Are Non-Magnetic Mechanisms Such As Temporal Solar Diameter Variations Conceivable for an Irradiance Variability?

Abstract

Irradiance variability has been monitored from space for more than two decades. Even if data are coming from different sources, it is well established that a temporal variability exists which can be set to as ≈ 0.1%, in phase with the solar cycle. Today, one of the best explanation for such an irradiance variability is provided by the evolution of the solar surface magnetic fields. But if some 90 to 95% can be reproduced, what would be the origin of the 10 to 5% left? Non magnetic effects are conceivable. In this paper we will consider temporal variations of the diameter of the Sun as a possible contributor for the remaining part. Such an approach imposes strong constraints on the solar radius variability. We will show that over a solar cycle, variations of no more than 20 mas of amplitude can be considered. Such a variability -far from what is reported by observers conducting measurements by means of ground-based solar astrolabes-may explain a little part of the irradiance changes not explained by magnetic features. Further requirements are needed that may help to reach a conclusion. Dedicated space missions are necessary (for example PICARD, GOLF-NG or SDO, scheduled for a launch around 2008); it is also proposed to reactivate SDS flights for such a purpose.

Advances in Global and Local Helioseismology: An Introductory Review

“…Since the f-mode frequencies provide an accurate estimate of the seismic radius, then it is interesting to investigate the variations of the solar radius during the solar activity cycle, which are quite important for understanding physical mechanisms of solar variability (e.g. [108]). Figure 23 shows the fmode frequency variations during the solar cycle 24, in 1997-2004, relative to the f-mode frequencies observed in 1996 during the solar minimum [109].…”

Solar Radius Measurements at Mount Wilson Observatory

“…Recently, a four-component model composed of quiet Sun, magnetic network, faculae and sunspots has been developed to explain 90%Y95% of the solar irradiance output (Solanki & Krivova 2004). Possible variations in the Sun's radius are not included in this model, and this could possibly explain, if not the whole of the remaining 5% Y10% , at least a part of it ( Fazel et al 2006;Rozelot et al 2004).…”

Reconstruction of total solar irradiance 1974–2009

“…Explanations for secular trends other than solar surface magnetic activity have also been proposed. E.g., Rozelot et al (2004) suggested that changes in the Sun's radius could account for long-term changes in the Sun. Harder et al (2009) proposed that there may have been a change in the photospheric temperature gradient that produced a spectrally dependent change, increasing irradiance at some wavelengths and decreasing at others.…”