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--200 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's equatorial magnetic field.