Global modeling of ULF waves at Mercury

Abstract

ULF waves in the ion cyclotron frequency range waves are regularly observed at Mercury's magnetosphere. Although previous statistical studies have shown that ULF waves are primarily compressional near the equator and transverse with linear polarization at higher latitude, the underlying reason for this distribution of wave polarization has not been understood. In order to address this key question, we have developed a two‐dimensional, finite element code that solves the full wave equations in global magnetospheric geometry. Using this code, we show that (1) efficient mode conversion from the fast compressional waves to the ion‐ion hybrid resonance occurs at Mercury consistent with previous calculations; (2) such mode‐converted waves globally oscillate similar to field line resonance at Earth; and (3) compressional wave energy is primarily localized near the equator, while field‐aligned transverse, linearly polarized waves generated by mode conversion at the ion‐ion hybrid resonance radiate to higher latitude. Based on these wave solutions, we suggest that the strong transverse component of observed ULF waves at Mercury in high magnetic latitude can be explained as excitation of the field line resonant waves at the ion‐ion hybrid resonance.

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

“…The IIH resonant waves appear only in E η showing field-aligned structure (See Figure 3a). Because the waves are initially launched as a pure FW mode, the waves exhibiting field-aligned structure in E η provide evidence of mode conversion [11]. The IIH resonant waves reach higher magnetic latitude, but the waves reflect before reaching the altitude where the frequency matches the oxygen gyrofrequency, which is unlike to the IIH resonant waves at Mercury [11].…”

“…Ref. [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.…”

“…The FW2D wave code has been successfully used to describe low frequency waves in Earth and Mercury's multi-ion magnetospheres (i.e., dipole geometry). The results include generation and propagation of externally driven ULF waves via mode conversion at Mercury [11]. Ref.…”

“…A recently developed 2D full-wave (FW2D) code using finite element method [11] overcomes these shortcomings using an approach that describes wave propagation, mode effects for arbitrary plasma and magnetic field configurations. The FW2D wave code has been successfully used to describe low frequency waves in Earth and Mercury's multi-ion magnetospheres (i.e., dipole geometry).…”

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

“…For Mercury, where the magnetic field is relatively weak, the wavelength of field-aligned modes can be comparable to the size of the magnetosphere. Therefore, IIH waves oscillate globally along the magnetic field lines for Mercury, similar to the field line resonance on Earth (Othmer et al 1999;Glassmeier et al 2003Glassmeier et al , 2004Klimushkin et al 2006;Kim et al 2008Kim et al , 2011aKim et al , 2013Kim et al , 2015a. On the other hand, on Earth, the magnetic field strength is larger and the wavelength is shorter, which typically localizes mode converted waves between the Buchsbaum cutoff locations, which occur at around 10 degrees latitude.…”