Timescales for electron quasi‐linear diffusion by parallel and oblique lower‐band chorus waves

Abstract

[1] The loss of relativistic electrons from the Earth's radiation belts can be described in terms of the quasi-linear pitch angle diffusion by cyclotron-resonant waves, provided that their frequency spectrum is broad enough. Chorus waves at large wave-normal angles with respect to the magnetic field are often present in CLUSTER and THEMIS measurements in the outer belt at moderate to high latitudes. An approximate analytical formulation of diffusion coefficients has been derived in the low-frequency limit, leading to a simplified analytical expression of diffusion coefficients and lifetimes for energetic trapped electrons. Large values of the wave-normal angles between the Gendrin and resonance angles are shown to induce important increases in diffusion, thereby strongly reducing the particle lifetimes (by almost two orders of magnitude). The analytical diffusion coefficients and lifetimes obtained here are found to be in a good agreement with full numerical calculations based on CLUSTER chorus waves measurements in the outer belt for electron energies ranging from 100 keV to 2 MeV. Such very oblique chorus waves could contribute to a predominantly perpendicular anisotropy of the global equatorial electron population on the dayside and to a relative isotropization at low energy under disturbed conditions. It is also suggested that they might play a significant role in pulsating auroras.

Chorus wave-normal statistics in the Earth's radiation belts from ray tracing technique

“…Our numerical model shows that the average value of the angle θ grows with the latitude that is in a good agreement with observations onboard Cluster satellite (Agapitov et al, 2011a), and shows that, at medium and high latitudes, the approximation based on constant value of the average angle becomes invalid. This difference in wave-normal distributions is known to be very important for the evaluation of the angular diffusion rates and can lead to an underestimation of the diffusion rates especially for electrons with small pitch angles, which spend a large portion of their time oscillating along the field line at high latitudes (Artemyev et al, 2012a,b;Mourenas et al, 2012). The distribution of X obtained in our calculation is very close (see Fig.…”

“…It is thought to contribute to electron acceleration (up to ∼ MeV energies) and losses into the ionosphere (Lyons and Thorne, 1973), especially during geomagnetic disturbances (see Horne, 2002;Chen et al, 2007;Shprits et al, 2008b). Since increases in the energetic electron flux are hazardous to satellites (Baker, 2002), manned space missions, or can drastically affect atmospheric chemistry (Thorne, 1977), it is crucial to improve the existing numerical models that specify and predict the dynamics of radiation belts (Bourdarie et al, 1996;Li et al, 2001;Glauert and Horne, 2005;Summers et al, 2007;Fok et al, 2008;Mourenas et al, 2012). These models are based on the quasi-linear approach where resonant wave-particle interactions are described in terms of particle pitch angle and energy diffusion (Kennel and…”

Field-aligned chorus wave spectral power in Earth's outer radiation belt

“…For each value of electron energy the most effective resonant interaction corresponds to a certain latitude domain. For example, MeV electrons with small equatorial pitch angles resonantly interact with whistler waves propagating parallel to the magnetic field at latitudes ∼ 30-40 • (Mourenas et al, 2012b;Artemyev et al, 2013a). Thus, the 3 times decrease of wave frequency between the equatorial plane and high latitudes can change the efficiency of scattering of small pitch angle electrons.…”

Parametric validations of analytical lifetime estimates for radiation belt electron diffusion by whistler waves

“…On the other hand, analytically derived formulas may be less accurate over some domains while retaining physically (and quantitatively) correct variations over a much broader parameter range. It is precisely the main goal of the present paper to investigate extensively the accuracy of the analytical lifetime model introduced in earlier articles (Mourenas and Ripoll, 2012;Mourenas et al, 2012b), by means of numerous comparisons with full numerical calculations over a wide parameter domain representative of the whole inner magnetosphere. Determining these quasi-linear lifetimes is indeed a prerequisite for a comprehensive radiation belt modeling in the magnetosphere of Earth as well as other planets .…”

“…1 from Mourenas et al, 2012b). The small-θ part g s = exp(−(tan θ − tan θ m ) 2 / tan 2 θ ) is assumed to be approximately Gaussian with a width of θ ≤ 45 • and a maximum at θ m ≈ 0 (corresponding in practice to θ m < θ ≤ 45 • ), with lower and upper bounds at θ lc = 0 and θ uc ∼ θ.…”