In situ Observations of CIRs on STEREO, Wind, and ACE During 2007 – 2008

Abstract

During the 2007 and 2008 solar minimum period, STEREO, Wind, and ACE observed numerous Corotating Interaction Regions (CIRs) over spatial separations that began with all the spacecraft close to Earth, through STEREO separation angles of ∼ 80 degrees in the fall of 2008. Over 35 CIR events were of sufficient intensity to allow measurement of He and heavy ion spectra using the IMPACT/SIT, EPACT/STEP and ACE/ULEIS instruments on STEREO, Wind, and ACE, respectively. In addition to differences between the spacecraft expected on the basis of simple corotation, we observed several events where there were markedly different time-intensity profiles from one spacecraft to the next. By comparing the energetic particle intensities and spectral shapes along with solar wind speed we examine the extent to which these differences are due to temporal evolution of the CIR or due to variations in connection to a relatively stable interaction region. Comparing CIRs in the 1996 – 1997 solar minimum period vs. 2007 – 2008, we find that the 2007 – 2008 period had many more CIRs, reflecting the presence of more high-speed solar wind streams, whereas 1997 had almost no CIR activity.