospheric electrodynamics during storms are more strongly controlled by local time and geomagnetic longitude than by storm intensity, although storm strength still modulates the magnitude and spatial extent of the response. Notably, 18.97% of positive phases occur during the daytime compared to 10.94% at night, underscoring the critical role of local time in modulating TEC variability.