  Halo dark matter (DM) particles could lose energy due to the scattering off
nuclei within the Earth before reaching the underground detectors of DM direct
detection experiments. This Earth shielding effect can result in diurnal
modulation of the DM-induced recoil event rates observed underground due to the
self-rotation of the Earth. For electron recoil signals from DM-electron
scatterings, the current experimental constraints are very stringent such that
the diurnal modulation cannot be observed for halo DM. We propose a novel type
of diurnal modulation effect: diurnal modulation in electron recoil signals
induced by DM-nucleon scattering via the Migdal effect. We set so far the most
stringent constraints on DM-nucleon scattering cross section via the Migdal
effect for sub-GeV DM using the S2-only data of PandaX-II and PandaX-4T with
improved simulations of the Earth shielding effect. Based on the updated
constraints, we show that the Migdal effect induced diurnal modulation of
electron events can still be significant in the low energy region, and can be
probed by experiments such as PandaX-4T in the near future.
