  We present the first systematic study of clouds observed during twilight on
Mars. We analyze images obtained by the Visual Monitoring Camera (VMC) on Mars
Express between 2007 and 2020. Using an automated retrieval algorithm we found
407 cases of clouds observed at twilight, in which the geometry of the
observations allows to derive the minimum altitude, revealing that many of
these clouds are in the mesosphere (above 40km and up to 90km). The majority of
these mesospheric clouds were detected in mid-latitudes at local autumn and
winter, a new trend only hinted at by previous studies. In particular, we find
a massive concentration of clouds in the southern mid-latitudes between Terra
Cimmeria and Aonia, a region where high altitude events have been previously
observed. We propose that there is an unknown mechanism in these regions that
enhances the probability to host high altitude clouds around the southern
winter solstice.
