PUBLICATIONS. Space Weather. Short-term variation of cosmic radiation measured by aircraft under constant flight conditions

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1 PUBLICATIONS RESEARCH ARTICLE Key Points: We newly measured the temporal variations of cosmic radiation at 9144 m Short-term variation of cosmic radiation can be independent with the neutron monitor data Correspondence to: J. Lee, jjlee@kasi.re.kr Citation: Lee, J., U.-W. Nam, J. Pyo, S. Kim, Y.-J. Kwon, J. Lee, I. Park, M.-H. Y. Kim, and T. P. Dachev (2015), Short-term variation of cosmic radiation measured by aircraft under constant flight conditions, Space Weather, 13, , doi:. Received 16 AUG 2015 Accepted 26 OCT 2015 Accepted article online 28 OCT 2015 Published online 21 NOV The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. Short-term variation of cosmic radiation measured by aircraft under constant flight conditions Jaejin Lee 1,2, Uk-Won Nam 1, Jeonghyun Pyo 1, Sunghwan Kim 3, Yong-Jun Kwon 1,4, Jaewon Lee 5, Inchun Park 5, Myung-Hee Y. Kim 6, and Tsventan P. Dachev 7 1 Korea Astronomy and Space Science Institute, Daejeon, South Korea, 2 University of Science and Technology, Daejeon, South Korea, 3 Department of Radiological Science, Cheongju University, Cheongju, South Korea, 4 School of Space Research, Kyung Hee University, Yongin, South Korea, 5 Republic of Korea Air Force, Gyeryong, South Korea, 6 Wyle Science, Technology, and Engineering Group, Houston, Texas, USA, 7 Space and Solar-Terrestrial Research Institute, Bulgarian Academy of Sciences (SSTRI-BAS), Sofia, Bulgaria Abstract The temporal variations in cosmic radiation on aircraft under constant flight conditions were measured by a Liulin detector. Rather than a commercial long-distance aircraft, we used a military reconnaissance aircraft performing a circular flight at a constant altitude over the Korean Peninsula. At 9144 m (30,000 ft), the mean and standard deviation of the radiation dose rate (among 35 measurements) was 2.3 and 0.17 μsv/h, respectively. The experiment yielded two observational results. First, the dose rate changed over a flight time of 5 7 h; second, no strong correlation was revealed between the cosmic rays observed from the ground-based neutron monitor and the radiation doses at aircraft altitude. These observations can provide insight into the short-term variation of cosmic radiation at aviation altitudes. When discarding various negligible factors, it is postulated that the changes in the geomagnetic field and the air density still could affect the variation of cosmic radiation at aircraft altitude. However, various factors are less known about the dependence on the cosmic radiation. Therefore, investigations of possible factors are also warranted at the monitoring points of space weather. 1. Introduction Cosmic radiation was first revealed in a balloon experiment conducted between 1911 and 1913 by the Austrian scientist Victor Francis Hess [Hess, 1912; Dorman, 2004]. As the altitude increased, the radiation detector installed on the balloon measured higher radiation doses, meaning that the atmospheric radiation field is formed not by particles ejected from the ground but by particles originating from space. These radiation particles were termed cosmic rays, as suggested by Millikan and Cameron [1926]. The secondary cosmic radiation created at flight altitudes (approximately 6 18 km) is complex, contributed by both galactic cosmic rays (GCR) originating from outside the solar system and solar cosmic rays that are accelerated by solar activity and collide with the Earth s atmosphere [Reitz, 1993; Badhwar, 1997]. Aircrew and other individuals working at flight altitudes are exposed to effective annual doses of 1 6 msv, higher than those of workers exposed to industrial and medical radiation [Beck et al., 1999; Bartlett, 2004]. Accordingly, the International Commission on Radiological Protection (ICRP) has advised that aircrew be classified as occupationally exposed to radiation [International Commission on Radiological Protection (ICRP), 1991]. Furthermore, Europe has also prepared a safety standard for radiation exposure, the EURATOM/96/29 Basic Safety Standard, later replaced by the EURATOM Directive 2013/59/EURATOM. Each member of the European Union has now established and implemented regulations based on this standard [EURATOM, 1996;EURATOM, 2014; Courades, 1999]. Radiation at flight altitude is difficult to measure as it comprises a complex mix of neutrons, protons, electrons, gamma rays, and muons; moreover, its energy is more widely distributed than near-ground radiation [Ferrari et al., 2001; Roesler et al., 2002]. Therefore, radiation detectors used in usual radiation protection areas are not suitable for measuring flight-altitude radiation. Instead, specially designed silicon detectors or tissueequivalent proportional counters are required [Rossi and Zaider, 1996; Nam et al., 2015]. An intensive radiation experiment on commercial aircraft was performed in Europe in the 1990s. The radiation dose at flight altitude has been statistically predicted from the radiation doses measured by diverse groups during one solar cycle [Lewis et al., 2002; Lewis et al., 2004; Lindborg et al., 2004; Wissmann, 2006]. These achievements have led to radiation calculation codes for assessing aircrew radiation exposure. Current radiation calculation software such as CARI-6, EPCARD, SIEVERT, JISCARD, and PCAIRE are well described in Bottollier-Depois et al. [2009]. LEE ET AL. SHORT-TERM VARIATION OF COSMIC RADIATION 797

2 Figure 1. Long-term variation of cosmic radiation at 9144 m altitude, 36.5 N latitude, and 128 E longitude calculated by the CARI-6 model. The cosmic radiation derived from the measurements by the ground-based neutron monitor should be anticorrelated with the sunspot numbers representing solar activity. Figure 1a plots the long-term variation of cosmic radiation exposure level in effective dose, calculated by CARI-6 over the Korean peninsula with the solar activity represented by sunspot numbers. The cosmic radiation doses of aircrew are calculated from the measurements of neutron monitors (Figure 1b) by taking into account altitude and geographic location, and the resultant radiation doses of aircrew at 9144 m is anticorrelated with solar activity shown in Figure 1a. However, the short-term relationship between aircraft-altitude radiation doses and neutron monitor measurements has yet to be properly studied. Cosmic radiation is known to be temporarily increased by solar energetic particles (SEP) accelerated by coronal mass ejections, particularly on polar regions [O Brien et al., 1996]. However, even in the absence of SEP events, the intensity of GCRs could be modulated by solar wind and changes in the Earth s magnetic field. Variations in the density and composition of the atmosphere above 9144 m can also modify the radiation dose. As these factors can change on hourly time scales, they should introduce short-term variations in the cosmic radiation. Because cosmic radiation increases with increasing altitude and decreasing cutoff rigidity, the temporal and spatial distributions of radiation doses on long-distance aircraft are difficult to distinguish. To measure the temporal distribution of the radiation dose while excluding the spatial variation, the aircraft must fly at a constant altitude within limited latitudes and longitudes. Such an experiment was conducted in Europe through the CAATER (Coordinated Access to Aircraft for Transnational Environmental Research) campaign in May This experiment aimed to compare the radiation doses measured by different detectors. To reduce the statistical uncertainty, seven detectors were installed in an aircraft under constant flight conditions and the radiation was measured at m and m. To observe the effect of cutoff rigidity on radiation dose, researchers measured the radiation intercepted during four flights (in total) above Aalborg, Denmark (57 N, 10 E), and Rome, Italy (42 N, 12 E) [Lillhök et al., 2007; Latocha et al., 2007]. However, although the CAATER campaign yielded meaningful data, four flights are insufficient for observing the temporal changes in cosmic radiation. We measured the radiation dose by installing a radiation detector on a Korean Air Force reconnaissance aircraft flying at a specific latitude, longitude, and altitude. This paper presents the 35 dose rates measured between 10 March and 17 September 2014 and discusses the temporal changes of cosmic radiation obtained through the experiment. Real progress in mitigating radiation risk requires an active international effort toward observing atmospheric radiation weather [Tobiska et al., 2015]. 2. Experiment An Air Force reconnaissance aircraft, RC-800, flying a circular path over the Korean Peninsula, was used for the cosmic radiation measurements [Lee et al., 2014]. The aircraft took off in the early evening (8 10 pm) and LEE ET AL. SHORT-TERM VARIATION OF COSMIC RADIATION 798

3 Figure 2. Area covered during the flight (red circle) over South Korea. landed during the early morning (3 5 am), giving an approximate flight duration of 5 7 h for each flight. Although the exact flight path cannot be disclosed because the aircraft is performing military duty, we can state that the aircraft remained over South Korea, as shown in Figure 2. The flight path was a 200 km radius centered at latitude 36.5 N and longitude 128 E, and the travel speed was approximately 700 km/h. Therefore, the aircraft passed over the same sites several times during one flight, so we can ignore variations in cosmic radiation caused by the changing location of the aircraft. Because the mission of this aircraft is to collect information from North Korea, it is required to fly at a constant altitude of 9144 m (FL300). This altitude refers to the pressure altitude, which may differ from the absolute altitude depending on the state of the atmosphere. The present study examines the changing levels of cosmic radiation at constant latitude, longitude, and altitude. From these measurements, we can quantify the difference between the measurement and the values calculated from radiation code that is generally used for calculating aircrew radiation doses. Figure 3. Ambient dose equivalents measured by a Liulin detector at the FL300 flight level. Error bars show the 95% confidence intervals. LEE ET AL. SHORT-TERM VARIATION OF COSMIC RADIATION 799

4 Table 1. Ambient Dose Equivalents (H*(10)) and Count Rates Measured by the Liulin Detector at an Altitude of FL300 (9144 m) Over South Korea With the Kp Index Ascending Time (UT) Descending Time (UT) H*(10) (μsv/h) Count Rate (Count/min) Kp Index 12:00 15:00 UT 15:00 18:00 UT 18:00 21:00 UT 21:00 24:00 UT : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : We adopted the radiation detector Liulin, which is equipped with a silicon sensor. The Liulin detector measures the linear energy transfer (LET) to the Si sensor, from which the radiation dose is calculated [Spurný and Dachev, 2002]. This detector was selected for its portability in both size and operation. Because it measures the energy transmitted to Si rather than a tissue equivalent material, the Liulin detector cannot accurately measure the ambient dose equivalent. However, its consistency with models and other radiation measuring devices has been confirmed [Spurny and Dachev, 2003; Spurný and Dachev, 2005; Dachev et al., 2015]. The Liulin detector provides meaningful data for the purpose of our experiment, namely, to observe the dose changes. We are not interested in the absolute accuracy of the radiation dose. The single Liulin detector was installed in an Air Force reconnaissance aircraft, and the ambient dose equivalent and count rate were measured every 30 s. Here we analyzed the measured data downloaded after the flight and compared them with ground-based neutron monitor data collected in Moscow and Mexico City. Figure 3 presents the short-term variations of the mean dose rates, H*(10), measured by the detector. Here the radiation doses were acquired during stable altitude maintenance of the aircraft until its descent. The error bars indicate the 95% confidence intervals. Note that the short-term variation of the cosmic radiation is larger than the long-term variation caused by solar activity (cf. Figure 1). The dose rates in Figure 3, relative to observation time, are presented in Table 1. For comparison, the count rates of neutrons produced by cosmic rays and the effective dose calculated by CARI-6 and EPCARD are shown in Figure 4. The pressure-corrected neutron monitor data measured on the LEE ET AL. SHORT-TERM VARIATION OF COSMIC RADIATION 800

5 Figure 4. Cosmic ray variations measured by ground-based neutron monitors Moscow (R = 2.48; navy blue), Mexico City (R = 8.28; purple), and the effective dose rates calculated by the cosmic radiation codes CARI-6 (red) and EPCARD (blue). ground in Moscow and Mexico City were obtained from Using the geomagnetic cutoff rigidity computer program developed by Smart and Shea [2005], we calculated the approximate cutoff rigidity of the protons over Korean Peninsula as 9.96 GV at 100 km altitude. The Fortran code is available from the Community Coordinated Modeling Center archive at ftp://hanna.ccmc.gsfc.nasa.gov/pub/modelweb/cosmic_rays/cutoff_rigidity_sw/. Although the cutoff rigidity considerably differs between Moscow (R = 2.43 GV) and Mexico City (R = 8.28 GV), the overall variations in the neutron monitor data appear similar. Therefore, we assumed similar tendencies of GCR changes over the Korean peninsula, although Korea lacks a neutron monitor. Note the dose rates measured from the aircraft changed by approximately 30% from 2.13 to 2.88 μsv/h, whereas the count rates measured by the neutron monitors changed by less than 10%. In cosmic radiation codes, the intensity of GCR affects the dose rate under constant flight conditions. In both the CARI-6 and EPCARD codes, the effective dose is estimated by the monthly means of the neutron monitor measurements. Hence, both of these codes ignore the changes in cosmic radiation dose over short periods (less than one month). Although the daily variation was comparatively large, the monthly averages of the neutron monitor data were more-or-less stable throughout the observation experiment (March to September 2014). Thus, the dose rates calculated by CARI-6 and EPCARD were also stable (see Figure 4). Because significant short-term variation appeared in the neutron monitor data, we asked the following Figure 5. Relationships between count rate and ambient dose equivalents measured by a Liulin detector aboard the aircraft. LEE ET AL. SHORT-TERM VARIATION OF COSMIC RADIATION 801

6 Figure 6. (a) Dose rates and (b) count rates measured on 17 May (red) and 10 June (blue) The faint grey signals show the unprocessed original data; bold solid lines are the running averages of the data measured at 50 points in each flight. question: by applying the daily instead of the monthly average GCR in the calculation of cosmic radiation dose, can we more accurately assess the aircrew radiation exposure? Our experiment would answer this question as no in following analysis. 3. Analysis The dose rate measurements exceeded the values calculated by CARI-6 or EPCARD (shown in Figures 3 and 4). Reasoning the inconsistency between the measured and the calculated values and/or determining the more reliableresults isbeyondthescope ofthispaper,whileitis known that the Liulin detector overestimates the dose in the area of high cutoff rigidity [Ploc et al., 2010]. To approach such problems, we require more observational data and a thorough understanding of the detector and cosmic radiation code. Moreover, because the measured dose is the ambient dose equivalent (operational dose), whereas the modeled values constitute theeffective dose (committed dose), the two quantities might not be directly comparable. Thus, this paper focuses on the observed values, which vary more widely than the values calculated by the cosmic radiation code. Figure 5 plots the relationship between the dose rates and particle count rates measured by the detector. Remind that the particle count rate is independent of the particles energy, but the dose rate is calculated from the deposited energy spectra. In general, the dose rate is directly proportional to the count rate; however, the data observed on 17 May and 17 September 2014 showed different tendencies. Although the behavior is anomalous at only two points, Figure 5 indicates that cosmic radiation can affect the count rate without quantitatively changing the dose rate. Comparing the measurements on 17 May and 10 June (normal case) of 2014, the dose rates (2.52 and 2.48 μsv/h, respectively) were very similar; however, the count rates were LEE ET AL. SHORT-TERM VARIATION OF COSMIC RADIATION 802

7 Figure 7. Normalized LET spectrogram measured with Liulin on 17 May (red), 10 June (dash), 11 June (dash dot) and 20 June (dot) recorded as and 39.08, respectively. The dose rates on both days are compared in detail in Figure 6a. Excluding the difference in-flight times (the flight was 1 h longer on 17 May than on 10 June), the measured dose rates were quite similar. Figure 6b shows the count rates measured on both dates; the count rate was clearly higher on 17 May than on 10 June. The high count rate indicates that the sensor detected a large number of radiation particles, which generally accompanies an increased dose value. However, as aforementioned, the dose rates measured on 17 May and 10 June2014 were very similar, whereas the count rates were significantly different. Figure 7 shows the comparison of LET spectra between anomalous case of 17 May and three normal cases of 10, 11, and 20 June Note that the LET spectra are similar in normal cases. The values in the low LET region were higher on 17 May than on 10 June; moreover, the spectrum of the latter date exhibited high values in the high LET area. From this result, it can be interpreted that the high count rate measured on 17 May corresponds to low LET, thus contributes little to the dose rate. The data measured on 17 September 2014 show a similar trend. At this stage, we cannot clarify why the count rate occasionally increased in the low LET region. The time profiles of the dose rates were similar on the anomalous and normal dates, ruling out malfunction of the detector. Thus, we presume that the cosmic radiation temporally varies at the flight altitude. A more detailed interpretation will be possible once more observational data from various detectors have accumulated. Figure 8. GCR count rates obtained from the ground-based Mexico City monitor versus the dose rates in the current experiment. LEE ET AL. SHORT-TERM VARIATION OF COSMIC RADIATION 803

8 Figure 9. Dose rate profiles measured while in-flight on 11 June (blue), 16 June (green), and 20 June (magenta) Faint grey signals show the unprocessed original data. Above we discussed the unusual phenomenon of constant dose rate and varying count rate. We now introduce a case in which the dose rate was particularly high. The dose rate on 16 June 2014 was 2.88 μsv/h, higher than the overall mean dose rate of 2.3 μsv/h. As shown in Table 1, several of the 35 observed dose rates were above the norm most likely because of external factors rather than errors in the measurement device. To understand why the radiation changes at flight altitudes, we plot the dose rates obtained from the Liulin detector versus the GCR count rates simultaneously observed by the neutron monitor in Figure 8. Although the two data sets are uncorrelated (correlation coefficient of 0.11), this does not imply noncorrelation between the cosmic ray counts and radiation doses at the flight altitude. As is already known, the number of galactic cosmic rays entering the Earth s atmosphere increases toward the minimum of the solar cycle, increasing the radiation dose at flight altitudes [Gleeson and Axford, 1968]. However, because the number of cosmic rays changed little during our observations (from March to September 2014), the dose rate was probably fairly consistent. This suggests that the changes in the observed dose rates are contributed almost entirely by factors other than the cosmic rays observed on ground. To understand the dose rate measured on 16 June 2014, which was highest among the obtained values, we plotted the detailed dose rate profiles observed in-flight in Figure 9. Data observed on 11 and 20 June 2014 (which are similar at 2.22 and 2.25 μsv/h, respectively) are also shown for comparison. On all 3 days, the aircraft took off at approximately the same time (12:00), completed its mission, and landed around 19:00. The three data sets showed similar trends immediately after takeoff; however, the radiation dose suddenly increased between 16:00 and 18:30 on 16 June, whereas it was constant or slightly decreased on the other 2 days. Therefore, the mean dose rate during the 16 June flight was increased within a narrow timeframe (2.5 h in the late afternoon). The cause of this increase cannot be ascertained at present. We postulate two possible reasons for the short-term variation of cosmic radiation. First, changing geomagnetic field could affect the intensity of the GCR entering the Earth s atmosphere. As shown in Table 1, the geomagnetic activity was typically subdued during the measurements, and no correlation between the Kp index and radiation was observed during these quiet periods. Note that the minimum GCR energy that can enter the atmosphere, known as the cutoff energy, is very sensitive to the geomagnetic field configuration [Smart and Shea, 2005]. Considering that the GCR flux decreases exponentially with energy, a small change in the cutoff energy could trigger a large number change in the GCR flux, sufficient to be detected as variable cosmic radiation at aircraft altitudes. Thus, small changes in the geomagnetic field would lead to the varying radiation doses at these altitudes. Second, the changes of the air density above the flight altitude might be a reason for the variation. Interactions between GCR particles and atmospheric gas produce secondary particles, which are absorbed by air during their downward transport. However, only a small amount of such radiation survives to ground level. Thus, the intensity of radiation is critically controlled by the column density of the atmospheric particles LEE ET AL. SHORT-TERM VARIATION OF COSMIC RADIATION 804

9 above the measurement points. Under constant pressure condition, the air number density could be changed by vertical wind velocity and the variation of air composition. While we cannot exactly describe highaltitude atmospheric state with lack of measured data, the dose rate could also be increased by reduction in the air column density above the aircraft altitude. In addition, artificial effects such as the amount of fuel and the presence of human bodies could affect the neutron thermalization process, which would consequently affect the counts/dose conversion. Acknowledgments The data of sunspot number are available to download from silso/home. The cosmic ray data observed by neutron monitor at Moscow and Mexico City are available to download from The Kp index used in this paper was downloaded from earths-magnetic-field/services/kp-index. The raw data of cosmic radiation measured by Liulin detector are available from the authors upon request This work was partially supported by the National Research Foundation of Korea (NRF) and grant funded by the Ministry of Science, ICT and Future Planning (MSIP) (No ) and Development of Technology for Cosmic Radiation Assessment on International Air Route funded by KINS. We wish to thank Korean Air Force for providing opportunity to measure cosmic radiation on reconnaissance aircraft. 4. Summary and Conclusion Aircrew experience higher radiation levels than workers exposed to medical or industrial radiation. Although their annual radiation doses are generally below 6 msv, aircrew are exposed to continuous radiation over long work hours. For this reason, the ICRP recommends the systematic control of radiation exposure doses for aircrew. Accordingly, many European countries have adopted radiation models that calculate and control the doses of annual radiation encountered by aircrew. In 2011, Korea also passed legislation for better management of aircrew radiation exposure. Radiation exposure doses of aircrew are assessed by radiation calculation models, because actual measurements are costly and no less effective than the models. Nevertheless, the models must be verified through continuous experimental measurements. Currently, radiation doses are measured by radiation detectors installed in commercial aircrafts. Because these aircrafts fly over several thousand kilometers, whether the differences between the calculated and measured values arise from temporal changes or flight paths are difficult to determine. In the current experiment, instead of a long-distance commercial aircraft, we used a military reconnaissance aircraft performing a circular flight within a certain area (over the Korean Peninsula) and measured the radiation at a specified flight altitude (9144 m) using a Liulin active detector installed on the aircraft. By this approach, we successfully measured the changing radiation dose over a limited area. The measured radiation dose was μsv/h. No direct correlation was observed between the galactic cosmic rays observed from the Neutron Monitor at the ground-based Moscow Station and the radiation doses measured in this experiment. If the cosmic ray fluxes are relatively constant, the radiation doses at flight altitudes might alter under other complex conditions. According to the current experimental results, the radiation dose at 9144 m can be reliably assessed by modeling the measured values. Among the 35 measurements, the mean and standard deviation of the dose rate at 9144 m were 2.3 and 0.17 μsv/h, respectively, corresponding to an error of 7.4% over the experimental period. Here we assumed that the performance of the measuring device was consistent over time and that the error occurred from unmeasurable factors such as temporal changes in the magnetic field or changes of the air density in the upper atmosphere above the flight altitude. Radiation doses at flight altitudes might be predictable if solar proton events were excluded [ICRP, 1997]. Two results emerged from the present experiment. First, the cosmic radiation particles (count rate) can increase at the specified flight altitude without any change in dose rate. Second, the dose rate can change on short time scales of several hours. Although the exact cause of these changes cannot be ascertained at present, both results highlight the role of short-term radiation-related phenomena. Currently, aircrew radiation doses are calculated from monthly averaged measurements of neutron monitors and depend on the flight path and duration. 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