The neutron monitor at the U.S. Amundsen-Scott Station, located at the geographic South Pole, has operated since 1964. Neutrons detected by such monitors are byproducts of nuclear interactions of cosmic rays (predominantly protons and helium nuclei) with Earth's atmosphere. South Pole is a unique location at high altitude and low geomagnetic cutoff rigidity. This installation is the lynchpin of the worldwide neutron monitor network at low energies and the primary link to spacecraft measurements at much lower energies. Central to the research is the need to understand the detector response to the radiation environment of the South Pole, particularly to determine the cause of a peculiar secular decline in cosmic rays intensity at South Pole throughout the ~50-year operating period of the neutron monitor. Understanding this decline is important because cosmic rays produce radionuclides such as Beryllium-10 that become trapped in the ice and are used to determine ice-core sample ages and precipitation levels over the Earth's Polar Regions. A full understanding of the production rate is vital to interpreting these data.
Recent opening of the IceCube Neutrino Observatory at South Pole, specifically the IceTop air shower array, has increased the value of neutron observation. In addition to its primary function as an extensive air shower detector, IceTop is highly sensitive to the intensity and spectrum of cosmic rays of energy formerly accessible only to the neutron monitors. IceTop and the neutron monitor are highly complementary to each other as the former is more sensitive in an absolute sense but responds to somewhat higher energy particles than does the monitor. IceTop responds primarily to the electronic component of the secondary particles whereas the neutron monitor responds primarily to the hadronic component. Together the detectors can determine not only the spectra but also the element composition of the primary particles.
Solid science, collaborative effort, international partners, and travel to Antarctica provide an ideal opportunity to achieve education and outreach goals. Operation of the neutron monitor at South Pole will become an undergraduate activity at University of Wisconsin-River Falls. Providing undergraduate and two-year college students with research experiences will allow students to make meaningful contributions to cutting-edge science.
Neutron monitor data are broadly employed by other research groups, with applications in cosmic ray research, solar-terrestrial relations, space weather, climatology, atmospheric physics, geophysics, and magnetospheric physics. Neutron monitors play a direct role in forecasting and specifying solar wind disturbances and in providing Ground Level Enhancement alerts relevant to the transpolar aviation. Improving the capability to forecast and characterize major space weather events has direct societal benefit.
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