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

<p>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 [<a href="http://onlinelibrary.wiley.com/doi/10.1029/2004JA010707/abstract$\#$jgra17631-bib-0016" rel="references:$\#$jgra17631-bib-0016" style="margin: 0px; padding: 0px; border: 0px; outline: 0px; font-size: 12px; vertical-align: baseline; color: rgb(0, 126, 138); font-family: Arial, \textquoterightLucida Grande\textquoteright, Geneva, Verdana, Helvetica, \textquoterightLucida Sans Unicode\textquoteright, sans-serif; line-height: 18px;"><em>Newell and Meng</em>, 1987</a>]. 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\&nbsp;<em>B</em>_<em>z</em>\&nbsp;\&lt; 0 nT) and nonlinearly (<em>B</em>_<em>z</em>\&nbsp;\&lt; -10 nT) with the IMF\&nbsp;<em>B</em>_<em>z</em>. The nonlinear effect can be explained by an\&nbsp;<em>L</em>^-3\&nbsp;dependence of the Earth\textquoterights equatorial magnetic field.</p>