Drift Resonance of Compressional ULF Waves and Substorm‐Injected Protons From Multipoint THEMIS Measurements

Abstract

A compressional Pc5 wave associated with localized hot proton injection was observed by the five THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft in the dusk sector of the Earth's magnetosphere at L ∼ 10R E on 21 May 2007. The wave magnetic field perturbation transverse to the background magnetic field was primarily poloidal, in agreement with the predominately azimuthal wave vector direction (with westward phase velocity). The observation followed two consecutive substorms, when the cloud of energetic particles comprised of the lower-energy protons from the earlier substorm was mixed with higher-energy protons from the subsequent one. The clear signatures of the wave-particle drift resonance of protons modulated by the wave were observed. The wave period was found to be about 2 times longer than the corresponding Alfvén wave eigenmode period on the same L-shells calculated with the THEMIS data. The increase of the particle energy with the distance from the Earth and the observed strong dependence of the wave frequency on the azimuthal wave number constitutes conditions for the gradient instability of the drift compressional mode (for the Alfvén mode one supposes the particle energy decrease with radial distance). Based on these results, we conclude that the observed wave was the drift compressional mode generated by the gradient instability.

Conjugate Ionosphere‐Magnetosphere Observations of a Sub‐Alfvénic Compressional Intermediate‐m Wave: A Case Study Using EKB Radar and Van Allen Probes

“…Thus, the wave can be generated only by the second mechanism: the gradient instability. Another example of the compressional wave likely generated by the gradient instability is presented in recent study by Rubtsov et al (). There is another compressional mode similar to the drift‐compressional mode, the drift mirror mode.…”

“…There are three possible reasons for the plasma instability leading to the high‐ m waves generation: inverted parts of the distribution function ( bump on tail distribution), strong gradients of the distribution function, and pressure anisotropy (Chen & Hasegawa, ; Karpman et al, ; Southwood, ). Recent examples of the high‐ m waves driven by the bump on tail instability are presented in Mager et al () and Liu et al (), by the gradient instability in Dai et al (), Rubtsov et al (), and Takahashi et al (), and by the pressure anisotropy in Rae et al (). An interest for the study of the high‐ m waves is caused by their ability to accelerate the charged particles up to very high energies (Ukhorskiy et al, ; Zong et al, ), modulate the fluxes of energetic particles (Chen & Hasegawa, ; Zong et al, ), and serve as substorm triggers (Rae et al, ).…”

Plasmapause surface wave oscillates the magnetosphere and diffuse aurora

“…9f) indicates that the free energy was provided by the hot plasma injections. Other internal instabilities, such as the ion drift resonances which can excite fundamental poloidal waves 50,51 and the Kelvin-Helmholtz instability which can excite magnetopause surface waves 7,8 , could certainly play a role at some point in the excitation of the ULF waves, in addition to the PSW-associated ULF waves. Drift wave perturbations or the effect from ULF waves in the vicinity of the plasmapause might also be possible interpretations to the observed waves.…”

Ballooning Instability in the Magnetospheric Plasma: Two‐Dimensional Eigenmode Analysis

“…The resulting plasma pressure, plasma pressure gradient, and profiles are shown in Figure 4 (two upper panels) for L fall = 7. These values are typical for the azimuthally small-scale waves observed in the magnetosphere (e.g., Yeoman et al, 2012;James et al, 2016;Rubtsov, Agapitov, et al, 2018;Takahashi et al, 2018). The calculated Ω 2 − (L) profile is shown in the lower panel of this figure.…”