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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: regular paper 20 Jan 2020

Submitted as: regular paper | 20 Jan 2020

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This preprint is currently under review for the journal ANGEO.

Lower Thermosphere response to solar activity: an EMD analysis of GOCE 2009–2012 data

Alberto Bigazzi, Carlo Cauli, and Francesco Berrilli Alberto Bigazzi et al.
  • University of Rome Tor Vergata, Via della Ricerca Scientifica, 1, Roma, Italy

Abstract. Forecasting the Thermosphere (Atmosphere's uppermost layer, from about 90 to 800 km altitude) is crucial to many space-related applications, from space mission design, to re-entry operations, to space surveillance. Thermospheric dynamics is directly linked to the solar dynamics through the solar UV input, which is highly variable, and through the solar wind and plasma fluxes, impacting Earth's magnetosphere. The solar input is non-periodic and non-stationary, with long-term modulations from the solar rotation and the solar cycle, and impulsive components, due to magnetic storms. Proxies of the solar input exist and may be used to forecast the thermosphere, such as the F10.7 radio flux and the MgII EUV flux. They relate to physical processes on the Solar atmosphere. Other indices, such as the Ap geomagnetic index, connect with Earth's geomagnetic environment. We analyse the proxies' time series comparing them with in-situ density data from the ESA/GOCE gravity mission, operational from March 2009 to November 2013, therefore covering the full rising phase of solar cycle XXIV, exposing the entire dynamic range of the solar input. We use Empirical Mode Decomposition (EMD), an analysis technique appropriate to non-periodic, multi-scale signals. Data are taken at an altitude of 260 km, exceptionally low for a LEO satellite, where density variations are the single most important perturbation to satellite dynamics.

We show that the synthesized signal from optimally selected combinations of proxies's basis functions, notably Mg II for the solar flux and Ap for the plasma component, shows a very good agreement with thermospheric data obtained by GOCE, during low and medium solar activity periods. In periods of maximum solar activity, density enhancements are also well represented. The Mg II index proves to be, in general, a better proxy than the F10.7 one, to model the solar flux, because of its specific response to the UV spectrum, whose variations have the largest impact over thermospheric density.

Alberto Bigazzi et al.

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Alberto Bigazzi et al.

Alberto Bigazzi et al.


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Publications Copernicus
Short summary
Forecasting the thermosphere (atmosphere's uppermost layer from 90 to 800 km altitude) is crucial to space mission design, spacecraft operations and space surveillance. Thermosphere is controlled by the Sun, through the highly variable solar EUV radiation and the solar wind. We show how two solar indices, Mg II and Ap, may be used in forecasting thermospheric density at 260 km, a very low altitude where the GOCE satellite has operated from 2009 to 2013, during the full rise of Solar Cycle XXIV.
Forecasting the thermosphere (atmosphere's uppermost layer from 90 to 800 km altitude) is...