Abstract
Abstract. We present a detailed study of waves with frequencies near the proton gyrofrequency in the high-altitude cusp for northward IMF as observed by the Cluster spacecraft. Waves in this regime can be important for energization of ions and electrons and for energy transfer between different plasma populations. These waves are present in the entire cusp with the highest amplitudes being associated with localized regions of downward precipitating ions, most probably originating from the reconnection site at the magnetopause. The Poynting flux carried by these waves is downward/upward at frequencies below/above the proton gyrofrequency, which is consistent with the waves being generated near the local proton gyrofrequency in an extended region along the flux tube. We suggest that the waves can be generated by the precipitating ions that show shell-like distributions. There is no clear polarization of the perpendicular wave components with respect to the background magnetic field, while the waves are polarized in a parallel-perpendicular plane. The coherence length is of the order of one ion-gyroradius in the direction perpendicular to the ambient magnetic field and a few times larger or more in the parallel direction. The perpendicular phase velocity was found to be of the order of 100km/s, an order of magnitude lower than the local Alfvén speed. The perpendicular wavelength is of the order of a few proton gyroradius or less. Based on our multi-spacecraft observations we conclude that the waves cannot be ion-whistlers, while we suggest that the waves can belong to the kinetic Alfvén branch below the proton gyrofrequency fcp and be described as non-potential ion-cyclotron waves (electromagnetic ion-Bernstein waves) above. Linear wave growth calculations using kinetic code show considerable wave growth of non-potential ion cyclotron waves at wavelengths agreeing with observations. Inhomogeneities in the plasma on the order of the ion-gyroradius suggests that inhomogeneous (drift) or nonlinear effects or both of these should be taken into account.
Highlights
The cusp regions of the terrestrial magnetosphere play an important role in the transfer of energy from the solar wind to the ionosphere, since the cusp magnetic field lines directly connect the solar wind with the Earth’s polar regions
We performed a preliminary analysis of the wave properties using the Wave Distribution Function (WDF) technique (Oscarsson, 1994), that is based on a homogeneous plasma model
From the multi-spacecraft cross-spectral analysis (Sect. 2.5) we concluded that the phase velocity projected on the C1-C2 separation axis should be lower than ∼300 km/s and we argued that the most probable wave vector is almost perpendicular with respect to the ambient magnetic field (k⊥ k ), in which case the perpendicular phase velocity would be even lower, vph,⊥ 100 km/s
Summary
The cusp regions of the terrestrial magnetosphere play an important role in the transfer of energy from the solar wind to the ionosphere, since the cusp magnetic field lines directly connect the solar wind with the Earth’s polar regions. The cusps are regions where efficient energization of the ionospheric plasma is taking place and energy is continuously transferred among different plasma populations. Most of these processes are due to different plasma waves. Intense waves with frequencies of the order of the ion gyrofrequency have often been observed in the cusp by several spacecraft at varying altitudes. Such waves are known to be important for energy redistribution between different particle populations, e.g. via ion-cyclotron resonance. Identification of the wave generation mechanisms and wave modes are essential for the understanding of the overall cusp energy conversion processes and particle transport
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