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The Forefront of Space Science

The Importance of Space Plasma Science at Saturn with Cassini
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What is special about Saturnís bow shock is that the shock is stronger than the space shock waves frequently encountered closer to the Sun, because of the different solar wind conditions at Saturn. The strength of a shock, given by Mach numbers, indicates how much energy has to be dissipated, and is thus available to boost some particles to very high energies. Saturnís bow shock is an important natural laboratory in which we can study particle acceleration at strong shock waves.

This topic of shock-acceleration is a major theme of astrophysics research because it is thought that very strong shock waves outside our solar system are primarily responsible for the high-energy particles that pervade our galaxy ĀEcosmic rays. Clouds of stellar debris expand following supernova explosions, and drive shock waves that we know can accelerate electrons to ultrarelativistic energies. To address the issue of how cosmic rays are produced, these supernova remnant shocks are widely studied using theory, simulations, and remote observations.

On rare occasions Saturnís bow shock becomes unusually strong, and is similar to a young supernova remnant shock in many respects. Cassini observations of Saturnís bow shock under these conditions may be the closest we will ever come to in situ measurements made at a supernova remnant.

On 3 February 2007 Cassini encountered Saturnís bow shock during a strong blast of solar wind, when all shock Mach numbers became higher than normal. Plasma densities, the upstream magnetic field strength, and the Alfvťn Mach number approached values expected at young supernova remnant shocks. At this time the magnetic field orientation upstream of the shock was quasi-parallel (see Figure 2). Although quasi-perpendicular shocks in the Solar System are known to significantly accelerate electrons, quasi-parallel shocks (which behave very differently) have never been seen to do this before.

Figure 1
Figure 2. The two different types of magnetic field orientation in front of a space shock wave. Magnetic field lines are shown as dark blue lines, and normals to the shock surface are shown as red arrows. Credit: ESA - C. Carreau.

However, on 3 February 2007 Cassini observed significant electron acceleration (to relativistic energies) at a quasi-parallel shock wave for the first time. This is very likely because of the high Mach numbers of Saturnís bow shock at this time. These observations suggest that at very strong shocks, like those surrounding young supernova remnants, quasi-parallel shocks become considerably more effective electron accelerators. This result is important because it implies that electron acceleration by strong astrophysical shocks may be more widespread than previously thought. This has implications for how we understand and model particle acceleration at distant shocks, where local conditions are unclear. This discovery has been made at only the beginning of a full analysis of the many crossings of Saturnís bow shock made by Cassini.

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