Parameterized lifetime of radiation belt electrons interacting with lower‐band and upper‐band oblique chorus waves

Abstract

[1] In order to evaluate the electron lifetimes from quasilinear bounce-averaged diffusion coefficients, a large amount of CPU time is required to compute scattering rates. To reduce computation time, we assume that the electron loss rates are a function of L shell and electron energy, allowing us to derive the parameterization of electron lifetime using a linear regression technique. We also present the parameterizations of lifetimes as a function of electron energy and ratios of plasma frequency to electron gyrofrequency at a fixed L shell. By considering the net loss rates due to lowerband (0.05-0.5 |W ce |) and upper-band (0.5-0.7 |W ce |) chorus waves propagating obliquely with respect to the ambient magnetic field, the parameterizations of electron lifetime for energies between 1 keV and 2 MeV are investigated in the outer radiation belt. The parameterized lifetimes indicate a strong functional dependence on electron energy, L shell, and the level of geomagnetic activity (Kp index), and are also dependent on the ratios of plasma frequency to electron gyrofrequency. During the storm time, the lifetimes for higher energy (>100 keV) electrons range from hours to days in the heart of the radiation belts. In contrast, the lifetimes for lower energy (<100 keV) electrons are mainly in the range from minutes to hours. Such parameterization efforts of radiation belt electron lifetimes are very significant for multidimensional simulations of the dynamics of the radiation belt and ring current electrons. Citation: Gu, X., Y. Y. Shprits, and B. Ni (2012), Parameterized lifetime of radiation belt electrons interacting with lower-band and upper-band oblique chorus waves,

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

“…Moreover, chorus waves are assumed here to be uniformly present up to λ + ≈ 40 • , while in reality intensity peaks between λ ≈ 15 • and λ ≈ 30 • on the dayside . While upper-band chorus is also assumed to be present in the numerical simulations by Gu et al (2012), its effect on lifetimes consists mainly in the suppression of the deep gap in diffusion between the Landau and first cyclotron resonance peaks. In such a case, Landau resonance is unimportant in lifetimes for moderately oblique waves at E > 100 keV, as noted by Gu et al (2012).…”

“…While upper-band chorus is also assumed to be present in the numerical simulations by Gu et al (2012), its effect on lifetimes consists mainly in the suppression of the deep gap in diffusion between the Landau and first cyclotron resonance peaks. In such a case, Landau resonance is unimportant in lifetimes for moderately oblique waves at E > 100 keV, as noted by Gu et al (2012). Upper-band chorus actually plays the same role in the outer belt as lightning-generated waves in the plasmasphere (which fill the trough in diffusion between Landau and cyclotron peaks of hiss waves; see Meredith et al, 2007).…”

“…Whistler mode waves scatter trapped electrons into the loss cone, where they are quickly removed by collisions with atmospheric particles (Kennel and Petschek, 1966;Trakhtengerts, 1966). The corresponding analytical lifetime expressions may be obtained either by fitting numerical calculations (as it was done for moderately oblique chorus waves by Shprits et al, 2007;Gu et al, 2012) or from complicated, approximate analytical developments (as it was done for parallel to very oblique hiss and chorus waves by Mourenas and Ripoll, 2012;Mourenas et al, 2012b,a). While numerical fits have their own advantages (a known accuracy over a restricted parameter domain), they also suffer from the limited parameter range that is considered and the associated assumptions on the wave and plasma conditions.…”

“…(9) of electron lifetimes for small θ-waves is compared with parameterized lifetimes recently obtained by fitting numerical calculations for an interaction with lower-band chorus waves dominant in the outer radiation belt. We use the same initial conditions as Shprits et al (2007) and Gu et al (2012): 100 pT storm-time chorus waves, dayside plasma trough density N e ∼ 100(3/L) 4 cm −3 for L > 3 from Sheeley et al (2001), and θ < π/4. Moreover, chorus waves are assumed here to be uniformly present up to λ + ≈ 40 • , while in reality intensity peaks between λ ≈ 15 • and λ ≈ 30 • on the dayside .…”

Radial diffusion simulations of the 20 September 2007 radiation belt dropout

“…As a compromise, Hwang et al (2013) used τ = 4/Kp, while Li et al (2014) used τ = 6/Kp. More physically, Shprits et al (2007) and Gu et al (2012) presented fits in L, E, and Kp to simple estimates of the timescale of pitch angle diffusion by chorus waves. The obvious problem with this approach is that it neglects energy diffusion, which tends to increase flux levels in the multikeV range.…”