Statistical evidence for O⁺ energization and outflow caused by wave-particle interaction in the high altitude cusp and mantle

Abstract. We present a statistical study of the low (<1 Hz) frequency electric and magnetic field spectral densities observed by Cluster spacecraft in the high altitude cusp and mantle region. At the O⁺ gyrofrequency (0.02–0.5 Hz) for this region the electric field spectral density is on average 0.2–2.2 (mV m⁻¹)² Hz⁻¹, implying that resonant heating at the gyrofrequency can be intense enough to explain the observed O⁺ energies of 20–1400 eV. The relation between the electric and magnetic field spectral densities results in a large span of phase velocities, from a few hundred km s⁻¹ up to a few thousand km s⁻¹. In spite of the large span of phase velocity, the ratio between the calculated local Alfvén velocity and the estimated phase velocity is close to unity. We provide average values of a coefficient describing diffusion in ion velocity space at different altitudes, which can be used in studies of ion energization and outflow. The observed average waves can explain the average O⁺ energies measured in the high altitude (8–15 R_E) cusp/mantle region of the terrestrial magnetosphere according to our test particle calculations.