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

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.

Segmentation of photospheric magnetic elements corresponding to coronal features to understand the EUV and UV irradiance variability

“…Even though the magnetic structure evolution and its influence on solar irradiance is not yet carried out on a fully automated basis, first results for the total solar irradiance (TSI) and ultraviolet (UV), visible, and infrared (IR) part of the spectrum were made available in recent years (Yeo et al 2014a,b;Wenzler et al 2006). Investigations on the UV and the extreme UV (EUV) part of the spectrum has been provided by Haberreiter et al (2014) Direct comparison of Ca ii K images and magnetograms shows the strong association between plage and strong field regions (Babcock & Babcock 1955;Leighton 1959;Howard 1959). Stepanov & Petrova (1959) gave the first magnetic flux versus Ca ii K intensity curves measured in calcium faculae (plages) with an electronic magnetograph.…”

Small-scale heating events in the solar atmosphere

“…Also, the question of whether the occurrence rate of the SSHEs shows a dependence on the solar activity, i.e., the magnetic field strength, is yet to be addressed. Ultimately, SSHEs could be the main driver of the solar cycle variation in coronal emission (see, e.g., Haberreiter et al 2014). Finally, this work is extremely relevant for the interpretation, and the potential observational evidence of the SSHEs with respect to the upcoming data from the Spectral Imaging of the Coronal Environment instrument (SPICE; Fludra et al 2013) and the Extreme UV imager (EUI; Halain et al 2014) on board the Solar Orbiter Mission.…”

EUV Irradiance Inputs to Thermospheric Density Models: Open Issues and Path Forward

“…Another approach is to cover the UV/EUV range by combinations of EUV measurements within discrete passbands (e.g., Cessateur et al, ; Dudok de Wit et al, ; Suess et al, ) or by using additional information, such as expanding the F 10.7 index with additional radio wavelengths that encompass emission from other layers of the solar atmosphere (Dudok de Wit et al, ) and, indirectly, serve the same purpose as the EUV measurements. Other researchers prefer to use spatially resolved images in the UV/EUV to estimate the contribution of different solar features (i.e., sunspots, plages, and coronal holes) to the spectral irradiance via semiempirical methods, that is, differential emission measure (Warren et al, ) or image segmentation and synthetic spectrum (Haberreiter et al, ). Another, more recent approach relies on empirical relationships between photospheric magnetic flux and EUV emission (e.g., Henney et al, , and references therein).…”