July 18,2017: On July 16th, a CME hit Earth’s magnetic field, sparking two days of geomagnetic storms and beautiful southern auroras. The solar storm cloud also swept aside some of the cosmic rays currently surrounding Earth. Spaceweather.com and the students of Earth to Sky Calculus launched a space weather balloon to the stratosphere hours after the CME arrived. We detected a 7% decrease in X-rays and gamma-rays (two tracers of secondary cosmic rays). Neutron monitors in the Arctic and Antarctic recorded similar decrements. For instance, these data from the Bartol Research Institute show a nearly 8% drop in cosmic ray neutrons reaching the South Pole:
This is called a “Forbush Decrease,” named after physicist Scott E. Forbush who first described it in the 20th century. Wherever CMEs go, cosmic rays are deflected by magnetic fields inside the solar storm clouds. As a result, when solar activity is high, cosmic radiation around Earth is relatively low–a yin-yang relationship that holds throughout all phases of the solar cycle.
Lately, cosmic rays around Earth have been intensifying as the solar cycle plunges toward minimum. The CME of July 16th reversed that trend–but only for a few days. Solar activity has returned to low levels and cosmic rays are on the rise again.
Why do we care about cosmic rays? For one thing, they penetrate commercial airlines, dosing passengers and flight crews so much that pilots are classified as occupational radiation workers. Some research shows that cosmic rays can seed clouds and trigger lightning, potentially altering weather and climate. Furthermore, there are studies ( #1, #2, #3, #4) linking cosmic rays with cardiac arrhythmias in the general population.