Technical Article

The L-Band solar flux retrieval from the Soil Moisture and Ocean Salinity (SMOS) mission: Theoretical algorithm and its validation

¹ Serco Italia SpA-for European Space Agency, ESA-ESRIN, Largo Galileo Galilei 1, 00044 Frascati, Italy
² University of Rome Tor Vergata, Department of Mathematics, Via della Ricerca Scientifica 1, 00133 Roma, Italy
³ Serco Italia SpA, Via Bernardino Alimena 111, 00133 Roma, Italy

^* Corresponding author: raffaele.crapolicchio@ext.esa.int

Received: 14 May 2025
Accepted: 25 September 2025

Abstract

The Soil Moisture and Ocean Salinity (SMOS) satellite, launched in November 2009, is equipped with the Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) payload, which collects high temporal resolution polarimetric data at 1.413 GHz (L-band), corresponding to a wavelength of 21.2 cm, with a time resolution of 2.4 s. The Sun is present in the SMOS images as an alias and is removed from the images to improve the accuracy of soil moisture and ocean salinity maps. This paper illustrates the algorithm’s theoretical basis for the retrieval of the solar flux from the SMOS dataset at the time scale of 50 min, and details the validation process and results. Validation and long-term stability are evaluated at daily and monthly scales, using the radio-telescopes and other solar activity proxies as references. Comparison with the F10.7 index and the flux at 1 GHz from the Nobeyama Radio Polarimeters (NoRP) shows good agreement with the SMOS solar flux dataset, with no significant seasonal or temporal drift, unlike data collected by the Radio Solar Telescope Network (RSTN) at 1.415 GHz. The SMOS solar flux spectrum reveals both the solar rotation and solar cycle frequencies, resembling the F10.7 index. The uncertainties in the 50 min time resolution dataset are assessed, and the precision of the daily data is further evaluated through auto-regressive modelling, comparing favourably to results obtained by the radio telescopes, with errors consistently below 3%. The correlation between SMOS solar flux and both the Wolf sunspot number and the Mg II proxy shows high agreement, and suggests that the correlation between the SMOS solar flux and the Mg II is mostly linear, and stronger than the one between the latter and the F10.7. Lastly, a prototype for solar radio burst detection is introduced, and a few solar radio burst events are discussed. This work demonstrates the added value of the SMOS solar flux in solar physics studies, thanks to its 15-year-long time series dataset and its proven stability. The full polarimetric measurements acquired in the L-band at high temporal resolution represent an innovative point of view for solar flux observations, alleging SMOS solar flux as a valid resource in the space weather field, complementing established solar indicators and contributing to Space Weather applications and studies. The SMOS solar flux at 50 min time resolution dataset is publicly accessible from Zenodo as daily files.