Ground Based Measurements of Galactic and Solar Cosmic Rays

EXECUTIVE SUMMARY The global network of ground based neutron monitors and muon telescopes constitutes a unique “instrument” in an armada of particle detection and measurement facilities now in space and to be deployed in the future. This network has a particle energy detection threshold of order 500 MeV, extending up to 50 GeV, with a full-sky field of view continuously making dozens of pointed measurements in key directions. It is sensitive to the highest energy solar cosmic rays and galactic cosmic rays, whose intensities and anisotropies are produced or heavily influenced by solar activity. It measures the part of the solar energetic particle (SEP) spectrum that is most influenced by the particle acceleration processes taking place close to the Sun. Theses process are affected by coronal turbulence, shock speed and strength and radius, and by turbulence far upstream of the activity. Some particles may be accelerated in flares if the magnetic topology is such that they can escape into interplanetary space to be detected at Earth. Monitoring of galactic cosmic rays provides information on interplanetary conditions not only at the radius of Earth, but also far out in the heliosphere through variations in the cosmic-ray intensities and anisotropies on time scales from minutes to decades. Combining these measurements with lower energy measurements from space based instruments provides a comprehensive data set that can be used to investigate a variety of solar and heliospheric phenomena. The network is now comprised of some new, but mostly aging, neutron monitors and directional muon detectors. Recently, air Čerenkov telescopes, primarily intended for TeV γ-ray astronomy, have been successfully performed differential measurements of the secondary muon component produced by solar and galactic cosmic rays. The network is not functioning as it once did or as envisioned during the International Geophysical Year, 1957-1958. Several key measurement sites have been closed or turned off, and the perceived lack of interest by the US makes foreign sites vulnerable to cuts and closings. There is little coordination between supporting organizations that are also fighting for their own existence New and inexpensive technology has had difficulty making its way into the field to modernize and greatly improve the performance of those stations now operating. Key recommendations to bring the network into a condition to support heliospheric physics include: