Whistler instability threshold condition of energetic electrons by kappa distribution in space plasmas
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Abstract:
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.Keywords:
Whistler
Kappa
Astrophysical plasma
Wave‐particle interactions driven by whistler mode waves in the inner Van Allan belt is an important loss process for the energetic electrons found in this region. In this paper we seek to investigate the significance of Whistler‐Induced Electron Precipitation (WEP) to inner belt electron lifetimes, by combining experimental satellite results, ionospheric remote observations, and global lightning distributions. We find that long‐term WEP driven losses at L = 2.23 are more significant than all other inner radiation belt loss processes for electron kinetic energies in the range 40–350 keV. This suggests a faster depletion rate and thus lower lifetimes than previously calculated. Thunderstorm generated whistlers are an important factor for determining the lifetimes of medium energy electrons in the inner belt.
Whistler
Electron precipitation
Lightning
Van Allen Probes
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Abstract Lightning generated whistlers (LGWs) play an important role in precipitating energetic electrons in the Earth's inner radiation belt and beyond. Wave burst data from the Van Allen Probes are used to unambiguously identify LGWs and analyze their properties at L < 4 by extending their frequencies down to ~100 Hz for the first time. The statistical results show that LGWs typically occur at frequencies from 100 Hz to 10 kHz with the major wave power below the equatorial lower hybrid resonance frequency, and their wave amplitudes are typically strong at L < 3 with an occurrence rate up to ~30% on the nightside. The lifetime calculation indicates that LGWs play an important role in scattering electrons from tens of keV to several MeV at L < ~2.5. Our newly constructed LGW models are critical for evaluating the global effects of LGWs on energetic electron loss at L < 4.
Whistler
Lightning
Van Allen Probes
Electron scattering
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