Temporal and Spatial Variation of the Solar Wind Bulk Properties from STEREO SWEA/PLASTIC by Multi-Spacecraft Analysis

The two STEREO spacecraft with nearly identical instrumentation were launched near solar activity minimum and they separate by about 45 degrees per year providing a unique tool to study the temporal and spatial evolution of the solar wind. We analyzed the solar wind bulk properties measured by the SWEA electron and the PLASTIC ion plasma instruments on board. We calculate the timelag between the STEREO A and B spacecraft considering their radial and longitudinal separation and time-shift the B measurements in order to forecast the A measurements. We show that the correlation between the forecasted and the real A datasets is very good. It decreases slightly as their timelag increases, which is due to the temporal evolution of the solar wind. We also find that this correlation clearly decreases when we compare structures of smaller spatial scales. As a result, the characteristic temporal and spatial changes in the solar wind bulk properties can be quantitatively determined.