Approaches and Challenges of the Neutron Monitor based Incoming Flux Correction for Cosmic-Ray Neutron Sensing

Cosmic Ray Neutron Sensing (CRNS) is a technique to measure water content, for example soil moisture, on the hectare scale through the measurement of epithermal neutrons. The neutrons are results of  particle showers in the earth's atmosphere caused by cosmic rays impinging on it. The abundance and global 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 incoming neutron flux correction method is ultimately to account for these heliospheric changes. Any neutron monitor based correction method has to overcome the uneven distribution of neutrons across latitudes, due to the earth's magnetic field.  There have been multiple, neutron monitor based, approaches developed, all of them based upon the assumption of linearity between the CRNS and the neutron monitor measurement. This assumption is challenged by multiple factors, most importantly geomagnetic and local conditions. Understanding the challenges and limitations of the linearity assumption is crucial to reliably correct CRNS measurements and produce a robust soil moisture product. Multiple correction methods have been evaluated and compared, with consideration towards the impact of different geomagnetic and local conditions.