SPACE WEATHER, VOL. , NO. , PAGES 1–9,

Aircraft radiation exposure during a high-energy solarenergetic particle event in October 2003

Christopher J. Mertens,1 Brian T. Kress,2 Michael Wiltberger,3 Steve R.Blattnig1, Tony S. Slaba4, Stanley C. Solomon3, and M. Engle5

Abstract.We present initial results from the Nowcast of Atmospheric Ionizing Radiation for Avi-

ation Safety (NAIRAS) model during the Halloween 2003 superstorm. The objective ofNAIRAS is to produce global, real-time, data-driven predictions of ionizing radiation forarchiving and assessing the biologically harmful radiation exposure levels at commercialairline altitudes. We have conducted an analysis of radiation exposure during a high-energysolar energetic particle (SEP) event in October 2003. We find that radiation received bya conceptus during a commercial polar flight may have reached as much as 70% of theInternational Commission on Radiological Protection recommended limit. Our analysisalso quantified the important influence of the storm-time and quiet-time magnetosphericmagnetic field on SEP atmospheric radiation exposure. The Halloween 2003 superstormis an ideal event to study magnetospheric influences on atmospheric radiation exposuresince this event was accompanied by a major magnetic storm and one of the largest ofthe last solar cycle. We find that neglecting geomagnetic storm effects during SEP eventscan underestimate the high-latitude radiation dose from 40% to over a factor of three,depending on the flight-path relative to the magnetosphere open-closed boundary.

1. Introduction

An important atmospheric state variable, driven by spaceweather phenomena, is the ionizing radiation field. The twosources of atmospheric ionizing radiation are: (1) the ever-present, background galactic cosmic rays (GCR), with ori-gins outside the solar system, and (2) the transient solarenergetic particle (SEP) events, which are associated witheruptions on the Sun’s surface lasting for several hours todays with widely varying intensity. Quantifying the levels ofatmospheric ionizing radiation is of particular interest to theaviation industry since it is the primary source of human ex-posure to high linear energy transfer (LET) radiation. Thisradiation is effective at directly breaking DNA strands inbiological tissue, or producing chemically active radicals intissue that alter the cell function, both of which can lead tocancer or other adverse health effects. Consequently, thereis increased concern about potential health outcomes amongpassengers and crew in the commercial and private aviationsector [Wilson et al., 2005a, 2003]. As a result, there is grow-ing need for a capability to predict the real-time radiationlevels at airline altitudes in order to: (1) provide a con-tinuous assessment of ionizing radiation exposure for track-ing aircrew exposure levels, (2) to provide time-critical data

1NASA Langley Research Center, Hampton, Virginia, USA.2Dartmouth College, Hanover, New Hampshire, USA.3National Center for Atmospheric Research, High Altitude

Observatory, Boulder, Colorado, USA.4Old Dominion University, Norfolk, Virginia, USA.5Hamline University, Saint Paul, Minnesota, USA.

Copyright by the American Geophysical Union.

Paper number .0148-0227/09/$9.00

during SEP events for aviation radiation risk evaluation andmitigation, and (3) provide an archived database of radia-tion exposure for assessing the impact of ionizing radiationon the global air transportation system as a whole, espe-cially in view of the current and future exponential growthin the number of polar routes. The polar region receives thelargest quantity of radiation because the shielding providedby Earth’s magnetic field rapidly approaches zero near thepole.

Currently under development is the Nowcast of Atmo-spheric Ionizing Radiation for Aviation Safety (NAIRAS)model [Mertens et al., 2008]. The goal of NAIRAS is to pro-vide a data-driven, global, real-time prediction of ionizingradiation exposure from the surface to approximately 100km. NAIRAS has adopted, as far as possible, the meteoro-logical weather forecasting paradigm of combining physics-based forecast models with data assimilation techniques.The physics-based model we use to transport the ionizingradiation through the atmosphere and calculate the dosimet-ric quantities is the High charge (Z) and Energy TRaNsport(HZETRN) code [Wilson et al., 1991]. While the quantityof observations relevant to radiation exposure predictions iscurrently too sparse to apply data assimilation techniquesper se, nevertheless, we utilize as much real-time measure-ment data as possible to characterize the sources of ionizingradiation incident at the top of the atmosphere and the ma-terial properties of the atmosphere necessary to understandthe transport and transmutations of the sources of ionizingradiation through the atmosphere.

There are a number of models currently in use for cal-culating GCR radiation exposure at aircraft altitudes. TheCARI-6 model utilizes a database of transport calculationsgenerated by the deterministic LUIN code for a wide varietyof geographic locations, altitudes, and solar activity levels[O’Brien et al., 2003, 1998]. The EPCARD model is based

1

2 MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE

on a similar approach, but uses the Monte Carlo FLUKAcode for the transport calculations [Schraube et al., 1999].PC-AIRE is a semi-empirical model based on fits to mea-surement data [Lewis et al., 2002]. Other aircraft radiationexposure models are described in the recent European Radi-ation Dosimetry Group report [Lindborg et al., 2004]. Cur-rently, the above models calculate SEP atmospheric radia-tion exposure post-storm on a case-by-case basis, althoughPC-AIRE incorporated low-earth orbit measurements to de-velop a simple extrapolation to SEP events [Lewis et al.,2002]. Recently, Copeland et at. [2008] calculated adult andconceptus aircraft exposure rates for 170 SEP events foryears 1986-2008 using the Monte Carlo MCNPX transportcode.

The main differences that distinguish the NAIRAS modelfrom the models discussed above are the following. Thephysics-based deterministic HZETRN transport calculationsused in NAIRAS are continuously updated using real-timemeasurements of the space radiation environment and of at-mospheric density versus altitude. Furthermore, both GCRand SEP atmospheric radiation exposures predictions areincluded in real-time. And finally, dynamical geomagneticeffects are routinely included in the NAIRAS radiation ex-posure calculations.

In this paper we focus solely on SEP aircraft radiationexposure and analyze a high-energy SEP event during theHalloween 2003 superstorm. We have two primary objec-tives: (1) assess the total exposure received for a few typ-ical high-latitude commercial flight paths during the SEPevent, and (2) diagnose the influence of geomagnetic stormeffects on SEP atmospheric radiation exposure. The firstobjective is important because the International Commis-sion on Radiological Protection (ICRP), the EnvironmentalProtection Agency (EPA), and the Federal Aviation Ad-ministration (FAA) consider crews of commercial aircraft asradiation workers [McMeekin, 1990; ICRP, 1991]. However,aircrew are currently not monitored for radiation exposure,nor are passengers aware of any potential radiation risks.Reproductive disorder, including prenatal injury, are a par-ticular concern [Lauria et al., 2006; Waters et al, 2000; andAspholm et al., 1999]. We find that approximately 70% ofthe ICRP prenatal exposure limit may have been reachedfor a representative polar route during the Halloween 2003high-energy SEP event.

The second objective is also important because we findthat geomagnetic storm effects have a profound effect onSEP atmospheric radiation exposure. The Halloween 2003superstorm is an ideal event to study geomagnetic effectssince this event contained a major magnetic storm and oneof the largest of the last solar cycle. The geomagnetic stormeffects are the consequence of solar wind-magnetosphere in-teractions that weaken the effective shielding of low-energySEP incident particles by the Earth’s magnetic field [Kresset al., 2009]. The effect of a weakened magnetic field, inresponse to a geomagnetic storm, is a significant increase inSEP atmospheric ionizing radiation. This result underscoresthe need for continued space environment measurements andaccurate models of solar wind-magnetosphere interactions.

The outline of the rest of the paper is as follows. In sec-tion 2 we describe the measurement data and space environ-ment models used to characterize the incident SEP particleflux at the top of the atmosphere, and the meteorologicaldata and models used to characterize atmospheric density.Section 3 describes the transport of the incident SEP par-ticle flux through the atmospheric and the calculation ofradiation exposure. The high-energy SEP event during theHalloween 2003 superstorm is analyzed in section 4. A sum-mary and conclusions are presented in section 5.

2. Data Input and Space EnvironmentModels

NAIRAS model predictions of atmospheric SEP radiation

exposure are driven by measurements from the atmosphere

and from space. Real-time satellite ion flux measurements

are used to derive the SEP proton and alpha fluence spectra

incident at the top of the atmosphere. The geomagnetic field

(internal field plus magnetospheric contributions) filters the

incident SEP fluence spectra by deflecting the lower energy

particles back out to space. This spectral filtering effect is

quantified by a canonical variable called the geomagnetic

cutoff rigidity. Satellite measurements of solar wind dynam-

ical pressure and the interplanetary magnetic field (IMF)

are used to characterize the magnetospheric contributions

to the cutoff rigidity. Global meteorological measurements

combined with data assimilation and forecasts models are

used to predict atmospheric depth as a function of altitude.

In the subsections below, we describe the data input and

space environment models used to derive: (1) the incident

SEP fluence spectra, (2) the geomagnetic cutoff rigidity, and

(3) atmospheric depth as a function of altitude.

2.1. SEP Fluence Spectra

The current understanding of SEP fluence is that the

energy spectrum is a result of injected particle seed pop-

ulations that are stochastically accelerated in a turbulent

magnetic field associated with a CME-driven interplanetary

shock [Tylka and Lee, 2006]. An ansatz that represents the

fluence spectrum for this shock acceleration mechanism was

given by Ellison and Ramaty [1985], where the spectrum has

the form

d2J

dEdΩ= Ca E−γa exp (−E/E0). (1)

The differential-directional fluence on the left hand side of

(1) has units of (cm2-sr-MeV/n)−1, and the energy (E) has

units of MeV/n (i.e., MeV/nucleon). The constant Ca is

related to the injected seed population far upstream of the

shock. The power-law energy dependence of the spectrum

is due to shock acceleration of the seed population by ran-

dom first-order Fermi acceleration (scattering) events in a

turbulent magnetic field, with the power index (γa) related

to the shock compression ratio. The exponential turnover

in (1) represents high-energy limits to the the acceleration

mechanism, such as escape from the shock region. Using the

above ansatz, the three parameters (Ca, γa, and E0) can be

determined by fitting (1) to fluence measurements.

Recently, Mewaldt et al. [2005] found that the Ellison-

Ramaty spectral form failed to fit NOAA/GOES fluence

measurements at the highest energy channels during the

Halloween 2003 SEP events. To circumvent this deficiency,

Mewaldt et al. proposed using a double power-law spectrum.

The low-energy spectrum is assumed to follow the Ellison-

Ramaty ansatz. The high-energy spectrum is assumed to

have a power-law energy dependence with a different power

index, such that

d2J

dEdΩ= Cb E−γb . (2)

MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE 3

The power-law ansatz in (1)-(2) can be merged into onecontinuous spectrum by requiring that the fluence spectrain (1)-(2) and their first derivatives are continuous at themerge energy. The result is given by the expression below:

d2J

dEdΩ= C E−γa exp (−E/E0),

E ≤ (γb − γa)E0 (3)

= C E−γb[(γb − γa)E0]

(γb−γa) exp (γb − γa)

,

E > (γb − γa)E0. (4)

Physically, the double power-law spectrum in (3)-(4) repre-sents SEP sources from two different injected seed popula-tions. For example, the low-energy spectrum, with γa powerindex and the e-folding energy E0, are likely associated withcorona (solar wind) seed populations while the high-energyspectrum, with γb power index, are likely associated withflare suprathermal seed populations [Tylka et al., 2005].

The NAIRAS model initially assumes the double power-law form in (3)-(4) for the SEP fluence spectrum and derivesthe fit parameters (C, γa, γb, and E0) by a non-linear least-square fit to differential-directional fluence measurements.The spectral fitting algorithm uses a Marquardt-Levenbergiteration technique [Brandt, 1999]. If the double power-lawspectrum fails to converge to the measurement data, the fit-ting procedure is restarted and the Ellison-Ramaty spectralform is assumed.

NAIRAS utilizes available real-time measurements of pro-ton and alpha differential-directional particle flux (cm2-sr-sec-MeV/n)−1 for the SEP spectral fitting described above.Fluence is obtained by time-integrating the particle fluxmeasurements. Low-energy proton data are obtained fromthe Electron, Proton, and Alpha Monitor (EPAM) instru-ment onboard the NASA Advanced Composition Explorer(ACE) satellite [Gold et al., 1998]. EPAM is composedof five telescopes and we use the LEMS120 (Low-EnergyMagnetic Spectrometer) detector, which measures ions at120 degrees from the spacecraft axis. LEMS120 is theEPAM low-energy ion data available in real-time, for rea-sons described by Haggerty et al. [2006]. The other pro-ton channels used in the SEP spectral fitting algorithmare obtained from NOAA’s Geostationary Operational En-vironmental Satellite (GOES) Space Environment Monitor(SEM) measurements. The Energetic Particle Sensor (EPS)and the High Energy Proton and Alpha Detector (HEPAD)sensors on GOES/SEM measure differential-directional pro-ton flux [Onsager et al., 1996]. We also generate additionaldifferential proton measurement channels by taking differ-ences between the EPS integral proton flux channels. Thechannels used to derive SEP alpha fluence spectra are alsoobtained from EPS measurements. We use 5-minute aver-aged ACE and GOES data to derive the SEP fluence spectra.

Figure 1 shows the time variation of a representative set ofproton flux measurements used to derive SEP fluence spec-tra for the Halloween 2003 storm period. The two panelsshow the GOES-11 EPS and HEPAD proton flux spectra,and the integral proton flux measurements, respectively. Bydefinition, a SEP event occurs when the >10 MeV integralproton flux exceeds 10 proton flux units (pfu ≡ cm−2 sr−1

sec−1). The SEP event threshold is denoted by the hori-zontal line on the integral proton flux panel. There are atotal of five SEP events during the Halloween 2003 stormperiod, which are denoted by the vertical lines in both pan-els in Figure 1. Different line styles are used to bound eachof the five events, and the event number is shown between

the vertical lines. Note that the onset of event 3 doesn’tfollow the conventional SEP threshold definition. It is clearfrom the integral proton flux that two events overlap: event3 arrives before event 2 decreases below the SEP thresholdlevel. However, there is an important distinguishing fea-ture between the two events. That is, the beginning of ourdefinition of event 3 is accompanied by a sudden increase inhigh-energy protons associated with the arriving SEP event,as noted by the sudden increase in the 510-700 MeV differ-ential proton flux measurements in Figure 1. Partitioningthe simultaneous SEP events 2 and 3 into separate eventsis useful for our study, since the high-energy portion of thedifferential proton flux distribution penetrates deeper in theatmosphere.

In this study, we will analyze atmospheric radiation expo-sure during SEP event 3, because of the comparatively largehigh-energy component of the proton spectrum associatedwith this event. In order to isolate geomagnetic effects, wederived the spectra shown in Figure 2. The horizontal linesin Figure 2 are the event-averaged differential-directional flu-ence measurements. The width of the horizontal lines cor-respond to the energy width of the measurement channels.The black lines are the proton and alpha fluence spectraderived using the double power-law spectrum and fittingtechnique describe above. The event-averaged fit parame-ters derived for SEP event 3 are given in Table 1. Thus,equations (3)-(4) and the parameters in Table 1 character-ize the incident SEP fluence spectra used in the analysis ofatmospheric exposure in this paper.

2.2. Geomagnetic Cutoff Rigidities

The geomagnetic field provides a form of momentumshielding by deflecting lower energy charged particles backout to space. The minimum momentum per unit charge thata vertically incident particle can have and still reach a givenaltitude above the earth is called the vertical geomagneticcutoff rigidity. The cutoff rigidity is a canonical variable forspecifying the minimum access energy of incident chargedparticles (SEP particles in this study) for transport throughthe atmosphere. Once the cutoff rigidity is known, the min-imum access energy is determined for each incident particleof charge Z and mass number A through the relativistic en-ergy equation, such that

E =

[√R2 (Z/A · amu · c2)2 + 1− 1

]· amu · c2, (5)

where E is kinetic energy per nucleon (MeV/n), R is verticalgeomagnetic cutoff rigidity (MV), c is the speed of light invacuum, and amu = 931.5 MeV/c2 (atomic mass unit).

The cutoff rigidity is determined by numerical solution ofcharged particle trajectories in the geomagnetic field [Smartand Shea, 2005, 1994]. The NAIRAS cutoff rigidities are cal-culated from code developed by the Center for IntegratedSpace Weather Modeling (CISM) at Dartmouth College.The CISM-Dartmouth geomagnetic cutoff model can be runusing several different empirical and physics-based mod-els [Kress et al., 2004]. In particular, the specification ofthe geomagnetic field due to Earth’s internal field source isprovided by the International Geomagnetic Reference Field(IGRF) model [Langlais and Mandea, 2000]. The magneto-spheric current systems are also important contributors toEarth’s total geomagnetic field. The real-time dynamical

4 MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE

response of the magnetospheric magnetic field to solar windconditions and IMF can be provided by the semi-empiricalT05 model [Tsyganeko and Sitnov, 2005], or by the Lyon-Feder-Mobarry (LFM) global MHD simulation code [Lyonet al., 2004]. Currently, the T05 model provides more ac-curate cutoff rigidities than the LFM MHD model, as de-termined by comparisons with satellite observations duringa Halloween 2003 geomagnetic storm [Kress et al., 2009].Thus, for the present work, the simulated real-time geomag-netic cutoff rigidities are calculated using the T05 model,and using the IGRF model for comparison.

Figure 3 shows the vertical cutoff rigidity over the north-ern hemisphere for three different models of the geomagneticfield. The left column is cutoff rigidity computed using theIGRF field. Since total flight-path exposure at aviation al-titudes do not change significantly (< ∼ 1%) for cutoffs lessthan 0.05 GV, we set the cutoffs to zero at geographic loca-tions poleward of the 0.05 GV contour (see the bold-white0.05 GV color contour in Figure 3). The middle columnin Figure 3 shows the cutoff rigidities computed using theT05 field under geomagnetically quiet conditions, October28 (0200 UT), prior to the onset of the Halloween 2003 SEPevent 3. One can see that even during magnetically quietconditions, the cutoff rigidities predicted from the T05 fieldare lower than predicted from the IGRF field, and the po-lar cap region (i.e., inside the bold-white 0.05 GV contourin Figure 3) is expanded to lower latitudes. A weaker fieldpredicted by the T05 model, compared to IGRF, is due inpart to the diamagnetic effect of the magnetospheric ringcurrent included in the T05 model. Lower cutoff rigiditiescorrespond to less momentum shielding and higher radiationexposure levels. The right column in Figure 3 shows thecutoff rigidities during peak geomagnetic storm conditions,October 29 (2100 UT), during SEP event 3. The cutoffs arelower at all latitudes compared to the two previous simula-tions, and the polar cap region has expanded to much lowerlatitudes than during the magnetically quiet period. Thesegeomagnetic effects are discussed in more detail in section 4.

2.3. Atmospheric Depth-Altitude

The atmospheric itself provides shielding from incidentcharged particles. The shielding of the atmosphere at agiven altitude depends on the overhead mass. Sub-dailyglobal atmospheric depth is determined from pressure versusgeopotential height and pressure versus temperature dataderived from the National Center for Environmental Predic-tion (NCEP) / National Center for Atmospheric Research(NCAR) Reanalysis 1 project [Kalnay et al., 1996]. TheNCAR/NCEP Reanalysis 1 project uses a state-of-the-artanalysis/forecast system to perform data assimilation usingpast data from 1948 to the present. The data products areavailable 4x daily at 0, 6, 12, and 18 UT. The spatial cover-age is 17 pressure levels in the vertical from approximatelythe surface (1000 hPa) to the middle stratosphere (10 hPa),while the horizontal grid is 2.5 x 2.5 degrees covering theentire globe.

NCEP/NCAR pressure versus geopotential height datais extended in altitude above 10 hPa using the Naval Re-search Laboratory Mass Spectrometer and Incoherent Scat-ter (NRLMSIS) model atmosphere [Picone et al., 2002].NCEP/NCAR and NRLMSIS temperatures are smoothlymerged at 10 hPa at each horizontal grid point. NRLM-SIS temperatures are produced at 2 km vertical spacing

from the altitude of the NCEP/NCAR 10 hPa pressuresurface to approximately 100 km. The pressure at theseextended altitudes can be determined from the barometriclaw using the NRLMSIS temperature profile and the knownNCEP/NCAR 10 hPa pressure level, which assumes the at-mosphere is in hydrostatic equilibrium and obeys the idealgas law. Finally, the altitudes and temperatures are linearlyinterpolated in log pressure to a fixed pressure grid from1000 hPa to 0.001 hPa, with six pressure levels per decade.The result from this step is pressure versus altitude at eachhorizontal grid point from the surface to approximately 100km.

Atmospheric depth (g/cm2) at each altitude level andhorizontal grid point is computed by vertically integratingthe mass density from a given altitude to the top of theatmosphere. The mass density is determined by the idealgas law using the pressure and temperature at each alti-tude level. The result from this step produces a 3-D grid-ded field of atmospheric depth. Atmospheric depth at anyspecified aircraft altitude is determined by linear interpola-tion along the vertical grid axis in log atmospheric depth.Figure 4 shows the atmospheric pressure over the northernhemisphere at 11 km on October 29, 2003 (2100 UT). This isthe atmospheric data used in the exposure rate calculationsin section 4.

3. SEP Transport and Dosimetry

In the previous section, we described the incident SEPfluence, cutoff rigidity, and atmospheric depth as a functionof altitude. In this section, we describe the transport of theincident SEP fluence through the atmosphere and the sub-sequent absorbed dose in tissue from the ionizing radiationfield.

SEP atmospheric transport is described by a coupled sys-tem of linear Boltzmann equations, which are derived on thebasis of conservation principles [Wilson et al., 1991]. Thetransport equation for the spectral fluence φj(x,Ω, E) forparticle type j is given by

Ω • ∇Φj(x,Ω, E) =∑

k

∫σjk(Ω,Ω′, E, E′)Φ(x,Ω′, E′)dΩ′dE′

−σj(E)Φ(x,Ω, E) (6)

where σj(E) and σjk(Ω,Ω′, E, E′) are the target mediummacroscopic cross sections. The σjk(Ω,Ω′, E, E′) aredouble-differential particle production cross sections thatrepresent all processes by which type k particles moving indirection Ω′ with energy E′ produce a particle of type jmoving in direction Ω with energy E, including radioactivedecay processes. The total cross section σj(E) of the targetmedium for each incident particle type j is

σj(E) = σj,at(E) + σj,el(E) + σj,r(E), (7)

where the first term refers to collisions with atomic electrons,the second term refers to elastic ion-nucleus scattering, orelastic neutron-neutron scattering, and the third term con-tains all relevant nuclear reactions. The corresponding dif-ferential cross sections are similarly ordered.

The coupled SEP transport equations in (6) are solved inthe NAIRAS model using NASA Langley Research Center’sdeterministic HZETRN code. HZETRN is used in a widevariety of radiation transport applications: e.g., the calcu-lation of dosimetric quantities for assessing astronaut risk

MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE 5

to space radiations on the International Space Station (ISS)and the Space Transportation System (STS) Shuttle, andfor design and validation of the Crew Exploration Vehicle(CEV) under the Constellation program, including realisticspacecraft and human geometry [Slaba et al., 2008a; Badaviet al., 2007a, 2005; Wilson et al., 2005b]. Extensive sum-maries of HZETRN laboratory and space-flight verificationand validation are found in recent reports by Badavi et al.,[2007a], Nealy et al., [2007]; and Wilson et al. [2004, 2005c].

For SEP proton and alpha particles incident at the topof the atmosphere, solutions to six coupled transport equa-tions are obtained: one for neutrons and five for light-ions (protons, deuterons, tritons, helium-3, and helium-4). A recent update to HZETRN includes a directionally-coupled forward-backward low-energy neutron transport al-gorithm, which is important for atmospheric radiation trans-port [Slaba et al., 2008b].

The energy deposited in a target medium by the radiationfield is the dose, which is given by

D(x) = K

∫

Ω

∫ ∞

0

S(E)Φ(x,Ω, E)dΩdE. (8)

In the above equation, S(E) is the stopping power(Mev/g/cm2) and K is a unit conversion factor (1.602x10−9)to convert dose to units of Gray (1 Gy = J/kg). Radiationhealth risk and the probability of biological damage dependnot only on the absorbed dose, but also on the type and en-ergy of the radiation causing the dose. This is taken into ac-count by weighting the absorbed dose by a factor related tothe quality of the radiation. The weighted absorbed dose hasbeen given the name dose equivalent by the ICRP [ICRP,1991]. The unit of dose equivalent is the Sievert (Sv). Doseequivalent (H(x)) is defined in terms of the LET dependentquality factor Q, such that

H(x) =

∫

L

Q(L)D(x, L)dL, (9)

where L is LET, which can be approximated by the stoppingpower in units of keV/um; D(x, L) is the spectral dose dis-tribution in terms of LET, and Q(L) is the LET-dependentquality factor. We use the Q-L relationship defined by theICRP 60 report [ICRP, 1991] to compute dose equivalentfor the projectile particles, combined with an approximatemethod of computing dose equivalent for the target frag-ments in tissue [Wilson et al., 1991]. The target fragmentsare all possible nuclear disintegration products from the tis-sue nuclei after being struck by a passing projectile particle.

The relationship between the probability of biologicaldamage and dose equivalent is found to also depend on theorgan or tissue irradiated. A further dosimetric quantity,called the effective dose, is defined to include the relativecontributions of each organ or tissue to the total biologicaldetriment caused by radiation exposure. The effective doseis the sum of weighted dose equivalents in all the organs andtissues. The organ/tissue weighting factors are given in theICRP 60 report [ICRP, 1991]. All recommended ICRP ra-diation exposure limits reported in this paper are defined interms of effective dose.

The assumptions and simplifications to our transport cal-culations and dosimetric predictions presented in the nextsection are briefly described here. We assume that the in-cident SEP fluence are isotropic in the calculation of dose

equivalent in (8)-(9), and the minimum access energies forthe incident fluence are determined by the vertical cut-off rigidities described in section 2.2. Future updates toNAIRAS will include directional effects on the cutoff rigidi-ties and their subsequent influence on the radiation expo-sure. The atmospheric dose equivalent is calculated at zerodepth in tissue after the radiation field has been transportthrough the atmosphere. This approach ignores the in-fluence of the aircraft structure on the radiation field anddoes not include the organ/tissue weighting factors usedin calculating effective dose. None of the existing atmo-spheric radiation models include aircraft structure effectsoperationally; however, some cases studies have been done.The aircraft may reduce the free-atmosphere radiation expo-sure by perhaps ∼ 10% for GCR exposure [Battistoni et al.,2005; Copeland et al., 2008] and less than 1% for SEP events[Copeland et al., 2008]. Our use of dose equivalent at zerodepth in tissue as a dosimetric quantity will overestimate theeffective dose for an adult – effective dose is the dosimetricquantity used in the ICRP recommended exposure limits –but may be a reasonable approximation for assessing radia-tion exposure in a conceptus, since the maternal abdomenprovides no effective shielding to the conceptus [Chen et al.,2005; Bartlett, 2004]. Future improvements to NAIRAS willinclude transport through the aircraft fuselage and the cal-culation of effective dose, utilizing the methodologies devel-oped for astronaut radiation risk assessment [Badavi et al.,2007b and Slaba et al., 2008a].

4. Analysis of Halloween 2003 Event

In this section we present our predictions of SEP doseequivalent rates and accumulated dose equivalent along rep-resentative high-latitude commercial routes during the Hal-loween 2003 SEP event 3 [10/29 (2100 UT) - 10/31 (2400UT)]. The incident SEP fluence and meteorological data arefixed in time in our calculations, which are given by theevent-averaged fluence and atmospheric depth-altitude datashown in Figures 2 and 4, respectively. On the other hand,we allow the cutoff rigidity to vary in time along the flighttrajectories, according to the magnetospheric magnetic fieldresponse to the real-time solar wind and IMF conditions[Kress et al., 2009]. The physics-based HZETRN code isused to transport the incident SEP fluence through the at-mosphere and calculate the dosimetric quantities. A uniquefeature of NAIRAS is the computational efficiency of the de-terministic HZETRN code and the ability to predict globalatmospheric radiation exposure from a physics-based trans-port code. Moreover, a primary objective of this paper isto diagnose geomagnetic storm effects on SEP atmosphericradiation exposure. The geomagnetic effects have not beenquantified sufficiently, and we find they have a profound ef-fect on SEP atmospheric radiation exposure. In a futurereport, we will allow the SEP fluence, cutoff rigidity, andatmospheric depth-altitude to all vary according to the real-time data input.

4.1. Global SEP Dose Distribution

Global SEP atmospheric radiation exposure are obtainedfrom a pre-computed database. The dose equivalent ratesare calculated on a fixed 2-D grid in atmospheric depth andcutoff rigidity. The atmospheric depth grid extends fromzero to 1300 g/cm2, and the cutoff rigidity grid extends fromzero to 19 GV. Both grids have non-uniform spacing with

6 MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE

the highest number of grid points weighted toward low cutoffrigidities and tropospheric atmospheric depths. The real-time cutoff rigidities are computed on the same 2.5 x 2.5horizontal grid as the NCEP/NCAR meteorological data.The pre-computed dose equivalent rates are interpolated tothe real-time cutoff rigidity and atmospheric depth specifiedat each horizontal grid point.

Figure 5 shows global snapshots of atmospheric doseequivalent rates over the northern hemisphere polar regionfor the Halloween 2003 SEP event 3. The dose equivalentrates are shown at three altitudes and for three differentmagnetic field models used in the cutoff rigidity simulations.The left column shows exposure rates using the IGRF field.The middle column shows exposure rates computed for ageomagnetically quiet time prior to the onset of SEP event3 using the T05 field (October 28, 2003, 0002 UT). Theright column shows the exposure rates using the T05 fieldat the peak of the geomagnetic storm (October 29, 2003,2100 UT) during SEP event 3. A typical cruising altitudefor a commercial high-latitude flight is 11 km. Overlaid onthe 11 km dose equivalent rate altitude surface are greatcircle routes for three representative high-latitude commer-cial flights: London, England (LHR) to New York, NewYork (JFK); Chicago, Illinois (ORD) to Stockholm, Sweden(ARN), and a combination of two great circle routes fromChicago, Illinois (ORD) to Beijing, China (PEK).

There are a number of striking features to be noted fromFigure 5. First, the representation of the geomagnetic fieldhas a significant influence on SEP atmospheric radiation ex-posure. Comparing the left and middle columns of Figure 5shows that even during geomagnetically quiet periods, themagnetospheric magnetic field weakens the overall geomag-netic field with a concomitant increase in radiation levels.This is seen as a broadening of the open-closed magneto-spheric boundary in the T05 quiet field compared to theIGRF field. The cutoffs are zero in the region of open ge-omagnetic field lines. Thus, dose equivalent rates based onthe IGRF field are underestimated even for magneticallyquiet times. During strong geomagnetic storms, as shown inthe third column of Figure 5, the area of open field lines arebroadened further, bringing large exposure rates to muchlower latitudes. Dose equivalent rates predicted using theIGRF model during a large geomagnetic storm can be sig-nificantly underestimated. The expansion of the polar re-gion high exposure rates to lower latitudes, due to geomag-netic effects, is quantified by calculating hemispheric aver-age dose equivalent rates from 45N to the pole. This isdenoted by “avg” in Figure 5. At 11 km, there is roughly a14% increase in the global-average dose equivalent rate usingT05 quiet-field compared to IGRF. During the geomagneticstorm, there is a ∼ 57% increase in the global-average doseequivalent rate using T05 storm-field compared to IGRF.

A second important feature to note in Figure 5 is thestrong altitude dependence due to atmospheric shielding.The exposure rates are very low at 5 km, independent ofgeomagnetic field model used. At 15 km, the exposure ratesare significantly higher than at 11 km. Figure 5 shows thatthe SEP dose equivalent rates increase (decrease) exponen-tially with increasing (decreasing) altitude. The SEP expo-sure rate altitude dependence is a fortunate feature for theaviation community, since radiation exposure can be signif-icantly reduced by descending to lower altitudes. Privatebusiness jets will receive more radiation exposure than com-mercial aircraft if mitigation procedures are not taken, sincebusiness jet cruising altitudes are roughly 12-13 km. The al-titude dependence of the SEP exposure rates is quantified

in Figure 5 by showing the maximum dose equivalent rate

at each altitude, which is the exposure rate at zero cutoff

rigidity (i.e., in the polar region of open geomagnetic field

lines). The maximum is denoted “max” in Figure 5. The

exposure rate increases on average by 170% per km between

5 km and 11 km. Between 11 km and 15 km, the exposure

rate increases on average by approximately 65% per km.

4.2. Dose on High-Latitude Flights

Before calculating radiation exposure along specified

flight paths, it’s constructive to examine a sample of dose

equivalent rate profiles at different cutoff rigidities from our

pre-computed database previously described for SEP event

3. Figure 6 shows SEP dose equivalent rates as a function of

atmospheric depth for cutoff rigidities from zero to 2.5 GV.

This figure clearly shows the exponential dependence of SEP

exposure on both cutoff rigidity and atmospheric depth. The

vertical lines indicate constant exposure rates necessary to

receive a total exposure of 1, 5, 10, and 20 mSv on a 8 hour

flight. A typical international, high-latitude flight is 8 hours.

The rationale for choosing the total exposure identified with

the vertical lines is as follows [Wilson et al., 2003]: 20 mSv

is the ICRP annual occupational radiation worker limit, 10

mSv is the National Committee on Radiological Protection

(NCRP) annual occupational exposure limit, 5 mSv is the

NCRP occasional public exposure limit, and 1 mSv is the

ICRP annual public and prenatal exposure limit. The cutoff

rigidities at high-latitudes are less than 1 GV. The typical

commercial airline cruising altitudes correspond to an atmo-

spheric depth between ∼ 200-300 g/cm2. Consequently, one

can see from Figure 6 that it’s possible for passengers on

high-latitude commercial flights during the Halloween 2003

SEP events to approach or exceeded the ICRP public and/or

prenatal radiation exposure limit. We examine this qualita-

tive assessment in more detail below.

Figure 7 shows the cutoff rigidities and dose equivalent

rates for the three representative high-latitude flights men-

tioned in the previous subsection. The left column is the

cutoff rigidities along the flight paths and the right column

is the corresponding dose equivalent rates along the flight

paths. The cutoff rigidities include both latitude and time-

dependent variations along the flight paths. The variations

of the exposure rates along the flight paths include latitudi-

nal variations in both atmospheric depth and cutoff rigidity.

The temporal variations in cutoff rigidity also map into the

variations of the exposure rates along the flight path. The

top row shows results for the LHR-JFK flight, while the

middle and bottom rows show results for the ORD-ARN

and ORD-PEK flights, respectively. Each panel in Figure 7

shows cutoff rigidities and corresponding dose equivalent

rates using the IGRF field (green lines) and T05 storm-

field (red lines) in the cutoff calculations. The largest differ-

ences in flight-path cutoff rigidities between IGRF and T05

storm-field models are for the LHR-JFK flight. The entire

LHR-JFK flight path is near the magnetosphere open-closed

boundary and is most sensitive to perturbations in cutoff

rigidity due to geomagnetic effects. Consequently, the expo-

sure rates along the LHR-JFK flight are most sensitive to

geomagnetic effects. The ORD-PEK polar route is the least

sensitive to geomagnetic suppression of the cutoff rigidity,

since most of the flight path is across the polar cap region

MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE 7

with open geomagnetic field lines. The influence of geomag-netic storm effects on the ORD-ARN flight is intermediatebetween a typical polar route and a flight along the NorthAtlantic corridor between the US and Europe.

The total dose equivalent along the three representativehigh-latitude flight paths are given in Table 2. The firstthree columns show total dose equivalent computed fromthe three models of the geomagnetic field used in this study:IGRF, T05 quiet-field, and T05 storm-field. The last threecolumns show various ratios between the total dose equiva-lent computed from the different geomagnetic field models.There are three major points to be noted from these results.One, the total dose equivalent predicted for the ORD-PEKpolar route for SEP event 3 during the Halloween 2003 stormis ∼ 70% of the ICRP public/prenatal effective dose limitof 1 mSv. Moreover, the ORD-ARN flight received a doseequivalent of 50% of the ICRP public/prenatal effective doselimit. As stated earlier, our use of dose equivalent is an over-estimation of the effective dose on which the ICRP limits arebased. On the other hand, our calculated dose equivalentmay be a reasonable approximation for conceptus exposure.Two, using the IGRF field to compute the cutoff rigiditycan underestimate the total dose equivalent from ∼ 40% forpolar routes to over a factor of three for flights along theNorth Atlantic corridor. Third, even for SEP events with-out an accompanying geomagnetic storm, using the IGRFfield in cutoff rigidity simulations can underestimate the to-tal dose equivalent by roughly 20-40% for US flights intoEurope.

Paramount to developing accurate and reliable aircraftradiation exposure models are continued comparisons withon-board aircraft dosimetry measurements. Getley et al.,[2005a,b] report dosimetry measurements on Quantas Flight107 from Los Angeles, California to New York, New Yorkon October 29, 2003. The measurement data were analyzedand reported in terms of ambient dose equivalent, a rea-sonable measurement proxy for effective dose. Most of thisflight was confided to geographic locations with geomagneticshielding sufficient to significantly suppress the SEP expo-sure relative to the background GCR exposure. The ringcurrent was beginning to build up – thereby weakening thegeomagnetic shielding and increase the exposure rate – asthe Quantus flight descended into New York. Thus, theQuantus flight missed the large polar SEP exposures thatwere extended to mid-latitudes by the geomagnetic stormeffects. These measurements are well-suited for testing theability of NAIRAS to model the reduction in the GCR expo-sure due to the Forbush decrease observed by the worldwidenetwork of ground-level neutron monitors during, which weplan to do in the future.

5. Summary and Conclusions

The NAIRAS model is a physics-based, global, data-driven nowcast of atmospheric ionizing radiation, with ra-diation exposure rates calculated from the surface to ap-proximately 100 km. At project completion, which is antic-ipated in mid-2011, NAIRAS will provide real-time predic-tions of radiation exposure for both background GCR andSEP events. In this paper we have conducted an analysisof atmospheric ionizing radiation exposure associated witha high-energy SEP event during the Halloween 2003 stormperiod. The two main objectives of this paper are the fol-lowing: (1) provide an estimate of the exposure received onrepresentative high-latitude commercial flights during the

high-energy SEP event, and (2) to diagnose the influenceof geomagnetic storm effects on SEP atmospheric radiationexposure. High-latitude flight paths are the routes most sus-ceptible to significant SEP radiation exposure, since the cut-off rigidity rapidly approaches zero near the magnetosphereopen-closed boundary.

The result from our first objective is that the radiationexposure during a representative polar flight was relativelyhigh, as well as other high-latitude flights. We find thata conceptus may have received as much as 70% of recom-mended ICRP prenatal exposure limit in one polar flight.SEP exposure rates increase (decrease) exponentially withincreasing (decreasing) altitude. Thus, SEP aircraft radi-ation exposure can be significantly reduced by descendingto lower altitudes. Business jet cruising altitudes are higherthan commercial aircraft. Consequently, private jets flyingsimilar high-latitude routes as the commercial airlines willreceive substantially more radiation if mitigation proceduresare not enacted. NAIRAS real-time radiation exposure ratepredictions during SEP events will enable the aviation com-munity to make informed decisions concerning radiation riskevaluation and reduction.

To achieve our second objective of diagnosing the geomag-netic storm effects on SEP radiation exposure, we calculatedthe atmospheric dose equivalent rates using event-averagedincident SEP proton and alpha fluence spectra and a staticatmospheric depth-altitude relation, while the cutoff rigiditywas calculated both statically and dynamically. The staticcutoff rigidities were simulated using the IGRF field. Thedynamic cutoff rigidities were simulated using the T05 field,which was allowed to respond to the real-time solar wind andIMF conditions. The dynamic cutoff rigidities were com-puted during a geomagnetically quiet period prior to thehigh-energy SEP event and during the peak of the geomag-netic storm associated with the high-energy SEP event. Thekey results of this study are as follows. One, ignoring solarwind-magnetosphere interactions during a strong geomag-netic storm in the calculation of cutoff rigidity can under-estimate the total exposure by approximately 40% to overa factor of three. Two, even during geomagnetically quietconditions, ignoring solar wind-magnetosphere interactionsin the computed cutoff rigidities can underestimate the to-tal exposure for flights along the North Atlantic corridor byroughly 20-40%. To achieve more accurate assessments ofaircraft radiation dose, the magnetospheric influence on thecutoff rigidities must be included routinely in atmosphericradiation exposure predictions.

In the future, we will assess the reliability and feasibil-ity of predicting the real-time geomagnetic cutoff rigiditiesusing the physics-based LFM MHD magnetic fields. TheLFM MHD code may be run as a stand alone model or cou-pled with other geospace models currently under develop-ment within CISM. For example, the LFM magnetosphericmagnetic fields may be coupled with the Thermosphere-Ionosphere Nested Grid (TING) model [Wang et al., 2004]and/or with the Rice Convection Model (RCM) [Toffoletto,2004], which models the ring current. The semi-empiricalT05 model provides more accurate cutoff rigidities than thestand alone LFM MHD model, as determined by compar-isons with satellite observations during a Halloween 2003geomagnetic storm [Kress et al., 2009]. This is mainly dueto the lack of a full kinetic description of the ring current inthe MHD model, which typically causes the LFM fields to be

8 MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE

too high. We anticipate that the fully coupled LFM-RCM-TING model currently under development will significantlyimprove the simulations of cutoff rigidities compared to thestand along LFM MHD model. Furthermore, the physics-based LFM-RCM-TING model will be able to incorporateshort time-scale dynamics not included in empirical mag-netospheric magnetic field models. When the code develop-ment within CISM reaches sufficient maturity, we will assessthe influence of short time-scale magnetospheric dynamicson the atmospheric ionizing radiation field using the fullycoupled LFM-RCM-TING model.

Additionally, our future efforts will build upon this workin five other ways. One, we will study directional effectson the cutoff rigidities and subsequent radiation exposurerates. Two, we will model the aircraft fuselage and fullbody organ and tissue exposure. Third, we will allow theSEP fluence and atmospheric pressure to vary with timeaccording to the real-time input data. Fourth, we will ana-lyze additional storm periods to further quantify the relativecontributions of SEP fluence, geomagnetic activity, and me-teorological variability on atmospheric radiation exposure.And five, we will compare our predictions with previous andfuture on-board aircraft dosimetric measurements.

Acknowledgments. This work was supported by the NASAApplied Science Program. CJM is is grateful to Xiaojing Xu(SSAI, Inc.) for improving the graphics and for helpful discus-sions with Dr. Barbara Grajewski (National Center for Occupa-tional Safety and Health).

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Steve R. Blattnig, NASA Langley Research Center, 2 WestReid St., Mail Stop 188E, Hampton, Virginia 23681-2199, USA(Steve.R.Blattnig@nasa.gov)

Tony S. Slaba, NASA Langley Research Center, 2 WestReid St., Mail Stop 188E, Hampton, Virginia, 23681-2199, USA(Tony.C.Slaba@nasa.gov)

Stanley C. Solomon, National Center for Atmospheric Re-search, High Altitude Observatory, 3080 Center Green, Boulder,Colorado, USA (stans@ucar.edu)

(Received .)

10 MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE

Figure 1. GOES-11 proton flux measurements: top panelshows the EPS/HEPAD differential-directional flux, thebottom panel shows the EPS integral-directional flux. Thecolor lines represent different GOES-11 proton channels andthe energy ranges of the channels are specified in the legend.The different vertical line styles bound the five SEP events,which are also numbered in both panels. See text for details.

Figure 2. Event-Averaged SEP fluence spectra for event 3 [10/29/2003 (2100 UT) - 10/31/2003 (2400 UT)].

Figure 3. Simulated vertical geomagnetic cutoff rigidityshown over the northern hemisphere in October 2003. Thecutoff rigidities in the left column were calculated using theIGRF model. The cutoffs in the middle column were calcu-lated using the T05 model during a geomagnetically quietperiod. Cutoff rigidities in the right column were calculatedusing the T05 model during the largest geomagnetically dis-turbed period of SEP event 3. Also shown are the magneticlatitude circles and the meridians at 0, 6, 12, and 18 mag-netic local time.

Figure 4. NCAR/NCEP Reanalysis 1 pressure levels at 11km corresponding to the date/time of the largest geomag-netically disturbed period of SEP event 3 (10/29/2003, 2100UT). Also shown are the magnetic latitude circles and themeridians at 0, 6, 12, and 18 magnetic local time.

Figure 5. Dose equivalent rates computed during Hal-loween 2003 SEP event 3. The three columns correspondto exposure rates calculated using the geomagnetic cutoffrigidities and magnetic field models shown in the previousfigure. The three rows are exposure rates calculated at dif-ferent altitudes. In each graph, the hemispheric averagedose equivalent rate (uSv/hr) is indicated by the value nextto “avg.” The maximum exposure rate is indicated by thevalue next to “max”. See text for definition of “avg” and“max.” Also shown are the magnetic latitude circles andthe meridians at 0, 6, 12, and 18 magnetic local time

Figure 6. Event-averaged dose equivalent rate for Hal-loween SEP event 3 [(10/29/2003, 2100 UT) - 10/31/2003(2400 UT)] as a function of atmospheric depth for variousgeomagnetic cutoff rigidities. Different vertical lines indicateconstant exposure rates required to reach the correspondingtotal exposure levels indicated in the legend for a 8-hr flight.

Figure 7. Geomagnetic cutoff rigidities (left column) anddose equivalent rates (right column) calculated during Hal-loween 2003 SEP event 3 along three representative flightpaths for a cruising altitude of 11 km. The green line rep-resents cutoff rigidities and exposure rates calculated usingthe IGRF model. The red lines represent cutoffs and expo-sure rates computed using the T05 model during the periodof largest geomagnetic activity of event 3. Note that thedose equivalent rate calculated using the IGRF model forthe LHR-JFK flight is scaled by a factor of two.

MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE 11

Table 1. Event-Averaged [10/29/03 (2100 UT) - 10/31/03 (2400 UT)] fit parameters

Particle C γa γb E0

Proton 8.565x106 1.191 3.493 30.78Alpha 3.782x106 1.682 4.992 105.3

Table 2. Total Dose Equivalent Using Different Geomagnetic Cutoff Models

Flight Dose Eq. Dose Eq. Dose Eq. Dose Eq. Ratio Dose Eq. Ratio Dose Eq. Ratio

Path T05Sa T05Qb IGRFc T05S/IGRF T05S/T05Q T05Q/IGRF(mSv) (mSv) (mSv)

JFK-LHR 0.194 0.080 0.060 3.23 2.42 1.33ORD-ARN 0.482 0.320 0.233 2.07 1.51 1.37ORD-PEK 0.687 0.577 0.485 1.42 1.19 1.19

a Cutoff rigidities computed from Tsyganenko (T05) storm fieldsb Cutoff rigidities computed from Tsyganenko (T05) quiet-time fieldsc Cutoff rigidities computed from IGRF fields

12 MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE

MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE 13

14 MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE

808060

4020

0

0

6

12

18

00.05

0.20.4

0.60.8

1 2

3 4

5 6

7 8

910

1112

1314

1516

1718

GV

808060

4020

0

0

6

12

18

8060

4020

0

06

1218

Vertical G

eom

agn

etic Cu

toff R

igid

ity

IGR

F F

ieldT

05 Qu

iet Field

Oct 28, 2003 U

T=2

T05 S

torm

Field

Oct 29, 2003 U

T=21

MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE 15

October 29 2003 (21 U

T)

NC

AR

/NC

EP

Reanalysis 1: P

ressure at 11 km

8060

4020

0

06

1218

Pressure (hP

a)

185.0

190.0

195.0

200.0

205.0

210.0

215.0

220.0

225.0

230.0

235.0

240.0

245.0

250.0

255.0

260.0

265.0

270.0

275.0

16 MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE

IGR

F F

ield

8070

6050

0

12

18

80

70

60

500

12

18

80

70

60

500

12

18

Lo

nd

on

, UK

- New

Yo

rk, NY

Ch

icago

, IL - S

tockh

olm

, SW

Ch

icago

, IL - B

eijing

, Ch

ina

max:210.3

max: 81.1

max: 8.0

avg: 49.2

avg: 19.2

avg: 2.1

10 20 30 40 50 60 70 80 90

100

110

120

130

uS

v/hr

T05 Q

uiet F

ieldO

ct 28, 2003 UT

= 2

8070

6050

0

12

18

80

70

60

500

12

18

80

70

60

500

12

18

Lo

nd

on

, UK

- New

Yo

rk, NY

Ch

icago

, IL - S

tockh

olm

, SW

Ch

icago

, IL - B

eijing

, Ch

ina

max:210.3

max: 81.5

max: 8.1

avg: 57.4

avg: 22.0

avg: 2.4

10 20 30 40 50 60 70 80 90

100

110

120

130

uS

v/hr

T05 S

torm

Field

Oct 29, 2003 U

T= 21

8070

6050

6

12

18 80

70

60

50

6

12

18 80

70

60

50

6

12

18 L

on

do

n, U

K - N

ew Y

ork, N

YC

hicag

o, IL

- Sto

ckho

lm, S

WC

hicag

o, IL

- Beijin

g, C

hin

a

max:210.3

max: 81.5

max: 8.4

avg: 80.8

avg: 30.2

avg: 3.2

10 20 30 40 50 60 70 80 90

100

110

120

130

uS

v/hr

Do

se for H

allow

een 2003 S

EP

(10/29 (2100 UT

) - 10/31 (2400UT

))

5km

11km

15km

MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE 17

18 MERTENS ET AL.: ATMOSPHERIC SEP RADIATION DOSE