  On July 25 2017 a multi-step Forbush decrease (FD) with the remarkable total
amplitude of more than 15\% was observed by MSL/RAD at Mars. We find that these
particle signatures are related to very pronounced plasma and magnetic field
signatures detected in situ by STEREO-A on July 24 2017, with a higher than
average total magnetic field strength reaching more than 60 nT. In the observed
time period STEREO-A was at a relatively small longitudinal separation (46
degrees) to Mars and both were located at the back side of the Sun as viewed
from Earth. We analyse a number of multi-spacecraft and multi-instrument (both
in situ and remote-sensing) observations, and employ modelling to understand
these signatures. We find that the solar sources are two CMEs which erupted on
July 23 2017 from the same source region on the back side of the Sun as viewed
from Earth. Moreover, we find that the two CMEs interact non-uniformly,
inhibiting the expansion of one of the CMEs in STEREO-A direction, whereas
allowing it to expand more freely in the Mars direction. The interaction of the
two CMEs with the ambient solar wind adds up to the complexity of the event,
resulting in a long, sub-structured interplanetary disturbance at Mars, where
different sub-structures correspond to different steps of the FD, adding-up to
a globally large-amplitude FD.
