Poleward expansion of the auroral oval and associated phenomena in the magnetotail during auroral substorms: 1.

The early phase of the auroral substorm is characterized by poleward motion of the poleward boundary of the auroral oval and by a poleward shift of the auroral electrojet. By Using records from a Canadian north-south chain of magnetic stations, data from Vela satellites, and published observations by the Imp 2 satellite, first-approximation meridian profiles of the concurrent changes in the auroral oval and in the plasma sheet are obtained. It is shown that a sudden enhancement of energetic electrons (Ee>45 kev) occurs in the magnetotail during the electrojet's poleward motion. In the 33 examples studied, the electron enhancement always occurred after the electrojet reached dipole latitude 69°. The length of the delay from this time strongly depends on the perpendicular distance of the detecting satellite from the neutral sheet; specifically, a satellite within 1 or 2 RE of the neutral sheet detects the electron enhancement about 20 minutes after this time or when the electrojet reaches dipole latitude ≈ 74°. The delay times depend less strongly on the satellite's radial distance from the earth (within the radial distance range of the observations, i.e., 11 to 19 RE). The sudden enhancement of energetic electrons is usually accompanied by a large increase of the number density of lower energy plasma particles. The energetic electron enhancements are thus associated with mass transport through the magnetotail and not caused by the passage of a ‘disturbance’ through the ambient plasma. The immediate source of the moving plasma is not known at present.