Abstract | The systematic monitoring of the solar wind in high-cadence and high-resolution heliospheric images taken by the Solar-Terrestrial Relation Observatory (STEREO) spacecraft permits study of the spatial and temporal evolution of variable solar wind flows from the Sun to 1 AU, and beyond. As part of the EU Framework (FP7) HELCATS project, we have generated a catalogue listing the properties of 190 corotating structures well-observed in images taken by the Heliospheric Imager (HI) instrument on-board STEREO-A from 2007 to 2014.This analysis suggests that most of the corotating density structures detected by the HI are density inhomogeneities advected by the slow solar wind that eventually become entrained by stream interaction regions. We have derived the spatial-temporal evolution of each of these corotating structures by using a well-established fitting technique. The mean radial propagation speed of the corotating structures is found to be 310 $\pm 30$ km/s. Such a low mean value corresponds to the terminal speed of the slow solar wind rather than the speed of stream interfaces, which is typically intermediate between the slow and fast solar wind speeds ($\sim$400 km/s). We predicted the arrival time of each corotating structure at different probes in the inner heliosphere and show that the speeds of the corotating density structures derived using our fitting technique track well the long-term variation of the radial speed of the slow solar wind during solar minimum years (2007-2008). Furthermore, we demonstrate that these features originate near the coronal neutral line that eventually becomes the heliospheric current sheet. |
