# PlasmaPy-NEI Documentation

The plasmapy_nei package is being developed as an affiliated package of PlasmaPy designed to perform non-equilibrium ionization modelling of plasma. Use cases include the solar wind and coronal mass ejections.

## Non-equilibrium ionization

Plasma that is kept at a constant temperature will eventually reach ionization equilibrium: a state where the ionization rate from an ion or neutral atom with charge $$Z$$ balances the recombination rate from an ion with charge $$Z+1$$. Ionization equilibrium is a valid assumption when the changes in temperature are much longer than the characteristic time scales for ionization and recombination. The assumption of ionization equilibrium is built into many analysis techniques, such as differential emission measure (DEM) analyses of the solar corona and other astrophysical plasmas. This assumption, when valid, greatly simplifies the interpretation of astrophysical spectra.

However, the temperature and density of solar and astrophysical plasma often changes on time scales shorter than the time scale for ionization and recombination. As a consequence, the plasma ends up in a state of non-equilibrium ionization (NEI) . For an example in solar physics, plasma in a coronal mass ejection (CME) drops in density while propagating out of the solar corona. At the lowest heights, the density is high enough that ionization and recombination can keep up with the temperature changes. As the plasma moves away from the Sun, the density drops and the ionization and recombination time scales exceed the propagation time scales. Eventually the charge states freeze out and remain roughly constant for longer than it takes for the plasma to depart the heliosphere. Similarly, low-density plasma that is suddenly heated by a supernova remnant shock wave will be out of ionization equilibrium for some time. In these situations, the charge state distributions must be found by evolving the time-dependent ionization equations.

The plasmapy_nei package is intended to enable students and scientists to perform NEI models of laboratory, heliospheric, and astrophysical plasma. The early versions of this package will account for collisional ionization, radiative recombination, and dielectronic recombination. The time advance is calculated using the eigenvalue method.

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