Ground-level neutron monitors (NMs) are essential tools for monitoring space weather events, including the detection and alerting of ground-level enhancement (GLE) events. This study presents findings from a neutron monitoring survey using two compact N50L neutron slab-based subsystems (N50L detectors encased in lead bricks, used as an analogue to a new NM design) deployed across various field sites in the United Kingdom (UK). Data from these N50L neutron slab-based subsystems were compared to readings from established NM-64 monitors with similar geomagnetic cutoff rigidities and accessed via the Neutron Monitor Database (NMDB). Observations showed that the N50L neutron slab-based subsystems’ cosmic ray (CR) count rates corresponded closely with NMDB trends, and count rates varied with altitude, latitude, and environmental factors within meters of the detectors. Key events observed during the campaign include Forbush decreases and GLE-74. The collected data will support the deployment of the new NM-2023 design initiative, specifically targeting the site of the first operational NM-2023 in the UK. Additionally, data from the N50L neutron slab-based subsystems were integrated with the University of Surrey’s Compact NM setup, enhancing GLE monitoring capabilities across UK cutoff rigidity. These preliminary measurements from the 4-NM-2023 prototype suggest it can achieve performance comparable to the 6-NM-64 monitor but with a reduced footprint, volume, mass, and cost, utilizing environmentally friendly, non-toxic gas-filled counters. A full 4-NM-2023 system is planned for deployment at the UK Met Office’s Camborne Observatory, with a 1-NM-2023 unit to be installed at Lancaster University.

A document by Cameron James Patterson. Click on the document to view its contents.

A document by Cameron James Patterson. Click on the document to view its contents.

Extreme fluctuations in the horizontal geomagnetic field (dBh/dt) may be generated at the Earth’s surface by electrical currents in the ionosphere and magnetosphere. Using a global database of 125 magnetometers covering several decades we present occurrence statistics for fluctuations exceeding the 99.97th percentile (P99.97) for both ramp changes (Rn) and the root-mean-square (Sn) of fluctuations over periods, τ, from 1 to 60 min and describe their variation with geomagnetic latitude and magnetic local time (MLT). Rates of exceedance are explained by reference to the magneto-ionospheric processes dominant in different latitude and MLT sectors, including ULF waves, interplanetary shocks, auroral substorm currents, and travelling convection vortices. By fitting Generalised Pareto tail distributions above P99.97 we predict return levels (RLs) for Rn and Sn over return periods up to 500 years. P99.97 and RLs increase monotonically with frequency (1/τ) (with a few exceptions at auroral latitudes) and this is well modelled by quadratic functions whose coefficients vary smoothly with latitude. For UK magnetometers providing 1-s cadence measurements, the analysis is extended to cover periods from 1 to 60 seconds and empirical Magnetotelluric Transfer functions are used to predict percentiles and return levels of the geoelectric field over a wide frequency range (2x10-4 to 4x10-2 Hz) assuming a sinusoidal field fluctuation. These results help identify the principal causes of field fluctuations leading to extreme geomagnetically induced currents (GIC) in ground infrastructure over a range of timescales and they inform the choice of frequency dependence to use with dBh/dt as a GIC proxy.