Variable time delays in the propagation of the interplanetary magnetic field

Simultaneous measurements of the interplanetary magnetic field (IMF) are obtained at various locations with four spacecraft, ACE, Wind, IMP‐8, and Geotail. We have devised a technique whereby the exact propagation delay time between ACE, at the L1 orbit, and each of the other three spacecraft can be derived from these measurements. This propagation delay is determined as a continuously varying function of time; when this measured delay is applied to all three components of the IMF measured by ACE, they will match the other satellites' IMF to a degree that is much better than expected. However, the actual time delays can vary by nearly an hour in either direction from the expected advection delays, and the lag times have significant changes that can occur on a timescale of a few minutes. These results are interpreted as due to the effects of tilted phase fronts that are changing orientation with time. We have used the delay measurements between multiple satellites to calculate the three‐dimensional orientation and temporal variations of the phase front. The best fit phase front plane usually lies within 4 R E or less from the four‐point measurements, indicating a lag resolution of a minute or less. Computer animations of the time‐varying phase fronts are used to illustrate their behavior. Orientations can change on short timescales. Our findings have implications for both basic research and “space weather” predictions. These results give a high confidence that the same IMF that is measured near L1 will most likely impact the Earth's magnetosphere, providing ample justification for use of spacecraft data in halo orbit at L1 for monitoring the upstream solar wind prior to its interacting with the magnetosphere. However, there is strong uncertainty in the timing of the arrival of the detailed IMF structures, and these delays will need to be considered.