By Neil Rogers (Lancaster University)
Many of us use the NASA “OMNI” database, which takes measurements from spacecraft (e.g., ACE or Wind) near the L1 Earth-Sun Lagrange point and uses them to predict solar wind conditions near the Earth’s bow shock nose. The question is: How accurate are these OMNI predictions? This is particularly topical given recent studies showing that measurement uncertainty could explain saturation in magnetospheric responses to solar wind driving (e.g., Sivadas et al. (2026), Nature, https://doi.org/10.1038/s41586-026-10757-4). We have compared 1-min resolution OMNI predictions of plasma density, velocity, and magnetic field with concurrent ‘ground truth’ measurements from two ESA Cluster spacecraft when they were located in the solar wind in years 2001 - 2023. After calibrating ‘systematic’ linear (instrumental) biases between pairs of spacecraft, we quantified and parameterised probability density functions (PDF) of the ‘stochastic’ differences between OMNI and Cluster. We found that many of these PDFs had a very narrow central peak (i.e., most differences were close to zero) but had ‘heavier tails’ in comparison with the Normal distribution (i.e., a greater likelihood of the largest differences). The Student’s-t distribution is a good fit for these cases, although we found the five-parameter Generalised Hyperbolic distribution provides the best characterisation of the rarest and largest differences (the outermost tails of the PDFs). Our paper provides a full quantification of both systematic and stochastic uncertainties, which could be used to place more realistic uncertainty bounds on analyses and forecasts that rely on OMNI data. Our database of times for which Cluster spacecraft were in the solar wind (~ 5000 hours) at various distances beyond the bow shock is also available to download: https://doi.org/10.5281/zenodo.18390327.
See publication for more details:
Rogers, N. C., Wild, J. A., & Grocott, A. (2026). Quantifying uncertainty in OMNI solar wind measurements projected from L1 to the Earth's bow shock. Journal of Geophysical Research: Space Physics, 131, e2026JA035221. https://doi.org/10.1029/2026JA035221

(a) Errors (OMNI minus Cluster-1) in the interplanetary magnetic field strength perpendicular to the Sun-Earth axis for all minutes in years 2001-2023. (b) Binned means and standard deviations of data in panel (a) and coefficients of fitted truncated Normal distributions (maximum likelihood estimates and 95% confidence intervals). (c) PDF of data in panel (a).