RAPID: Neutron Monitors in the Twenty-First-Century

University Of Delaware, Newark DE

Investigators

Abstract

A neutron monitor is a ground-based instrument that measures the number of high-energy particles striking Earth's atmosphere from space. Because the intensity of cosmic rays hitting Earth is not uniform, it is important to place neutron monitors at multiple locations to form a complete picture of cosmic rays in space. Neutron monitors are capable of detecting Solar Energetic Particles (SEPs) as well as background neutrons produced by the ever-present Galactic Cosmic Rays (GCR). They produce decades-long records of stable and reliable cosmic ray intensity measurements. Furthermore, observations from wide-spread networks of monitors can be analyzed to provide high-time resolution measurements of the most energetic and powerful of solar particle events, Ground Level Enhancements (GLEs). The University of Delaware (UD) neutron monitors have suffered from lack of external funding over the past few years, and they are currently minimally supported through institutional funds (from the UD and the Bartol Research Institute). However, this funding will end in July of 2019, and plans are in place to dismantle the stations beginning of June, 2019, unless external funding can be secured. The purpose of this one-year RAPID project is to restore the North American network to a functional condition and support operation for one year until longer term support can be secured. The historic neutron monitor data have shown that at each station the count rate varies with the solar activity cycle. This is the phenomenon of solar modulation of galactic cosmic rays. To investigate the origin of the phenomenon, one cannot rely on the count rates of one neutron monitor: hence, one needs to derive the cosmic ray intensity as a function of particle energy or particle rigidity. Since each neutron monitor is sensitive to primary cosmic rays above some geomagnetic cutoff rigidity, one can combine measurements of stations at different latitudes. The reliability, stability and robustness of multi-decade measurements is critical for understanding the variability of the Sun. Of particular interest is the fact that the GCR intensity during the last two solar minimum is the greatest over the period covered by the observations by a significant factor. This would not have been known without neutron monitors. The current network of North American neutron monitors, combined with Russian operated stations are strategically located to provide precise, real-time, 3-dimensional measurements of the cosmic-ray angular distribution. Because of the combined effects of Earth's magnetic field and atmosphere, high latitude sites have superior directional sensitivity relative to low latitude sites. At the high latitude sites, where the UD's neutron monitors are located, the SEPs are in-focus, while cosmic rays arriving at low and mid latitude stations arrive from widely dispersed directions. The complete network of high latitude neutron monitors allows a complete picture of when, where, and from what direction SEPs strike Earth. In addition to space weather forecasting, data from neutron monitors, such as those to be collected during this one-year RAPID project, are also used in support of other science investigations, beyond studies of cosmic rays and heliospheric physics. One use of neutron monitor data is to monitor the flux of neutrons produced by cosmic rays for a broad range of practical applications, including detecting nuclear threats for homeland and national security, calculating the radiation dose to airplane crews and passengers, understanding the rate of single-event upsets (soft errors) in microelectronic devices, measuring soil and snow moisture content, and calculating the production rate of cosmogenic radionuclides used for atmospheric tracers and nuclear treaty verification. For all these applications, neutrons and other secondary particles produced in the atmosphere and surface materials by galactic cosmic rays (and occasionally by solar particles) are either the source of the effect or an important background. This one-year RAPID project will provide an educational experience for an undergraduate student to help maintain and calibrate the neutron monitor station in Newark, Delaware. The research agenda of this RAPID project supports the Strategic Goals of the AGS Division in discovery, learning, diversity, and interdisciplinary research. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.