BOW SHOCK

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The Earth's Bow Shock

The bow shock is the nonlinear wave which stands in the solar wind flow upstream of the Earth's magnetosphere, at which the solar wind plasma is heated and decelerated in preparation for diversion around the magnetosphere. The dissipation processes at the shock depend on the properties of a collisionless plasma, and lead to a rich range of energetic particles and plasma waves.

In an ordinary gas, a shock wave forms when an obstacle is placed in a flow which is supersonic, i.e., the relative speed between flow and obstacle is greater than the sound speed. For example, a supersonic aircraft has an associated shock wave. In a plasma, such as the solar wind, the situation is complicated by the existence of other wave modes in addition to sound waves. However, the principle remains true, and the solar wind flow speed is greater than the sound speed and the Alfven speed, so that a plasma shock is formed, due to the obstacle created by the Earth's magnetosphere.

Shock waves can arise in a number of different situations in astrophysics. For example, an explosive event may generate supersonic flows which impact the surrounding gas, and this leads to a blast shock wave, where there is a limited amount of energy associated with the shock (e.g., super nova remnant, or solar flare). In the case of the magnetized planets in the solar system, the magnetosphere presents an essentially impermeable obstacle, and a steady shock wave is formed which stands at a relatively constant distance from the planet in the upstream flow. This is called a bow shock, in analogy to the bow wave in water ahead of a ship.

At the bow shock the magnetic field, density and temperature all increase as the solar wind transits the shock. This is similar to what happens in a fast magnetosonic wave (i.e., fast mode MHD wave). Thus the bow shock is an example of a fast mode collisionless shock, and the governing characteristic wave speed is that of the fast magnetosonic wave.

In an ordinary gas the dissipation necessary at a shock is provided by collisions between the gas molecules, so that the width of the shock is associated with the dissipational scale length which is of the order of the collision mean free path. In space plasmas the mean free path between collisions is so large that the system is essentially collisionless. Consequently the dissipation is supplied by plasma processes related to the gradients at the shock, particularly changes in the electric and magnetic field. The plasma shock processes consequently govern the width and internal structure of the shock layer. The Earth's bow shock has a width of between roughly 100 km and 1000 km, depending on the shock and plasma parameters.

The bow shock surrounds the magnetosphere on its upstream side, and is about 15 R_E (Earth radii) from the Earth at the "nose" of bow shock, i.e., at the subsolar point. However, the position of the shock is highly variable (from 12 to 20 R_E) and depends on the solar wind parameters, most importantly the ram pressure. The shock position changes on the time scale of minutes in response to changes in the solar wind, and the motion of the shock can be in the range 10-100 km/s. One important consequence is that observations of the bow shock are usually due to the shock moving over the spacecraft, rather than to the motion of the spacecraft around its orbit. The motion of the shock allows a translation from a time series of measurements to a cross-section through the shock giving its spatial structure. This can obviously become impossible if the shock is unsteady.

The overall shape of the bow shock has been investigated by collating large sets of shock crossings determined from measurements of the magnetic field. Most models used to fit the observations are cylindrically symmetric conic sections (i.e., the surface generated by rotating a parabola or hyperbola around its symmetry axis), with suitable scaling to take into account variation of the solar wind ram pressure. Locating the bow shock at any given time is not yet a precise operation, since the magnetospheric obstacle is not rigid, but also responds to changes in the solar wind pressure and magnetic field orientation.

The bow shock is important for different reasons. The solar wind flow is processed by the shock before it hits the magnetosphere, so the coupling between solar wind and magnetosphere is mediated by the shock and its downstream region (the magnetosheath). More importantly the bow shock is an accessible example of an astrophysical shock in a collisionless plasma. Such shocks are believed to be common in a wide range of astrophysical situations, and the Earth's bow shock allows us to study in detail the shock processes such as dissipation, wave generation, and particle acceleration. A spacecraft crossing the shock takes between seconds and minutes, and in that time instrumentation can record the electric and magnetic fields and particle distribution functions at high resolution. This has allowed an analysis of the plasma processes producing the necessary dissipation at the shock, and how these processes, and hence the shock structure, are controlled by variations in the solar wind parameters. Observations of the bow shock show that it is a rich source of waves and energetic particles, and in situ observations have provided something like a plasma laboratory, so that much of the physics of collisionless shocks has been deciphered.