Engineering topics

Sojka, J. J.

Publications and source records attributed to Sojka, J. J..

At least 37 records · Page 2

NASA NTRS1991

Model and observation comparison of the universal time and IMF by dependence of the ionospheric polar hole

The polar ionospheric F-region often exhibits regions of marked density depletion. These depletions have been observed by a variety of polar orbiting ionospheric satellites over a full range of solar cycle, season, magnetic activity, and universal time (UT). An empirical model of these observations has recently been developed to describe the polar depletion dependence on these parameters. Specifically, the dependence has been defined as a function of F10.7 (solar), summer or winter, Kp (magnetic), and UT. Polar cap depletions have also been predicted /1, 2/ and are, hence, present in physical models of the high latitude ionosphere. Using the Utah State University Time Dependent Ionospheric Model (TDIM) the predicted polar depletion characteristics are compared with those described by the above empirical model. In addition, the TDIM is used to predict the IMF By dependence of the polar hole feature.

Sojka, J. J.↗

NASA NTRS1990

The longitude dependence of the dayside F region trough - A detailed model-observation comparison

The nighttime main F-region trough extends into the sunlit afternoon sector. This trough feature exhibits both a strong magnetic-activity dependence and a longitude (UT) dependence. Whalen (1987), using IGY ionosonde data, showed that both of these effects are readly extracted from f0F2 observations. This study shows that the longitude effect is the same as that contained in the Utah State University time-dependent ionospheric model. It arises from the offset of the geomagnetic axis from the geographic axis. The magnetic-activity dependency is associated with the westward convection in the afternoon sector. It is also contained in the ionospheric model via the empirical magnetospheric convection model.

Sojka, J. J.↗

NASA NTRS1990

A test of convection models for IMF Bz north

The Utah State University Ionospheric Model was run to obtain diurnally reproducible ionospheric densities and temperatures for summer and winter conditions using both distorted two-cell and three-cell convection patterns. Differences due to the different convection patterns manifest themselves in the depth and location of polar holes in the F-region electron density. While the total depth of the model holes is a characteristic of the diurnally reproducible pattern, the features appear and are recognizable within 0.5 h. Langmuir probe data from 41 DE-2 passes, during which the IMF Bz component was northward, have been qualitatively checked against the model predictions. The cross polar cap electron density profiles of a large majority of the passes more closely conform to the distorted two-cell runs for both polarities of the IMF By component. This test can be generalized to rule out proposed convection patterns based on the presence/absence and position of polar electron density holes.

Maynard, N. C.↗

NASA NTRS1989

Global scale, physical models of the F region ionosphere

Consideration is given to the development and verification of global computer models of the F-region which simulate the interactions between physical processes in the ionosphere. The limitations of the physical models are discussed, focusing on the inputs to the ionospheric system such as magnetospheric electric field and auroral precipitation. The possibility of coupling ionospheric models with thermospheric and magnetospheric models is examined.

Sojka, J. J.↗

NASA NTRS1989

A three-dimensional time-dependent model of the polar wind

A time-dependent three-dimensional multiion model of the polar wind was developed, which covers the altitude range of from 120 to 9000 km and takes into account supersonic ion outflow, shock formation, and ion energization during plasma expansion events. The model was used to study the temporal response of global polar wind to changing magnetospheric conditions, for the winter solstice and for solar-minimum conditions in the northern polar region. Graphs illustrating temporal changes with changes in T(e), T(i), and T(n) along the dawn, the trough, and the dusk convection trajectories and in the O(+), O, and H densities along the same convection trajectories are presented together with conntours of the H(+) and the O(+) densities along the three convection trajectories.

Schunk, R. W.↗

NASA NTRS1989

Modeled F region response to auroral dynamics based upon Dynamics Explorer auroral observations

Auroral images from the Dynamics Explorer 1 (DE 1) scanning auroral imager have been combined with in situ auroral precipitation data from the DE 2 low-altitude plasma instrument, to form a time-dependent global auroral energy flux model. This model has both good time (12 min) and spatial (100 km) resolution compared to that currently available for global-scale ionospheric and thermospheric modeling. The development and comparison of this model with others are discussed. Data from an aurorally active period, November 25, 1981, are presented and used as a case study for this model. Using a global ionospheric model, the effect of the DE auroral model is contrasted with that of a conventional empirical auroral energy flux model. Major differences in the modeled F region ionosphere are predicted from this comparative study.

Sojka, J. J.↗

NASA NTRS1989

Theoretical study of the seasonal behavior of the global ionosphere at solar maximum

The seasonal behavior of the global ionosphere was studied using a time-dependent three-dimensional physical model (developed by Shunk and his coworkers) of the ionosphere at altitudes between 120 and 800 km. This model accounts for field-aligned diffusion, cross-field electrodynamic drifts both the equatorial region and at high latitudes, interhemispheric flow, thermospheric winds, polar wind escape, energy-dependent chemical reactions, neutral composition changes, ion production due to solar EUV radiation and auroral precipitation, thermal conduction, diffusion-thermal heat flow, and local heating and cooling processes. The model studies were carried out for both June and December solstice conditions at solar maximum and for low geomagnetic activity. The ionospheric features predicted by the model agreed qualitatively with the available measurements.

Sojka, J. J.↗

NASA NTRS1989

Modelling ionospheric density structures

Large-scale density structures are a common feature in the high-latitude ionsphere. The structures were observed in the dayside cusp, polar cap, and nocturnal auroral region over a range of altitudes, including the E-region, F-region and topside ionosphere. The origins, lifetimes and transport characteristics of large-scale density structures were studied with the aid of a three-dimensional, time-dependent ionospheric model. Blob creation due to particle precipitation, the effect that structured electric fields have on the ionosphere, and the lifetimes and transport characteristics of density structures for different seasonal, solar cycle, and interplanetary magnetic field (IMF) conditions were studied. The main conclusions drawn are: (1) the observed precipitation energy fluxes are sufficient for blob creation if the plasma is exposed to the precipitation for 5 to 10 minutes; (2) structured electric fields produce structured electron densities, ion temperatures, and ion composition; (3) the lifetime of an F-region density structure depends on several factors, including the initial location where it was formed, the magnitude of the perturbation, season, solar cycle and IMF; and (4) depending on the IMF, horizontal plasma convection can cause an initial structure to break up into multiple structures of various sizes, remain as a single distorted structure, or become stretched into elongated segments.

Schunk, R. W.↗

NASA NTRS1988

Dynamics Explorer guest investigator

The research has focused on using the SAI auroral images as a high resolution auroral precipitation input to the USU global scale ionospheric model. From the global scale modeling viewpoint, these images offer unique spatial and temporal resolution since all prior studies have used empirical auroral models. These latter models are devoid of storm, substorm, or discrete oval features. The research focused on the problems in converting images to energy flux; using LAPU data to calibrate these energy fluxes; using the USU Time Dependent Ionospheric Model (TDIM) to look at the ionospheric consequences of this structure; and then using DE-2 in-situ observations to compare with the TDIM ionospheric parameters. In carrying out these studies, several additional investigations cropped up which were pursued to help meet the overall goals. The foremost difficulty in carrying out the TDIM modeling in conjunction with the high resolution DE auroral model was that of defining an appropriate ionospheric convection pattern. Under northward conditions this pattern is very complex. In order to study Theta aurora or in general northward IMF conditions, a new model was required. Hence, a study was completed to supply this new model to drive the TDIM as a function of the IMF. With the DE auroral model having adequate resolution to show structure on the 100's of km and all model electric fields being devoid of such structure, an investigation was pursued to find out the effects of structures in the electric field on the F-region.

Sojka, J. J.↗

NASA NTRS1988

A model study of how electric field structures affect the polar cap F region

The ionospheric polar F region's response to a series of electric field structures is studied theoretically, modeling individual electric field structures as elongated two-cell Volland patterns. A semiempirical model of a single electric field structure is developed, and specific structure polar cap electric field models are described. The results of using these models as an input to the Utah State University time-dependent ionospheric model (TDIM) are presented. The implications of the results for present-day 'smooth' input simulations are considered.

Sojka, J. J.↗

NASA NTRS1987

Dynamics Explorer guest investigator

Four objectives were accomplished during this reporting period. The visible auroral image conversion algorithms were compated with algorithms developed by Dr. M. H. Rees for data at different wavelengths. In the study 630 and 557 nm images were used to deduce the auroral energy flux and characteristic energy of the precipitating auroral electrons. The data for Southward IMF, B sub y negative conditions were collected and put into global format. A total of 55 sets of auroral images were obtained, and then converted to energy flux and characteristic energy data sets. The shortcoming of representing the high latitude convection pattern as a smooth function was written up and submitted to the Journal of Geophysical Research. A series of midlatitude corotational model runs were performed to quantitatively show how the F region varied as a function of electric field, topside number flux, and a topside heat source.

Sojka, J. J.↗

NASA NTRS1987

A theoretical study of the lifetime and transport of large ionospheric density structures

A three-dimensional time-dependent ionospheric model was used to study the spatial and temporal evolution and transport of large-scale high-density ionospheric structures for a range of solar cycle, seasonal, and IMF conditions. Both density depletions and enhancements were considered. It was found that, depending on the IMF, horizontal plasma convection can cause an initial structure to break up into multiple structures of various sizes, to become stretched into elongated segments, or to remain as a single distorted structure. The lifetime of an F-region density structure depends on several factors, including its magnitude, the initial location where it was formed, the season, the solar cycle, and the convection pattern. For example, in summer, the effects of a large density structure can disappear in a few hours or last as long as nine hours, while in winter the effects can persist for 24 hours. The passage of perturbed plasma flux tubes through sunlit and auroral regions can significantly increase the lifetime of plasma enhancements.

Schunk, R. W.↗

NASA NTRS1987

Theoretical study of the high-latitude ionosphere's response to multicell convection patterns

A time-dependent three-dimensional model of the high-latitude ionosphere is used to study the characteristic ionospheric signatures associated with two-, three-, and four-cell plasma convection patterns. It is found that, for two-cell convection, the antisunward flow of plasma from the dayside into the polar cap acts to maintain the densities in this region in winter. For four-cell convection, the two additional convection cells in the polar cap are in darkness most of the time, and the resulting O(+) decay acts to produce twin polar holes that are separated by a sun-aligned ridge of enhanced ionization due to theta-auroral precipitation. For three-cell convection, only one polar hole forms in the total electron density, and an additional O(+) depletion region develops near noon. In this region there are strong electric fields, high ion temperatures, and an enhanced rate of O(+) - NO(+) conversion.

Sojka, J. J.↗

NASA NTRS1987

Theoretical study of the effect of ionospheric return currents on the electron temperature

A time-dependent, three-dimensional model of the high-altitude ionosphere is presently used to study the effects of field-aligned ionospheric return currents on auroral electron temperatures for different seasonal and solar cycle conditions, as well as for different upper boundary heat fluxes. The average, large scale, return current densities, which are a few microamps/sq m, are too small to affect auroral electron temperatures. The thermoelectric effect exhibits a pronounced solar cycle and seasonal dependence, and its heat transport corresponds to an upward flow of electron energy which can be either a source or sink of electron energy depending on altitude and geophysical conditions.

Schunk, R. W.↗

NASA NTRS1987

Investigation into alternative statistical auroral oval models, stage 1

The statistical model sensitivity to different methods of binning the NOAA/TIROS data base was examined. Each partial auroral pass was analyzed to locate the latitude of peak electron energy flux. Then using this location as the final reference latitude, all other data was binned relative to it. The next line of attack was to bin according to where the latitudinal profile center of gravity lay. This procedure overcomes the problem of very narrow intense features controlling the binning. Again the same data sets were analyzed and a binning made. These profiles look better and indeed look similar to the original auroral model. The passes were then binned relative to the equatorward boundary. The problems associated with the equatorward edge were resolved.

Bowline, M.↗

NASA NTRS1986

Theoretical study of the electron temperature in the high-latitude ionosphere for solar maximum and winter conditions

The T(e) variation in the high-latitude ionosphere at altitudes between 120 and 800 km has been modeled for solar maximum, winter solstice, and strong magnetic activity conditions. The calculated electron temperatures are consistent with the plasma densities and ion temperatures computed from a time-dependent ionospheric model. Heating rates for both solar EUV and auroral precipitation were included. In general, the predicted UT variation of the electron temperature that results from the displacement between the magnetic and geographic poles is only a few hundred degrees. However, in sunlit trough regions, T(e) hot spots develop, and these hot spots show a marked UT variation, by as much as 2500 K. The dominant parameter controlling the T(e) variation above 200 km is the magnetospheric heat flux into the ionosphere, which is essentially unknown. For realistic values of the magnetospheric heat flux, the maximum electron temperature ranges from 5000 to 10,000 K at 800 km. A magnetospheric heat flux is particularly effective in enhancing trough electron temperatures. In general, the electron heat flux at high altitudes is uniquely related to the electron temperature and gradient, except on auroral field lines where thermoelectric heat flow is important.

Schunk, R. W.↗

NASA NTRS1986

An interplanetary magnetic field dependent model of the ionospheric convection electric field

An IMF-dependent model of the magnetospheric electric field at ionospheric altitudes has been developed based on published observations, qualitative models, and a limited understanding of the electric field source. The empirical inputs are discussed, and the model is presented in an ionospheric convection situation where corotation is an important ingredient. This leads to a description of sunward ionospheric plasma transport in the polar cap for northward IMF orientations. The validity of the model is discussed, and areas in which more empirical results are required are specified.

Sojka, J. J.↗

NASA NTRS1986

EISCAT velocity patterns for theoretical plasma convection models

Theoretical line-of-sight velocities, as would be observed by the EISCAT radar, are computed for idealized models of plasma convection in the polar ionosphere. The calculations give the velocity as a function of range and Universal Time. For several variants of the Volland and Heelis convection models, how the maxima, minima and reversals of velocity depend on beam azimuth is examined. The analysis is designed to be applied to data from the UK-POLAR experiment, an example of which is shown.

Rishbeth, H.↗