Probing the 2-D Geometry of CIRs at Solar Minimum: Observations From STEREO

The twin STEREO observatories provide a unique opportunity to study the two-dimensional in-ecliptic geometry of structures in the solar wind. At the start of 2007 the AHEAD (A) and BEHIND (B) satellites were near Earth. By the end of the year they will each be separated from Earth by about 20 degrees longitudinally, and from each other by about 40 degrees. This arrangement is well suited to test the conceptual picture of CIRs in the ecliptic plane. The orbital radius of STEREO A is about 0.98 AU, while STEREO B is at about 1.03 AU. If A and B were at the same line of longitude they would be less than 2000 Earth radii apart. Observations show this small radial separation becomes important when predicting the arrival time at Earth of streams observed with STEREO B; stream fronts were seen to arrive at A and B almost simultaneously in mid 2007 when the spacecraft were separated by about 7 degrees. This suggests the leading edge of the stream is roughly aligned with the solar wind garden hose angle. We report on observations of the leading edges of co-rotating high-speed streams, and discuss the implications for space weather forecasting.