Abstract Local acceleration driven by whistler mode chorus waves largely accounts for the enhancement of radiation belt relativistic electron fluxes, whose favored region is usually considered to be the plasmatrough with magnetic local time approximately from midnight through dawn to noon. On 2 October 2013, the Van Allen Probes recorded a rarely reported event of intense duskside lower band chorus waves (with power spectral density up to 10 −3 nT 2 /Hz) in the low‐latitude region outside of L =5. Such chorus waves are found to be generated by the substorm‐injected anisotropic suprathermal electrons and have a potentially strong acceleration effect on the radiation belt energetic electrons. This event study demonstrates the possibility of broader spatial regions with effective electron acceleration by chorus waves than previously expected. For such intense duskside chorus waves, the occurrence probability, the preferential excitation conditions, the time duration, and the accurate contribution to the long‐term evolution of radiation belt electron fluxes may need further investigations in future.
The database of ECH waves based on observations from Van Allen Probes between September 17, 2012 and October 13, 2019. The probe and the year of observations are marked in the data file names. The columns from left to right are: the mean value of the SuperMAG electrojet index in the previous hour, L-Shell, the magnetic local time, the magnetic latitude, the wave amplitude of band 1to band 4.
We report correlated observation of enhanced electromagnetic ion cyclotron (EMIC) waves and dynamic evolution of ring current proton flux collected by Cluster satellite near the location L = 4.5 during March 26-27, 2003, a nonstorm period (D-st > -10). Energetic (5-30 keV) proton fluxes are found to drop rapidly (e.g., a half hour) at lower pitch angles, corresponding to intensified EMIC wave activities. By adopting a Gaussian fit to the observed spectra of EMIC waves, we present two-dimensional (2D) numerical simulations which demonstrate that EMIC wave can yield such decrements in proton flux within 30 minutes, consistent with the observational data. The current result provides a further understanding of ring current dynamics driven by wave-particle interaction under different geomagnetic activities.
Abstract Electromagnetic whistler‐mode chorus and electrostatic electron cyclotron harmonic (ECH) waves can contribute significantly to auroral electron precipitation and radiation belt electron acceleration. In the past, linear and nonlinear wave‐particle interactions have been proposed to explain the occurrences of these magnetospheric waves. By analyzing Van Allen Probes data, we present here the first evidence for nonlinear coupling between chorus and ECH waves. The sum‐frequency and difference‐frequency interactions produced the ECH sidebands with discrete frequency sweeping structures exactly corresponding to the chorus rising tones. The newly generated weak sidebands did not satisfy the original electrostatic wave dispersion relation. After the generation of chorus and normal ECH waves by hot electron instabilities, the nonlinear wave‐wave interactions could additionally redistribute energy among the resonant waves, potentially affecting to some extent the magnetospheric electron dynamics.
Abstract Plasmaspheric hiss is one of the important plasma waves controlling radiation belt dynamics. Its spatiotemporal distribution and generation mechanism are presently the object of active research. We here give the first report on the shock‐induced disappearance of plasmaspheric hiss observed by the Van Allen Probes on 8 October 2013. This special event exhibits the dramatic variability of plasmaspheric hiss and provides a good opportunity to test its generation mechanisms. The origination of plasmaspheric hiss from plasmatrough chorus is suggested to be an appropriate prerequisite to explain this event. The shock increased the suprathermal electron fluxes, and then the enhanced Landau damping promptly prevented chorus waves from entering the plasmasphere. Subsequently, the shrinking magnetopause removed the source electrons for chorus, contributing significantly to the several‐hours‐long disappearance of plasmaspheric hiss.
Observational studies clearly reveal that natural space plasmas generally possess a pronounced non‐Maxwellian high‐energy tail distribution that can be well modeled by a generalized Lorentzian (kappa) distribution. In this study we consider the whistler mode wave instability driven by the anisotropy condition ( T ⊥ / T ∥ > 1) of energetic electrons modeled with a typical kappa distribution in the presence of a cold plasma population. We use a linear theory to study the instability threshold condition for two typical plasma regions of interest: the higher‐density (or a weakly magnetized) region and the lower‐density (or a strongly magnetized) region. We find that (1) as in the case for a regular bi‐Maxwellian, the energetic electron anisotropy T ⊥ / T ∥ is subject to the threshold condition of this whistler instability, and the instability threshold condition obeys a general form T ⊥ / T ∥ − 1 = S /β ∥ α , with a narrow range of the fitting parameter 0.25 ≤ α ≤ 0.52 over 0.01 ≤ β ∥ ≤ 2.0; (2) the instability threshold condition in the higher‐density (or a weakly magnetized) region is generally lower than that in the lower‐density (or a strongly magnetized) region, specifically, with the fitting parameter range 0.3 ≤ S ≤ 5.0 in the higher‐density (or a weakly magnetized) region, while 0.32 ≤ S ≤ 6.94 in the lower‐density (or a strongly magnetized) region; and (3) the instability threshold condition for the kappa distribution generally decreases as the spectral index κ increases and tends to the lowest limiting values of the bi‐Maxwellian as κ → ∞. The results above may present a further insight into the nature of this instability threshold condition for the whistler mode waves in the outer radiation belts of the Earth, the inner Jovian magnetosphere, or other space plasmas where an anisotropic hot electron component and a cold plasma component are both present.
Abstract Previous studies have shown that auroral kilometric radiation (AKR) can play an important role in the magnetosphere‐atmosphere coupling and has the right‐handed extraordinary (R‐X), left‐handed ordinary (L‐O) and left‐handed extraordinary (L‐X) modes. However, the L‐X mode has not been directly observed in the lower latitude magnetosphere yet, probably because of its very limited frequency range. Here, using observations of the Arase satellite on 6 September 2018, we present an AKR event with two distinct bands (8–20 and 300–1000 kHz) around the location: L = 8 and latitude = −37°. The low (high) band is identified as the L‐X (R‐X) mode based on the polarization and frequency ranges. Simulations of 3‐D ray tracing show that most of ray paths with 14 (11 and 18) kHz pass (miss) the location of Arase, basically consistent with observations. Our study provides direct evidence that the L‐X mode can propagate from high latitudes downward to lower latitudes.
Abstract Van Allen radiation belt electrons exhibit complex dynamics during geomagnetically active periods. Investigation of electron pitch angle distributions (PADs) can provide important information on the dominant physical mechanisms controlling radiation belt behaviors. Here we report a storm time radiation belt event where energetic electron PADs changed from butterfly distributions to normal or flattop distributions within several hours. Van Allen Probes observations showed that the flattening of butterfly PADs was closely related to the occurrence of whistler‐mode chorus waves. Two‐dimensional quasi‐linear STEERB simulations demonstrate that the observed chorus can resonantly accelerate the near‐equatorially trapped electrons and rapidly flatten the corresponding electron butterfly PADs. These results provide a new insight on how chorus waves affect the dynamic evolution of radiation belt electrons.
Abstract Van Allen radiation belts are typically two zones of energetic particles encircling the Earth separated by the slot region. How the outer radiation belt electrons are accelerated to relativistic energies remains an unanswered question. Recent studies have presented compelling evidence for the local acceleration by very-low-frequency (VLF) chorus waves. However, there has been a competing theory to the local acceleration, radial diffusion by ultra-low-frequency (ULF) waves, whose importance has not yet been determined definitively. Here we report a unique radiation belt event with intense ULF waves but no detectable VLF chorus waves. Our results demonstrate that the ULF waves moved the inner edge of the outer radiation belt earthward 0.3 Earth radii and enhanced the relativistic electron fluxes by up to one order of magnitude near the slot region within about 10 h, providing strong evidence for the radial diffusion of radiation belt relativistic electrons.
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.
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