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The Science of SCOPE

The SCOPE mission strategy

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SCOPE and the simulation studies

Link to Cross-Scale (ESA)

Shocks

Shock waves can be found in everywhere in the Universe. They are created when a super-sonic plasma flow is slowed down to subsonic flow when encountering an obstacle such as a magnetosphere, or a region of slow solar wind. These plasma structures are highly dynamic and play an important role in space plasma processes including particle acceleration, dissipation, and wave generation. Because of the large mean free path of the ions in the space, all these processes occur without collisions and therefore they are called: collisionless.

Since, these processes occur almost anywhere in the Universe on different temporal and spatial scales, understanding the shocks that are accessible to us gives us insight into many physical processes throughout the Universe. A bow shock is formed in front of the Earth and other planets in our solar system when the super-sonic solar wind encounters the magnetospheres of these objects. Coronal mass ejections and high speed plasma streams emerging from the Sun create interplanetary shocks which propagate in the heliosphere and accelerated plasma on their way to the termination shock - a boundary to the interstellar medium. The largest and strongest shock waves are found in astrophysical settings. It is thought they play important role for energy release in the super nova remnants, gamma ray bursts, etc. However, because of the large distance these shocks they are not easily accessible. All the information on physical processes occurring at these shocks is inferred by remote observations.

The near Earth space is THE only a place where one can take in-situ spacecraft observations in order to perform detailed exploration of the physical processes in the vicinity of a collisionless shock. This gives us the ground truth that can be used as empirical tests of the universal shock theories.

Most recent results from space missions, such as Cluster, indicate that the observed quantity is a composite of physical processes, which operate on very different spatial scales. SCOPE will unveil the multi-scale nature of collisionless shock process by addressing the following three key scientific objectives:


shock

movie
(6.7MB)
A movie showing results from one of the most recent huge-scale full particle simulations of a shock in a collisionless plasma. Dynamic and multi-scale nature of the shock behavior is impressive. Indeed, the shock front set-up by MHD-scale dynamics is capable of producing very energetic electrons.

 

1. How is the upstream flow energy dissipated and partitioned at the shock transition region?

Previous observations and numerical simulation studies performed in 1980s revealed the basic structure of the transition region of the Earth’s bow shock. Above the critical Alfven Mach number, the Joule dissipation at the shock transition region cannot dissipate enough energy for the shock formation. In this case, a plasma instability created by the upstream flow and reflected ions at the shock potential provide additional dissipation. Several possible candidates of plasma instabilities, which may provide dissipation at the shock have been proposed. However, none of these instabilities can explain the dissipation process in all details. Most recently, Cluster-II observations show that there is observational evidence that shock reformation as well as ion reflection is highly affected by the electron scale and not just be the ion scale as previously assumed. SCOPE will address these scientific topics in detail by an improved payload and higher resolution data.

2. How far up/downstream is the shock dissipation region extend?

The reflected and accelerated particles as well as electromagnetic fluctuation are expected to be observed away from the shock surface toward far upstream/down stream region and form an additional dissipation layer. This layer may contribute to energy partition, i.e. what fraction of the solar wind energy is converted into heat, magnetic field fluctuations, and downstream bulk flow. Therefore, not only micro-scale processes but also macro-scale processes are involved in energy dissipation. Currently there is no multi-scale mission, which is able cover the spatial and temporal scales. SCOPE will be the first mission addressing these highly interesting science topics associated with simultaneous observations at different scales.

3. How are particles are accelerated at shocks and what determines the maximum energy gain?

The theory of the diffusive shock acceleration has been widely accepted and it was thought to be a standard theory. However, whether the assumptions of this theory are really correct or not still remains uncertain. For example, one of the important problems is the so-called "injection problem." Ions have to be pre-accelerated in order to freely cross between up and downstream regions. These ions are then reflected between the converging upstream and downstream waves to be accelerated by first order Fermi acceleration. However, how the particles are injected into the "Fermi cycle" is not been fully understood. Only in-situ observation can provide us with further information and insights to resolve this problem.