10.06. 2014

Research Review #2

Much of our understanding of collisionless shocks comes from studies of the terrestrial bow shock upstream of the magnetosphere, which serves to slow down and thermalize the incident supersonic and super-Alfvenic plasma in the solar wind. Over the last decades, multi-spacecraft observations and computer simulations have helped to enhance our understanding of the physical process involved, and progress is still being made. In particular, 3D simulations of the shock front are now becoming feasible, something which will allow full spatial and temporal investigations of the shock processes. Some of the problems that yet remain unresolved are for example how the highly energetic field-aligned beams observed in the quasi-parallel section of the bow shock (i.e. where the shock normal is within 45 degrees of the upstream magnetic field) gain their initial energies, and the possible non-stationary features and reformation of the quasi-perpendicular shock front (i.e. where the angle between the shock normal and the upstream magnetic field exceeds 45 degrees) which has earlier been considered as a stable boundary.
Below is a short summary of four papers published within the last year that investigate several such issues, and which are all directly relevant to the work conducted within the SHOCK project.
Shocklets, SLAMS, and field-aligned ion beams in the terrestrial foreshock.
Wilson, L. B. III,  A. Koval,  D. G. Sibeck,  A. Szabo,  C. A. Cattell,  J. C. Kasper,  B. A. Maruca,  M. Pulupa,  C. S. Salem, and  M. Wilber, J. Geophys. Res. Space Physics, 118, 957–966, doi:10.1029/2012JA018186, 2013.
Wilson III et al. [2013] study the acceleration of field-aligned beams (FAB) in the terrestrial foreshock. Such beams are generally observed at the quasi-perpendicular section of the shock, however, using data from the Wind spacecraft, the authors identify two such events at Earth’s quasi-parallel bow shock and thus show that beams can be accelerated to high energies also in that section of the bow shock. These beams are found to have thermal energies of ~80-850 eV, and bulk velocities on the order of 1.3-2.4 times that of the solar wind. The beams are found during a period of shocklets and short large-amplitude magnetic structures (SLAMS), associated with strong ion and electron heating. As the peak intensity of the field-aligned beams are just upstream of the SLAMS, and no ion beams are seen within the SLAMS, they conclude that the SLAMS are the most likely source of the beams, and that they may be generated at local quasi-perpendicular sections of the structures. Alternatively, the opposite may also be true: the SLAMS could have been caused by and ion-ion beam interaction driven by the FAB themselves. Wilson III et al. reject this scenario for two reasons: (1) The FAB are only observed upstream of the SLAMS, and never within the structures, and (2) the instability threshold for the ion-ion is too low to produce the SLAMS observed.
The acceleration of thermal protons at parallel collisionless shocks: Three-dimensional hybrid simulations
Guo, F., and J. Giacalone, Astrophys. J., 773, 2, doi:10.1088/0004-637X/773/2/158, 2013.
Guo and Giacalone [2013] present 3D simulations of a quasi-parallel shock front, with focus on the generation of the high-energy tail of the ion distribution. The acceleration of these ions are generally thought to be due to diffusive shock acceleration (DSA) where the ions gyrate across the discontinuity and gradually gain energy from the upstream electric field. However, in order for this to happen, the low-energy ions first need to undergo a preliminary acceleration and pitch-angle scattering for DSA to become active. This could either happen through leakage from the downstream plasma, or by scattering of the gyrating ions at the shock ramp by low-frequency waves. The exact nature of this process is currently under debate, and it is the main topic of the paper.  
In order to investigate this issue, the authors present computer simulation runs with 1D, 2D and 3D models, all with similar setup. This is done to assure consistency in their interpretations of the data. Using the 3D model they trace the history of a set of representative high-energy particles, and show that these indeed originate from the shock region itself. The ions can gyrate and drift along the shock front for many gyro periods, and thereby gain the energy necessary to explain the observations. This is in agreement with previous work, and it can be viewed as a first validation of the processes in 3 dimensions.
Magnetosheath filamentary structures formed by ion acceleration at the quasi-parallel bow shock.
Omidi, N.,  D. Sibeck,  O. Gutynska, and  K. J. Trattner,  J. Geophys. Res. Space Physics, 119, 2593–2604, doi:10.1002/2013JA019587, 2014.
Omidi et al. [2014] report results from a set of global 2.5-D electromagnetic hybrid simulations of the terrestrial bow shock and its interaction with the magnetosheath and the magnetosphere, covering a span of upstream Mach numbers and IMF cone angles. In the simulation runs, they find that the quasi-parallel section of the bow shock can generate filamentary plasma structures that extend far into the magnetosheath, associated with field-aligned enhancements of the plasma temperature and density. These structures, which they term “magnetosheath filamentary structures” (MFS) exist for most of the upstream conditions tested, they form as a direct result of the ion dissipation at the shock at the shock, and they lead to periodic variations in the local plasma density and ion temperature in the downstream magnetosheath with variation up to 30-50% of the background level. Similar perturbations can also be seen in the magnetic field close to the shock, but these tend to damp out further into the magnetosheath. In the paper, the authors also provide a series of cross-sectional cuts in the simulation plane which give clues to what these structures may look like in spacecraft data, however, it remains to be seen if the existence of these types of structures can be confirmed observationally. They are predicted to have a spacing of ~0.5-1 Earth radius (approximately 30-60 ion skin depths), which for a magnetosheath flow velocity of 100-300 km/s gives expected frequencies on the order of 0.01–0.1 Hz.
On the shape and motion of the Earth's bow shock.
Meziane, K., T.Y. Alrefay, and A.M. Hamza, Planet. Space Sci., 93–94, 1–9, 2014
The location of the bow shock is continuously adapting to the upstream conditions in order to keep the upstream and downstream media in equilibrium, with mass, momentum and energy conserved across the shock ramp. In this statistical study, Meziane et al. [2014] investigates this motion of the shock using a set of 133 crossings of the terrestrial bow shock by the Cluster spacecraft. By using data from when the spacecraft separation was short compared to the spatial inhomogeneities of the shock front (here restricted to ~600 km), measurements of the 4 spacecraft can be used to derive both the velocity and the direction of motion of the shock front.  The results of the study shows that standard gas dynamic models can adequately predict the shock position for high Mach numbers, and that the shock velocity most often is within +-100 km/s. Events with higher velocities, up to 300 km/s, are generally associated with discontinuities in the solar wind dynamic pressure. They also show that velocity observed on these occasions can be adequately predicted by comparing the change in pressure over time with the equation for the shock stand-off location.

[Written by Torbjorn Sundberg]
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