What is space radiation?

Radiation may be defined as energy in transit in the form of high-speed particles and electromagnetic waves. Radiation is divided into two categories: ionizing radiation and non-ionizing radiation.

Ionizing radiation is radiation with sufficient energy to remove electrons from the orbits of atoms resulting in charged particles. The effect on the medium the particles transit depends on the energy loss rate of the particles and the characteristics of the medium. The quantity used to characterize the interaction is LET.Linear Energy Transfer Low LETLinear Energy Transfer particles include X-rays, gamma rays, beta particles, and low energy protons. Generally, these interactions occur with the outer-most electrons of the target atom. High LETLinear Energy Transfer particles include high energy protons, alpha particles, and atomic nuclei. These particles result in significant ionization of the atoms along the path of transit and several electrons may be removed from a target atom as the particle passes.

Neutrons are indirectly ionizing. Since neutrons have no charge, they pass through material without interacting with the electrons. Typically the neutron will undergo scattering through neutron-nucleus interactions and transfers some kinetic energy to the target atom. Eventually the neutron will be absorbed by a nucleus. The nucleus becomes unstable and results in nuclear decay of nuclear transmutation where the nucleus splits into two or more parts sharing the neutron's incident kinetic energy. The components of the radioactive decay or transmutation are ionizing radiation products.

Non-ionizing radiation is radiation that interacts with the atoms that make up the material it traverses, but does not have sufficient energy to remove electrons from their orbits. Examples are microwaves, lasers, and visible light.

Space radiation consists primarily of ionizing radiation which exists in the form of high-energy, charged particles. There are three naturally occurring sources of space radiation: SEPSolar Energetic Particle events, GCRsGalactic Cosmic Ray, and trapped radiation.

Space Radiation Source: SEPSolar Energetic Particle Events

SEPSolar Energetic Particle events are the injection of energetic electrons, protons, alpha particles, and heavier particles from the Sun into interplanetary space. These particles are accelerated to near relativistic speeds by the interplanetary shock waves which precede fast CMEsCoronal Mass Ejection and are typically associated with solar flares. The most energetic particles arrive at Earth within tens of minutes of the event on the Sun. They temporarily enhance the radiation in interplanetary space around the magnetosphere, and they may penetrate to low altitudes in the polar regions of Earth.

The frequency of SEPSolar Energetic Particle events depends on Sun's level of activity which is characterized by an approximately 11-year cycle called the solar cycle -- broken down into Solar Maximum when activity is highest and Solar Minimum when activity is lowest. The number of flares, CMEsCoronal Mass Ejection, and SEPSolar Energetic Particle events increases toward Solar Maximum. The figure below demonstrates this by comparing the number of sunspots (a proxy for solar activity) and the number of times SRAGSpace Radiation Analysis Group was called into Mission Control due to an SEPSolar Energetic Particle event.

ISES Sunspot number progression
ISESInternational Space Environment Service sunspot number progression (courtesy of NOAA National Oceanic and Atmospheric Administration/SWPCSpace Weather Prediction Center) from Jan 2000 to Feb 2018. The purple line represents the number of sunspots each month, the blue line represents the smoothed average of sunspots, and the vertical red lines represent each time SRAGSpace Radiation Analysis Group was called into Mission Control due to an SEPSolar Energetic Particle event.

Solar flares are characterized by a highly concentrated, explosive release of energy, usually in the form of X-rays. In just several minutes, flares may heat material to many millions of degrees and release as much energy as a billion megatons of TNT. Groups of sunspots, especially those with complex magnetic field configurations, are often the sites of solar flares. Most flares do not pose a threat to crewed missions since the X-rays produced cannot penetrate spacecraft material.

Some of the most dramatic space weather effects occur in association with CMEsCoronal Mass Ejection. These are huge bubbles of plasma (ionized atomic matter with high kinetic energy) threaded with a magnetic field that is ejected from the Sun's corona (outer atmosphere). A large CMECoronal Mass Ejection can contain a billion tons of matter that can be accelerated up to several hundred million miles per hour. CMEsCoronal Mass Ejection are often associated with solar flares and prominence eruptions, but they can occur in the absence of either of these processes. Near Solar Maximum, the Sun produces about three CMEsCoronal Mass Ejection per day, whereas near Solar Minimum, CMEsCoronal Mass Ejection are seldomly produced. The faster CMEsCoronal Mass Ejection have outward speeds considerably greater than that of the normal solar wind (~400 km/s), and they produce large shock waves in the solar wind as they plow through it. Some of the solar wind ions are accelerated by the shock wave, and they become a source of intense and long-lasting energetic particle enhancements in interplanetary space.

Anatomy of a Large Solar Energetic Particle Event
Diagram summarizing the steps of an SEPSolar Energetic Particle event. (Wings In Orbit, Scientific and Engineering Legacies of the Space Shuttle, 1971-2010 NASA/SP-2010-3409, Published 2010. Page 454.)

Except for the Apollo and upcoming Artemis missions to the Moon, NASANational Aeronautics and Space Administration's crewed spaceflight missions have taken place within the protective cocoon of the Earth's magnetosphere. Between the Apollo 16 and 17 missions, one of the largest solar proton events ever recorded occurred. It is indeed fortunate that the timing of this event did not coincide with one of the Apollo missions. As NASANational Aeronautics and Space Administration prepares to send astronauts back to the Moon and eventually Mars, radiation protection for crew members remains one of the key technological issues which must be resolved.

Space Radiation Source: GCRGalactic Cosmic Rays

GCRGalactic Cosmic Rays originate outside the solar system and are thought to be the remnants of supernovas. They consist of fully ionized atoms ranging from protons up to uranium nuclei, although the majority of particles are iron or lighter. The amount of these particles is very low. However, since they travel very close to the speed of light, and because some of them are composed of very heavy elements such as iron, they produce intense ionization as they pass through matter.

GCRGalactic Cosmic Rays are influenced by the Sun's magnetic field which extends well beyond Earth, and therefore the flux of GCRGalactic Cosmic Rays observed at Earth fluctuates over the course of a solar cycle (see figure below). The flux peaks during Solar Minimum and declines toward Solar Maximum. GCRGalactic Cosmic Rays are the main source of daily radiation to astronauts outside of Earth's geomagnetic field. The Earth's magnetic field provides significant protection at latitudes near the Equator, but provide little to no protection near Earth's magnetic poles since these regions are open to interplanetary space.

Solar modulation of GCRs
Solar modulation of GCRGalactic Cosmic Rays. Top panel shows solar activity represented by the number of sunspots. Bottom panel shows the flux of GCRGalactic Cosmic Rays. As solar activity peaks, GCRGalactic Cosmic Ray flux is minimal. Similarly, as solar activity is minimal, GCRGalactic Cosmic Rayflux peaks. (Nuntiyakul, W., et al., "Latitude Survey Investigation of Galactic Cosmic Ray Solar Modulation during 1994-2007". ApJ, Volume 795(1):11, 2014 DOI: 10.1088/0004-637X/795/1/11).

Space Radiation Source: Trapped Radiation

The rotation of the Earth's molten iron core creates electric currents that produce a dipole magnetic field around the Earth -- similar to that of an ordinary bar magnet. This magnetic field extends several thousand kilometers out from the surface of the Earth. The Sun produces a constant stream of particles which billow out into space and travel at almost 1 million miles per hour. This stream of particles, called the solar wind, varies in intensity with the amount of surface activity on the Sun. The solar wind is mainly composed of protons and electrons, about 8% alpha particles, and small amounts of heavier ions such as C, N, O, Ne, Mg, Si, S, and Fe. The charged particles of the solar wind cannot easily penetrate the Earth's magnetic field. The interaction of the particles and the magnetic field forms a shock front around which the particles are deflected like water around the bow of a ship. The solar wind compresses and confines the magnetic field on the side toward the Sun and stretches it out into a long tail on the night side. The cavity formed by this process is called the magnetosphere. This cavity shelters the surface of the Earth from constant bombardment by charged particles.

Artist's rendering of trapped radiation.
An artist's rendition showing the influence of the solar wind on the Earth's magnetic field. Note how the magnetic field is compressed on the sun-facing side and elongated on the opposite side.

Not all of the particles are deflected by the magnetosphere, however, and some become trapped in the Earth's magnetic field. These trapped particles contribute to two doughnut-shaped structures within the magnetosphere called the Van Allen radiation belts. The inner belt mainly contains protons with energies exceeding 10 MeVMega Electron Volt. The outer belt contains mainly electrons with energies up to 10 MeVMega Electron Volt. The charged particles which compose the belts circulate along the Earth's magnetic field, travel along the field bouncing between North and South poles, and slowly drift across the magnetic field parallel to the Equator.

Part of the inner Van Allen belt dips down to about 200 km into the upper region of the atmosphere over the southern Atlantic Ocean off the coast of Brazil. This region is known as the SAASouth Atlantic Anomaly. The dip results from the fact that the magnetic axis of the Earth is tilted approximately 11 degrees from the spin axis, and the center of the magnetic field is offset from the geographical center of the Earth by 280 miles. Flights in LEOLow Earth Orbit typically traverse a portion of the SAASouth Atlantic Anomaly six or seven times a day, and a significant fraction of the radiation exposure received comes from these SAASouth Atlantic Anomaly passes.

South Atlantic Anomaly
South Atlantic Anomaly and Van Allen Belts

Earth's magnetosphere get disturbed when a large magnetic structure in space such as a CMECoronal Mass Ejections passes by Earth. The larger the southward component of the structure's magnetic field, the stronger the disturbance. This disturbance is called a geomagnetic storm. Geomagnetic storms result in changes to the radiation belts, energization of trapped radiation, heating of the ionosphere, and more. Storm conditions allow particles to reach previously unattainable altitudes and lower latitudes. Such storms are accompanied by enhanced displays of the Aurora Borealis (northern hemisphere) and Aurora Australis (southern hemisphere). Auroras are created by collisions between the particles and atmospheric gases. The collisions raise the energy levels of oxygen and nitrogen atoms; as these excited atoms lose energy, they release some of it in the form of light.

South Atlantic Anomaly
Aurora Borealis created from a CMECoronal Mass Ejection that reached Earth on Aug 2, 2021. Photo taken from the ISSInternational Space Station by Expedition 65 crew.