9:30 AM | *Sun goes blank again as it heads towards next minimum...low solar activity could play an important role in the upcoming winter season*

The sun is blank again today as it heads towards the next solar cycle minimum.  This marks the 70th day of 2017 in which the sun has been completely blank. Image courtesy spaceweather.com/NOAA

Overview
The sun is blank again today and this marks the 70th day of the year in which there have been no visible sunspots which makes up nearly a quarter of the time in 2017.  Historically weak solar cycle 24 continues to transition away from its solar maximum phase and towards the next solar minimum. The last solar minimum was historically long and deep lasting from 2008 to 2010. The blank look to the sun will increase in frequency over the next couple of years leading up to the next solar minimum - probably to be reached in late 2019 or 2020. By one measure, the current solar cycle is the third weakest since record keeping began in 1755 and it continues a weakening trend since solar cycle 21 peaked in 1980.  One of the impacts of low solar activity is the increase of cosmic rays that can penetrate into the Earth’s upper atmosphere and this, in turn, can impact clouds on Earth.  In addition, there is reason to believe that low solar activity can play an important role in winter weather conditions in the central and eastern US as it is well-correlated with more frequent "high-latitude blocking" events.  

Comparison of all solar cycles since 1755 in terms of accumulated sunspot number anomalies from the mean value at this stage of the solar cycle. Solar cycle 24 is indicated by the arrow.  Plot courtesy publication cited below, authors Frank Bosse and Fritz Vahrenholt

Third weakest solar cycle since 1755
An analysis of the current solar cycle (#24) finds it to be the third weakest since 1755 in terms of accumulated sunspot number anomalies from the mean value at this stage of the solar cycle (publication released early 2017).  In fact, the researchers claim that there have been only two weaker cycles since observations began in 1755.  Solar cycle 24 began in 2008 after a historically long and deep solar minimum which puts us more than eight into the current cycle. The plot (above) shows accumulated sunspot anomalies from the mean value after cycle start and only solar cycles 5 and 6 had lower levels going all the way back to 1755.  The mean value is noted at zero and solar cycle 24 is running 3817 spots less than the mean at the time of the study.  The seven cycles preceded by solar cycle 24 had more sunspots than the mean. Solar cycles have been in a weakening trend since solar cycle 21 peaked around 1980 and there are some predictions that the next solar cycle (#25) could be even weaker than the current one.  

Daily observations of the number of sunspots since 1 January 1900 according to Solar Influences Data Analysis Center (SIDC). The thin blue line indicates the daily sunspot number, while the dark blue line indicates the running annual average. The recent low sunspot activity is clearly reflected in the recent low values for the total solar irradiance. Data source: WDC-SILSO, Royal Observatory of Belgium, Brussels. Last day shown: 31 October 2017. (Graph courtesy climate4you.com)

An increase in cosmic rays and important consequences
One of the consequences of extended periods of low solar activity is that it can result in an increase in cosmic rays that can penetrate into the Earth’s upper atmosphere.  Galactic cosmic rays are high-energy particles originating from space that impact the Earth’s atmosphere. Most of the incoming cosmic ray particles are protons and they actually arrive as individual particles – not in the form of a ray as the term “ray” would suggest. Usually, cosmic rays are held at bay by the sun's magnetic field, which envelops and protects all the planets in the solar system. But the sun's magnetic shield is weakening as the current solar cycle heads towards the next solar minimum and this allows more cosmic rays to reach the Earth’s atmosphere.

Stratospheric radiation
Spaceweather.com has led an effort to monitor radiation levels in the stratosphere with frequent (almost weekly) high-altitude balloon flights over California. The findings confirm the notion that indeed cosmic rays have been steadily increasing in recent months as solar cycle 24 heads towards the next solar minimum. In fact, there was an 13% increase of stratospheric radiation from March 2015 into May 2017.  The sensors that are sent to the stratosphere track increasing levels of radiation by measuring X-rays and gamma-rays which are produced by the crash of primary cosmic rays into Earth's atmosphere. An increase in cosmic ray penetration during periods of low solar activity can make this a more dangerous time for astronauts as the increase in potent cosmic rays can easily shatter a strand of human DNA according to spaceweather.com.

Cosmic rays have been steadily increasing in recent months during historically weak solar cycle 24 which is heading towards the next solar minimum; courtesy spaceweather.com

The monitoring of cosmic rays by spaceweather.com is now going global.  Recently, they have developed launch sites in three continents: North America, South America and in Europe above the Arctic Circle.  The purpose of launching balloons from so many places is to map out the distribution of cosmic rays around our planet.  For more information on this study visit the “Intercontinental Space Weather Balloon Network”.  The increase in the penetration of cosmic rays into the Earth’s atmosphere is expected to continue for months to come as solar activity continues to drop as we head towards the next solar minimum expected around late 2019 or 2020.
 
Clouds
Some researchers have held the belief that cosmic rays hitting Earth's atmosphere create aerosols which, in turn, seed clouds and thereby help in the formation of low clouds. Svensmark and Friis-Christensen (1997) suggested that galactic cosmic rays enhance low cloud formation, explaining variations on the order of 3 percent global total cloud cover over a solar cycle. A 3 percent cloud cover change corresponds to a radiative net change of about 0.5 W/m2 and this would make cosmic rays an important player in weather and climate. Other researchers, however, have been dubious.  The skeptics have maintained that although some laboratory experiments have supported the idea that cosmic rays help to seed clouds, the effect is likely too small to substantially affect the cloudiness of our planet and have an important impact on climate.

A follow-up study published in the Aug. 19th, 2016 issue of Journal of Geophysical Research: Space Physics supports the idea of an important connection between cosmic rays and clouds with a link between sudden decreases in cosmic rays to changes in Earth's cloud cover. These rapid decreases in the observed galactic cosmic ray intensity are known as “Forbush Decreases” and tend to take place following coronal mass ejections (CMEs) in periods of high solar activity. When the sun is active (i.e., solar storms, CMEs), the magnetic field of the plasma solar wind sweeps some of the galactic cosmic rays away from Earth.  In periods of low solar activity, more cosmic rays bombard the earth.  The term “Forbush Decrease” was named after the American physicist Scott E. Forbush, who studied cosmic rays in the 1930s and 1940s. The research team identified the strongest 26 “Forbush Decreases” between 1987 and 2007, and looked at ground-based and satellite records of cloud cover to see what happened.  In a recent press release, their conclusions were summarized as follows: "[Strong “Forbush Decreases”] cause a reduction in cloud fraction of about 2 percent corresponding to roughly a billion tonnes of liquid water disappearing from the atmosphere."

Typical upper-atmosphere (500 mb) height anomaly pattern during years of low solar activity; courtesy NOAA/NCEP

"High-latitude blocking"
Research and empirical observations have shown that low solar activity tends to be well correlated with frequent "high-latitude atmospheric blocking" patterns. The analog years plot (above) shows upper-level (500 mb) height anomalies in previous winter seasons of low solar activity years (i.e., during solar minimum phases).  Abnormally strong high pressure tended to dominate near Greenland and Iceland (orange, red regions) in these years which typically results in more sustained cold air outbreaks for the central and eastern US.  While there are no doubt other factors to consider, it is fair to say that there is likely an increased chance for more frequent "high-latitude blocking" scenarios this upcoming winter season when compared to other years given the very strong chance for low solar activity to continue through the upcoming winter season.  There is more detailed information on the "high-latitude blocking" phenomenon and its potential impact on the upcoming winter season in the "2017-2018 Winter Outlook by Vencore Weather".

Final Thoughts
While the frequency of solar storm activity generally lessens during periods of low solar activity (e.g., during solar minimum phases), there is actually some evidence that suggests the severity does not diminish.  In fact, the most famous solar storm of all now known as “The Carrington Event” took place in 1859 during an overall weak solar cycle (#10).  In addition, other solar activity, such as coronal holes that unleash streams of solar material out into space, can amplify the auroras at Earth's poles.  The bottom line, a lack of sunspots does not mean the sun's activity stops altogether and it needs to be constantly monitored - even during periods of a blank sun.

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com