Influence of wave normal angles on hiss‐electron interaction in Earth's slot region

Abstract

Wave‐particle interaction which occurs in the radiation belts is generally determined by variations in wave normal angles. Using the Gaussian wave normal angle ( X=tanθ) distribution, we study the influence of peak wave normal angle ( Xm0.3em0.3em=0.3emtanθm) on gyroresonance between plasmaspheric hiss waves and energetic electrons in the slot regions L = 2.5, 3.0, and 3.5. The bounce‐averaged diffusion coefficients are calculated for different Xm = 0,1,3,5, and then the phase space density (PSD) evolutions of energetic electrons driven by hiss waves are simulated over a continuous energy range 0.2–5 MeV. As Xm increases, diffusion coefficients basically decrease mainly from the medium to a critical pitch angle αc for Ek≤1.0 MeV but increase at lower pitch angles for Ek>1.0 MeV. Differences of diffusion coefficients between Xm = 0 and 1 are close but become substantial as Xm≥3. Hiss can cause substantial drops in electron PSDs for Ek≤0.5 MeV and Xm≤1 from the loss cone αL to αc. For Ek≥1.0 MeV, such PSD drops become much smaller and confined in the lower pitch angles close to αL for each Xm. In contrast, electron PSD increases for Ek≤1.0 MeV above αc at L = 2.5 and 3.0, probably because momentum diffusion coefficients increase steeply above αc. The current results demonstrate that gyroresonance between plasmaspheric hiss and energetic electrons in the slot region is strongly associated with variations of peak wave normal angles, which should be integrated into future global modeling of radiation belt dynamics.

Wave‐driven gradual loss of energetic electrons in the slot region

“…Around e = 70 ∘ , the pitch angle diffusion rates exhibit a "gap" with low values, not conductive to the transport of near equatorially trapped electrons toward the loss cone. This is consistent with previous results [Meredith et al, 2009;Gao et al, 2015;Zhu et al, 2015]. The diffusion rates with different wave amplitudes and electron densities are sequentially input into the STEERB model to simulate the radiation belt electron flux j = p 2 F evolution.…”

“…Based on previous study [ Thorne et al , ], the waves are assumed to occur in the latitudinal range of | λ |<45°. The tangent of wave normal angles is assumed to obey a Gaussian distribution, with the center X m = 0 (waves propagate approximatively aligned with the ambient field in the observation), lower cutoff X 1 = 0, the upper cutoff X 2 = 10, and the half‐width X ω = 2.2 [ Gao et al , ]. We average the wave spectra observed by RBSP A at L = 3 and MLT =18 during five passages (Figure ) to obtain the wave‐frequency distribution.…”

Propagation characteristics of plasmaspheric hiss: Van Allen Probe observations and global empirical models

“…Besides the wave amplitude influence, the electron pitch angle diffusion coefficient also depends on hiss WNAs during the wave particle resonance [ Artemyev et al ., ; Gao et al ., ]. Since the small substorm‐dependent variation of the hiss WNAs is ignorable in comparison with their large latitudinal variation, the spatial distribution model of global hiss propagation angles (WNAs) can be simplified as a function of spatial location (L, MLT, and MLAT) and where α is the same as that in equation and the maximum L‐value also corresponds to the plasmapause location; other fitting coefficients are shown in Table .…”

“…On the one hand, the pitch angle diffusion coefficient of resonant electrons is proportional to the wave amplitude [Summers et al, 2007a[Summers et al, , 2007b. On the other hand, the oblique whistler mode waves (e.g., hiss) can cause faster electron pitch angle scattering than the parallel waves with the same amplitude [Li et al, 2014], and the WNA effect is pronounced for the low pitch angle electrons above 1 MeV [Artemyev et al, 2012;Gao et al, 2015]. Therefore, both hiss wave amplitude and WNA are very important for the radiation belt modeling.The amplitudes of global hiss waves have been extensively investigated [Meredith et al, 2004;Kim et al, 2015;Li et al, 2015b].…”

Storm Time Evolution of Outer Radiation Belt Relativistic Electrons by a Nearly Continuous Distribution of Chorus

“…The electron flux reduction at L = 4-6 during the main phase is probably due to both adiabatic loss (e.g., Dst effect) (Kim et al, 2010;Su et al, 2011) and nonadiabatic processes (e.g., gyroresonance between electrons with hiss (Gao et al, 2015;He et al, 2016) or EMIC waves (Ni et al, 2015;Wang et al, 2016)). Electron flux enhancements in the period 06:30-15:21 UT tend to be more associated with chorus-electron interaction instead of Dst effect because Dst did not vary much (ΔDst ≈ 6 nT in 06:30-09:54 UT and 32 nT in 09:54-15:21 UT (Figure 1a)).…”