Proton and electron heating by radially propagating fast magnetosonic waves

Abstract

Abstract. We investigate the propagation, growth, and decay of fast magnetosonic waves in the Earth's magnetosphere which are believed to contribute to proton heating up to energies of a few hundred eV near the magnetic equator. We construct a model of the proton and electron distribution functions from spacecraft data and use the HOTRAY code to calculate the path-integrated growth and decay of the waves over a range of L shells from L = 2 to L = 7. Instability calculations show that the waves are excited at very large angles of propagation with respect to the magnetic field, ;bm 89 ø, at the harmonics of the proton gyrofrequency up to the lower hybrid resonance frequency O$LH R by a proton ring distribution at energies of the order of 10 keV. As a "rule of thumb", we find that growth is possible for co > 30•qH+ when the ring velocity exceeds the Alfvdn speed and for co < 30[qH+ when v•t > 2v/i. For propagation in the meridian plane, waves generated just outside the plasmapause grow with large amplification as they propagate away from the Earth but eventually lose energy to plasma sheet electrons at energies of a few keV by Landau damping. The waves grow to large amplification at frequencies just below CVoeH•t. For inward propagation we find that waves generated just outside the plasmapause can propagate to L m 2 with very little attenuation, suggesting that waves observed well inside the plasmasphere could originate from a source region just outside the plasmapause. Strong wave growth only occurs for large angles of propagation, and thus the waves are confined to within a few degrees of the magnetic equator. Waves generated near geostationary orbit and which propagate toward the Earth are absorbed by Doppler-shifted cyclotron resonance when they propagate into a region where vR < va. Cyclotron resonant absorption causes pitch angle scattering and heating transverse to the ambient magnetic field. The amount of absorption, and hence transverse proton heating, increases significantly as the thermal proton temperature is increased up to 100 eV, suggesting a feedback process. Ray tracing shows that transverse heating of the thermal proton distribution is most likely to occur just outside the plasmapause where v•4 is large. Since proton ring distributions are formed during magnetic storms at ring current energies, we suggest that fast magnetosonic waves provide an additional energy loss process for ring current decay.

Propagation and linear mode conversion of magnetosonic and electromagnetic ion cyclotron waves in the radiation belts

“…Figure 1a shows an example of the dispersion curves for wave propagation at an angle of 89 ∘ with respect to the magnetic field direction. Magnetosonic waves can be generated above the proton cyclotron frequency (Ω H ) on this branch[e.g., Horne et al, 2000]. The dispersion curves here were calculated for a cold plasma using the HOTRAY code [Horne, 1989] using the diffusive equilibrium plasma density model [Inan and Bell, 1977] and a dipole magnetic field.…”

“…It has been generally accepted that magnetosonic waves can be generated by a ring distribution in the proton and heavier ion distributions [e.g., Horne et al, 2000] at energies of tens to hundreds of keV and that EMIC waves can be generated by a temperature anisotropy [e.g., Sakaguchi et al, 2013]. However, unless there is a very large convection electric field, it is not clear how such ion distributions can form in the inner belt to generate waves observed near L = 1.5.…”

Analytical approximation of transit time scattering due to magnetosonic waves

“…MS waves are believed to derive their energy from unstable "ring" distributions in the ion population [e.g., Gurnett, 1976;Perraut et al, 1982;Boardsen et al, 1992;Horne et al, 2000;Meredith et al, 2008;Xiao et al, 2013;Ma et al, 2014], which form mostly on the dayside as a result of the overlap of the westward drifting energetic ions (due to gradient curvature drift) and eastward drifting cold ions (due to the corotation electric field), with a null near ∼10 keV [e.g., Chen et al, 2010].…”

Multiple harmonic ULF waves in the plasma sheet boundary layer: Instability analysis

“…Perraut et al [1982] identified them as magnetosonic waves, propagating in a direction almost perpendicular to B 0 , and suggested that the observed harmonic structure was related to nonlinear effects of locally generated waves rather than the superposition of multiple waves each generated at their local proton gyrofrequency. Boardsen et al [1992] and Horne et al [2000] modeled an instability that could be responsible for these waves using a "subtracted bi-Maxwellian" distribution function (with a population of lower T ? subtracted from a population with higher T ?…”