Abstract
Abstract. Unusually large-amplitude morning Pc5 magnetic pulsations during the recovery phase of the huge magnetic storm in November 2003 have been studied by using ground-based multi-point observations. Two main spectral Pc5 enhancements were observed: at f≈2 mHz, which featured slowly increasing frequency with decreasing latitude, and at f≈3 mHz, which was latitude independent. The Pc5 pulsations were observed at wide range of latitudes (more than 10°) with the same very strong amplitude (up to 500nT) and with the same polarization. Only the 3-mHz peak was clearly seen in the spectra of pulsating auroral radio absorption, as observed by the Finnish riometer chain. Short and localized bursts of PiB (f~50–100 mHz) magnetic pulsations and simultaneous short bursts of energetic electron precipitation were observed in the morning sector, as well. The beginning of the large-amplitude morning Pc5 activity occurred simultaneously with a substorm onset in the evening and midnight sectors. However, the spectra of pulsations in the morning and evening sectors were different. They were compared with spectra of IMF and solar wind parameters, measured by ACE spacecraft. The similarity between the spectra of morning Pc5 and IMF By was found, but the spectra of evening Pi3 pulsations were similar to the spectra of solar wind density variations. The Pc5 and PiB pulsations, as well as bursts of the auroral radio absorption, suddenly disappeared, when the solar wind density abruptly dropped. We suppose that the ~2-mHz Pc5 geomagnetic pulsations could be attributed to field line resonance (FLR), however, the 3-mHz oscillations were apparently non-resonance origin. Keywords. Magnetospheric physics (MHD waves and instabilities; Solar wind-magnetosphere interaction; Storms and substorms)
Highlights
Several magnetic superstorms occurred in October– November 2003
During the recovery phase of this storm, unusually strong Pc5 geomagnetic pulsations appeared in the morning sector, when a sharp density enhancement in the solar wind arrived at the dayside magnetopause, at ∼03:00 UT
There are two principally different ways for coupling the solar wind energy into Pc5 range pulsations observed on the ground: (1) the fast solar wind streams lead to an enhanced Kelvin-Helmholtz instability on the flanks of the magnetosphere, mainly on the dawn flank and that arouses enhanced ULF waves inside the magnetosphere through surface mode (e.g. Miura, 1992) or by cavity/waveguide modes (e.g. Samson et al, 1992; Mann et al, 1999); (2) fluctuations in the solar wind can directly drive ULF waves in the magnetosphere (Kepko et al, 2002; Stephenson and Walker, 2002) or lead to an enhanced excitation of the field line resonance or cavity/waveguide magnetospheric waves
Summary
Several magnetic superstorms occurred in October– November 2003 (see, e.g. the special issue of Geomagnetism and Aeronomy, Vol 45, No 1, 2005). During the recovery phase of the storm, on 21 November at ∼03:00– 06:00 UT, several dense transients were detected by ACE, and at ∼06:00 UT the solar wind density suddenly dropped to a background level. During the recovery phase of this storm, unusually strong Pc5 geomagnetic pulsations appeared in the morning sector, when a sharp density enhancement in the solar wind arrived at the dayside magnetopause, at ∼03:00 UT. To study short period geomagnetic pulsations (f=50–100 mHz), we used the induction magnetometer data at the Finnish (OUL, ROV, SOD, IVA and KIL) stations. The locations of stations in the IMAGE, SAMNET, Finnish riometer and pulsation magnetometer arrays are shown in the Table 1
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