The sun continues to be very quiet and it has been without sunspots this year more than half the time as we approach what is likely to be a deep solar minimum. In fact, all indications are that the upcoming solar minimum which is expected to begin later this year may be even quieter than the last one which was the deepest in nearly a century. Solar cycle 24 has been the weakest sunspot cycle with the fewest sunspots since cycle 14 peaked in February 1906. Solar cycle 24 continues a recent trend of weakening solar cycles which began with solar cycle 21 that peaked around 1980. The last time the sun was this blank in a given year on a percentage basis was 2009 during the last solar minimum when 71% of the time was spotless. That last solar minimum actually reached a nadir in 2008 when an astounding 73% of the year featured a spotless sun - the most spotless days in a given year since 1913. One of the natural impacts 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. 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.
Galactic cosmic rays are high-energy particles originating from outside the solar system that can impact the Earth’s atmosphere. Our first line of defense from cosmic rays comes from the sun as its magnetic field and the solar wind combine to create a 'shield' that fends off cosmic rays attempting to enter the solar system. 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 varies globally by about 15% over a solar cycle because of changes in the strength of the solar wind, which carries a weak magnetic field into the heliosphere, partially shielding Earth from low-energy galactic charged particles.
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 nearly four 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.
During the last solar minimum in 2009, radiation peppering Earth from deep space reached a 50-year high at levels never before seen during the satellite era - and we’re getting close to those same levels and a new record is certainly on the table. Ground-based neutron monitors and high-altitude cosmic ray balloons are registering a new increase in cosmic rays. The Oulu neutron monitor in Finland, which has been making measurements since 1964, reports levels measured in April 2019 are now only percentage points below the satellite era maximum reached in 2009.
Consequences of increasing cosmic rays: 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.
In this publication, 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”.
Consequences of increasing cosmic rays: 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.
Consequences of increasing cosmic rays: 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.
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