The second example discovery was based on Cassini observations at the edge of Saturnís magnetosphere ĀEthe planetary magnetopause. This boundary is clearly a common feature of all planetary magnetospheres, in our solar system and beyond. Although the solar wind is mainly forced to flow around a magnetopause, fundamental plasma processes can operate at the boundary that break down the shielding of near-planet space by the planetary magnetic field. It is crucial to understand these energy transport processes if we are to understand how energy flows from the Sun to the magnetized planets.
Possibly the most important of these energy transport processes is magnetic reconnection, which releases magnetic energy, accelerates particles, and allows the solar wind to enter the magnetosphere. In the case of Earthís magnetosphere, it is magnetopause reconnection that principally drives energy flow through the system. Reconnection is a fundamental plasma process that occurs in a range of diverse environments, including at other planetary magnetopauses.
Like at other planetary magnetopauses, the solar wind that encounters Saturnís magnetopause has been processed by the planetary bow shock, making it hotter and denser. However, compared to conditions at Earthís magnetopause, some important (dimensionless) plasma parameters are significantly different at Saturnís magnetopause, because Saturnís bow shock is much stronger than Earthís, as we have already discussed. These different solar wind conditions produce stronger particle drifts within Saturnís magnetopause boundary, which generally prohibit reconnection from taking place (see Figure 3). Essentially, these drifts become so fast that they ďblow awayĀEthe reconnection flow pattern, suppressing the reconnection process itself.
This is another important result because it suggests that how solar wind energy enters a planetary magnetosphere differs between the magnetised planets in the Solar System. In particular, because planetary bow shocks become stronger with the distance of a planet from the Sun, plasma conditions at planetary magnetopauses further from the Sun should be less favourable for reconnection. Reconnection is expected to be restricted to limited occasions when magnetic field lines become almost exactly anti-parallel to each other at the magnetopauses of planets in the Outer Solar System.
This discovery also has broad implications because it is relevant for exoplanetary astrophysics, for example. Planets around other stars are frequently being identified, and many of these planets are likely to be both magnetised and immersed in a flow of charged particles from their parent star (like the solar wind). The Cassini result concerning magnetopause reconnection that we have discussed here is clearly relevant for how energy flows through space around these distant and mysterious planets, as well as planets in our own solar system. This understanding may influence our assessments of whether these exoplanets are likely to have atmospheres, and are capable of supporting life.
In summary, our understanding of space plasmas in the Solar System is constantly evolving. Thanks to spacecraft like Cassini that are exploring planetary systems in detail for the first time, we can make significant fundamental plasma physics discoveries. As well as being highly relevant for research into solar system space environments, the current wealth of in situ measurements made by Cassini and other spacecraft represent a valuable source of information about distant astrophysical environments that are of great scientific interest.