Comparison and evaluation of neutron monitor based incoming corrections for Cosmic Ray Neutron Sensing

Abstract

Cosmic Ray Neutron Sensing (CRNS) is a technique to measure water content, for example soil moisture, on the hectare scale through the measurementof epithermal neutrons. The neutrons are results of particle showers in the earth’s atmosphere caused by cosmic rays impinging on it. The abundance andglobal distribution of neutrons is changed in time through different factors. On the largest scale, the heliosphere and therefore the solar cycle greatly affect the amount of galactic cosmic rays that are able to reach earth. Large solar events, such as Forbush decreases, also cause rapid changes in the cosmic ray flux. The aim of any correction method is ultimately to account for these heliospheric changes. The next smaller scale is the earth’s magnetic field. The biggest consequence here is an uneven distribution of neutrons across latitudes, as with increasing cut-off rigidity less cosmic rays are able to reach the atmosphere. This presents a challenge to correction methods based on neutron monitors, since any neutron measurement is always localised to specific geomagnetic and atmospheric conditions. Lastly, the abundance of neutrons at different energy bands is strongly tied to the local conditions. In particular hydrogen abundance is of vital importance. The CRNS technique is based upon the sensitivity of neutrons to hydrogen, which is dependent on the energy of any given neutron. As these neutrons are still dependent on heliospheric and geomagnetic conditions, their measurement needs to be corrected for the amount and variation in incoming radiation. Neutron monitor based correction methods are tackling two challenges at the same time. Firstly, a neutron signal, free of local interference, needs to be extracted from the neutron monitor data. This signal is representative of the heliospheric conditions localised through the geomagnetic conditions to the neutron monitor site. Secondly, the signal needs to be localised to the point of the CRNS measurement, which rarely coincides with the position of a neutron monitor. Multiple correction methods have been evaluated and compared, with consideration towards both challenges.