The NOAA SWPC monthly solar cycle update has been published here, and after a big spike last month, the sunspot count is down again. There’s an even bigger drop though in the Ap geomagnetic index, as seen and discussed below the Continue reading line.
10.7 centimeter radio flux was down slightly too.
But here’s the really interesting part, the Ap geomagnetic index plummeted to a value of 2, equal to the previous 12 year minimum set in November 2009.
Dr. Leif Svalgaard offers these comments via email:
Ap is based on mostly Northern Hemisphere stations [11 North, 2 South] and is somewhat biased [having less activity in northern winter]. This is in addition to a general semiannual variation http://www.leif.org/research/Semiannual-Comment.pdf
with minima at the solstices. The definitive Ap values are determined by Potsdam and can be found here: http://isgi.latmos.ipsl.fr/lesdonne.htm
Real-time values [preliminary the last 15 days] are available here http://www.geomag.bgs.ac.uk/data_service/data/magnetic_indices/apindex.html
SWPC [NOAA] also compute preliminary real-time values. These computed values are truncated, so if, for instance, Ap = 9.99 it is reported [and plotted] as 9.00. SWPC is not very good at updating their graphs with definitive values, so one should not make too strong statements based on their graphs. The value for December, 2011 is a case in point. It is plotted as 2, but the real value is estimated [by BGS] to be 4.1.
The Aa index is based on one northern and one southern station, so does not suffer from some of the problems Ap has. The index can also be calculated from solar wind data: Aa = 1/6 BVo^2, where the solar wind magnetic field B is in nT and the solar wind speed Vo is in units of 100 km/s. Here is computed [blue and green curves] vs. observed [red curve] values since 2005: http://www.leif.org/research/Aa-Since-2005.png
You can see that geomagnetic activity is low, but not as low as at the end of 2009.
The reason for the low activity is that the solar wind speed is low [365 km/s]. This often happens near solar maximum.
UPDATE: David Archibald adds this graph and narrative -
Dr Svalgaard’s comment re solar wind and solar maximum might be misinterpreted to suggest that Ap Index is lowest at solar maximum. The opposite is true as shown by this graph from of the Ap Index from 1932.
The Ap Index is back below the floor established by all the previous solar minima. This is important, and there is a correlation between low Ap Index and cooling.