Compressional ULF waves in the outer magnetosphere: 1. Statistical study

Abstract

Using 14 months of magnetic field and plasma data observed by the ISEE 1 and 2 spacecraft, ULF waves of period 2–20 min in the outer magnetosphere were studied. Statistical properties of the ULF waves can be summarized as follows: (1) Intense compressional waves are a persistent feature near the two flanks of the magnetosphere. They are mainly polarized in a meridian plane with comparable compressional and transverse amplitudes and have larger amplitude at magnetic latitudes below 20° than at higher latitudes. The magnetic pressure perturbations for the waves are in antiphase with the plasma pressure perturbations; (2) Transverse waves polarized in the azimuthal direction (azimuthal waves) are found to be mainly a nightside phenomenon. Their appearances on the nightside magnetosphere seem to be associated with substorm activity; (3) Compressional wave power and plasma β, the ratio between the plasma pressure and the magnetic field pressure, are correlated, but ULF wave power in the above noted period range is not significantly correlated with AE, plasma pressure or VA; (4) The compressional waves are most likely to be generated internally in the regions where plasma β and field line curvature are large.

THEMIS observations of compressional pulsations in the dawn-side magnetosphere: a case study

“…Compressional pulsations with periods ranging from 100 to more than 600 s are common in the equatorial morning and afternoon magnetosphere at distances beyond L=8 (e.g., Baumjohann et al, 1987;Takahashi et al, 1990a;Anderson et al, 1990;Zhu and Kivelson, 1991;Vaivads et al, 2000Vaivads et al, , 2001 when the plasma β is large (Zhu and Kivelson, 1994;Rae et al, 2007). At dusk they occur during geomagnetically disturbed intervals, are associated with plasma injections, and propagate westward (sunward) at the 10-20 km s −1 gradient-curvature drift velocities expected for 10-30 keV ions (Barfield and McPherron, 1972;Takahashi et al, 1985).…”

THEMIS observations of compressional poloidal pulsations in the dawnside magnetosphere: A case study

“…Among these waves, compressional pulsations with periods from 3 to 15 min play a prominent role in both the dawn and dusk magnetospheres at distances beyond L = 8 [e.g., Baumjohann et al, 1987;Takahashi et al, 1990;Anderson et al, 1990;Zhu and Kivelson, 1991;Vaivads et al, 2000Vaivads et al, , 2001 where the plasma β is large [Zhu and Kivelson, 1994;Rae et al, 2007].…”

“…Case and statistical studies reveal that the waves typically exhibit azimuthal wavelengths from 2000 to 7000 km and therefore azimuthal wave numbers from 15 to 120 [e. g., Takahashi et al, 1985Takahashi et al, , 1990Zhu and Kivelson, 1991;Lessard et al, 1999;Vaivads et al, 2001].…”

Compressional Pc5 type pulsations in the morningside plasma sheet

“…Statistical studies of spacecraft data (Anderson et al, 1990;Hedgecock, 1976;Takahashi et al, 1990;Zhu and Kivelson, 1991;Lessard et al, 1999) show that compressional Pc5 pulsations are dominant pulsations at large distances, L ≥ 8, but are less common at synchronous orbit. They have highest occurrence rates in the morning and afternoon sectors.…”