Electrostatic fluctuations near upper-hybrid frequency, which are sometimes accompanied by multiple-harmonic electron cyclotron frequency bands above and below the upper-hybrid frequency, are common occurrences in the Earth's radiation belt, as revealed through the twin Van Allen Probe spacecrafts. It is customary to use the upper-hybrid emissions for estimating the background electron density, which in turn can be used to determine the plasmapause locations, but the role of hot electrons in generating such fluctuations has not been discussed in detail. The present paper carries out detailed analyses of data from the Waves instrument, which is part of the Electric and Magnetic Field Instrument Suite and Integrated Science suite onboard the Van Allen Probes. Combined with the theoretical calculation, it is shown that the peak intensity associated with the upper-hybrid fluctuations might be predominantly determined by tenuous but hot electrons and that denser cold background electrons do not seem to contribute much to the peak intensity. This finding shows that upper-hybrid fluctuations detected during quiet time are not only useful for the determination of the background cold electron density but also contain information on the ambient hot electrons population as well.

It is well known that electromagnetic ion cyclotron (EMIC) waves play an important role in controlling particle dynamics inside the Earth's magnetosphere, especially in the outer radiation belt. In order to understand the results of wave‐particle interactions due to EMIC waves, it is important to know how the waves are distributed and what features they have. In this paper, we present some statistical analyses on the spatial distribution of EMIC waves in the low Earth orbit by using Swarm satellites from December 2013 to June 2017 (~3.5 years) as a function of magnetic local time, magnetic latitude, and magnetic longitude. We also study the wave characteristics such as ellipticity, wave normal angle, peak frequency, and wave power using our automatic wave detection algorithm based on the method of Bortnik et al. (2007, https://doi.org/10.1029/2006JA011900). We also investigate the geomagnetic control of the EMIC waves by comparing with geomagnetic activity represented by Kp and Dst indices. We find that EMIC waves are detected with a peak occurrence rate at midlatitude including subauroral region, dawn sector (3–7 magnetic local time), and linear polarization dominated with an oblique propagating direction to the background magnetic field. In addition, our result shows that the waves have some relation with geomagnetic activity; that is, they occur preferably during the geomagnetic storm's late recovery phase at low Earth orbit.

The electromagnetic proton firehose instability is driven by excessive parallel temperature anisotropy, T∥ > T⊥ (or more precisely, parallel pressure anisotropy, P∥ > P⊥) in high-beta plasmas. Together with kinetic instabilities driven by excessive perpendicular temperature anisotropy, namely, electromagnetic proton cyclotron and mirror instabilities, its role in providing the upper limit for the temperature anisotropy in the solar wind is well-known. A recent Letter [Seough et al., Phys. Rev. Lett. 110, 071103 (2013)] employed quasilinear kinetic theory for these instabilities to explain the observed temperature anisotropy upper bound in the solar wind. However, the validity of quasilinear approach has not been rigorously tested until recently. In a recent paper [Seough et al., Phys. Plasmas 21, 062118 (2014)], a comparative study is carried out for the first time in which quasilinear theory of proton cyclotron instability is tested against results obtained from the particle-in-cell simulation method, and it was demonstrated that the agreement was rather excellent. The present paper addresses the same issue involving the proton firehose instability. Unlike the proton cyclotron instability, however, it is found that the quasilinear approximation enjoys only a limited range of validity, especially for the wave dynamics and for the relatively high-beta regime. Possible causes and mechanisms responsible for the discrepancies are speculated and discussed.

Enhanced whistler instability produced by suprathermal electrons upstream of the earth's bow shock AIP Conf.The electromagnetic ion (proton) cyclotron instability is important for regulating the excessive development of perpendicular temperature anisotropy in the solar wind, for instance, when it is compressed in the vicinity of the Earth's magnetosheath environment. A recent letter [Seough et al., Phys. Rev. Lett. 110, 071103 (2013)] successfully employed the quasilinear kinetic theory to explain the observed temperature anisotropy upper bound. The present paper rigorously examines the reliability of the quasilinear theory by making a direct comparison against results from the particle-in-cell simulation method. It is found that the quasilinear approach is indeed a valid firstcut theoretical tool in the study of proton cyclotron instability. V C 2014 AIP Publishing LLC.

Physics of electromagnetic ion cyclotron (EMIC) waves is complicated by inclusion of heavy ions. In particular, He+ ions in the magnetosphere have long been considered to play important roles. Motivated by recent observations, we examine the effect of the inclusion of hot anisotropic He+ ions in addition to the usual hot anisotropic protons. We solve the kinetic dispersion relation for this examination and find the following results. First, inclusion of hot anisotropic He+ ions leads to the growth of EMIC waves at frequencies below the He+ gyrofrequency (He band) and a reduction of the EMIC wave growth rates (or damping of the waves) at frequencies between the proton and He+ gyrofrequencies (H band). Second, this effect is more dramatic for higher temperatures of He+ that would play a role in damping EMIC waves for both frequency bands and especially for cases without a He+ temperature anisotropy. Lastly, the effect is more prominent for cold plasma dominant conditions such as the region inside the plasmasphere or plume than for hot proton dominant conditions such as the region outside the plasmasphere. We propose that this last effect can at least partially explain the satellite observations indicating the preferred (though not exclusive) occurrence of He band waves inside the plasmasphere for the times when hot anisotropic He+ ions are supplied from the plasma sheet and ring current.

Magnetospheric compression due to impact of enhanced solar wind dynamic pressure Pdyn has long been considered as one of the generation mechanisms of electromagnetic ion cyclotron (EMIC) waves. With the Van Allen Probe‐A observations, we identify three EMIC wave events that are triggered by Pdyn enhancements under prolonged northward interplanetary magnetic field (IMF) quiet time preconditions. They are in contrast to one another in a few aspects. Event 1 occurs in the middle of continuously increasing Pdyn while Van Allen Probe‐A is located outside the plasmapause at postmidnight and near the equator (magnetic latitude (MLAT) ~ −3°). Event 2 occurs by a sharp Pdyn pulse impact while Van Allen Probe‐A is located inside the plasmapause in the dawn sector and rather away from the equator (MLAT ~ 12°). Event 3 is characterized by amplification of a preexisting EMIC wave by a sharp Pdyn pulse impact while Van Allen Probe‐A is located outside the plasmapause at noon and rather away from the equator (MLAT ~ −15°). These three events represent various situations where EMIC waves can be triggered by Pdyn increases. Several common features are also found among the three events. (i) The strongest wave is found just above the He+ gyrofrequency. (ii) The waves are nearly linearly polarized with a rather oblique propagation direction (~28° to ~39° on average). (iii) The proton fluxes increase in immediate response to the Pdyn impact, most significantly in tens of keV energy, corresponding to the proton resonant energy. (iv) The temperature anisotropy with T⊥ > T|| is seen in the resonant energy for all the events, although its increase by the Pdyn impact is not necessarily always significant. The last two points (iii) and (iv) may imply that in addition to the temperature anisotropy, the increase of the resonant protons must have played a critical role in triggering the EMIC waves by the enhanced Pdyn impact.

Van Allen radiation belt is characterized by energetic electrons and ions trapped in the Earth's dipolar magnetic field lines and persisting for long periods. It is also permeated by high‐frequency electrostatic fluctuations whose peak intensity occurs near the upper hybrid frequency. Such a phenomenon can be understood in terms of spontaneous emission of electrostatic multiple harmonic electron cyclotron waves by thermal plasmas. In the literature, the upper hybrid fluctuations are used as a proxy for determining the electron number density, but they also contain important information concerning the energetic electrons in the radiation belt and possibly the ring current electrons. The companion paper analyzes sample quiet time events and demonstrates that the upper hybrid fluctuations are predominantly emitted by tenuous population of energetic electrons. The present paper supplements detailed formalism of spontaneous thermal emission of multiple‐harmonic cyclotron waves that include upper hybrid fluctuations.

Low Earth orbit satellites frequently encounter Pc1 pulsations, but most have been observed with limited latitudinal extent or short lifetime. In this study we analyze two large‐scale Pc1 pulsations (both latitudinally wide and long‐lasting) generated by ionospheric ducting effect using Swarm and ground magnetometers on 25 June and 3 September 2015. Swarm observed the 25 June pulsations on both dayside and nightside during the storm time substorm (a strong geomagnetic storm on 23 June with Dst =  − 204 nT). We found the Pc1 pulsations were pervasive in both magnetic local time sectors of dayside and nightside for at least 2 hr. Another large Pc1 pulsation on 3 September was observed during a nonstorm substorm period. We conclude that (1) ionospheric ducting can transmit Pc1 waves to a wide range of L shells, (2) geomagnetic storm is not a prerequisite for such large‐scale ducting, and (3) wave intensity can abruptly decrease across sharp gradients in the ionospheric plasma density.