  We demonstrate that global observations of high-energy cosmic rays contribute
to understanding unique characteristics of a large-scale magnetic flux rope
causing a magnetic storm in August 2018. Following a weak interplanetary shock
on 25 August 2018, a magnetic flux rope caused an unexpectedly large
geomagnetic storm. It is likely that this event became geoeffective because the
flux rope was accompanied by a corotating interaction region and compressed by
high-speed solar wind following the flux rope. In fact, a Forbush decrease was
observed in cosmic-ray data inside the flux rope as expected, and a significant
cosmic-ray density increase exceeding the unmodulated level before the shock
was also observed near the trailing edge of the flux rope. The cosmic-ray
density increase can be interpreted in terms of the adiabatic heating of cosmic
rays near the trailing edge of the flux rope, as the corotating interaction
region prevents free expansion of the flux rope and results in the compression
near the trailing edge. A northeast-directed spatial gradient in the cosmic-ray
density was also derived during the cosmic-ray density increase, suggesting
that the center of the heating near the trailing edge is located northeast of
Earth. This is one of the best examples demonstrating that the observation of
high-energy cosmic rays provides us with information that can only be derived
from the cosmic ray measurements to observationally constrain the
three-dimensional macroscopic picture of the interaction between coronal mass
ejections and the ambient solar wind, which is essential for prediction of
large magnetic storms.
