A case study of a radially polarized Pc4 event observed by the Equator-S satellite

R Cramm,K H Fornacon,W Baumjohann,C Othmer,E Georgescu,H U Auster,K H Glassmeier

doi:10.1007/s00585-000-0411-5

Abstract

Abstract. A 16 mHz Pc4 pulsation was recorded on March 17, 1998, in the prenoon sector of the Earth's magnetosphere by the Equator-S satellite. The event is strongly localized in radial direction at approximately L = 5 and exhibits properties of a field line resonance such as an ellipticity change as seen by applying the method of the analytical signal to the magnetic field data. The azimuthal wave number was estimated as m \\approx 150. We discuss whether this event can be explained by the FLR mechanism and find out that the change in ellipticity is more a general feature of a localized Alfvén wave than indicative of a resonant process.Key words: Magnetospheric physics (MHD waves and instabilities)

Highlights

Ultralow-frequency (ULF) pulsations are the manifestation of magnetohydrodynamic (MHD) plasma waves (AlfveÂ n, 1942; Dungey, 1954)
Characteristics of ULF pulsations such as frequency, wave number, localization, and polarization depend on the excitation mechanism
A prominent external excitation mechanism for ULF waves is the KelvinHelmholtz instability generating surface waves at the magnetopause (e.g. Fujita et al, 1996; Engebretson et al, 1998). Such waves propagate into the magnetosphere as isotropic fast modes and can couple resonantly to shear AlfveÂ n waves via the ®eld line resonance (FLR) process (Tamao 1965; Chen and Hasegawa, 1974; Southwood, 1974). This process has become a paradigm in understanding the features of a majority of observable ULF pulsations in the Pc3±5 range

Summary

Introduction

Ultralow-frequency (ULF) pulsations are the manifestation of magnetohydrodynamic (MHD) plasma waves (AlfveÂ n, 1942; Dungey, 1954). Fujita et al, 1996; Engebretson et al, 1998) Such waves propagate into the magnetosphere as isotropic fast modes and can couple resonantly to shear AlfveÂ n waves via the ®eld line resonance (FLR) process (Tamao 1965; Chen and Hasegawa, 1974; Southwood, 1974). This process has become a paradigm in understanding the features of a majority of observable ULF pulsations in the Pc3±5 range. We discuss how the event ®ts into present theories on the excitation of ULF pulsations

The event

Discussion

Summary and conclusions