Whistler‐mode extremely low frequency hiss emissions commonly exist in the plasmasphere and the plasmaspheric plume and contribute to the precipitation loss of the radiation belt electrons. How these ...hiss waves are generated remains a critical unanswered question. Here we report the large‐amplitude (up to 1.5 nT) hiss waves in the plasmaspheric plumes, nearly an order of magnitude stronger than previous observations. These waves are found to propagate toward higher latitudes, and the corresponding frequency dependence of wave power can be qualitatively (but not quantitatively) explained by the modeled linear instability of hot electrons near the equator. At the high‐frequency end of hiss spectra, the discrete rising tones are shown to emerge, similar to the situation of whistler‐mode chorus in the plasmatrough. These data and modeling suggest that these large‐amplitude hiss waves were generated within the plasmaspheric plume probably through a combination of linear and nonlinear instabilities of hot electrons.
Key Points
The large‐amplitude (up to 1.5 nT) hiss propagated toward higher latitudes in the plasmaspheric plumes
The hiss wave power showed a similar frequency dependence to the modeled linear instability of hot electrons
The hiss frequency‐time spectrum consisted of a noisy band and a series of triggered rising tones
Magnetosonic waves inside and outside the plasmasphere differ statistically in occurrence rate, frequency, and intensity. How the density interface separates magnetosonic waves inside and outside the ...plasmasphere remains not fully understood. Here we report an experimental test made with the Van Allen Probes mission from the plasmaspheric plume through the low‐density channel to the plasmaspheric core. Our linear instability analysis and two‐dimensional full‐wave modeling support that the magnetosonic waves propagate from elsewhere to the channel, undergo reflection and transmission at the flanking plasmaspheric density interfaces and eventually exhibit drastic differences in intensity and frequency coverage between neighboring regions. Such a mesoscale (tens of wavelength wide) interface with a strong refractive index gradient allows the transformation of incident waves to surface waves and consequently filters waves in both frequency and orientation. This unexpected filtering pattern could commonly occur at the plasmaspheric boundary and eventually affect the global distribution of magnetosonic waves.
Plain Language Summary
Magnetosonic waves play an important role in the coupling between the ionosphere and magnetosphere and the evolution of the magnetospheric radiation environment. Previous global surveys have shown that magnetosonic waves inside and outside the plasmasphere differ significantly in occurrence rate, frequency, and intensity. In fact, magnetosonic waves are not confined near the source region but are able to propagate over a broad range of radial distances and magnetic local times. An obvious question arises as to how the density interface separates magnetosonic waves inside and outside the plasmasphere. Previous modeling and analysis suggested that the mesoscale (tens of wavelengths) density interface allowed the free penetration of magnetosonic waves from outside to inside the plasmasphere. In contrast, our data and modeling here show that such a mesoscale interface with a strong refractive index gradient allows the transformation of incident waves to surface waves and consequently filters waves in both frequency and incident angle. This unexpected filtering pattern could commonly occur at the plasmaspheric boundary and eventually affect the global distribution of magnetosonic waves.
Key Points
A tens of wavelength wide plasmaspheric density interface separated the magnetosonic waves with differing intensity and frequency coverage
The local proton Bernstein instabilities are unable to explain the drastic differences between magnetosonic waves in neighboring regions
The mesoscale plasmaspheric density interfaces can filter the inward penetrating magnetosonic waves in both frequency and incident angle
In this study, we develop a new code which uses a hybrid finite difference (HFD) method to solve a 2‐D bounce‐averaged energy–pitch angle diffusion equation in the outer radiation belts. We implement ...the numerical algorithm by adopting a split operator technique, an implicit scheme for dealing with diagonal diffusion coefficients, and an alternative direction implicit scheme for off‐diagonal (or cross) diffusion coefficients. We show that our code HFD can successfully overcome the unstable problem when the large and rapidly varying cross diffusion coefficients are included. Particularly, we examine whether resonant interactions with obliquely propagated whistler mode chorus and hiss waves result in a net acceleration or loss of relativistic electrons in the outer radiation belt L = 4.5. Numerical simulations show that chorus waves can yield significant acceleration of relativistic electrons, while hiss waves are primarily responsible for scattering equatorially mirroring electrons toward the loss cone.
Plasmaspheric hiss plays a key role in shaping the radiation belt environment, whose origin remains under active debate. Using the wave and particle data of Van Allen Probes, Geostationary ...Operational Environmental Satellites, and Time History of Events and Macroscale Interactions during Substorm spacecraft, we here examine the nightside plasmaspheric hiss generation during a substorm. The substorm‐electron injection caused the plasmapause to shrink promptly from Lpp=6.6 to 5.1. Corresponding to the azimuthal drift of the injected electrons, the plasmaspheric hiss was intensified gradually from nightside to dayside. Particularly, in the inner postmidnight plasmasphere free from the substorm injection, the instantaneous peak amplitude of hiss reached 0.9 nT. The enhanced hiss within the locally unchanged plasma must originate from other spatial regions. Our data and modeling demonstrate that the large‐amplitude hiss was generated by the substorm‐injected electrons drifting into the outer postmidnight plasmasphere, rather than linked to the nightside chorus suffering strong Landau damping or the dayside chorus/hiss propagating azimuthally to the nightside plasmasphere.
Plain Language Summary
Plasmaspheric hiss, a naturally occurring electromagnetic emission (0.1–2.0 kHz) in the dense cold plasma surrounding the Earth, can precipitate energetic electrons from the Van Allen radiation belts into the atmosphere. Since its discovery in 1969, the origin of plasmaspheric hiss has remained a puzzle to be solved. We here use the data of seven magnetospheric spacecraft to investigate the nightside plasmaspheric hiss generation during a substorm. Corresponding to the azimuthal drift of substorm‐injection front, the plasmaspheric hiss is found to be intensified gradually from nightside to dayside. In the inner nightside plasmasphere free from the substorm injection, the instantaneous peak amplitude of hiss is shown to increase from less than 40 pT to 0.9 nT. Within the locally unchanged plasma, the substorm‐enhanced hiss must originate from other spatial regions. Our data and modeling support the previously proposed hypothesis that the nightside hiss is excited by the hot electrons in the outer plasmasphere and then propagates to the inner plasmasphere. This experimental confirmation will allow further developments in modeling and forecasting of the plasmaspheric hiss spatiotemporal distribution and the Earth's radiation belt behavior.
Key Points
The substorm‐electron injection peeled off the nightside plasmasphere and excited the chorus outside the plasmapause
The azimuthally drifting hot electrons intensified the plasmaspheric hiss gradually from nightside to dayside
The nightside large‐amplitude hiss might be generated by the hot electrons drifting into the outer plasmasphere
Using our recently introduced hybrid finite difference method, we develop a three‐dimensional (3‐D) code to solve a fully bounce‐averaged pitch‐angle and energy diffusion equation, including radial ...diffusion and particularly the cross‐diffusion terms. We show that our 3‐D code can successfully prevent the unstable numerical problems resulting from the large and rapidly varying cross diffusion coefficients. We present one of the first simulations to examine the effects of radial diffusion and chorus‐electron interaction with/without cross diffusion terms on the radiation belt electron dynamics. Simulated results demonstrate that chorus waves may yield significant accelerations of energetic (∼MeV) electrons, leading to peaks in phase space density (PSD), which are subsequently smoothed by inward and outward radial diffusion. Moreover, neglecting cross‐diffusion rates generally produces relatively large overestimates in the PSD evolution, implying that cross diffusion terms are very critical in the 3‐D modeling of radiation belts dynamics. However, test case simulations show that such overestimates are sensitively dependent on the initial conditions together with wave models, deserving further thorough investigations.
Magnetosonic waves are highly oblique whistler mode emissions transferring energy from the ring current protons to the radiation belt electrons in the inner magnetosphere. Here we present the first ...report of prompt disappearance and emergence of magnetosonic waves induced by the solar wind dynamic pressure variations. The solar wind dynamic pressure reduction caused the magnetosphere expansion, adiabatically decelerated the ring current protons for the Bernstein mode instability, and produced the prompt disappearance of magnetosonic waves. On the contrary, because of the adiabatic acceleration of the ring current protons by the solar wind dynamic pressure enhancement, magnetosonic waves emerged suddenly. In the absence of impulsive injections of hot protons, magnetosonic waves were observable even only during the time period with the enhanced solar wind dynamic pressure. Our results demonstrate that the solar wind dynamic pressure is an essential parameter for modeling of magnetosonic waves and their effect on the radiation belt electrons.
Key Points
A sudden decrease in the solar wind dynamic pressure caused the prompt disappearance of magnetosonic waves
A sudden increase in the solar wind dynamic pressure caused the prompt emergence of magnetosonic waves
The adiabatic deceleration or acceleration of the ring current protons explained the evolution of magnetosonic waves
Magnetosonic waves play a potentially important role in the complex evolution of the radiation belt electrons. These waves typically appear as discrete emission lines along the proton gyrofrequency ...harmonics, consistent with the prediction of the local Bernstein mode instability of hot proton ring distributions. Magnetosonic waves are nearly dispersionless particularly at low harmonics and therefore have the roughly unchanged frequency time structures during the propagation. On the basis of Van Allen Probes observations, we here present the first report of magnetosonic harmonic falling and rising frequency emissions. They lasted for up to 2 hr and occurred primarily in the dayside plasmatrough following intense substorms. These harmonic emission lines were well spaced by the proton gyrofrequency but exhibited a clear falling (rising) frequency characteristic in a regime with the temporal increase (decrease) of the proton gyrofrequency harmonics. Such unexpected structures might be produced by the nonlinear interactions between the locally generated magnetosonic waves at the proton gyrofrequency harmonics and a constant frequency magnetosonic wave propagating away from the Earth.
Plain Language Summary
Magnetosonic waves confined near the magnetospheric equator can scatter and accelerate the energetic electrons in the Van Allen radiation belts. Their precise generation and propagation processes remain the subjects of ongoing research and controversy. Here we report two new types of magnetosonic frequency‐time structures: harmonic falling and rising frequency emissions. Available data and numerical estimations tend to support that these unusual structures were produced by the nonlinear interactions between the locally generated magnetosonic waves at the proton gyrofrequency harmonics and a constant frequency magnetosonic wave propagating away from the Earth. These unexpected frequency‐time structures reported here bring new insights on the wave generation and propagation, which will be of high interest to the radiation belt and space plasma physics communities.
Key Points
Magnetosonic harmonic falling/rising frequency emissions were spaced by the local proton gyrofrequency
Discrete magnetosonic emission lines crossed the proton gyrofrequency harmonics over a long period (up to 2 hr)
Magnetosonic harmonic falling/rising frequency emissions primarily occurred in the dayside plasmatrough following intense substorms
The adiabatic transport process is introduced into our recently developed three‐dimensional physics‐based electron radiation belt model (STEERB, Storm‐Time Evolution of Electron Radiation Belt) via ...adopting a time‐varying Hilmer‐Voigt geomagnetic field. The current STEERB model contains more complete physical processes: adiabatic transport, radial diffusion, and various in situ wave‐particle interactions. In particular, the influence of adiabatic transport on storm time radiation belt electron dynamics is investigated by some idealized simulations. It is found that the adiabatic transport alone (without plume hiss and electromagnetic ion cyclotron (EMIC) waves) is unable to reproduce the observed main phase loss of energetic outer radiation belt electron fluxes in the presence of a strong chorus‐driven acceleration process. However, these adiabatic and nonadiabatic processes for radiation belt electron dynamics are coupled to each other. The adiabatic transport, together with radial diffusion and cyclotron resonant interactions with chorus, plume hiss, and EMIC waves, contributes significantly to the main phase loss and the recovery phase enhancement of energetic electron fluxes. In the absence of adiabatic transport, the energetic outer radiation belt electron fluxes are found to be overestimated by a factor of 5–30 over all the pitch angles during the main phase and to be underestimated by a factor of 2–5 at larger pitch angles (αe > 50°) during the recovery phase. These numerical results suggest that the adiabatic transport in a time‐varying geomagnetic field model should be incorporated into the future radiation belt models for space weather application.
Key Points
We introduce the adiabatic transport into the radiation belt model (STEERB)
Influence of adiabatic transport on radiation belt dynamics is investigated
Adiabatic process contributes obviously to energetic electron dynamics
We present a three‐dimensional code (storm‐time evolution of electron radiation belt, STEERB) for the storm‐time evolution of electron radiation belt following the preceding work. STEERB code is ...based on the solution of three‐dimensional modified Fokker‐Planck equation, which covers the inner and outer radiation belts with incorporation of the Coulomb collisions, radial diffusion due to magnetic and electric field perturbations, and local pitch angle, energy, and cross‐pitch angle‐energy diffusion due to various wave‐particle interactions. It is implemented by a split operator technique, in conjunction with the recently developed hybrid finite difference method for local wave‐particle interaction, and the fully implicit finite difference method for radial diffusion as well as Coulomb collisions. The resulting numerical model is robust, efficient, and easily parallelizable. Some of the dominant characteristics of electron radiation belt during both quiet and active periods can be well reproduced by STEERB code. Test simulations are performed to evaluate the respective roles of radial diffusion, cyclotron resonant interaction with chorus and plume waves in the global radiation belt dynamics, the sensitivity of numerical results to the uncertainty of wave spectrum, and the importance of cross‐pitch angle‐energy diffusion in the three‐dimensional simulations.
Suprathermal electrons are a major heat source of ionospheric plasma. How the suprathermal electrons evolve during their bounces inside the plasmasphere is a fundamental question for the ...magnetosphere‐ionosphere coupling. On the basis of Van Allen Probes observations and quasi‐linear simulations, we present here the first quantitative study on the evolution of suprathermal electrons under the competition between Landau heating by whistler mode hiss waves and Coulomb collisional cooling by background plasma inside a plasmaspheric plume. We show that the Landau heating can prevail over the collisional cooling for >50 eV electrons and cause the field‐aligned suprathermal electron fluxes to increase by up to 1 order of magnitude within 1.5 hr. Our results imply that the plasmaspheric plume hiss waves could mediate energy from the ring current electrons to the ionospheric plasma.
Plain Language Summary
The ionospheric plasma temperature variation can affect the Earth's atmospheric escape and the spacecraft orbit decay. A major heat source of the ionospheric plasma is the suprathermal (several eV to hundreds of eV) electrons, which can bounce along the magnetic field lines inside the plasmasphere. How these suprathermal electrons evolve during their bounces is a fundamental question for the magnetosphere‐ionosphere coupling. In the past, the plasmaspheric suprathermal electrons were usually considered to gain energy from the ring current ions through Coulomb collisions and wave‐particle interactions. Here we show that strong whistler mode hiss waves can grow from the instability of the ring current electrons in the plasmaspheric drainage plume, and their Landau heating can prevail over the collisional cooling by background plasma for >50 eV electrons. The enhanced field‐aligned suprathermal electrons could eventually heat the ionospheric plasma. These results have significant implications for understanding the energy transfer process from the magnetosphere to the ionosphere.
Key Points
The collisional cooling by background plasma counteracts largely the Landau heating by hiss waves for <50 eV electrons
The Landau heating can increase the field‐aligned fluxes of >50 eV electrons by 1 order of magnitude within 1.5 hr
Plasmaspheric plume hiss could mediate energy from the ring current electrons through the suprathermal electrons to the ionospheric plasma
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