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Solar Particle Radiation

Solar Particle Radiation

The Sun radiates more than just life-sustaining light into the solar system. At irregular intervals, it also produces bursts of high-energy particles. These solar particles have energies that range from 30,000 electron volts to 30 billion electron volts per nucleon and consist primarily of protons (96% of the total number of nuclei) and helium nuclei (3%). The remaining particles are ions of elements that are common in the solar atmosphere, such as carbon, nitrogen, oxygen, neon, magnesium, silicon, and iron, as well as small numbers of even heavier elements. The processes that produce high-energy protons and ions also accelerate electrons to at least 20 million electron volts. Collisions between energetic particles and the solar atmosphere also produce neutrons and gamma rays . All these particles flow outward from the Sun into the heliosphere , where they can affect space systems and are a major concern for astronaut safety.

The Origins of Solar Particles

Until recently, it was thought that solar energetic particles came only from flares . Now solar physicists know that they are produced in both flares and coronal mass ejections. Flares occur when stressed magnetic fields in solar active regions release their energy. The energy appears as both heated plasma and energetic particles, some of which stream out along magnetic field lines into the heliosphere. Because they come from a small area on the Sun, the energetic particles follow a narrow set of field lines and affect only a small region of the heliosphere. Flare-generated energetic particle events tend to be impulsive, meaning that the flux of particles measured near Earth rises and decays rapidly, often within a day.

Coronal mass ejections are the result of a large-scale restructuring of the magnetic field in the solar corona . In this process significant amounts of plasma are ejected into the heliosphere. Usually a coronal mass ejection includes the eruption of a solar prominence and often is accompanied by a flare. The fastest coronal mass ejections travel at speeds above 800 kilometers per second (500 miles per second) and drive shock waves, which accelerate coronal plasma and solar wind into energetic particle events. Since the coronal mass ejection is a large-scale event, the accelerated particles cover a much broader region of the heliosphere than is the case for particles accelerated in flares alone. Coronal mass ejection-associated energetic particle events tend to be gradual, sometimes lasting for many days.

The Impact of Solar Particle Radiation

Because solar energetic particles have been stripped of some or all their electrons, they are positively charged and must follow the magnetic field lines away from the Sun. Near Earth, they are prevented from directly penetrating the near-Earth environment by the magnetosphere that surrounds the planet. Some particles can penetrate in the polar regions where Earth's magnetic field lines connect more directly with the space environment. There they produce fade-outs of radio communication at high latitudes and can bombard high-flying aircraft, including commercial flights. During a solar particle event, a passenger on a high-flying supersonic aircraft can receive a radiation dose equivalent to about one chest X ray an hour.

Energetic particles are stopped when they strike other matter. When this happens, they give up their energy to that material. On an orbiting satellite, energetic particle exposure degrades the efficiency of the solar-cell panels used to provide operating power. A large energetic particle event can also damage sensitive electronic components, leading to the failure of critical subsystems and loss of the satellite.

When energetic particles strike living tissue, the transfer of energy to the atoms and molecules in the cellular structure causes the atoms or molecules to become ionized or excited. These processes can break chemical bonds, produce highly reactive free radicals , and produce new chemical bonds and cross-linkage between macromolecules . Cells can repair small amounts of damage from low doses of particle radiation. Higher doses overwhelm this ability, resulting in cell death. If the dose of radiation is high enough, entire organs can fail to function properly and the organism dies.

Radiation doses are measured in rads or grays, where 1 gray equals 100 rads. One rad equals 100 ergs of absorbed energy per gram of target matter. The potential for radiation to cause biological damage is called the dose equivalent, which is measured in rems or sieverts, where 1 sievert equals 100 rems. The dose equivalent is simply the dose (in rads or grays) multiplied by the so-called radiation weighting factor, which depends on the type of radiation and other factors. The average American receives 360 millirems a year, and a typical X ray gives a patient 50 millirems (a millirem is a thousandth of a rem). The National Aeronautics and Space Administration (NASA) limits exposure to radiation absorbed by the skin to 600 rems for an astronaut's career, with additional limits of 300 rems per year and 150 rems for every thirty-day period.

A large solar particle event can produce enough radiation to kill an unprotected astronaut. For example, the large solar storm of August 1972 would have given an unshielded astronaut on the Moon a dose equivalent of 2,600 rems, probably resulting in death. Shielding can reduce the radiation levels, but the amount required for a large solar particle event is too large for an entire Mars-bound spacecraft or lunar surface base. Instead, a highly shielded storm shelter is necessary. This must be combined with a warning capability to give astronauts who are away from the shelter sufficient time to seek safety.

Solar activity is monitored continuously from specially designed ground-based observatories and from the National Oceanic and Atmospheric Administration's geostationary operational environmental satellites. These satellites continuously observe the solar flux in soft X rays and monitor energetic particles at the satellite location. A sudden increase in soft X rays signifies a solar flare. Coronal mass ejections are not currently monitored continuously, but ground-based observatories can often detect the disappearance of a solar filament, which is usually related to a coronal mass ejection. Significant solar particle events occur much less frequently than flares and coronal mass ejections, so many false alarms are possible. Thus, with current observing systems, astronauts must always be able to seek a sheltered environment within the roughly one-hour period that it takes for the particle radiation to rise to dangerous levels. This limits, for example, the distances away from a lunar base that an astronaut can safely explore.

see also Living on Other Worlds (volume 4); Solar Wind (volume 2); Space Environment, Nature of the (volume 2); Sun (volume 2).

John T. Mariska


Odenwald, Sten. "Solar Storms: The Silent Menace." Sky and Telescope 99, no. 3(2000):50-56.

Phillips, Kenneth J. H. Guide to the Sun. Cambridge, UK: Cambridge University Press, 1992.

Reames, Donald V. "Particle Acceleration at the Sun and the Heliosphere." Space Science Reviews 90 (1999):413-491.

Internet Resource

Space Environment Center Homepage. National Oceanic and Atmospheric Administration, Space Environment Center. <>.

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