Quantification of Energetic Electron Precipitation Driven by Plume Whistler Mode Waves, Plasmaspheric Hiss, and Exohiss
Wen LiXiaochen ShenQianli MaLuisa CapannoloRun ShiR. J. RedmonJ. V. RodriguezG. D. ReevesC. KletzingW. S. KŭrthG. B. Hospodarsky
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Abstract Whistler mode waves are important for precipitating energetic electrons into Earth's upper atmosphere, while the quantitative effect of each type of whistler mode wave on electron precipitation is not well understood. In this letter, we evaluate energetic electron precipitation driven by three types of whistler mode waves: plume whistler mode waves, plasmaspheric hiss, and exohiss observed outside the plasmapause. By quantitatively analyzing three conjunction events between Van Allen Probes and POES/MetOp satellites, together with quasi‐linear calculation, we found that plume whistler mode waves are most effective in pitch angle scattering loss, particularly for the electrons from tens to hundreds of keV. Our new finding provides the first direct evidence of effective pitch angle scattering driven by plume whistler mode waves and is critical for understanding energetic electron loss process in the inner magnetosphere. We suggest the effect of plume whistler mode waves be accurately incorporated into future radiation belt modeling.Keywords:
Hiss
Whistler
Plasmasphere
Electron precipitation
Pitch angle
Van Allen Probes
A long-standing problem in the field of space physics has been the origin of plasmaspheric hiss, a naturally occurring electromagnetic wave in the high-density plasmasphere (roughly within 20,000 kilometers of Earth) that is known to remove the high-energy Van Allen Belt electrons that pose a threat to satellites and astronauts. A recent theory tied the origin of plasmaspheric hiss to a seemingly different wave in the outer magnetosphere, but this theory was difficult to test because of a challenging set of observational requirements. Here we report on the experimental verification of the theory, made with a five-satellite NASA mission. This confirmation will allow modeling of plasmaspheric hiss and its effects on the high-energy radiation environment.
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Abstract We report a rare event of intense plasmaspheric hiss and chorus waves simultaneously observed at the same L shell but different magnetic local times by Van Allen Probes and Magnetospheric Multiscale. Based on the measured waves and electron distributions, we calculate the bounce‐averaged diffusion coefficients and subsequently simulate the temporal evolution of electron distributions. The simulations show that the dynamics of tens to hundreds of keV electrons are jointly controlled by hiss and chorus. The dynamics of MeV electrons are dominantly controlled by hiss near the loss cone but by chorus at intermediate to large pitch angles. The simulated electron distributions driven by combined diffusion can reproduce the majority of the observations. Our results provide a direct observational evidence that hiss and chorus can simultaneously occur at the same electron drifting shells due to the irregular plasmasphere and highlight the importance of their combined effect on electron dynamics.
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Chorus
Van Allen Probes
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Pitch angle
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Abstract. Plasmaspheric hiss was observed by Van Allen Probe B in association with energetic electron injections in the outer plasmasphere. The energy of injected electrons coincides with the minimum resonant energy calculated for the observed hiss wave frequency. Interestingly, the variations in hiss wave intensity, electron flux and ultra low frequency (ULF) wave intensity exhibit remarkable correlations, while plasma density is not correlated with any of these parameters. Our study provides direct evidence for the first time that the injected anisotropic electron population, which is modulated by ULF waves, modulates the hiss intensity in the outer plasmasphere. This also implies that the plasmaspheric hiss observed by Van Allen Probe B in the outer plasmasphere (L > ∼ 5.5) is locally amplified. Meanwhile, Van Allen Probe A observed hiss emission at lower L shells (< 5), which was not associated with electron injections but primarily modulated by the plasma density. The features observed by Van Allen Probe A suggest that the observed hiss deep inside the plasmasphere may have propagated from higher L shells.
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Abstract Plasmaspheric hiss is an electromagnetic wave mode that occurs ubiquitously in the high‐density plasmasphere and contributes crucially to the dynamic behavior of the Earth's Van Allen radiation belts. While plasmaspheric hiss is commonly considered to be a broadband emission with frequencies from ∼100 Hz to several kHz, here we report Van Allen Probes measurements of unambiguous banded signatures of plasmaspheric hiss, uniquely characterized by an upper band above ∼200 Hz, a lower band below ∼100 Hz and a power gap in between. Banded plasmaspheric hiss occurs with the probability ∼8% in the postnoon sector within 2.5–5.0 Earth radii, showing strong dependence on geomagnetic and solar wind conditions. Observations also suggest that banded hiss waves result possibly from two combined sources, the upper band originating from the transformation of chorus waves propagating from outside the plasmasphere, and the lower band from localized excitation inside the plasmasphere, which however requires future investigation. The banded hiss waves shed new light on the evolution of the Earth's radiation belts and have implications for understanding whistler‐mode waves in planetary magnetospheres.
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Abstract We present a global survey of energetic electron precipitation from the equatorial magnetosphere due to hiss waves in the plasmasphere and plumes. Using Van Allen Probes measurements, we calculate the pitch angle diffusion coefficients at the bounce loss cone, and evaluate the energy spectrum of precipitating electron flux. Our ∼6.5‐year survey shows that, during disturbed times, hiss inside the plasmasphere primarily causes the electron precipitation at L > 4 over 8 h < MLT < 18 h, and hiss waves in plumes cause the precipitation at L > 5 over 8 h < MLT < 14 h and L > 4 over 14 h < MLT < 20 h. The precipitating energy flux increases with increasing geomagnetic activity, and is typically higher in the plasmaspheric plume than the plasmasphere. The characteristic energy of precipitation increases from ∼20 keV at L = 6–∼100 keV at L = 3, potentially causing the loss of electrons at several hundred keV.
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Abstract The two classes of whistler mode waves (chorus and hiss) play different roles in the dynamics of radiation belt energetic electrons. Chorus can efficiently accelerate energetic electrons, and hiss is responsible for the loss of energetic electrons. Previous studies have proposed that chorus is the source of plasmaspheric hiss, but this still requires an observational confirmation because the previously observed chorus and hiss emissions were not in the same frequency range in the same time. Here we report simultaneous observations form Van Allen Probes that chorus and hiss emissions occurred in the same range ∼300–1500 Hz with the peak wave power density about 10 −5 nT 2 /Hz during a weak storm on 3 July 2014. Chorus emissions propagate in a broad region outside the plasmapause. Meanwhile, hiss emissions are confined inside the plasmasphere, with a higher intensity and a broader area at a lower frequency. A sum of bi‐Maxwellian distribution is used to model the observed anisotropic electron distributions and to evaluate the instability of waves. A three‐dimensional ray tracing simulation shows that a portion of chorus emission outside the plasmasphere can propagate into the plasmasphere and evolve into plasmaspheric hiss. Moreover, hiss waves below 1 kHz are more intense and propagate over a broader area than those above 1 kHz, consistent with the observation. The current results can explain distributions of the observed hiss emission and provide a further support for the mechanism of evolution of chorus into hiss emissions.
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Abstract 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 L pp =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.
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Whistler
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