1. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 1 Retrieval of electron density profiles from GRACE radio occultation data N. Jakowski 1, C. Mayer 1, J. Wickert 2, S.Heise 2, W. Köhler 2 German Aerospace Center (DLR), Institute of Communications and Navigation, Neustrelitz, Germany GeoForschungsZentrum (GFZ), Telegrafenberg, Potsdam, Germany

2. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 2 Outline Ionospheric Radio Occultation (IRO) measurements Retrieval technique and validation Coincidence conditions for simultaneous occultations at different satellites Comparison of retrieval parameters obtained at GRACE, CHAMP and COSMIC satellites Summary and conclusions

3. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 3 Satellite missions GRACE, CHAMP, COSMIC COSMIC GRACE CHAMP

4. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 4 Sounding of the ionosphere by using radio waves Vertical Sounding (VS) Incoherent Scatter Radar (ISR) e.g. EISCAT n e (h) height h / km Electron Density h max n emax Total profile below h max ISR: Backscatter of radio waves on charged particles up to about 1000 km height (upper panel) VS: Reflection of radio waves at the bottomside ionosphere (lower panel) if f=f p (plasma frequency) IRO: ionospheric radio occultation measurements provide a rather new technique for measuring the vertical electron density distribution of the ionosphere Ionosphere (60 -1000 km height) : Highly variable plasma density generated by solar radiation and energetic particles Traditional sounding by using radar techniques

5. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 5 GPS sounding of the ionosphere onboard a Low Earth Orbiting (LEO) satellite GPS Satellite LEO LEO Orbit Occultation (1Hz)

6. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 6 Retrieval of electron density profiles n e (h) Differential phase, no TEC calibration required Inversion Jakowski et al., GRL, 29, No.10, 95-1, 2002 Model assisted retrieval nene ~ Topside ionosphere/plasmasphere Orbit height h GRACE: 450km

7. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 7 Validation of CHAMP IRO data with ionosonde data Ionosonde station Juliusruh / Germany cross section diameter: 12 deg Bias: ≤ 0.5 MHz RMS: ≤ 1MHz Jakowski et al., in C. Reigber, H. Lühr, P. Schwintzer, and J. Wickert (Eds.), Earth Observation with CHAMP, Results from Three Years in Orbit, Springer-Verlag Berlin, pp. 447-452, 2005

8. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 8 Electron density profiles derived from radio occultations at three different satellites Triple coincidences of occultations enable comparison of retrievals obtained from radio occultation measurements at three satellites

9. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 9 Distribution of measurements in local time sector All local time sectorsare covered Data base (profiles,coincidences) January (March) – September 2007 GRACE: 16309 157 CHAMP: 35580 2742 6449 COSMIC: 558222

10. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 10 Spatial distribution of measurements Coincidences cover all geographic locations Coincidence: Δt < 10min distance < 8 deg Availability of 6 COSMIC satellites demonstrate a high potential for ionospheric monitoring

11. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 11 Intercomparison between electron density profiles retrieved from different satellites Good agreement between electron density profiles derived from different satellites and differnt retrieval techniques small bias small standard deviation n e / 1000 cm -3

12. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 12 Intercomparison between height of maximum electron density profiles hmF2 retrieved from different satellites Good agreement of hmF2 retrievals from different satellites Bias < 5 km Standard deviation < 25 km

13. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 13 Intercomparison between maximum electron density NmF2 retrieved from different satellites Good agreement of NmF2 retrievals from different satellites Bias < 1 x10 3 cm -3 Standard deviation < 1.3 x10 5 cm -3 NmF2 / 1000 cm -3

14. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 14 „Ionosonde parameters“ from GRACE GRACE data contribute to explore the ionospheric electron density structure on global scale In combination with other satellite missions such as CHAMP, TerraSAR-X and COSMIC the three-dimensional electron density structure may be derived σσ

15. Joint International GRACE Science Team Meeting and DFG SPP 1257 Symposium, 15-17 Oct. 2007, GFZ Potsdam Folie 15 Summary & Conclusions GRACE radio occultation measurements have been analyzed from day 59 – 261 in 2007. Within this period 16309 profiles have been retrieved, i.e. about 80 electron density profiles per day. Comparison of IRO data derived from GRACE, CHAMP and Formosat-3 measurements reveal good agreement. Biases and standard deviations agree with CHAMP-ionosonde comparison Validation of IRO data must continue to be fully accepted in the ionospheric community Basic ionospheric features are well reproduced in GPS sounding data from GRACE. There is an outstanding potential of multisatellite data analysis for ionospheric research and space weather monitoring Acknowledgement The authors are grateful to the international GRACE and CHAMP teams for maintaining the operation and reception of the GRACE satellite since 2000. We thank also the Formosat- 3/COSMIC team for making available their data products.