# Field Listing¶

The most fundamental quantities associated with any calculation done in ares are the gas density, species fractions and the gas temperature.

## Species Fractions¶

Our naming convention is to denote ions using their chemical symbol (in lower-case), followed by the ionization state, separated by an underscore. Rather than denoting the ionization state with roman numerals, we simply use integers. For example, neutral hydrogen is h_1 and ionized hydrogen is h_2.

Here is a complete listing:

• Neutral hydrogen fraction: 'h_1'
• Ionized hydrogen fraction: 'h_2'
• Neutral helium fraction: 'he_1'
• Singly-ionized helium fraction: 'he_2'
• Doubly-ionized helium fraction: 'he_3'
• Electron fraction: 'e'
• Gas density (in $$g \ \text{cm}^{-3}$$): 'rho'

These are the default elements in the history dictionary, which is an attribute of all ares.simulations classes.

We also generally keep track of the ionization and heating rate coefficients:

• Rate coefficient for photo-ionization, k_ion.
• Rate coefficient for secondary ionization by photo-electrons, k_ion2.
• Rate coefficient for photo-heating, k_heat.

Each of these quantities are multi-dimensional because we store the rate coefficients for each absorbing species separately.

## Two-Zone IGM Models¶

For calculations of the reionization history or global 21-cm signal, in which we use a two-zone IGM formalism, all quantities described in the previous sections keep their usual names with one important change: they now also have an igm or cgm prefix to signify which phase of the IGM they belong to. The igm phase is of course short for inter-galactic medium, while the cgm phase stands for the circum-galactic medium (really just meant to indicate gas near galaxies).

• Kinetic temperature, igm_Tk.
• HII region volume filling factor, cgm_h_2.
• Neutral fraction in the bulk IGM, igm_h_1.
• Heating rate in the IGM, igm_k_heat.
• Volume-averaged ionization rate, cgm_k_ion.

There are also new (passive) quantities, like the neutral hydrogen excitation (or spin’’ temperature), the 21-cm brightness temperature, and the Lyman-$$\alpha$$ background intensity:

• 21-cm brightness temperature: 'igm_dTb'.
• Spin temperature: 'igm_Ts'.
• $$J_{\alpha}$$: 'igm_Ja'.

Each of these are only associated with the IGM grid patch, since the other phase of the IGM is assumed to be fully ionized and thus dark at 21-cm wavelengths.