March 17, 2022: The March 13th CME did more than spark bright auroras. It also wiped out a lot of cosmic rays. Neutron monitors at the Sodankyla Geophysical Observatory in Oulu, Finland, recorded a sharp drop in cosmic radiation just after the CME arrived:
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 coronal mass ejection (CME) sweeps past Earth and pushes galactic cosmic rays away from our planet. Radiation from deep space that would normally pepper Earth’s upper atmosphere is briefly wiped out.
There’s something odd about this Forbush decrease. It’s a double dip decrease. Cosmic rays dropped precipitously on March 13th–then they surged midday on March 14th–then they dropped precipitously again. The up-and-down may be a sign of structure inside the CME.
As Solar Cycle 25 intensifies, more and more CMEs will sweep past Earth. Forbush decreases will become increasingly common and may even begin to overlap. This will cause a persistent decline in cosmic rays around our planet.
A recent paper in the Astrophysical Journal looked at the last two solar cycles and compared the daily rate of CMEs to the strength of cosmic rays. The plot, above, shows the results. At the peak of Solar Cycle 24, the sun was spitting out more than 5 CMEs per day; at the same time, galactic cosmic rays (GCRs) dropped more than 60%.
Evidence is mounting that new Solar Cycle 25 will be stronger than Solar Cycle 24. If so, CMEs will be more abundant and cosmic rays even more depressed–a welcome reduction for astronauts, air travelers, and even some mountain climbers.
Meanwhile, the March 13th Forbush decrease is still underway. Cosmic rays remain depressed 4 full days after the CME arrived.