Storm Warning – Understanding the risks associated with extreme space weather events
In recent years, concerns have been raised around the potential for space weather events to adversely affect critical infrastructure here on Earth. In order to manage such hazards, we need to understand them – so, what are space weather events, and what impact can they have?
INTRODUCTION
Space weather is a natural consequence of the behaviour of the Sun and Galactic Cosmic Rays (GCRs), and relates to their interaction with the Earth’s magnetic field and atmosphere.
The sun is a highly dynamic body, and many processes can affect solar activity, including solar flares (see Figure 1). Where a sudden increase of energetic particles at ground level is induced by solar activity, this can result in dramatic changes in potential radiation exposure at lower altitudes, causing fluctuations in power systems and disrupting electronic devices, with a potential significant effect on critical infrastructure and plant.
NATURAL RADIATION
Exposure to low levels of natural background radiation is part of everyday life, and most people are not aware of this exposure and the potential risks to health. Personal behaviour can affect the dose received – for example taking a commercial flight from the UK to the USA results in a ~80μSv effective dose (Ref. 2), or about 4% of the average UK annual background radiation dose.
In contrast, exposure to elevated levels of ionising radiation (>100mSv), such as those possible during a severe space weather event, has been noted by the UK Health Protection Agency, as having the potential to cause damage to DNA, lead to mutations, uncontrolled cell division and lead to malignancy (Ref 2).
Figure 1 – Photograph of the Sun (December 19, 1973), from the third and final manned Skylab mission, showing one of the most spectacular solar prominences ever recorded, spanning more than 588,000 kilometres (365,000 miles) across the solar surface (Ref. 1)
SPACE WEATHER
Space weather is best understood as the variation in the Earth’s surrounding space environment, and is caused mainly by energetic charged particles from the Sun that interact with the Earth’s magnetic field. The most visible sign of this interaction is the Aurora, where charged particles enter the atmosphere at the poles, channelled by the Earth’s magnetic field lines. Reassuringly, this magnetic field and the atmosphere largely protect us on the ground from potential exposure to these energetic particles; however, there are some space weather events that can result in dramatic changes in potential radiation exposure at lower altitudes.
GROUND LEVEL ENHANCEMENTS
When energetic particles from Solar Energetic Particle (SEP) events, driven by shocks from Coronal Mass Ejections (CMEs), hit the atmosphere, a large influx of protons can result in showers of secondary particles, especially neutrons, which can potentially reach ground level if high enough in energy to penetrate the atmosphere. These events are called Ground Level Enhancements (GLEs) (Figure 2)
GLE events involve the interaction of energetic particles over ~350MeV in energy with the Earth’s atmosphere. These energies are high enough to interact with atomic nuclei of the atmosphere, generating a cascade of secondary particles at ground level. This air shower can consist of neutrons, protons, electrons, pions, muons and others, with high energy neutrons (>10 MeV) being the main concern.
Higher energy particles observed during a GLE event have sufficient energy to penetrate the magnetosphere, even at non-polar regions, and result in a cascade of secondary particles at lower latitudes.
There have been 73 GLEs recorded since measurements began in the 1940s – approximately one GLE event per year. Although these are difficult to predict with constantly varying solar conditions, they show some alignment with the solar maximum. The largest GLE ever recorded was in 1956, with the observed neutron count rate at one station (Leeds, UK) increasing by ~4760% (15-minute average) (Ref. 4).
WEATHER EFFECTS
The vulnerability of power transmission systems to the effects of geomagnetically induced current surges, and the subsequent potential for widespread black-outs, are generally well known, largely thanks to the collapse of the Hydro-Quebec power network in 1989 during a severe solar storm.
What is less well known is the potential for GLEs to disrupt electronic devices. Semiconductor devices made of silicon are particularly vulnerable. The most common result is a bit flip (a 1 changing to a 0 or vice versa), affecting running processors or a rapid energy discharge in power devices, with local effects ranging from none to temporary or permanent malfunction.
With the widespread use of semiconductors in modern communication, control and protection systems, modern plant are potentially vulnerable to associated equipment loss or spurious behaviour during a severe GLE, together with coincidental loss of offsite power.
Figure 2 – Cascade of particles within the atmosphere from incident protons during a GLE event (Ref. 3)
BAD WEATHER DAY
In an extreme example, for a nuclear power station, a severe GLE impacting reactor control and supporting safety systems could, in principle, result in a complete loss of their functionality if they are solely based on microcontroller technology. It is worth noting, however, that being the product of scattering and location-specific impacts, adverse events are stochastic in nature, not certain, and currently difficult to assess in terms of probability.
Although loss or impairment of affected equipment is unlikely to prevent a reactor trip, the subsequent operation of supporting safety systems, e.g., electrical, air supply, etc. may require manual interaction to ensure post-trip cooling.
There could also be an impact on electronic systems that support plant operators, such as monitoring, warnings and alarms, which may behave spuriously or fail, causing confusion. Internal and offsite communications may also be unavailable, preventing operators from co-ordinating response efforts or requesting assistance.
Performing the necessary manual actions during a severe GLE, in a timely manner with a lack of reliable indication, could be extremely challenging; as could correctly diagnosing any false alarms, without any systems currently available to alert them of an occurring GLE.
While it is clear that the potential consequences of a severe GLE could be serious for a nuclear power station, evaluating the frequency of such accidents is fraught with uncertainty. Further research is needed to characterise the spectrum of event frequency versus GLE energies at varying locations, and the vulnerability of device types and their effects in probabilistic terms, before the associated risk can be quantified. Only then will we be in an informed position to decide whether the hazards of space weather merit specific design provisions (such as shielding or use of analogue protection systems).
CONCLUSION
As the effects of space weather on people and equipment become better understood, we have the capability to identify the wider potential consequences on safety-related plant, indication and operator response.
To understand the associated risk, however, will require more research into not only the frequency (and forecasting) of severe events at varying locations, but also their inherently stochastic effects on electronic devices.
References
- Attribution: Skylab III – Experiments ATM, NASA, https://archive.org/details/S74-23458
- S F Mobbs, C R Muirhead and J D Harrison, Risks from Ionising Radiation, https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/337141/HPA-RPD-066_for_website2.pdf
- Attribution: Dr Ben Clewer, Surrey Space Centre, https://www.surrey.ac.uk/news/surrey-develop-new-telescope-measure-extreme-space-weather[DM1]
- John W. Bieber, John Clem, Paul Evenson, Roger Pyle, Marc Duldig, John Humble, David Ruffolo, Manit Rujiwarodom and Alejandro Sáiz, Largest GLE in Half a Century: Neutron Monitor Observations of the January 20, 2005 Event, https://www.researchgate.net/publication/234202262_Largest_GLE_in_Half_a_Century_Neutron_Monitor_Observations_of_the_January_20_2005_Event
