The Influence of Spacecraft Latitudinal Offset on the Accuracy of Corotation Forecasts. Issue 8 (11th August 2021)

• Record Type: Journal Article
• Authors: Turner, Harriet; Owens, Mathew J.; Lang, Matthew S.; Gonzi, Siegfried
• Is Part Of: Space weather. Volume 19:Issue 8(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-08-11
• Subjects: Corotation -- data assimilation -- solar wind -- space weather forecasting; Space environment -- Periodicals; 551.509992
• DOI: 10.1029/2021SW002802
• Languages: English
• ISSNs: 1542-7390

Abstract

Abstract: 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. Plain Language Summary: Space weather can damage our technologies, from power lines to satellites, as well as pose a hazard to humans in space. Forecasting space weather requires prediction of solar wind, a continuous outflow of material from the Sun, conditions in near‐Earth space. A simple way to achieve this is "corotation, " which assumes the structure of the solar wind is unchanging, but simply rotates around with the Sun. Thus, solar wind which sweeps past one spacecraft will arrive at Earth some time later. This forecast will be less accurate when the spacecraft is far from Earth, as we need to wait longer for the solar wind to rotate around, during which time its structure may have changed. If the spacecraft is at a different latitude to Earth, it will also create problems for the forecast. In this study, we quantify the contribution of these factors to the forecast error. This knowledge will improve corotation forecasts, but also aid in other, more sophisticated, forecast techniques. By defining how close spacecraft need to be to Earth to sample the same solar wind, it also helps define where future space weather monitoring spacecraft should be positioned. Key Points: Solar wind speed corotation forecast error is affected by solar activity and spacecraft longitudinal and latitudinal separation Latitudinal separation has little effect when it is below 6°, but increasing importance above this A period in solar minimum gives a 46% increase in mean absolute error (MAE) from low to high latitudinal offset for Solar Terrestrial Relations Observatory (STEREO)‐A and B corotations