Leif Svalgaard a well known solar scientist is giving a presentation in Oslo on Monday titled Solar Activity Past Present and Future. The whole presentation is rather detailed and at times complex, but I found slide 34 very interesting, as Leif mentions the potential for a Maunder like Grand Minimum in the Sun’s future. Slide 34 is below, with his comentary below the graphic.
And something is happening with the Sun. The F10.7 flux has from the beginning of the data [in 1947] had a firm and consistent relationship with the sunspot number, so that one could use one as a proxy for the other [we used that in Slide 22]. Using that relationship we can calculate what sunspot number to expect for a given F10.7 flux and compare it with observations. The ratio between the observed and the synthetic sunspot numbers should scatter a bit around unity, as it does nicely up to about 1990. From then on, the observed SSN falls progressively below the expected values. This using both the SIDC and the SWPC [NOAA] numbers [upper left panel].
Mount Wilson Observatory calculates for each day the fraction [called the Magnetic Plage Strength Index, MPSI] of the solar disk covered with magnetic fields outside of spots [so called 'plages']. That fraction has a strong solar cycle variation; on average the SSN is a function MPSI: SSN = 55 MPSI. As for F10.7, we can calculate the expected SSN for a given MPSI and form the ratio between the observed and the calculated numbers [lower panel, left].
That fraction has been falling significantly during the same time when the SSN and F10.7 deviated from each other.Ever since Wolf, there has been on average about 10 spots per sunspot group. This is the reason for the 10 appearing in the sunspot number formula SSN = 10*Groups + Spots. Over the past couple of cycles the number of spots per group has dropped by about a third, so groups are losing the smaller spots.
Livingston & Penn have observed the magnetic field at the darkest point in every sunspot [within their observing time window] and find that the average field has decreased by 20% over the same time as the other effects we have been describing. The ‘bottom’ of the distribution seems to be cut off at 1500 Gauss, below which sunspots do not seem to form.
All of these effects are unprecedented in the observational data and tells us that the Sun is changing in ways not seen before. Or have we seen this before? During the Maunder Minimum cosmic rays were still modulated, the spicule ‘forest’ in the chromosphere was still observed, so there were healthy magnetic fields, yet few spots were visible. Perhaps they just didn’t form as they used to, being an extreme example of the trends we are seen now…
The full presentation can be found HERE.