| Summary: | A ground-based optical study of polar arc characteristics is presented. An All-sky Camera (ASC) and Meridian Scanning Photometer (MSP) were operated at Eureka (89° N, Corrected Geomagnetic Latitude), Northwest Territories, over winter periods from 1990 to 1994 with 656 polar arcs detected by the ASC. Polar arcs were observed on average 8% of the time (above 300 R, 557.7 nm) by the ASC, with a diurnal variation peaking around local midnight. The amplitude of the diurnal variation was about 37% of the mean occurrence rate. This may be due to the fact that the viewing area of the ASC in the ionosphere was magnetically connected to a magnetotail region which was closer to the plasma sheet around the local midnight hours. Polar arcs were detected by the MSP with a threshold of 50 R 30% of the time from 557.7 nm emissions and 49% of the time from 630.0 nm emissions. These suggest that the precipitated electrons exciting the polar arcs were often very soft. It was also found that the mean frequencies of polar arcs over the winter periods correlated with solar activity (10.7 cm flux or sunspot number). Most polar arcs (78%) occurred with northward IMF Bz. However, there were significant numbers (16%) observed with the IMF Bz stably southward. Polar arcs also tended to occur with the IMF Bx near zero and the IMF By slightly positive. One half of the polar arcs (above 300 R, 557.7 nmm) had lifetimes less than 20 minutes. There were 25 solar wind dynamic pressure enhancements detected by the IMP-8 satellite during the 1991-92 and 1992-93 winter periods. All the enhancements were followed by polar arcs. A short time delay (18 min on average) between the enhancements and the polar arcs with southward IMF Bz suggests direct access of solar wind particles into the polar ionosphere and an open magnetospheric configuration in the polar cap region. A positive correlation between the intensity of 630.0 nm emission of the polar arcs and the magnitude of the enhancements supports the direct access of the solar wind particles. The long time delay (70 min on average) for polar arc appearance following the enhancements with northward IMF Bz suggests the particle source for the polar arcs in this case was located in the magnetotail. This finding shows the solar wind dynamic pressure enhancements do generate or induce energetic electron precipitation in the polar cap with either northward or southward IMF Bz. Mapping of polar arcs by using the Tsyganenko magnetic field model (T89) suggests electron source regions for polar arcs are mostly located in the magnetotail lobe region. The mapping supports a bifurcated magnetotail model or an expanded plasma sheet (plasma sheet boundary) model. Suggestions for future work include the relation between the energy of precipitated electrons and the polarity of the IMF, generation of field-aligned electric fields, the relation between the lifetimes of polar arcs and stability of the northward IMF Bz or solar wind dynamic pressure, and development of fine structure in polar arcs in relation to plasma instabilities in the magnetosphere or the ionosphere.
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