Cosmic rays live

What does a neutron monitor measure?

Cosmic rays are atomic nuclei accelerated to colossal energies by the explosion of distant stars. Hitting the atmosphere, they trigger showers of particles whose neutrons are counted at ground level. The Oulu station, in Finnish Lapland, has kept that count without interruption since 1964; the figure shown is the number of neutrons per minute, corrected for the atmospheric pressure that absorbs them.

The curve breathes opposite to the Sun: when solar activity is high, the magnetic field carried by the solar wind deflects part of the cosmic rays and the count drops by 10 to 20%; at solar minimum it climbs back. After a coronal mass ejection, the flux falls sharply by a few percent within hours, then recovers over a week: that is a Forbush decrease, visible on the 7-day view.

Cosmic rays help ionise the lower ionosphere, the upper wall of the Schumann cavity; that is one of the links between this page and the 7.83 Hz curve.