Before the era of space exploration the nature of the interplanetary space environment was the subject of much debate. However, in situ measurements made by numerous spacecraft have conclusively revealed that charged particles (plasma) continually flow away from the Sun, forming the solar wind. Although generally very low density, and thus invisible to the naked eye, the solar wind represents a significant and continuous energy flux from the Sun to the planets. Magnetised planets like Earth are sheltered form the solar wind by their strong magnetic field, producing a cavity in the flow known as a planetary magnetosphere. The other magnetised planets (Mercury, Jupiter, Saturn, Uranus, and Neptune) also have magnetospheres, although there are many differences between these complex systems.
Research aimed at revealing how space plasmas in the Solar System work is often motivated by our desire to understand how the solar wind interacts with our own planetís magnetic field. This interaction is responsible for Earthís auroral emissions, and the geomagnetic storms that can damage satellites and endanger astronauts. A fleet of spacecraft continuously monitor near-Earth space, but we are still far from able to accurately predict space weather. Spacecraft have visited other planets and regions of the Solar System, and what has become clear is that data returned by these more distant explorers is crucial for testing and improving our understanding of all space plasma systems, including our own planetís magnetosphere.
In this article I focus on another important reason why we study space plasmas: To learn about environments beyond the Solar System. These environments are so far away that in situ measurements will almost certainly not be made in our lifetimes, if ever. However, many such astrophysical systems are sufficiently ionized and low density to be considered collisionless plasmas, fundamentally the same as the space plasmas we continuously sample closer to our own star. As a result, the importance of in situ measurements in different regions of the Solar System for astrophysics research should not be under-estimated.
To give examples of this, here I discuss two recent solar system plasma science discoveries that have resulted from the analysis of data taken by the Cassini spacecraft at the planet Saturn.
Cassini began its orbital tour of Saturn in July 2004, and will continue to orbit the planet until September 2017. A number of instruments mounted on the spacecraft allow us to study local plasmas and magnetic fields (see Figure 1). Saturn is ~10 times further from the Sun than Earth, making Cassini the furthest planetary orbiter to date. Solar wind conditions at Saturn are very different to those at Earth, allowing Cassini to study plasmas with properties rarely, or never, encountered by spacecraft closer to the Sun. As a result, Cassini can potentially make fundamental plasma physics discoveries that have broad implications.
The first example discovery was based on Cassini observations made at Saturnís bow shock. Shock waves are familiar in Earthís atmosphere (e.g. the shock waves around a supersonic aircraft), but they can also form in very low-density plasma environments like the supersonic solar wind. Saturnís bow shock stands in the solar wind in front of Saturnís magnetosphere (see Figure 1). A similar planetary bow shock wave is present at every solar system magnetosphere.