Solar extreme ultraviolet variability of the quiet Sun

Abstract

The last solar minimum has been unusually quiet compared to the previous minima (since space-based radiometric measurements are available). The Sun's magnetic flux was substantially lower during this minimum. Some studies also show that the total solar irradiance during the minimum after cycle 23 may have dropped below the values known from the two minima prior to that. For chromospheric and coronal radiation, the situation is less clear-cut. The Sun's 10.7 cm flux shows a decrease of ∼4% during the solar minimum in 2008 compared to the previous minimum, but Ca ii K does not. Here we consider additional wavelengths in the extreme ultraviolet (EUV), specifically transitions of He i at 584.3 Å and O v at 629.7 Å, of which the CDS spectrometer aboard SOHO has been taking regular scans along the solar central meridian since 1996. We analysed this unique dataset to verify if and how the radiance distribution undergoes measurable variations between cycle minima. To achieve this aim we determined the radiance distribution of quiet areas around the Sun centre. Concentrating on the last two solar minima, we found out that there is very little variation in the radiance distribution of the chromospheric spectral line He i between these minima. The same analysis shows a modest, although significant, 4% variation in the radiance distribution of the TR spectral line O v. These results are comparable to those obtained by earlier studies employing other spectral features, and they confirm that chromospheric indices display a small variation, whereas in the transition region a more significant reduction of the brighter features is visible.

Multichannel autocalibration for the Atmospheric Imaging Assembly using machine learning

“…4, we find that both the dimmed and undimmed 1D histograms have a skewed shape, with a dominant peak at lower intensities and extended tails at higher intensities. A skewed distribution for the quiet-Sun intensities like this has been reported by various previous studies (see Shakeri et al 2015), where they were modeled either as a sum of two Gaussians (Reeves et al 1976) or as a single log-normal distribution (Griffiths et al 1999;Fontenla et al 2007). Despite an increased number of free parameters in a double-Gaussian fitting, Pauluhn et al (2000) showed that the observed quiet-Sun intensity distribution could be fitted significantly better with a single log-normal distribution.…”

“…A similar assumption in measuring the instrument sensitivity of the Solar and Heliospheric Observatory (SOHO, Domingo et al 1995) CDS was also adopted by Del Zanna et al (2010), who assumed that the irradiance variation in the EUV wavelengths is mainly due to the presence of ARs on the solar surface and that the mean irradiance of the quiet Sun is essentially constant throughout the solar cycle. There is evidence of small-scale variations in the intensity of the quiet Sun when observed in the transition region (Shakeri et al 2015), but their contribution is insignificant compared to that of their AR counterparts. We used this idea for our baseline model as described in this section.…”

“…However, no standard candles exist in the solar atmosphere at these wavelengths because the solar corona is continuously driven and structured by evolving magnetic fields, which cause localized and intermittent heating. This causes even the quiet-Sun brightness in the EUV channels to vary significantly, depending on the configuration of the small-scale magnetic fields (Shakeri et al 2015, and the references therein). On the one hand, the Sun may not be bright enough to appear in the hotter EUV channels such as AIA 94 Å.…”

Non-normal distribution of the top-of-atmosphere satellite optical measurements over calibration sites

“…From that distribution, the mode was selected as the statistical estimate that minimizes the influence from solar cycle radiance variations. Furthermore, the analysis in Shakeri, Teriaca, and Solanki (2015), uses the 'contrast ratio' to characterize the radiance distribution, defined as the ratio between the lower and higher quantiles, which is selected either as 5% or 10%. The authors argue that this technique cancels out the effects of correction and calibration factors and permits the study of subtle solar changes.…”

“…The authors argue that this technique cancels out the effects of correction and calibration factors and permits the study of subtle solar changes. Both studies (Del Zanna et al (2010) and Shakeri, Teriaca, and Solanki (2015)) indicate that the incorporation of additional parameters and quantile information of the TOA reflectance/radiance distribution to understand variations of either the instrument or vicarious test site is important.…”