Solar Activity and Spotless Days

Weather Eye
with John Maunder

The NASA solar physics website, and other websites such as the Royal Observatory of Belgium, include information on sunspot numbers, spotless days, the ‘Maunder Minimum' and sunspot cycle predictions.

A sunspot is a relatively dark, sharply defined region on the solar disc – marked by an umbra, dark area, which is 2000 degrees cooler than the effective photospheric temperature. The average diameter of a sunspot is 4000 km, but they can exceed 200,000 km.

The sunspot index is updated monthly and is available from 1749. The last time the value was much above 200 was in August 1990.

Solar observers have noticed that since mid-2016, the Sun has occasionally been devoid of sunspots. These spotless disks will gradually become a familiar feature as the solar cycle is heading for its next minimum, currently expected by the end of this decade and probably has already started. The number of spotless days can vary significantly from one solar cycle transit to another. For example, during the previous minimum solar cycle 24 (around 2008), 817 spotless days were recorded, whereas the minimum period leading into solar cycle 23 (around 1996) counted only 309 such blemishless days.

The current solar cycle 24 will gradually give way to the new solar cycle 25, and several consecutive days and even periods of 30 or more consecutive days without sunspots will become the norm. the Belgium SILSO website has created a “Spotless Days page”. This page contains graphs and tables on the accumulated number of spotless days, stretches of spotless days, and comparisons to other solar cycles.

The previous minimum (solar cycle 24) surprised many solar scientists and solar observers by being the deepest in nearly 90 years. Will the upcoming solar cycle minimum show as many spotless days, or will solar cycle 25 take off much faster than expected? The “Spotless Days page” provides a front-row seat on the current status of the solar cycle minimum and the number of spotless days. Enjoy!

Since 1849, there have been 114 years (including 2019) with at least one spotless day. The chart below shows the 25 years with the highest number of spotless days. 1913 had 311 spotless days, while 2008 ranks fourth in years with a spotless sun (265 days). With 208 days, 2018 (green bar) has nested itself into the top 25 years with most spotless days.

As of September 22, 2019 there have been 189 days in 2019 without sunspots, and with 100 days remaining in the calendar year, the forecast by many solar scientists is that the 2019 will have over 250 days with sunspots, which would make 2019 the ninth most “spotless” year since records began in 1849.

The ‘Maunder Minimum' is the name given to the period from 1645 to 1715 when the number of sunspots – ‘storms' on the sun – became almost zero. The period is named after the solar astronomer Edward Walter Maunder (1851-1928), who was working at The Royal Observatory at Greenwich when he discovered the dearth of sunspots during this period.

During one 30-year period within the Maunder Minimum there were only about 50 sunspots compared with a more typical 40,000. Maunder was a driving force in the foundation of the British Astronomical Association and a Fellow of the Royal Astronomical Society.The sun was well observed during the period of the Maunder Minimum and this lack of sunspots is well documented. This period of solar inactivity corresponded to a climatic period called the ‘Little Ice Age' when in Europe rivers that were normally ice-free, froze and snow fields remained at low altitudes throughout the year. There is evidence the sun had similar periods of inactivity during the years 1100-1250 and 1460-1550. Sunspots generally follow a cycle of about 11 years, but cycles have varied from eight-15 years.

The connection between solar activity and the earth's climate is an area of ongoing and sometimes controversial research.Time will tell whether the sun will once again go into another ‘Maunder Minimum' within the lifetime of the present generation with very many spotless days , but if this happens we're likely to have a much colder climate for a few decades.

The sunspot number for most days during the last 3 months is zero.

For further information on a range of weather and climate matters see:


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