Observations of large magnetospheric electric fields during the onset phase of a substorm

Abstract

A large (30‐mV/m peak) impulsive westward electric field was observed in the midnight, low latitude, dipole L = 7.5 region of the earth's magnetospheric tail at the onset of a large substorm. The measurements were made with the long, 179‐m baseline, cylindrical electric field probes carried by ISEE 1. The electric field impulse was coincident with a sharp 60 nT steplike change in the x component of the magnetic field at the satellite and the onset of a sharp 60 nT decrease in the H component of the field at a magnetic observatory near the subconjugate point. The 2‐min envelope of the westward Ey field correlates with the time derivative ∂Bx/∂t of the collapsing magnetic field attributed to the decrease in the cross‐tail current. Associated with this inductive impulse, large electric field variations are also observed on time scales of tens of seconds to tens of milliseconds. The low‐frequency (10 s) wave variations show a coherent phase relationship between the electric and magnetic field that changes from correlative to anti‐correlative during the event. Higher frequency (t < 1 s) turbulent electrostatic fields of similar magnitude are present throughout the event. These large time‐dependent electric fields seem to have the proper amplitude, duration, and timing relative to the auroral substorm sequence to explain the energetic proton enhancements frequently observed near midnight both at synchronous orbit and in the magnetotail at times of substorm onsets. The azimuthal direction of Ey (westward) for this event appears to conflict with the IMP 6 observations of an eastward Ey at higher latitudes under similar conditions. However, these observations are in agreement when magnetotail collapse is viewed in terms of a sharp reduction in the anti‐solar tension on auroral field lines as opposed to interpretations based solely on the effects of a sudden decrease in the cross‐tail current sheet.

Particle Transport and Energization Associated with Disturbed Magnetospheric Events

“…We consider the electric field to be accompanied by a consistent magnetic field, forming a pulse that propagates towards the Earth. Correlation studies which support this picture of consistent inductive E and B fields have been previously performed [8].…”

Phase bunching during substorm dipolarization

“…In particular, it was mentioned above that the technique adopted to model field line dipolarization leads to a global reconfiguration of the magnetosphere which clearly is unrealistic [e.g., Nagai, 1991]. Also, we considered a smooth relaxation of the magnetic field whereas in situ measurements reveal numerous spikes on the dipolarization timescale [e.g., Aggson et al, 1983]. We furthermore neglected propagation effects associated with local disruption of the cross-tail current [e.g., Jacquey et al, 1991].…”

Substorm associated magnetotail energetic electrons pitch angle evolutions and flow reversals: Cluster observation

“…Early studies of the energetic electrons' pitch angle distributions during substorms included both modeling work and observational work. Modeling and observation work have shown that substorm induced electric fields accelerate electrons [Moore et al, 1981;Mauk, 1986;Aggson et al, 1983;Quinn and Southwood, 1982;Hesse, 1993, 1996;Birn et al, 1998Birn et al, , 2000.…”