Answer to Question #1266 Submitted to "Ask the Experts"

Q

What is the increase of ionizing radiation that one receives from a coronal mass ejection or solar flare that is aimed directly at the earth? Even though the atmosphere filters the radiation, there must be some increase.

A

The amount of any radiation increase depends strongly on where one is located. If you are in a spacecraft outside the Earth's magnetic field, the radiation doses can be quite large (as much as tens of Gray—1 Gy = 100 rad), depending on how much spacecraft shielding there is around you. If you are in a spacecraft, such as the Space Shuttle or International Space Station, in Low-Earth Orbit, the doses are lower (up to tens of milligray)—specific values depending upon the altitude and inclination of the orbit and the amount of shielding provided by the spacecraft. In high-altitude aircraft (~65,000 ft), near the poles, doses from solar particle events are measurable, but not above current limits for the general public. For most aircraft altitudes (below 45,000 ft), doses are negligible, as they are anywhere on the surface of the Earth. The most severe events can generate measurable fluxes of neutrons at ground level from the interactions of the solar protons with the atmosphere. But these fluxes are low enough that they do not pose any health hazard. The University of New Hampshire provides ground-level measurements of some of these fluxes. A good source of information is the Proceedings of the 1998 Annual Meeting of the National Council on Radiation Protection and Measurements (NCRP), which were published in the November 2000 issue of Health Physics.

Larry Townsend, PhD
University of Tennessee