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

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 E y field correlates with the time derivative ∂B x /∂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 E y (westward) for this event appears to conflict with the IMP 6 observations of an eastward E y 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.