Based on 10 years of European Incoherent Scatter (EISCAT) radar data, we have investigated the variation of ionospheric conductivities, electric fields, and currents over magnetic local time (MLT). Pedersen and Hall currents are generally enhanced especially on the nightside, and the MLT dependencies of these two types of ionospheric currents is similar, as previously found for field‐aligned currents and auroral electrojets, respectively. We readdress the question to what extent the conductivity or the electric field contributes to the ionospheric current in different periods of MLT. On average, higher Pedersen conductivities are seen at 2000–0800 MLT in comparison with other MLT intervals, and these conductivities crucially contribute to Pedersen currents over two MLT sectors, around midnight and in the late morning. However, when the Pedersen current is stronger, not only the Pedersen conductivity but also the electric field becomes higher statistically on the nightside. Hall conductivities are also higher at about 2000–0800 MLT, showing two maxima, around midnight and in the late morning, and they increase more strongly than Pedersen conductivities on a statistical basis. Thus the nightside electrojets are mainly due to high Hall conductivities.

Abstract. Simultaneous Common Program Two experiments by the EISCAT UHF radar at Tromsø and the EISCAT Svalbard radar at Longyearbyen from 00:00 to 15:00 UT on 22 September 1998 and 9 March 1999 have been utilized to investigate distributions of the ion and neutral temperatures in the E-region between 105 and 115 km. During the experiments, soft particle precipitations in the dayside cusp were observed over the Svalbard radar site by the Defense Meteorological Satellite Program (DMSP) F11 satellite. It is found that the dayside electric field in the regions of the lowlatitude boundary of the polar cap and the cusp was greater and more variable than that in the auroral region. The ion temperature, parallel to the geomagnetic field at Longyearbyen, was higher than that at Tromsø during the daytime from 06:00 to 12:00 UT. The steady-state ion energy equation has been applied to derive neutral temperature under the assumption of no significant heat transport and viscous heating. The estimated neutral temperature at Longyearbyen was also higher than that at Tromsø. The ion and neutral energy budget was discussed in terms of the ion frictional heating and the Joule heating. The results indicate two possibilities: either the neutral temperature was high in the low latitude boundary of the polar cap and the cusp, or the heat transport by the polar cap neutral winds toward the dayside sector was significant.

Dusk scatter event (DUSE) (first reported by Ruohoniemi et al. [1988]) is one of the most reproducible features among the SuperDARN radar backscatter within the subauroral ionosphere. Hosokawa et al. [2001] analyzed the scattering occurrence percentage of the Northern Hemisphere SuperDARN radars in a statistical fashion and pointed out that the region where the DUSE appears has a close relationship with the duskside end of the midlatitude trough in longitudinal direction. They proposed a model explaining the generation of the DUSE which employs a Sunward density gradient at the duskside edge of the trough and an ambient poleward electric field. In order to confirm the model proposed by Hosokawa et al. [2001], we have investigated two DUSE events which had been observed by the CUTLASS Finland radar and the EISCAT UHF system simultaneously. Consequently, when the CUTLASS Finland radar observed the DUSE, a Sunward directed density gradient was observed by the EISCAT in the vicinity of the DUSE. After the passage of the DUSE the EISCAT observed an ion temperature enhancement which suggested that the EISCAT entered the trough through its duskside edge. These observational facts suggest that the geometry of the parameters around the DUSE is quite consistent with the model proposed by Hosokawa et al. [2001]. In addition, the EISCAT observed an enhancement of the poleward electric field around the DUSE, and a signature of the substorm was identified by the ground magnetometer on the nightside. We suggest generation mechanisms of the trough responsible for the DUSE during substorm conditions in terms of the role of the enhanced subauroral electric field and discuss a relationship between the DUSE and the other substorm‐related phenomena.

Abstract. Attempting to derive the field-aligned current (FAC) density using the EISCAT radar and to understand the role of the ionosphere on closing FACs, we conducted special radar experiments with the EISCAT radar on 9 October 1999. In order to derive the gradient of the ionospheric conductivity (grad ) and the divergence of the electric field (div E) nearly simultaneously, a special experiment employed an EISCAT radar mode which let the transmitting antenna sequentially point to four directions within 10 min; two pairs of the four directions formed two orthogonal diagonals of a square.Our analysis of the EISCAT radar data disclosed that P div E and E · grad P produced FACs with the same direction inside a stable broad arc around 05:00 MLT, when the EISCAT radar presumably crossed the boundary between the large-scale upward and downward current regions. In the most successfully observed case, in which the conductances and the electric field were spatially varying with little temporal variations, the contribution of P div E was nearly twice as large as that of E · grad P . On the other hand, the contribution of (b × E) · grad H was small and not effective in closing FACs.The present EISCAT radar mode along with auroral images also enables us to focus on the temporal or spatial variation of high electric fields associated with auroral arcs. In the present experiment, the electric field associated with a stable arc was confined in a spatially restricted region, within ∼ 100 km from the arc, with no distinct depletion of electron density. We also detected a region of the high arcassociated electric field, accompanied by the depletion of electron density above 110 km. Using auroral images, this region was identified as a dark spot with a spatial scale of over 150 × 150 km. The dark spot and the electron depletion were likely in existence for a limited time of a few minutes.