Energetic electron precipitation during high-speed solar wind stream driven storms

Electron precipitation from the Earth's inner magnetosphere transmits solar variability to the Earth's upper atmosphere and may affect surface level climate. Here we conduct a superposed epoch analysis of energetic electrons observed by the NOAA POES spacecraft during 42 high-speed solar wind stream (HSS) driven geomagnetic storms to determine the temporal evolution and global distribution of the precipitating flux. The flux of trapped and precipitating E > 30 keV electrons increases immediately following storm onset and remains elevated during the passage of the HSS. In contrast, the trapped and precipitating relativistic electrons (E > 1 MeV) drop out following storm onset and subsequently increase during the recovery phase to levels which eventually exceed the prestorm levels. There is no evidence for enhanced precipitation of relativistic electrons during the MeV flux drop out, suggesting that flux drop outs during the main phase of HSS-driven storms are not due to precipitation to the atmosphere. On average, the flux of precipitating E > 30 keV electrons is enhanced by a factor of similar to 10 during the passage of the high-speed stream at all geographic longitudes. In contrast, the precipitating relativistic electron count rate is observed to peak in the region poleward of the South Atlantic Anomaly. During the passage of the high-speed stream, the flux of precipitating E > 30 keV electrons peaks in the region from 2100 to 1200 magnetic local time at low L (4 < L < 7) and in the prenoon sector at high L (7 < L < 9), suggesting that chorus waves are responsible for the precipitation of E > 30 keV electrons in both regions.