Clette, Frédéric

Talk presented at 2022 Sun-Climate Symposium, Madison, WI, USA on 2022-05-17

Abstract: The F10.7cm radio flux is one of the primary long-term indices of solar activity. Beyond its solar physics applications, F10.7 is used as base proxy of the Sun's UV irradiance for various ionospheric models and predictions. As it broadly extends the direct space-based measurements of the solar spectral irradiance, F10.7 thus plays a unique role in the understanding of the long-term Sun-Earth coupling. By using the principles implemented for the re-calibration of the sunspot number (SN) series and by comparing the F10.7 series to the new upgraded SN version 2, we investigate the relation between those two global measures of solar activity, from 1947 to the present, and we verify a few key properties of the F10.7 long-term variations. We find that: - the F10.7/SN relation is very closely linear, down to almost null activity - a low-range non-linearity of this relation appears for temporally-averaged values (monthly, yearly means). A data simulation shows that it is purely induced by the temporal smoothing, and that it varies with the width of the averaging. - the quiet-Sun F10.7 background flux depends on the duration of the quiet (spotless) intervals, ranging from 74 sfu for single-day intervals down to 67 sfu for the true "all-quiet" value. This reconciles the many seemingly incompatible values published until now. - an upward 10.5% scale jump occurs in the F10.7 series in 1980, cutting the full series into two halves, which are each fully homogeneous. We conclude on the need to include a corresponding correction in all F10.7-based solar and geophysical applications spanning the entire length of the F10.7 record.

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