The Sun imaged through clouds on 5 October 2012 shows a few spots in its southern hemisphere. Photo Nick Lomb
The Sun provides us with a good opportunity to study the behaviour of a star from close-up. Observing it can even be done in the daytime! The easiest features to observe on the Sun are sunspots, the dark spots that appear sporadically on its visible surface, though other features such as prominences that appear on the edge of the Sun can also be seen with special telescopes that only show light from excited hydrogen atoms.
These and other features demonstrating activity on the Sun have a characteristic pattern of variation. Over a period of roughly 11 years the number of spots increases over a few years to a maximum number and then drops more slowly to another minimum. From the historical record scientists have a good idea of the past variation of sunspot numbers since 1700 and since 1750 each sunspot cycle has a number. The current cycle is number 24.
Predicting the time and size of the maximum of a sunspot cycle is difficult in advance, but becomes easier once the cycle has started. For the current cycle the prediction is that it will peak in about 12 months’ time, that is, in about October 2013 at a size that will make it the least active solar cycle for over a century.
Now, in a paper submitted to the journal Solar Physics on 12 September 2012, Richard Altrock of the Space Weather Center of Excellence at the appropriate location of Sunspot, New Mexico, USA suggests that the sunspot cycles of the north and south hemispheres of the Sun have diverged. He says that his observations indicate that the maximum in the northern hemisphere has already happened in the middle of 2011, while the peak in the southern hemisphere will not happen until early 2014. If he is right, then this is very strange, but feasible behaviour by the Sun.
A large prominence on the south-west edge of the Sun observed in the red light of hydrogen atoms on the morning of 21 October 2012. Image Monty Leventhal OAM
Richard Altrock observes different features to most observers. He uses a special device called a coronograph at a solar observatory at Sacramento Peak in New Mexico to image the Sun’s faint, but extremely hot, outer atmosphere, the corona. Normally, the corona can only be seen at a total eclipse of the Sun, but coronographs can block the light coming directly from the Sun’s bright disc and by carefully suppressing its scattered light allow observation of the corona.
With the coronagraph Altrock observes emission by highly ionised iron atoms or ions (FeXIV) in a region about seven per cent of the Sun’s width away from its edge. By observing the last three solar cycles with the same instrument he has identified patterns that he calls the Rush to the Poles and the Extended Solar Cycle and which can indicate the timing of a sunspot cycle maximum.
For the present cycle the patterns are very messy until the two hemispheres are examined separately. Then the indications are that the northern hemisphere of the Sun has already reached maximum in 2011 and that in the southern hemisphere the activity is still climbing towards a maximum.
The suggestion that the link in the activity between the two hemispheres has been broken is a fascinating one. Only time will tell if Altrock is correct, but shortly on 14 November 2012, if clouds permit, anyone in Cairns or its vicinity will get a direct view of the corona during the total eclipse and maybe get some extra clues about the Sun’s present and future activity.