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data.mat is PIC data for Figure 1, the rest is from the linear theory for Figure1 to Figure 6.

We report plasma wave observations of equatorial magnetosonic waves at integer harmonics of the local gyrofrequency of doubly-ionized helium (He++). The waves were observed by Van Allen Probe A on ...

Van Allen Probes observations are used to statistically investigate plasmaspheric hiss wave properties. This analysis shows that the wave normal direction of plasmaspheric hiss is predominantly field aligned at larger L shells, with a bimodal distribution, consisting of a near‐field aligned and a highly oblique component, becoming apparent at lower L shells. Investigation of this oblique population reveals that it is most prevalent at L < 3, frequencies with f/fce>0.01 (or f > 700 Hz), low geomagnetic activity levels, and between 1900 and 0900 magnetic local time. This structure is similar to that reported for oblique chorus waves in the equatorial region, perhaps suggesting a causal link between the two wave modes. Ray tracing results from HOTRAY confirm that it is feasible for these oblique chorus waves to be a source of the observed oblique plasmaspheric hiss population. The decrease in oblique plasmaspheric hiss occurrence rates during more elevated geomagnetic activity levels may be attributed to the increase in Landau resonant electrons causing oblique chorus waves to be more substantially damped outside of the plasmasphere. In turn, this restricts the amount of wave power that can access the plasmasphere and evolve into oblique plasmaspheric hiss. These results confirm that, despite the difference in location of this bimodal distribution compared to previous studies, a direct link between oblique equatorial chorus outside of the plasmasphere and oblique hiss at low L shells is plausible. As such, these results are in keeping with the existing theory of chorus as the source of plasmaspheric hiss.

Linear Vlasov theory and particle-in-cell (PIC) simulations for electromagnetic fluctuations in a homogeneous, magnetized, and collisionless plasma are used to investigate a fast magnetosonic wave event observed by the Van Allen Probes. The fluctuating magnetic field observed exhibits a series of spectral peaks at harmonics of the proton cyclotron frequency Omega p and has a dominant compressional component, which can be classified as fast magnetosonic waves. Furthermore, the simultaneously observed proton phase space density exhibits positive slopes in the perpendicular velocity space, partial derivative f(p)/partial derivative v(perpendicular to) > 0, which can be a source for these waves. Linear theory analyses and PIC simulations use plasma and field parameters measured in situ except that the modeled proton distribution is modified to have larger partial derivative f(p)/partial derivative v(perpendicular to) under the assumption that the observed distribution corresponds to a marginally stable state when the distribution has already been scattered by the excited waves. The results show that the positive slope is the source of the proton cyclotron harmonic waves at propagation quasi-perpendicular to the background magnetic field, and as a result of interactions with the excited waves the evolving proton distribution progresses approximately toward the observed distribution.

In this paper, we perform a 1-D particle-in-cell (PIC) simulation model consisting of three species, cold electrons, cold ions and energetic ion ring, to investigate spectral structures of magnetosonic waves excited by ring distribution protons in the Earth's magnetosphere, and dynamics of charged particles during the excitation of magnetosonic waves. As the wave normal angle decreases, the spectral range of excited magnetosonic waves becomes broader with upper frequency limit extending beyond the lower hybrid resonant frequency, and the discrete spectra tends to merge into a continuous one. This dependence on wave normal angle is consistent with the linear theory. The effects of magnetosonic waves on the background cold plasma populations also vary with wave normal angle. For exactly perpendicular magnetosonic waves (parallel wave number k||=0), there is no energization in the parallel direction for both background cold protons and electrons due to the negligible fluctuating electric field component in the parallel direction. In contrast, the perpendicular energization of background plasmas is rather significant, where cold protons follow unmagnetized motion while cold electrons follow drift motion due to wave electric fields. For magnetosonic waves with a finite k||, there exists a non-negligible parallel fluctuating electric field, leading to a significant and rapid energization in the parallel direction for cold electrons. These cold electrons can also be efficiently energized in the perpendicular direction due to the interaction with the magnetosonic wave fields in the perpendicular direction. However, cold protons can be only heated in the perpendicular direction, which is likely caused by the higher-order resonances with magnetosonic waves. The potential impacts of magnetosonic waves on the energization of the background cold plasmas in the Earth's inner magnetosphere are also discussed in this paper.

Based on the Van Allen Probe A observations from 1 October 2012 to 31 December 2014, we develop two empirical models to respectively describe the hiss wave normal angle (WNA) and amplitude variations in the Earth's plasmasphere for different substorm activities. The long-term observations indicate that the plamsaspheric hiss amplitudes on the dayside increase when substorm activity is enhanced (AE index increases), and the dayside hiss amplitudes are greater than the nightside. However, the propagation angles (WNAs) of hiss waves in most regions do not depend strongly on substorm activity, except for the intense substorm-induced increase in WNAs in the nightside low-L region. The propagation angles of plasmaspheric hiss increase with increasing magnetic latitude (MLAT) or decreasing radial distance (L value). The global hiss WNAs (the power-weighted averages in each grid) and amplitudes (medians) can be well reproduced by our empirical models.

Utilizing simultaneous twin Van Allen Probes observations of whistler mode waves at variable separations, we are able to distinguish the temporal variations from spatial variations, determine the coherence spatial scale, and suggest the possible mechanism of wave modulation. The two probes observed coherently modulated whistler mode waves simultaneously at an unexpectedly large distance up to ~4.3 RE over 3 h during a relatively quiet period. The modulation of 150–500 Hz plasmaspheric hiss was correlated with whistler mode waves measured outside the plasmasphere across 3 h in magnetic local time and 3 L shells, revealing that the modulation was temporal in nature. We suggest that the coherent modulation of whistler mode waves was associated with the coherent ULF waves measured over a large scale, which modulate the plasmaspheric density and result in the modulation of hiss waves via local amplification. In a later period, the 500–1500 Hz periodic rising-tone whistler mode waves were strongly correlated when the two probes traversed large spatial regions and even across the plasmapause. These periodic rising-tone emissions recurred with roughly the same period as the ULF wave, but there was no one-to-one correspondence, and a cross-correlation analysis suggests that they possibly originated from large L shells although the actual cause needs further investigation.

The dynamics of relativistic electrons traveling through a parallel-propagating, monochromatic electromagnetic ion cyclotron (EMIC) wave in the Earth's dipole field are investigated via test particle simulations. Both resonant and nonresonant responses in electron pitch angle are considered and the differences between the two are highlighted. Nonresonant electrons, with energies below the minimum resonant energy down to 100s keV, are scattered stochastically in pitch angle and can be scattered into the atmospheric loss cone. The nonresonant effect is attributed to the spatial edge associated with EMIC wave packets. A condition for effective nonresonant response is also provided. This effect is excluded from current quasi-linear theory, can be a potentially important loss mechanism of relativistic and subrelativistic electrons in the radiation belts.

Three mechanisms have been proposed to explain relativistic electron flux depletions (dropouts) in the Earth's outer radiation belt during storm times: adiabatic expansion of electron drift shells due to a decrease in magnetic field strength, magnetopause shadowing and subsequent outward radial diffusion, and precipitation into the atmosphere (driven by EMIC wave scattering). Which mechanism predominates in causing electron dropouts commonly observed in the outer radiation belt is still debatable. In the present study, we evaluate the physical mechanism that may be primarily responsible for causing the sudden change in relativistic electron pitch angle distributions during a dropout event observed by Van Allen Probes during the main phase of the 27 February 2014 storm. During this event, the phase space density of ultrarelativistic (>1 MeV) electrons was depleted by more than 1 order of magnitude over the entire radial extent of the outer radiation belt (3

Electromagnetic ion cyclotron (EMIC) waves have been proposed to cause efficient losses of highly relativistic (>1 MeV) electrons via gyroresonant interactions. Simultaneous observations of EMIC waves and equatorial electron pitch angle distributions, which can be used to directly quantify the EMIC wave scattering effect, are still very limited, however. In the present study, we evaluate the effect of EMIC waves on pitch angle scattering of ultrarelativistic (>1 MeV) electrons during the main phase of a geomagnetic storm, when intense EMIC wave activity was observed in situ (in the plasma plume region with high plasma density) on both Van Allen Probes. EMIC waves captured by Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes and on the ground across the Canadian Array for Real-time Investigations of Magnetic Activity (CARISMA) are also used to infer their magnetic local time (MLT) coverage. From the observed EMIC wave spectra and local plasma parameters, we compute wave diffusion rates and model the evolution of electron pitch angle distributions. By comparing model results with local observations of pitch angle distributions, we show direct, quantitative evidence of EMIC wave-driven relativistic electron losses in the Earth's outer radiation belt.

Unipolar pulses of kinetic Alfven waves (KAW) are first observed in the near-Earth plasma sheet (NEPS) associated with dipolarizations during substorm expansion phases. Two similar events are studied with Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations during substorms on 3 February 2008 and 7 February 2008. The unipolar pulses were located at a trough-like Alfven speed profile in the northern plasma sheet at a distance of 10–11 R E from Earth. The dominant wave components consist of a southward δ E z toward the neutral plane and a +δ B y toward the dusk. The |δ E z |/|δ B y | ratio was in the range of a few times the local Alfven speed, a strong indication of KAW nature. The wave Poynting flux was earthward and nearly parallel to the background magnetic field. The pulse was associated with an earthward field-aligned current carried by electrons. These observational facts strongly indicate a KAW eigenmode that is confined by the plasma sheet but propagates earthward along the field line. The KAW eigenmode was accompanied by short timescale (1 min) dipolarizations likely generated by transient magnetotail reconnection. The observed polarity of the KAW field/current is consistent with that of the Hall field/current in magnetic reconnection, supporting the scenario that the Hall fields/current propagate out from reconnection site as KAW eigenmodes. Aurora images on the footprint of THEMIS spacecraft suggest that KAW eigenmode may power aurora brightening during substorm expansion phase.

Magnetosonic waves, also known as equatorial noise emission, were found to have discrete frequency structures, which is consistent with instability caused by proton ring distribution. Nonetheless, nondiscrete structure, i.e., a broadband spectrum over a continuous frequency range, has been reported. We investigate the question whether proton ring distribution can generate nondiscrete spectra for perpendicularly propagating magnetosonic waves. We propose discrete and nondiscrete characteristics of the local instability for explaining the observation of discrete, continuous, and mixed spectra. The criterion for transition from discrete and continuous instability is given, γ >∼ Ωh/2, where γ is wave growth rate and Ωh is proton cyclotron frequency. The condition is verified by particle-in-cell simulation using more realistic electron-to-proton mass ratio and speed of light than in previous studies. Such criterion of generating a continuous spectrum can be tested against simultaneous in situ measurement of wave and particle. We also find that the modes at low Ωh harmonics, including the fundamental Ωh, can be still excited through nonlinear wave-wave coupling, even when they are neutral modes (γ = 0) according to the linear kinetic theory. Comparison with magnetosonic waves in cold plasma limit and electromagnetic ion Bernstein mode is also discussed.