[current solar image courtesy NASA]
Overview We are now about halfway through the solar maximum phase of the current solar cycle, 24, but it is remarkably weak relative to previous cycles. In fact, solar cycle 24 is on a pace that would make it the weakest cycle in one hundred years. Not since solar cycle 14 which peaked in 1906 have we experienced such a weak solar cycle. Solar cycle 24 began after an unusually deep solar minimum that lasted from 2007 to 2009. In fact, in 2008 and 2009, there were almost NO sunspots, a very unusual situation that had not happened for almost a century.
Consequences of a weak solar cycle First, the weak solar cycle has resulted in rather benign “space weather” in recent times with generally weaker-than-normal geomagnetic storms. Research suggests that this is likely due to reduced magnetic field strength inside the coronal mass ejections that shoot off the sun’s surface. The milder space weather reduces drag on satellites which makes it easier to keep them in orbit; however, it also allows space junk to last longer which can pose problems for operational satellites.
Second, it is pretty well understood that solar activity has a direct impact on temperatures at very high altitudes in a part of the Earth’s atmosphere called the thermosphere. This is the biggest layer of the Earth’s atmosphere which lies directly above the mesosphere and below the exosphere. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation and are highly dependent on solar activity.
Finally, if history is a guide, it is safe to say that weak solar activity for a prolonged period of time can have a negative impact on global temperatures in the troposphere which is the bottom-most layer of Earth’s atmosphere - and where we all live. There have been two notable historical periods with decades-long episodes of low solar activity. The first period is known as the “Maunder Minimum”, named after the solar astronomer Edward Maunder, and it lasted from around 1645 to 1715. The second one is referred to as the “Dalton Minimum”, named for the English meteorologist John Dalton, and it lasted from about 1790 to 1830. Both of these historical periods coincided with below-normal global temperatures in an era now referred to by many as the “Little Ice Age”. In addition, research studies in just the past couple of decades have found a complicated relationship between solar activity, cosmic rays, and clouds on Earth. This research suggests that in times of low solar activity where solar winds are typically weak; more cosmic rays reach the Earth’s atmosphere which, in turn, has been found to lead to an increase in certain types of clouds that can act to cool the Earth.
Outlook The increasingly likely outcome for a weak solar cycle continues the recent downward trend in sunspot cycle strength that began over twenty years ago with solar cycle 22. If this trend continues for the next couple of cycles, then there would likely be more talk of another “grand minimum” for the sun. Some solar scientists are already predicting that the next solar cycle, 25, will be even weaker than this current one. However, it may be just a bit too early for high confidence in these predictions since some solar scientists believe that the best predictor of future solar cycle strength involves activity at the sun’s poles during a solar minimum and the next solar minimum is still likely several years away.