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Description
The solar radio flux index F10.7 denotes the spectral flux density of solar radiation at a wavelength of 10.7 cm or 2.8 GHz and is the most important parameter for solar activity alongside the sunspot number. F10.7 is also a central input parameter for models of the ionosphere and thermosphere and is therefore relevant for radio communication, navigation and all applications relying on LEO satellites, such as satellite internet or remote sensing. Despite its importance, F10.7 is only provided worldwide by one civilian agency – the Dominion Radio Astrophysical Observatory (DRAO) in Penticton, Canada – while the military-run Radio Solar Telescope Network (RSTN) monitors the sun's activity at 2.7 GHz (i.e. at a similar frequency). In this context, Elvidge et al. recently posed the legitimate question in the journal Space Weather: “What to Do When the F10.7 Goes Out?” [1].
The good news: with the Solar Flux Telescope CLT at the Wettzell Geodetic Observatory (GOW), which went into operation at the beginning of 2024, a second civilian facility, the Federal Agency for Cartography and Geodesy, is helping to ensure that this does not happen. In order to link DRAO and RSTN, measurements are taken at both 2.7 and 2.8 GHz. The analog receiving system of the radio telescope is designed for a flexible selection of any number of measurement frequencies between 1 and 6 GHz with bandwidths of up to 100 MHz. The F10.7 measurements are currently framed by three additional measurement frequencies each above (at 3.3, 5.0 and 6.0 GHz) and below in the L-band (at 1.0, 1.4 and 1.7 GHz). The frequency 1.0 GHz corresponding to a wavelength of 30 cm was chosen to match and complement the F30 measurements conducted at the Nobeyama radio observatory in Japan, which are especially relevant for satellite drag models such as the DTM-2013 [2]. At night, the CLT addresses its second task and monitors the quality of Galileo navigation signals.
Designing the CLT, a key criterion was the robustness of the measurement system including an automated interference detection. The signal processing chain which amplifies the received analog signal and converts it to digital is therefore encapsulated in a temperature-stabilized box directly behind the telescope’s feed. Each measurement cycle (comprising eight measurement frequencies) takes around four seconds. This high temporal resolution enables a detailed recording of dynamic processes, such as the enormous solar radio bursts that preceded the geomagnetic storm in the past year on May 10 and 11. For long term stability, careful gain calibration is performed regularly.
The CLT is complemented by additional sensors, nemely by magnetometers and GNSS scintillation receivers to connect solar radio flux measurements to possible impairing effects on satellite navigation.
[1] Elvidge, S., Themens, D. R., Brown, M. K., & Donegan-Lawley, E. (2023). What to do when the F10.7 goes out?, Space Weather, 21, e2022SW003392. https://doi.org/10.1029/2022SW003392
[2] Bruinsma, S. (2015). The DTM-2013 thermosphere model, J. Space Weather Space Clim., 5 A1, https://doi.org/10.1051/swsc/2015001
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