On the Effect of the Seed Population, a Comparison of Three Low Energy Boundaries in the BAS Radiation Belt Model

Abstract:

Seed electrons with energies ranging from a few to a several hundred keV can be injected into the Earth’s radiation belt region from the night-side plasmasheet. A number of mechanisms may then act on the injected population, subsequently accelerating the electrons to considerably higher energies. Understanding how changes in the lower energy population affect the production of >1 MeV electrons may help us to better understand the occurrence of high energy electrons in the radiation belts. Three different low energy boundaries are used in the BAS Radiation Belt Model: one generated from a statistical method derived from Combined Release and Radiation Effects Satellite (CRRES) data, another based on Van Allen Probe observations, and one from Polar Operational Environmental Satellites (POES) integral flux data taken at low earth orbit. A novel method has been developed to convert the POES integral flux data to differential flux at 90 degree pitch angle, allowing the data set to be used to formulate the low energy boundary. The POES constellation measures a cross-section of the radiation belt region every ~25 min, from up to 6 magnetic local time planes, potentially capturing short lived electron injections not included in the other two boundaries. Using the 3D BAS Radiation Belt Model, the effects of the three different low energy boundaries are studied for a number of events and the results are compared to observations. We show that even small variations between the low energy boundaries resulted in differences in the amount of high energy electrons produced. Model results obtained using the boundary formed from POES low earth orbit data consistently showed a better agreement to observations than model outputs using the statistical method for the low energy boundary. Our results indicate that the duration of the active period and the timeframe over which changes on the low energy boundary occur are both important for the acceleration of >1 MeV electrons.