  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 the study of the spatial and temporal
evolution of variable solar wind flows from the Sun out to 1~AU, and beyond. As
part of the EU Framework 7 (FP7) Heliospheric Cataloguing, Analysis and
Techniques Service (HELCATS) project, we have generated a catalogue listing the
properties of 190 corotating structures well-observed in images taken by the
Heliospheric Imager instruments on-board STEREO-A. We present here one of very
few long-term analyses of solar wind structures advected by the background
solar wind. This analysis confirms that most of the corotating density
structures detected by the heliospheric imagers comprises a series of 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 density structures by using a
well-established fitting technique. The mean radial propagation speed of the
corotating structures is found to be $311 \pm 31$ km~s$^{-1}$. We predicted the
arrival time of each corotating density structure at different probes. We 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.
