Poloidal ULF oscillations in the dayside magnetosphere: a Cluster study

P T I Eriksson,L G Blomberg,A D M Walker,K.-H Glassmeier

doi:10.5194/angeo-23-2679-2005

P T I Eriksson, L G Blomberg + Show 2 more

Open Access

https://doi.org/10.5194/angeo-23-2679-2005

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Abstract

Abstract. Three ULF wave events, all occurring in the dayside magnetopshere during magnetically quiet times, are studied using the Cluster satellites. The multi-point measurements obtained from Cluster are used to determine the azimuthal wave number for the events by means of the phase shift and the azimuthal separation between the satellites. Also, the polarisation of the electric and magnetic fields is examined in a field-aligned coordinate system, which, in turn, gives the mode of the oscillations. The large-inclination orbits of Cluster allow us to examine the phase relationship between the electric and magnetic fields along the field lines. The events studied have large azimuthal wave numbers (m~100), two of them have eastward propagation and all are in the poloidal mode, consistent with the large wave numbers. We also use particle data from geosynchronous satellites to look for signatures of proton injections, but none of the events show any sign of enhanced proton flux. Thus, the drift-bounce resonance instability seems unlikely to have played any part in the excitation of these pulsations. As for the drift-mirror instability we conclude that it would require an unreasonably high plasma pressure for the instability criterion to be satisfied.Keywords. Ionosphere (Wave propagation) – Magnetospheric physics (Plasma waves and instabilities; Instruments and techniques)

Highlights

Waves with small azimuthal length scale in the Pc5 frequency range have been studied since the 1930s (Rolf, 1931) there are still open questions regarding their generation mechanism, largely due to insufficient data for a complete picture of both the waves and the particles
The high values (∼100) of the azimuthal wave number found in all three events are consistent with the poloidal mode type polarisation of the wave magnetic field
LRE r α where ρeq is the density at the equator, L is the McIlwain parameter, RE is the radius of the Earth, r the geocentric distance to a point on the field line and α is the scaling index

Summary

Introduction

Waves with small azimuthal length scale in the Pc5 frequency range have been studied since the 1930s (Rolf, 1931) there are still open questions regarding their generation mechanism, largely due to insufficient data for a complete picture of both the waves and the particles. In the limit when the azimuthal wavelength goes to zero, the eastward magnetic field component becomes zero This is usually termed the poloidal mode. Oscillations that occur during active times are normally observed in the dusk sector and are termed storm-time pulsations Another proposed excitation mechanism is the drift-mirror instability (Chen and Hasegawa, 1991; Walker et al, 1982). In the rest frame of the plasma the mirror instability grows without oscillations (zero real frequency) This instability is a viable excitation mechanism at L>8, where the magnetic field becomes weak enough to allow β to exceed unity during normal conditions. There are a number of spacecraft observations of large-m ULF pulsations and two reviews of these are given by Anderson (1994) and Takahashi (1988) The majority of these studies were with satellites near the equatorial plane and some utilised multi-spacecraft configurations.

Instrumentation

Observations and data analysis

Results

Discussion and conclusions