Evaluating the OMNI Database: Statistical Analysis of Time-Shifted L1 Data Versus Direct Near-Earth Solar Wind Observations

This study presents a comprehensive statistical comparison of solar wind measurements between the OMNI database (collected at L1 and shifted to the Earth's bow shock nose), and near-Earth solar wind observations from MMS, Cluster, and THEMIS missions near the bow shock nose. Using a threshold-based classification methodology, the analysis encompasses approximately 353 days (MMS), 283 days (Cluster), and 125 days (THEMIS) of solar wind intervals that are compared to OMNI data. Bisector regression analysis reveals that the anti-sunward flow component (V_x) demonstrates exceptional agreement across all missions with near-unity slopes and correlation coefficients of 0.92 for THEMIS and 0.97 for both MMS and Cluster. However, perpendicular velocity components show progressively degraded performance: V_y exhibits correlation coefficients of 0.63-0.77 with intercepts ranging from 21.57 km/s (MMS) to 47.49 km/s (THEMIS), while V_z shows lower correlations (0.42-0.72) with intercepts of 4.73-11.94 km/s. Ion density measurements reveal systematic mission-specific biases: MMS and THEMIS show ion density regression slopes below unity (0.59 and 0.54, respectively), while Cluster shows a slope above unity (1.14) compared to OMNI measurements. Magnetic field measurements show higher consistency, with near-unity slopes and correlation coefficients exceeding 0.84 for B_x and B_y components. The northward magnetic field component (B_z) exhibits elevated variance ratios and reduced correlations across all missions, reaching as low as 0.74 for THEMIS. These results quantify inherent uncertainties in cross-platform solar wind comparisons and assess the accuracy of time-shifted solar wind measurements in the OMNI database as proxies for near-Earth conditions. Based on the presented statistics, OMNI-equivalent measurements from near-Earth missions can be generated as alternative data sources to support the upcoming SMILE mission, multispacecraft studies, and magnetohydrodynamic simulations that require accurate upstream boundary conditions.