Quantifying Uncertainty in OMNI Solar Wind Measurements Projected From L ₁ to the Earth's Bow Shock

Abstract

Many routine measurements of the solar wind plasma and interplanetary magnetic field (IMF) are made at the L ₁ Sun–Earth Lagrange point; therefore, it is helpful to characterize the errors introduced in propagating these measurements to the near‐Earth environment. We have compiled over 5,000 hr of plasma and magnetic field measurements (1‐min averages) at two European Space Agency Cluster satellites when they were just outside the bow shock (BS) for years 2001–2023 and compared them with measurements from NASA's OMNI measurements made near L ₁ and projected to the BS nose. We describe a method for determining BS crossings and we retain only high quality instrument data, not subject to caveats. After calibrating systematic relative biases between different spacecraft instruments, empirical probability density functions (PDFs) of the differences between OMNI and Cluster measurements were determined and fitted by shape functions with up to five parameters. The error PDFs are heavy‐tailed (with high kurtosis), with small biases, and standard deviations of: 1.5 cm ⁻³ (proton density), 14 km s ⁻¹ (bulk speed), 0.7 nT (field magnitude), 1.2 nT (transverse field), and 39° (IMF orientation described by “clock angle”).