  We present a concept study of a solar wind forecasting method for Earth,
based on persistence modeling from STEREO in-situ measurements combined with
multi-viewpoint EUV observational data. By comparing the fractional areas of
coronal holes (CHs) extracted from EUV data of STEREO and SoHO/SDO, we perform
an uncertainty assessment derived from changes in the CHs and apply those
changes to the predicted solar wind speed profile at 1AU. We evaluate the
method for the time period 2008-2012, and compare the results to a persistence
model based on ACE in-situ measurements and to the STEREO persistence model
without implementing the information on CH evolution. Compared to an ACE based
persistence model, the performance of the STEREO persistence model which takes
into account the evolution of CHs, is able to increase the number of correctly
predicted high-speed streams by about 12%, and to decrease the number of missed
streams by about 23%, and the number of false alarms by about 19%. However, the
added information on CH evolution is not able to deliver more accurate speed
values for the forecast than using the STEREO persistence model without CH
information which performs better than an ACE based persistence model.
Investigating the CH evolution between STEREO and Earth view for varying
separation angles over ~25-140{\deg} East of Earth, we derive some relation
between expanding CHs and increasing solar wind speed, but a less clear
relation for decaying CHs and decreasing solar wind speed. This fact most
likely prevents the method from making more precise forecasts. The obtained
results support a future L5 mission and show the importance and valuable
contribution using multi-viewpoint data.

Solar and Stellar Astrophysics