Evolution of the magnetotail energetic‐electron population during high‐speed‐stream‐driven storms: Evidence for the leakage of the outer electron radiation belt into the Earth's magnetotail

For 15 high‐speed‐stream‐driven geomagnetic activations (weak storms) in 2006–2007, the temporal behaviors of the outer electron radiation belt at geosynchronous orbit and the energetic‐electron population of the magnetotail are compared via superposed‐epoch averaging of data. The magnetotail measurements are obtained by using GPS‐orbit measurements that magnetically map out into the magnetotail. Four temporal phases of high‐speed‐stream‐driven storms are studied: (1) the pre‐storm density decay of the electron‐radiation belt, (2) the electron‐radiation‐belt density dropout near the time of storm onset, (3) the rapid density recovery a few hours after dropout, and (4) the heating of the electron radiation belt during the high‐speed‐stream‐driven geomagnetic activity. In all four phases the behaviors of the outer electron radiation belt and of the energetic‐electron population in the magnetotail are the same and simultaneous. The physical explanations for the behavior in phase 1 (decay), phase 2 (dropout), and phase 4 (heating) lie in the dipolar regions of the magnetosphere: hence for those three phases it is concluded that the temporal behavior of the energetic‐electron population in the magnetotail mimics the behavior of the outer electron radiation belt. Behavior attributable to physical processes in the dipole is seen in the magnetotail energetic‐electron population: this implies that the origin of the energetic‐electron population of the magnetotail is “leakage” or “outward evaporation” from the outer electron radiation belt in the dipolar magnetosphere.