Electron Density and Ionisation Balance of the Local Interstellar Cloud from Published Line and In-Situ Values

The Local Interstellar Cloud is only partly ionised, and its free electrons set both its pressure and the charge-exchange rates that shape the heliosphere boundary. Combining published neutral densities with the hydrogen and helium ionisation fractions returned by photoionisation models of the local radiation field gives an electron density of 0.073 per cubic centimetre, a proton density of 0.063 per cubic centimetre, and a total particle density of 0.352 per cubic centimetre, for a thermal pressure P/k of 2,462 K per cubic centimetre. The electron density agrees with the range inferred from magnesium and carbon fine-structure diagnostics, which supports the model ionisation fractions. The numbers are computed from stated published inputs, not measured here, and the deposited script exposes every step.