A Parametric Study of Oxygen Ion Cyclotron Harmonic Wave Excitation and Polarization by an Oxygen Ring Distribution

Oxygen ion cyclotron harmonic (OCH) waves are electromagnetic emissions with frequencies near the harmonics of the oxygen ion cyclotron frequency. They are ubiquitously observed in the Earth’s magnetosphere. These waves can be excited by an energetic O+ ring distribution. Here, we perform a parametric study of OCH waves by an O+ ring distribution. We investigate the effects of ring concentration (ηho), velocity (vr), temperature (Tr), total O+ concentration (ηo), and wave normal angles (WNAs) on the wave growth rate and polarization. We find that four-wave modes are related to OCH waves. The growth rates and frequency range increase with ηho and ηo and decrease with Tr. The peak growth rate roughly follows the first peak of Jn2 (square of the Bessel function corresponding to the O+ ring) or cold plasma wave modes, which can be used to explain the vr and WNA dependences. OCH waves shift from the transverse mode to the compressional mode as vr increases. This work used TACC to perform particle-in-cell (PIC) simulation (part of Figure 1 of this work) and was published in Journal of Geophysical Research-Space Physics.Texas Advanced Computing Center (TACC

Statistical characteristics of ionospheric hiss waves

In this study, we use the observations of electromagnetic waves by DEMETER satellite to investigate propagation characteristics of low altitude ionospheric hiss. In an event study, intense hiss wave power is concentrated over a narrow frequency band with a central frequency that decreases as latitude decreases, which coincides to the variation of local proton cyclotron frequency fCH. The wave propagates obliquely to the background magnetic field and equatorward from high latitude region. We use about 6 years' observations to statistically study the dependence of ionospheric hiss wave power on location, local time, geomagnetic activity and season. The results demonstrate that the ionospheric hiss power is stronger on the dayside, under higher geomagnetic activity, in local summer and confined near the region where the local fCH is equal to the wave frequency. To explain the concentration of wave power, a ray tracing simulation is performed and reproduced the wave propagation process

Particle‐in‐cell simulation of electron cyclotron harmonic waves driven by a loss cone distribution

Electron Cyclotron Harmonic (ECH) waves driven by a loss cone distribution are studied in this work by self‐consistent particle‐in‐cell simulations. These waves have been suggested to play an important role in diffuse auroral precipitation in the outer magnetosphere. However, particle simulation of this instability is difficult because the saturation amplitude of the wave driven by a realistic size loss cone distribution is very small. In this work we use an extraordinarily large number of particles to reduce simulation noise so that the growth and saturation of ECH waves can be investigated. Our simulation results are consistent with linear theory in terms of growth rate, and with observation in terms of wave amplitude. We demonstrate that the heating of cold electrons is negligible and non‐resonant, different from previous conclusions, and suggest that the saturation of the wave is caused by the filling of the loss cone of hot electrons

Frequency-dependent modulation of whistler-mode waves by density irregularities during the recovery phase of a geomagnetic storm

Density irregularities near the plasmapause are commonly observed and play an important role in whistler-mode wave excitation and propagation. In this study, we report a frequency-dependent modulation event of whistler-mode waves by background density irregularities during a geomagnetic storm. Higher-frequency whistler waves (near 0.5 fce, where fce is the equatorial electron cyclotron frequency) are trapped in the density trough regions due to the small refractive index near the parallel direction, while lower-frequency whistler waves (below 0.02 fce) are trapped in the density crest regions due to the refractive index maximum along the parallel direction. In addition to the modulation, we also find that, quantitatively, the wave amplitude of the higher- (lower-) frequency whistler-mode waves is anti-correlated (correlated) with the relative plasma density variation. Our study suggests the importance of density irregularity dynamics in controlling whistler-mode wave intensity, and thus radiation belt dynamics

Direct evidence reveals transmitter signal propagation in the magnetosphere

Signals from very-low-frequency transmitters on the ground are known to induce energetic electron precipitation from the Earth's radiation belts. The effectiveness of this mechanism depends on the propagation characteristics of those signals in the magnetosphere, and in particular whether the signals are ducted or nonducted along channels of enhanced plasma density, analogous to optical fibers. Here we perform a statistical analysis of in-situ waveform data collected by the Van Allen Probes satellites that shows that nonducted propagation dominates over ducted propagation in both the occurrence and intensity of the waves. Ray tracing confirms that the latitudinal distribution of wavevectors corresponds to nonducted as opposed to ducted propagation. Our results show the dominant mode of propagation needed to quantify transmitter-induced precipitation and improve the forecast of electron radiation belt dynamics for the safe operation of satellites

Modulation of chorus intensity by ULF waves deep in the inner magnetosphere

Previous studies have shown that chorus wave intensity can be modulated by Pc4-Pc5 compressional ULF waves. In this study, we present Van Allen Probes observation of ULF wave modulating chorus wave intensity, which occurred deep in the magnetosphere. The ULF wave shows fundamental poloidal mode signature and mirror mode compressional nature. The observed ULF wave can modulate not only the chorus wave intensity but also the distribution of both protons and electrons. Linear growth rate analysis shows consistence with observed chorus intensity variation at low frequency (f <∼ 0.3fce), but cannot account for the observed higher-frequency chorus waves, including the upper band chorus waves. This suggests the chorus waves at higher-frequency ranges require nonlinear mechanisms. In addition, we use combined observations of Radiation Belt Storm Probes (RBSP) A and B to verify that the ULF wave event is spatially local and does not last long

Resonant scattering of energetic electrons by unusual low-frequency hiss

Abstract We quantify the resonant scattering effects of the unusual low-frequency dawnside plasmaspheric hiss observed on 30 September 2012 by the Van Allen Probes. In contrast to normal (∼100-2000 Hz) hiss emissions, this unusual hiss event contained most of its wave power at ∼20-200 Hz. Compared to the scattering by normal hiss, the unusual hiss scattering speeds up the loss of ∼50-200 keV electrons and produces more pronounced pancake distributions of ∼50-100 keV electrons. It is demonstrated that such unusual low-frequency hiss, even with a duration of a couple of hours, plays a particularly important role in the decay and loss process of energetic electrons, resulting in shorter electron lifetimes for ∼50-400 keV electrons than normal hiss, and should be carefully incorporated into global modeling of radiation belt electron dynamics during periods of intense injections

Source of the low-altitude hiss in the ionosphere

We analyze the propagation properties of low-altitude hiss emission in the ionosphere observed by DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions). There exist two types of low-altitude hiss: type I emission at high latitude is characterized by vertically downward propagation and broadband spectra, while type II emission at low latitude is featured with equatorward propagation and a narrower frequency band above ∼fcH+. Our ray tracing simulation demonstrates that both types of the low-altitude hiss at different latitude are connected and they originate from plasmaspheric hiss and in part chorus emission. Type I emission represents magnetospheric whistler emission that accesses the ionosphere. Equatorward propagation associated with type II emission is a consequence of wave trapping mechanisms in the ionosphere. Two different wave trapping mechanisms are identified to explain the equatorial propagation of Type II emission; one is associated with the proximity of wave frequency and local proton cyclotron frequency, while the other occurs near the ionospheric density peak