Determination of a geomagnetic storm and substorm effects on the ionospheric variability from GPS observations at high latitudes

Abstract

The aim of this work is to characterize the ionospheric electron content variability during a standard and simple geomagnetic storm, and substorms during it. The analysis is based on tying the geomagnetic disturbances including the signatures of the current wedge formed during the substorm expansion phase, with the variability of ionospheric vertical total electron content (VTEC) in local time; for this reason the VTEC is computed for complete geographical longitude coverage at subauroral and auroral latitudes. The study is based on the geomagnetic storm befallen on April 6 and 7, 2000 (near the equinox) and the TEC are computed from global positioning system (GPS). The main results can be divided into three groups: (a) when the geomagnetic storm starts between pre-midnight and dawn, a minimum of VTEC is recorded, lasting all the long day (ionospheric storm negative phase); also the nighttime electron content may decrease below the corresponding for quiet days; but near the 60 ∘ of geomagnetic latitude the ionization polar tongue can be observed at noon, superimposed to the negative phase; (b) computed by GPS stations placed lower than 50 ∘ , when the geomagnetic storm starts between dawn and noon the VTEC recorded a positive phase, but if it starts at noon a dusk effect is recorded; those located between 50 ∘ and 60 ∘ show a sudden increase and later sudden decrease to nocturnal values, (c) when the geomagnetic storm starts between afternoon and sunset, at stations located lower than 50 ∘ a dusk effect and an ionospheric negative phase during the next day are recorded, but if the GPS stations are located at higher latitude than 50 ∘ the VTEC representation shows the nocturnal end of the ionization polar tongue. Expansion phases of substorms are shown as small VTEC variations recorded for a short time: decreases if the substorm happens between dawn and midday; enhancements during the fall of the ionospheric positive phase. From the comparison with the results obtained by other methods, the GPS analysis proves to be an adequate method for studying globally the ionospheric variability at these latitudes.