Vertical total electron content and geomagnetic perturbations at mid- and sub-auroral southern latitudes during geomagnetic storms

摘要

Several new space geodesy techniques allow us to analyze the behavior of the vertical total electron content (VTEC) with high spatial and temporal resolution. This study is based on the VTEC computed from global positioning system (GPS) satellite signals that are recorded from observatories located at mid- and sub-auroral southern latitudes. The geomagnetic disturbances are analyzed using the Dst and AL geornagnetic indices and geornagnetic field variations which are recorded from an observatory close to one of the GPS stations and from observatories located at equivalent geomagnetic latitudes but in the Northern Hemisphere. The study is focused on two consecutive geomagnetic storms, which happened on October 4 and 5, 2000, characterized by two flips of the interplanetary magnetic field. During this perturbed period, the substorms are evidenced by the AL index and by the field variations recorded by the geomagnetic observatories. We also analyze a substorm effect that occurred during a geornagnetic storm.Variations in f(0)F(2) are currently considered to study the geomagnetic storm effects on the ionosphere. Our results show that at mid- and subauroral southern latitudes the behavior of the VTEC evidences the "dusk" effect (positive ionospheric storm after noon) in a similar way to f(0)F(2) variations. Similar geomagnetic conditions can be inferred from the Dst index for both geomagnetic storms but a quick rise of the VTEC and the dusk effect is only observed on the first stormy day. The positive ionospheric storm is followed by a negative phase that lasts until October 6. The second geornagnetic storm starts when the negative phase of the first ionospheric storm is still deployed and the ionosphere/ plasmasphere system conditions do not allow a new positive ionospheric storm.The AL index and the geomagnetic field variations allow us to recognize the expansion phase of the substorm due to the presence of the electromagnetic wedge that couples the magnetosphere and ionosphere at high latitudes. Fluctuations in the VTEC computed from the GPS observatories are in tough agreement with the influence of the upward and downward field-aligned currents of the wedge. (C) 2004 Elsevier Ltd. All rights reserved.