Is There a Super Cycle of Solar Cycles?
Usually the number of sunspots vary in an eleven year cycle known as Schwabe Cycle, which is better known as just Solar Cycle. We are currently in the Solar Cycle 25 meaning that it is the 25th Solar Cycle ever since scientific documentation of sunspot numbers was started in 1775. The periodicity of sunspot number was clearly identified by Samuel Heinrich Schwabe after an initial observation for 17 years and later continued for another 23 years. But it was Rudolf Wolf who started numbering the Solar Cycles from 1775 based on observations of Schwabe and others.
Amateur Radio operators are very much concerned about sunspot numbers and Solar Cycles. There is an ongoing discussion on QRZ.COM forum whether Solar Cycle 25 is better or worse than Solar Cycle 24. When the sunspot numbers decrease, ionospheric ionization on the Earth decreases, leading to poorer propagation on HF bands. I have personally experienced this long back in late 1980s when I started as an amateur radio operator. Initially the propagation was good on 40m and later the propagation dropped while propagation improved on 80m. To take advantage of that, I added an 80m coil to my homebrew radio in those days and operated on 80m as well.
When solar activity is high, higher bands like 10m are better while 80m and 160m propagation improves when solar activity decreases leading to lower ionization in the ionosphere. Now coming back to the original question of whether there is a super cycle of Solar Cycles, which came up as a discussion on one of the QRZ.COM forums. N2LKR after observing the pattern of historical solar cycles noted a sine wave pattern in the peaks of Solar Cycles. That raised a question of whether there is a super cycle of Solar Cycles.
On posting this querry on the HamSCI or Ham Radio Science Citizen Investigation Google Group, K9LA came up with a detailed reply. He mentioned that while Schwabe Cycle has a periodicity of 11 years, Hale Cycle has periodicity of around 22 years during which Sun’s magnetic field goes through its full cycle. The bigger Gliessberg Cycle has a periodicity of approximately 90 years. K9LA also brought out a nice graph which showed the sine wave like periodicity of maximum smoothed sunspot number starting from Solar Cycle 1 to Solar Cycle 24. He also mentioned that it is possible to go back in history to get evidence of solar activity by checking Carbon 14 deposition.
K9LA further posted a graph showing Carbon 14 data right from 800 AD 2000 AD! He explained that when the change in C-14 is positive, Sun’s magnetic field is weakest, corresponding to a solar minimum period and lets in galactic comic rays that eventually deposit C-14 into trees. When the change in C-14 is negative, the Sun’s magnetic field is strongest corresponding to a solar maximum period and keeps out galactic cosmic rays. He also mentioned that there appears to be a 205-year De Vries/Suess cycle and a 2300-year Halstatt cycle! Truly mind boggling types of Solar Cycles indeed!
Now what is the Maunder Minimum shown in the cover picture? It was a period of prolonged sunspot minimum around 1645 to 1715, when sunspots were very rare. Dalton minimum was another period of less marked decrease in sunspot numbers between 1790 and 1820. Increased sunspot activity starting in 1914 with Solar Cycle 15, peaking by Solar Cycle 19 in late 1950s and coming down with the peak of Solar Cycle 23 in 2000 has been called as the Modern Maximum. There is a lot more information on Solar Cycles for those interested, in the Wikipedia article on Solar Cycle and linked articles.