Today marks the 6th day in a row that the sun is blank and the 36th time this year - already more spotless days than all of 2016. In what has turned out to be a historically weak solar cycle (#24), the sun continues to transition away from its solar maximum phase and towards the next solar minimum. In April 2010, the sun was emerging from the last solar minimum which was historically long and deep. 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 can have many important consequences.
Third weakest solar cycle since 1755
A recent publication has analyzed the current solar cycle and has found that when sunspot anomalies are compared to the mean for the number of months after cycle start, 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 years into the current cycle. The plot (above) shows accumulated sunspot anomalies from the mean value after cycle start (97 months ago) 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. The seven cycles preceded by solar cycle 24 had more sunspots than the mean.
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. Simply put, as sunspot numbers decline, cosmic rays intensify. 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.
An increase in 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 has been a 13% increase of stratospheric radiation over California from March 2015 to May 6th, 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.
Some important consequences
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. In addition, there are other consequences of increasing cosmic rays according to spaceweather.com including the penetration of commercial airlines, dosing passengers and flight crews enough that pilots are classified as occupational radiation workers. Furthermore, there are studies linking cosmic rays with cardiac arrhythmias in the general population. 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 complicated place to navigate for astronauts.
The connection of cosmic rays to 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 clouds. 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 recent study published in the Aug. 19th issue of Journal of Geophysical Research: Space Physics supports the idea of an important connection between cosmic rays and clouds. According to spaceweather.com, a team of scientists from the Technical University of Denmark (DTU) and the Hebrew University of Jerusalem has linked 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 led by Jacob Svensmark of DTU 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."
The monitoring of cosmic rays by spaceweather.com is now going global. In recent months, 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 plunges toward the next solar minimum expected around late 2019 or 2020.
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