Statistical Analysis of Multi-spacecraft Observations on CIR Evolution

STEREO-BEHIND (STB) and STEREO-AHEAD (STA) drifted behind and ahead of ACE around the Sun, with longitudinal separation increasing day by day. CIR driven by high speed solar wind from coronal holes swept STB, ACE, and STA sequentially. Currently, solar wind observations on ACE are the direct basis for Earth space environment forecasting. Based on interplanetary magnetic field and solar wind observations on STB and ACE, assuming high speed stream transmitted uniformly and radially from coronal holes, characteristics of CIR travel time from STB to ACE with their longitudinal separation increased from 0° to 70° was discussed. Statistical variation characteristics of high speed stream parameters were analyzed. The following was found out. Firstly, CIR arrival time on ACE could be predicted by CIR observations on STB. The average and maximum errors of travel time was 0.217d and 0.952d, respectively. 61% of all events reached ACE earlier than theoretical travel time. The maximum errors of travel time increased with longitudinal separation. The errors of travel time was related to observation differences in CIR speed on STB and ACE. The average and maximum errors of travel time were reduced to 0.194d and 0.489d respectively after being corrected by speed variances. Secondly, Linear correlation coefficient of maximum CIR speed on STB and ACE was 0.84. Characteristic parameters of the same CIR on STB and ACE such as speed and proton temperature showed the least difference, while proton density and total pressure showed relatively large difference. The average and maximum range of difference on CIR characteristic parameters observed by STB and ACE, such as maximum speed, maximum proton temperature, maximum total pressure, maximum magnetic field and maximum magnetic southward components, increased with longitudinal separation, while they decreased on maximum proton density. Thirdly, CIR events with larger maximum range and average of observation difference between STB and ACE concentrated in longitude 10°, 25° and 47°, corresponding to CR2059-2060, CR2065-2066, CR2077-2078, which had larger errors on travel time. These results showed that, there was a strong similarity between CIR observations on STB and ACE. Therefore, CIR observations on STB could be used as reference of CIR characteristics on ACE, and provided basis for forecasting of Earth space environment disturbance.