D. T. Decker

Parameterized Ionospheric Model: A Global Ionospheric Parameterization Based on First Principles Models

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+).

Modeling Polar Cap F-Region Patches Using Time Varying Convection

Here the authors present the results of computerized simulations of the polar cap regions which were able to model the formation of polar cap patches. They used the Utah State University Time-Dependent Ionospheric Model (TDIM) and the Phillips Laboratory (PL) F-region models in this work. By allowing a time varying magnetospheric electric field in the models, they were able to generate the patches. This time varying field generates a convection in the ionosphere. This convection is similar to convective changes observed in the ionosphere at times of southward pointing interplanetary magnetic field, due to changes in the B[sub y] component of the IMF.

Modeling daytime F layer patches over Sondrestrom

A comprehensive, time-dependent, high-latitude, one-species F region model has been developed to study the various physical processes which are believed to affect the polar cap plasma density distributions as a function of altitude, latitude, longitude, and local time. These processes include production of ionization by solar extreme ultraviolet radiation and particle precipitation; loss through charge exchange with N 2 and O 2 ; and transport by diffusion, neutral winds, and convection E×B drifts. In our initial calculations we have modeled highly structured plasma densities characterized by digisonde observations at Sondrestrom using both a time-dependent global convection pattern and spatially localized regions of transient high-speed flow

Modeling the formation of polar cap patches using large plasma flows

Recent measurements made with the Sondrestrom incoherent scatter radar have indicated that the formation of polar cap patches can be closely associated with the flow of a large plasma jet. In this paper, we report the results of a numerical study to investigate the role of plasma jets on patch formation, to determine the temporal evolution of the density structure, and to assess the importance of O+ loss rate and transport mechanisms. We have used a time-dependent model of the high-latitude F region ionosphere and model inputs guided by data collected by radar and ground-based magnetometers. We have studied several different scenarios of patch formation. Rather than mix the effects of a complex of variations that could occur during a transient event, we limit ourselves here to simulations of three types to focus on a few key elements. The first attempt employed a Heelis-type pattern to represent the global convection and two stationary vortices to characterize the localized velocity structure. No discrete isolated patches were evident in this simulation. The second modeling study allowed the vortices to travel according to the background convection. Discrete density patches were seen in the polar cap for this case. The third case involved the use of a Heppner and Maynard pattern of polar cap potential. Like the second case, patches were seen only when traveling vortices were used in the simulation. The shapes of the patches in the two cases of moving vortices were defined by the geometrical aspect of the vortices, i.e. elliptical vortices generated elongated patches. When we “artificially” removed the Joule frictional heating, and hence any enhanced O+ loss rate, it was found that transport of low density plasma from earlier local times can contribute to ∼60% of the depletion. We also found that patches can be created only when the vortices are located in a narrow local time sector, between 1000 and 1200 LT and at latitudes close to the tongue of ionization.

Modeling boundary blobs using time varying convection

The Global Theoretical Ionospheric Model (GTIM) has been used to study a mechanism which generates F-region electron density enhancements known as boundary blobs. The model calculates the O{sup +} density as a function of altitude, latitude, and local time. It includes the effects of production of ionization by solar extreme ultraviolet radiation and electron precipitation; loss through charge exchange with N{sub 2} and O{sub 2}; and transport by diffusion, neutral winds, and ExB convection drifts. Using time-dependent convection patterns that previously were used to study polar cap patch formation, it was found that patches can be convected out of the polar cap and swept sunward by the dusk convection cell. The resulting structures have many of the features associated with boundary blobs: extended in local time and altitude, narrow in latitude, located in the return flow region of the aurora, and densities up to a factor of 8 over background. These results are the first quantitative, first principles verification and extension of the trajectory modeling. 9 refs., 5 figs.

Comparisons of TOPEX and Global Positioning System total electron content measurements at equatorial anomaly latitudes

Discrepancies exist between vertically measured ionospheric total electron content (TEC) and slant measurements of TEC that are converted to vertical with the use of a mapping function. Vertical measurements of TEC that are determined by the TOPEX altimeter are compared with equivalent vertical TEC values that are derived from the Global Positioning System (GPS) constellation at latitudes −40° to +40° and longitudes 180° to 360° during periods in 1993, 1994, and 1995. Also, comparisons are made with the Phillips Laboratory parameterized ionospheric model (PIM) predictions of vertical and equivalent vertical TEC from the same observation points. A trend of disagreement in maximum and minimum TEC values is observed between TOPEX and GPS passes that involve measurements within 20° to the south and to the north of the geomagnetic equator. PIM model predictions, although not exact in value, are consistent in configuration with these observations of overestimation as well as underestimation of TEC. It is shown that the errors are dependent on not only elevation angle but also azimuth of the line-of-sight direction. The elevation mapping function that relates the line-of-sight TEC to vertical TEC and other assumptions that are made in the application of the ionospheric shell model may be contributing factors to the slant-to-vertical conversionerrors.

Improving IRI-90 low-latitude electron density specification

At low latitudes under moderate to high solar conditions, a number of comparisons between the international reference ionosphere (IRI-90) model of F region electron density profiles and observed profiles measured by the Jicamarca incoherent scatter radar indicate that during the daytime the observed profile shape can be much broader in altitude than that specified by IRI-90, while at night, just after sunset, observed F2 peak altitudes are significantly higher than what is specified by IRI-90. Theoretically derived ionospheric parameters such as F2 peak density (NmF2), F2 peak altitude (hmF2), and profile shape, which are contained in the parameterized ionospheric model (PIM), have been shown in some cases to be in better agreement with Jicamarca observations. This paper describes a new low-latitude option for IRI-90 that uses five ionospheric parameters derived from PIM: the bottomside profile half thickness, NmF2 hmF2, and two parameters of a topside Chapman profile. The generation of electron density profiles using these five parameters is presented, as well as a description of how these parameters can be implemented into the IRI-90 model.

Determination of Ionospheric Electron Density Profiles From Satellite UV Emission Measurements

This paper addresses the problem of using satellite ultraviolet measurements to deduce the ionospheric electron density profile (EDP). The ionospheric processes that produce the ultraviolet emissions differ from region to region, so it is necessary to consider separate approaches for the various ionospheric subregions. We will discuss approaches suitable for (1) the midlatitude daytime ionosphere, (2) the midlatitude nighttime ionosphere, and (3) the undisturbed auroral E-layer.

Determination Of Daytime Midlatitude Electron Density Profiles From Satellite UV And In-Situ Data

This paper addresses the problem of determining accurate, real-time ionospheric electron density profiles (EDP) using passive UV and other sensor measurements from satellites. This is done by using real-time satellite data to constrain the geophysical parameters that appear in an ab initio theoretical daytime midlatitude ionospheric model, creating what we call the AFGL constrained EDP model. In February and March 1987 a series of coincident measurements were made by the Millstone Hill incoherent scatter radar and the Polar BEAR satellite multispectral UV imager. These observations of electron density profiles and daytime UV airglow give us an excellent opportunity to test the AFGL constrained EDP model with observational data. Using the satellite UV data and the radar-determined plasma data at one altitude (in-situ satellite plasma measurements were not available), we apply the constrained AFGL model to compute the EDP from about 90 to 600 km. By comparing the model results to the EDP measured by the radar, we show that the AFGL constrained EDP model predicts the EDP more accurately than can empirical models, such as the International Reference Ionosphere (IRI), which contain no real-time data.