Abstract/Summary

Routine in‐situ solar wind observations from L5, located 60° behind Earth in its orbit, would provide a valuable input to space‐weather forecasting. One way to ulitise such observations is to assume that the solar wind is in perfect steady state over the 4.5 days it takes the Sun to rotate 60° and thus near‐Earth solar wind in 4.5‐days time would be identical to that at L5 today. This corotation approximation is most valid at solar minimum when the solar wind is slowly evolving. Using STEREO data, it has been possible to test L5‐corotation forecasting for a few months at solar minimum, but the various contributions to forecast error cannot be disentangled. This study uses 40+ years of magnetogram‐constrained solar wind simulations to isolate the effect of latitudinal offset between L5 and Earth due to the inclination of the ecliptic plane to the solar rotational equator. Latitudinal offset error is found to be largest at solar minimum, due to the latitudinal ordering of solar wind structure. It is also a strong function of time of year; maximum at the solstices and very low at equinoxes. At solstice, the latitudinal offset alone means L5‐corotation forecasting is expected to be less accurate than numerical solar wind models, even before accounting for time‐dependent solar wind structures. Thus, a combination of L5‐corotation and numerical solar wind modelling may provide the best forecast. These results also highlight that three‐dimensional solar wind structure must be accounted for when performing solar wind data assimilation.