Propagation of whistler-mode chorus to low altitudes: divergent ray trajectories and ground accessibility

• Main Authors: J. Chum, O. Santolík
• Format: Article
• Language: English
• Published: Copernicus Publications 2005-12-01
• Series: Annales Geophysicae
• Online Access: https://www.ann-geophys.net/23/3727/2005/angeo-23-3727-2005.pdf

We investigate the ray trajectories of nonductedly propagating lower-band chorus waves with respect to their initial angle θ₀, between the wave vector and ambient magnetic field. Although we consider a wide range of initial angles θ₀, in order to be consistent with recent satellite observations, we pay special attention to the intervals of initial angles θ₀, for which the waves propagate along the field lines in the source region, i.e. we mainly focus on waves generated with &theta₀ within an interval close to 0° and on waves generated within an interval close to the Gendrin angle. We demonstrate that the ray trajectories of waves generated within an interval close to the Gendrin angle with a wave vector directed towards the lower L-shells (to the Earth) significantly diverge at the frequencies typical for the lower-band chorus. Some of these diverging trajectories reach the topside ionosphere having θ close to 0°; thus, a part of the energy may leak to the ground at higher altitudes where the field lines have a nearly vertical direction. The waves generated with different initial angles are reflected. A small variation of the initial wave normal angle thus very dramatically changes the behaviour of the resulting ray. Although our approach is rather theoretical, based on the ray tracing simulation, we show that the initial angle θ₀ of the waves reaching the ionosphere (possibly ground) is surprisingly close - differs just by several degrees from the initial angles which fits the observation of magnetospherically reflected chorus revealed by CLUSTER satellites. We also mention observations of diverging trajectories on low altitude satellites.