Statistics of Uncertainty in OMNI Solar Wind Measurements

Rogers, Neil and Wild, Jim and Grocott, Adrian (2025) Statistics of Uncertainty in OMNI Solar Wind Measurements. In: Autumn MIST 2025, 2025-11-28 - 2025-11-28, Geological Society of London, Burlington House, Piccadilly. (In Press)

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Abstract

We present a statistical evaluation of uncertainties associated with NASA’s OMNI database of solar wind (SW) parameters (ion velocity, density, and magnetic field) measured at the L1 Sun-Earth Lagrange point by comparison with ESA Cluster spacecraft measurements from just beyond the Earth’s bow shock (BS) in years 2001–2023. We first calibrate and remove systematic instrument biases between Cluster-1 or -3 and ACE or Wind spacecraft (which contributed to OMNI). Uncertainty probability distributions are then recorded and found to be best-fitted by a Generalised Hyperbolic distribution which has excellent agreement in the tails (extreme quantiles). All SW parameter uncertainties are moderately heteroskedastic, with variances increasing with parameter magnitude. Uncertainty metrics are also demonstrably affected by Cluster’s proximity to the ion foreshock region. We also evaluate correlations between different SW parameter uncertainties as these are helpful in propagating uncertainties to SW–magnetosphere coupling functions which combine SW parameters. Errors (of approximately +/- 6 min) associated with the OMNI estimates of L1 to BS nose propagation times are determined by cross-correlating field orientation (‘clock angle’) time series. By applying these optimal lags to the data, we demonstrate a modest reduction in SW parameter uncertainties.