and particles from solar particle events (SPEs) are two major components1 of the space radiation environment that pose radiation risk to astronauts during space missions away from the protective zone of Earth’s magnetic field. The GCR and SPE environments in the solar system have a strong correlation with the approximately 11-year solar cycle.
Note that much of the material in this Overview section is contained in Heliophysics—Evolving Solar Activity and the Climates of Space and Earth, in the chapter by J.R. Jokipii (2010).2
Galactic cosmic rays constitute a major part of the space radiation environment near Earth. GCRs are very energetic charged particles (electrons and atomic nuclei) that are believed to be accelerated by vast, spheroidal blast waves from supernova explosions that propagate in the interstellar gas. The accelerated cosmic rays enter the heliosphere on their way to the inner solar system and Earth. In the process they are changed, and so understanding their transport is essential to understanding the space radiation environment.
The heliosphere is a vast spheroidal cavity in the local interstellar plasma, some 150 to 200 astronomical units (AU) in size, created by a supersonic, radial flow of plasma, called the solar wind, that flows outward from the Sun. The spatial scale of the heliosphere is determined by both the Sun and the back pressure of the surrounding interstellar plasma and magnetic field. Far from the Sun, the outward-flowing solar wind is spread over such a large volume that it can no longer continue out into the surrounding interstellar plasma. Because the wind is flowing supersonically (faster than waves can propagate), the supersonic flow ends at a spheroidal shock wave, which is called the heliospheric termination shock, where the flow changes suddenly to a subsonic (slower than the wave speed) outward flow.
The interstellar plasma is moving at about 26 km/sec relative to the heliosphere, pushing it in on one side. Beyond the termination shock, the solar plasma continues to flow outward, but it is deflected and eventually turns to flow in the same direction as the interstellar plasma, forming a large, trailing, heliospheric tail. The interstellar medium also contains neutral atoms, and these also play a role in the interaction of the heliosphere with the interstellar medium, although the effects are small and may be neglected.
Energetic particles including cosmic rays pervade the heliosphere, as they do all regions of low-enough density in the universe. The energetic particles are in four basic types: galactic cosmic rays, anomalous cosmic rays, interplanetary energetic particles, and solar energetic particles. This discussion concentrates primarily on galactic cosmic rays. They come from the galaxy, where they are thought to be accelerated by supernova blast waves. They envelop the heliosphere with a very nearly constant, isotropic bath. The particles are then partially excluded from the inner parts of the heliosphere. Therefore, their intensity reflects the varying properties of the heliosphere. GCRs have a typical energy of 1 GeV and are present continuously, but fluctuate on a variety of timescales. Solar cosmic rays are produced sporadically by the Sun, at considerably lower energies than those of the galactic particles (see Figure 2.1). Their spectrum is also a much more rapidly decreasing function of energy. The time-averaged intensity of these two types of cosmic rays, as a function of energy, is illustrated in Figure 2.1, where the solar particles are a solar-cycle average. The average spectrum over time is therefore dominated by the GCRs, although for short periods (hours to days) the solar particles can be quite intense.
The intensity of GCRs in the inner solar system is observed to vary with time over a wide variety of timescales. The time variations of galactic particles are due to variations in the solar wind and its entrained magnetic field, which are accessible to direct measurement. There exists a generally accepted physical model that can account quantitatively for these modulations.
1Trapped-particle models are not covered here because they contribute very little to the organ dose for missions aboard the International Space Station or missions to the Moon or to Mars.
2J.R. Jokipii, The heliosphere and cosmic rays, Chapter 9 in Heliophysics—Evolving Solar Activity and the Climates of Space and Earth (C.J. Schrijver and G.L. Siscoe, eds.), Cambridge University Press, New York, 2010. Copyright © 2010 Cambridge University Press. Used with permission.