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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.

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A Pc4 ultralow frequency wave was detected at spacecraft B of the Van Allen Probes at the plasmapause. A distinctive feature of this wave is the strong periodical modulation of the wave. It is assumed that this modulation is a beating of oscillations close in frequency: at least two harmonics with frequencies of 15.3 and 13.6 MHz are found. It is shown that these harmonics can be the eigenmodes of the transverse resonator at the local maximum of the Alfvén velocity. In addition, the observed wave was in a drift resonance with energetic 80-keV protons and could be generated by an unstable bump on tail distribution of protons simultaneously observed with the wave. The estimate of the azimuthal wave number m made from the drift resonance condition gives a value of about −100, that is, it is a westward propagating azimuthally small-scale wave. Plain Language SummaryThe study presented in this paper can be considered as the first direct experimental confirmation of the existence of the transverse Alfvénic resonators on the plasmapause exited by the kinetic instabilities in the hot plasma component. An Alfvén wave was detected by the Radiation Belt Storm Probes spacecraft B spacecraft on 23 October 2012 at the outer edge of the plasmapause where Alfvén velocity has a local maximum. The wave had strong amplitude modulation of the magnetic and electric fields. This amplitude modulation was the result of the superposition of (at least) two harmonics with close frequencies 15.3 and 13.6 mHz. We suppose that these harmonics are eigenmodes of the transverse Alfvénic resonator at the plasmapause. Moreover, the energetic proton fluxes were strongly modulated by the wave at energies near 80 keV. Assuming the drift wave-particle resonance, we found that the wave was azimuthally small-scale westward propagating wave with the azimuthal wavelength at the geomagnetic equator about 0.3 Earth's radius. The wave was probably generated by the instability caused by the inverted (bump on tail) proton distribution for energies below 84.4 keV.

A case study of shortwave radar observations of magnetospheric Pc5 ULF waves (wave periods of 150–600 s) that occurred on 26 December 2014 in the nightside magnetosphere during substorm activity is presented. The radar study of waves in the magnetosphere is based on analysis of scattering from field‐aligned irregularities of the ionospheric F layer. Variations of their trueE→×trueB→ drift velocity at F layer heights are associated with the wave electric field. Analysis of the observations from the Ekaterinburg (EKB) radar shows that the frequency f of the observed wave depends on the azimuthal wave number m (positive correlation of about 0.90): an increase in frequency from 2.5 to 5 mHz corresponds to increased m number from 20 to 80. Of the known types of waves in the magnetosphere corresponding to the Pc5 range, only drift compressional waves have such azimuthal dispersion: the frequency of the drift compressional mode is directly proportional to the azimuthal wave number and the gradient‐curvature drift velocity of energetic particles in the magnetic field. This wave has a kinetic nature and represents the most common kind of the compressional modes, demanding for its existence only finite pressure and plasma inhomogeneity across magnetic shells.