Comparative heliophysics

Our knowledge of conditions – past, present, and future – throughout the solar system is rapidly advancing. We can specify the details of the evolution of the Sun, and can represent the conditions of its magnetic activity over time with fair certainty. We have explored – both remotely and in situ – many of the “worlds” throughout the solar system, from Mercury to Pluto, including asteroids and comets. Along with an appreciation of the diversity of conditions in space around all these environments comes the realization that the conditions here on Earth are remarkably just right for the only world that we know to sustain life. Specifically, Earth is in an orbit that has kept it within the range in orbital distances within which the planet has maintained liquid water at its surface throughout the past several billions of years. It orbits far enough from the Sun that tidal forces could not lock Earth's rotation to its orbital motion, thus allowing solar irradiance to be effectively distributed over the sphere on the relatively short time scale of the terrestrial day. Moreover, since the Late Heavy Bombardment some 4 billion years ago, Earth has received limited environmental debris (asteroids, comets, …). The impacts of the Sun's magnetic activity on the Earth have also been relatively benign, without overly detrimental effects of flares and coronal mass ejections. Moreover, the Earth is shielded well enough from galactic cosmic rays by both solar wind and terrestrial magnetism. And then there are the added conditions related to the internal properties of the planet that contribute to its ability to sustain life, including plate tectonics and dynamo action. In short, the Earth's characteristics are “just right” (see, e.g., Ch. 4 in Vol. III, and Ward and Brownlee, 2000, and also Vidotto et al., 2014, and references therein, for select topics on cool stars and space weather and planetary habitability).

All of these circumstances conspire to a relatively stable environment, both in terms of the terrestrial climate and in terms of what we nowadays call “space weather”. In order to understand the processes that create these environmental conditions, and to appreciate the influences of space weather on the planet that is our only known compatible habitat, we need to advance a field of science called “heliophysics”.