Model of Electromagnetic Ion Cyclotron Waves in the Inner Magnetosphere

Abstract:

The He-band electromagnetic ion cyclotron (EMIC) waves are studied using our global model of ring current (RC) ions, electric field, plasmasphere, and EMIC waves. In contrast to the approach by Gamayunov et al. [2009], however, we do not use the bounce-averaged kinetic equation for waves but instead use a complete, non bounce-averaged, equation to model EMIC wave power spectral density. The major results of our study can be summarized as follows. (1) The thermal background level for EMIC waves is too low to allow waves to grow up to the observable level during one pass between the “bi-ion latitudes” (the latitudes where the given wave frequency is equal to the O⁺-He⁺ bi-ion frequency) in conjugate hemispheres. As a consequence, quasi-field-aligned EMIC waves are not typically produced in the model if the thermal background level is used, but routinely observed in the Earth’s magnetosphere. To overcome this model-observation discrepancy we suggest a nonlinear energy cascade from the lower frequency range of ULF waves into the frequency range of EMIC wave generation as a possible mechanism supplying the needed level of seed fluctuations that guarantees growth of EMIC waves during one pass through the near equatorial region. The EMIC wave development from a suprathermal background level shows that EMIC waves are quasi-field-aligned near the equator, while they are oblique at high latitudes, and the Poynting flux is predominantly directed away from the near equatorial source region in agreement with observations. (2) An abundance of O⁺ strongly controls the energy of oblique He-band EMIC waves that propagate to the equator after their reflection at “bi-ion latitudes”, and so it controls a fraction of wave energy in the oblique normals. (3) The RC O⁺ not only causes damping of the He-band EMIC waves but also causes wave generation in the region of highly oblique wave normal angles, typically for θ > 82^o, where a growth rate γ > 10^-2 rad/s is frequently observed. The instability is driven by the loss-cone feature in the RC O⁺ distribution function. (4) The oblique and intense He-band EMIC waves generated by RC O⁺ in the region L~2-3 may have an implication to the energetic particle loss in the inner radiation belt. Acknowledgments: This paper is based upon work supported by the National Science Foundation under Grant Number AGS-1203516.