Monitoring the Intensity of Cosmic Rays From Outer Space

The general nature and purpose of this experiment was to measure variations in the number or intensity of cosmic ray particles reaching Earth. The neutron monitor itself, nicknamed “Cosmo” by the Observatory staff, was in operation at the summit from 1955 to 2006 and returned to operation in 2014. Originally under the sponsorship of the Geophysics Directorate of the United States Air Force Cambridge Research Laboratories, it has now been taken over by UNH’s Space Science Center within the Institute for the Study of Earth, Oceans, and Space.

Most cosmic rays are protons (positively-charged subatomic particles) originating within our galaxy, possibly from the explosion of large stars. Cosmic ray particles continually bombard Earth at almost the speed of light. Some of these particles have energies of over 1000 billion electron volts, much greater than can be achieved in any man-made equipment such as the giant particle accelerators used in nuclear physics research.


When a cosmic ray proton strikes the top of the atmosphere, it collides with an air molecule. In this energetic collision, many kinds of other “secondary” particles are produced and a cascade of particles develops (Fig. 1) The most common secondary particles generated by these chain reactions are muons, mesons, protons, and neutrons; Cosmo is designed to detect primarily just neutrons.

In addition to the monitor on the summit, UNH operated another monitor in Durham. The results from this station was compared with the summit results in studies of differences in time and altitude. The Durham station is close to sea level and recorded an intensity approximately one-quarter that recorded on the summit. This is because cosmic ray particles must travel through more atmosphere to reach sea level, and many of them are absorbed.

Cosmo has recorded variations in cosmic ray intensity over time: long-term, sudden decreases, and sudden increases.

The long-term variation occurs over an 11-year period with a maximum in the cosmic ray intensity during the minimum of the solar activity and vice versa. Sudden decreases, called Forbush events, are associated with powerful solar events, such as solar flares and eruptions called “coronal mass ejections.” When a coronal mass ejection occurs, a gas cloud is emitted from the sun and travels to the earth. When the earth is enveloped in the large “cloud” the cosmic ray intensity is reduced, since fewer particles from the Galaxy can reach Earth.

Sudden increases in intensity are less common. Increases are produced by high energy protons emitted from the sun during solar flares and coronal mass ejections.

The University of New Hampshire’s Neutron Monitor project is the longest running research project on the summit of Mount Washington and is one of many directed toward extending our basic knowledge of the world around us.


Brian Fitzgerald, Director of Science & Education
603-356-2137 ext. 225