Sunspot cycles

In a previous post, I had discussed how Galileo had begun making systematic observations of sunspots with a telescope starting in 1610.  Science has been observing sunspots ever since.  This led to the discovery of a sunspot cycle in 1843 by German astronomy Samuel Heinrich Schwabe (1789-1875).  This cycle averages 11 years (rounded off) but can range from 9 to 14 years in length.

Solar maximum                                                             Solar minimum

Each 11-year cycle goes from a solar minimum to a solar maximum and then back to a solar minimum again.  At a solar maximum, there are a lot of sunspots on the surface of the Sun. At a solar minimum, there are few to no sunspots.

The numbering scheme for solar cycles was developed by Swiss astronomer Rudolph Wolf (1816-1893) who began with the cycle starting in March of 1755.  The last solar cycle, number 23, peaked around April 2000 and we're currently in cycle 24, which began on January 8, 2008, and is expected to peak in May of 2013.  The current solar cycle seemed to be slow getting started and has exhibited about 50% less sunspot activity than expected.

One of the interesting things about the sunspot cycle is that its intensity varies cycle to cycle - at least over the past few hundred years of observation.  Keep in mind that the Sun has been around for four-and-a-half billion years.  If sunspot cycles have always averaged 11-years (highly improbable), then there would have been over 400 million sunspot cycles (and we've only observed a couple of dozen!).  Astronomers have been carefully observing, with satellites, only for the past couple of cycles.

Shortly after the earliest scientific studies of sunspots by Galileo, the Sun went quiet.  From about 1645 to 1715, sunspots practically disappeared from the Sun (that's a span of six or so sunspot cycles).  It's been named after English astronomy Edward Maunder (1851-1928).  Other minimums include the Dalton Minimum (1790-1830) and the Spörer Minimum (1460-1550) named for an event identified by the radiocarbon (¹⁴C) concentration in tree rings (¹⁴C is produced in the atmosphere strongly correlates with solar activity).

In more modern times we seem to be in a period of increased solar activity (except for the current sunspot cycle).  One of the most intense cycles was number 19 which peaked around 1960.  Why the variations?

I'll save that for the next post...