Observation and theory of Pc 5 waves with harmonically related transverse and compressional components

Abstract

The properties of 23 magnetic pulsation events observed by the AMPTE CCE spacecraft are studied. These events are selected on the basis of the field magnitude which oscillated at the second harmonic of a simultaneously present transverse oscillation. The events have a second harmonic period of 80–600 s (roughly the Pc 5 range), are observed in cluster in the dawn (0300–0800 magnetic local time, MLT) and dusk (1600–2100 MLT) sectors, and are localized near the magnetic equator. Although the azimuthal wave number, m, estimated from an ion finite Larmor radius effect, is generally large (|m| ∼ 50), there is a marked difference between the events observed in the dawn and dusk sectors. In the dawn sector the waves have low frequencies (1–5 mHz), indicate left‐hand polarization with respect to the ambient magnetic field, and propagate eastward with respect to the spacecraft. In the dusk sector the waves have high frequencies (5–15 mHz), indicate right‐hand polarization, and propagate westward. We suggest that the waves are all westward propagating in the plasma rest frame and that local‐time‐dependent Doppler shift is the reason for the local time dependence of the wave properties. The drift mirror instability is considered to be the mechanism for exciting the westward propagating waves. An analytical formula for the ion flux oscillations is derived on the basis of the nonlinear gyrokinetic theory. The observed correlation between the ion flux and the parallel magnetic field perturbation δB∥ can be adequately explained with this analytical formula.

THEMIS observations of electron cyclotron harmonic emissions, ULF waves, and pulsating auroras

“…[15] The ULF wave characteristics described above are nearly identical to those investigated by Takahashi et al [1990] in terms of the frequency range, the propagation direction, the azimuthal wave number, and most crucially, the presence of second-order harmonics of the compressional components seen by the equatorial probes. More substantial statistics of the events with similar features were performed by Anderson et al [1990] and Lessard et al [1999], who found that these waves occur predominantly at L > 7.…”

“…More substantial statistics of the events with similar features were performed by Anderson et al [1990] and Lessard et al [1999], who found that these waves occur predominantly at L > 7. Takahashi et al [1990] suggested the drift-mirror mode as their underlying mechanism; the distinct anisotropy of CPS ions shown on all three probes provides support to this proposal. Such anisotropy existed before the substorm and the wave appearance, but it is reasonable to conceive that the substorm expansion modified the local magnetospheric condition to make it more liable to the instability threshold and in turn triggered the waves.…”

“…The idea is not theoretically new. Chiu et al [1983] proposed that the spatial modulation of the equatorial magnetic field and ambient plasma by a mirrormode wave, which is also the suggested mechanism of our observed ULF waves [see also Takahashi et al, 1990], can modulate the growth rate of the electron cyclotron waves, either electromagnetic or electrostatic, and in turn modulate the precipitation rate of auroral electrons. Also, AshourAbdalla and Kennel [1978] found that the local growth rate and spatial amplification of the ECH instability are crucially controlled by the "cold" electron plasma and/or its ratios to the "hot" population in density and temperature.…”

“…This speed is still substantially smaller than that of the in situ observed ULF waves (∼1°/min), even taking into account the Earth's rotation (∼0.25°/min). It is well known that the propagation speed of a drift ULF wave is crucially controlled by the ambient plasma convections [e.g., Takahashi et al, 1990], which is presumably eastward in the postmidnight sector, but decreases in magnitude in a transition from the outer to the inner magnetosphere. Therefore, the discrepancy between the observed eastward speed of the ULF wave and that of the PsAs gives indirect clue that there was likely a radial separation between the probes and the magnetospheric origins of the PsAs; the former were located somewhat outward of the latter, as we shall discuss later in section 4.…”

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.…”