7:15 AM | *Solar minimum continues…cosmic rays near highs in the satellite era*

The sun has been spotless for 17 consecutive days as observed here by the Helioseismic and Magnetic Imager (HMI) on Wednesday, May 20th. Courtesy NASA/Solar Dynamics Observatory (SDO)

Overview

The sun continues to be very quiet and it has been without sunspots 77% of the time this year which is the same exact percentage experienced during all of 2019.  In fact, last year turned out to be the quietest year in terms of sunspots since 1913 with 281 spotless days as the solar minimum phase intensified from the year before.  Back-to-back years of very high levels of spotlessness on the sun would certainly support the notion this is indeed a noteworthy and deep solar minimum.  Solar minimum represents the end of solar cycle #24 which featured the fewest number of sunspots since solar cycle 14 peaked in February 1906. Some of the predictions for solar cycle #25 suggest that it may peak in July 2025 and continue the trend of weakening solar cycles that began around 1980 when solar cycle 21 peaked in sunspot activity. 

One of the natural effects of decreasing solar activity is the weakening of the ambient solar wind and its magnetic field which, in turn, allows more and more cosmic rays to penetrate the solar system. In fact, one measurement indicates that cosmic ray activity is very close to an all-time high for the satellite era. The intensification of cosmic rays can have important consequences on such things as Earth’s cloud cover and climate, the safety of air travelers, and as a possible trigger mechanism for lightning.  

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: 30 April 2020. Last diagram update: 3 May 2020. Courtesy climate4you.com.

Solar minimum and the intensification of cosmic rays 

Galactic cosmic rays are high-energy particles originating from outside the solar system that can reach the Earth’s atmosphere. The planet's atmosphere and magnetic field combine to form a formidable ‘shield’ against space radiation, protecting life on the surface. The shielding action of the sun is strongest during solar maximum and weakest during solar minimum with the weakening magnetic field and solar wind.  The intensity of cosmic rays can vary globally by about 15% over a solar cycle because of changes in the strength of the solar wind.

Earth is in no great peril from the extra cosmic rays. In fact, humans have weathered storms much worse than this. Hundreds of years ago, cosmic ray fluxes were at least 200 percent higher than they are now according to space.com. Researchers know this because when cosmic rays hit the atmosphere, they produce an isotope of beryllium, 10 Be, which is preserved in polar ice. By examining ice cores, it is possible to estimate cosmic ray fluxes more than a thousand years into the past. Even with the recent surge, cosmic rays today are much weaker than they have been at times in the past millennium.

Neutron monitors have long been considered a “gold standard” for monitoring cosmic rays on Earth. According to the latest data from neutron counters at the University of Oulu’s cosmic ray station in Finland, cosmic rays are near an all-time high (upper, right) since measurements began in the 1960’s (i.e., within one percentage point when using the space age averages as a point of comparison). Data source: https://cosmicrays.oulu.fi/; spaceweather.com

One of the ways to measure the influx of cosmic rays is through a neutron monitor ground-based detector which is designed to measure the number of high-energy charged particles reaching the Earth’s upper atmosphere.  For historical purposes the incoming particles are called cosmic “rays”, but they are, in fact, particles, predominantly protons and helium nuclei. Most of the time, a neutron monitor records galactic cosmic rays and their variation within the 11-year sunspot cycle and 22-year magnetic cycle.  The neutron monitor was invented at the University of Chicago in 1948 and the “18-tube” monitor is a large instrument weighing about 36 tons (info courtesy Wikipedia). The Sodankyla Geophysical Observatory at the University of Oulu neutron monitor in Finland, which has been making measurements since 1964, currently is reporting levels that are very close to the satellite era maximum reached in 2009 (last solar minimum).

Stratospheric radiation has been rising over the past few years as the sun began to head into the current solar minimum phase. Evidence for this rise comes from a campaign of almost weekly high-altitude balloon launches sponsored by spaceweather.com. Since March 2015, there has been a ~22% increase in X-rays and gamma-rays over central California where hundreds of balloons have been launched in this long-term effort.

Solar minimum and an increase in stratospheric radiation

Another 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 a long-term effort 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 the sun has headed into a solar minimum phase.  

Consequences of increasing cosmic rays

1)     Cloud cover/climate

The correlation between cosmic rays and cloud cover over a solar cycle was first reported by Svensmark and Friis-Christensen in 1997. A more recent study by Svensmark published in the August 2016 issue of Journal of Geophysical Research: Space Physics continues to support the idea of an important connection between cosmic rays and clouds. Yet another study in 2019 supports the idea of a connection between cosmic rays and cloud cover and even to an increased strength in the East Asian winter monsoon.

In the original publication from 1997, the authors found that “the observed variation of 3–4% of the global cloud cover during the recent solar cycle is strongly correlated with the cosmic ray flux. This, in turn, is inversely correlated with the solar activity. The effect is larger at higher latitudes in agreement with the shielding effect of the Earth's magnetic field on high-energy charged particles. The above relation between cosmic ray flux and cloud cover should also be of importance in an explanation of the correlation between solar cycle length and global temperature that has been found”.

2)     Threat to air travelers

Not only can an increase of cosmic rays have an impact on Earth’s cloud cover and climate, it is of special interest to air travelers.  Cosmic radiation at aviation altitudes is typically 50 times that of natural sources at sea level. Cosmic rays cause "air showers" of secondary particles when they hit Earth's atmosphere. Indeed, this is what neutron monitors and cosmic ray balloons are measuring--the secondary spray of cosmic rays that rains down on Earth. Secondary cosmic rays penetrate the hulls of commercial aircraft, dosing passengers with the whole body equivalent of a dental X-ray even on ordinary mid-latitude flights across the USA. International travelers receive even greater doses (source). The International Commission on Radiological Protection has classified pilots as occupational radiation workers because of accumulated cosmic ray doses they receive while flying. Moreover, a recent study by researchers at the Harvard School of Public Health shows that flight attendants face an elevated risk of cancer compared to members of the general population. They listed cosmic rays as one of several risk factors.

3)     Possible lightning trigger

Finally, there has been some research suggesting there is a connection between cosmic rays and lightning (paper 1, paper 2).  When cosmic rays smash into molecules in our atmosphere, the collisions create showers of subatomic particles, including electrons, positrons, and other electrically charged particles. This shower of electrons would collide into still more air molecules, generating more electrons. All in all, cosmic rays could each set off an avalanche of electrons and trigger lightning. 

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com

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