Assessment of the Radiation Environment at Commercial Jet Flight Altitudes During GLE 72 on 10 September 2017 Using Neutron Monitor Data
aa r X i v : . [ phy s i c s . s p ace - ph ] M a r Assessment of the Radiation Environment atCommercial Jet Flight Altitudes During GLE 72 on 10September 2017 Using Neutron Monitor Data
A.L. Mishev and I.G. Usoskin
Space Climate Research Unit, University of Oulu, Finland. Sodankyl¨a Geophysical Observatory, University of Oulu, Finland.March 14, 2019
Abstract
As a result of intense solar activity during the first ten days of September, a ground levelenhancement occurred on September 10, 2017. Here we computed the effective dose ratesin the polar region at several altitudes during the event using the derived rigidity spectra ofthe energetic solar protons. The contribution of different populations of energetic particlesviz. galactic cosmic rays and solar protons, to the exposure is explicitly considered andcompared. We also assessed the exposure of a crew members/passengers to radiation atdifferent locations and at several cruise flight altitudes and calculated the received doses fortwo typical intercontinental flights. The estimated received dose during a high-latitude, 40kft, ∼
10 h flight is ∼ µ Sv.Keywords:Solar eruptive events, Ground level enhancement, Neutron Monitor, Rigidity spectra of solarenergetic protons,Exposure to radiation and received doses for crew members/passengersFor contact: alexander.mishev@oulu.fi
Intense solar activity took place during the first ten days of September 2017. This time periodwas among the most flare productive of the ongoing solar cycle 24. The solar active region12673 produced several X-class flares and coronal mass ejections (CMEs), leading to a moder-ate solar energetic particle (SEP) event, followed by a stronger, more energetic one, which wasobserved even at the ground level by several neutron monitors (NMs) (see the International GLEdatabase http://gle.oulu.fi ), i.e., the ground level enhancement (GLE) 72 event on Septem-ber 10, 2017. The GLE 72 was related to an X8.2 solar flare, which peaked at 16:06 UT. Itproduced a gradual SEP event. At ground level, the event onset was observed at ≈ R e l a t i ve i n c r ease , % UT [hh:mm]
DOMC DOMB SOPO SOPB FSMT
Figure 1: 15-min averaged count rate variations of NMs with maximal increases during GLE 72on September 10, 2017. The DOMC and SOPO correspond to standard NMs at Dome-C andSouth Pole stations, DOMB and SOPB correspond to the lead free NMs at at Dome-C and SouthPole stations. Data are available at http://gle.oulu.fi .in Fig. 1. The maximal count rate increases were observed by high-altitude standard and lead-free, i.e. without Pb producer, monitors at Concordia station, 75.06 S, 123.20 E, 3233 m abovesea level (a.s.l.), (DOMC/DOMB, 10–15 % above the pre-increase levels), South Pole 2820 ma.s.l. (SOPO/SOPB, 5–8 %) and at the sea level Forth Smith - FSMT ( ≈ Spurny et al. ,2002;
Vainio et al. , 2009, and references therein). While cosmic rays (CRs) of galactic originpermanently govern the radiation environment in the global atmosphere, particles of solar origin,specifically during strong SEP and GLE events can considerably enhance the flux of secondaryCR particles in the atmosphere. Primary CR particles penetrate into the atmosphere and induce acomplicated nuclear-electromagnetic-muon cascade, producing large amount of various types ofsecondary particles, viz. neutrons, protons, γ , e − , e + , µ − , µ + , π − , π + , distributed in a wide en-ergy range, which eventually deposit their energy and ionize the ambient air ( Bazilevskaya et al. ,2008;
Asorey et al. , 2018). Hence, CR particles determine the complex radiation field at flight2ltitudes (
Spurny et al. , 1996;
Shea and Smart , 2000).Assessment of the radiation exposure, henceforth exposure, at typical flight altitudes is an im-portant topic in the field of space weather (e.g.
Baker , 1998;
Latocha et al. , 2009;
Lilensten and Bornarel ,2009;
Mertens et al. , 2013;
Mertens , 2016, and references therein). Individual accumulated dosesof the cockpit and cabin crew are monitored and crew members are regarded as occupationalworkers (
ICRP , 2007;
EURATOM , 2014). The contribution of galactic cosmic rays (GCRs) to theexposure can be assessed by computations and/or using corresponding data sets for solar modula-tion and reference data (e.g.
Menzel , 2010;
Meier et al. , 2018, and references therein), consideringexplicitly the altitude, geographic position, solar activity, geomagnetic conditions (
Spurny et al. ,2002;
Shea and Smart , 2000;
Tobiska et al. , 2018). On the other hand, the assessment of exposureduring GLEs can be rather complicated, because of their sporadic occurrence and a large vari-ability of their spectra, angular distributions, durations and dynamics (
Gopalswamy et al. , 2012;
Moraal and McCracken , 2012). For a precise computation of the exposure during a GLE event,it is necessary to possess appropriate information about the energy and angular distribution of theincoming high-energy particles (
Kuwabara et al. , 2006). Such computations are performed on acase-by-case basis for individual events (e.g.
Sato et al. , 2018).Here, we computed the effective dose rates during GLE 72 at several cruise flight altitudes. Weemployed a recently developed model and procedure, the details are given in
Mishev and Usoskin (2015) and
Mishev et al. (2017). We calculated the exposure over the globe and the receiveddoses of crew members/passengers for typical intercontinental flights.
Using a model briefly described below and actual records from the global NM network, we de-rived the rigidity spectra and angular distributions of solar protons for GLE 72, see details in
Mishev et al. (2018). Estimates of GLE characteristics viz. rigidity/energy spectra and angulardistributions can be performed using the NM data and a corresponding model of the global NMnetwork response (e.g.
Shea and Smart , 1982;
Cramp et al. , 1997). In this study we employeda method described in great detail elsewhere (
Mishev et al. , 2014;
Mishev and Usoskin , 2016).Modelling of the global NM response was performed using a recently computed NM yield func-tion (
Mishev et al. , 2013;
Gil et al. , 2015;
Mangeard et al. , 2016), which results in an improvedconvergence and precision of the optimization (
Mishev et al. , 2017).Here, we assume the rigidity spectrum of the GLE particles to be a modified power law similarto
Vashenyuk et al. (2008): J || ( P ) = J P − ( γ + δγ ( P − )) (1)where J || ( P ) is the differential flux of solar particles with a given rigidity P in [GV] arriving fromthe Sun along the axis of symmetry, whose direction is defined by the geographic coordinates Ψ (latitude) and Λ (longitude), γ is the power-law spectral exponent and δγ is the correspond-ing rate of steepening of the spectrum. The pitch-angle distribution (PAD) is assumed to be asuperposition of two oppositely directed (Sun and anti-Sun) Gaussians: G ( α ) ∼ exp ( − α / σ ) + B ∗ exp ( − ( α − π ) / σ ) (2)3 -2 -1 R e l a t i ve p r o t on f l u x B F l u x [ P r o t on m - s r - s - G V - ] R [GV]A
Pitch Angle [deg]
Figure 2: GLE particles rigidity spectra and PAD during GLE 72 on September 10, 2017, detailsare given in Table 1. Time (UT) corresponds to the start of the five minute interval over which thedata are integrated. The black solid line of the left panel denotes the GCR particle flux computedon period corresponding to GLE 72 occurrence.where α is the pitch angle, i.e., the angle between the charged particle’s velocity vector and thelocal magnetic field direction, σ and σ are parameters corresponding to the width of the PAD,and B corresponds to the contribution of the particle flux arriving from the anti-Sun direction.The rigidity spectrum and PAD are derived by minimizing the functional form F which is thesum of squared differences between the model ∆ N i N i mod . and measured ∆ N i N i exp . relative increases ofNMs: F = m ∑ i = "(cid:18) ∆ N i N i (cid:19) mod . − (cid:18) ∆ N i N i (cid:19) exp . (3)over m NM stations, where ∆ N i and N i are the the count rate increase due to solar protons and thepre-event background counts due to GCRs of the i -th NM, respectively. Herein, the minimiza-tion of F is performed using a variable regularization similar to that proposed by Tikhonov et al. (1995) employing the Levenberg-Marquardt method (
Levenberg , 1944;
Marquardt , 1963). The4oodness of the fit is based on residual D (Equation 4) (e.g. Himmelblau , 1972;
Dennis and Schnabel ,1996). D = r ∑ mi = h(cid:16) ∆ N i N i (cid:17) mod . − (cid:16) ∆ N i N i (cid:17) meas . i ∑ mi = ( ∆ N i N i ) meas . (4)During the analysis, the background due to GCRs was averaged over two hours before theevent’s onset, and the Forbush decrease started, on September 7, 2017, was explicitly consideredin our analysis. Here we present the derived SEP characteristics, expanding the time intervalreported in Mishev et al. (2018). The derived rigidity spectra of GLE particles were found to berelatively hard during the event onset (see Fig.2a) for a weak event and a softening of the spectrathroughout the event was derived (e.g.
Mishev et al. , 2017, 2018). The derived spectral indexafter the event onset is in very good agreement with other estimates (e.g.
Kataoka et al. , 2018).After 17:15 UT the energy distribution of the GLE particles was described by a pure power-lawrigidity spectrum. In addition, it was recently shown that this event was softer at high energiesthan average GLEs, but revealed hard spectrum at low energies (e.g.
Cohen and Mewaldt , 2018).The angular distribution of the high energy solar particles broadened out throughout the eventand was wide, except for the event onset (see Fig.2b). We assumed an isotropic SEP flux forconservative assessment of the exposure similarly to
Copeland et al. (2008). The derived spectraand angular distributions will be integrated into the GLE database (
Tuohino et al. , 2018).
For the calculation of the effective dose rates during GLE 72 we employed a recently devel-oped numerical model, which is based on pre-computed effective dose yield functions from high-statistics Monte Carlo simulations. These yield functions are the response of ambient air at agiven altitude h above sea level as the effective dose to a mono-energetic unit flux of primary CRparticle entering the Earth’s atmosphere.The effective dose rate at a given atmospheric altitude h a.s.l. induced by primary CR particlesis given by the expression: E ( h , T , θ , ϕ ) = ∑ i Z ∞ T cut , i ( P cut ) Z Ω J i ( T ) Y i ( T , h ) d Ω ( θ , ϕ ) dT , (5)where P cut is the local geomagnetic cut-off rigidity, Ω is a solid angle determined by the angles ofincidence of the arriving particle θ (zenith) and ϕ (azimuth), J i ( T ) is the differential energy spec-trum of the primary CR at the top of the atmosphere for nuclei of type i (proton or α − particle) and Y i is the corresponding yield function. The integration is over the kinetic energy above T cut , i ( P cut ) ,which is defined by P cut for a nuclei of type i . The full description of the model with the corre-sponding look-up tables of the yield functions at several altitudes a.s.l. and comparison withreference data is given elsewhere ( Mishev and Usoskin , 2015).Here we computed the effective dose rate during GLE 72 using newly derived SEP spectraand angular distributions on the basis of NM data (details are given in Section 2) and Eq. 5.5
000 4000 6000 8000 10000 12000 14000 160000510152025 E ff ec t i ve do se r a t e [ S v h - ] Altitude a.s.l. [m]
Figure 3: Computed maximal effective dose rate as a function of altitude a.s.l. during the mainphase of GLE 72 on September 10, 2017. The dashed lines encompass the 95 % confidenceinterval .The exposure during GLE events is defined as a superposition of the GCRs and SEPs contribu-tions. The radiation background due to GCR was computed by applying the force field modelof galactic cosmic ray spectrum (
Gleeson and Axford , 1968;
Burger et al. , 2000;
Usoskin et al. ,2005) with the corresponding parametrization of local interstellar spectrum (e.g.
Usoskin et al. ,2005;
Usoskin and Kovaltsov , 2006), where the modulation potential is considered similar to
Usoskin et al. (2011). For the computation of the exposure we do not consider the depressionof GCRs due to the Forbush decrease, started on September 7, 2017. This results in a conserva-tive approach for the contribution of GCRs to the exposure with eventual overestimation of thebackground exposure. Accordingly, the characteristics of energetic solar protons used in Eq. 5were taken from Fig.1. The flux of incoming GLE particles was assumed to be isotropic, whichis consistent with the derived angular distribution and allows one to assess conservatively theexposure (e.g.
Copeland et al. , 2008).In this way we computed the effective dose rate during GLE 72 at several typical for cruiseflight altitudes, namely 30 kft (9 100m), 35 kft (10 670m), 40 kft (12 200m) and 50 kft (15 200m)6.s.l.. The effective dose rate was estimated also at high-mountain altitude of about 3000 and5000 m a.s.l. using the yield functions by
Mishev (2016). These computations were performedfor a high-latitude region with a low cut-off rigidity P cut < µ Sv.h − at altitude of 50 kft a.s.l., 11–13 µ Sv.h − at alti-tude of 35 kft a.s.l. and about 10 µ Sv.h − at altitude of 30 kft a.s.l., during the main phase of theevent, i.e., between 17:00 and 18:30 UT. During the late phase of the event (after 21:00 UT), theexposure decreases to roughly 20 µ Sv.h − , 12 µ Sv.h − and about 10 µ Sv.h − at altitudes of 50,35 and 30 kft a.s.l., respectively. The contribution of solar protons to the exposure considerablydecreases during the late phase of the event.The distribution of the exposure over the globe is determined by the cut-off rigidity, whichis computed here using a combination of Tsyganenko 1989 (external) ( Tsyganenko , 1989) andIGRF (internal) (
Langel , 1987) geomagnetic models. This combination allows one to computestraightforwardly the cut-off rigidity with a reasonable precision (
Kudela and Usoskin , 2004;
Kudela et al. , 2008;
Nevalainen et al. , 2013). An example of the distribution of the exposureas a function of the geographic coordinates for altitude of 50 kft a.s.l. during the main phaseof GLE 72 is given in Fig. 4. The distribution of the effective dose rate reveals a maximum atpolar and sub-polar regions and rapidly decreases at regions with higher cut-off rigidity. Similarcomputations were performed for lower cruise flight altitudes, the results are presented in Fig. 5(35 kft a.s.l.) and Fig. 6 (30 kft a.s.l.). Computations for the late phase of the event depict sim-ilar distributions of the exposure, but with lower values. Those results are valid for the polarregions, while at low latitudes there is no notable change of the expected exposure, which is dueto GCRs. Moreover, even a slight increase of the exposure at low latitudes is expected, becauseof the recovery of the Forbush decrease, but not considered here.The exposure decreases significantly as a function of increasing cut-off rigidity. Below 30 kft,as well as at regions with P cut ≥ µ Sv on a flight fromHelsinki (HEL), Finland to Osaka (KIX), Japan (departure time 17:10 UT, 9h 30m duration,altitude 40 kft), 110 µ Sv from Helsinki to New York–JFK (departure time 15:20 UT, 8h 40mduration, altitude 40 kft), respectively. Here, We do not consider change of the flight altitudeduring the ascending and the landing phase in order to conservatively assess the exposure. Inboth cases, the flight routes are along the great circle. Despite the shorter HEL–JFK flight, one7
60 120 180 240 300 360-90-60-300306090
Sv.h -1 L a t i t ud e ( N o r t h ) Longitude (East)
Figure 4: Distribution of the effective dose rate as a function of the geographic coordinates ataltitude of 50 kft due to high energy GLE and GCR particles during the main phase of GLE 72on September 10, 2017.would receive larger exposure, mostly because of the polar route. In addition, the HEL–JFK flightis during the main phase of the event, while HEL–KIX flight is during the main and late phase ofthe event, because of the later departure, according the actual flight information.These results related to radiation environment during GLE 72 are compared with other similarestimates (e.g.
Copeland et al. , 2018;
Kataoka et al. , 2018;
Matthi¨a et al. , 2018). A good agree-ment, in the order of 10–14 %, at altitude of 50 kft with the exposure reported by
Copeland et al. (2018) is achieved. At lower levels the difference increases to 40–55 % at altitude of 40 kft andto 75 % at altitude of 35 kft, respectively. In all cases our model reveals greater exposure. Thedifferences are consistent with recent reports (e.g.
B¨utikofer and Fl¨uckiger , 2013, 2015). Theyare most likely due to the slightly different SEP spectra derived using NM data (our analysis),compared to GOES data analysis (e.g.
Copeland et al. , 2018).8
60 120 180 240 300 360-90-60-300306090
Sv.h -1 L a t i t ud e ( N o r t h ) Longitude (East)
Figure 5: Distribution of the effective dose rate as a function of the geographic coordinates ataltitude of 35 kft due to high energy GLE and GCR particles during the main phase of GLE 72on September 10, 2017. 9
60 120 180 240 300 360-90-60-300306090
Sv.h -1 L a t i t ud e ( N o r t h ) Longitude (East)
Figure 6: Distribution of the effective dose rate as a function of the geographic coordinates ataltitude of 30 kft due to high energy GLE and GCR particles during the main phase of GLE 72on September 10, 2017. 10
Summary and Discussion
In this study we presented reconstruction of rigidity spectrum and PAD of solar energetic protonsduring the GLE 72 using data from the global neutron monitor network. Using the reconstructedspectrum we assessed the exposure for crew members/passengers at several typical cruise flightaltitudes in a polar region, assuming a conservative isotropic approach of the GLE particles an-gular distribution. We also conservatively calculated the received doses for two typical inter-continental flights: HEL– KIX (departure time 17:10 UT, 9h 30m duration, altitude 40 kft) andHEL–JFK (departure time 15:20 UT, 8h 40m duration, altitude 40 kft). We conclude that duringa weak GLE event such as GLE 72 on September 10, 2017, the upper limit of the radiation expo-sure over a single flight is about 100 µ Sv, with contribution of GCRs of about 60–65 µ Sv, doesnot represent an important space weather issue. Usually, the pilots receive annually more than theannual general public limit of 1 mSv (e.g.
EURATOM , 2014), with the majority receiving around3 mSv (e.g.
Bennett et al. , 2013). However, the exposure during GLEs should be monitored. Thepresented results can be compared with other similar estimates.The exposure at cruise flight altitudes during strong SEP events can be significantly enhancedcompared to quiet periods. It is a superposition of contributions of GCRs and SEPs. As a result,during strong SEP events and GLEs, crew members/passengers may receive doses well above thebackground level due to GCRs (e.g.
Matthi¨a et al. , 2009;
Tuohino et al. , 2018). While the back-ground exposure due to GCRs can be assessed by computations and/or on the basis of appropriatemeasurements, the estimation of the exposure due to high energy SEPs is rather complicated andit is performed retrospectively. Occurring sporadically, GLEs differ from each other in spectraand duration, and are therefore usually studied case by case. Deep and systematic study of the ex-posure during GLEs provides a good basis for further assessment of space weather effects relatedto accumulated doses at aviation flight altitudes and allows one to compare and adjust possibleuncertainties in the existing methods and models in this field.
Acknowledgements
This work was supported by the Academy of Finland (project No. 272157, Center of ExcellenceReSoLVE and project No. 267186). French-Italian Concordia Station (IPEV program n903 andPNRA Project LTCPAA PNRA14 00091) is acknowledged for support of DOMC/DOMB stationsas well as the projects CRIPA and CRIPA-X No. 304435 and Finnish Antarctic Research Pro-gram (FINNARP). We acknowledge neutron monitor database (NMDB) and all the colleaguesand PIs from the neutron monitor stations, who kindly provided the data used in this analysis,namely: Alma Ata, Apatity, Athens, Baksan, Dome C, Dourbes, Forth Smith, Inuvik, Irkutsk,Jang Bogo, Jungfraujoch, Kerguelen, Lomnicky ˇStit, Magadan, Mawson, Mexico city, Moscow,Nain, Newark, Oulu, Peawanuck, Potchefstroom, Rome, South Pole, Terre Adelie, Thule, Tixie.The NM data are available on-line at International GLE database http://gle.oulu.fi .11 eferences
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