NAIRAS aircraft radiation model development, dose climatology, and initial validation

Abstract

[1] The Nowcast of Atmospheric Ionizing Radiation for Aviation Safety (NAIRAS) is a real-time, global, physics-based model used to assess radiation exposure to commercial aircrews and passengers. The model is a free-running physics-based model in the sense that there are no adjustment factors applied to nudge the model into agreement with measurements. The model predicts dosimetric quantities in the atmosphere from both galactic cosmic rays (GCR) and solar energetic particles, including the response of the geomagnetic field to interplanetary dynamical processes and its subsequent influence on atmospheric dose. The focus of this paper is on atmospheric GCR exposure during geomagnetically quiet conditions, with three main objectives. First, provide detailed descriptions of the NAIRAS GCR transport and dosimetry methodologies. Second, present a climatology of effective dose and ambient dose equivalent rates at typical commercial airline altitudes representative of solar cycle maximum and solar cycle minimum conditions and spanning the full range of geomagnetic cutoff rigidities. Third, conduct an initial validation of the NAIRAS model by comparing predictions of ambient dose equivalent rates with tabulated reference measurement data and recent aircraft radiation measurements taken in 2008 during the minimum between solar cycle 23 and solar cycle 24. By applying the criterion of the International Commission on Radiation Units and Measurements (ICRU) on acceptable levels of aircraft radiation dose uncertainty for ambient dose equivalent greater than or equal to an annual dose of 1 mSv, the NAIRAS model is within 25% of the measured data, which fall within the ICRU acceptable uncertainty limit of 30%. The NAIRAS model predictions of ambient dose equivalent rate are generally within 50% of the measured data for any single-point comparison. The largest differences occur at low latitudes and high cutoffs, where the radiation dose level is low. Nevertheless, analysis suggests that these single-point differences will be within 30% when a new deterministic pion-initiated electromagnetic cascade code is integrated into NAIRAS, an effort which is currently underway.

Numerical model for computation of effective and ambient dose equivalent at flight altitudes

“…The difference between the effective dose E and the ambient dose equivalent H*(10) is shown in Figure A.3. One can see that the H*(10) is a conservative approach for the exposure in a low rigidity cut-off regions, while the effective dose is a conservative approach above some 5 GV in accord with a recent study (Mertens et al 2013). Moreover, our model computations demonstrate better agreement with the reference data (Figs.…”

“…In general it slightly overestimates the effective dose. However, it is not a conservative estimate for cosmic radiation exposure at aviation altitudes according to Mertens et al (2013) and the work presented here, specifically in high rigidity regions. Nevertheless, it is regarded as an acceptable approximation for effective dose at aircraft altitudes Mertens et al 2013).…”

“…2 and A.1). The modelled ambient dose equivalent tends to slightly overestimate the dose in the low rigidity cut-off region and underestimate it in high rigidity cut-off regions, similarly to Mertens et al (2013). The slight underestimation of the reference data at the high level of the rigidity cut-off could be explained by the contribution of thunderstorm radiation, widely discussed in Dwyer et al (2010) and Drozdov et al (2013).…”

“…At present, several models have been proposed, aiming to estimate the dose rate (effective and/or ambient dose equivalent) at flight altitudes due to primary CR radiation (Schraube et al 2000;Ferrari et al 2001;Roesler et al 2002;Lewis et al 2005;Matthiä et al 2008;Sato et al 2008;Mishev & Hristova 2012;Mertens et al 2013;Al Anid et al 2014;Mishev 2014;Mishev et al 2015). According to the definition, the absorbed dose is the energy deposited in a medium by ionizing radiation per unit mass.…”

Assessment of spectral and angular characteristics of sub-GLE events using the global neutron monitor network

“…It was demonstrated that the aircrew exposure can be estimated on the basis of a full simulation of the atmospheric cascade (Ferrari et al, 2001;Roesler et al, 2002). Several models have been proposed, aiming to estimate the radiation dose rate (effective and/or ambient dose equivalent) at flight altitudes (Schraube et al, 2000;Ferrari et al, 2001;Roesler et al, 2002;Lewis et al, 2005;Sato et al, 2008;Matthiä et al, 2008;Mertens et al, 2013;. In this study, we employed a recently proposed numerical model for computation of effective and/or ambient dose equivalent at flight altitudes, the details are given elsewhere .…”