Ground Level Enhancement Event Monitor (GLEEM)

The risks posed by space weather are becoming more widely recognised, and they are now listed on the UK National Risk Register. In particular, hard solar energetic particle (SEP) events containing a substantial flux of particles with energies greater than 300 MeV pose a considerable risk. Ground level neutron monitors detect such solar events, termed ground level enhancement (GLE) events, at the Earth's surface and have done so since the 1940s. Typically there is around one GLE event per year and they have durations from 1 to 12 hours, the largest event observed with instruments was measured in Leeds on 23 February 1956. Besides other ground-detectable space weather phenomena, GLE events have the potential to disrupt critical national infrastructures, such as the power grid, transport (aviation and rail), satellite applications and communications, and safety critical electronic control systems. Deducing space weather radiation at the top of the Earth's atmosphere from measurements made by neutron monitors on the Earth's surface requires a globally distributed network of monitors and models that simulate the physics of particle interactions in the Earth's atmosphere. The Met Office is responsible for reporting space weather risks to government departments and civil aviation, among others, and has recognised that it does not have sufficient capabilities to provide the necessary services for space weather radiation hazards. For example there are only 50 ground level neutron monitors worldwide still operational, none of these are located in the UK. The design of existing monitors and their instrumentation have changed very little over the last sixty years, they rely on detector materials that are either highly toxic (boron trifluoride) or expensive (helium-3), and are large and bulky instruments containing lead shielding.

Concerns over the use of these materials in other applications involving neutron detection has led to the development of a plethora of alternative detection technologies. Despite the wide range of alternative neutron detectors now available, very few are suitable for the specific application requirements of ground-level neutron monitoring, where high detection efficiency and several decades of stability are essential.

During the design phase, GLEEM evaluated an alternative detector technology (boron coated straws) that promised the greatest potential to fulfil these specific application requirements. This detector technology was developed for unattended safeguards monitoring, among other applications, where similar challenges exist. Our findings showed that, currently, fully modernised helium-3 detectors remain the most viable option. Our new design is optimised for cost savings, compactness and most efficient use of helium-3. It is designed to produce comparable results to a typical monitor in the existing network and is suited for unattended operation in relatively remote locations. The GLEEM implementation phase now aims to commission and demonstrate a prototype network of the new monitor design. The monitor will be deployed and tested at an existing meteorological field site to verify that such instruments can produce comparable results to those from existing ground level neutron monitors, and potentially enhance existing global capabilities. As proof of concept, a network of one complete instrument and one partial instrument will be demonstrated as part of a test deployment, to provide a compatible data stream for incorporation into the Met Office Space Weather Operations Centre (MOSWOC) and feed into the airborne radiation models being developed as part of SWIMMR N2, the NERC funded SWIMMR Aviation Risk Modelling (SWARM) project. Ultimately, GLEEM aims to construct and operate a significantly cheaper instrument, re-introduce monitoring in the UK and facilitate a major increase in space weather monitoring worldwide.