Following up from the previous posting, Sun and Climate Change Part 1-Solar Activity, we will continue to examine the connection between the Sun and the Earth’s climate.
Sun’s Magnetic Field
On average, the Sun’s magnetic field is thought to be only about twice as strong as Earth’s magnetic field. The magnetic field is generated by the rotation of the Sun acting like a giant electromagnet. But it has local fields of enormous strength, something in the range of 8000 times greater that Earth’s field. It is believed that the cause of these local fields is the differential rotation of the Sun’s latitudes. The observed rotation at the equator is 25 days and near the poles about 35 days is due to the fact that the Sun is made up of plasma and hot gases. This results in the magnetic field becoming twisted and erupting from the surface in these local fields. Where this enormous magnetic force exists, you find Sunspots, flares, and coronal mass ejections (CME). The Sun has a cycle of about 11 years from minimum to maximum and back to minimum magnetic activity. This cycle can be observed by the numbers of Sunspots formed on the surface of the Sun. During a cycle, the Sunspot number increase until the Sun’s polarity “flips”. The Sun’s magnetic polar north flips and points south. This usually is the point at which the so-called solar maximum is reached. The activity on the Sun begins to decrease. The cycle eventually reaching a point where very few Sunspots are observed. This is the completion of a cycle.
Sunspots are the product of the enormous magnetic fields (thousands of times stronger than Earths magnetic field) created on the Sun. They appear as dark spots. The Spots are cooler than the surrounding surface of the Sun. NASA says that the Spots are about 3700K versus 5700K for the surrounding photosphere.
Sunspots are probably not the best indicators of the Sun’s activity but the better indicators have little history where as Sunspots have been recorded for hundreds of years. What make them especially interesting is that the Earth’s climate and Sunspots have a high degree of correlation. Periods where the Earth’s climate has cooled off appear to coincide with periods of few Sunspots and periods of warmer climate seem to coincide with periods of high Sunspot counts.
The telescopic recording of Sunspots began with Galileo using the newly invented telescope to observe the sunspots in 1610. He and later others began to count the sunspots.
When a solar cycle is forecast, the y-axis is usually Sunspot numbers and the x-axis is time. The current solar cycle is 24. On the chart below, the blue line is the monthly average of Sunspots. It shows cycle 23 declining from its high and then cycle 24 as it begins to rise. The orange line was the predicted shape of cycle 24 by NASA experts but the actual Sunspot numbers are lower than forecast. The experts have found it necessary to continue to reduce the numbers (height) of the forecast monthly Sunspot numbers. In 2006 they forecast a height of cycle 24 at a sunspot number of 156 to 180. The following chart is from a posting by Anthony Watts on his blog, WattsUpWithThat and can be seen here.
The following chart shows the latest (Dec 2010) prediction of NASA and how they have found it necessary to continue to lower their expectations over time to new “high Sunspot number” of 64!!! Although some people think they have a system that allows them to predict these numbers, its clear that no one knows enough to make any forecast that’s good enough to bet on it. The NASA forecast has been downgraded four times since the March 2006 prediction.
(The chart courtesy of Ira Glickstein on WUWT blog and can be seen here.)
There is one additional factor that the reader needs to be aware of, and that is comparing Sunspot counts of old and those of today could be problematic. The pinpricks that are counted on the sun today with the high-powered equipment were probably not even noticed in years past. So when it is reported that the sunspot count during period of the Maunder Minimum in lower than today, you cannot be sure we are comparing apples and apples. Some discussion about this variable can be found here.
But no matter how you count them, the cycle 24 sunspots are out of the norm. Cycle 24 is being compared to Cycle 5 which occurred at the time of the Daulton Minimum. The indications are Earth’s climate is in for a period of cooling.