Statistics of whistler mode waves in the outer radiation belt: Cluster STAFF‐SA measurements

Abstract

AbstractELF/VLF waves play a crucial role in the dynamics of the radiation belts and are partly responsible for the main losses and the acceleration of energetic electrons. Modeling wave‐particle interactions requires detailed information of wave amplitudes and wave normal distribution over L‐shells and over magnetic latitudes for different geomagnetic activity conditions. We performed a statistical study of ELF/VLF emissions using wave measurements in the whistler frequency range for 10 years (2001–2010) aboard Cluster spacecraft. We utilized data from the STAFF‐SA experiment, which spans the frequency range from 8 Hz to 4 kHz. We present distributions of wave magnetic and electric field amplitudes and wave normal directions as functions of magnetic latitude, magnetic local time, L‐shell, and geomagnetic activity. We show that wave normals are directed approximately along the background magnetic field (with the mean value of θ—the angle between the wave normal and the background magnetic field, about 10°–15°) in the vicinity of the geomagnetic equator. The distribution changes with magnetic latitude: Plasmaspheric hiss normal angles increase with latitude to quasi‐perpendicular direction at ∼35°–40° where hiss can be reflected; lower band chorus are observed as two wave populations: One population of wave normals tends toward the resonance cone and at latitudes of around 35°–45° wave normals become nearly perpendicular to the magnetic field; the other part remains quasi‐parallel at latitudes up to 30°. The observed angular distribution is significantly different from Gaussian, and the width of the distribution increases with latitude. Due to the rapid increase of θ, the wave mode becomes quasi‐electrostatic, and the corresponding electric field increases with latitude and has a maximum near 30°. The magnetic field amplitude of the chorus in the day sector has a minimum at the magnetic equator but increases rapidly with latitude with a local maximum near 12°–15°. The wave magnetic field maximum is observed in the night sector at L>7 during low geomagnetic activity (at L∼5 for Kp>3). Our results confirm the strong dependence of wave amplitude on geomagnetic activity found in earlier studies.