Reconstruction of the solar EUV irradiance from 1996 to 2010 based on SOHO/EIT images

Margit Haberreiter,Benjamin Mampaey,Giulio Del Zanna,Seth Wieman,Cis Verbeeck,Véronique Delouille

doi:10.1051/swsc/2014027

Abstract

The solar Extreme UltraViolet (EUV) spectrum has important effects on the Earth’s upper atmosphere. For a detailed investigation of these effects it is important to have a consistent data series of the EUV spectral irradiance available. We present a reconstruction of the solar EUV irradiance based on SOHO/EIT images, along with synthetic spectra calculated using different coronal features which represent the brightness variation of the solar atmosphere. The EIT images are segmented with the SPoCA2 tool which separates the features based on a fixed brightness classification scheme. With the SOLMOD code we then calculate intensity spectra for the 10–100 nm wavelength range and each of the coronal features. Weighting the intensity spectra with the area covered by each of the features yields the temporal variation of the EUV spectrum. The reconstructed spectrum is then validated against the spectral irradiance as observed with SOHO/SEM. Our approach leads to good agreement between the reconstructed and the observed spectral irradiance. This study is an important step toward understanding variations in the solar EUV spectrum and ultimately its effect on the Earth’s upper atmosphere.

Highlights

The solar Extreme UltraViolet (EUV) spectrum, which covers the wavelength range from 1 to 120 nm, varies on timescales of minutes to days, the 27-day solar rotation period, and the 11-year solar cycle
We present a reconstruction of the solar EUV irradiance based on SOlar and Heliospheric physics Observatory (SOHO)/Extreme UV Imaging Telescope (EIT) images, along with synthetic spectra calculated using different coronal features which represent the brightness variation of the solar atmosphere
Using synthetic spectra for each of the features, along with the area coverages derived from the SPoCA2 procedure described in the previous section, we determine the solar spectral irradiance S as given in Eq (2): Sðk; tÞ

Summary

Introduction

The solar Extreme UltraViolet (EUV) spectrum, which covers the wavelength range from 1 to 120 nm, varies on timescales of minutes to days, the 27-day solar rotation period, and the 11-year solar cycle. The peculiar 2008–2009 solar minimum gave rise to the question whether a long-term trend can be observed, both in the EUV emission of the Sun and in the parameters of the Earth’s upper atmosphere. Whether the long-term trend is positive or negative is still a topic of current debate (e.g., Lean et al 2011a; Laštovicka 2013). At the same time the variability of the EUV spectrum has considerable impact on the upper Earth’s atmosphere, i.e., on the density, temperature, and total electron content A detailed understanding of the EUV variability requires consistent long-term observations from space. All space instruments are subject to degradation, which makes the gathering of a homogeneous time series more challenging. The degradation of current EUV instruments in space has been discussed in detail by BenMoussa et al (2013)

Results

Discussion

Conclusion