Off‐equatorial chorus occurrence and wave amplitude distributions as observed by the Polar Plasma Wave Instrument

Abstract

Determining the global distribution of chorus wave power in the off‐equatorial region (i.e., magnetic latitude λ > 15°) is a crucial component of understanding the contribution of chorus to radiation belt acceleration and loss. In this paper we use a database of chorus power spectral density observations from the Plasma Wave Instrument (PWI) Sweep Frequency Receiver (SFR) on the Polar spacecraft to generate separate distributions of wave occurrence rate and magnetic field amplitude as a function of space and geomagnetic activity. Previous studies focused on a band‐integrated and time‐averaged data product to characterize the global distribution of wave power. Using a slightly different technique, we first estimate the wave amplitude from the peak wave power spectral density for times when chorus is observed. The mean wave amplitude at a given location is then multiplied by the wave occurrence rate to yield the time‐averaged amplitude. We present the spatial distributions of wave occurrence rate, mean amplitude, and time‐averaged amplitude in the region of maximum statistics, λ > 15° and R = 4−8 RE. We find that waves of significant amplitude (>10 pT) can be observed in all local time sectors, but significant wave occurrence (>20%) is confined to the dawn and noon local time sectors. Wave mean and time‐averaged amplitudes are also highest in the dawn and noon sectors. The spatial extent of regions with high time‐averaged amplitude is primarily defined by regions of high occurrence rate. Time‐averaged amplitudes exceeding ∼6 pT are observed up to a magnetic latitude of 40° at dawn and 50° at noon, while at midnight and dusk the time‐averaged amplitude tends to be below that value. We also examine the geomagnetic and solar wind dependence of the spatial distribution of wave occurrence, mean amplitude, and time‐averaged amplitude. In the off‐equatorial region (λ > 15°), wave amplitude and occurrence on the nightside increase dramatically during disturbed geomagnetic and solar wind conditions. In contrast, waves on the dayside occur over a wider range of activity, and even during quiet conditions, mean and time‐averaged amplitudes at noon significantly exceed amplitudes at midnight for disturbed times. In the dusk sector, observation of waves is mostly limited to quiet conditions, and during those times, mean amplitudes at dusk exceed those at midnight and approach amplitudes observed in the dawn sector. Parallel investigation of the independent variability of occurrence and amplitude provides a more complete picture of the chorus wave environment, particularly for application to modeling radiation belt dynamics on both short and long time scales.