Studying solar-wind conditions is central to forecasting the impact of space weather on Earth. Under the assumption that the structure of this wind is constant in time and co-rotates with the Sun, solar-wind and thereby space-weather forecasts have been made quite effectively. Such co-rotation forecasts are well studied with decades of observations from STEREO and near-Earth spacecraft. Forecast accuracy is primarily determined by three factors: i) the longitudinal separation of spacecraft from Earth determines the corotation time (and hence forecast lead time) [δ t] over which the solar wind must be assumed to be constant, ii) the latitudinal separation (or offset) between Earth and spacecraft [δθ]] determines the degree to which the same solar wind is being encountered at both locations, and iii) the solar cycle, via the sunspot number (SSN), acts as a proxy for both how fast the solar-wind structure is evolving and how much it varies in latitude. However, the precise dependencies factoring in uncertainties are a mixture of influences from each of these factors. Furthermore, for high-precision forecasts, it is important to understand what drives the forecast accuracy and its uncertainty. Here we present a causal inference approach based on information-theoretic measures to do this. Our framework can compute not only the direct (linear and nonlinear) dependencies of the forecast mean absolute error (MAE) on SSN, Δ θ , and Δ t , but also how these individual variables combine to enhance or diminish the MAE. We provide an initial assessment of this with the potential of aiding data assimilation in the future.
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FunderWe have benefited from sunspot data provided by the Royal Observatory of Belgium SILSO, RGO/SOON sunspot-latitude and -area data collated by David Hathaway and Lisa Upton, and OMNI data provided by NASA/SPDF. We benefited from useful discussions as part of the International Space Science Institute (ISSI, Bern) team “Magnetic open flux and solar-wind structuring in interplanetary space” (2019 – 2021) led by Manuela Temmer.
This work was part-funded by Science and Technology Facilities Council (STFC) grant number ST/V000497/1.
© 2023, The Author(s).
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science