The sun is currently featuring one lonely visible sunspot region and this relatively blank look is becoming more and more common as solar cycle 24 heads towards the next solar minimum. In fact, there have been 42 days in 2017 with a completely blank sun – already ten more days than all of last year – and this makes up almost one-quarter of the time for this year. Solar cycle 24 has turned out to be historically weak with the lowest number of sunspots since cycle 14 peaked more than a century ago in 1906. In fact, 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 natural impacts of decreasing solar activity is the increase of cosmic rays that can penetrate into the Earth’s upper atmosphere and this can have many important consequences.
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 and its solar winds sweep them aside when they pass by Earth. As the sun plunges towards a minimum phase, there is typically less and less solar activity (e.g., solar storms, coronal mass ejections), and the weakening magnetic field and solar wind provides less and less shielding for the Earth.
New evidence of an increase in stratospheric radiation
One way to monitor cosmic ray penetration into the Earth’s upper atmosphere is to measure stratospheric radiation over an extended period of time. “Spaceweather.com” has led an effort for more than two years to monitor radiation levels in the stratosphere over California with frequent high-altitude helium balloon flights. These balloons contain sensors which detect X-rays and gamma-rays in the energy range 10 keV to 20 MeV and are produced by the crash of primary cosmic rays into Earth's atmosphere. These energies span the range of medical X-ray machines and airport security scanners. The findings confirm the notion that indeed cosmic rays have been steadily increasing over California 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 2017.
The monitoring of stratospheric radiation by “Spaceweather.com” has now been extended to New England and their findings not only show the same kind of cosmic ray increase is happening as over California, but at an even higher rate. Balloons have been flown over Maine and New Hampshire four times since 2015 – most recently on June 15th, 2017 – and though the data are sparse compared to the much more frequently sampled US west coast, the results are pretty clear. Stratospheric radiation has increased over New England by 19% in this time period compared to the 13% increase seen over California. This difference seen between California and New England suggests the Earth’s magnetic field provides different levels of protection in different areas of the planet.
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. A study published in the August 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 tons of liquid water disappearing from the atmosphere."
Other 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. [Dose rates are expessed as multiples of sea level. For instance, we see that boarding a plane that flies at 25,000 feet exposes passengers to dose rates ~10x higher than sea level. At 40,000 feet, the multiplier is closer to 50x]. 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.
Meteorologist Paul Dorian