EQUATORIAL ELECTRIC-FIELD OBSERVATIONS

Both DC and AC measurements of equatorial electric fields from the San Marco D satellite are reported. These measurements were made with double floating probe sensors and have yielded a surprising number of new phenomena and effects in regions of equatorial spread F. The electric fields observed inside equatorial bubbles at altitudes greater than 400km are characterized by large DC eastward electric fields (approximately 10 mV/m up to 50 mV/m) which can be associated with large plasma upward convection velocities (500 m/s to 2500 m/s). The vertical electric fields show bubble tilting inclinations that are often westward and more rarely eastward. All of the upward convecting bubble events examined show a striking and systematic lack of conservation of the vertical component of the electric field vector on either side of the structures. This lack of conservation corresponds to the component tangential to the bubble walls and is of the order of 1 mV/m leading to the conclusion that the bubbles are collapsing inward at the rate of some tens of meters per second. The largest updrafting events are shown to involve displacement currents implying that some bubbles evolve into Alfvenic disturbances at high altitudes. The AC measurements can be divided into wavelengths less than or greater than a gyro-radius. At the longer wavelengths the AC electric fields (RMS) are shown to often track the DC electric field variations implying that the eastward currents are continuous through the bubble irregularities. At shorter wavelengths the measured RMS electric fields are shown to be often closely correlated with plasma gradients at the walls of the bubbles.