Quoted from: https://ccmc.gsfc.nasa.gov/modelweb/ionos/pim.html

Parameter: Electron density, TEC

Brief Description:
The Parameterized Ionospheric Model (PIM) is a climatology model built from combining model output from the Global Theoretical Ionospheric Model (GTIM) model for low and middle latitude with output from the TDIM model for high latitudes and from the empirical Gallagher et al. [Adv. Space Res., 8, 15-24, 1988] model for plasmaspheric altitudes. The altitude profiles obtained with these models was then represented by empirical orthonormal functions (EOF). The resulting set of several million coefficients constitutes the PIM model. EOFs have the advantage of a simple and straightforward fitting process different from the non-linear schemes required for Chapman functions. But EOFs do not include the peak density and height as explicit parameters. To get these parameters the full profile has to be constructed and then a peak-finding algorithm has to be applied. PIM consists of portable FORTRAN source code and a large database of coefficients for the orthogonal function expansion. For user specified geophysical conditions and spatial coordinates, PIM produces electron density profiles (EDPs) between 90 and 25000 km altitude, corresponding critical frequencies and heights for the ionospheric E and F2 regions, and Total Electron Content (TEC).

The Parameterized Real-Time Ionospheric Specification Model (PRISM) is a version of PIM with data assimilation capabilities. The GTIM evolved from a low-latitude, to a mid-latitude and finally global model. It computes O+, H+ and He+ densities along magnetic field lines from 90 to 1600 km. In addition to the standard empirical input models for thermospheric parameters, solar EUV, ExB drift pattern, and precipitating energetic electrons, GTIM also requires the ion and electron temperatures as input parameters. To provide an easy accessible version of the GTIM model, output parameters were computed on a global grid and then fitted with Chapman functions. The set of Chapman parameters constitutes the Semi-Empirical Low Latitude Ionospheric Model (SLIM). An analytical version of the SLIM model, the Fully-Analytical Ionospheric Model (FAIM), was produced on the basis of the Chiu [1975] model formalism by altering the low latitude portion of this model so that it reproduces the SLIM profiles. The GTIM, SLIM, FAIM, PIM and PRISM family of models was developed at the US Air Force Research Laboratories.

Availability: CPI website

References:
R.E. Daniell, L.D. Brown, D.N. Anderson, M.W. Fox, P.H. Doherty, D.T. Decker, J.J. Sojka, and R.W. Schunk, Parameterized Ionospheric Model: A Global Ionospheric Parameterization Based on First Principle Models, Radio Science, 30, 5, 1499-1510, 1995