Validation of Global Ionospheric Specification (GIS) Electron Density Measurements During Solar Minimum to Solar Maximum Under Quiet and Disturbed Geomagnetic Conditions

Abstract

Global Ionospheric Specification (GIS) three‐dimensional electron density profiles constructed by assimilating the line‐of‐sight total electron content (TEC) from global navigation satellite system (GNSS) radio occultation (RO) measurements by FORMOSAT‐7/COSMIC‐2 (F7/C2) and ground‐based receiver network from international GNSS service, constitute a unique ionospheric database with 1‐hr cadence. Using the data spanning over half a solar cycle since the release of GIS as an operational Level 3 data product of F7/C2 mission, a comprehensive evaluation of the peak electron density (NmF2) is carried out by identifying data conjunctions with Global Ionosphere Radio Observatory (GIRO) Digisonde observations. The results reveal an overall correlation of 0.93, with GIS overestimating GIRO NmF2 by 6.4%. While the values do not show any specific dependence on geomagnetic activity, the correlation (bias) reduces (increases) when only high latitude (>35°) observations are included where no RO measurements are assimilated. A similar tendency is observed when only low solar activity years (F10.7 ≤ 100) are considered. The detailed local time dependence of the correlation and bias are examined in this study, with lowest correlation and large relative bias occurring during 0400 LT—0600 LT. The results show large local time variation of bias depending on latitude region and solar activity. Overall, the bias ranges from 10% to 30% during 1000 LT—2300 LT, whereas GIS may underestimate NmF2 by 5%–15% during 0700 LT—1000 LT. The results also demonstrate that GIS reveals identical day‐to‐day variations as seen in GIRO measurements and compares well with TECs from Global Ionosphere Map and ground GNSS receivers.