| Using various ground observations at South Pole, Antarctica (invariant magnetic latitude {dollar}-74spcirc{dollar}) and its conjugate point, Frobisher Bay, Canada, we have studied the following aspects of nightside polar cap auroral activity: the appearance and disappearance of polar cap auroras (diffuse and discrete) associated with substorms and interplanetary magnetic field (IMF) variations; auroral optical emission line intensities; and the seasonal variation of auroral conjugacy. The observations show that the polar cap auroras usually fade away before the expansive phase of a substorm and bright auroral arcs reach high latitude ({dollar}-74spcirc{dollar}) near the recovery phase. Just before the auroras fade away the discrete polar cap auroral arcs, which are usually on the poleward boundary of the diffuse aurora, intensify for 1 to 2 minutes. The observations also indicate the the IMF may have stronger control over polar cap auroral activity than do substorms. A search for energy spectral variation of precipitating electrons using the intensities of 630.0 nm (O) and 427 nm ({dollar}Nsbsp{lcub}2{rcub}{lcub}+{rcub}{dollar}) auroral emission lines reveals no dramatic changes in the energy spectrum; instead, the data show possible atmospheric scattering and geometric effects on the photometric measurements while the bright auroral arc is moving into the polar cap. The conjugate observations show that the stormtime auroral electrojet current, which is associated with the bright auroral arc, in most cases reaches higher (lower) latitudes in the winter (summer) hemisphere. An asymmetric plasma sheet (with respect to the neutral sheet) is proposed, which expands deeper into the winter lobe, under a tilted geomagnetic dipole. Accordingly, the winter polar cap would have smaller area and the auroral electrojet would be at higher latitude. |
