The spatial extent of the solar wind

While orbiting the Sun, the Earth and the other planets in our solar system are exposed to an outflow of plasma from the Sun. This is the solar wind, which largely determines the particle environment of the planets, as well as their magnetospheric properties. Long-term exposure to the solar wind may have had significant effects on planetary atmospheres in some cases. Mars is a particularly interesting example, as the Martian atmosphere was once much thicker than it is today, and the erosive effects of solar wind exposure are commonly believed to be responsible for its depletion (see Ch. 8; also Jakosky et al., 1994; Atreya et al., 2013).

The solar wind does not expand indefinitely. Eventually it runs into the interstellar medium (ISM), the extremely low-particle-density environment that exists in between the stars. Our Sun is moving relative to the ISM that surrounds the planetary system, so we see a flow of interstellar matter in the heliocentric rest frame, coming from the direction of the constellation Ophiuchus. The interaction between the solar wind and the ISM flow determines the large-scale structure of our heliosphere, which basically defines the extent of the solar wind's reach into our Galactic environment. Other stars are naturally surrounded by their own “astrospheres” (alternatively “asterospheres”) defined by the strength of their own stellar winds, the nature of the ISM in their Galactic neighborhoods, and their relative motion.

Much as the Earth's magnetosphere plays a role in protecting the Earth's atmosphere from direct and potentially damaging exposure to the solar wind, the global heliosphere has its own role to play in shielding the planets that exist inside it from high-energy particles, called Galactic Cosmic Rays (GCRs), that permeate the Galaxy (see Vol. II, Ch. 9; and Vol. III, Ch. 9). Some of these GCRs do penetrate into the inner heliosphere and encounter Earth. The extent to which they have influenced our planet and the evolution of life is debatable, but cosmic rays cause radiation damage and mutations in DNA (Dartnell, 2011). There are also claims that cosmic rays may be involved in atmospheric processes such as lightning formation and cloud formation (Carslaw et al., 2002; Shaviv, 2005a; Svensmark et al., 2009).