Abstract
Abstract. A poleward-progressing 630 nm optical feature is observed between approximately 0100 UT and 0230 UT (0400 MLT to 0530 MLT) by a meridian-scanning photometer (MSP) located at Ny Ålesund, Svalbard. Simultaneous coherent HF radar measurements indicate a region of poleward-expanding backscatter with rapid sunward plasma flow velocity along the MSP meridian. Spatial maps of the backscatter indicate a stationary backscatter feature aligned obliquely with respect to the MSP meridian, which produces an impression of poleward-expansion as the MSP progresses to later MLT. Two interpretations of the observations are possible, depending on whether the arc system is considered to move (time-dependent) or to be stationary in time and apparent motion is produced as the MSP meridian rotates underneath it (time-independent). The first interpretation is as a poleward motion of an east-west aligned auroral arc. In this case the appearance of the region of backscatter is not associated with the optical feature, though the velocities within it are enhanced when the two are co-located. The second interpretation is as a polar arc or theta aurora, common features of the polar cap under the prevailing IMF northwards conditions. In this case the backscatter appears as an approximately 150 km wide region adjacent to the optical arc. In both interpretations the luminosity of the optical feature appears related to the magnitude of the plasma flow velocity. The optical features presented here do not generate appreciable HF coherent backscatter, and are only identifiable in the backscatter data as a modification of the flow by the arc electrodynamics.
Highlights
The SuperDARN coherent HF radars (Greenwald et al, 1995) are designed to employ backscatter from highlatitude ®eld-aligned ionospheric plasma density irregularities as tracers of the bulk plasma motion under the inuence of the convection electric ®eld, and as a diagnostic tool for the investigation of large-scale magnetospheric-ionospheric coupling
As the meridianscanning photometer (MSP) meridian is to the west of the region of backscatter, it must be assumed that the arc has constant geomagnetic latitude along its length to allow the location of the optical feature to be compared with the observedows
If the same interpretation is applied to the optical observations, i.e. that the luminosity features are static and that motion along the MSP meridian is only apparent motion, the arcs can be projected onto the geomagnetic latitude (GMLAT)/magnetic local time (MLT) frame
Summary
The SuperDARN coherent HF radars (Greenwald et al, 1995) are designed to employ backscatter from highlatitude ®eld-aligned ionospheric plasma density irregularities (radar aurora) as tracers of the bulk plasma motion under the inuence of the convection electric ®eld, and as a diagnostic tool for the investigation of large-scale magnetospheric-ionospheric coupling. At E region altitudes the ion-neutral collision frequency is suciently high to constrain the ion population to the neutralow speed, whereas the electrons move under the inuence of the convection electric ®eld with an E B drift. The luminous feature of an arc is the optical manifestation of an upward ®eld-aligned current (FAC) sheet carried by precipitating electrons, enhancing the ionospheric conductivity along its length. The present study examines the observations of ionospheric convectionow measured by the CUTLASS Finland HF radar in the vicinity of a polewardprogressing optical luminosity feature observed by a meridian-scanning photometer located at Ny AÊ lesund, Svalbard The comparison of both observational techniques allows possible interpretations of the observations that would not have been possible with either data set independently. The Doppler velocity gives an estimate of the radar line-of-sight component of the
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