The Galactic Environment of the Sun
The heliosphere appears to protect the inner solar system from the vagaries of the interstellar medium
Pieces of interstellar matter are constantly passing through our solar system. These galactic visitors—atomic particles and bits of dust—flow through interplanetary space and may collide with the major bodies in the solar system—the earth and the other planets. Although each particle is microscopic, their total mass in the solar system is enormous. Indeed, about 98 percent of the gaseous fraction in the heliosphere—the volume of space filled by the solar wind?consists of interstellar material! How do these particles interact with a planet's environment? Do they have a significant impact on a planet's atmosphere? No one knows.
These questions take on special significance in light of the fact that the interstellar medium—the sun's galactic environment—is not a homogeneous substrate. Astronomers have discovered that interstellar material is organized into clouds of dust and gas with elaborate features resembling filaments, worms, knots, loops and shells. Within a relatively small region of space, just a few thousand light-years across, the interstellar medium may exhibit a broad range of temperatures, densities and compositions. Indeed, astronomers' understanding of the interstellar medium has been up-ended in the past decade as the physical and structural complexity of interstellar material has unfolded. It is now evident that the sun must have experienced a broad range of galactic environments in its 5-billion-year history.
How does the solar system respond to a change in its galactic environment? Part of the answer must lie with the solar wind—the hot, ionized gas that blows out from the sun. As it happens, the solar wind modulates what can (and cannot) flow into the solar system. Given that the solar wind is itself a variable phenomenon (changing in strength periodically with an 11-year solar cycle), the relation between the solar wind and the invading particles from the interstellar medium is in constant flux. Sorting out the complex interplay between these dynamic phenomena has become a fascinating area of research. Here we describe what space scientists and astrophysicists have learned about the sun's galactic environment and its interaction with the solar system.