The KBR-band experiment of the GRACE and GRACE-FO missions have been processed to obtain in situ plasma density observations of the topside ionosphere along the satellite orbits. These data have been applied to study our near Earth’s space environment of the ionosphere. The GRACE missions have been critical in complementing the spatial and temporal coverage of global data sets of plasma density from dedicated instrumentation onboard the CHAMP and Swarm missions. The most recent major geomagnetic storm occurred in May 2024. GRACE-FO has largely contributed to observing and explaining the low latitude ionospheric response to the enhanced solar energy input.
Prof. Dr. Claudia Stolle, Leibniz Institute of Atmospheric Physics at the University of Rostock, Germany
GRACE and GRACE-FO as ionospheric space weather missions
The GRACE missions have realized an internationally unparalleled project of monitoring the variable Earth’s gravity field. In this area, their data have led to scientific findings with outstanding novelties. In addition, the opportunities for the spacecraft’s observations have tremendous value for making GRACE space weather monitoring missions and extending the missions’ objectives to a multidisciplinary use.
GRACE and GRACE-FO continuously measure the distance between the twin satellites using a K-band ranging system. This radio wave is in a frequency band, which is affected by plasma density variations in the ionosphere it propagates through. Therefore, the integrated total electron content between the two spacecraft can be derived and, divided by the satellites’ distance, leads to a measure of average in situ electron density (Xiong et al., 2010; Schreiter et al., 2023). The mission’s specifications allow a multiple-year data set over two solar cycles (since 2002) with a temporal and spatial resolution of 0.1 Hz and about 200 km, respectively. Its particular benefit is its complementarity with other operating missions, such as CHAMP and Swarm, by expanding the global coverage of ionospheric data in longitude and altitude (see Figure 1).
The GRACE missions’ ionospheric data have largely contributed to characterize the altitude distribution of low-latitude ionospheric plasma “bubbles” (Xiong et al., 2010) which are known to seriously affect GNSS operations (e.g., Crane, 2016; Xiong et al., 2018). They have been important data sets in enhancing empirical models by the specification of the otherwise rarely covered topside ionosphere (e.g., Bilitza and Xiong, 2021, Smirnov et al., 2023), and it was used to characterize the ionospheric response to the recent major geomagnetic storm in May 2024 (Das et al., 2025), among other applications.
The May-2024 geomagnetic storm event
On 10 May 2024, Earth experienced a major geomagnetic storm with geomagnetic Hp30-indices (Matzka et al., 2024) reaching values up to 11+. Here, GRACE-FO has been important to characterizing the space weather response of the low-latitude ionosphere. Das et al. (2025) showed, that the strength of the so-called equatorial ionization anomaly has increased by 240 % compared to pre-storm values at pre-sunset times of about 17 LT (LT: local time, see Figure 2). During pre-sunrise times, at around 05 LT, GRACE-FO detected exceptional plasma “bubbles” reaching magnetic latitudes of 30° and higher. This corresponds to altitudes of 3000 km above the magnetic equator. This observation is in contrast to regular, non-storm occurrences of “bubbles” confined to pre-midnight hours only and with equatorial altitudes of 1000 km. In a combined analysis with plasma density observations of the Swarm A and Swarm B satellites (at 07/19 LT and 11/23 LT, respectively) these extreme events could be explained by strong variations in the low-latitude ionospheric electric field which drove strong plasma convection.
Referring to the “constellation” observations through the combination of the CHAMP and GRACE satellites during the Halloween storm in October 2003 (Hp30 = 12), Das et al. (2025) also demonstrated that the strength of the equatorial ionization anomaly was larger in magnitude during the Halloween storm, but the relative increase of the anomaly has been of similar order of magnitude during the Halloween and the May-2024 storms at noon and pre-sunset hours. The Halloween and the May-2024 storms have been the strongest geomagnetic storms during the satellite era.
Data availability
The plasma density data of the Swarm and GRACE-FO missions are provided by the European Space Agency (ESA) at https://earth.esa.int/eogateway/missions/swarm/data. Hp30 indices are available through the GFZ Helmholtz Centre for Geosciences at https://kp.gfz.de/hp30-hp60.
References
- Bilitza, D., Xiong, C. (2021). A solar activity correction term for the IRI topside electron density model. Adv. Space Res. 68 (5), 2124–2137. https://doi.org/10.1016/j.asr.2020.11.012.
- Crane, L. (2016). What causes GPS signal loss on satellites like Swarm?, Eos, 97, https://doi.org/10.1029/2016EO059257.
- Das, S. K., Stolle, C., Yamazaki, Y., Rodríguez-Zuluaga, J., Wan, X., Kervalishvili, G., et al. (2025). On the F-region ionospheric plasma density distribution and irregularities response during the May-2024 geomagnetic storm observed by LEO satellites. Geophysical Research Letters, 52, e2025GL115780. https://doi.org/10.1029/2025GL115780
- Matzka, J., Bronkalla, O., da Silva, M. V., Kervalishvili, G., Rauberg, J., Korte, M., Yamazaki, Y., (2024). Geomagnetic Hpo index(V3.0). GFZ Data Services. https://doi.org/10.5880/Hpo.0003
- Schreiter, L., Stolle, C., Rauberg, J., Kervalishvili, G., van den Ijssel, J., Arnold, D., et al. (2023). Topside ionosphere sounding from the CHAMP, GRACE, and GRACE-FO missions. Radio Science, 58, e2022RS007552. https://doi.org/10.1029/2022RS007552
- Smirnov, A., Shprits, Y., Prol, F. et al. (2023). A novel neural network model of Earth’s topside ionosphere. Sci Rep 13, 1303. https://doi.org/10.1038/s41598-023-28034-z
- Xiong, C., Park, J., Lühr, H., Stolle, C., Ma, S. (2010). Comparing plasma bubble occurrence rates at CHAMP and GRACE altitudes during high and low solar activity. - Annales Geophysicae, 28, 1647-1658. doi.org/10.5194/angeo-28-1647-2010
- Xiong, C., Stolle, C., Park, J. (2018). Climatology of GPS signal loss observed by Swarm satellites. - Annales Geophysicae, 36, 679-693. doi.org/10.5194/angeo-36-679-2018