Supplementary memorandum submitted by
Professor Clive Dyer (SAGE 05a)
INQUIRY INTO SCIENTIFIC ADVICE AND EVIDENCE
IN EMERGENCIES: (III) SOLAR STORMS
ADDITIONAL INFORMATION ON RADIATION HAZARDS
TO SPACECRAFT AND AIRCRAFT
1. INTRODUCTION
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.
2. RADIATION EFFECTS
ON SATELLITES
USED IN
THE GLOBAL
NAVIGATION SYSTEM
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.
3. RADIATION EFFECTS
ON AIRCRAFT
AND AIRCREW
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
| 
