# Astrospheres


## Euler equations

In the literature a variety of (multi-) fluid (Euler-) equations with different extensions is used ( Pogorelov et al. 2009, Bouquet et al. 2000, <Fahr et al. 2000, Jun et al. 1994, Downes & Drury 2014).

The set of Euler equations, the continuity -, momentum -, and energy equation can be combined into:
\begin{eqnarray} \label{eq:1} \del \begin{bmatrix} \rho_{j} \\ \rho_{j} \vec{v}_{j} + \mathcal{P}_{1} \vec{F}_{rad} \\ E_{j} + \mathcal{P}_{2} E_{rad}\\ \vec{B} \end{bmatrix} +\div \begin{bmatrix} \rho_{j} \vec{v}_{j} \\ \rho_{j} \vec{v}_{j}\vec{v}_{j} + P_{j} \widehat{I} + \mathcal{P}_{3} \vec{F}_{rad} - \dfrac{\vec{B}\vec{B}}{4\pi}\\[0.25cm] (E_{j} + P_{j} )\vec{v}_{j} + \mathcal{P}_{4} \vec{F}_{rad} -\dfrac{\vec{B}(\vec{B}\cdot\vec{v_{j}})}{4\pi} \\ \vec{v}_{j}\vec{B}-\vec{B}\vec{v}_{j} \end{bmatrix} =\\ \begin{bmatrix} 0 \\ \rho_{j} \vec{F} + \div \widehat{\sigma} - \vec{\nabla} P_{CR} \\ \rho_{j} \vec{v}_{j}\cdot\vec{F} + \div (\vec{v}_{j}\cdot\widehat{\sigma}) - \div \vec{Q} - R_{L} -\vec{v}_{j}\cdot\vec{\nabla} P_{CR}\\0\end{bmatrix} + \begin{bmatrix} S_{j}^{c} \\ \vec{S}_{j}^{m}\\S_{j}^{e}\\\vec{A}\end{bmatrix} \end{eqnarray}
where the left-hand side is written in a conservative form. $$\vec{v}_{j}, \rho_{j}, E_{j}, P_{j}$$ are the fluid velocity, mass density, total energy density and pressure of species $$j$$, $$\widehat{I}$$ is the unit tensor, $$\widehat{\sigma}$$ the viscosity/stress tensor, $$\vec{F}$$ is an external force per unit mass and volume , $$\vec{Q}$$ is the heat flow, $$S_{j}^{r}$$ are sources and sinks caused by charging processes in the continuity and energy equation $$r\in\{c,e\}$$ and $$\vec{S_{j}^{m}}$$ those of the momentum equation. $$R_{L}$$ is a cooling function. The parameters with the subscript $$_{rad}$$ describe the momentum $$\vec{F}_{rad}$$ and energy $$E_{rad}$$ coupling to the radiation transport, while $$P_{CR}$$ that to cosmic rays. The $$\mathcal{P}_{k}, k\in\{1,2,3,4\}$$ are constants. Finally, $$\vec{A}_{j}$$ describes the ambipolar diffusion between the neutrals and the ions.

Under construction