Solar Modulation Corrections for Cosmic-ray Soil and Snow Sensors Using
the Global Neutron Monitor Network
Abstract
Cosmic-ray neutron sensors (CRNS) have been used in many studies for
measuring soil moisture and snow pack over intermediate scales.
Corrections for geomagnetic latitude, barometric pressure and
atmospheric humidity are well established, however, corrections for the
effect of solar activity on neutron count rates have been overly
simplistic, typically relying on one neutron monitor station and
accounting for latitude and elevation crudely or not at all. Recognizing
the lack of a generalised and scientifically robust approach to neutron
intensity correction, we developed a new approach for correcting CRNS
count rates based on analysis of data from 110 quality-controlled
neutron monitor stations from around the world spanning more than seven
decades. Count rates from each monitor were plotted against the count
rates from Climax, CO, USA or Jungfraujoch, Switzerland depending on the
time period covered. Relationships between relative counting rates at
the site of interest versus the reference neutron monitors were found to
be strongly linear. The dimensionless slope of this linear relation,
referred to as τ, was shown to increase with increasing geomagnetic
latitude and elevation. This dependence of τ on geomagnetic latitude and
elevation was represented using an empirical relationship based on a
single reference neutron monitor. This generalised approach enables τ to
be derived for any location on Earth and also lends itself to roving
CRNS studies. The correction procedure also includes a
location-dependent normalisation factor which enables easy substitution
of an alternative reference neutron monitor into the correction
procedure.