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At least 451 records · Page 25

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NASA NTRS1985

The SO2 atmosphere and ionosphere of Io - Ion chemistry, atmospheric escape, and models corresponding to the Pioneer 10 radio occultation measurements

Some numerical models of the SO2 atmosphere and ionosphere of Io at the time of the Pioneer 10 fly-by are calculated. It is shown that the formation of the observed ionosphere in the downstream direction may have required the precipitation of electrons, and that solar EUV radiation alone cannot account for it. In a comparison with Pioneer 10 radio occultation measurements, an electron impact in the range 500-800 eV was found to agree with Pioneer 10 radio occultation data for a SO2 atmosphere with a surface density of 4 x 10 to the 10th per cu cm. The relatively narrow energy range and flux required for incident electrons suggests that a fraction of a torus plasma compresses the ionosphere. It is pointed out on the basis of the model calculations that a weak magnetic field may be associated with Io.

Kumar, S.↗

NASA NTRS1985

Long-term (solar cycle) and seasonal variations of upflowing ionospheric ion events at DE 1 altitudes

In the investigation conducted by Yau et al. (1984), the occurrence frequency distribution of upflowing ionospheric ions (UFI) in the auroral and polar cap ionosphere up to 23,300 km altitude was determined using the Energetic Ion Composition Spectrometer (EICS) data acquired in the first orbital cycle (18 months) of Dynamics Explorer 1 (DE 1). The present paper is concerned with the seasonal and long-term variations in the occurrence and composition characteristics of upflowing ions. The observed solar cycle and seasonal dependences of upflowing ionospheric ion occurrence morphology are summarized schematically in a graph. It is found that the acceleration altitude of O(+) UFI is modulated by the atmospheric scale height, which increases with increasing solar activity (EUV flux). The O(+) UFI occurrence frequency (and hence O(+) outflow) increases at solar maximum.

Yau, A. W.↗

NASA NTRS1985

The geomagnetic mass spectrometer - Mass and energy dispersions of ionospheric ion flows into the magnetosphere

Observations of ion flows in the polar magnetosphere, made by the retarding ion mass spectrometer on NASA's Dynamics Explorer (DE) 1, are compared with those made simultaneously in the topside ionosphere by the ion drift meter on the lower-altitude DE 2 spacecraft. The results show the dayside auroral ionosphere to be a significant and highly persistent source of plasma for the magnetosphere. The upwelling ionospheric ions are spatially dispersed, according to both their energy and mass, by the combined actions of the geomagnetic field and the dawn-to-dusk convection electric field, in an effect analogous to the operation of an ion mass spectrometer.

Lockwood, M.↗

NASA NTRS1986

The ionosphere of Uranus - A myriad of possibilities

A one-dimensional model has been used to study the effects of exospheric temperature, methane and water influx, ionospheric outflow, and electron precipitation on the composition and structure of the ionosphere of Uranus. Peak ion concentrations range from 1000 to 1 million per cu cm with a wide variation in peak altitude, which depends strongly on the exospheric temperature. In all the cases considered, H(+) is the major ion in the topside ionosphere. At altitudes near or below the peak, H3(+) and CH5(+) can dominate, depending on the magnitude of CH4 and H2O influx. Atomic hydrogen column depths above the methane absorbing layer exceed 10 to the 17th per sq cm and can produce large (400 R) emissions of resonantly scattered Lyman-alpha. In the sunlit polar cap, electron precipitation with energy fluxes of 0.6 to 1.0 erg/sq cm s results in direct production of Lyman-alpha emissions that exceed 1 kR.

Chandler, M. O.↗

NASA NTRS1985

On the dynamo generation of flux ropes in the Venus ionosphere

Small scale magnetic field structures or 'flux ropes' observed in the ionosphere of Venus can be interpreted as the result of a kinematic dynamo process acting on weak seed fields. The seed fields result from the prevailing downward convection of magnetic flux from the vicinity of the ionopause, while small scale fluctuations in the velocity of the ionospheric plasma, which can be caused by collisional coupling to gravity waves in the neutral atmosphere, provide the mechanism by which the field is twisted and redistributed into features of similar scale. This mechanism naturally explains some of the average properties of flux ropes such as the variation of their characteristics with altitude and solar zenith angle. It also elucidates the relationship between the large scale and small scale ionospheric magnetic fields.

Luhmann, J. G.↗

NASA NTRS1985

Transient tropospheric electric fields resulting from sudden changes in ionospheric conductivity

Electric field mapping in the earth's atmosphere has been a research subject for more than 20 years. The present paper is concerned with the downward mapping of an ionospheric electric field into the troposphere following a 'sudden' - change in the atmospheric conductivity profile. The formulation is limited to the case of a static magnetic field which is vertical to the earth's surface. The obtained results are, therefore, most applicable at high latitudes. It is assumed that the 'sudden' change occurs within a fraction of a second and is sustained for a time of the order of at least several seconds. It is pointed out that such changes in ionospheric conductivity can occur as a result of sudden solar particle events (SPE) or particle precipitation into the lower ionosphere. Attention is given to theory, electric field calculations, and the obtained results.

Dejnakarintra, M.↗

NASA NTRS1987

Yosemite Conference on Ionospheric Plasma in the Magnetosphere: Sources, Mechanisms and Consequences, meeting report

The sixth biennial Yosemite topical conference and the first as a Chapman Conference was held on February 3 to 6, 1986. Due to the recent changes in our perception of the dynamics of the ionospheric/magnetospheric system, it was deemed timely to bring researchers together to discuss and contrast the relative importance of solar versus terrestrial sources of magnetospheric plasma. Although the solar wind was once thought to dominate the supply of plasma in the Earth's magnetosphere, it is now thought that the Earth's ionosphere is a significant contributor. Polar wind and other large volume outflows of plasma have been seen at relatively high altitudes over the polar cap and are now being correlated with outflows found in the magnetotail. The auroral ion fountain and cleft ion fountain are examples of ionospheric sources of plasma in the magnetosphere, observed by the Dynamics Explorer 1 (DE 1) spacecraft. The conference was organized into six sessions: four consisting of prepared oral presentations, one poster session, and one session for open forum discussion. The first three oral sessions dealt separately with the three major topics of the conference, i.e., the sources, mechanisms, and consequences of ionospheric plasma in the magnetosphere. A special session of invited oral presentations was held to discuss extraterrestrial ionospheric/magnetospheric plasma processes. The poster session was extended over two evenings during which presenters discussed their papers on a one-on-one basis. The last session of the conferences was reserved for open discussions of those topics or ideas considered most interesting or controversial.

Gallagher, D. L.↗

NASA NTRS1985

The Venus ionosphere

Physical properties of the Venus ionosphere obtained by experiments on the US Pioneer Venus and the Soviet Venera missions are presented in the form of models suitable for inclusion in the Venus International Reference Atmosphere. The models comprise electron density (from 120 km), electron and ion temperatures, and relative ion abundance in the altitude range from 150 km to 1000 km for solar zenith angles from 0 to 180 deg. In addition, information on ion transport velocities, ionopause altitudes, and magnetic field characteristics of the Venus ionosphere, are presented in tabular or graphical form. Also discussed is the solar control of the physical properties of the Venus ionosphere.

Bauer, S. J.↗

NASA NTRS1986

Parallel acceleration and transport of ions from polar ionosphere to plasma sheet

The effect of the convection electric field in accelerating ions that escape from the polar ionosphere is investigated. It is shown that at high altitudes the velocity component of the ions along the magnetic field may be increased by more than an order of magnitude. The highest velocities are acquired by ions that escape from the region of the ionosphere that is connected along magnetic field lines to the dayside cusps. During disturbed times, ions from that region intercept the center plane of the magnetotail in one to two hours at radial distances exceeding about 6 earth radii. Investigation of the resulting O(+) properties in the center plane, viz., their locations, number densities, and energies, indicates that the polar ionosphere near the cusps is the principal source of the O(+) observed in the plasma sheet. Moreover, a study of the ion motion at quiet and disturbed times indicates that the increase of O(+) in the plasma sheet with increasing AE values (Lennartsson and Shelley, 1986) is due mainly to an increase in the source of O(+) rather than alteration of its transport path.

Cladis, J. B.↗

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

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 NTRS1987

Ionospheric electron-content measurements during the second space-plasma negative-ion experiment (SPINEX-2)

The second space-plasma negative-ion experiment (SPINEX-2), a chemical-release active experiment to investigate negative-ion effects in the ionospheric F region, is described by Mendillo et al. (1982). This paper describes the electron-content measurements in somewhat more detail than would be appropriate there. The circumstances of the experiment, particularly the use of a vehicle with a very high spin rate, presented some unusual challenges during interpretation of the electron-content data. These are described. The resulting profiles show clearly that the chemical release caused a very significant 'hole' in the ionosphere. Under certain fairly realistic assumptions, the actual number of free electrons removed from the region of the peak of the ionospheric F layer is estimated to be about 4 x 10 to the 25th. The same assumptions lead to a simple radial distribution of the depleted region about the rocket trajectory in the neighborhood of the release.

Fulford, J. A.↗

NASA NTRS1988

Estimating Electron Content Of The Ionosphere

Method for estimation total electron content of ionosphere. Based on measurements of signals transmitted from global positioning satellites (GPS's). Ionospheric delays obtained by measuring differential arrival times of signals at two different frequencies. Since GPS observations cover certain regions of sky, location of space probe to be calibrated might not overlap GPS field of view. Certain assumptions made about behavior of ionosphere so total electron content estimated in any direction from receiver.

Lanyi, G. E.↗

NASA NTRS1987

Current to the ionosphere following a lightning stroke

A simple analytical expression for calculating the total current waveform to the ionosphere after a lightning stroke is derived. The validity of this expression is demonstrated by comparison with a more rigorous computer solution of Maxwell's equations. The analytic model demonstrates that the temporal variation of the current induced in the ionosphere and global circuit and the corresponding return current in the earth depends on the conductivity profile at intervening altitudes in the middle atmosphere. A conclusion is that capacitative coupling may provide tighter coupling between the lower atmosphere and the ionosphere than usually considered, in both directions, which may help to explain observations which seem to indicate that magnetospheric phenomena may in some instances trigger lightning.

Hale, L. C.↗

NASA NTRS1987

Ion escape fluxes from the terrestrial high-latitude ionosphere

In this paper, the hydrodynamic transport equations for H(+) and O(+) are solved, including the important dynamic, collisional, and chemical effects that operate in the F region ionosphere below regions of ion acceleration. It is found that the most important parameter controlling the amount of O(+) in plasma outflows is the total ion flux demand imposed on the ionosphere by the higher-altitude acceleration region. The O(+) content is further modulated by the temperature of the exosphere and the resultant composition in the topside ionosphere, and by the location of the lower boundary of the ion acceleration region relative to the crossover altitude, where O and H have equal densities. As solar activity increases, the limiting O(+) escape flux increases, while the limiting H(+) escape flux decreases.

Barakat, A. R.↗

NASA NTRS1987

Photochemistry of planetary ionospheres

The dominant photochemical reactions taking place in the ionospheres of Venus, Saturn, and Comet P/Halley are presented. It is shown that the differences in the ionospheres of these celestial bodies result from the different chemistry, energetics, and dynamics of the respective atmospheres. The role of photochemistry in the formation of the individual ionospheres is discussed.

Nagy, Andrew F.↗

NASA NTRS1987

Spatial and temporal variations of the ion velocity measured in the Venus ionosphere

Temporal and spatial deviations of ion velocity from the dominant flow of the Venusian ionosphere were detected in data collected from a retarding potential analyzer (RPA) aboard the Pioneer-Venus orbiter spectrometer. The ion velocity measurements were analyzed for the first 3.5 Venus years of the Pioneer-Venus mission, approximately through orbit 780. The deviations of ion velocity from the dominant velocity of the Venusian ionosphere, which generally flows nightward and is almost symmetric about the sun-Venus axis, affect both the ionospheric structure and dynamics. Two examples of departure from steady symmetric flow that were measured by the RPA are discussed.

Miller, K. L.↗

NASA NTRS1987

Current review of the Jupiter, Saturn, and Uranus ionospheres

The ionospheres of the major planets Jupiter, Saturn, and Uranus are reviewed in light of Pioneer and Voyager observations. Some refinements to pre-Voyager theoretical models are required to explain the results, most notably the addition of significant particle ionization from 'electroglow' and auroral processes and the need for additional chemical loss of protons via charge exchange reactions with water. Water from the Saturn rings has been identified as a major modifier of the Saturn ionosphere and water influx from satellites and/or meteorites may also be important at Jupiter and Uranus as well, as evidenced by the observed ionospheric structure and the identification of cold stratospheric carbon monoxide at Jupiter.

Waite, J. H., Jr.↗