Far-ultraviolet signature of polar cusp during southward IMF Bz observed by TIMED/Global Ultraviolet Imager and DMSP

The coincident TIMED/Global Ultraviolet Imager (TIMED/GUVI) optical and DMSP particle observations have revealed new features of the optical signature of the polar cusp under a southward interplanetary magnetic field (IMF). We have found that cusp auroras usually take the shape of a thin arc with a width around 100\textendash200 km. This provides the first far-ultraviolet evidence of the narrow cusp under a southward IMF [Newell and Meng, 1987]. The cusp auroras could extend down to 0800 magnetic local time (MLT) in the morningside and 1400 MLT in the duskside. Its length is about a few thousand kilometers. A large solar wind density, speed, and IMF are necessary conditions for GUVI to observe the cusp aurora. We found that the cusp location at 1200 MLT changes linearly (-10 nT\ Bz\ \< 0 nT) and nonlinearly (Bz\ \< -10 nT) with the IMF\ Bz. The nonlinear effect can be explained by an\ L-3\ dependence of the Earth\textquoterights equatorial magnetic field.