Global Bottomside Ionospheric Modeling Based on GNSS‐RO Observations From the Tianmu‐1 Constellation

Radio occultation (RO) observations provide a crucial means for estimating the global Vertical Bottomside Electron Content (VBEC) in the ionosphere. However, the limited number of low Earth orbit (LEO) satellites in existing RO missions restricts the capability for high‐precision reconstruction of global VBEC spatiotemporal variations. The Tianmu‐1 (TM‐1) constellation significantly improving the global coverage and resolution of RO measurements. In this study, TM‐1 GNSS RO data were used for the first time to retrieve global VBEC during a geomagnetic storm period (10–12 May 2024) and a geomagnetically quiet period (21–23 July 2024). Based on spatial interpolation, global bottomside ionospheric maps (GBIMs) were constructed. The GBIM derived from TM‐1 (TM‐1 GBIM) and that derived from COSMIC‐2 (COSMIC‐2 GBIM) were then compared with the IGS global ionospheric maps (GIMs) to assess their accuracy and consistency. The results indicate that the TM‐1 GBIM exhibits strong spatiotemporal consistency with the IGS GBIM. During the geomagnetically quiet period, the TM‐1 GBIM performs better, with a root mean square (RMS) of 6.78 TECU, compared to an RMS of 9.68 TECU during the geomagnetic storm period. Further error analysis shows that the modeling errors of TM‐1 GBIM and COSMIC‐2 GBIM exhibit consistent spatiotemporal patterns. In particular, a systematic underestimation of VBEC occurs in the low‐ and mid‐latitude regions from 08:00 to 19:00 LT, resulting in a pronounced double‐peak structure resembling the equatorial ionization anomaly. Moreover, geomagnetic storms expand this double‐peak error structure from 08:00–19:00 LT to 08:00–24:00 LT.