Correlating near‐Earth interplanetary magnetic fields: Foreshock effects

Interplanetary magnetic field (IMF) observations by ISEE 1 and IMP 8 were correlated to reveal the effects of upstream waves on IMF predictions. Past studies using spacecraft just outside the Earth's bow shock and far upstream at the L1 libration point attributed frequently poor (≤0.5) correlation coefficients to short IMF scale lengths and difficulties in estimating time delays. We find that the correlation coefficients for two near‐Earth spacecraft are actually worse than those for a spacecraft at the L1 point and one just outside the bow shock: 48% of the near‐Earth correlation coefficients for the IMF magnitude are poor (<0.5), and only 17% are good (>0.8). We attribute the poor result to two causes: (1) high‐frequency 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 cases with poor correlation, and the intervals of nearly constant IMF account for the remaining 20% of the cases. While correlation coefficients do not increase 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 have reduced correlation coefficients in previous IMF correlation studies. While the significance of the foreshock waves on the solar wind input into the magnetosphere deserves further study, there is no obstacle to predicting solar wind input into the magnetosphere during intervals with poor correlation coefficients but low IMF variance.