Geoeffective interplanetary scale sizes derived from regression analysis of polar cap potentials

This paper presents a physical interpretation of linear regression comparisons between measured and predicted values of the polar cap potential and the interplanetary electric field (IEF). We define a “geoeffective” scale size in the solar wind L G as spanning all interplanetary equipotentials that touch the magnetopause. Simple dimensional analysis considerations suggest that L G is proportional to the slope of the linear regression curve. The regression intercept represents potential from sources other than the IEF. Four independent data sets indicate that L G is only a few R E wide in the Y GSM direction. This has implications for the reliability of space weather forecasting. Most IEF observations from subsolar libration orbits are taken many RE from the Sun‐Earth line. Good correlations with near‐Earth variations require that the IEF has a Y GSM scale size that is significantly larger than L G .