Routine stereo imaging by TWINS began on 15 June 2008, during an extremely weak geomagnetic storm whose Dst index never fell below -40 nT, as compared to a nominal Dst of -100 nT for classification as a storm. During the TWINS prime mission (2008–2010), an extended and unprecedented solar minimum (from solar cycle 23) prevailed, bringing with it very calm magnetospheric conditions ranging from dead quiet to mildly disturbed. During this time period TWINS observed numerous weak storms, roughly once every 27 days (corresponding to the solar rotation period and triggered by solar corotating interaction regions (CIRs).