Correlating near-Earth interplanetary magnetic fields: Foreshock effects

Interplanetary magnetic field (IMF) observations by ISEE 1 and IMP 8 were c orrelated to reveal the effects of upstream waves on IMF predictions. Past studies using spacecraft just outside the Earth's bow shook and far upstrea m at the L1 libration point attributed frequently poor (less than or equal to0.5) correlation coefficients to short IMF scale lengths and difficulties in estimating time delays. We find that the correlation coefficients for t wo near-Earth spacecraft are actually worse than those for a spacecraft at the Ll point and one just outside the bow shock: 48% of the near-Earth corr elation coefficients for the IMF magnitude are poor (<0.5), and only 17% ar e good (>0.8). We attribute the poor result to two causes: (1) high-frequen cy waves and diamagnetic effects in the foreshock and (2) intervals of low IMF variance. Of these two, high-frequency waves account for 80% of the cas es with poor correlation, and the intervals of nearly constant IMF account for the remaining 20% of the cases. While correlation coefficients do not i ncrease with solar wind density while both spacecraft are in the solar wind , they do increase when one or both spacecraft lie within the foreshock. We argue that foreshock waves and intervals of low IMF variance must also hav e reduced correlation coefficients in previous IMF correlation studies. Whi le the significance of the foreshock waves on the solar wind input into the magnetosphere deserves farther study, there is no obstacle to predicting s olar wind input into the magnetosphere during intervals with poor correlati on coefficients but low IMF variance.