OVERVIEW

Environment Definition

The particles associated with ionizing radiation are categorized into three main groups relating to the source of the radiation: trapped radiation belt particles, cosmic rays, and solar flare particles. Results from recent satellite studies suggest that the source of the trapped radiation belts (or Van Allen belts) particles seems to be from a variety of physical mechanisms: from the acceleration of lower-energy particles by magnetic storm activity, from the trapping of decay products of energetic neutrons produced in the upper atmosphere by collisions of cosmic rays with atmospheric particles, and from solar flares. Solar proton events are associated with solar flares. Cosmic rays originate from outside the solar system from other solar flares, nova/supernova explosions, or quasars.
 

Fig 1 - Trapped particles spiral back and forth along magnetic field lines.

The Earth's magnetic field concentrates large fluxes of high-energy, ionizing particles including electrons, protons, and some heavier ions. The Earth's magnetic field provides the mechanism which traps these charged particles within specific regions, called the Van Allen belts. The belts are characterized by a region of trapped protons and both an inner and an outer electron belt. The radiation belt particles spiral back and forth along the magnetic field lines ( Fig. 1). Because the Earth's approximate dipolar field is displaced from the Earth's center, the ionizing radiation belts reach their lowest altitude off the eastern coast of South America. This means as particles travel into this region they will reach lower altitudes, and particle densities will be anomalously high over this region. This area is termed the South Atlantic Anomaly (SAA). For the purpose of this document, the term "cosmic rays" applies to electrons, protons, and the nuclei of all elements from other than solar origins. Satellites at low inclination and low altitude experience a significant amount of natural shielding from cosmic rays due to the Earth’s magnetic field. A small percentage of solar flares are accompanied by the ejection of significant numbers of protons. Solar proton events occur sporadically, but are most likely near solar maximum. Events may last hours or up to more than a week, but typically the effects last 2 to 3 days. Solar protons add to the total dose and may also cause single-event effects in some cases ( Figs. 2 and Figs. 3).

 

Fig 2 - The density of shielding needed to reduce the average radiation dose from a large solar proton event.

  

Fig 3 - Variation of solar flare proton events as a function of solar activity.

The high-energy particles comprising the radiation environment can travel through spacecraft material and deposit kinetic energy. This process causes atomic displacement or leaves a stream of charged atoms in the incident particle’s wake. Spacecraft damage includes decreased power production by solar arrays, failure of sensitive electronics, increased background noise in sensors, and radiation exposure of the spacecraft crew. Modern electronics are becoming increasingly sensitive to ionizing radiation.
 

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