Correlation of auroral power with the polar cap index

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Abstract

We compare auroral power (AP) as derived from a global imager with the polar cap ( PC ) index for the periods of December 1996 through January 1997 (winter) and June 1997 through July 1997 (summer). The auroral power is inferred from auroral luminosity in the N 2 Lyman‐Birge‐Hopfield bands (λ ∼ 160–180 nm) acquired from the ultraviolet imager on board the ISTP/GGS program's Polar satellite in the Northern Hemisphere. The PC index (1‐min time resolution) is based on the horizontal component of the magnetometer recordings from a single near‐geomagnetic polar station at Thule (85.4° corrected geomagnetic latitude). A fair correlation exists between the hemispheric auroral power and PC ( r = 0.70), with higher correlation for winter ( r = 0.77) compared to summer ( r = 0.69). Also, PC correlates with nightside auroral power much better than with dayside auroral power. The best AP‐ PC correlation took place in the pre‐dawn and post‐dusk sectors. The seasonal and diurnal effects are a result of competition between the ionospheric convection currents and field‐aligned currents manifested by seasonal and diurnal variations in ionospheric conductivity. Finally, a comparison of the AP‐ PC correlation between substorm and non‐substorm times indicates that substorm electrojets contribute little, if not at all, to the PC index.

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