Protecting passengers from exposure to solar flares: Kyoto University quantifies the economic loss of aircraft operation plan changes

Professor Yosuke Yamashiki, SIC Human Spaceology Center, Graduate School of Advanced Integrated Studies in Human Survivability, Kyoto University, Aioi Nissay Dowa Insurance Co. Ltd, Japan Atomic Energy Agency, and the National Institute of Maritime, Port and Aviation Technology, have jointly announced that they have succeeded in quantifying the economic loss risk associated with changes in aircraft operation plans caused by solar radiation exposure. They analyzed the frequency and intensity of solar flares that occurred over the past 2000 years. They also analyzed the four-dimensional spatial-time series data of the dose rate of solar radiation. Through their research they found that solar flares causing changes to low-altitude flights or cancellations to prevent exposure occurred once every 17 years. The cost of countermeasures is estimated to reach US $1,500 per year for long-distance flights operating daily, indicating that it is a feasible countermeasure. The results are expected to be useful for risk countermeasures during solar flares and were published in the September 2 issue of Scientific Reports, a British academic journal.

Generally, the higher the altitude and latitude of the aircraft, the higher the exposure dose. Cosmic radiation exposure to aircraft passengers involves radiation from other galaxies and the Sun. The intensity of galactic cosmic rays is at a low rate of less than 10 μSv/h at the standard cruising altitude (12 km). However, the intensity of solar radiation increases abruptly when a huge solar flare occurs, reaching 2 mSv/h, although this is only for a short time.

As the general annual exposure limit is 1 mSv/h, it is considered necessary to adopt measures, such as cancellation of flights and lowering of flight altitude during these events. Therefore, this study group estimated the costs of countermeasures and evaluated the occurrence frequency of solar flares necessary for the cost estimation.

First, the maximum cumulative radiation exposure and radiation exposure rate for 80 incidents from five solar flares, eight flight paths, and two cruising altitudes (altitudes of 12 and 9 km) were evaluated using the four-dimensional spatial-time series data of solar radiation exposure estimated using the warning system for aviation exposure to solar energetic particles (WASAVIES). In addition, they quantified the relationship between the frequency of solar flares and their intensity based on cosmic ray neutron observations and radiocarbon analysis results retroactively over the past 2000 years. For the solar radiation intensity, they developed and analyzed not only the conventionally used event integrated intensity (EII) but also the peak event intensity (PEI), which reflects the effect of an instantaneous increase in the solar radiation intensity. The results showed that the frequency of solar flares with a maximum cumulative radiation dose exceeding 1 mSV for each route condition occurs once every 47 years. It was also found that the occurrence frequency of solar flares exceeding the maximum radiation dose rate (80 μSv/h: classified as severe exposure) is once every 17 years.

Next, to minimize radiation dose, they formulated a protocol that reduced the cruising altitude when radiation exceeds the threshold at the standard cruising altitude (12 km) but is possible at 9km, but cancels if the flight cannot operate at 9 km. Furthermore, they estimated the economic loss required for altitude reduction and cancellation. It was estimated that the economic loss risk per year for daily long-distance flights reaches US $500 if regulations are imposed on cumulative dose and approximately US $1,500 if regulations are imposed on maximum dose rates. This economic loss is not very high compared to other aviation risks such as volcanic eruption, indicating the possibility of introducing regulations.