06:20pm Tuesday 12 December 2017

Change in solar activity brings increased radiation risk to air passengers

The researchers say this a serious concern because our present day engineering, design, operation and insurance of vulnerable technology is based on past experience from the space age and does not yet account for long-term change in space climate.

Image of a coronal mass ejection (CME) on June 7, 2011, recorded in ultraviolet light by the Solar Dynamics Observatory

The scientists have paid particular attention to the radiation effects on aircraft crew and passengers on long-distance flights.

The Sun has been in a ‘grand solar maximum’ which has already lasted longer than any other such maximum in the past 9.3 millennia and is expected to end soon. The changes in near-Earth space that will result will return Earth to conditions that last prevailed before the advent of susceptible modern operational systems, such as spacecraft, power distribution grids and aircraft.

The study says that at cruise altitudes of commercial aviation, particularly at higher latitudes, high-energy ionising radiations such as Solar Energetic Particles (SEPs) and Galactic Cosmic Rays (GCRs) pose threats through single event upsets in electronics critical to flight safety, and through the radiation exposure of crew and passengers.

GCRs are high energy particles generated by supernovae explosions in our galaxy and, because of the shielding effect of our Sun’s magnetic field, they give a continuous radiation dose throughout the solar system that increases during a solar activity minimum and decreases during a solar maximum. SEPs are bursts of energetic particles that are formed from supersonic ejections of material from the solar atmosphere.

The work was led by Mike Lockwood, professor of Space Environment Physics at the University of Reading and his PhD student Luke Barnard. Professor Lockwood said: “Radiation standards for avionics are based only on experience from the current grand solar maximum and there are no regulations concerning passenger exposure.”

The International Commission on Radiological Protection (ICRP) recommends a 1 mSv limit for the annual dose received by a member of the public. Dosages during a flight depend on path, duration and altitude as well as on the level of solar activity. A commercial eight-hour polar flight during the 2003 ‘Halloween’ SEP event would have given 70% of this recommended annual limit and it is estimated that the largest known SEP event, the ‘Carrington event’ of 1859, would have given 20 times the limit.

Even without a SEP event, GCRs mean that the recommended limit corresponded to only five round trips (of two such flights each) during the recent solar minimum. The researchers at Reading have studied how these dosages are likely to increase in the future as solar activity declines.

Professor Lockwood said: “All the evidence suggests that the Sun will shortly exit from a grand solar maximum that has persisted since before the start of the space age. This analysis shows that the risk of the space-weather effects is considerably enhanced over the next century compared to the space age thus far.”

The researchers use past experience to predict that there is an 8% chance of the Sun falling to grand minimum conditions over the next 40 years, giving enhanced dosages of GCR radiation which is of concern for aircraft avionics, crew and passengers. The risk of large SEP events is higher at middling levels of solar activity and so is initially enhanced in this case but then decreases to almost zero during the grand minimum itself.

A more likely scenario, predicted from the mean of all previous examples, is a more modest rise in the GCR fluxes but an enhanced risk of a large SEP event. There is only a 5% chance that the SEP risk and the GCR hazard will increase only slightly above those for the present solar cycle.

The study noted that both GCR and SEP events will be amplified by the probable continuation of the decrease in the geomagnetic field, and the shielding it provides, that has been observed over the past 160 years.

‘Predicting Space Climate Change’ by L. Barnard, M. Lockwood, M.A. Hapgood, M.J. Owens, C.J. Davis and F. Steinhilber appears in Geophysical Research Letters, doi:10.1029/2011GL048489

ENDS

For more information please contact Rona Cheeseman in the press office on 0118 378 7388 or email r.cheeseman@reading.ac.uk

Notes to editors

The Department of Meteorology at the University of Reading is internationally renowned for its excellent teaching and research in atmospheric, oceanic and climate science. Established in 1965, it is the only UK university which offers a full range of undergraduate and postgraduate courses in meteorology.

In 2005 Meteorology at Reading was awarded the Queen’s Anniversary Prize for Higher and Further Education.

In the latest Research Assessment Exercise (2008), 75% of its research was graded as world-leading or internationally excellent. It is the highest-graded department focusing on the fundamental science of weather and climate in the UK.

Additional notes and a glossary of terms and acronyms

SEP stands for Solar Energetic Particle. Small events are common but large events occur more rarely. The size of an event is gauged by the flux of particles above a threshold energy and for the size of event studied by the researchers at Reading, there are of order 1-10 events per century, depending on the level of solar activity. Recent research indicates that large SEP events occur most frequently for average levels of solar activity. It is thought that the activity must be high enough to generate large CME events, but the yield of SEP particles is higher if the interplanetary magnetic field (IMF) is low because the CME is more “super-Alfvénic” (the equivalent of supersonic when a magnetic field is present and the medium is ionised). The combination of these two effects is thought to make large SEP events more common at middling solar activity levels.

CME stands for Coronal Mass Ejection. It is the ejection of a vast amount of material (of order 1 million, million, million, million kg) from the solar atmosphere into the solar system. They travel at super-sonic and super-Alfvénic speeds (several hundred kilometres per second) and as a result a shock front forms ahead of them that accelerate SEP particles to very high energies. CMEs are more frequent when solar activity is high.

IMF stands for interplanetary magnetic field. This magnetic field is generated in the Sun and dragged out of the solar atmosphere by the solar wind flow. It is important for two reasons. Firstly, it shields the Earth from galactic cosmic rays, so that the flux of cosmic rays reaching Earth decreases as solar activity increases (and vice-versa). Secondly, the efficiency of SEP production by a CME increases with decreasing IMF.  The strength of the IMF varies with solar activity levels.

Solar activity. This is a general term for the production of magnetic field in the convection zone of the Sun (the outer third), its eruption through the solar surface (where is gives sunspots), and its emergence through the solar atmosphere to become the IMF (see above). Solar activity varies over a decadal-scale cycle (which usually averages 11 years in length) and over centuries as the Sun oscillates between grand minima and grand maxima

The Maunder Minimum. The most recent of the grand minima in solar activity which took place between about 1650-1700. Cosmogenic isotopes show that there have been 12 such minima in the last 9300 years.

Grand solar maxima. A series of high activity solar cycles. The cosmogenic isotope record shows that there have been 24 such maxima in the past 9300 years. Recent decades (since 1920) form the longest-lived grand solar maxima in that interval.

GCR stands for galactic cosmic rays. These particles are accelerated at the shock fronts ahead of violent events in our galaxy, such as supernovae explosions. They form a continuous flux of particles. The number reaching Earth decreases as the strength of the IMF increases.

Radiation doses. Nations have their own legislation setting the allowed limits of ionising radiation dose that individuals should be exposed to. These limits are generally lower for members of the public than for workers who deal with radioactive material as the latter have protection and medical screening not afforded to the public. These normally relate to the integrated dose over a whole year and apply on the ground within the nation concerned. There are no such legal limits for passengers and crew within an aircraft but there are recommendations by The International Commission on Radiological Protection (ICRP).


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