STEREO observations of shock formation in the solar wind

During solar minimum, the majority of shocks observed at 1 AU are associated with interactions between fast and slow streams, appearing as the pressure ridge between the streams narrows, and as the fast mode speed becomes smaller than the velocity jump between streams. Shocks are believed to strengthen when following shock waves catch up with the leading shock. STEREO observations confirm this behavior for very weak shocks. We can also put limits on the location of the shock-forming region by examining the periods of alignment between Venus and STEREO. In a period in which we see nine shocks at 1 AU, we see no shocks at 0.72 AU. Hence, stream interactions from 0.72 AU to 1 AU are responsible for the development of most of the shocks observed at 1 AU during solar minimum.