Simultaneous triggered VLF emissions and energetic electron distributions observed on POLAR with PWI and HYDRA

We report simultaneous observations of energetic 1-20 keV electrons and VLF emissions triggered within the plasmasphere by pulses from ground based VLF transmitters, using the PWI and HYDRA instruments on the POLAR spacecraft. The 1-20 keV electrons have the correct energy to interact with the input pulses through gyoresonance. Emissions are generated by the pulses only when the particle flux is enhanced well above background and the particle pitch angle distribution is very highly anisotropic, with the average equatorial pitch angle exceeding ∼ 75°. Because of these high pitch angles, the particles are trapped typically within 7° of the magnetic equator. Only pulses which propagate within whistler mode ducts are observed to trigger emissions. The observed pitch angle anisoptropies are much larger than those assumed in present models of the VLF emission triggering phenomenon, and thus our observations provide a new starting point for understanding the emission process.