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# Shocks and discontinuities (magnetohydrodynamics)

Shocks and discontinuities are transition layers where the plasma properties change from one equilibrium state to another. The relation between the plasma properties on both sides of a shock or a discontinuity can be obtained from the conservative form of the magnetohydrodynamic (MHD) equations, assuming conservation of mass, momentum, energy and of $\nabla \cdot \mathbf{B}$.

## Rankine-Hugoniot jump conditions for MHD

The jump conditions across a MHD shock or discontinuity are referred as the Rankine-Hugoniot equations for MHD. In the frame moving with the shock/discontinuity, those jump conditions can be written:

ρ1vn1 = ρ2vn2,
Bn1 = Bn2,
$\rho_1 v_{n1}^2+ p_1 + \frac{B_{t1}^2}{2 \mu_0}=\rho_2 v_{n2}^2+ p_2 + \frac{B_{t2}^2}{2 \mu_0},$
$\rho_1 v_{n1} \mathbf{v_{t1}} - \frac{\mathbf{B_{t1}}B_{n1}}{\mu_0}= \rho_2 v_{n2} \mathbf{v_{t2}} - \frac{\mathbf{B_{t2}}B_{n2}}{\mu_0},$
$\left(\frac{\gamma}{\gamma-1}\frac{p_1}{\rho_1}+\frac{v_1^2}{2}\right)\rho_1 v_{n1}+\frac{v_{n1} B_{t1}^2}{\mu_0}-\frac{B_{n1}(\mathbf{B_{t1}}\cdot \mathbf{v_{t1}})}{\mu_0}=\left(\frac{\gamma}{\gamma-1}\frac{p_2}{\rho_2}+\frac{v_2^2}{2}\right)\rho_2 v_{n2}+\frac{v_{n2} B_{t2}^2}{\mu_0}-\frac{B_{n2}(\mathbf{B_{t2}}\cdot \mathbf{v_{t2}})}{\mu_0},$
$(\mathbf{v} \times \mathbf{B})_{t1} = (\mathbf{v} \times \mathbf{B})_{t2},$

where ρ, v, p, B are the plasma density, velocity, (thermal) pressure and magnetic field respectively. The subscripts t and n refer to the tangential and normal components of a vector (with respect to the shock/discontinuity front). The subscripts 1 and 2 refer to the two states of the plasma on each side of the shock/discontinuity

## Contact and tangential discontinuities

Contact and tangential discontinuities are transition layers across which there is no particle transport. Thus, in the frame moving with the discontinuity, vn1 = vn2 = 0.

Contact discontinuities are discontinuities for which the thermal pressure, the magnetic field and the velocity are continuous. Only the mass density and temperature change.

Tangential discontinuities are discontinuities for which the total pressure (sum of the thermal and magnetic pressures) is conserved. The normal component of the magnetic field is identically zero. The density, thermal pressure and tangential component of the magnetic field vector can be discontinuous across the layer.

## Shocks

Shocks are transition layers across which there is a transport of particles. There are three types of shocks in MHD: slow-mode, intermediate and fast-mode shocks.

Intermediate shocks are non-compressive (meaning that the plasma density does not change across the shock). They are also isentropic. They are sometimes referred as rotational discontinuities. All thermodynamic quantities are continuous across the shock, but the tangential component of the magnetic field can "rotate".

Slow-mode and fast-mode shocks are compressive and are associated with an increase in entropy. Across slow-mode shock, the tangential component of the magnetic field decreases. Across fast-mode shock it increases.

The type of shocks depend on the relative magnitude of the upstream velocity in the frame moving with the shock with respect to some characteristic speed. Those characteristic speeds, the slow and fast magnetosonic speeds, are related to the Alfvén speed, vA and the sonic speed, cs as follows:

$a_{\mathrm{slow}}^2 = \frac{1}{2} \left[\left(c_s^2 + V_A^2\right)-\sqrt{\left(c_s^2+V_A^2\right)^2-4c_s^2V_{An}^2}\,\right],$
$a_{\mathrm{fast}}^2 = \frac{1}{2} \left[\left(c_s^2 + V_A^2\right)+\sqrt{\left(c_s^2+V_A^2\right)^2-4c_s^2V_{An}^2}\,\right],$

where VAn is the normal component of the Alfvénic speed.

The normal component of the slow shock propagates with velocity aslow in the frame moving with the upstream plasma, that of the intermediate shock with velocity VAn and that of the fast shock with velocity afast.

## Example of shocks and discontinuities in space

• The Earth's bow shock, which is the boundary where the solar wind's speed drops due to the presence of Earth's magnetosphere is a fast mode shock. The termination shock is a fast-mode shock due to the interaction of the solar wind with the interstellar medium.
• Magnetic reconnection can happen associated with a slow-mode shock (Petschek or fast magnetic reconnection) in the solar corona.[1]
• The existence of intermediate shocks is still a matter of debate. They may form in MHD simulation, but their stability has not been proven.
• Discontinuities (both contact and tangential) are observed in the solar wind, behind astrophysical shock waves (supernova remnant) or due to the interaction of multiple CME driven shock waves.
• The Earth's magnetopause is generally a tangential discontinuity.[2]