Nonducted propagation of chorus emissions and their observation

Chorus emissions are one of the most intense electromagnetic waves of whistler mode observed in the Earth's magnetosphere. They are generated near the magnetic equator plane and consist of more or less regularly repeating rising (falling) tones. We focus on the propagation of this emission from the source region. Based on plasma density measurements and recent satellite observations, we consider that chorus emissions may in many cases propagate obliquely to magnetic field lines. We study the influence of initial wave normal angle distribution on the chorus propagation using the ray tracing method for different frequencies. Our special interest is the estimation of the ray bundle width. As for the initial dimension of the source, we use the published values: ∼100 km for the transverse dimension (with respect to magnetic field), and the longitudinal extent of ∼2000 km. We conclude that the merging of the ray bundles corresponding to different frequencies defines the frequency bandwidth of the emission that can be observed at a particular point. We estimate that the initial spread of ∼10° is sufficient so that the typical bandwidth ∼1 kHz could be observed. We also show that there may be regions, where ray bundle focuses for a specific frequency and a specific initial wave normal spread, leading thus to enhanced energy density at this frequency. In the experimental part, we show examples of the observation of nonducted propagating emission recorded on the MAGION 5 satellite.