We describe a parameterized ionospheric model (PIM), a global model of theoretical ionospheric climatology based on diurnally reproducible runs of four physics based numerical models of the ionosphere. The four numerical models, taken together, cover the E and F layers for all latitudes, longitudes, and local times. PIM consists of a semianalytic representation of diurnally reproducible runs of these models for low, moderate, and high levels of both solar and geomagnetic activity and for June and December solstice and March equinox conditions. PIM produces output in several user selectable formats including global or regional latitude/longitude grids (in either geographic or geomagnetic coordinates), a set of user specified points (which could lie along a satellite orbital path), or an altitude/azimuth/elevation grid for a user‐specified location. The user selectable output variables include profile parameters (ƒ0F2, hmF2, total electron content, etc.), electron density profiles, and ion composition (O+, NO+, and O2+).

Daytime double maxima (twin peaks or bite-outs) in the ionospheric total electron content (TEC) at middle and lower latitudes are found to be related to substorm signatures shown in both auroral electrojet and ring current variations. Case studies reveal that during substorm onset and recovery phases, the penetration of magnetospheric convection electric fields and their subsequent "overshielding" effects may be the major dynamical sources of these events. A theoretical lowlatitude ionospheric model is used to simulate the dynamical effects of electric field disturbances on F region electron density and TEC. It is demonstrated that the diurnal double maxima in TEC can be created by a combined effect of E x B drift and altitude-dependent F region chemical loss. The required zonal electric fields are found to have greater penetration efficiency in the early evening sector and their latitudinal requirements appear to change with local time. The time scales for the modeled penetration and overshielding effects are 2-3 hours. Modeling results also show that considerable structuring in the local time variation of the ionospheric "equatorial anomaly" can occur due to the interplay of convection electric field penetration and overshielding effects. The possible cause of the midday bite-out ionospheric disturbances by the meridional winds associated with traveling atmospheric disturbances (TADs) is also addressed in modeling studies, but the specialized nature of the required TADs makes this a less well understood substorm-related mechanism.1. Sources of Ionospheric DisturbanceAt middle latitudes, the ionospheric total electron content has been shown to exhibit a major response to geomagnetic storms and substorms [e.g., Mendillo, 1971; Lanzerotti et al., 1975; Mendillo and Klobuchar, 1975; Essex et al., 1981; Titheridge and Buonsanto, 1988; PrSlss et al., 1991; Huang and Gheng, 1991]. This response is believed to be mainly due to a combined effect of dynamical and chemical processes. Thermospheric circulation variations due to Joule heating and energetic particle precipitations in the auroral zone [Rishbeth, 1975; Volland, 1979; Fuller-Rowell and Rees, 1981; Robie et al., 1982, 1987], and electric field perturbations caused by penetration processes [e.g., Vasyliunas, 1972; Jaggi and Wolf, 1973; Testud et al., 1975; Nopper and C'arovillano, 1978; Kamide and Matsushita, 1981; Senior and Blanc, 1984; Earle and Kelley, 1987; Spiro et al., 1988; Fejer et al., 1990a, 1990b] as well as disturbance dynamo processes [Blanc and Richmond, 1980; Fejer et al., 1983], are thought to be major sources of the dynamical disturbances in the middle-and lower-latitude ionosphere. During substorms, traveling atmospheric disturbances (TADs), also triggered by high-latitude Joule heating and particle precipitations, can cause traveling ionospheric disturbances (TIDs) on a global scale [e.g., Yeh and Liu, 1974; Testud et al.Paper number 93JA00502. 0148-0227193193JA-00502$05.00 et al., 1991]. As F region charged particles move upward or downward, ...

A data base of equivalent slab thickness observations covering nearly two complete solar cycles has been obtained at a mid‐latitude site (Hamilton, Massachusetts). The mean variations are described both qualitatively, in terms of changes to profile shapes, and quantitatively, in terms of a simple numerical model. A number of currently available ionospheric models have also been used to predict values of slab thickness (τ = total electron content/Nmax) over Hamilton, Massachusetts, and comparisons of these values with those observed provided validation of the models. Adjustments to the model profiles were suggested in a couple of cases.

A new system has been devised to automatically scale digital ordinary‐ray ionograms. The system has been developed for use at mid‐latitudes and has been trained on a full set of ionograms that are typical of the region. This paper describes the stages adopted in forming, recognizing, and scaling traces from the various ionospheric layers. Examples of the output available with this system and an error analysis of autoscaled parameters are presented as evidence of the system's capabilities.

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