Group-standing whistler-mode waves observed as 1 Hz waves in the solar wind

Abstract:

"1 Hz waves" have been generally observed in the solar wind around many solar system bodies: Mercury [Le et al., 2013], Venus [Orlowski et al., 1990], Earth [e.g., Heppner et al., 1967; Russell et al., 1971], the Moon [Nakagawa et al., 2003; Halekas et al., 2006], Mars [Brain et al., 2002], Saturn [Orlowski et al., 1992], and comets [Tsurutani et al., 1987]. The waves mostly exhibit narrowband spectra with the frequency around 1 Hz and left-hand polarizations. They are whistler mode waves Doppler shifted significantly to be reversed the polarization in the spacecraft frame [Fairfield, 1974]. A variety of energy sources of the waves have been proposed including reflected ions and electrons from the bow shock, temperature anisotropies in the foreshock, cross-field drift in the foot region, and shock front perturbations. In the present study, in order to clarify the generation process of the waves, we suggest that it is necessary to understand effects on the observed wave spectra in the propagation process.

We propose the group-standing effects [Tsugawa et al., 2014] which can explain the observed frequency, wave spectra and wave vector direction of 1 Hz waves observed by Kaguya around the Moon and by Geotail around the terrestrial bow shock. The results suggest that 1 Hz waves are nearly stagnating in the spacecraft frame. Since most of the properties of 1 Hz waves are determined by the group-standing effects, it is difficult to clarify the generation process merely from the observed properties. Based on the group-standing effects, we reveal the essential differences of the waves around the Moon and the terrestrial bow shock, which should reflect their different generation processes.