Particle acceleration via multiple reflections off a thin astrophysical shock (snapshot)
In the vicinity of very thin astrophysical shocks, a particle acceleration mechanism known as multiple reflection, or surfing, becomes particularly efficient. This mechanism was proposed by Zank et al., 1996 (Journal of Geophysical Research, 101, A1, 457-477).
The illustration shows the trajectory of an ion that is initially slow, with an energy of only a few keV, that is accelerated to higher energies after several interactions with a thin shock. The ion is shown as it approaches the shock (lower left) and is reflected off its surface; it then undergoes a process known as shock drift acceleration, during which it stays in the shock transition region and drifts back and forth across the shock surface, gaining energy as a result. After that, the ion is shown to repeatedly bounce off the shock surface, being gradually energised in the process and eventually gaining enough energy (of the order of 0.5 MeV or beyond) to pass through the shock (upper right).
Data from ESA's Cluster mission have revealed that the bow shock formed by the solar wind as it encounters Earth's magnetic field is remarkably thin: it measures only 17 kilometres across. Thin astrophysical shocks such as this are likely sites for acceleration mechanisms such as multiple reflection, which may be a solution to the injection problem in cosmic accelerators. If a shock can be this thin, particles 'surfing' along it may be accelerated to a sufficiently high energy threshold that they can then be fed to different mechanisms that accelerate them to very high energies, well beyond 1 GeV, such as those reported in cosmic ray studies.