The space radiation environment is composed of multiple particles from different sources. Depends on theirs sources, particles have different energies and different fluxes in space. Energy of particles are usually given in the units of electronvolt (eV). To describe density and dynamics of particles the measure of flux is used. Usually flux tells us how many particles will cross one square centimeter of space within one second.

Solar radiation

Solar radiation is most important aspect of space radiation. Sun is a dynamic object, and change its activity level in 11-years cycles. Sun still loss its mass by emission into space. Slow stream of particles is from time to time disturbed by eruptive events.

Slow type of solar activity

Sun heats up its “Corona”, expands it and releases its outer part into space – release solar wind. In slow type of solar activity the solar wind comprising mostly electrons and protons (95% of ejected mas), helium ions (4.8%) and small percent of other particles. The speed of particles is about 900 km/s. The flux of low-energy particles is high, and flux of high-energy particles is low.

Eruptive type of solar activity

Solar Energetic Particle Events

In effect of eruptive events, The Sun emits hi-energetic nuclei into space. The speed of particles exceeds 2000 km/s and flux is several order of magnitude higher, than during slow type of emission. We can observe two kinds of events:

  • Solar flare
    sudden flash of increased brightness on the Sun, usually observed near its surface and in close proximity to a sunspot group.
  • Solar prominence
    large, bright, gaseous feature extending outward from the Sun’s surface, often in a loop shape. Prominences are anchored to the Sun’s surface, and extend outwards into the Sun’s corona.

During a solar prominence eruption or very often during solar flares, it happens that significant matter of Sun reach Suns corona and is thrown into space. This throw of matter is named Coronal Mass Ejection (CME).

solar flare
Solar Flare Erupts with EMC on Sept. 8, 2010/Credit: NASA
Solar Prominence of August 25, 2010
Solar Prominence of August 25, 2010/ Credit: NASA

Galactic cosmic rays (GCR)

Stream of particles which coming outside the solar system. The particles cams about Earth from every direction, theirs flux is low but energy of single particle maybe very huge and can reach even 3*1020eV (at the moment mankind cannot speedup particles to such big energies). The source of GCR is unknown. The flux of GCR particles comprises 83% of protons, 13% ammonium ions, 3% electrons, and 1% other high-energy particles.

Trapped radiation

Earths magnetic field catches charges particles inside two radiation belts, named Van Allen belts from name of its discoverer. The belts consists mostly protons and electrons and less number of heavy ions of helium, nitrogen and oxygen. The flux of particles is very high.

structure of trapped radiation
The structure of the Van Allen radiation belts (idealized)/ Fortescue, Spacecraft Systems Engineering

Inner Van Allen belt

Is situated in a shell space above the meridian plane within latitude range +-40o. The shell space above the equator is a at altitude range from 1200 km to 9540 km (altitude range in Earth radius(L): from 1.2L to 2.5L) . The inner belt is dominated by hi-energetic protons. The belt has symmetric shape around Earth.

Outer Van Allen belt

Is situated in a shell space above the meridian plane within latitude range +-55o to +-75o. The altitude range above equator is in range from 11450 km to 70000 km ( range in Earth radius(L): 2.8L to 12L). The belt is dominated by hi-energetic electrons. On its ‘shadowed site’ the belt it much more wider than on its ‘sunny site’.

Magnetic field and radiation belts

Earth has weak but width magnetic field. Solar wind is a plasma, in which flow very high electric currents which produce electromagnetic field. Magnetic field of The Earth interacts with magnetic field carried by solar wind. The sum of the magnetics fields influences on tracks of charged particles around the Earth. The solar wind is responsible for asymmetry of outer van Allen belt.

van Allen belts and magnetic field
Magnetic layer of the earth and radiation/ Mengfei Yang, Fault‐Tolerance Techniques for Spacecraft Control Computers

South Atlantic Anomaly (SAA)

Because of the inhomogeneity of the earth’s magnetic field, in the area of South Atlantic the inner van Allen belt sink deep into atmosphere to altitude 200 km.

SAA on altitude 500km
The anomaly at an altitude of approximately 560 kilometers, ROSAT Guest Observer Facility. Retrieved October 16, 2007.

Secondary radiation

When original high‐energy level particles penetrate a spacecraft’s material or atmosphere, a nuclear reaction is produced, which in turn excites secondary particles and rays, including the strong penetrative types, such as bremsstrahlung and neutrons.

Summarizing space ionization radiation

plot of space ionization radiation eergy and density
Space radiation environment/Mengfei Yang, Fault‐Tolerance Techniques for Spacecraft Control Computers

At the plot above we can see complexity of space radiation. Particles from Sun, from outside The Solar System (GCR) and trapped inside radiation belts make together space radiation environment. Fortunately the most destructivle particles, these with the highest energy, have the lowest flux, so it is less possible for spaccrafts to be hit by them during missions.

Space weather

The Sun is decisive about shape of radiation around The Earth. During stable solar activity solar wind, GCR levels and van Allen belts are quite stable. Eruptive events on Sun have impact on earths magnetosphere an thus on tracks of nuclei particles, changes radiation belts by moving it toward the Earth, decrease level of GCR and inject particles to the Earths atmosphere. The prediction of all the phenomenas depends on observation of The Sun, and is similar to forecast the weather by observing clouds. Space weather may have impact on work artificial satellites, working electric devices on Earth and on our natural environment.

References

  • Arnold Hanslmeier (2004) The Sun and space wheather. Kluwer Academic Publisher, New York.
  • Mengfei Yang, Gengxin Hua, Yanjun Feng, Jian Gong (2017) Fault-Tolerance Techniques for Spacecraft Control Computers. Wiley, .
  • Peter Fortescue, Graham Swinerd, John Stark (2011) Spacecraft Systems Engineering 4th Edition. Wiley, .