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Magnetosphere & Solar Protons

Introduction

The Earth is continuously bombarded with radiation, both optical radiation as light, and ionising radiation such as gamma rays, X-rays and charged particles. The sources of these different types of radiation are many and varied - here we will discuss the origin and nature of ionising radiation that is incident on the Earth. Beyond the Earth's atmosphere, in the magnetosphere of the Earth, there is much more ionising radiation than at the Earth's surface, and this has direct implications for the operation of satellites and Earth-orbiting space craft. The energy deposited by such radiation in spacecraft materials, particularly electronic materials such as integrated circuits, can cause significant damage.

In general, high levels of ionising radiation encountered by an Earth-orbiting satellite comprise both electromagnetic radiation, such as X-rays and gamma rays, and particles such as electrons (e-), protons (p+), alpha-particles and other, heavier ions. The X-rays and gamma rays, which nearly all emanate from the Sun, do not cause a significant damage to satellites, however this is not the case for the charged particles. The particles encountered in space are generally known as cosmic-rays. In fact there are different types of cosmic rays that can be distinguished according to their source, and these are galactic cosmic-rays (GCRs) and solar cosmic-rays (SCRs).

In the laboratory on Earth we are used to observing the interactions of charged particles, such as electrons(beta particles) and alpha particles produced by radioisotope sources. For example, a very common laboratory source of alpha particles is the unstable isotope 241-Am, which emits alpha particles with an energy of about 5.49 MeV. Alpha particles of this energy can be easily and safely contained in the laboratory as they penetrate only very shallow distances in materials (eg. about 20 micrometers into silicon). However the radiation environment in space contains a variety of very energetic charged particles than can penetrate through the skins of spacecraft and cause considerable damage to the internal control circuits. For example, protons with an energy of 10 MeV can penetrate a 1 millimetre thick aluminium wall of a satellite. The primary sources of high-energy particles encountered by a satellite in Earth-orbit are:

  • geomagnetically trapped particle populations (i.e. the Van Allen belts);
  • galactic cosmic-rays (i.e. interplanetary protons, electrons and heavy ions);
  • solar particle events (protons, alphas, and other heavy ions).

These sources of radiation are not necessarily steady, for example the rate of solar protons incident on the Earth is strongly related to the level of activity of our Sun. When the Sun is actively producing solar flares, the proton fluxes reaching the Earth can be particularly high. Additionally, these bursts of solar protons can then be trapped in the Van Allen belts, and so 'persist' in the Earth's magnetosphere.

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