First observations of poleward large-scale traveling ionospheric disturbances over the African sector during geomagnetic storm conditions

摘要

This paper presents first observations of poleward traveling ionospheric disturbances (TIDs) during strong geomagnetic conditions over the African sector. By analyzing different data sets we have observed both positive and negative ionospheric responses during the storm period of 08–10 March 2012. Considering the African region as a whole, three longitudinal sectors were strategically selected to establish the entire regional response. On both sides of the geomagnetic equator, results show poleward shift in peak total electron content (TEC) enhancements/depletions at different times which are associated to large-scale TIDs. The observed phenomena are linked to the global ionospheric response and electrodynamics. The understanding has been established using data from International GNSS Service receiver network, radio occultation electron density profiles, derived E × B drift measurements from magnetometer observations and regional ground-based and satellite data. Contrary to other related studies, generated regional TEC perturbation maps were not enough to show obvious directions of the large-scale TIDs due to insufficient data over the northern hemispheric part of the African sector. There appears to be a switch between positive and negative storm phases during the same storm period especially in the Southern Hemisphere part of the African region where "enough" data were available. However, a detailed analysis revealed that the positive storm phase corresponded to the expansion of the equatorial ionization anomaly (EIA) toward some parts of midlatitude regions (and possibly with the contribution from low-latitude electrodynamics associated to equatorial electrojet), while the other part recorded a negative storm phase due to storm-induced changes from the auroral origin. We have observed a simultaneous occurrence of both poleward and equatorward propagating TIDs over the African sector during the same geomagnetic storm period. Our results show that short-lived large-scale TIDs are possibly launched by the equatorial electrojet, while the EIA expansion contributes (through modulation) to the poleward propagation of the disturbances. Temporal variation of TEC perturbations on a storm day over the entire African sector showed the existence of large-scale TIDs during the main and recovery phases which can travel poleward up to 20° latitude. The amplitudes of the TIDs have range ±2 total electron content unit, 1 TECU = 10~(16) elm~(?2), period of 2 h and virtual velocities of 250 ± 59–750 ± 95 m/s in midlatitude regions and up to 990 ± 65 m/s within the EIA region.