The sun is currently in the midst of a deep solar minimum and it is about to reach an historic milestone. So far this year the sun has been blank (i.e., no visible sunspots) for 266 days and, barring any major surprises, it’ll reach 269 days early next week which will be the quietest year in terms of sunspots since 1913 when the sun was spotless for 311 days. In fact, the current stretch of consecutive spotless days has reached 29 and for the year the sun has been blank 77% of the time. The current record-holder in the satellite era for spotless days in a given year is 2008 when the sun was blank for 268 days making the 2008-2009 solar minimum the deepest since 1913.
Solar minimum is a normal part of the 11-year sunspot cycle, but the last one and the current one have been far deeper than most. One of the consequences of a solar minimum is a reduction of solar storms and another is the intensification of cosmic rays. The just ended solar cycle 24 turned out to be one of the weakest in more than a century – continuing a weakening trend that began in the 1980’s – and, if the latest forecasts are correct, the next solar cycle will be the weakest in more than 200 years.
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. 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.
High-altitude balloons have been launched on a periodic basis in recent years to monitor stratospheric radiation associated with the influx of cosmic rays and they have shown a steady increase since 2015 (campaign sponsored by spaceweather.com). In this set of measurements, cosmic rays have increased by about 13% during the past four years over the central part of California. At another location, the neutron monitor at the University of Oulu’s cosmic ray station in Finland recorded levels earlier this month that were within a percentage point of the satellite era record.
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