Magnetic Fields (MHD)

Mera reads RAMSES MHD (ideal magnetohydrodynamics) outputs and exposes the magnetic field for analysis. RAMSES evolves B with a constrained-transport scheme, so the field is stored as the six face-centred components B_{x,y,z}_left and B_{x,y,z}_right in the ordinary hydro files (there are no separate magnetic-field files). The physically meaningful cell-centred field is the average of the two opposing faces,

\[B_i = \tfrac12\,(B_{i,\text{left}} + B_{i,\text{right}}), \qquad i\in\{x,y,z\}.\]

How Mera detects and names MHD variables

In an MHD hydro file the variable order is

density, vx, vy, vz, B_x_left, B_y_left, B_z_left, B_x_right, B_y_right, B_z_right, [non-thermal], pressure, [scalars…]

so the thermal pressure sits at index 11, not 5 (index 5 is B_x_left). Mera handles this automatically across RAMSES versions:

With a hydro_file_descriptor.txt (post-2019 and 2025 outputs) Mera reads the variable names directly and maps them to its canonical symbols (density→:rho, velocity_*→:vx/:vy/:vz, pressure→:p at its true index, B_*_{left,right}→:b*_{left,right}).
Without a descriptor (older outputs) Mera uses the community heuristic (matching yt): a 3-D run with nvar ≥ 11 is treated as MHD (the constrained-transport module adds the three B_right components). A short @info line is printed when this heuristic is applied.

Either way you get canonical names and the cell-centred field :bx, :by, :bz.

Ambiguous no-descriptor case

Without a descriptor, a hydro run that happens to carry exactly six passive scalars also has nvar = 11 and would be read as MHD. Modern RAMSES writes the descriptor, which removes the ambiguity; if you hit this, the columns are still available positionally (:var6…).

A reproducible example (yt sample dataset)

The yt project hosts a small RAMSES MHD test (a 3-D MHD tube). Download and extract it:

# in a shell:
#   curl -LO https://yt-project.org/data/ramses_mhd_128.tar.gz
#   tar -xzf ramses_mhd_128.tar.gz

using Mera
info = getinfo(27, "ramses_mhd_128")          # prints the MHD-layout info note
gas  = gethydro(info)

# canonical names: pressure is correct (index 11), B faces are present
getvar(gas, :p)            # thermal pressure
getvar(gas, :bx)           # cell-centred Bx = ½(bx_left + bx_right)
getvar(gas, :T, :K)        # temperature, from the *correct* pressure

Derived magnetic quantities

All of these are built-in getvar quantities computed from the cell-centred field — no manual arithmetic needed — and each takes the units shown:

# field magnitude |B|  (field-strength units: :Gauss, :muG, :microG, :nG, :Tesla)
Bmag    = getvar(gas, :bmag)          # code units
Bmag_uG = getvar(gas, :bmag, :muG)    # μG

# magnetic pressure P_mag = B²/8π  and plasma β = P_thermal / P_mag
Pmag = getvar(gas, :pmag, :Ba)        # magnetic pressure [barye = erg/cm³]  (also :g_cm_s2)
beta = getvar(gas, :beta)             # plasma β (dimensionless)

# Alfvén speed and magnetic energy per cell
vA   = getvar(gas, :v_alfven, :km_s)  # v_A = |B|/√(4πρ)
Emag = getvar(gas, :e_magnetic, :erg) # magnetic energy per cell = (B²/8π)·V_cell

# magnetosonic Mach numbers
mA = getvar(gas, :mach_alfven)   # M_A = |v| / v_A
mf = getvar(gas, :mach_fast)     # fast:  v_f = √(c_s² + v_A²)
ms = getvar(gas, :mach_slow)     # slow:  v_s = c_s·v_A/√(c_s² + v_A²)

Almost no new units were needed: B reuses the field-strength scales (:Gauss, :muG, :microG, :nG, :Tesla), magnetic pressure/energy-density reuse the pressure scales (:Ba, :g_cm_s2), the Alfvén speed reuses the velocity scales (:km_s, :cm_s), and the magnetic energy reuses :erg. (:nG, nanogauss, was added via a new ScalesType003 so pre-existing mera files still load.)

The exact formulas (incl. the RAMSES code-unit convention P_mag = B²/2 and the Alfvén-speed conversion) are listed in How Quantities Are Computed.

Projecting the magnetic field

The cell-centred components project like any other field — e.g. a mass-weighted map of :bx, or a column-density map alongside it:

using CairoMakie
sd = projection(gas, :sd, :Msol_pc2; direction=:z)
bx = projection(gas, :bx;            direction=:z)   # mass-weighted ⟨Bx⟩ map
heatmap(bx.maps[:bx])

On an MHD run the first-look dashboard does this for you: quicklook(output) adds a face-on |B| panel and reports the |B| and plasma-β ranges automatically.

Caveats

Mera reads ideal-MHD RAMSES outputs (the constrained-transport B faces). Non-ideal terms (e.g. resistivity) are not separate fields.
:bx/:by/:bz are the cell-centred average of the faces; the raw faces remain available as :bx_left, :bx_right, … if you need the divergence-free face representation.
On a non-MHD run, :bx/:by/:bz, the derived quantities (:bmag, :pmag, :beta, :v_alfven, :e_magnetic) and the magnetosonic Mach numbers all error with a clear message.