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.

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[1] In the current study we perform a comparative analysis of the gyroresonant interactions of whistler mode waves with radiation belt electrons in the magnetospheres of Earth, Jupiter, and Saturn. Our primary goal is to evaluate the effect of resonant wave-particle interactions with chorus waves and determine whether chorus waves can produce net acceleration or net loss of radiation belt electrons on the outer planets. The ratio of plasma frequency to gyrofrequency is a key parameter that determines the efficiency of the pitch angle and energy resonant scattering. We present a comparison of statistical maps of the ratio of plasma frequency to gyrofrequency for Jupiter, Saturn and Earth in terms of radial distance and latitude. Preliminary maps of the plasma frequency to gyrofrequency ratio and 2D simulations of pitch angle and energy diffusion using the Versatile Electron Radiation Belt (VERB) indicate that the Kronian plasma environment is not likely to support as efficient gyroresonant interactions with whistler mode chorus waves as in the Terrestrial or Jovian environments. Inefficiency of the local acceleration by whistler mode waves in the Kronian environment raises important questions about the origin of the relativistic electrons in the Saturn's radiation belts. Two-dimensional diffusive simulations of local acceleration and loss to the atmosphere using the VERB code confirm previous suggestions that the acceleration of electrons may be very efficient in the outer radiation belt of Jupiter. However, sensitivity simulations also show that the result of the competition between acceleration and loss in the Jupiter's magnetosphere strongly depends on the currently unknown latitudinal distribution of chorus waves that will be provided by the upcoming Juno mission. If waves extend to high latitudes, it is likely that the loss rates due to whistler mode waves will exceed energization rates.Citation: Shprits, Y. Y., J. D. Menietti, X. Gu, K. C. Kim, and R. B. Horne (2012), Gyroresonant interactions between the radiation belt electrons and whistler mode chorus waves in the radiation environments of Earth, Jupiter, and Saturn: A comparative study,

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To investigate the hot plasma effects on the cyclotron‐resonant interactions between electromagnetic ion cyclotron (EMIC) waves and radiation belt electrons in a realistic magnetospheric environment, calculations of the wave‐induced bounce‐averaged pitch angle diffusion coefficients are performed using both the cold and hot plasma dispersion relations. The results demonstrate that the hot plasma effects have a pronounced influence on the electron pitch angle scattering rates due to all three EMIC emission bands (H+, He+, and O+) when the hot plasma dispersion relation deviates significantly from the cold plasma approximation. For a given wave spectrum, the modification of the dispersion relation by hot anisotropic protons can strongly increase the minimum resonant energy for electrons interacting with O+ band EMIC waves, while the minimum resonant energies for H+ and He+ bands are not greatly affected. For H+ band EMIC waves, inclusion of hot protons tends to weaken the pitch angle scattering efficiency of >5 MeV electrons. The most crucial differences introduced by the hot plasma effects occur for >3 MeV electron scattering rates by He+ band EMIC waves. Mainly due to the changes of resonant frequency and wave group velocity when the hot protons are included, the difference in scattering rates can be up to an order of magnitude, showing a strong dependence on both electron energy and equatorial pitch angle. Our study confirms the importance of including hot plasma effects in modeling the scattering of ultra‐relativistic radiation belt electrons by EMIC waves.

[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,

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.

Very-Low-Frequency (VLF) transmitters operate worldwide mostly at frequencies of 10–30 kilohertz for submarine communications. While it has been of intense scientific interest and practical importance to understand whether VLF transmitters can affect the natural environment of charged energetic particles, for decades there remained little direct observational evidence that revealed the effects of these VLF transmitters in geospace. Here we report a radially bifurcated electron belt formation at energies of tens of kiloelectron volts (keV) at altitudes of ~0.8–1.5 Earth radii on timescales over 10 days. Using Fokker-Planck diffusion simulations, we provide quantitative evidence that VLF transmitter emissions that leak from the Earth-ionosphere waveguide are primarily responsible for bifurcating the energetic electron belt, which typically exhibits a single-peak radial structure in near-Earth space. Since energetic electrons pose a potential danger to satellite operations, our findings demonstrate the feasibility of mitigation of natural particle radiation environment.