Resonant scattering of central plasma sheet protons by multiband EMIC waves and resultant proton loss timescales

Abstract

This is a companion study to Liang et al. (2014) which reported a “reversed” energy‐latitude dispersion pattern of ion precipitation in that the lower energy ion precipitation extends to lower latitudes than the higher‐energy ion precipitation. Electromagnetic ion cyclotron (EMIC) waves in the central plasma sheet (CPS) have been suggested to account for this reversed‐type ion precipitation. To further investigate the association, we perform a comprehensive study of pitch angle diffusion rates induced by EMIC wave and the resultant proton loss timescales at L = 8–12 around the midnight. Comparing the proton scattering rates in the Earth's dipole field and a more realistic quiet time geomagnetic field constructed from the Tsyganenko 2001 (T01) model, we find that use of a realistic, nondipolar magnetic field model not only decreases the minimum resonant energies of CPS protons but also considerably decreases the limit of strong diffusion and changes the proton pitch angle diffusion rates. Adoption of the T01 model increases EMIC wave diffusion rates at > ~ 60° equatorial pitch angles but decreases them at small equatorial pitch angles. Pitch angle scattering coefficients of 1–10 keV protons due to H+ band EMIC waves can exceed the strong diffusion rate for both geomagnetic field models. While He+ and O+ band EMIC waves can only scatter tens of keV protons efficiently to cause a fully filled loss cone at L > 10, in the T01 magnetic field they can also cause efficient scattering of ~ keV protons in the strong diffusion limit at L > 10. The resultant proton loss timescales by EMIC waves with a nominal amplitude of 0.2 nT vary from a few hours to several days, depending on the wave band and L shell. Overall, the results demonstrate that H+ band EMIC waves, once present, can act as a major contributor to the scattering loss of a few keV protons at lower L shells in the CPS, accounting for the reversed energy‐latitude dispersion pattern of proton precipitation at low energies (~ keV) on the nightside. The pitch angle coverage for H+ band EMIC wave resonant scattering strongly depends on proton energy, L shell, and field model. He+ and O+ band EMIC waves tend to cause efficient scattering loss of protons at higher energies, thereby importantly contributing to the isotropic distribution of higher energy (> ~ 10 keV) protons at higher L shells on the nightside where the geomagnetic field line is highly stretched. Our results also suggest that scattering by H+ band EMIC waves may significantly contribute to the formation of the reversed‐type CPS proton precipitation on the dawnside where both the wave activity and occurrence probability is statistically high.

Sensitivity of EMIC Wave‐Driven Scattering Loss of Ring Current Protons to Wave Normal Angle Distribution

“…To evaluate the scattering loss of ring current protons by resonant interactions with EMIC waves, we use the Full Diffusion Code (Cao et al, ; Cao, Ni, et al, ; Ni et al, , , ; Shprits & Ni, ) to calculate the quasi‐linear bounce‐averaged pitch angle diffusion coefficients by obliquely propagating EMIC waves in a multiion (H + , He + , and O + ) plasma. In the present study, we adopt a nominal wave amplitude of 1 nT and the results for arbitrary wave amplitude can be easily obtained, since diffusion coefficients are proportional to the square of wave amplitude in the quasi‐linear regime (Albert, ; Summers et al, ).…”

“…Then we choose the small value of these two latitudes as the maximum latitude of wave confinement. As in previous studies (Cao et al, ; He et al, ; Ni et al, ), we assume that the ambient electron density remains constant along the field line. Our calculations of EMIC wave‐induced quasi‐linear scattering rates of ring current protons include contributions from the N = −20 to 20 cyclotron harmonic resonances and the Landau resonance ( N = 0).…”

Understanding the Driver of Energetic Electron Precipitation Using Coordinated Multisatellite Measurements

“…The EMIC waves interact with protons and electrons via cyclotron resonance and are more efficient in pitch angle scattering than energy diffusion for electrons (Summers et al, ). Ring current protons with energies of tens of keV can be scattered into the loss cone (Cao et al, ; Summers et al, ; Xiao et al, ) and produce proton aurora (Miyoshi et al, ; Yuan et al, ). The minimum energy (E min ) of electrons resonating with EMIC waves depends strongly on various parameters, such as wave frequency spectrum, plasma density, and ion composition (W. Li et al, ; Meredith et al, ; Summers & Thorne, ).…”

Bounce Resonance Scattering of Radiation Belt Electrons by Low‐Frequency Hiss: Comparison With Cyclotron and Landau Resonances

“…To make quantitative comparisons among the bounce resonance, cyclotron resonance and Landau resonance‐induced pitch angle scattering of electrons, the Full Diffusion Code (Cao et al, ; Ni et al, , ; Shprits & Ni, ) is used to calculate the quasi‐linear bounce‐averaged pitch angle scattering rates due to cyclotron and Landau resonances with low‐frequency hiss. The resonance condition for cyclotron and Landau resonant interactions between electrons and electromagnetic waves is given by (Summers et al, ; Summers & Thorne, ) where ω = 2 πf is the wave frequency, k ∣∣ is the parallel component of wave number, v ∣∣ is the electron parallel velocity, Ω e is the nonrelativistic electron gyro‐frequency, γ = (1 − v 2 / c 2 ) −1/2 is the Lorentz factor, and N is the resonance harmonic.…”