Locations of chorus emissions observed by the Polar Plasma Wave Instrument

We performed a statistical study of the locations of chorus emissions observed by the Polar spacecraft's Plasma Wave Instrument (PWI) from March 1996 to September 1997, near the minimum of solar cycles 22/23. We examined how the occurrence of chorus emissions in the Polar PWI data set depends upon magnetic local time, magnetic latitude, L shell, and L*. The Polar PWI observed chorus most often over a range of magnetic local times extending from about 2100 MLT around to the dawn flank and into the dayside magnetosphere near 1500 MLT. Chorus was least likely to be observed near the dusk flank. On the dayside, near noon, the region in which Polar observed chorus extended to larger radial distances and higher latitudes than at other local times. Away from noon, the regions in which chorus occurred were more restricted in both radial and latitudinal extent. We found that for high-latitude chorus near local noon, L* provides a more reasonable mapping to the equatorial plane than the standard L shell. Chorus was observed slightly more often when the magnitude of the solar wind magnetic field B-SW was greater than 5 nT than it was for smaller interplanetary magnetic field strengths. We also found that near solar minimum, chorus is twice as likely to be observed when the solar wind speed is greater than 450 km/s than it is when the solar wind speed is less than 450 km/s.