Analytical approximation of transit time scattering due to magnetosonic waves

Abstract

Recent test particle simulations have shown that energetic electrons traveling through fast magnetosonic (MS) wave packets can experience an effect which is specifically associated with the tight equatorial confinement of these waves, known as transit time scattering. However, such test particle simulations can be computationally cumbersome and offer limited insight into the dominant physical processes controlling the wave-particle interactions, that is, in determining the effects of the various wave parameters and equatorial confinement on the particle scattering. In this paper, we show that such nonresonant effects can be effectively captured with a straightforward analytical treatment that is made possible with a set of reasonable, simplifying assumptions. It is shown that the effect of the wave confinement, which is not captured by the standard quasi-linear theory approach, acts in such a way as to broaden the range of particle energies and pitch angles that can effectively resonate with the wave. The resulting diffusion coefficients can be readily incorporated into global diffusion models in order to test the effects of transit time scattering on the dynamical evolution of radiation belt fluxes.

The Radiation Belt Electron Scattering by Magnetosonic Wave: Dependence on Key Parameters

“…We see that MS waves with a wider latitudinal extent cause stronger Landau resonance, simply because the particle undergoes longer‐time interaction when traversing the wave field. This result is consistent with the analytical expression developed by Bortnik et al (), where a factor of appears in the analytical expression of the diffusion coefficients. In addition, while the Landau resonance line does not change, the Landau resonance region becomes narrower for increasing λ w .…”

“…In addition, while the Landau resonance line does not change, the Landau resonance region becomes narrower for increasing λ w . This is consistent with the exponential part of the analytical expressions of Bortnik et al (), which imply that the wave confinements can broaden the range of particle energies and pitch angles that can effectively scattered by MS waves.…”

“…In contrast, in the wave field of a small latitudinal extent, the test particle simulation results show a clear signature of additional transit‐time scattering effect (Bortnik & Thorne, ). It has been known that QLT may hardly capture a broad transit‐time region in ( E k , α ) space (Bortnik & Thorne, ; Bortnik et al, ) and is valid only if the wave spatial confinement is longer than about two wavelengths (Li et al, ). We emphasize from the present study that the latitudinal extent of MS waves on the basis of observations should be carefully taken into consideration with appropriate ways when evaluating the impact of MS waves on radiation belt electrons.…”

Survey of the frequency dependent latitudinal distribution of the fast magnetosonic wave mode from Van Allen Probes Electric and Magnetic Field Instrument and Integrated Science waveform receiver plasma wave analysis

“…Bortnik and Thorne [2010] and Bortnik et al [2015] show that energetic electrons can experience transit time scattering by these waves. Bortnik and Thorne [2010] and Bortnik et al [2015] show that energetic electrons can experience transit time scattering by these waves.…”

Propagation of equatorial noise to low altitudes: Decoupling from the magnetosonic mode

“…Natural electromagnetic emissions of equatorial noise [Russell et al, 1970;Gurnett, 1976] are receiving increased attention because of their possible importance for electron dynamics in the radiation belts [Horne et al, 2007;Li et al, 2014;Bortnik et al, 2015;Shprits, 2016;Walker et al, 2015a;Ma et al, 2016]. These emissions are composed of a complex system of many spectral lines with spacing ranging from a few hertz to a few tens of hertz, linked to ion cyclotron frequencies in local or distant sources [Gurnett, 1976;Santolík et al, 2002a;Ma et al, 2014].…”