A statistical study of the spatial distribution and source-region size of chorus waves using Van Allen Probes data

The spatial distribution and source-region size of chorus waves are important parameters for understanding their generation. In this work, we analyze over 3 years of continuous wave burst-mode data from the Van Allen Probes mission and build a data set of rising-tone and falling-tone chorus waves. For the L shell range covered by Van Allen Probes data (3:5 <= L <= 7), statistical results demonstrate that the sector where rising tones are most likely to be observed is the dayside during geomagnetically quiet and moderate times and the dawn side during active times. Moreover, rising-tone chorus waves have a higher occurrence rate near the equatorial plane, while the falling-tone chorus waves have a higher possibility to be observed at lower L shell and higher magnetic latitudes. By analyzing the direction of the Poynting wave vector, we statistically investigate the chorus sourceregion size along a field line, and compare the results with previous theoretical estimates. Our analysis confirms previous conclusions that both rising-tone and falling-tone chorus waves are generated near the equatorial plane, and shows that previous theoretical estimates roughly agree with the observation within a factor of 2. Our results provide important insights into further understanding of chorus generation.