  Forecasting Solar Energetic Particle (SEP) fluence, as integrated over an SEP
event, is an important element when estimating the effect of solar eruptions on
humans and technology in space. Current real-time estimates are based on SEP
measurements at a single location in space. However, the interplanetary
magnetic field corotates with the Sun approximately 13$^\circ$ each day with
respect to Earth, thus in 4 days a near-Earth spacecraft will have changed
their connection about 60$^\circ$ from the original SEP source. We estimate the
effect of the corotation on particle fluence using a simple particle transport
model, and show that ignoring corotation can cause up to an order of magnitude
error in fluence estimations, depending on the interplanetary particle
transport conditions. We compare the model predictions with STEREO observations
of SEP events.
