with John Maunder
A sunspot is a relatively dark, sharply defined region on the solar disc - marked by an umbra (dark area) which is 2000 degrees Celsius cooler than the effective photosphere temperature.
The average diameter of a sunspot is 4000 km, but can exceed 200,000 km.
The NASA Solar Physics website (and other web sites such as the Royal Observatory of Belgium) includes information on sunspot numbers, the ‘Maunder Minimum', and sunspot cycle predictions.
The sunspot index is updated monthly and available from 1749. The last time the monthly sunspot number was above 100 for any significant period of time was September 2002 when the value was 109.6, and the last time the value was above 200 was in August 1990 when the value was 200.3.
The peak of latest solar cycle Number 24 was reached in April 2014, with a maximum of the 13-month smoothed sunspot number at 81.8. Since then, solar activity has steadily declined. The monthly mean sunspot number is now around 30-45.
Solar cycle 24 proved to be 30% weaker than the previous solar cycle, which reached 119.7 in July 2000, and thus belongs to the category of moderate cycles, like cycles 12 to 15, which were the norm in the late 19th and early 20th century.
As this late maximum comes more than 5 years after the preceding minimum in December 2008, cycle 24 has now entered its long declining phase, as none of the past observed cycles had longer delays between minimum and maximum.
Therefore, the average solar activity should progressively decrease towards a minimum around 2020. However, over the next 2 or 3 years, we can still expect strong but brief peaks of activity caused by the appearance of a few big complex groups, a typical feature of the late phase of solar cycles.
We are currently over six years into solar cycle 24. This the smallest sunspot cycle since solar cycle 14 which had a maximum of 64.2 in February of 1906.
The “Maunder Minimum” period is named after the solar astronomer Edward Walter Maunder (1851-1928), who while working at The Royal Observatory at Greenwich discovered the dearth of sunspots during the 1650-1700 period.
Time will tell whether the sun will once again go into another “Maunder Minimum” within the lifetime of the present generation, and what affect it will have on our climate.
For further information see: https://sites.google.com/site/climatediceandthebutterfly/