Abstract
Abstract. We report observations of the cusp/cleft ionosphere made on December 16th 1998 by the EISCAT (European incoherent scatter) VHF radar at Tromsø and the EISCAT Svalbard radar (ESR). We compare them with observations of the dayside auroral luminosity, as seen by meridian scanning photometers at Ny Ålesund and of HF radar backscatter, as observed by the CUTLASS radar. We study the response to an interval of about one hour when the interplanetary magnetic field (IMF), monitored by the WIND and ACE spacecraft, was southward. The cusp/cleft aurora is shown to correspond to a spatially extended region of elevated electron temperatures in the VHF radar data. Initial conditions were characterised by a northward-directed IMF and cusp/cleft aurora poleward of the ESR. A strong southward turning then occurred, causing an equatorward motion of the cusp/cleft aurora. Within the equatorward expanding, southward-IMF cusp/cleft, the ESR observed structured and elevated plasma densities and ion and electron temperatures. Cleft ion fountain upflows were seen in association with elevated ion temperatures and rapid eastward convection, consistent with the magnetic curvature force on newly opened field lines for the observed negative IMF By. Subsequently, the ESR beam remained immediately poleward of the main cusp/cleft and a sequence of poleward-moving auroral transients passed over it. After the last of these, the ESR was in the polar cap and the radar observations were characterised by extremely low ionospheric densities and downward field-aligned flows. The IMF then turned northward again and the auroral oval contracted such that the ESR moved back into the cusp/cleft region. For the poleward-retreating, northward-IMF cusp/cleft, the convection flows were slower, upflows were weaker and the electron density and temperature enhancements were less structured. Following the northward turning, the bands of high electron temperature and cusp/cleft aurora bifurcated, consistent with both subsolar and lobe reconnection taking place simultaneously. The present paper describes the large-scale behaviour of the ionosphere during this interval, as observed by a powerful combination of instruments. Two companion papers, by Lockwood et al. (2000) and Thorolfsson et al. (2000), both in this issue, describe the detailed behaviour of the poleward-moving transients observed during the interval of southward Bz, and explain their morphology in the context of previous theoretical work.Key words: Ionosphere (ionosphere - magnetosphere interactions; auroral ionosphere; plasma temperature and density)
Highlights
In December 1998, an extended campaign of measurements took place involving observations of the EISCAT Svalbard radar, the EISCAT Tromsù VHF radar and a number of other diagnostics including meridian scanning photometers at Longyearbyen and Ny AÊ lesund
The major motivations of these experiments were to take advantage of the dark conditions during the noon sector, which allowed optical observations of the cusp to be made from Svalbard, and the possibility to combine these optical observations with data from the EISCAT Svalbard radar which ran a new radar experiment to probe the whole range of altitudes from 90 to 1000 km
The associated 557.7 nm emission appeared to be well poleward of the red-line emission, indicating that it arose from an altitude considerably greater than the 120 km assumed in plotting the EISCAT Svalbard radar (ESR) location
Summary
In December 1998, an extended campaign of measurements took place involving observations of the EISCAT Svalbard radar, the EISCAT Tromsù VHF radar and a number of other diagnostics including meridian scanning photometers at Longyearbyen and Ny AÊ lesund. The major motivations of these experiments were to take advantage of the dark conditions during the noon sector, which allowed optical observations of the cusp to be made from Svalbard, and the possibility to combine these optical observations with data from the EISCAT Svalbard radar which ran a new radar experiment to probe the whole range of altitudes from 90 to 1000 km. The observing campaign, which lasted from December 11th to 22nd, yielded a large amount of interesting data. McCrea et al.: ESR and EISCAT observations of the response of the cusp and cleft
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