The neutron monitor records the number of cosmic rays hitting the monitor over time. Figure 5 is a graph showing both cosmic ray intensity recorded by the Inuvik neutron monitor and the solar cycle over a 30 year period. The sun continually expels matter (in the form of solar wind plasma) and magnetic fields. The expulsion occurs at a higher rate during high solar activity. On average, every eleven years solar activity is high. Plasma and magnetic fields spread out from the sun and create a barrier, which galactic cosmic rays must struggle through. Thus, when the sun is active, fewer galactic cosmic rays reach Earth.

Figure 5. Cosmic ray intensity, and solar activity cycle over a 30 year period. During periods of high solar activity, fewer cosmic rays penetrate the atmosphere and make it to ground level. The cosmic ray data were recorded by the Inuvik neutron monitor. (Graph courtesy of the Solar-Terrestrial Physics Section, HIA)

Cosmic ray monitors as well as long-term cycles record short-term events. One of these events is termed a Forbush decrease. A Forbush decrease occurs when the sun releases an exceptionally large burst of matter and magnetic disturbance. The disturbance sweeps away some of the cosmic rays in its path. When the disturbance passes earth a Forbush decrease is seen on the neutron monitor. These disturbances typically travel at a speed of 400-1000 km/s, and take 2-4 days to travel from the sun to the earth. Cosmic ray intensity dips within a few hours, and then slowly recovers over the next few days. Figure 6 shows the large Forbush decrease on March 24, 1991 recorded by the Inuvik neutron monitor.

Figure 6. A large Forbush decrease was detected by the Inuvik neutron monitor between 3 am and 12 noon on March 24, 1991, shown by the dark shaded area. A magnetic storm prevented many cosmic rays from entering the atmosphere. The cosmic ray intensity recovered within a few days. (Graph courtesy of the Solar-Terrestrial Physics Section, NRC Canada)

Another event recorded by neutron monitors is caused by solar flares. Solar flares erupt form  the surface of the sun during high solar activity. Occasionally solar particles accelerate to such a high energy (greater than 400 million electron volts) that the neutron monitor sees them. Thus, while galactic cosmic rays are less common during high solar activity, solar cosmic rays are more common. The flare is recorded as sharp spike, and then decreases, usually within 24 hours, to previous values. Figure 7 illustrates how Inuvik, Deep River, and Goose Bay locations responded to a flare on May 24th, 1990.

Figure 7. A large solar flare was recorded by Inuvik, Deep River, and Goose Bay neutron monitors on May 24, 1990. The flare is recorded as a sharp spike of intense cosmic rays, and then usually decreases within an hour. Monitors at different locations across the country recorded different strengths of cosmic rays. (Graph courtesy of Solar-Terrestrial Physics Section, HIA)