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.

This paper investigates the conditions of the ballooning instability of the coupled Alfvén and slow magnetoacoustic modes in the dipole model of Earth's magnetosphere taking into account plasma and magnetic field inhomogeneity in the direction along the magnetic field lines. The diamagnetic condition (meaning vanishing perturbation of the total pressure) is satisfied. It was shown that the instability develops on the slow magnetoacoustic oscillation branch, but the instability threshold is determined by the coupling with the Alfvén mode. The symmetric (with respect to the magnetic equator) modes were found to be more unstable than antisymmetric ones. In this case, the instability threshold depends on plasma compressibility: the finite sound velocity raises the instability threshold. For all other equal conditions, the instability threshold decreases with the decrease in the field line curvature radius on the equator.

Magnetic azimuthally small-scale (azimuthal wave numbers m ≫ 1) pulsations in Pc4-5 bands (45-600 s periods; Jacobs et al., 1964) on the dayside of the Earth's magnetosphere are intensively studied in recent years. The high-m waves are observed by satellites, high-frequency radars (Shi et al., 2018, and references therein), and optical manifestations of auroral undulations (Motoba et al., 2015). Generally, these waves propagate westward (m < 0; see Chelpanov et al., 2018), but some authors reported eastward propagating waves (e.g., Chelpanov et al., 2019;Yamamoto et al., 2019). These waves are believed to be excited through drift or drift-bounce resonance with ∼1-100 keV protons (Min et al., 2017;Takahashi et al., 2018). Electron flux oscillations in a wide energy range were found to correlate with Pc4-5 waves as well (Ren et al., 2017(Ren et al., , 2018. The energy transfer from particles to the wave going through internal instabilities caused by non-Maxwellian proton distribution or phase space density radial gradient (Southwood et al., 1969). Usually, Pc4-5 waves are associated with the MHD Alfvén waves (e.g., Dai et al., 2013), although sometimes they can be identified with the drift-compressional modes (Mager et al., 2019;Rubtsov et al., 2018). According to previous studies, the dayside high-m Pc4-5 waves usually appear as a consequence of magnetic storms (

In the inner magnetosphere dominated by the dipole magnetic field, energetic electrons of 90° pitch angle drift faster than those of other pitch angles. Electron flux oscillations initiated by injections or drift resonances will be observed earlier at a 90° pitch angle, that is, 90°‐leading boomerang‐shaped stripes on pitch angle distributions(PADs) are expected. In this work, reversed‐boomerang stripes (90° observed last) are first reported from the observations of RBSP‐A at L‐shell ∼5.9 on 11 March 2016. The corresponding solar wind dynamic pressure is over 10 nPa, which suggests that strong compression on the magnetosphere changes the magnetic field and drift frequencies of energetic electrons. Test particles running in an image‐dipole magnetic field (off‐equatorial minima exists at dayside large L‐shell) reproduce the abnormal (90° slower) drift velocities at L‐shell >5.8 but are normal at L‐shell <4.5 dominated by a dipole field. Normal boomerang stripes on PADs are indeed observed by RBSP‐B at L‐shell ∼4.0, indicating the solar wind dynamic pressure effect.

No abstract

This paper is concerned with the transverse structure of the ballooning instability in a two-dimensionally inhomogeneous model of the magnetosphere, which takes into account inhomogeneity both across the magnetic shells and along the field lines. According to the previous studies, the ballooning instability can develop at steep fall of the plasma pressure from the Earth. In this paper, the region of negative plasma pressure gradient is assumed to be sharply localized across the magnetic shells. It causes the localization of the unstable perturbation in the same direction. In this region, the perturbation has a resonator-like structure, where only modes with discrete set of the growth rate values can exist. The azimuthal wave number of the unstable mode must exceed some critical value, which is determined by the width of the localization region.

Magnetohydrodynamic (MHD) waves play a crucial role in the plasma processes of stellar atmospheres and planetary magnetospheres. Wave phenomena in both media are known to have similarities and unique traits typical of each system. MHD waves and related phenomena in magnetospheric and solar physics are studied largely independently of each other, despite the similarity in properties of these media and the common physical foundations of wave generation and propagation. A unified approach to studying MHD waves in the Sun and Earth's magnetosphere opens up prospects for further progress in these two fields. The review examines the current state of research into MHD waves in the Sun’s atmosphere and Earth's magnetosphere. It outlines the main features of the wave propagation media: their structure, scales, and typical parameters. We describe the main theoretical models applied to wave behavior studies; discuss their advantages and limitations; compare characteristics of MHD waves in the Sun’s atmosphere and Earth’s magnetosphere; and review observation methods and tools to obtain information on waves in various media.

We present the results of the complex study of ionospheric parameter variations during two geomagnetic storms, which occurred on April 12–15, 2016. The study is based on data from a set of radiophysical and optical instruments. Both the storms with no sudden commencement were generated by high-speed streams from a coronal hole. Despite the minor intensity of the storms (Dst ≥ –55 and –59 nT), we have revealed a distinct ionospheric response to these disturbances. A negative response of electron density and F2-layer critical frequency was observed during the main phase of both the storms. The amplitude of the negative response was higher for the second storm. The period of negative electron density deviations was accompanied by an increase in the peak height, as well as by the downward plasma drift in the evening and night hours, which is not typical of quiet conditions. We have also recorded sharp peaks in the AATR (Along Arc TEC Rate) index and in total electron content noise spikes on average 2–2.5 times. This indicates an intensification of small-scale ionospheric disturbances caused by disturbed geomagnetic conditions and high substorm activity.