Gyroharmonic emissions induced by energetic ions in the equatorial plasmasphere

Abstract

The general problem of the excitation of ion gyroharmonic electrostatic and electromagnetic waves by energetic ions (E ≳ 0.1 MeV) in a thermal plasma as observed in the earth's plasmasphere is considered. The thermal plasma is taken to be predominantly H+ with trace higher Z ion constituents of ionospheric origin such as He+ and O+. This plasma component has a Maxwellian distribution and supports the waves which may be excited by energetic ions with Z ≳ 1. The energetic ions possess a power law distribution in energy with strong pitch angle anisotropy. Coupling of the free energy source of the energetic ions to the waves is obtained via the velocity dependence of the ion gyrofrequency Ωα. Owing to constraints imposed by free energy coupling, wave amplification, and ion cyclotron damping, the dominant growth rates are found for nearly perpendicularly propagating fast magnetosonic waves closely confined to the magnetic equator. Growth is restricted to frequencies immediately below the ion gyroharmonics by the resonance condition and implies line widths of ∼ 10−3Ωα. The fast magnetosonic waves are found to have a significant electromagnetic component which decreases with increasing gyroharmonic number. The polarization becomes almost entirely electrostatic where the fast magnetosonic and ion Bernstein branches merge above the lower hybrid frequency. When it is compared to observations of ion gyroharmonic emissions, the theory implies that there exist a significant number of charge states for Z ≳ 2 MeV ions in the plasmasphere.

Magnetosonic waves above fc (H+) at geostationary orbit: GEOS 2 results

“…This conclusion was also drawn by Curtis and Wu [1979] but is at variance with ray tracing, which suggests radial propagation [Perraut, 1982]. The MSWs, which are known to remain bounded in the equatorial region, generally propagate azimuthally, as evidenced by the direction of the electric field phase angle.…”

“…As a matter of fact, the convective growth of an azimuthally propagating MSW is much larger than that of a radially propagating wave, as discussed by Curtis and Wu [1979]. The rays having a k vector perpendicular to the geomagnetic field stay in the vicinity of the equatorial plane all the time, and whatever the direction of the k vector, they deviated quickly outward.…”

“…From electric field measurements we are able to derive the basic parameters of the polarization ellipse: the magnitude of the major axis; the orientation angle • of the ellipses, which is defined as the angle between the major axis and some reference direction; the ellipticity e; and the sense of rotation.We will use the fluid equations of the cold plasmas, since the growth rate is found to be largest just in the region where the approximation of cold plasma is valid (especially, the growth rate has maxima at the crossings of the cold plasma dispersion curve and the lines ntlp)[Curtis and Wu, 1979; Perraut et al, 1982]. The data differ from the two other events, since the peaks are rather wide, which might be caused by the injection of energetic ions over a broad spatial region, corresponding to different values of the magnetic field.…”

Radiation belt electron acceleration induced by gyroresonant interaction with magnetosonic waves

“…The generation source for MS waves is the ring distribution of energetic (∼10 keV) protons injected from the (Curtis and Wu 1979;Meredith et al 2008;Gary et al 2010;Chen et al 2010Chen et al , 2011Xiao et al 2013). These waves primarily stay close to the geomagnetic equator by a few degrees (Russell et al 1970;Santolík et al 2002;Němec et al 2005) and can propagate perpendicularly to the background geomagnetic field both inside and outside the plasmasphere (Chen and Thorne 2012;Xiao et al 2012b).…”

Quenching of Equatorial Magnetosonic Waves by Substorm Proton Injections

“…The Bernstein mode instability of hot protons with a velocity ring distribution can destabilize the magnetosonic waves at the quasi-perpendicular normal angles (Boardsen et al, 1992;Curtis & Wu, 1979;Gary et al, 2010;Gulelmi et al, 1975;Horne et al, 2000;Liu et al, 2011;Min et al, 2018;Perraut et al, 1982). Near the source region, the magnetosonic waves are characterized as the emission lines along the proton gyrofrequency harmonics in the high-resolution frequency-time spectrograms (e.g., Balikhin et al, 2015;Perraut et al, 1982).…”