Wave normal angles of magnetospheric chorus emissions observed on the Polar spacecraft

Abstract

Using data from the High Frequency Waveform Receiver on board the Polar spacecraft, 1,765 and 993 wave normal angles have been analyzed for 13 orbits containing upper band magnetospheric chorus emissions and 15 orbits containing lower band emissions, respectively. The purpose of this study is to characterize the distribution of the polar wave normal angle, θ, for chorus emissions as a function of magnetic latitude, λ. Understanding wave normal angles is an important step in evaluating resonant wave‐particle interactions. For upper band chorus, wave normal angles tend to remain at or rise toward the resonance cone angle for low latitudes and midlatitudes but move away from the resonance cone angle at higher latitudes. For lower band chorus, wave normal angles with values θ < 20° have the highest probability of occurrence in the latitude range of 10°–50°. Just off the equator, 10° ≤ λ < 25°, there exists a secondary occurrence peak in the range of 50° ≤ θ < 70°. The probability of observing these higher wave normal angles decreases with increasing latitude. The time‐averaged Poynting flux, S, is much larger for lower band chorus waves, which have a mean value of 8.5 × 10−8 W/m2, than for upper band chorus waves, which have a mean value of 1.4 × 10−9 W/m2. S is fairly evenly distributed about its median value, 3.1 × 10−10 W/m2, for all wave normal angles for upper band chorus but deceases as θ increases for lower band chorus.