Chatanika Radar observations of ionospheric and field‐aligned currents

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Abstract
A technique for estimating field‐aligned current densities from Chatanika (64.9° invariant latitude) incoherent scatter radar measurements is presented and used to examine characteristics of ionospheric and field‐aligned currents at auroral latitudes as functions of magnetic local time and substorm activity. The field‐aligned current densities are obtained as the divergence of height‐integrated ionospheric currents measured at different latitudes. In this paper the one‐dimensional divergence of the horizontal currents is taken to approximate the current density of a field‐aligned current sheet with variations of the horizontal current parallel to the current sheet assumed to be negligible. It is found that the field‐aligned current densities thus deduced from the Chatanika radar data agree with those obtained from simultaneous magnetic observations by the Triad satellite, when the divergence for the radar estimates is taken either perpendicular to the direction of the field‐aligned current sheet inferred from the Triad data or simply along the electric field direction. The following are general features of the estimated field‐aligned currents: (1) Downward (or upward) field‐aligned currents with densities of 10−6–10−5 A/m² are typically obtained in the evening (or morning) sector, particularly equatorward of Chatanika. The observations thus correspond to the equatorward portion of the field‐aligned current system normally observed by more direct methods. (2) During periods of increased geomagnetic activity the observed densities are enhanced, and at times the current directions are reversed. The latter feature presumably occurs during expansion of the auroral oval, when the poleward portion of the current system (in which the current directions are the opposite of those of the equatorward half) moves within the region probed by the radar.