Scientific advice and evidence in emergencies - Science and Technology Committee Contents

Supplementary memorandum submitted by Professor Clive Dyer (SAGE 05a)




  Further to my submission prior to the oral session on 10 November, and having listened to most if the session, I offer the following clarifying information.


  The orbits used by both the US GPS and the planned European Galileo satellites are Medium Earth Orbits (MEO) which pass through the heart of the outer radiation belt where they experience intense fluxes of energetic electrons. These electron fluxes show large time variations which are driven by geomagnetic storms. The largest storms are caused by coronal mass ejections which arrive at earth within about a day of being observed to leave the Sun. The electrons lead to cumulative effects from dose and damage but also to more immediate spacecraft charging and discharging within hours to days.

  In addition, such orbits are almost fully exposed to solar particle events (energetic protons and heavier ions) which start to arrive at the same time that the flare event is observed (ie they travel at close to the speed of light). However due to scattering in the interplanetary medium the enhancement is usually spread over a day or two. These particles also contribute to dose and damage but can also greatly enhance the rates of single event effects in microelectronics.

  These spacecraft must be carefully engineered on the basis of environment specifications which embody the totality of knowledge since space measurements were commenced in the late 1950's. Margins are then applied to ensure that the system will survive. Confidence levels are applied to the environment to ensure low probabilities that the specification will be exceeded.

  The question that needs to be posed is whether the confidence levels and margins are adequate to cope with a Carrington size event. Clearly the US GPS system has a good track record to date, although it is believed that a few anomalies do occur and of course there has not been exposure to a Carrington size event. The specifications are not widely published but possibly include a military specification to cover artificial enhancements; this would aid in providing resilience to natural events. It is noteworthy that the system carries radiation monitors to improve knowledge of the environment and warn of enhancements. Knowledge of the environment enables rapid reaction to make the system safe.

  The European Galileo system is currently in the early stages of construction. The question should be asked of this and other critical systems as to whether the environment specification confidence levels and the margins applied are able to cope with a Carrington size event. In addition warning monitors should be carried.

  In general spacecraft projects are well aware of radiation effects and spacecraft are engineered accordingly. Prospects for survival should be good but the above questions should be posed to the design authorities.


  As stated before, radiation hazards to both people and electronics on aircraft are less widely appreciated despite being very significant. Some of the key points are re-emphasised below:

    (1) For high latitude routes at conventional altitudes (33,000 to 39,000 feet) a Carrington size event could deliver a dose to aircrew and passengers 10 to 20 times the annual limit for the general public and pregnant aircrew. Such routes are not confined to transpolar but also include London to USA and Japan. If the particles arrive while a previous geomagnetic storm is in progress the hazard can extend into the tropics.

    (2) At the same time electronics in avionics could show very large upset rates and possible failures and these could compromise flight control systems and safety. For example a Gigabyte of modern memory could show an upset every second during a Carrington size event. It is virtually impossible for aircraft to be manufactured using the very limited range and quantity of radiation-hardened parts that are produced for certain military systems. Hence commercial off-the-shelf (COTS) electronics are employed and are susceptible to single event effects so that system level solutions are required to ensure reliability.

    (3) The energetic particles arrive at close to the speed of light and events are often over within a few hours making prediction nearly impossible.

    (4) At present radiation monitors are not routinely flown on aircraft. There are attempts by a few research groups to keep some in the air to possibly capture a solar particle event but these are not used operationally to give a real-time warning and response system. Concorde was compelled to carry a warning monitor but this has not been extended to subsonic flights despite the fact that they are used on routes that experience more radiation than Concorde; this is due to the length of travel and higher latitudes flown by certain subsonic flights exceeding the influence of the higher altitudes used by the supersonic Concorde. Many pilots would welcome such warning monitors (notably the European Cockpit Association) but it would appear that the airline industry and the aviation authorities are currently not taking account of this hazard and its possible mitigation. Widespread use of monitors would have the additional advantage of ensuring that the hazards from background cosmic rays are genuinely monitored (ie measured), as opposed to reliance on calculations, which are currently employed for "monitoring" crew dose.

Professor Clive Dyer

21 November 2010

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