Inferring magnetospheric heavy ion density using EMIC waves

Abstract

We present a method to infer heavy ion concentration ratios from electromagnetic ion cyclotron (EMIC) wave observations that result from ion‐ion hybrid (IIH) resonance. A key feature of the IIH resonance is the concentration of wave energy in a field‐aligned resonant mode that exhibits linear polarization. These mode‐converted waves at the IIH resonance are localized at the location where the frequency of a compressional wave driver matches the IIH resonance condition, which depends sensitively on the heavy ion concentration. This dependence makes it possible to estimate the heavy ion concentration ratio. In this paper, we evaluate the absorption coefficients at the IIH resonance at Earth's geosynchronous orbit for variable concentrations of He+ and wave frequencies using a dipole magnetic field model. We find that the resonance only occurs over a limited range of wave frequency such that the IIH resonance frequency is close to but not exactly the same as the crossover frequency. Using the wave absorption and EMIC waves observed from the GOES 12 satellite, we demonstrate how this technique can be used to estimate the He+ concentration of around 4% near L = 6.6 assuming electron‐H+‐He+ plasma.

Full‐wave modeling of EMIC waves near the He+ gyrofrequency

“…The simulation in Figure 4 particularly shows that additional heavier ions are necessary for EMIC wave propagation to the ground. Therefore, additional mode conversion from RHP waves to fieldaligned waves, such as mode conversion at the ion-ion hybrid resonance [e.g., Lee et al, 2008, 2015a, 2015b, 2015c, may be necessary. Therefore, additional mode conversion from RHP waves to fieldaligned waves, such as mode conversion at the ion-ion hybrid resonance [e.g., Lee et al, 2008, 2015a, 2015b, 2015c, may be necessary.…”

2D full-wave simulation of waves in space and tokamak plasmas

“…[6,24] calculated the efficiency of mode conversion of a radially propagating compressional wave at the IIH resonance using a simplified 1D slab cold plasma model that captures the essential features of the IIH resonance, however, global structure at Earth's magnetosphere has not been investigated.…”

“…Since linear polarization occurs only near the crossover frequency or at oblique propagation near the multi-ion hybrid resonances, the linearly polarized EMIC waves are suggested to occur at such critical frequencies [6,[24][25].…”

“…EMIC waves are also considered to be a dominant radiation belt loss mechanism leading to rapid pitch angle scattering radiation belt electrons of almost all energies on the time-scale of seconds with subsequent loss to the ionosphere [4]. In addition, these waves have also been used as a diagnostic tool to measure plasma densities [5][6].…”

Localization of Ultra-Low Frequency Waves in Multi-Ion Plasmas of the Planetary Magnetosphere

“…These waves have a significantly different polarization from EMIC waves, which are excited by proton temperature anisotropy (Cornwall 1965;Kennel & Petschek 1966;Williams & Lyons 1974a, b;Taylor & Lyons 1976). Because the incoming FW absorption at the IIH resonance (the generation of linearly polarized EMIC waves) occurs at a limited wave frequency and heavy ion density ratio, linearly polarized waves can be used as a diagnostic tool to estimate the heavy ion density ratio (Kim et al 2015c).…”