# Introduction

Athena is a grid-based code for astrophysical magnetohydrodynamics (MHD). It was developed primarily for studies of the interstellar medium, star formation, and accretion flows. Athena has been made freely available to the community in the hope that others may find it useful.

The current version (v4.2) implements algorithms for the following physics:

• compressible hydrodynamics and MHD in 1D, 2D, and 3D,
• special relativistic hydrodynamics and MHD,
• ideal gas equation of state with arbitrary $γ$ (including $γ = 1$, an isothermal EOS),
• an arbitrary number of passive scalars advected with the flow,
• self-gravity, and/or a static gravitational potential,
• Ohmic resistivity, ambipolar diffusion, and the Hall effect,
• both Navier-Stokes and anisotropic (Braginskii) viscosity,
• both isotropic and anisotropic thermal conduction,

In addition, Athena allows for the following grid and parallelization options:

• Cartesian or cylindrical coordinates,
• static (fixed) mesh refinement,
• shearing-box source terms, and an orbital advection algorithm for MHD,
• parallelization using domain decomposition and MPI. A variety of choices are also available for the numerical algorithms, such as different Riemann solvers and spatial reconstruction methods.

The code has been developed using the GNU development tools, maintaining strict adherence to ANSI standards, thus it should be possible to configure, compile, and run the code on any platform that supports these standards. Athena has been run on everything from a Mac laptop to a 25,000 processor Cray XT4.

Equations Solved: What system of equations does the code actually solve?

Documentation: Tutorial, User Guide, Programmer Guide, and research papers describing the numerical algorithms in Athena.

Gallery: Images and links to research papers that include results from Athena.

Tests: Suite of 1D, 2D, and 3D hydrodynamic and MHD problems used to validate Athena. Potentially useful to anyone developing algorithms for MHD.

Parallel Performance: Results from strong and weak scaling tests of Athena, up to 10^5^ cores.

People: The list of collaborators developing Athena.

Is Athena Galilean invariant? Well, for resolved solutions, yes.