3:00 PM | *The sun has gone completely blank*

There are no visible sunspots on the most current solar image; courtesy NASA/SDO, spaceweather.com

Overview
The sun has gone completely blank.  This may not last too long, but at least for now, there are no visible sunspots – a sure sign of an approaching solar minimum - and this is the first spotless day on the sun since 2014.  In fact, there has been only one spotless day on the sun since 2011 – until today that is.  The current solar cycle is the 24th since 1755 when extensive recording of solar sunspot activity began. Solar cycle number 24 is the weakest solar cycle in more than a century with the fewest sunspots since cycle 14 peaked in February 1906.  

Sunspot numbers for solar cycles 22, 23 and 24 which shows a clear weakening trend; courtesy Dr. David Hathaway, NASA/MSFC

Info on solar maximum phase
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 (current location indicated by arrow above).  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 which 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 (above) shows a clear weakening trend in solar cycles since solar cycle 22 peaked around 1990. 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).    

Top "sunspotless" days since 1849; last solar minimum produced 3 of these years

Consequences of a solar minimum
Contrary to popular belief, solar minimum is not a period of complete quiet and inactivity as it is associated with numerous interesting changes.  First, cosmic rays surge into the inner solar system with relative ease during periods of solar minimum.  Galactic cosmic rays coming from outside the solar system must propagate upstream against the solar wind and a thicket of solar magnetic fields.  Solar wind decreases and sun’s magnetic field weakens during solar minimums making it easier for cosmic rays to reach the Earth.  This is a more dangerous time for astronauts as the increase in potent cosmic rays can easily shatter a strand of human DNA.  Also, during years of lower sunspot number, the sun’s extreme ultraviolet radiation (EUV) drops and the Earth’s upper atmosphere cools and contracts.  With sharply lower aerodynamic drag, satellites have less trouble staying in orbit— a good thing. On the other hand, space junk tends to accumulate, making the space around Earth a more dangerous place for astronauts.

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 (number 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.

400 years of sunspots with Maunder and Dalton Minimums; courtesy wikipedia

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 (above). 

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.

Outlook
This 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 – and the current blank look to the sun is liable to become more and more frequent in the months to come. 

Meteorologist Paul Dorian

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