Electron loss rates from the outer radiation belt caused by the fillingof the outer plasmasphere: The calm. before the storm

摘要

Measurements from seven spacecraft in geosynchronous orbit are analyzed todetermine the decay rate of the number density of the outer electron radiation belt prior tothe onset of high-speed-stream-driven geomagnetic storms. Superposed-data analysis isused with a collection of 124 storms. When there is a calm before the storm, the electronnumber density decays exponentially before the storm with a 3.4-day e-folding time:beginning about 4 days before storm onset, the density decreases from —4 x 10-4 cm-3 to—1 x 10-4 cm 3. When there is not a calm before the storm, the number densitydecay is very small. The decay in the number density of radiation belt electrons is believedto be caused by pitch angle scattering of electrons into the atmospheric loss cone as theouter plasmasphere fills during the calms. This is confirmed by separately measuringthe density decay rate for times when the outer plasmasphere is present or absent. Whilethe radiation belt electron density decreases, the temperature of the electron radiation beltholds approximately constant, indicating that the electron precipitation occurs equally atall energies. Along with the number density decay, the pressure of the outer electronradiation belt decays, and the specific entropy increases. From the measured decay rates,the electron flux to the atmosphere is calculated, and that flux is 3 orders of magnitude lessthan thermal fluxes in the magnetosphere, indicating that the radiation belt pitch anglescattering is 3 orders weaker than strong diffusion. Energy fluxes into the atmosphere arecalculated and found to be insufficient to produce visible airglow.