Executive Summary Chapter 1 Introduction History, heritage and operation
Cosmic-ray neutron method for measuring area-average soil moisture
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- Physical principle
- Conversion of neutron data to soil moisture
- Funding sources
Cosmic-ray neutron method for measuring area-average soil moisture Marek Zreda, U. Arizona Rationale for development 
Figure 1. Left: Water and energy mass balance at the land surface; red labels indicate pools and fluxes of water measurable using cosmic-ray neutrons. Right: Cosmic-ray neutron interactions with air and soil. Tracks of two neutrons are shown in the lower panel. Neutron n1 was absorbed in soil and removed from the pool of neutrons measurable by cosmic-ray probe above the surface; neutron n2 went back to the atmosphere and is measurable there. These tracks are copied onto the left panel (red lines). Soil moisture is the most important part of the water and energy cycle (Fig. 1). It plays a critical role in weather and seasonal climate forecasting, and in linking water, energy, and biogeochemical cycles over land. It should be measured at the scale that is useful for land-surface processes and hydrology (100 m – 1 km). Conventional methods measure soil moisture either at a point (eg, time-domain reflectometry) or over large areas (eg, satellite microwave instruments). They have to be upscaled or downscaled, respectively, to provide data at the useful scale, which is impractical and unreliable. The recently developed cosmic-ray method (Zreda et al., 2008, 2012; Desilets et al., 2010) has a hectometer footprint (Desilets and Zreda, 2013; Köhli et al., 2015), and is, therefore a good scale integrator of soil moisture for land-surface and hydrological studies. Physical principle The cosmic-ray method (Zreda et al., 2008, 2012; Desilets et al., 2010) takes advantage of the extraordinary sensitivity of cosmogenic low-energy, moderated neutrons of energy between 1 eV and 1000 eV to hydrogen present in materials at the land surface (Fig. 1). Most neutrons on earth are cosmogenic. Primary cosmic-ray protons collide with atmospheric nuclei and unleash cascades of energetic secondary neutrons that interact with terrestrial nuclei and produce fast (evaporation) neutrons at the land surface. The fast neutrons that are produced in air and soil travel in all directions within the air-soil-vegetation continuum, and in this way an equilibrium concentration of neutrons is established. The equilibrium is shifted in response to changes in the water present above and below the land surface, for example in soil. Adding water to soil results in more efficient moderation of neutrons by the soil, causing a decrease of fast neutron intensity above the soil surface, where the measurement is made. Removing water from the soil has the opposite effect. The resultant neutron intensity above the land surface is inversely proportional to soil water content (Zreda et al., 2008, 2012). Instruments 
Figure 2. Cosmic-ray soil moisture probe installed at Marshall Lake, Colorado, USA. For description of the components, see Fig. 9 in Zreda et al. (2012). Low-energy cosmogenic neutrons are measured using proportional counters (Knoll, 2000), which are sensitive to thermal neutrons (median energy of 0.025 eV), shielded by a layer of plastic that shifts the energy sensitivity of the counter to neutrons of the desired energy (>1 eV). The cosmic-ray probe (Fig. 2) is powered using a solar panel paired with a rechargeable battery, and is equipped with an Iridium satellite modem or a cellular modem for real-time telemetry. It can be operated almost anywhere in the world, except areas with insufficient day light. A stationary neutron probe of this type is implemented in the Cosmic-ray Soil Moisture Observing System (Fig. 3), or COSMOS (Zreda et al., 2012; cosmos.hwr.arizona.edu); therefore, it is sometimes called “COSMOS probe”. A mobile COSMOS detector is a bigger version of the stationary probe, additionally equipped with a GPS system (Chrisman and Zreda, 2013). Conversion of neutron data to soil moisture The measured neutron, normalized for variations in pressure, humidity and incoming neutron intensity (Zreda et al., 2012), is converted to soil moisture using the response function, such as that developed by Desilets et al. (2010). Other local measurements needed for the conversion are atmospheric pressure, temperature and water vapor. Additionally, the knowledge of temporal variations of the intensity of high-energy cascade neutrons is necessary to assess the strength of the source function for low-energy evaporation neutrons. Those data are generated from measurements with neutron monitors. The feasibility of soil-moisture monitoring using the cosmic-ray method relies on the availability of real-time neutron monitor data. 
Figure 3. The COsmic-ray Soil Moisture Observing System (COSMOS) consists of approximately 100 probes of the type shown in Fig. 2. The Australian probes displayed here belong to the CosmOz network. Not shown here are other networks that either already exist or that are under construction, most notably the TERENO network in Germany and the COSMOS-UK network in the United Kingdom. Funding sources Most of the work on the development and applications of the cosmic-ray soil moisture method was funded by the US National Science Foundation (NSF). The development work was funded through grants from the Hydrology program between 2001 and 2010 (grants EAR-0126241, EAR-0636110). The installation of COSMOS network in 2009-2013 was supported by a grant from the Mid-size Infrastructure program administerd by the Atmospheric Sciences program (grant ATM-0838491). Additional relevant work on cosmic-ray neutron variations in space and time was funded by the Geochemistry program between 1999 and 2004 (grants EAR-0001191 and EAR-0126209). References Chrisman B., and Zreda M. (2013): Quantifying mesoscale soil moisture with the cosmic-ray rover. Hydrology and Earth System Sciences 17, 5097-5108. Desilets D. et al., 2010. Nature's neutron probe: Land-surface hydrology at an elusive scale with cosmic rays. Water Resources Research 46, W11505. Desilets D., and Zreda M., 2013. Footprint diameter for a cosmic-ray soil moisture probe: Theory and Monte Carlo simulations. Water Resources Research 49, 3566-3575. Knoll, G.F., 2000. Radiation detection and measurement. Wiley, New York, 802 p. Köhli M. et al., 2015. Footprint characteristics revised for field-scale soil moisture monitoring with cosmic-ray neutrons. Water Resources Research 51, 5772–5790. Zreda M. et al., 2008. Measuring soil moisture content non-invasively at intermediate spatial scale using cosmic-ray neutrons. Geophysical Research Letters 35, L21402. Zreda, M. et al., 2012. COSMOS: the COsmic-ray Soil Moisture Observing System. Hydrology and Earth System Sciences 16, 4079-4099. Glacier Dynamics Ian Howat Byrd Polar and Climate Research Center Ohio State University Direct, continuous measurements of ice sheet surface mass balance are lacking, particularly in the accumulation zone where the surface snow and firn varies in density. Nearly all of our knowledge of surface mass variability comes from snow pit and ice core stratigraphy, providing annual resolution with relatively large uncertainties that are inadequate for constraining meteorological models. Further, little information is available on how the density of the firn layer changes with time, hampering efforts to estimate mass change from altimetry measurements. We are testing cosmic ray sensing technology to obtain the first high-accuracy (mm-scale) and continuous measurements of ice sheet mass balance that can be directly compared with models. The technology is simple in concept: we measure the attenuation of neutrons impacting a sensor as it is buried by accumulating mass. To obtain accurate measurements of mass accumulation, however, this signal must be corrected for variations due to atmospheric pressure and solar fluctuations. For the later, the neutron monitors maintained by the Bartol Research Institute provide a critical reference series. Following the successful pilot deployment of our snow mass balance sensors, we aspire to deploy a large network of sensors in both Greenland and Antarctica. The Bartol monitors at Thule, Labrador, South Pole and McMurdo Station will be critical for this effort. Evolution of the geomagnetic field and its influence on cosmic radiation measurements Peggy Shea and Don Smart As cosmic radiation measurements became more precise, the inadequacy of the geomagnetic latitude or cutoff rigidities derived from the dipole components of the geomagnetic field became increasing apparent in the satisfactory ordering of cosmic ray data. The 1950s saw the advent of digital computers being used for scientific data analysis. Using a computer at MIT, K. G. McCracken developed a computer code to trace the orbits of charged particles through a high order (6th degree simulation) of the geomagnetic field for Epoch 1955 (McCracken, et al., 1962). The original purpose was to investigate charged particle access to neutron monitors for the ground-level solar cosmic ray events (GLEs) in 1960. This trajectory-tracing code was also used to compute the geomagnetic cutoff rigidity for the neutron monitor at Port aux Français, Kerguelen Islands to study the GLE increases on 12 and 15 November 1960. Freon and McCracken (1962) found that these trajectory-derived calculations of the vertical cutoff rigidity provided a better fit to the solar proton event data than previous available values. Using this same basic trajectory computer code (and many computer hours), Shea et al. (1965) determined "effective vertical cutoff rigidity values" for neutron monitor locations.  In subsequent years newer models of the geomagnetic field became available, primarily under the auspices of the International Association of Geomagnetism and Aeronomy (IAGA). With improvements in the speed of computers, Shea and Smart continued their cutoff rigidity calculations using these newer field models which were developed from both ground and satellite data. Shea (1971) found a decrease in the vertical cutoff rigidity values in the Latin American region from 1955 to 1970 and predicted that neutron monitors in that area should observe an apparent increase in the galactic cosmic ray intensity from 0.8 to 2.0 percent between successive solar minima. This result was initially viewed with skepticism; however, an apparent increase in the galactic cosmic radiation at Huancayo, Peru between two solar minima was reported by Cooper and Simpson (1979), and this was attributed to a decrease in the geomagnetic cutoff rigidity.
Figure 2. Cosmic ray intensity data obtained on an airline flight between South Africa and New York City in October 1976 as plotted against vertical cutoff rigidities calculated using the 1965.0 geomagnetic field model (top) and against vertical cutoff rigidities appropriate for October 1976 (bottom). The "upper" section of the curve (between 8 and 12 GV) in the top panel are the intensity data obtained in the southern hemisphere between South Africa and the equatorial region; the "lower" section of the same curve are the intensity data obtained in the northern hemisphere between the equatorial region and New York City. Using neutron monitor data from an aircraft flight between Johannesburg and New York City in October 1976, König and Stoker (1981) reported that the latitude curves above and below the cosmic ray equator were displaced when ordered by the geomagnetic cutoff rigidity values for 1965. Calculating the cutoff rigidity values for the Epoch of the flights, Shea and Smart (1990) found that while the vertical cutoff rigidity values had decreased in time in the South Atlantic region, they had markedly increased in the North Atlantic region by as much as one percent per year between 1965 and 1980 (Figure 1). Using the appropriate cutoff rigidity for the Epoch of the measurement resulted in a satisfactory ordering of the cosmic radiation intensity over the latitude survey (Figure 2). 
Figure 3 illustrates the change in the magnitude of the dipole term of the geomagnetic field over 400 years. While the magnitude of the geomagnetic field is decreasing, the higher order components of the field are both increasing and decreasing. The resulting overall changes are not distributed uniformly over the world. For the period 1955-1995, this has the effect of an increase in the vertical cutoff rigidity values in the North Atlantic area (including the east coast of the USA) and a decrease in the Latin American region. As an example, the vertical cutoff rigidity for Mexico City in 1955 was 9.45 GV; in 1995 was 8.02 GV - a decrease of 15.1% over 40 years. For Mt. Washington, NH, the change in vertical cutoff rigidity was an increase over time: 1.25 GV in 1955 and 1.58 GV in 1995 - a 26.4% increase. The cutoff rigidity for the Climax, Colorado neutron monitor remained approximately constant over this time interval, and the cosmic ray intensity at this station has been used by many scientists as a benchmark for the long term time variations of the galactic cosmic radiation. The rapidly evolving geomagnetic field has several implications for cosmic ray research. The calculation of the cutoff rigidities for the proper Epoch of the geomagnetic field would be essential for studies of the long term galactic cosmic radiation, latitude surveys, relativistic solar proton events and for computing the radiation dose along specific airline flight paths. A world map of vertical cutoff rigidities for Epoch 2010 is shown in Figure 4. 
REFERENCES Cooper, J.F., and J. A. Simpson, Origin of the large-scale difference in the observed magnetic rigidity dependence of the cosmic radiation for two solar cycles - 1954-65 and 1965-1976, 16th International Cosmic Ray Conference, Conf. Pap., 12, 176-181, 1979. König, P. J. and P. H. Stoker, Displaced isorigidity contours in the North Atlantic for 1975, J. Geophys. Res., 86, 219-233, 1981. McCracken, K.G., U.R. Rao, and M.A. Shea, The Trajectories of Cosmic Rays in a High Degree Simulation of the Geomagnetic Field, Massachusetts Institute of Technology Technical Report No. 77, NYO-2670, August 1962. Shea, M.A., Changes in Neutron Monitor Response and Vertical Cutoff Rigidities Resulting from Secular Variations in the Geomagnetic Field, 12th International Conference on Cosmic Rays, Hobart, Conference Papers (University of Tasmania), 3, 859-864, 1971. Shea, M.A. and D.F. Smart, The Influence of the Changing Geomagnetic Field on Cosmic Ray Measurements, J. Geomag. Geoelectr., 42, 1107-1121, 1990. Shea, M.A., D.F. Smart, and K.G. McCracken, A Study of Vertical Cutoff Rigidities Using Sixth Degree Simulations of the Geomagnetic Field, J. Geophys. Res., 70, 4117-4130, 1965. Directory: ~clem -> NMworkshop2015 -> paper paper -> Executive Summary Chapter 1 History, heritage and operation paper -> Journal: The Astrophysical Journal Download 9.21 Mb. Share with your friends:
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