Knowledge of the ambient solar wind is important for accurate space weather forecasting. A simple-but-effective method of forecasting near-Earth solar wind speed is “corotation,” wherein solar wind structure is assumed to be fixed in the reference frame rotating with the Sun. Under this approximation, observations at a source spacecraft can be rotated to a target location, such as Earth. Forecast accuracy depends upon the rate of solar wind evolution, longitudinal and latitudinal separation between the source and target, and latitudinal structure in the solar wind itself. The time-evolution rate and latitudinal structure of the solar wind are both strongly influenced by the solar cycle, though in opposing ways. A latitudinal separation (offset) between source and target spacecraft is typically present, introducing an error to corotation forecasts. In this study, we use observations from the Solar Terrestrial Relations Observatory (STEREO) and near-Earth spacecraft to quantify the latitudinal error. Aliasing between the solar cycle and STEREO orbits means that individual contributions to the forecast error are difficult to isolate. However, by considering an 18-month interval near the end of solar minimum, we find that the latitudinal-offset contribution to corotation forecast error cannot be directly detected for offsets <6°, but is increasingly important as offsets increase. This result can be used to improve solar wind data assimilation, allowing representivity errors in solar wind observations to be correctly specified. Furthermore, as the maximum latitudinal offset between L5 and Earth is ≈5°, corotation forecasts from a future L5 spacecraft should not be greatly affected by latitudinal offset.

中文翻译：

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航天器纬度偏移对共转预报精度的影响

了解环境太阳风对于准确的空间天气预报很重要。预测近地太阳风速的一种简单但有效的方法是“共转”，其中假设太阳风结构固定在随太阳旋转的参考系中。在这种近似下，源航天器的观测可以旋转到目标位置，例如地球。预报的准确性取决于太阳风演化的速率、源和目标之间的纵向和纬度分离以及太阳风本身的纬度结构。太阳风的时间演化速率和纬度结构都受到太阳活动周期的强烈影响，尽管以相反的方式。源航天器和目标航天器之间通常存在纬度分离（偏移），这会给共转预测带来误差。在这项研究中，我们使用来自日地关系天文台 (STEREO) 和近地航天器的观测来量化纬度误差。太阳周期和 STEREO 轨道之间的混叠意味着很难分离对预测误差的个体贡献。然而，通过考虑太阳活动极小期结束附近的 18 个月间隔，我们发现对于 <6° 的偏移无法直接检测到纬度偏移对共转预测误差的贡献，但随着偏移的增加而变得越来越重要。该结果可用于改进太阳风数据同化，从而正确指定太阳风观测中的代表性误差。此外，由于 L5 和地球之间的最大纬度偏移为 ≈5°，