Observed relationships between electric fields and auroral particle precipitation

Abstract

Simultaneous electric field and plasma observations with the low‐altitude polar‐orbiting satellite Injun 5 have provided a comprehensive survey of convection electric fields and their association with magnetospheric plasma phenomena. The most prominent features of the convection electric fields are reversals located at high magnetic latitudes, with generally antisunward convection poleward and sunward convection equatorward of the electric field reversal location. The electric field reversal is interpreted as the boundary between open and closed magnetic field lines. During local day the electric field reversal is observed to coincide with the equatorward boundary of the polar cusp. The plasma flow in the dayside polar cusp region is dominantly E‐W, away from the stagnation point, the convection velocities typically being ∼1 km sec−1. At local evening, ‘inverted V’ electron precipitation bands are observed near or at the position of the electric field reversal. In the local late‐evening sector the electric field reversal becomes less distinct, and often no single well‐defined electric field reversal can be identified. In all cases the inverted V electron precipitation events are closely associated with large, typically >30 mv m−1, irregular electric field fluctuations with time scales of a few seconds or less. Often these fluctuations comprise distinct ‘spikes’ of a few seconds duration or less, which can be identified with distinct boundaries or other features of the electron precipitation. In the local midnight sector, convection electric fields of >50 mv m−1 associated with plasma sheet electrons have been observed extending deep into the magnetosphere, equatorward of the electron (E > 45 kev) trapping boundary. These convection electric fields are characterized by considerably smaller fluctuations relative to those observed within the inverted V electron precipitation bands. To investigate the electric field and plasma interrelationships during a polar magnetic substorm, a series of passes obtained before and during a substorm is presented. Large antisunward convection velocities were detected over the polar cap several tens of minutes before the onset of the expansive phase of the substorm. These convection velocities gradually decreased during the decay phase of the substorm. Our measurements of enhanced antisunward flow over the polar cap region are generally consistent with several current ideas concerning the origin of substorms.

High-latitude day side electric field and particle measurements

“…The electric field that we found to be 50 to 100 mv/m in the cusp corresponds to a convection velocity of 1 to 2 km/sec. This value is in agreement with the measurements of Heppner (1972) and Gurnett and Frank (1973). Thus, the width at the orifice should be 100 to 200 times the width in the ionosphere or the angular width should be 10 to 20 times the ionospheric angular width assuming the boundary to be at 10 Re.…”

Observations of interactions between interplanetary and geomagnetic fields

“…Figure 3 shows schematically the idealized topologies for reconnection on closed field lines, resulting in closed loops, and reconnection on open field lines, resulting in a macroscopic X-type neutral line configuration. Akasofu (1973) has suggested that reconnection in the tail could be responsible for the inverted 'V' events described by Frank and Ackerson (1972) and Gurnett and Frank (1973), which occur -19 -poleward of the 40-kev trapping boundary and chiefly in the evening sector.…”

Magnetospheric convection and the high-latitudeF2ionosphere

“…In Figure 1 I have simply connected the flow lines of closed field tubes across the cap with a suggestion of divergence around the pole. The electric field measurements of Gurnett and Frank (1973), Cauffman and Gurnett (1971), and Maynard (1972) suggest that the flow may be typically faster near the boundary of closed field lines (l km/sec) and slower interior to the boundary (0.5 km/sec). On occasion the flow velocity is apparently uniform across the entire cap and may rise to 3 km/sec.…”