Survey of Poynting flux of whistler mode chorus in the outer zone

摘要

We present a survey of whistler mode chorus emissions based on high-telemetry rate data of the Plasma Wave Instrument on board the Polar spacecraft. Using simultaneous measurements of full vectors of the electric field and the magnetic field we calculate the Poynting vectors of chorus, and we parameterize the observations by their L~* coordinate. Our new analysis of the angle between the direction of the Poynting vector and the magnetic field line confirms previous results on propagation of the main chorus band away from the equator. Our systematic results also prove the existence of the magnetospherically reflected component of chorus found previously by case studies. We observe weak outer zone waves that propagate toward the equator. These waves can be attributed to a poleward reflected component of equatorial chorus or chorus generated in the dayside pockets of a low magnetic field strength. We analyze the probability distribution of the Poynting flux of chorus, and we find that it shows a "heavy tail" feature that can be modeled by a power law or lognormal model. This result has consequences for theories of chorus generation and for methods of statistical characterization of chorus intensities in the frame of radiation belt modeling. We find that the Poynting flux of dayside chorus increases with L~* toward the outer zone at L~* > 6. We estimate that approximately 1 of the Poynting flux of chorus in the outer zone could be sufficient to accelerate electrons in the outer Van Allen radiation belt on the timescale of days.