The dependence of plasma and magnetic field correlations in the solar wind on geomagnetic activity

The solar wind structures measured by four spacecraft (ISEE 1, ISEE 3, IMP 8, and Wind) are correlated and examined in the light of their geoeffectiveness. The average plasma and magnetic field correlations range from about 0.6 to 0.7 during quiet periods and increase substantially during the geomagnetic storms. Using the Ap and Dst indices as a measure for the level of geomagnetic disturbance, we find that correlations of plasma density, |B|, Bz and By are about 0.85 for the most geoeffective events. Earth's foreshock significantly degrades the correlation when one or both spacecraft are within ∼30 RE of Earth during quiet periods, and this degradation extends beyond 50 RE during geoeffective periods. Excluding the foreshock effect, the correlations show little dependence on the spacecraft separation along the Sun‐Earth line, but depend strongly on the spacecraft separation in the direction perpendicular to the Earth‐Sun line, consistent with previous findings. The decrease in correlation in the perpendicular direction is less severe during geoeffective periods, suggesting that geoeffective features have larger scale sizes. The front normals of geoeffective features are predominantly in the propagation direction, consistent with radially outward propagating ejecta.