Sunspot counts have just reached their lowest level since 2011. The sun has been completely spotless on 23 days in 2016 and the blank look is increasing in frequency as it heads towards the next solar minimum. The next solar minimum is expected to be deep and extended - probably reached around 2019 or 2020. The current solar cycle is the 24th since 1755 when extensive recording of solar sunspot activity began and it is the weakest in more than a century with the fewest sunspots since cycle 14 peaked in February 1906. Low solar activity is known to have consequences on Earth’s weather and climate and it also is well correlated with an increase in cosmic rays that reach the upper part of the atmosphere.
Consequences of low solar activity
There are some known consequences on Earth’s weather and climate from extended periods of low solar activity. First, low solar activity during winter seasons have been well correlated with higher geopotential heights than normal across the northern latitudes and this typically leads to frequent “blocking” events which, in turn, often results in cold air outbreaks for the eastern US. In fact, the expectation for low solar activity during this upcoming winter was a main factor on our colder-than-normal Winter Outlook for the Mid-Atlantic region.
In addition to its impact on high-latitude atmospheric “blocking” patterns, low solar activity has a direct impact on the ability of cosmic rays to reach the Earth’s atmosphere. Specifically, as sunspot activity goes down, there is an increase in cosmic rays penetrating 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.
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 new study just 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."
Cosmic ray activity on the rise
Given the fact that we are experiencing one of the weakest solar cycles in more than a century and it is headed towards the next solar minimum, it would not be surprising to have relatively high cosmic ray penetration into the Earth’s atmosphere. In fact, 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 levels during this weakening solar activity phase of solar cycle 24 have been intensifying with an 11% increase since March 2015. 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 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 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.
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.
Meteorologist Paul Dorian