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

Abstract

Electromagnetic ion cyclotron (EMIC) waves are known to be excited by the cyclotron instability associated with hot and anisotropic ion distributions in the equatorial region of the magnetosphere and are thought to play a key role in radiation belt losses. Although detection of these waves at the ground can provide a global view of the EMIC wave environment, it is not clear what signatures, if any, would be expected. One of the significant scientific issues concerning EMIC waves is to understand how these waves are detected at the ground. In order to solve this puzzle, it is necessary to understand the propagation characteristics of the field‐aligned EMIC waves, which include polarization reversal, cutoff, resonance, and mode coupling between different wave modes, in a dipolar magnetic field. However, the inability of ray tracing to adequately describe wave propagation near the crossover cutoff‐resonance frequencies in multi‐ion plasmas is one of reasons why these scientific questions remain unsolved. Using a recently developed 2‐D full‐wave code that solves the full‐wave equations in global magnetospheric geometry, we demonstrate how EMIC waves propagate from the equatorial region to higher magnetic latitude in an electron‐proton‐He+ plasma. We find that polarization reversal occurs at the crossover frequency from left‐hand polarization (LHP) to right‐hand (RHP) polarization and such RHP EMIC waves can either propagate to the inner magnetosphere or reflect to the outer magnetosphere at the Buchsbaum resonance location. We also find that mode coupling from guided LHP EMIC waves to unguided RHP or LHP waves (i.e., fast mode) occurs.

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

“…Such localization of the EMIC waves, which is predicted by Ref. [28], is similar to the LHP EMIC waves [12]. Ref.…”

“…Ref. [12] suggested that polarization reversal and mode conversion occurs at the crossover frequency, where the left-hand (LHP) and right-hand (RHP) polarization modes cross each other in the dispersion relation, from LHP to RHP polarization and such RHP EMIC waves can propagate to the inner magnetosphere, which is consistent with 1D full-wave calculations [13,14].…”

“…[11] showed that efficient mode conversion from the fast compressional waves to the ion-ion hybrid (IIH) resonance occurs at Mercury and such mode-converted waves globally oscillate similar to field-line resonance at Earth. Mode coupling, refraction and reflection of internally driven field-aligned propagating left-handed EMIC waves at [12] has also been demonstrated. Ref.…”

“…Ref. [12] also showed that the EMIC waves generated in the LHP propagate along the field line and reach the heavier ion cyclotron frequency. However, the waves cannot tunnel through the cut-off and heavier ion cyclotron frequency even though the heavy ion concentration ratio is 5% (because the stop gap is too large compared with the wavelength).…”

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

“…In Earth's magnetosphere, as these waves propagate along B 0 , the wave normal angle increases and becomes nearly 90° when waves reach the Buchsbaum resonance. The waves then reflect toward a higher L-shell and lower magnetic latitude (Kim & Johnson 2015). Because both reflected IIH waves (Kim et al 2015a) and LHP EMIC waves (Kim & Johnson 2015) at the Buchsbaum resonance propagate to the different L-shells in the dipole field configuration, how 2D/3D heavier ion density structures in the planetary magnetosphere relate to propagation characteristics of the IIH and LHP EMIC waves remains as future work.…”