Resonant scattering of outer zone relativistic electrons by multiband EMIC waves and resultant electron loss time scales

Abstract

To improve our understanding of the role of electromagnetic ion cyclotron (EMIC) waves in radiation belt electron dynamics, we perform a comprehensive analysis of EMIC wave‐induced resonant scattering of outer zone relativistic (>0.5 MeV) electrons and resultant electron loss time scales with respect to EMIC wave band, L shell, and wave normal angle model. The results demonstrate that while H+‐band EMIC waves dominate the scattering losses of ~1–4 MeV outer zone relativistic electrons, it is He+‐band and O+‐band waves that prevail over the pitch angle diffusion of ultrarelativistic electrons at higher energies. Given the wave amplitude, EMIC waves at higher L shells tend to resonantly interact with a larger population of outer zone relativistic electrons and drive their pitch angle scattering more efficiently. Obliquity of EMIC waves can reduce the efficiency of wave‐induced relativistic electron pitch angle scattering. Compared to the frequently adopted parallel or quasi‐parallel model, use of the latitudinally varying wave normal angle model produces the largest decrease in H+‐band EMIC wave scattering rates at pitch angles < ~40° for electrons > ~5 MeV. At a representative nominal amplitude of 1 nT, EMIC wave scattering produces the equilibrium state (i.e., the lowest normal mode under which electrons at the same energy but different pitch angles decay exponentially on the same time scale) of outer belt relativistic electrons within several to tens of minutes and the following exponential decay extending to higher pitch angles on time scales from <1 min to ~1 h. The electron loss cone can be either empty as a result of the weak diffusion or heavily/fully filled due to approaching the strong diffusion limit, while the trapped electron population at high pitch angles close to 90° remains intact because of no resonant scattering. In this manner, EMIC wave scattering has the potential to deepen the anisotropic distribution of outer zone relativistic electrons by reshaping their pitch angle profiles to “top‐hat.” Overall, H+‐band and He+‐band EMIC waves are most efficient in producing the pitch angle scattering loss of relativistic electrons at ~1–2 MeV. In contrast, the presence of O+‐band EMIC waves, while at a smaller occurrence rate, can dominate the scattering loss of 5–10 MeV electrons in the entire region of the outer zone, which should be considered in future modeling of the outer zone relativistic electron dynamics.

Resonant scattering of energetic electrons in the outer radiation belt by HAARP-induced ELF/VLF waves

“…Here, we use both narrowband limit of quasi-linear theory (Albert, 2007;, and the Full Diffusion Code (FDC) (Ni et al, 2008(Ni et al, , 2011(Ni et al, , 2015 to compute the quasi-linear local diffusion coefficients at the magnetic equator for comparisons with the test particle diffusion coefficients.…”

“…The details of multiple electron energy and pitch-angle variation with time due to resonant interactions with both monotonic waves and multi-frequency waves are presented respectively to evaluate the average effect of wave-induced localized pitch-angle scattering and energy diffusion. Moreover, the diffusion coefficients calculated by our test particle code will be compared with the diffusion rates in the narrowband limit of quasi-linear theory (Albert, 2007;, and that obtained using the Full Diffusion Code (FDC) (Ni et al, 2008(Ni et al, , 2011(Ni et al, , 2015.…”

Space Weather Effects in the Earth’s Radiation Belts

“…Finally, EMIC waves are known to interact strongly with highly energetic electrons in the MeV range, particularly during active storm times (Albert 2003;. EMIC waves occur in frequency bands and new studies indicate that the H + -band is most efficient at scattering up to a few MeV, while the He + -and O + -bands are more significant for higher energies (Ni et al 2015). Interestingly, Usanova et al (2014) showed that EMIC waves primarily affect only the lower pitch angles, essentially leaving the core distribution unchanged.…”

Electron radiation belt dynamics during magnetic storms and in quiet time

“…Sufficiently convincing arguments for the mechanism of electron dropout at the magnetopause have been put forward in [Dmitriev, Chao, 2003;Shprits et al, 2006, Millan, Thorne, 2007Kim et al, 2008;Saito et al, 2010;Matsumura et al, 2011;Turner et al, 2012;Hudson et al, 2014;Lazutin, 2016]. In the quasitrapping region, drift shells are not closed; on the night side, electrons drift with adiabatic invariants remaining unchanged to the morning magnetosphere-magnetopause boundary; protons, to the evening boundary (and vice versa on the dayside); then they go from the closed field lines to the turbulent magnetosheath.…”

“…Existing reviews on radiation belts (RB) [Parks, Winkler 1968;Vernov et al, 1969;Friedel et al, 2002;Millan, Thorne, 2007;Shprits et al, 2008a] describe in sufficient detail both the structure of RB and its formation. According to the traditional theory [Tverskoy, 1964[Tverskoy, , 1965, the RB formation is attributed to the combination of slow radial diffusion of electrons driven by small magnetic field pulses with losses in the atmosphere due to pitch-angle diffusion.…”