Chatanika Radar observations of the latitudinal distributions of auroral zone electric fields, conductivities, and currents

Abstract

The latitudinal distributions of convection electric fields, height‐integrated Hall (ΣH) and Pedersen (ΣP) conductivities, and horizontal currents in the auroral ionosphere have been measured with the Chatanika incoherent scatter radar for the range of invariant latitudes 63°–68°. Approximately 60 hours of data were obtained for three geomagnetically disturbed days: January 18, May 16, and May 17, 1974. During this observation period the general electric field properties, as functions of latitude and time, were as follows: (1) Strong electric fields in the range 50–100 mV/m were typically directed northward in the afternoon‐evening sector and southward in the morning sector; (2) the electric field strength tended to increase with increasing latitude, with latitudinal scale lengths of 1°–3°. (3) In several instances a reversal of the electric field direction occurred at the highest latitudes probed, corresponding to antisunward plasma convection near the boundary of the polar cap. (4) The Harang discontinuity in the electric field was seen in the midnight sector (2200–0100 MLT) as a complex rotation of the electric field vector counterclockwise from northward through westward to southward, beginning at the highest latitudes, as the radar moved in local time through the Harang discontinuity. The Harang discontinuity region was often slanted in local time and latitude from the northwest to the southeast. Strong westward electric fields were seen dividing regions of northward and southward electric fields on the western and eastern sides, respectively, of the discontinuity. Association of the electric fields with the conductivities showed that (5) those early evening sector electric fields which were embedded within diffuse auroral precipitation associated with conductivities ΣP = 8–12 mhos and ΣH = 16–24 mhos tended to have less latitudinal and temporal structure than did midnight‐morning sector electric fields embedded within active and highly structured precipitation and conductivities ΣP = 10–16 mhos and ΣH = 20–60 mhos and that (6) the local time transition between the diffuse precipitation‐conductivity zone in the evening sector and the harder, active precipitation‐conductivity zone in the midnight‐morning sector coincided with the Harang discontinuity in the electric field. Association of the electric fields with interplanetary magnetic field (IMF) and AE index data showed that (7) during periods of southward IMF and increased AE activity the observed electric field strengths were enhanced, and in several instances, reversals of the electric field direction occurred at the highest latitudes. These data imply that during such periods a latitudinal belt of strong electric fields, associated with the auroral oval, expanded into the region probed by the radar and that the reversals of the electric field occurred near the poleward edge of this belt (i.e., the boundary of the polar cap). Comparison of the electric fields with optical auroral data from DMSP and all‐sky photographs showed that: (8) near a westward traveling auroral surge in the evening sector during one substorm the electric field was strongly westward near the leading westward edge or ‘nose’ of the surge, while northward electric fields were found equatorward and eastward of the surge's nose; (9) an auroral substorm in the premidnight hours brought about a brief premature southward transition of the electric field, suggesting a westward ‘jump’ of the Harang discontinuity; (10) in the morning hours, sharp reductions of the southward electric field strengths were seen in regions of bright active aurora, large electric fields often appearing immediately poleward of the high‐latitude borders of these auroral regions; and (11) the electric field vectors were generally directed perpendicular to the alignment of nearby auroral forms.