Historically weak solar cycle 24 continues to transition away from its solar maximum phase and towards the next solar minimum. There have already been 11 spotless days during 2017 and this follows 32 spotless days that occurred during the latter part of 2016. 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 has some 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 more than eight years into the current cycle. The plot (below) 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.
An increase in cosmic rays
One of the consequences of extended periods of low solar activity is that it can result in an increase in stratospheric radiation. Specifically, as sunspot activity goes down, there is an increase in cosmic rays that penetrate into the Earth’s upper atmosphere. Cosmic rays are high-energy photons and subatomic particles accelerated in our direction by distant supernovas and other violent events in the Milky Way. 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.
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 an 11% increase of stratospheric radiation since March 2015 into late 2016. 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. The increase in the penetration of cosmic rays into the Earth’s atmosphere is expected to continue for years to come as solar activity plunges toward the next solar minimum expected around 2019-2020.
The possible 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 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. 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."
Other impacts of cosmic rays
Finally, in addition to its possible impact on clouds and climate, 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. 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 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. Vencore Weather will continue to monitor their findings over the next several months as solar cycle 24 heads towards the next solar minimum. For more information on this study visit the “Intercontinental Space Weather Balloon Network”.
Meteorologist Paul Dorian