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At least 883 records · Page 49
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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

A model of ionospheric image structure underneath a braking, cross-field plasma jet

A plasma jetting across the geomagnetic field above the ionosphere tends to brake by ohmic dissipation of Pedersen currents. The braking can affect the ionosphere underneath if the associated Pedersen drifts are intense and prolonged enough to cause cumulative image structuring. Here, such image structuring is studied for the parameter regime of forthcoming releases from the Combined Release and Radiation Effects Satellite, involving photoionization of kilograms of barium vapor moving at orbital velocity. The resultant structuring in the upper E-region offers possible diagnostic telltales of the braking process.

Jacobson, Abram R.↗
NASA NTRS1986

Atmospheres and ionospheres of the outer planets and their satellites

Essential aspects of physical and chemical processes in the atmospheres and ionospheres of the outer planets are examined in an introductory overview of current knowledge, intended both for graduate students of planetary science and for practicing scientists. Chapters are devoted to bulk atmospheric compositions, thermal structures, cloud structures, vertical mixing, photochemistry, the ionospheres, and the satellites. Measurement data are compiled in tables, and extensive graphs and diagrams are provided.

Atreya, Sushil K.↗
NASA NTRS1987

A one-dimensional time-dependent model of the magnetized ionosphere of Venus

The behavior and time evolution of the large-scale magnetic fields and plasma of the dayside Venus ionosphere are studied using a one-dimensional model. The coupled continuity, momentum, and Maxwell's equations are solved simultaneously for O(+), O2(+), and H(+), and the magnetic field. The calculated magnetic field profiles are in good agreement with Pioneer Venus orbiter magnetometer observations. The magnetic field structure is quasi-steady for slow changes of the solar wind dynamic pressure. The peak at 165 km is maintained by downward convection from higher altitudes. The time scale for the decay of the field by the pure one-dimensional vertical diffusion/convection process is several hours unless the flux is resupplied from the top of the ionosphere.

Shinagawa, H.↗
NASA NTRS1987

The solar wind-magnetosphere-ionosphere current-voltage relationship

The global current-voltage relationship for the solar wind-magnetosphere-ionosphere system (SW-M-I) is investigated, restricting the study to strong southward IMF. The dynamo presently identified is on open field lines, and it operates at close to short circuit and at a fraction of the available power output. Control of the dynamo by ionospheric conductivity is discussed. Implications of the simulation results, including the relationship between open and closed field dynamos, the effect of solar wind conditions, the control of reconnection on the bow, the size of the open field line region, and the effects of increased auroral conductivity, are discussed.

Fedder, J. A.↗
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

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

Description of the mean behaviour of ionospheric plasma temperatures

A status report on the empirical modeling of ionospheric electron and ion temperatures is given with special emphasis on the models used in the International Reference Ionosphere (IRI). Electron temperature models have now reached a state where reliable prediction of the mean altitudinal, latitudinal and diurnal variations is possible. These models are largely based on satellite measurements, but comparisons with incoherent scatter radar measurements have shown excellent agreement. Variations with season and magnetic and solar activity seem to be small and are not yet included consistently in these models. Similar to the electron temperature, the ion temperature shows the largest variations with altitude, latitude and local time. But due to the larger mass, these variations are smoother and more steady in the case of the ions and therefore easier to model. Nevertheless, very few ion temperature models exist. The IRI model takes advantage of the observed concurrence of the ion temperature with the neutral temperature at low altitudes and with the electron temperature at high altitudes.

Bilitza, Dieter↗
NASA NTRS1987

Magnetosphere, ionosphere and thermosphere; Proceedings of the Topical Meeting of the Twenty-sixth COSPAR Plenary Meeting, Toulouse, France, June 30-July 11, 1986

Papers are presented on satellite observations of new particle and field signatures associated with SAR arc field lines at magnetospheric heights, the structure of the polar oval from simultaneous observations of the optical emissions and particle precipitations during the period of high solar activity between 1981 and 1982, and SAR-arcs and emissions in the main trough of the electron concentration. Also considered are a model calculation of hydrogen Balmer emissions under various modes of proton precipitation, electron precipitation near L = 4, and an investigation of the thermosphere-ionosphere interaction by means of the neutral post-storm effect. Other topics include auroral electric field penetration into the middle-latitude trough, field-aligned currents and related phenomena in the cust, and the positive phase of ionospheric storms and its connection with the dayside cusp.

Schmerling, E. R.↗
NASA NTRS1988

Can the high latitude ionosphere support large field-aligned ion drifts?

A three-dimensional time-dependent model of the ionosphere is used to examine recent results on vertical drift velocities and electron densities in the high latitude ionosphere. Upper limits for the downward ion velocity were found to be smaller than those obtained from previous measurements. The downward force in the model was arbitrarily increased to try to account for the narrow electron density profiles. It is noted that if the common volume measurement is made in a region of O(+) precipitation then the line profile would not be Doppler shifted when viewed off-zenith, and small field-aligned velocities and narrow profile widths would result.

Sica, R. J.↗
NASA NTRS1988

Ionospheric convection inferred from interplanetary magnetic field-dependent Birkeland currents

Computer simulations of ionospheric convection have been performed, combining empirical models of Birkeland currents with a model of ionospheric conductivity in order to investigate IMF-dependent convection characteristics. Birkeland currents representing conditions in the northern polar cap of the negative IMF By component are used. Two possibilities are considered: (1) the morning cell shifting into the polar cap as the IMF turns northward, and this cell and a distorted evening cell providing for sunward flow in the polar cap; and (2) the existence of a three-cell pattern when the IMF is strongly northward.

Rasmussen, C. E.↗
NASA NTRS1988

Ionospheric traveling convection vortices observed near the polar cleft - A triggered response to sudden changes in the solar wind

Analysis of 20-second resolution magnetometer data from an array of temporary stations operated around Sondre Stromfjord, Greenland, during the summer of 1986 shows the signatures of localized ionospheric traveling convection vortices. An example of an isolated event of this kind observed near 08 local time is presented in detail. This event consists of a twin vortex pattern of convection consistent with the presence of two field-aligned current filaments separated by about 600 km in the east-west direction. This system of currents is observed to move westward (tailward) past the array of stations at about 4 km/sec. The event is associated with relative quiet time ionospheric convection and occurs during an interval of northward IMF. It is, however, associated with a large fluctuation in both the Z and Y components of the IMF and with a large sudden decrease in the solar wind number density. The propagation of the system is inconsistent with existing models of FTE current systems, but nevertheless appears to be related to a readjustment of the magnetopause boundary to a sudden change in the solar wind dynamic pressure and/or to a change in reconnection brought about by a sudden reorientation of the IMF.

Friis-Christensen, E.↗
NASA NTRS1988

Observation of an ionospheric disturbance caused by a gamma-ray burst

A first observation of an ionospheric disturbance from a gamma-ray burst is reported. The burst, GB30801, occurred at 22:14:18 UT on August 1, 1983, and was one of the strongest ever observed. The total fluence was 0.002 erg/sq cm, most of which occurred in the first 4 s of the burst. Simultaneously, a change was observed in the amplitude of a VLF radio signal from a transmitter in Rugby, England indicative of an ionospheric disturbance. Weaker disturbances were also recorded at the same receiving site on signals from VLF stations in Annapolis, Maryland and Lualualei, Hawaii. The times of the burst and the disturbances are coincident within the 10-s resolution of the VLF recording system. No similar disturbances were observed within 60 hr around the time of the burst. In the future, a network of VLF burst monitors may provide measurements of the total ionizing energy fluence from a burst, as well as some limited directional information.

Fishman, G. J.↗
NASA NTRS1988

Viking electron temperature measurements - Evidence for a magnetic field in the Martian ionosphere

Further analysis of the Viking RPA data has now provided measurements of the thermal electron temperature in the upper Martian ionosphere. It is found that Te is several thousand degrees K, i.e., only of the order of twice the ion temperature. The sum of all the measured partial plasma pressures, including ions and suprathermal electrons, has a minimum value of about 5 x 10 to the -10 dyn/sq cm near 350 km and is found to be insufficient to balance the measured electron pressure in the shocked solar wind near 1000 km altitude, by a factor of the order of 4. Thus there is no doubt that a magnetic field of at least 30 to 40 nT permeates the ionosphere. This conclusion is not inconsistent with previous assessments, but it now has a firm observational basis. These data do not uniquely establish whether the magnetic field is intrinsic or induced, but our assessment is that a significant intrinsic moment is not required.

Hanson, W. B.↗
NASA NTRS1988

Effect of field-aligned potential drop in a global magnetosphere-ionosphere coupling model

Effects of field-aligned potential drops on the magnetosphere-ionosphere coupling in a steady state are studied on a global ionospheric scale. It is shown that a constant-current generator can support a larger field-aligned potential drop than a constant voltage generator under similar conditions. The magnetospheric convection pattern is distorted more in the constant current generator case than in the constant voltage generator case. The main difference between a constant current generator and a constant voltage generator is found to lie in their ability to adjust the vorticity of the magnetospheric convection. The results show that a constant current generator allows the vorticity of the magnetospheric convection to adjust so that the field-aligned current can be kept constant under the loading influence of the field-aligned potential. On the other hand, a constant voltage generator by definition cannot adjust the vorticity of the magnetospheric convection to maintain the field-aligned current under the loading influence of the field-aligned potential.

Kan, J. R.↗
NASA NTRS1988

Lightning-related fields in the ionosphere

Lightning-related transient electric fields and conduction current densities due to the redistribution of charge by the return stroke, but not directly associated with the high-intensity current channel, are investigated via a simulation of Maxwell's equations. The investigation presents evidence to suggest that the lightning-related charge redistribution and subsequent relaxation, rather than the high intensity current, is the source of the major component of the transient field and energy coupled to the ionosphere. A time domain finite element analysis is employed in the simulation, using a conductivity profile obtained simultaneously with electric field measurements which agree, in general terms, with the simulations. It is concluded that prior depictions of these phenomena underestimate the fraction of the total energy that couples to and dissipates within the ionosphere and magnetosphere.

Baginski, M. E.↗
NASA NTRS1988

A comparison of mapped and measured total ionospheric electron content using global positioning system and beacon satellite observations

Total ionospheric electron contents (TEC) were measured by global positioning system (GPS) dual-frequency receivers developed by the Jet Propulsion Laboratory. The measurements included P-code (precise ranging code) and carrier phase data for six GPS satellites during multiple five-hour observing sessions. A set of these GPS TEC measurements were mapped from the GPS lines of sight to the line of sight of a Faraday beacon satellite by statistically fitting the TEC data to a simple model of the ionosphere. The mapped GPS TEC values were compared with the Faraday rotation measurements. Because GPS transmitter offsets are different for each satellite and because some GPS receiver offsets were uncalibrated, the sums of the satellite and receiver offsets were estimated simultaneously with the TEC in a least squares procedure. The accuracy of this estimation procedure is evaluated indicating that the error of the GPS-determined line of sight TEC can be at or below 1 x 10 to the 16th el/sq cm. Consequently, the current level of accuracy is comparable to the Faraday rotation technique; however, GPS provides superior sky coverage.

Lanyi, Gabor E.↗
NASA NTRS1989

Dynamics of the middle atmosphere in winter (Dynamics). Interrelation between the different variations of turbulent diffusion and ionospheric absorption originating in the middle atmosphere

The turbulent diffusion coefficient was computed from the parameters of sporadic E layers using the wind shear theory of midlatitude sporadic E and models of the ionosphere as well as that of the neutral upper atmosphere. The turbulent diffusion coefficient obtained for the period of circulation disturbances associated with stratospheric warmings and for the intervals of the winter anomaly indicate changes similar to the ionospheric absorption of radio waves, in the former case decreased, in the latter case increased values. This may hit at the role of turbulent transport in the formation of these anomalies. On the basis of these findings, a seasonal variation of the turbulent diffusion coefficient having a minimum in summer and an increase of this parameter with increasing geomagnetic activity are anticipated.

Bencze, P.↗