The occurrence of bright (>0.0016 ergs cm-2 s-1 sr-1) polar rain is found to be determined by interplanetary magnetic field (IMF) conditions. An automated search of 11 years of DMSP particle precipitation data (1984--1994) was used to identify polar rain. Comparison was made with 30-min segments of appropriately lagged 15-s IMP 8 magnetic field data. Bright polar rain away from the dayside merging line occurred almost exclusively under conditions favorable for rapid merging (Bz2.5|Bz|). This implies that it occurs only on recently merged field lines and that during northward IMF open field lines have a distinctly different character, threading the magnetopause much further downtail than during southward IMF. The magnitude of this result was found to depend crucially on the distance from the Earth--Sun line of the IMF monitor, suggesting that unless the monitor is restricted to being within ~10 RE of the Earth--Sun line, statistical correlations with IMF are likely to suffer contamination from roughly one in five events.¿ 1997 American Geophysical Union