Multispacecraft Observations of Solar Wind Plasma Propagation

Using measurements from multiple upstream interplanetary spacecraft, including ACE, Wind, Genesis and IMP-8, we examine the variations of plasma parameters (density, speed, and temperature) as the solar wind propagates between spacecraft. Solar wind structures are often imagined to lie along planar phase fronts that propagate radially, but may be oriented at arbitrary angles to the flow. The difference in arrival time of particular structures at different spacecraft, called the lag time, is typically used to infer an orientation of the assumed planar phase front. We find that the lag time appropriate to match observed features between any two spacecraft can vary on short time scales, changing by tens of minutes over intervals of minutes to hours. That variability suggests that either the planar fronts must change speed or direction as they propagate from one spacecraft to another, or the structures themselves are not actually planar. Using three-spacecraft observations, we present examples which do not appear to be consistent with simple planar structures.