The current solar cycle, #24, is the weakest solar cycle in more than a century and it is now heading towards the next solar minimum phase which would be the beginning of solar cycle #25. The last solar minimum phase lasted from 2007 to 2009 and it was historically weak. In fact, it produced three of the most spotless days on the sun since the middle 1800’s (bar graph below). The current solar cycle is the 24th solar cycle since 1755 when extensive recording of solar sunspot activity began. Solar cycle 24 is currently on pace to be the weakest sunspot cycle with the fewest sunspots since cycle 14 peaked in February 1906. Solar cycle 24 continues a recent trend of weakening solar cycles which began with solar cycle 22 that peaked around 1990.
The latest solar image (above) shows a rather quiet looking sun with only a couple of visible sunspot regions (indicated by arrows). With no sunspots actively flaring, the sun's X-ray output has flatlined. The number of nearly or completely spotless days should increase over the next few years as we continue to move away from the solar maximum phase of cycle 24 and approach the next solar minimum phase and the beginning of solar cycle 25 (current location indicated by arrow below).
The sun goes through a natural solar cycle approximately every 11 years. The cycle is marked by the increase and decrease of sunspots which are visible dark regions on the sun’s surface and cooler than surroundings. The greatest number of sunspots in any given solar cycle is designated as the “solar maximum" and the lowest number is referred to as the “solar minimum” phase. We are currently more than seven years into Solar Cycle 24 and it appears the solar maximum of this cycle was reached in April 2014 during a spike in activity. Going back to 1755, there have been only a few solar cycles in the previous 23 that have had a lower number of sunspots during its maximum phase. The peak of activity in April 2014 was actually a second peak in solar cycle 24 that surpassed the level of an earlier peak that occurred in March 2012. While many solar cycles are double-peaked, this is the first one in which the second peak in sunspot number was larger than the first peak. The sunspot number plot below also shows a clear weakening trend in solar cycles since solar cycle 22 peaked around 1990.
Sunspot numbers for solar cycles 22, 23 and 24 which shows a clear weakening trend; courtesy Dr. David Hathaway, NASA/MSFC
Consequences of weak solar cycles
There can be important consequences from weak solar cycles; especially, if they are part of a long-term pattern. First, this particular weak solar cycle has resulted in rather benign “space weather” in recent times with generally weaker-than-normal geomagnetic storms. By all Earth-based measures of geomagnetic and geoeffective solar activity, this cycle has been extremely quiet. However, while a weak solar cycle does suggest strong solar storms will occur less often than during stronger and more active cycles, it does not rule them out entirely. In fact, the famous "superstorm" Carrington Event of 1859 occurred during a weak solar cycle (#10). In addition, there is some evidence that most large events such as strong solar flares and significant geomagnetic storms tend to occur in the declining phase of the solar cycle. In other words, there is still a chance for significant solar activity in the months and years ahead.
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 any guide, it is safe to say that weak solar activity for a very prolonged period of time (several decades) can have a cooling 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 (below).
Both of these historical periods coincided with colder-than-normal global temperatures in an era that is now referred to by many scientists as the “Little Ice Age”. One of the reasons prolonged periods of weak solar activity may be associated with colder global temperatures has to do with a complicated relationship between solar activity, cosmic rays, and clouds on Earth. Research studies in recent years have found 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.
The increasingly likely outcome for another historically weak solar cycle continues the recent downward trend in sunspot cycle strength that began over thirty years ago during solar cycle 22. If this trend continues for the next few cycles, then there would likely be increasing talk of another “grand minimum” for the sun which correlates to an extended decades-long period of low solar activity. Some solar scientists are already predicting that the next solar cycle will be even weaker than this current one which has been historically weak. However, it is just too early for high confidence in those predictions since many solar scientists believe that the best predictor of future solar cycle strength involves activity at the sun’s poles during a solar minimum phase – something we are now rapidly approaching.
Meteorologist Paul Dorian