  To accurately quantify the effect of solar proton events (SPEs) on the
atmosphere requires a good estimate of the particle energy deposition in the
middle atmosphere (60- 100 km) and how the energy is distributed globally.
Protons in the energy range 1-20MeV, depositing their energy in the middle
atmosphere, are subject to more complex dynamics with strong day-night
asymmetries compared to higher-energy particles. Our study targets six SPEs
from 2003 to 2012. By using measurements from the Medium Energy Proton and
Electron Detector on all available Polar Orbit Environment Satellites (POES),
we show that in the main phase of geomagnetic storms the dayside cutoff
latitudes are pushed poleward, while the nightside cutoff latitudes have the
opposite response, resulting in strong day-night asymmetries in the energy
deposition. These features cannot bemeasured by the frequently used
Geostationary Operational Environmental Satellites (GOES). Assuming that the
protons impact the polar atmosphere homogeneously above a fixed nominal
latitude boundary will therefore give a significant overestimate of the energy
deposited in the middle atmosphere during SPEs. We discuss the magnetospheric
mechanisms responsible for the local time response in the cutoff latitudes and
provide a simple applicable parameterization which includes both dayside and
nightside cutoff latitude variability using only the Dst, the northward
component of the interplanetary magnetic field, and solar wind pressure. The
parameterization is utilized on the GOES particle fluxes, and the resulting
energy deposition successfully captures the day-night asymmetry in good
agreement with the energy deposition predicted from the POES measurement.
