Effect of solar energetic particle (SEP) events on the radiation exposure levels to aircraft passengers and crew: Case study of 14 July 2000 SEP event

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003ja010343, 2004 Effect of solar energetic particle (SEP) events on the radiation exposure levels to aircraft passengers and crew: Case study of 14 July 2000 SEP event R. H. A. Iles, 1 J. B. L. Jones, 2 G. C. Taylor, 3 J. B. Blake, 4 R. D. Bentley, 1 R. Hunter, 5 L. K. Harra, 1 and A. J. Coates 1 Received 4 December 2003; revised 2 July 2004; accepted 5 August 2004; published 5 November [1] We investigate the circumstances required for aircrew and passengers to experience an increased radiation exposure rate from a solar energetic particle (SEP) event occurring during a flight. The effects of the 14 July 2000 National Oceanic and Atmospheric Administration S3 class SEP event are examined using ground-based and satellite measurements together with coincident measurements made using a tissue equivalent proportional counter (TEPC) on board a Virgin Atlantic Airways flight from London Heathrow to Hong Kong. In this paper we present the first measurements made during a SEP event using a TEPC at flight altitudes. Our results indicate that there were no increased radiation levels detected during the flight due to the SEPs, but the measurements agreed well with the CARI-6 model calculations made using a heliocentric potential value derived immediately prior to the SEP event. In addition, a prolonged increase in the >85 MeV particle flux is observed for up to 2 days after the SEP onset by the SAMPEX spacecraft at latitudes >55. INDEX TERMS: 2104 Interplanetary Physics: Cosmic rays; 2118 Interplanetary Physics: Energetic particles, solar; 7514 Solar Physics, Astrophysics, and Astronomy: Energetic particles (2114); 7519 Solar Physics, Astrophysics, and Astronomy: Flares; 6630 Public Issues: Workforce; KEYWORDS: radiation, cosmic, solar, aircrew, exposure Citation: Iles, R. H. A., J. B. L. Jones, G. C. Taylor, J. B. Blake, R. D. Bentley, R. Hunter, L. K. Harra, and A. J. Coates (2004), Effect of solar energetic particle (SEP) events on the radiation exposure levels to aircraft passengers and crew: Case study of 14 July 2000 SEP event, J. Geophys. Res., 109,, doi: /2003ja Introduction [2] Following publication of the 1990 International Commission on Radiological Protection 60 recommendations, there has been a need for further investigations into the radiation exposure levels experienced by aircrew [International Commission on Radiological Protection, 1991; O Sullivan and Zhou, 1999]. A number of research initiatives have since reported on areas including the physics of cosmic radiation [Heinrich et al., 1999], in-flight measurements, and validation of atmospheric radiation dose models [Beck et al., 1999; Schraube et al., 1999; Tommasino, 1999]. There has also been continued development of atmospheric radiation dose models, including CARI, which is based on the LUIN radiation transport code; PC-AIRE, an empirical code [Lewis et al., 2001]; and EPCARD, which uses a Monte Carlo particle interaction 1 Mullard Space Science Laboratory, University College London, Dorking, UK. 2 Virgin Atlantic Airways Ltd., Crawley, UK. 3 National Physical Laboratory, Teddington, UK. 4 Space Sciences Department, The Aerospace Corporation, El Segundo, California, USA. 5 Civil Aviation Authority, Gatwick, UK. Copyright 2004 by the American Geophysical Union /04/2003JA simulation called FLUKA. More recently, there has also been some work done to model the atmospheric radiation response to solar particle events [Dyer et al., 2003; O Brien and Sauer, 2000, 2003]. [3] Adoption of European Union (EU) legislation in May 2000 [Council of the European Union, 1996] requiring that EU member states monitor and record occupational exposure of aircrew to cosmic radiation has further increased the importance of accurate validation of atmospheric radiation dose models. At present, many airlines use computer models to assess the level of cosmic radiation exposure to aircrew rather than flying monitors on each flight because of a number of impracticalities in installing monitors aboard an aircraft fleet. The tissue equivalent proportional counter (TEPC) is a suitable instrument for in-flight measurement of cosmic radiation [Lindborg et al., 1999]. However, to place a TEPC permanently on board would require an aircraft power supply, space for a small suitcase-sized object, and space for extra weight, all of which are not readily available on most aircraft. Perhaps even more importantly, there are also extensive security checks and regulations that any new part of an aircraft must be subject to before it can be installed on the aircraft. This all leads to substantial added cost. Therefore, at present it is more costeffective to do postflight model calculations to determine the route doses. Obviously, it is crucial to determine the adequacy of such modeling. 1of9

2 [4] With the aims of providing precise validation of current models and much needed investigations into the possible effects of solar energetic particle (SEP) events, January 2000 saw the start of a collaborative project involving the Mullard Space Science Laboratory, Virgin Atlantic Airways, the UK Civil Aviation Authority, and the National Physical Laboratory. The project entails performing dose measurements on Virgin Atlantic aircraft around the world using TEPCs manufactured by Far West Technology. To date, measurements have been made on over 700 flights, with dose information being recorded every minute. In particular, we are able to show how the radiation levels vary during the flight in response to changes in the aircraft s location in the geomagnetic field and altitude and to the solar activity effects upon cosmic ray fluxes and the intensity of solar energetic particles. The project s aims include validation of radiation dose models currently used by many airlines to assess aircrew exposure and evaluation of variations in the cosmic radiation levels due to solar activity, in particular, those effects not accounted for by the models. Evaluating solar particle events in the atmosphere is particularly difficult because of the lack of in-flight measurements of such events. The low frequency and unpredictable nature of SEP events and the discontinuous manner of flying (a lot of time spent on the ground) mean that only a few such events have been measured at aircraft altitudes. Most notable are the measurements made on board Concorde in September and October 1989 when a number of large SEP events occurred. The measurements were made with the cosmic radiation effects and activation monitor designed to monitor radiation effects of concern to electronics. The measurements made on board Concorde recorded instantaneous neutron flux increases of up to a factor of 10 and flight average increases of up to a factor of 6 [Dyer et al., 2003]. Measurements made during ground level event (GLE) 60 on 15 April 2000 have also been made using a silicon-based spectrometer. The results showed a Prague to New York flight route dose increase of 44% over the same flight made during quiet conditions [Spurný and Dachev, 2003]. [5] Here we present TEPC measurements taken on board a flight from London to Hong Kong that took off 12 hours after the onset of the large 14 July 2000 SEP event. The measurements are compared with those taken during a flight on the same route a week later and with the results obtained from the CARI-6 atmospheric radiation dose model. The comparisons show no increased dose for the flight during the SEP event. Using observations of the SEP event at ground level neutron monitors along the flight route together with satellite measurements from SAMPEX and GOES, we provide an explanation as to why no increased dose was detected during the flight. 2. Instrumentation and Model 2.1. TEPC [6] The in-flight radiation dose measurements were taken using a prototype Hawk TEPC that was manufactured by Far West Technology, Goleta, California. The TEPC is a proportional counter 5 inches in diameter. The filling gas is lowpressure propane that simulates tissue such that the passage of a charged particle through the gas will deposit the same amount of energy as in 2 mm of tissue [Taylor et al., 2002]. This thickness is sufficiently small that the distribution of deposited energies can be interpreted as a track-averaged lineal energy transfer spectrum (more commonly called a lineal energy spectrum), particularly in the case of cosmic radiation, where the particle energies in question are reasonably high. The instrument also provides a measure of the total dose equivalent by the appropriate application of quality factors and has been calibrated in a number of neutron and photon fields [Taylor et al., 2002], including the Conseil European pour la Recherché Nucleaire-European Union high-energy reference field facility [Mitaroff and Silari, 2002]. Furthermore, the spectrum can be separated into nucleonic and gamma ray components by virtue of their different ionization densities in small volumes. In summary, the TEPC is a particularly good instrument for the dosimetry of cosmic radiation fields, as it is sensitive to all components of the cosmic radiation spectrum that can have an impact on the health of tissue [Lewis et al., 2001; Taylor et al., 2002] Ground Level Neutron Monitors [7] Ground level neutron monitors measure predominantly the neutron, proton, and muon components of a particle cascade generated by the interaction of a primary cosmic ray with the atmosphere [Hughes and Marsden, 1966]. These secondary particles finally interact with the lead casing surrounding the ground level detector. The number of interactions is then measured through the reaction product neutrons. The ground level neutron monitor stations provide an indication of the energy and intensity of primary cosmic rays and SEPs. The Earth s magnetic field acts as an energy (momentum) selector, providing maximum shielding at the magnetic equator with a cutoff rigidity of 17 GV and becoming nil at magnetic latitudes above 60 [Simpson, 2000]. The rigidity of a cosmic ray particle is defined as R = pc/ze in units of GeV per charge denoted as GV, where p is particle momentum, c is the speed of light in a vacuum, Z is the atomic number, and e is the charge. The atmosphere then provides a further barrier. It is suggested that a SEP energy spectrum with more than 100 protons (cm 2 s 1 sr 1 ) at energies greater than 100 MeV at the top of the atmosphere is required to generate a GLE. However, the intensity of the event will ultimately depend on the hardness of the SEP spectrum GOES 10 Energetic Particle Sensor and High-Energy Proton and Alpha Detector [8] The GOES 10 satellite is in a geosynchronous orbit that is at an altitude of 35,800 km (6.6 R E from the center of the Earth). It is located in the outer zone of the Earth s radiation belts but is still well within the trapping region which typically extends out to 10 R E on the dayside of the Earth. At this altitude the geomagnetic cutoff is nil for our purposes. Here we use the energetic particle sensor (EPS) measurements of protons >10 MeV and >100 MeV and the high-energy proton and alpha detector (HEPAD) measurements from the following proton energy channels: MeV, MeV, MeV, and >850 MeV [NASA, 1996] SAMPEX [9] SAMPEX was launched in July 1992 into a low Earth orbit with an altitude of km, an 82 inclination, 2of9

3 about CARI is available at AAM-600/Radiation/600radio.html.) Figure 1. First several hours of the 14 July 2000 solar energetic particle (SEP) event as seen by SAMPEX proton/ electron telescope (PET). Note the sharpness of the geomagnetic cutoffs and the great variability of the SEP intensity over the polar caps. and an orbital period of 1 1/2 hours [Baker et al., 1993]. Thus SAMPEX traverses both polar caps 16 times per day, each time measuring the geomagnetic cutoff as a function of rigidity and the uniformity of the SEPs over the polar cap. The SAMPEX satellite looks upward in the polar regions along the geomagnetic field lines and thus measures solar particles destined to impact the upper atmosphere. The proton/electron telescope (PET) [Cook et al., 1993] aboard SAMPEX measures the energy spectra of protons from 18 to 250 MeV and helium nuclei from 18 to 350 MeV nucleon 1 of solar, interplanetary, and galactic origins and the energy spectra of solar flare and precipitating electrons from 0.4 to 30 MeV. Here we use an energy channel that primarily measures protons >85 MeV. [10] Figure 1 shows the first several hours of the 14 July 2000 SEP event as seen by SAMPEX. Note the sharpness of the geomagnetic cutoffs and the great variability of the SEP intensity over the polar caps CARI-6 Model [11] CARI-6 is a radiation dose model developed by the Civil Aerospace Medical Institute, which is the medical certification, research, and education wing of the Federal Aviation Administration s Office of Aerospace Medicine. The computer program calculates the effective dose received from galactic cosmic radiation during a flight. The code takes into account variations in altitude, geographic location, and changes in the geomagnetic field and solar activity that influence the intensity of galactic cosmic rays entering the Earth s atmosphere. The solar modulation is input through the heliocentric potential, an interplanetary magnetic field index [O Brien, 1979]. The CARI-6 model is based on a database of results produced by the LUIN radiation transport code. (More information 3. Results 3.1. Solar Energetic Particle Event [12] The visible flare erupted on 14 July 2000 at 1003 UT near the center of the solar disk. The X-ray constituent peaked 24 min later, registering an X5.9 on the National Oceanic and Atmospheric Administration (NOAA) brightness scale. The most energetic of the SEPs arrived at Earth at 1030 UT [Belov et al., 2001]. This event was classified as a strong S3 class radiation storm; that is, the >10 MeV proton flux level exceeded 10 3 particles cm 2 s 1 sr 1. Interpreted biologically, NOAA s space weather scales (NOAA Space Weather Scale for Solar Radiation Storms, available at #SolarRadiationStorms) state that radiation hazard avoidance recommended for astronauts on Extra-Vehicular Activity (EVA); passengers and crew in commercial jets at high latitudes may receive low-level radiation exposure (approximately one chest x-ray). This was the largest SEP in 6 years; more intense SEP events can occur but are rare. Thus this event is of great interest for our studies of the effects of radiation on flight crews. [13] Figure 2 shows measurements made by the EPS and HEPAD instruments on GOES 10 for 14 July A significant increase in the flux of protons was observed at all energies from a factor of at >10 MeV to a factor of 18 at >850 MeV. There is a clear difference, however, between the evolution of the less energetic EPS measurements, which reach a maximum flux for >10 MeV close to 1200 UT on 15 July, and the evolution of the more energetic HEPAD measurements, which peak at the onset of the SEP event and then decay, rapidly at first and then more gradually. Also in Figure 2 the time interval during which the London to Hong Kong flight was airborne with the TEPC monitor on board is indicated. Despite missing the onset of the flare the flight is airborne when the proton flux is still high, although decreasing, at almost all energies. The one exception is the lowest-energy GOES channel, the >10 MeV energy channel, which continued to increase after takeoff. [14] Evaluation of the GOES spectrum during the event reveals a spectral slope of approximately 6 during the onset (1100 UT on 14 July 2000), which had softened to 9 by 2200 UT on 14 July 2000 when the flight was airborne. These values agree with the spectra determined by Duldig [2001] using ground level measurements. Duldig [2001] calculated a spectral slope of 6 during the rising phase that softened progressively to between 8 and 9 by 2000 UT on 14 July In-Flight Measurements [15] In order to identify possible increases in the measured dose experienced by the Virgin flight during the SEP we compare the observations with those made with the same instrument on the same flight route a week later. We also provide a comparison with the output from the CARI-6 dose model. [16] Both flights took off at 2100 UT. The first flight took off on 14 July 2000 when the >100 MeV flux was at 3of9

4 Figure 2. GOES 10 energetic particle sensor (EPS) (>10 MeV, grey dotted; >100 MeV, black dotted) and high-energy proton and alpha detector (HEPAD) ( MeV, black; MeV, dark grey; MeV, medium grey; >850 MeV, light grey) proton flux channels. The grey shaded area indicates when the London to Hong Kong flight was airborne with the tissue equivalent proportional counter ions cm 2 s 1 sr 1, 11 hours after the arrival of the solar energetic particles at Earth; the second flight took off on 21 July 2000 during a period of negligible solar energetic particles indicated by a >100 MeV flux of ions cm 2 s 1 sr 1, a factor of 10 4 less. The similarity of the two flight routes, shown in Figure 3, allows direct comparison to be made by splitting the measurements into flight levels and plotting them as a function of magnetic latitude versus longitude. This takes into account any variation in the dose measurements that would be caused by differences in location, especially altitude and magnetic latitude. [17] The CARI-6 output was generated by inputting the 14 July flight route but using the heliocentric potential for the last hour unaffected by the event before the start of the event. This was because CARI-6 is not currently designed to determine the additional effect of SEPs, and so no heliocentric potential is provided during the event for 14 July. The heliocentric potential does, however, take into account the effect of the Forbush decrease. Thus CARI-6 calculates the dose that we would expect to see if there was no SEP event on 14 July, while still accounting for the effect of the Forbush decrease. Figure 3. Summary of the London to Hong Kong flight route for 14 and 21 July indicated by the solid and dotted lines, respectively. 4of9

5 Figure 4. Total dose equivalent plotted as a function of magnetic latitude and longitude for four different flight levels. The red, blue, and black traces represent the 14 and 21 July onboard measurements and the CARI-6 results, respectively. [18] Figure 4 shows the results at four different flight levels. The 1 min integrated measurements of the dose equivalent were smoothed using a running boxcar averaging technique with a 9 min window. Throughout the 12 hour flight on 14 July 2000 the measured dose rate did not deviate significantly from either the model results of CARI-6 or the measurements made 1 week later on the same flight route but in the absence of a SEP event. Thus, despite the large increase in the energetic solar particle flux, there was not an increase in the radiation intensity at the locations and time of passage of the 14 July flight Ground Level Measurements [19] Figure 5 shows the flight route on 14 July 2000 and the location of ground-based neutron monitors near the route. These are overplotted on a contour map of the ground level cosmic ray effective cutoff rigidities measured in GV. In conjunction with Figure 5, Figure 6 shows the measurements at the three ground level monitors, Kiel, Moscow, and Alma Ata-B, with the times of the flights indicated by the shaded areas. [20] On 14 July the arrival of the highest-energy solar particles is observed at the ground stations Kiel and Moscow as a spike in the counts that rises 6 7%. Simultaneously, there is no increase observed at Alma Ata-B that is located at a lower latitude along the flight route. The ground level neutron monitors Kiel and Moscow are at latitudes of 54.3 and 54.5, which correspond to cutoff rigidities of 2.29 and 2.46 GV, respectively. In contrast, Alma Ata-B is at a latitude of 43, which corresponds to a much higher cutoff rigidity of 6.7 GV. The ground level measurements made at the early stages of the SEP event indicate that the SEP spectrum extended to a maximum energy of 6 7 GeV [Belov et al., 2001], consequently resulting in little or no observation at Alma Ata-B. Significantly, the duration of the increase due to the SEPs at Kiel and Moscow is relatively brief, only 4 6 hours, and is therefore over before the departure of the 14 July Virgin Atlantic flight. [21] There is a large decrease in the neutron measurements in Figure 6 observed at all three ground stations on 13 July, the day prior to the flare. This effect, called a Forbush decrease (named after its discoverer), is explained in more detail in section 4. It is important to 5of9

6 Figure 5. Isorigidity contours of vertical cosmic ray effective ground level cutoff in GV. Overplotted is the London to Hong Kong flight route and the location of the ground level neutron monitors at Kiel, Moscow, and Alma Ata-B [Shea and Smart, 1999]. note, however, that the decrease in the galactic cosmic ray (GCR) intensity measured by the neutron monitors is approximately the same for 14 July ( 8%) and 21 July ( 7%). Because of the depressed GCR intensity resulting from the Forbush decrease that began the day prior to the SEP event, the peak in neutron counts measured at Kiel and Moscow during the SEP event did not exceed the yearly average GCR intensity at these stations SAMPEX Observations [22] Figure 7 shows the measurements made by the PET instrument for protons >85 MeV for a 3 day period starting 13 July Figure 7 shows the particle count rate as a function of time versus magnetic latitude. The flight profile of the 14 July London to Hong Kong route can be seen overplotted in red. Note that the aircraft remained below the geomagnetic cutoff of the SEPs through the flight. Below that is a plot of the observed counts at the Apatity ground level neutron monitor over the same period. [23] At the onset of the SEP event the proton counts at SAMPEX increased by 3 orders of magnitude, from 5 6 counts s 1 up to almost counts s 1 at latitudes >55 (compare Figure 1). At the same time, the counts at Apatity ground level monitor increased by 27% but then quickly decayed over the next 6 hours, as expected from shock-accelerated SEPs. In contrast, the flux of energetic protons observed by SAMPEX was significant at high latitudes for the next 20 hours. The SAMPEX data also revealed the typical very steep cutoff in the SEP intensity at magnetic latitudes of close to 55. Unfortunately, the 14 July 2000 flight missed the onset of the SEP event and therefore the most energetic particles that were detected at both SAMPEX and Apatity ground level monitors. In addition, the flight remained at magnetic latitudes just below the geomagnetic cutoff at 55, so no measurements were made in the region where the proton flux measured by SAMPEX remained high. It should be noted that the particles measured at SAMPEX later in the event were not detected at the ground by neutron monitors. 4. Discussion [24] The onset of a significant SEP event was seen by GOES EPS and HEPAD instruments, the SAMPEX instruments, and others not discussed in this paper. Both spacecraft record a considerable increase in the energetic particle flux during the interval in which the London to Hong Kong flight was airborne. However, there was a considerable difference in the evolution of the less energetic GOES Figure 6. Measurements made at the cosmic ray ground stations at Kiel, Moscow, and Alma Ata-B. The shaded areas indicate when the 14 and 21 July flights were airborne. 6of9

7 Figure 7. (top) SAMPEX/PET particle intensity plotted as a function of time versus magnetic latitude. (The peaks seen at prior to the event are a result of instrument switching transients and so for our purposes should not be considered.) Overplotted is the London to Hong Kong flight route for 14 July 2000 (red). (bottom) Coincident measurements made at the Apatity ground level neutron monitor for comparison. EPS measurements, particularly the >10 MeV energy channel, compared with the more energetic GOES HEPAD measurements. The continued increase in the >10 MeV proton flux is contrary to the temporal behavior of the atmospheric radiation measured at ground level. This behavior is a result of the fact that the >10 MeV protons have insufficient energy to cause any noticeable contribution to the increased atmospheric radiation during SEP events. The continued rise in relatively low energy protons during a SEP event is believed to be due to the continued acceleration of the SEPs by an interplanetary shock propagating in the direction of the Earth. This difference in particle behavior as a function of energy is significant because the NOAA radiation storm alert is based on the >10 MeV proton flux, which is not necessarily indicative of the much more energetic solar energetic particle component that creates enhanced radiation at ground level and also at aircraft cruise altitudes. Perhaps a more useful measure would be based upon a more energetic solar particle spectrum that extends to the energies provided by the HEPAD instrument, although the threshold here may well be too high. During the 14 July event the GOES spectrum was calculated at the onset to be 6, but by the time the flight was airborne this had already softened to 9. [25] Following the early phase of the SEP event, the risk of receiving an increased radiation dose at aircraft altitudes becomes greatly reduced. The ground level and satellite measurements indicate a decrease in the SEP intensity and energy. These observations are confirmed by the in-flight measurements that show no increase in the atmospheric radiation levels for the 14 July 2000 Virgin Atlantic flight from London to Hong Kong. [26] However, the in-flight measurements taken on 14 and 21 July 2000 were both made during a period of globally depressed GCR intensity due to an extended Forbush decrease (FD). This FD was caused by a large coronal mass ejection (CME) that erupted on 13 July and was followed by another CME on 15 July. CMEs can increase the modulation of the GCRs entering the heliosphere, thus leading to a reduction in the intensity of GCRs on Earth. This shielding effect of the CME could mean a noticeable decrease in the radiation dose measured at aircraft altitudes from GCR. In this case the FD resulted in an 8% reduction in the GCR intensity measured at Kiel ground level neutron monitor on 14 July and later by a similar amount (7%) on 21 July The similar levels of solar modulation of the GCR intensity are also implied by the commensurate heliocentric potentials of 1218 and 1192 MV for 14 and 21 July, respectively. It is therefore fair to assume that the comparison between the two flights is valid and not biased by the effects of the FD, and thus no increased dose was observed during the SEP event. However, it should also be 7of9

8 noted that the size of the effect of the FD on the dose measured at aircraft altitude may vary from that detected at ground level. The precise effect of a FD on measurements made at aircraft altitudes is beyond the scope of this paper but will be the subject of future research. [27] In addition, it is important that the results from CARI-6 appear to provide a reasonably accurate estimate of the dose rate during the latter part of the SEP event when using the heliocentric potential value calculated immediately prior to the event. This is significant for airlines that currently use CARI-based models for the calculation of occupational exposure levels for aircrew. [28] Prior to the SEP event the FD caused an 8% decrease in the GCR intensity at Kiel which compares to an 7% increase during the early stages of the SEP event also at Kiel. Therefore the FD more than canceled out the effect of the SEP event measured at ground level at this location. A similar effect could be observed at aircraft altitudes, although because of the strong variation of dose with altitude during a SEP event the effect of the FD may be relatively smaller. [29] During the latter half of the SEP event, SAMPEX observes a substantial flux of energetic protons at latitudes above the geomagnetic cutoff at 55 of which there is no indication in the ground level measurements. It is not known to what extent, if at all, these particles may contribute to the atmospheric radiation at aircraft altitudes that may not be seen on the ground. (It should be mentioned that SAMPEX did observe protons with energies of hundreds of MeV early in the event before the aircraft took off.) In any event, the aircraft was below the cutoff throughout the flight as noted in section 3.4. [30] Comparison with GLE 42, which has been recently modeled by O Brien and Sauer [2000, 2003], suggests that during a similar time during GLE 42, only a 10% increase in the cosmic ray flux is observed at high latitudes. In addition, during SEP events, there is a very strong variation in the radiation levels with altitude. This variation, in turn, depends on the incident SEP particle spectrum which varies from one event to the next, and thus the resulting atmospheric radiation levels will also vary. Therefore it is suggested that further modeling together with more in situ measurements of SEP events is required to accurately evaluate the possible effect on the atmospheric radiation at aircraft altitudes. 5. Conclusions [31] In this paper we have studied the effect of a SEP event utilizing a number of different observing locations and detectors in order to better understand the significance of SEPs with respect to the potential risk of increased radiation exposure to aircrew and passengers. We draw the following conclusions: [32] 1. No increased radiation levels were detected during the 14 July 2000 SEP event on a Virgin Atlantic flight from London to Hong Kong that took off late on 14 July. [33] 2. An extended Forbush decrease resulted in measurements at the ground level neutron monitors along the flight route during the SEP event that did not exceed the yearly average. Forbush decrease effects would also be expected to reduce the dose received at aircraft altitudes during SEP events. [34] 3. CARI-6 model calculations using a heliocentric potential value from immediately prior to the SEP event were in good agreement with the measurements taken on the London to Hong Kong flight during the SEP event. [35] 4. Substantial fluxes of >85 MeV protons were observed destined for the top of the atmosphere at high latitudes (>55 ) for more than 2 days after the onset of the SEP event. [36] 5. The aircraft was never above the geomagnetic cutoff during the flight. Thus an increased dose to the aircraft because of the presence of SEPs is easily understood. [37] 6. A more accurate atmospheric radiation alert could be based on a much more energetic proton spectrum than is currently provided by the NOAA radiation storm warning scale. [38] 7. Low-altitude satellite measurements provide direct measurements of the geomagnetic cutoff as a function of time and thus can tell precisely whether an aircraft operates above the time-varying cutoff during a flight. Furthermore, the substantial irregularities in the polar cap intensities that frequently occur (see Figure 1) make uncertain the extrapolation of proton intensities from geostationary altitude to the upper atmosphere above an aircraft. [39] In the case of the flight on 14 July 2000 it was a combination of factors that meant no increased atmospheric radiation levels being detected. Understanding these conditions may help mitigate the impacts of SEP events on aircrew and passengers as well as on the aircraft itself. However, further in-flight measurements during SEP events are needed before we can fully assess the effect of SEPs at aircraft altitudes and consequently provide accurate annual exposure levels for aircrew. [40] Acknowledgment. Shadia Rifai Habbal thanks Clive Dyer and Keran O Brien for their assistance in evaluating this paper. References Baker, D. N., G. M. Mason, O. Figueroa, G. Colon, J. G. Watzin, and R. M. 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