Memorandum submitted by Professor Clive
Dyer (SAGE 05)
Please find enclosed my submission concerning
solar storms concentrating on radiation hazards to spacecraft
and aircraft.
I am writing to you in my private capacity and
not as a member of any particular company or body and the views
expressed are mine and not necessarily that of any company with
which I have been associated. However I believe that these views
are well founded and backed up by peer-reviewed research publications
in the open literature. I am happy to provide further advice and
papers if required. I trust that the following summary biography
illustrates my credentials in this area.
Following degrees at Christ's College Cambridge
and Imperial College London I have spent some 40 years in the
aerospace industry mainly researching radiation environments and
their effects on electronics and personnel. After positions with
NASA Goddard Space Flight Center, USA and the Royal Naval College
Greenwich I spent most of my career at Farnborough where the Royal
Aircraft Establishment has evolved to become QinetiQ. There I
founded and developed a research team to study radiation environments,
effects and hardening and became Senior Fellow and Chief Scientist(Space).
The initial emphasis was on Space Systems but in the late 1980s
I was one of the first to identify potential problems for avionics
and this aspect has steadily grown. I am co-author of several
standards including the IEC TS62396 standard for SEE in avionics.
I have published some 167 papers in the open literature including
35 on aircraft radiation environments and effects. I retired from
full-time employment at QinetiQ in 2008 but continue to supply
consultancy in my areas of expertise. In July 2010 I was honoured
with "The Radiation Effects Award" of the IEEE Nuclear
and Plasma Sciences Society.
I hope that this submission is useful to the
committee.
Clive Dyer
Professor Clive Dyer, MA (Cantab.), PhD (Lond.),
DIC.
INQUIRY INTO SCIENTIFIC ADVICE AND EVIDENCE
IN EMERGENCIES: (III) SOLAR STORMS
SUBMISSION ON RADIATION HAZARDS TO SPACECRAFT
AND AIRCRAFT
1. INTRODUCTION
Solar storms comprise bulk emissions of high
speed ionised gas (coronal mass ejections), which take a day or
two to reach earth where they disturb the earth's magnetic field,
and/or acceleration of particles to high energies which take a
matter of minutes to arrive at earth leading to a radiation hazard
and ionisation of the upper atmosphere. These storms can produce
a range of effects on technological systems and could in extreme
situations lead to an emergency situation. Such effects include:
(i) Disruption of the National Grid from geomagnetic
storms;
(ii) Disruption of communications due to ionospheric
disturbances;
(iii) Disruption of global positioning signals
from satellites due to ionospheric disturbances;
(iv) Damage or down time of key satellites caused
by enhanced levels of ionising radiation;
(v) Dose to air crew and passengers and disruption
to avionics due to enhanced ionising radiation from solar particle
events.
Of course a sinister synergy of all the above
would lead to the most severe emergency and this could indeed
occur from a sequence of solar storms over several days. However,
in this submission I will concentrate in items iv and v as they
are my major area of expertise and in the case of v, in my opinion,
the least appreciated and understood.
2. RADIATION
EFFECTS ON
SPACECRAFT
During certain solar storms energetic protons
and heavier ions are accelerated and arrive at earth within about
10 minutes with the enhanced levels lasting from hours to days.
In addition major geomagnetic storms can lead to enhanced levels
of trapped electrons in the radiation belts. Both types of environment
enhancement are significant at key orbits such as geosynchronous
orbit, used by communication and broadcast satellites, and at
orbits used by global positioning systems.
The effects of such radiation on spacecraft
range from cumulative dose and damage, such as solar array degradation,
to more immediate problems from electrostatic discharge or single
particle induced upsets in on-board computers and memories.
Although the experience of the space industry
is extensive, engineering mistakes are still made and losses and
outages still occur in extreme events. Most systems are specified
to the worst radiation levels measured since the beginning of
the space age. However this experience is limited and more severe
events have occurred historically, such as the Carrington event
of 1 September 1859. In addition modern microelectronics is becoming
increasingly susceptible to individual particles of radiation
(single event effects-SEE) due to their higher density and performance
requirements.
In general there is good international communication
on understanding environments and effects. However commercial
operators will always limit the degree of hardening to that which
they consider to be cost-effective. For critical systems there
is a need for greater communication and understanding to safeguard
the infrastructure. At present initiatives tend to be bottom-up
from concerned scientists.
3. RADIATION
EFFECTS ON
AIRCRAFT AND
AIR CREW
Solar storms can disrupt communications and
navigation signals to aircraft due to the disturbed ionosphere.
In addition certain events can lead to enhanced levels of ionising
radiation at aircraft altitudes. Again synergy of such effects
in conjunction with ground level problems is a recipe for an emergency.
The earth's atmosphere in conjunction with the
geomagnetic field provides considerable protection against both
cosmic rays and solar particle events. However the protective
layer of the atmosphere is reduced to about one third at normal
subsonic cruising altitudes and to one tenth at supersonic altitudes
leading to background radiation levels that are 300 to 1,000 times
higher than at sea level. As a result air crew are the most highly
exposed occupation with long haul crew receiving typically 4 to
6 milliSieverts (mSv) per year [1], the upper level being limited
by guidelines accompanying the current legislation [2]. The milliSievert
is a measure of effective radiation dose which is used to assess
the probability of long term effects such as cancer. By comparison
the average sea level dose is 2 to 3 mSv per year (from both rocks
and cosmic rays) while medical diagnostic doses range from 0.004
mSv for a dental X-ray to 0.06 mSv for a chest X-ray.
A small but very important subset of solar particle
events can generate particles of sufficient energy to enhance
radiation levels at aircraft altitudes [3]. Because they are also
detectable by ground level monitors (in general large area neutron
monitors) these are frequently referred to as ground level events
(GLEs). While increases at ground level can be up to a factor
50 (as measured at Leeds on 23 February 1956), increases at flight
altitudes can be 1000-fold leading to effective dose rates of
several mSv/hr, hence exceeding annual flight limits in one flight
if no avoiding action is taken [4]. In this regard it should be
noted that in Europe the general public and pregnant air crew
are restricted to 1 mSv per year and 1 mSv per term of pregnancy
respectively. FAA guidelines further limit exposure in pregnancy
to no more than 0.5 mSv in a month. If the geomagnetic field is
not disturbed there is a steep gradient of dose with respect to
magnetic latitude and such problems occur only with high latitude
flights. However this includes flights on some of the most densely
populated routes, such as from UK and Europe to North America
and Japan. Fortunately such large GLEs are rare and it is estimated
that about six events since 1942 (the start of ground level monitoring)
would have exceeded the 1 mSv legal limit for an example flight
from London to Los Angeles at 39,000 feet [5]. Indirect evidence
of solar particle radiation from ice core samples shows that the
1859 event could have been four times worse than any of these
[6]. If the geomagnetic field is highly disturbed when the particles
arrive, then much lower latitudes can be exposed. Indeed the 1859
event could have given significant exposure down to the tropics.
Fortuitously the growth of civil aviation has been accompanied
by a quieter sun. However there was a wake-up call on 20 January
2005 when a major GLE gave a factor 50 increase in the Antarctic
region corresponding to effective dose rates of 3 mSv/hr at cruising
altitudes. Fortunately for aviation this was very short lived
and localised, northern hemisphere rates being an order of magnitude
lower.
Another problem that has become increasingly
evident since about 1990 is the effect of radiation on avionics
via the single event effects mechanism mentioned above. There
is now a considerable body of evidence of upsets in flight systems
and hard failures in certain electronics [7]. These have been
shown to correlate with cosmic ray fluxes [8] but fortunately
during this time no very large ground level events have occurred.
For such events significant numbers of upsets could occur in a
single flight leading to possible flight hazards [9]. For example
an autopilot system was found to upset on average every 200 flight
hours and return control to the pilot. If a major GLE had occurred
before this problem was eventually fixed such an upset could have
occurred every hour making safe flight very difficult. Upsets
from cosmic rays are now starting to be considered in accident
investigations [10]. Technical specifications to account for and
limit radiation hazards in avionics have been available since
2006 [11, 12] but it is not clear that they are universally applied
and there is plenty of pre-2006 equipment in flight.
At present there are no viable methods for predicting
GLEs and the exposure of aircraft. Ground level monitors are diminishing
in number and by the time such information reached aircraft it
would be too late as maximum rates are reached in a matter of
10 minutes or so. Attempts are made to estimate the dose received
after the event but even here the accuracy is limited by the lack
of information, typically to about a factor 2. Concorde (and supposedly
all aircraft operating above 49,000 feet) was compelled to carry
a radiation warning monitor [2]. However this has not been extended
to other aircraft despite the fact that subsonic routes at high
latitude are in fact more exposed than Concorde due to the latitude
effect exceeding the influence of the higher altitude [4].
Many pilots would like to carry monitors to
measure their radiation environment and warn of enhancements and
this is reflected in a recent letter (25 September 2009) to the
European Commission from the European Cockpit Association (ECA),
which represents more than 38,000 commercial pilots. They make
a number of recommendations on controlling exposure to air flights
of which the following are relevant to solar particle events:
Sample ECA Recommendations:
8. Every flight of an aircraft should
be considered as a planned exposure situation (ICRP 103).
9. Solar energetic particle events
and other sudden increases in radiation should be classified as
emergency exposure situations (ICRP 103). Accordingly, an action
plan ("Emergency Response Plan") should be established,
using reference values ("Dose Constraints") and measures
in the case of current or anticipated radiation increases.
10. All aircraft with a maximum operating
altitude of more than 10,000 m (approx. 33,000 ft) shall be equipped
with a warning device to detect sudden increases in dose rate.
During flight, the cockpit crew shall have the display of the
warning function plainly visible.
11. Flight crews shall be provided
with regular information of actual and forecasted solar activity.
At present there appears to have been no movement
by government bodies to accommodate these recommendations.
There are currently solar particle event warnings
from US NOAA and FAA but these are based on satellite measurements
of much lower energy particles. It is not clear what notice is
taken of them and if it is there is likely to be over-reaction
and unnecessary grounding of flights as there are many more low
energy events affecting spacecraft than there are high energy
events affecting aircraft (about 10:1).
There is clear need for government action and
international agreement to protect aircraft and crew. The situation
is somewhat analogous to the recent "volcanic ash" emergency,
where there was apparently a lack of monitoring and a lack of
agreed fly/no-fly criteria. Only with further action will there
be a balance between safety on the one hand and unnecessary economic
chaos from the widespread grounding of flights on the other. The
aviation industry has been fortunate since 1956 but the Sun's
history will one day repeat itself.
4. GOVERNMENT
ADVICE AND
PREPAREDNESS
In relation to the five questions posed by the
Committee my opinions with respect to the above hazards are as
follows:
1. The hazards and risks are as above and have
largely been identified by basic research. Sometimes the relevant
industry will admit to problems following probing by scientists
and allow sufficient flight data to prove their cause. However
often there is a cloak of commercial sensitivity. In my opinion
the Government is totally unprepared for such an emergency.
2. Use of advice in this area is extremely limited.
A Cosmic Ray Advisory Group was once established (by DfT and CAA)
to consider the implementation of air crew dose legislation. However
this appears to have been abandoned without any ongoing methodology
for dealing with solar particle events.
3. Major obstacles arise from fragmentation of
knowledge across a wide variety of bodies. Government needs to
ensure joined-up thinking and coordination. There appears to be
very little power to enforce anything. There are too many vested
commercial and departmental interests. The situation has not been
helped by the commercialisation of research activity over the
last decade with consequent lack of objective advice to government.
4. There appears to be no strategic coordination
at all.
5. International coordination is extremely important.
Scientists communicate well at the research level but this does
not appear to be reflected in coordination between legislative
bodies. There is a lack of UK support for ESA Space Weather activities
which limits UK influence.
REFERENCES [1] L
Lindborg, D T Bartlett, P Beck, I R McAulay, K Schnuer, H Scraube,
F Spurny, "Compilation of measured and calculated data: A
report of EURADOS WG5," May 2004.
[2] Joint Aviation Authorities (JAA) JAR-OPS
1.390 Cosmic Radiation, 2001.
[3] C S Dyer, A J Sims, J Farren, and J Stephen,
Measurements of solar flare enhancements to the single event upset
environment in the upper atmosphere, IEEE Trans. on Nucl. Sci.,
37, 1929-1937, Dec. 1990.
[4] Clive Dyer, Fan Lei Alex Hands, Peter Truscott,
"Solar particle events in the QinetiQ atmospheric radiation
model," IEEE Trans. Nucl. Sci. vol. 54, no.4, 1071-1075,
2007.
[5] P Lantos, N Fuller, "History of the
solar particle event radiation doses on-board aeroplanes using
a semi-empirical model and Concorde measurements," Radiation
Protection Dosimetry, Vol. 104, no. 3, pp. 199-210, 2003.
[6] M A Shea, D F Smart, K G McCracken, G A M
Dreschhoff, H E Spence, "Solar proton events for 450 years:
The Carrington event in perspective," Adv. Space Res.,
Vol. 38, pp. 232-238, 2006.
[7] J Olsen et al., "Neutron-Induced
Single Event Upsets in Static RAMs Observed at 10 Km Flight Altitude,"
IEEE Trans. Nucl. Sci., 40, 77, (April, 1993).
[8] E Normand, "Correlation of In-flight
Neutron Dosimeter and SEU Measurements with Atmospheric Neutron
Model," IEEE Trans. Nucl. Sci., 48, 1996 (2001).
[9] C S Dyer, F Lei, S N Clucas, D F Smart, M
A Shea, "Solar particle enhancements of single event effect
rates at aircraft altitudes," IEEE Trans. Nucl. Sci., vol.
50, No. 6, pp. 2038-2045, Dec. 2003.
[10] Australian Transportation Safety Bureau
(ATSB) Transportation Safety Report, Aviation Occurrence Investigation
AO-2008-070, Interim Factual No 2, "In-flight upset 154 km
west of Learmonth, WA 7 October 2008," Nov. 2009.
[11] R Edwards, E Normand, C Dyer, "Technical
standard for atmospheric radiation single event effects (SEE)
on avionics electronics," 2004 IEEE Radiation Effects Data
Workshop Record, IEEE 04TH8774, pp 1-5, 2004.
[12] International Electrotechnical Commission,
Process Management For Avionics—Atmospheric Radiation Effects—Part
1: Accommodation of atmospheric radiation effects via single event
effects within avionics electronic equipment, IEC TS 62396-1,
2006.
Professor Clive Dyer,
MA (Cantab.), PhD (Lond.), DIC.
5 September 2010
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