Interpretation of the cross-correlation function of ACE and STEREO solar wind velocities using a global MHD Model

摘要

Measurements from the ACE and STEREO A and B spacecraft are allowing an unprecedented view of the structure of the three-dimensional heliosphere. One aspect of this is the degree to which the measurements at one spacecraft correlate with those at the other. We have computed the cross-correlation functions (CCFs) for all three combinations of ACE and STEREO A and B in situ observations of the bulk solar wind velocity as the spacecraft moved progressively farther away from one another. Our results confirm previous studies that the phase lag between the signals becomes linearly larger with time. However, we have identified two intervals where this appears to break down. During these "lulls," the CCF reveals a phase lag considerably less than that which would be predicted based only on the angular separation of the spacecraft. We modeled the entire STEREO time period using a global MHD model to investigate the cause for these "lulls." We find that a combination of time-dependent evolution of the streams as well as spatial inhomogeneities, due to the latitudinal separation of the spacecraft, are sufficient to explain them.