Plasmaspheric depletion and refilling associated with the September 25, 1998 magnetic storm observed by ground magnetometers at L = 2

Abstract

Abstract. The plasmaspheric mass density at L •_ 2 was monitored by two IGPP/LANL ground magnetometer stations during the magnetic storm on September 25, 1998. Even at this low latitude the plasma density dropped significantly to ___ 1/4 of the pre-storm value. The total electron content (TEC) inferred by GPS signals also shows a sizable decrease during the storm. The observations suggest that the convection caused by the strong electric field associated with the magnetic storm eroded the plasmasphere as low as L = 2, which is a much lower latitude than that expected from the estimated potential drop across the polar cap together with a simple model of the magnetospheric convection pattern.

ULF fluctuations of the geomagnetic field and ionospheric sounding measurements at low latitudes during the first CAWSES campaign

“…A similar effect (i.e. a depletion at an unexpected low L-shell) was previously found with the same ULF technique at L=2 by Chi et al (2000) for the geomagnetic storm of 25 September 1998. For both storms the depletion (by a factor of 3 for the 25 September 1998 storm) is observed during the recovery phase with a prolonged phase of northward IMF.…”

“…Equatorial plasmaspheric mass densities (ρ o ) given in Table 2 have been inferred from f R , assuming, as is usually done, a radial power law dependence of the density along the dipole field line (ρ∝ r −m ; Schulz, 1996). We have chosen m = 5, which is a value higher than the values (3-4) typically used at higher latitudes Chi et al, 2000). This choice was done in order to take into account that at our latitudes a significant fraction of the field line is characterized by a very steep density variation (Sutcliffe et al, 1987).…”

“…In the last recent years there has been increasing use of ground-based ULF magnetic measurements to remote sense the plasma mass density in the magnetosphere Chi et al, 2000;Clilverd et al, 2003;Berube et al, 2005). This method consists of determining the frequency of the eigen-oscillations of a given geomagnetic field line (usually the field line midway between two latitudinally separated recording stations).…”

Plasmaspheric electron content in the GPS ray paths over Japan under magnetically quiet conditions at high solar activity

“…The calculations are repeated from 3 to 16 iteration days (for the lowest to the highest field line) so that the model outputs on the last iteration day represent an ionosphere-plasmasphere system with a filled plasmasphere, which corresponds to the situation after a series of magnetically quiet days (Carpenter and Park, 1973;Chi et al, 2000). The model outputs on the last iteration day are regridded at intervals of 25 km in altitude up to GPS altitude, 2…”

Hodograph method to estimate the latitudinal profile of the field-line resonance frequency using the data from two ground magnetometers

“…These methods utilize the characteristic feature of FLRgenerated ULF waves: Since the amplitude and phase of FLR-generated ULF waves sharply changes across the resonance latitude while the superposed signals are more global and almost the same at the two stations, taking the difference of the data from the two adjacent stations can cancel out the most of the non-FLR signals and extract even weak FLR signals. These methods have been applied to Pc3-4 pulsation data from two ground stations in middle to low latitudes, and the FLR parameters at a point (usually regarded as the midpoint) between the two stations have been successfully obtained (e.g., Baransky et al, 1985Baransky et al, , 1989Kurchashov et al, 1987;Green et al, 1993;Waters et al, 1994;Russell et al, 1999;Chi et al, 2000;Menk et al, 2000).…”