Solar cycle activity versus cycle length. The activity is the sum of the monthly sunspots for the entire cycle. SC24 (in red) is still provisional, and the dashed arrow indicates a possible path it might follow until the solar minimum takes place. The Dalton Minimum (purple), Gleissberg Minimum (blue), and Modern Maximum (orange) cycles are indicated. The Modern Maximum is a period of seven consecutive high activity solar cycles in a period that coincides with high anthropogenic CO2 emissions and global warming (1935-2005). The longest such stretch of high solar activity known.
Solar cycle 24 is ending and we are approaching a time of minimal solar activity between solar cycles 24 and 25, known as a solar minimum. Despite claims that we understand how the Sun works, our solar predictive skills are still wanting, and the Sun continues to be full of surprises.
The surprising 2008 solar minimum
Solar scientists did not pay much attention to the early warning signs that the Sun was behaving differently during solar cycle 23 (SC23), and to most the surprise came when the expected solar minimum failed to show up in 2006. The SC23-24 minimum took place two years later (Dec 2008, according to SIDC), and despite showing only a tiny difference in total solar irradiation compared to previous minima of the space age, it displayed significantly reduced solar wind speed and density, extreme-UV flux was 10% reduced, the polar fields were 50% smaller, and the interplanetary magnetic field strength was 30% below past minima. In response to the changes in the Sun, the density of the Earth thermosphere dropped 20% lower than in previous minima. In 2007
Svalgaard & Cliverproposed a floor to the interplanetary magnetic field at 1 AU in the ecliptic plane of 4.6 nT based upon 130 years of data. This floor has implications for the solar wind during grand minima. After the solar minimum, in 2011,
Cliver & Ling were forced to revise down the floor to 2.8 nT, a 40 percent reduction! The SC23-24 minimum was truly shocking to solar scientists, showing them how little they knew of what happens to the Sun when it becomes very inactive. And it was just a centennial-type solar minimum, not a grand-type solar minimum.
Comment: Volcanic acitivty appears to be on the rise: