A study of magnetosphere dynamics during auroral electrojet events by observations of energetic electron intensity changes at synchronous orbit

A statistical study of over 1600 events has established that a close temporal correlation exists between the start of energetic electron directional intensity changes observed at synchronous orbit and the intensification of electrojet currents in the polar ionosphere. This result provides strong evidence that both phenomena are physically linked as part of a large‐scale disturbance and that the electrojet system is closely coupled to other current systems in the distant magnetosphere. The statistical pattern in local time of these electrojet‐associated electron intensity variations consists of decreases observed in the pre‐midnight sector, increases observed over a broad region symmetrical about midnight and spectrum softening increases observed post‐midnight. In the local evening sector intensity decreases and increases are observed with equal probability and typically occur as two parts of a single time sequence such that the decrease directly precedes the increase. Furthermore, while the start times for the decrease and the subsequent increase are both well‐correlated with a marked electrojet intensification the increase is also well‐correlated with a mid‐latitude positive bay but the decrease is not. This difference together with the fact that decreases systematically precede increases in the local time region where both are observed strongly suggests that the decrease corresponds to a distinct ‘precursor’ of the increase event which follows. These results are important for understanding the temporal development of magnetospheric disturbances and are also not consistent with ‘the fault‐line’ concept which suggests that the pre‐midnight decrease corresponds to a simultaneous increase event occurring further eastward in local time. The intensity decreases are closely correlated with a decrease in the local magnetic field and have been interpreted in terms of the electrons responding to a diamagnetic reconfiguration resulting from an enhancement of the particle energy density due to convection of plasma sheet protons into the pre‐midnight region. The subsequent intensity increases are accompanied by an increase in the local magnetic field and can be associated with a tail‐field collapse at expansion onset.