Nonlinear evolution of the EMIC waves in homogenous and inhomogeneous systems

Details Nonlinear evolution of the EMIC waves in homogenous and inhomogeneous systems Nonlinear evolution of the EMIC waves in homogenous and inhomogeneous systems

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm51c..05o&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SM51C-05

Physics

[2716] Magnetospheric Physics / Energetic Particles: Precipitating, [2753] Magnetospheric Physics / Numerical Modeling, [2772] Magnetospheric Physics / Plasma Waves And Instabilities

Scientific paper

Nonlinear evolution of the EMIC waves is investigated using 2.5-D electromagnetic hybrid (kinetic ions, fluid electrons) simulations to understand their role in pitch angle scattering and energization of the plasma in the magnetosphere. The plasma consists of cold and energetic protons and heavier ions such as helium and oxygen. Starting with a homogenous system, we examine the generation and evolution of the waves driven by temperature anisotropy of the energetic protons with perpendicular temperature larger than parallel. The results show an intricate interplay between the waves and the various populations of ions that takes place over thousands of proton gyroperiods (Ωp-1). Initially, ion cyclotron waves propagating primarily along the magnetic field grow with frequency of ~ 0.4 Ωp and reach saturation within a few hundred Ωp-1. After ~ 1000 Ωp-1 these waves get partially damped in association with the growth of compressional waves propagating perpendicular to the magnetic field with the same frequency as the parallel waves. This process completes at ~ 3300 Ωp-1 by which time both waves have comparable amplitudes at about 1.5% of the background level. In addition to this nonlinear coupling process, phase trapping of the cold ions by the ion cyclotron waves results in the generation and subsequent damping of electrostatic structures along the magnetic field with zero frequency. After time ~ 3300 Ωp-1, the parallel propagating waves remain at the same amplitude for additional tens of thousands of proton gyroperiods, while the compressionals waves get continuously damped. Examination of the parallel and perpendicular energies of the ions shows that until time ~ 6600 Ωp-1 when the energetic protons become isotropic, their pitch angle scattering is the dominant process with two thirds of the loss in their perpendicular energy going towards the parallel energy and the remaining going to the energization of the cold protons and heavier ions. After 6600 Ωp-1, the energetic protons start losing energy in both perpendicular and parallel directions while the cold ions continue to get heated. This energy transfer takes place through the ion cyclotron waves whose spectrum in frequency and wavelength broadens with time. Given the long time associated with this nonlinear evolution, a question that comes to mind is the extent to which all of these processes can be operative in the inhomogeneous magnetosphere where waves propagate out of the source region. To address this question, we also show results from hybrid simulations in a dipolar field with a finite sized source region near the equator.

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