© NASA/SDOThe sun on Sept. 22, 2019--as blank as a billiard ball.
Could northern summer 2019 go down in history as "the summer without sunspots"? From June 21st until Sept 22nd, the sun was blank more than 89% of the time. During the entire season only 6 tiny sunspots briefly appeared, often fading so quickly that readers would complain to Spaceweather.com, "you've labeled a sunspot that doesn't exist!" (No, it
just disappeared.) Not a single significant solar flare was detected during this period of extreme quiet.
This is a sign that Solar Minimum is underway and probably near its deepest point. For 2019 overall (January through September), the sun has been blank 72% of the time, comparable to annual averages during the
century-class Solar Minimum of 2008 (73%) and 2009 (71%). The current Solar Minimum appears to be century-class as well, meaning you have to go back to the beginnning of the 20th century to find lulls in solar activity this deep.
Contrary to the sound of it, "Solar Minimum" is not boring. During this phase of the solar cycle, the sun's magnetic field weakens, allowing
cosmic rays to enter the solar system. This doses astronauts and possibly air travelers with extra radiation. The
sun also dims, especially at extreme ultraviolet wavelengths, causing the upper atmosphere to
cool and collapse. Space junk accumulates in Earth orbit as a result. Finally, streams of solar wind
punch through the sun's weakening magnetic field, lashing Earth with gaseous material that can cause geomagnetic storms. (One such stream is due later this week on Sept. 27-28.)
Comment: NASA predicts the weakest solar activity in 200 years