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A very interesting paper was published recently by Klaus-Peter Schroeder and colleagues (Astronomy & Astrophysics, 540, A130, 2012). A link to this paper is:

In this paper, the authors examine the latest solar minimum in 2009 compared to the chromospheric flux of a sample of solar-type stars, a measure of stellar activity comparable to the solar sunspot index. To be precise, they used the Mt. Wilson S-index, which is a measure of excess (chromospheric flux) in the 3933 A line of ionized calcium. This quantity can be measured in other stars (unlike the sunspot index) and provides a suitable proxy of stellar activity. It turns out that the scatterplot of S-index vs. stellar temperature (in terms of color index B-V -- just a way of measuring stellar surface temperature) has a minimum locus (Fig 3 of the Schroeder et al. paper). No star with a chromospheric flux (or stellar activity) below this locus are observed. These are Maunder minimum stars in a very low state of activity.

The authors also monitored the Sun during the recent 2008-2009 solar minimum, when the solar S-index was very low by historical standards. It turns out that on several occasions during early 2009, the solar S-index actually touched the Maunder minimum locus (which is S=0.150 for the Sun, for those interested). One such date was on Feb 6, 2009 when the Sun looked like this in the light of the hydrogen Balmer-alpha (H-alpha) line at 6563 A (a good diagnostic of chromospheric activity).

A completely featureless solar disk is seen, which is very unusual. For comparison, take a look at today's Sun in H-alpha:

What this result means is that for the first time we *know* what the Sun looked like during the Maunder minimum -- it looked like the Sun on Feb 6, 2009 (and other days around then). We also know what the MM total solar irradiance is -- it's the 1360 W/m^2 observed in Feb 2009.

There has been a whole industry of recent trying to reconstruct past solar TSI (total solar irradiance -- the total solar flux reaching the Earth). The results obtained depended critically upon the assumed state of the Maunder-minimum (MM) Sun. For example, Shapiro et al. (2011) assumed *all* magnetic activity vanished during the MM and predicted a TSI some 6 W/m^2 lower than present. The reality, as shown by Schroeder et al. is that a small amount of small-scale magnetic field remains even during MM times, and the TSI is not nearly so reduced. Instead it only drops by about 1.0 W/m^2 from mean solar TSI values, for a total range of about 2.0 W/m^2 due to activity variability. This is a much smaller effect than claimed for some previous TSI reconstructions.

One important consequence of this is with regard to the climate sensitivity -- it must be fairly high because we observe substantial effects on global temperature (about 0.5 C peak to peak) due to small changes in TSI. Compare this to expected changes in global surface temperature for a simple planet with no other feedbacks. Then the expected change in surface temperature due to radiative equilibrium considerations would be T_surf*(delta_TSI/TSI)/4 = 0.11 C [where T_surf= 288 K, TSI= 1360 W/M^2, delta_TSI= 2.0 W/m^2]. This is just 20% of the actual temperature response, which suggests a high climate sensitivity.

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