Global distribution of wave amplitudes and wave normal angles of chorus waves using THEMIS wave observations

Abstract

The global distribution of chorus wave amplitudes and their wave normal angles is investigated using high-resolution wave spectra and waveform data from THEMIS for lower-band and upper-band chorus separately. Statistical results show that large amplitude chorus (>300 pT) occurs predominantly from premidnight to postdawn and is preferentially observed at lower L shells (<8) near the magnetic equator. However, strong or moderate chorus extends further into the afternoon sector and to higher L shells. For lower-band chorus, strong waves (>50 pT) tend to have wave normal angles of <20° and their wave normal angles become even smaller with increasing wave amplitudes. For modest waves, the wave normal angles are distributed over a broad range with a major peak at <20° and a secondary peak at 60°-80°. Wave normal angles of lower-band chorus are generally smaller on the dayside than on the nightside possibly due to the more uniform and more compressed magnetic field configuration on the dayside. Lower-band chorus becomes more oblique with increasing latitude on the dayside, whereas on the nightside the probability of observing oblique chorus decreases at higher latitudes. Compared to lower-band chorus, the properties of upper-band chorus are somewhat different. Upper-band chorus is considerably weaker in magnetic wave amplitudes, shows tighter confinement to the magnetic equator (<10°), and occurs at smaller L shells (<8). Furthermore, wave normal angles of upper-band chorus are generally larger than those of lower-band chorus, but the occurrence rate still peaks at wave normal angles of <20°, particularly for strong upper-band chorus. Copyright 2011 by the American Geophysical Union.