CONJUGATE OCCURRENCE OF THE ELECTRIC-FIELD FLUCTUATIONS IN THE NIGHTTIME MIDLATITUDE IONOSPHERE

The DE 2 satellite observed electric field fluctuations on the topside of the nighttime midlatitude ionosphere. They extended several hundre d kilometers in the latitudinal direction with wavelengths of several tens of kilometers, and their amplitudes were a few millivolts per met er, Such fluctuations were often observed at magnetically conjugate po ints in the northern and southern hemispheres, These electric field fl uctuations are perpendicular to the geomagnetic field, They are not ac companied by any significant plasma depletion or electron temperature variations. Magnetic field fluctuations are sometimes observed simulta neously with electric field fluctuations, We interpret that these fluc tuations are caused by field-aligned currents which flow from the iono sphere in one hemisphere to the conjugate point in the other hemispher e, The power spectrum of these midlatitude electric field fluctuations follows a power law of the form Power proportional to f(-n), with the spectral index n of 3.5 to 4.5, which is steeper than that of the ele ctric; field fluctuations in the high-latitude ionosphere or in the eq uatorial ionosphere. This phenomenon may be related to other ionospher ic phenomena, for example, the F region field-aligned irregularities o r spread-F, observed by ground-based methods such as the MU radar, but the relationship is not clear.