Which solar EUV indices are best for reconstructing the solar EUV irradiance?

Abstract

The solar EUV irradiance is of key importance for space weather. Most of the time, however, surrogate quantities such as EUV indices have to be used by lack of continuous and spectrally resolved measurements of the irradiance. The ability of such proxies to reproduce the irradiance from different solar atmospheric layers is usually investigated by comparing patterns of temporal correlations. We consider instead a statistical approach. The TIMED/SEE experiment, which has been continuously operating since Feb. 2002, allows for the first time to compare in a statistical manner the EUV spectral irradiance to five EUV proxies: the sunspot number, the f10.7, Ca K, and Mg II indices, and the He I equivalent width.Using multivariate statistical methods, we represent in a single graph the measure of relatedness between these indices and various strong spectral lines. The ability of each index to reproduce the EUV irradiance is discussed; it is shown why so few lines can be effectively reconstructed from them. All indices exhibit comparable performance, apart from the sunspot number, which is the least appropriate. No single index can satisfactorily describe both the level of variability on time scales beyond 27 days, and relative changes of irradiance on shorter time scales.

Tests of Sunspot Number Sequences: 1. Using Ionosonde Data

“…Thus ionising solar EUV irradiance is more important in controlling foF2 at Washington than it is at Slough, where the composition effect (on loss rates) dominates. EUV emission (particularly at the softer end of the spectrum) is enhanced through the presence around sunspots of plages and faculae (Dudok de Wit et al, 2008), and hence foF2 is expected to be more dependent on both sunspot numbers and facular area at Washington than at Slough.…”

Solar Weather Event Modelling and Prediction

“…All these models rely on one or a few indices that only partly describe the multiple facets of solar activity. Commonly used indices have limits as shown by Dudok de Wit et al (2008) who demonstrated that no index is representative of the variability of the EUV spectrum at all wavelengths. Therefore, accurate forecasting is limited to the uncertainties of correlations between proxies and spectral irradiance in all of these models, whatever their (numerous) qualities are.…”

Observations of Photospheric Dynamics and Magnetic Fields: From Large-Scale to Small-Scale Flows

“…This kind of approach allows to test the contribution of various components, for example the quiet sun (Withbroe 2006). It is also of great interest to consider information in the UV part of the spectrum, such as done by Dudok de Wit et al (2008) or Woods (2008), because the observed variations are much larger and the impact on the earth expected to be very significant.…”