Polar Ultraviolet Imager observations of global auroral power as a function of polar cap size and magnetotail stretching

The Polar Ultraviolet Imager (UVI) instrument can quantitatively determine important magnetospheric descriptors, notably including substorm onset time and global auroral output. Previous research has related the input variabl es of the magnetospheric system, namely solar wind parameters, to various o utput variables. However, a complex system such as the magnetosphere includ es, in addition to inputs and outputs, state variables. Polar cap flux and magnetotail stretching are two such that can be estimated from the Defense Meteorological Satellite Program (DMSP) series satellites. We herein determ ine that both polar cap flux, Phi (c), and the magnetotail stretching index , b2i, do correlate well with 40-min averages of nightside auroral power ob served by UVI. There were a total of n = 638 distinct 40-min intervals with in which b2i, Phi (c), and nightside auroral power could be determined. The correlations with premidnight auroral power were r = 0.72 for Phi (c) and r = -0.76 for b2i. The multiple-correlation coefficient of these two variab les with nightside auroral power was 0.81. These sample correlations are fa r better than the sample correlations of solar wind input variables to nigh tside auroral power. Thus accurate space weather forecasting can demonstrab ly benefit greatly by monitoring current magnetospheric state variables (no wcasting), rather than attempting to reproduce output variables solely from solar wind inputs. Attempts to predict substorm onsets were less successfu l. Although the average polar cap flux prior to onset is larger than normal , the difference is not large enough to have significant predictive capabil ity. Specifically, polar cap flux averaged 404 +/- 133 and 422 +/- 148 MWb for the entire years of 1996 and 1997, respectively, while the polar cap fl ux at the time of substorm onset averaged 455 +/- 143 MWb.