Van Allen Probes EFW Filterbank First Results: Geophysical Distributions of Large Amplitude Chorus Occurrence

We present a survey of large amplitude chorus waves using Van Allen Probe EFW filterbank data. The filterbank records at eight times per second the largest amplitude as well as the average amplitude from any number of selectable channels in 7 or 13 frequency bins. Thus the filterbank instrument effectively counts the number of discrete chorus elements. In the first year of the Van Allen Probe mission the two satellites have surveyed a large percentage of the chorus source region during solar maximum. The results presented represent the first survey of large amplitude chorus at L<5. The survey is divided into two periods. In the first, 2012-09-25 to 2013-03-15, the filterbank recorded 13 frequency channels of DC-coupled electric field from the E12 booms, allowing sufficient frequency resolution to separate lower and upper band chorus. Starting on 2013-03-16 the frequency resolution was reduced to seven channels but with the addition of seven frequency channels of spin axis search coil magnetic field (SCMw) from EMFISIS. Geophysical distributions of chorus occurrence are presented as a function of amplitude, DST and AE index. Results indicate that large amplitude chorus (>20 mV/m) are a near constant feature of the chorus source region down to L=3 during active times, with the largest amplitude electric fields typically found immediately outside of the plasmapause. Large amplitude chorus waves have been shown to interact very strongly with high-energy electrons causing pitch angle scattering and energization. Thus the ubiquity of large amplitude chorus waves during active times suggests that they may play an important role in the dynamics of the radiation belts.