Recent effort in studies of variation in total solar irradiance (TSI) has gone mainly into trying to determine whether the Sun may have dimmed enough during the Maunder Minimum to account for the 17th century climate cooling seen in most reconstructions of global temperature over the past millennium. Dimming by the small factor ( ~ 0.04% with climate-appropriate smoothing ) expected from spots, faculae and network is too small to influence climate models. Speculative TSI variations 4-5 larger can drive climate models, but they were based on stellar photometry that has since been retracted mainly because the supposed quiet solar analog stars under study turned out to be less Sun-like than originally suggested. These speculative TSI reconstructions were widely considered to have solved the problem of Sun and climate until a few years ago. Their retraction has caused some consternation in the climate modeling community.
More robust stellar photometric measurements on stars more ( rather than less) active than the Sun, may prove more useful. Their relevance applies to understanding TSI variation expected at activity levels exceeding that encountered during cycle 19, in the late 1950’s. A rapid increase of cyclic photometric variation has been reported,with increasing stellar activity. The simplest explanation is that, in more active stars, the dimming due to spots of increasing size dominates the brightening due to faculae. In the present Sun, the contributions almost cancel, but in such stars, the cancellation breaks down and variability increases. The stars also dim (rather than brightening like the present Sun) with increasing activity. Even in the present Sun, the ratio of spot/facular area increases at the highest activity levels seen in cycle 19. This suggests that a “hyperactive” Sun of somewhat higher sunspot number than seen in the 1950’s could become several times more variable in TSI, and also dimmer, rather than brighter with activity level increase. This particularly relevant to the recent finding from deep ocean sediments, that Holocene climate in seems to have been modulated by solar activity on a millennial time scale.
It is unclear how likely such a hyperactive Sun might have been in the past , or be in the future. Recent reconstructions of activity level from C14 record disagree on whether the present activity level has been exceeded in the past 10,000 yrs. Also, it is questionable how well radioisotope records can be used to place upper limits on solar activity. The reason is that they assume C14 and Be10 are produced by GCR’s whose flux at the Earth is modulated in inverse proportion to solar activity. However, at least for Be10 the production by solar CR’s is not negligible, even at recent activity levels. Therefore, one expects that for much higher activity levels, the solar CR flux will overwhelm the GCR contribution, and the correlation with solar activity will change from negative to positive. This means that the relation between activity level and radioisotope record would no longer be single-valued. The possibility of distinctly higher activity levels deserves closer investigation than it has received so far.
In summary, stellar photometry has not been as helpful as was hoped for understanding the extent of solar dimming at the lowest solar activity levels, but it may be more useful for gauging the luminosity behavior of a more active Sun.
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