On the importance of accurate solar wind measurements for studying magnetospheric dynamics

We have examined the validity of assuming that the solar wind observed at the L1 Lagrange point is a good predictor of the environment at the Earth by using solar wind observations during a magnetic storm from four spacecraft (ACE, Wind, IMP‐8, and Geotail). The storm on 23 and 24 May 2000 occurred when a magnetic cloud reached the Earth. We carried out a running cross‐correlation analysis between the solar wind parameters observed on pairs of the solar wind spacecraft and found good agreement early in the storm. The peak cross‐correlations between magnetic field components exceeded 90% throughout this interval, while the peak cross‐correlations between the dynamic pressure observations was about 70% or better. The main differences were at higher frequencies. However, later after the main cloud had passed the Earth the peak cross‐correlations decreased, especially between ACE at L1 and spacecraft nearer the Earth. Late in the storm the monitors were not observing the same solar wind. We examined the effect of these differences by using our global magnetohydrodynamic simulation to model the magnetosphere using observations from three monitors. Early in the event we found very good agreement between simulated magnetospheres based on input from different solar wind monitors but later the agreement became very bad. If we assume that the monitors near the Earth provide the best measurement of the solar wind that interacts with the magnetosphere, then later in the magnetic storm the solar wind observations at ACE were not a good indication of the solar wind reaching the Earth.