Magnetosonic waves above fc (H+) at geostationary orbit: GEOS 2 results

The fast magnetosonic waves in the frequency range 1–11 Hz (i.e., above the proton gyrofrequency Ωp) are considered. The electric wave field δE (mostly less than 0.5 mV/m) has an elliptic polarization in the plane perpendicular to B. While the magnetic wave field δB is strictly polarized along the ambient magnetic field, the ellipticity and the sense of rotation of the electric field polarization ellipses are crucial parameters for the identification of the wave mode. The waves are right‐hand polarized, and the ellipticity is usually about 0.2. The main axis of the ellipse is close to the azimuthal direction. The wave vector k is almost parallel to δE, and thus the fast magnetosonic waves propagate at small angles to the azimuthal direction. The wavelengths are usually between 150 and 300 km. The δE/δB ratio of the waves is between νA and υA(1 + ω²/Ωp²)1/2, where υA is the Alfvén velocity. In one case a burst of pure electrostatic modes near the second harmonic of the proton gyrofrequency was observed.