| Most cosmic rays come from the galaxy, though some are produced by eruptive events
on the Sun’s surface. These high energy cosmic ray particles can affect satellites,
causing single event upsets. Scientists have postulated that cosmic rays can affect
the earth by causing changes in weather. Cosmic rays can cause clouds to form in the
upper atmosphere, after the particles collide with other atmospheric particles in
our troposphere. The process of a cosmic ray particle colliding with particles in
our atmosphere and disintegrating into smaller pions, muons, and the like, is called
a cosmic ray shower. These particles can be measured on the Earth’s surface by neutron
monitors.Ground-based neutron monitors detect variations in the approximately 500
Mev to 20 GeV (low energy) portion of the primary cosmic ray spectrum. This class
of cosmic ray detector is more sensitive in the approximate 500 Mev to 4 GeV portion
of the cosmic ray spectrum than are cosmic ray muon detectors. The portion of the
cosmic ray spectrum that reaches the Earth’s atmosphere is controlled by the geomagnetic
cutoff which varies from a minimum (theoretically zero) at the magnetic poles to a
vertical cosmic ray cutoff of about 15 GV (ranging from 13 to 17) in the equatorial
regions. (Note: GeV is a unit of energy, GV is a unit of magnetic rigidity). The primary
cosmic ray particles interact with the atmosphere and generate secondaries, some of
which will reach the surface of the Earth. In high latitude regions of the Earth,
where the geomagnetic cutoff is low, the lower threshold response of the neutron monitor
is controlled by the atmospheric mass (about 1030 grams at sea level) which limits
the response threshold of the neutron monitor to primary radiation of about 430 MeV.
(At the South Pole, where the surface is 2820 M above sea level, the reduced atmospheric
mass lowers the primary radiation detection threshold to about 300 MeV). At mid-latitudes
or equatorial latitudes, the detection threshold is controlled by the geomagnetic
cutoff. Neutron monitors at high altitudes have higher counting rates than neutron
monitors at lower altitudes because of the atmospheric absorption of the cosmic ray
secondaries generated near the top of the atmosphere. When the secondary cosmic rays
interact in the monitor, (actually in lead surrounding the counters) they cause nuclear
disintegrations, or stars. These stars are composed of charged fragments and neutrons
typically in the energy range of tens to hundreds of MeV, even up to GeV energies.
As a result of these high energy nuclear interactions, there will be more secondary
fragments generated than incident particles and hence there is a multiplier effect
for the counters. The neutrons are moderated and then counted using Boron tri-fluoride
(BF3) proportional counters which are efficient thermal neutron detectors; hence the
name neutron monitor. The original design is often designated as an IGY neutron monitor.
A description of this type of instrument is given by Simpson, (Annals of the IGY,
Vol. 4, pp. 351-373, 1957). The NM-64 or super neutron monitor was developed for the
IQSY (International Quiet Sun Years 1964-65) when instruments with a higher counting
capacity were required. A description of this type of neutron monitor is given by
Carmichael (Annals of the IQSY, Vol. 1, pp. 178-197, 1968). Super neutron monitors
are often designated as xx-NM-64 where xx is the number of tubes in the monitors.
An 18-NM-64 at high latitude has a counting rate of approximately 1 million counts
per hour or 0.1 percent statistics. The neutron monitor and supermonitor digital data
for the time period 1953 to present are available at http://spidr.ngdc.noaa.gov and
consist of over 100 stations’ hourly (UT) values of: (a) counting rates corrected
for atmospheric pressure effects; (b) uncorrected counting rates; and (c) atmospheric
pressure data. If it is not possible to have all three types of data, the most important
is the counting rate corrected for atmospheric pressure.NGDC also holds paper archives
for some cosmic ray Ionization Chamber monitors and some meson telescopes, mostly
during the period of the International Geophysical Year (IGY) and International Quiet
Sun Years (IQSY) 1957-1968.
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