Field-aligned currents and ionospheric parameters deduced from EISCAT radar measurements in the post-midnight sector

• Main Authors: Sugino, M., Fujii, R., Nozawa, S., Nagatsuma, T., Buchert, S. C., Gjerloev, J. W., Kosch, M. J.
• Other Authors: Solar-Terrestrial Environment Laboratory Nagoya (STEL), Nagoya University, Communications Research Laboratory, 4-2-1 Nukui-kita, Swedish Institute of Space Physics Uppsala (IRF), National Research Council, NASA Goddard Space Flight Center (GSFC), Department of Communications Systems
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2002
• Subjects: [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; EISCAT
• Online Access: https://hal.science/hal-00317127; https://hal.science/hal-00317127/document; https://hal.science/hal-00317127/file/angeo-20-1335-2002.pdf

International audience 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 S) 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 S P div E and E · grad S 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 S P div E was nearly twice as large as that of E · grad S P . On the other hand, the contribution of ( b × E ) · grad S 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 arc-associated 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.