High-Mach-number collisionless shock dynamics: theory and simulations versus multi-point measurements in space

Dear ISSI committee, we would like to submit a proposal of the team to work on
“High-Mach-number collisionless shock dynamics: theory and simulations versus
multi-point measurements in space.”

Collisionless shocks are very ubiquitous in space plasmas and play a fundamental role in a number of astrophysical environments, among which are the heliosphere, supernova remnants (SNR), gamma ray bursts (GRB), jets from black holes, and others. They are intensively studied experimentally in the near-Earth environment aboard satellites (Cluster, ISEE, AMPTE, Wind, Polar and many others), in the interplanetary space, and in the vicinity of other planets (Ulysses, Voyager, and Phobos), as well as by means of remote sensing techniques making use of radio observations of the Sun and astrophysical objects such as supernova remnants, jets etc. The most of astrophysical shocks are presumably high-Mach-number shocks.
From the very beginning of the collisionless shock studies it was found that there exists a critical Mach number. In the shocks with Mach numbers larger than this critical one, the conductivity and viscosity cannot provide sufficient dissipation. A new dissipation mechanism should be invoked and one of these mechanisms was found to be an ion reflection. However, for the purely perpendicular case it was also indicated that there can exist the saturation of the number of reflected ions (Kennel et al, 1985, Hada et al., 2003).
Theoretical studies (Krasnoselskikh, 1985, Galeev et al., 1987, 1988a,b) and computer simulations (Quest, 1985, 1986; Lembege and Savoini, 1992) of high-Mach-number shocks showed that there can exist a shock front instability that leads to a dynamic behavior of shocks, later this phenomenon was called a reformation of the shock front structure. For oblique shocks, the reformation was found to be related to the conditions for existence of the standing whistler wave train within the front.
Computer simulations of high-Mach-number shocks in two-dimensional geometry led to the discovery of large-scale “ripples,” the electromagnetic field variations moving along the shock front. Such new instability can result in appearance of additional mechanisms of the energy dissipation and new mechanisms of particle acceleration.
The Cluster satellites give a unique possibility to use in situ observations of collisionless shocks as a natural laboratory to study the fundamental process of the shock front formation and dynamics. Multi-point measurements, which provide a unique possibility to distinguish between spatial and temporal variations, allow one to identify different types of phenomena such as the structures that are quite similar to “ripples.” The very first results showed clear manifestations of the nonstationary behavior of the shock front and to study variations of different parameters. Statistical studies resulted in the observation of small-scale structure of the electric and magnetic field profiles, this can also be considered as a manifestation of the dynamic behavior of the shock front.
Such studies are extremely important because they can provide a reliable tool for the remote sensing of the Coronal Mass Ejections (CME) related to interplanetary shocks, that is one of the goals of the Space Weather program and one of the scientific objectives of the International Living with the Star program.
In the proposed collaboration we plan to combine theoretical studies with the analysis of the observational data obtained in the in situ measurements, mostly in the Earth’s bow shock, to achieve better understanding of the dynamics of high-Mach-number shocks in space plasmas.
An International Team we would like to form will study the problems outlined above. To this end, it includes experimentalists, who are experts of data analysis of different kinds of data such as electric and magnetic fields and different kinds of particles, as well as theoreticians and specialists on computer simulation of collisionless shocks.

The topics we are going to work on are:

• Dynamics of high-Mach-number shocks seen by different instruments aboard Cluster satellites.
• Comparison of characteristics observed by Cluster satellites and predictions of theoretical studies and computer simulations.
• Relationship between particle distributions and waves observed in a close vicinity of the shock front.
• Nonlinear processes in the shock front.
• Acceleration of electrons and ions resulting from the shock dynamics.

The final goal is to write several research papers on the topics mentioned above, together with a review paper to summarize new results and current state of research, and to develop closer collaboration between different groups working in this field of common interest.