The Influence of Spacecraft Latitudinal Offset on the Accuracy of Corotation Forecasts

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 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 18month 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 solarwind 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.

Rate of Change of Large-Scale Solar-Wind Structure

“…Thus, a difference in solar-wind properties could be the result of time evolution and/or spatial structure in latitude. Over 27.27 days, Earth has θ ≤ 3.5 • , thus we expect the θ effect on the observed solar-wind properties to be relatively small (Owens et al, 2019;Laker et al, 2021;Turner et al, 2021).…”

“…Thus, the STEREO spacecraft, in conjunction with near-Earth spacecraft, provide a close approximation to the 'ideal' dataset described above. However, due to orbits being in the ecliptic plane (which is inclined to the solar equator by an angle of 7.25 • , resulting in differences in solar latitude between spacecraft) and the longitudinal separation of the spacecraft progressing in step with the solar cycle, it is difficult to isolate the effects of time evolution, latitudinal differences and the solar cycle from this dataset (Turner et al, 2021).…”

“…Using the OMNI dataset we cannot completely remove the effect of latitudinal offset, θ (see also Turner et al, 2021) or transients resulting from either unidentified ICMEs and/or turbulent and mesoscale structures (Viall, DeForest, and Kepko, 2021;Verscharen, Klein, and Maruca, 2019). Thus, it is useful to also examine simulation results that attempt to capture only the ambient solar wind.…”

Predictive Capabilities of Corotating Interaction Regions Using STEREO and Wind In‐Situ Observations

“…The differences in heliographic longitude between STEREO‐B and Wind increased from 0.2° to 165° since the launch of STEREO‐B until its loss at the end of 2014, while the latitudinal separations show an annual variation with an amplitude that increased with the longitude separation, reaching a maximum value of −15° to 15° in 2014. It should be noted that as longitude and latitude increase, solar activity transits from solar minimum to maximum, making the isolation of a single factor that affects the predictive capabilities of CIRs more challenging (Turner et al., 2021). The magnetic field intensity, solar wind bulk velocity, and ion density are the three key solar wind parameters, which are used in empirical models (Temerin & Li, 2002; Wang et al., 2003) to predict the corresponding geomagnetic storm's intensity (Dst index).…”

Using in situ solar-wind observations to generate inner-boundary conditions to outer-heliosphere simulations – I. Dynamic time warping applied to synthetic observations

“…it looks only back in time, which enables it to also be used for forecasting (e.g. Kohutova et al 2016;Thomas et al 2018;Turner et al 2021). When the solar wind structure (in the rotating frame of the Sun) is evolving with time, this produces sharp discontinuities in the reconstructed solar wind at longitudes where new observations are introduced.…”