Sun’s activity on the rise
COMPARED with many other stars, the sun is relatively stable. That’s good for us: indeed, if not for that, it is unlikely that we would exist. However, ‘relatively’ is the important word: the sun is not completely unchanging. It has activity that rises and falls, and right now this activity is on the rise once, heading toward a maximum in about 2025.
Up to a few centuries ago, it was thought that the sun was an unchanging body, but in the early 1600s Galileo Galilei discovered that there were dark spots on its surface that temporarily appeared. We call these sunspots, and in the 19th Century it was discovered by Heinrich Schwabe that the number of sunspots peaked about every 11 years.
Our studies have shown that the sunspots are phenomena related to the sun’s magnetic field, and are places at which the sun’s temperature is about 2000C cooler than its typical 5800C. Sunspots appear dark only in contrast to their much brighter surroundings.
The apparent passage of the sunspots across the face of the sun from day to day was also an indication that the sun is rotating. It does not rotate as a solid body, though: at its equator, it turns once every 25.38 days, but the period is several days longer near its poles.
We also know of other phenomena that peak over this same cycle. For example, solar flares release enormous amounts of energy and are produced by collapsing magnetic fields. We have also found that when the activity is at a maximum, the sun’s overall output of energy is slightly higher than at solar minimum.
Astronomers plot the sun’s activity in two main ways. One is to record the daily sunspot number, which is calculated using a formula related to both the total number of sunspots and the number of sunspot groups. The other is to record the flux – or flow – of solar energy at the radio wavelength of 10.7cm. These records show that the sun reached its minimum activity in 2020, and that it is now clearly on the rise, with the next maximum predicted for sometime between about late 2024 and mid-2025.
Interestingly, the peak levels actually vary considerably from peak to peak, as does the length of the cycle. The maxima around 1990 and 2000 were much stronger than the following one, which was the broad (and rather ‘late’) peak of 2012-2014. We can’t be sure how strong the peak in about three years will be, although some astronomers feel that it will be a little stronger than the previous one.
So, what does solar maximum mean for us?
The most obvious visible sign of solar activity for us in Tasmania is the peak in displays of the aurora australis, which is caused by charged particles from the sun arriving
at Earth and causing ‘geomagnetic storms’.
During these, the particles collide with our upper atmosphere, and make it glow with light. Indeed, Tasmania is famous for its visibility of the ‘southern lights’ as they are also called and at the planetarium, I am often asked by mainlanders about the best times to come to Tasmania for a good view. Unfortunately, they are impossible to predict well in advance, but certainly the best times are the few years centred on solar maximum.
However, the effect on
Earth does have its down side. It can interfere with radio communications, and even generate currents in power grids that can cause serious blackouts. Artificial satellites, too, can be damaged by the effects of solar activity. These can also put the health of human space travellers at serious risk.
Solar maximum is also when there is the greatest amount of ionisation in Earth’s ionosphere, which is a series of layers of charged particles surrounding our planet. At these times, it is more difficult for radio astronomers to
study the sky at very low radio frequencies, as the ionosphere blocks these radio waves. Therefore, such research is best carried out during times of solar minimum. Important low-frequency observations were made by the maverick independent radio astronomer
Grote Reber near Bothwell, and by several University of Tasmania researchers, during the minima of the mid-1960s and mid-1970s.
Martin George is principal astronomer at the Ulverstone Planetarium in Tasmania.