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A Sharp Reduction In Cosmic Rays

Astronauts on the International Space Station got a break from their daily dose of cosmic rays on March 24th when radiation levels suddenly dropped more than 14%. What happened? A coronal mass ejection (CME) swept the radiation aside:

This is called a "Forbush decrease," named after American physicist Scott Forbush who studied cosmic rays in the early 20th century.  It happens when a CME sweeps past Earth and pushes galactic cosmic rays away from our planet. Radiation from deep space that would normally pepper space stations, satellites, and Earth’s upper atmosphere is briefly wiped out.

In the graph, above, the Forbush decrease is shown as a downward spike in neutron counts recorded at the Sodankyla Geophysical Observatory in Oulu, Finland. Neutrons are a well-known proxy for cosmic rays. When cosmic rays strike Earth's upper atmosphere, they produce a spray of neutrons that reach the ground below. Sensors in Oulu count these "secondary cosmic rays" to monitor activity in space.

This week's Forbush decrease is the biggest of Solar Cycle 25 (so far), exceeding the previous record-holder on Nov. 3-4, 2021 when a potent Cannibal CME dropped neutron counts by 11%.


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