Glancing blows of three interplanetary shocks caused an unexpectedly
large magnetic storm on November 4-6, 2023, which was popular for
citizen scientists because of the surprising appearance of the
crimson-red auroras world-wide in middle latitudes. Based on the
analysis of the in-situ interplanetary magnetic field data at DSCOVR and
STEREO-A, we show that the multi-step main phase of the magnetic storm
is explained by the shock pileup, i.e. slow interplanetary shock was
caught up from behind by the fast one, and the multi-step prolonged
recovery phase by the remnant structure associated with the shock
pileup.