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At least 289 records · Page 16
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NASA NTRS1989

Solar wind interaction with the ionosphere of Venus inferred from radio scintillation measurements

The observation of S-band (2.3 GHz) radio scintillations in the ionosphere of Venus by the Pioneer Venus Orbiter is reported. In situ plasma measurements and propagation calculations show that the scintillations are caused by electron density irregularities in the topside ionosphere of Venus below the ionopause. It is suggested that these topside plasma irregularities are associated with the penetration of large-scale magnetic fields in the ionosphere. It is found that the disturbed plasma and the scintillations are a manifestation of high-dynamic solar wind interaction with the ionosphere.

Woo, Richard↗
NASA NTRS1989

Ionospheric effects of the extreme solar activity of February 1986

During February 1986, near the minimum of the 11 year Solar sunspot cycle, after a long period of totally quiet solar activity (R sub z = 0 on most days in January) a period of a suddenly enhanced solar activity occurred in the minimum between solar cycles 21 and 22. Two proton flares were observed during this period. A few other flares, various phenomena accompanying proton flares, an extremely severe geomagnetic storm and strong disturbances in the Earth's ionosphere were observed in this period of enhanced solar activity. Two active regions appeared on the solar disc. The flares in both active regions were associated with enhancement of solar high energy proton flux which started on 4 February of 0900 UT. Associated with the flares, the magnetic storm with sudden commencement had its onset on 6 February 1312 UT and attained its maximum on 8 February (Kp = 9). The sudden enhancement in solar activity in February 1986 was accompanied by strong disturbances in the Earth's ionosphere, SIDs and ionospheric storm. These events and their effects on the ionosphere are discussed.

Boska, J.↗
NASA NTRS1989

Kinetic modeling of the Saturn ring-ionosphere plasma environment

A time-independent kinetic plasma model was developed on the basis of the Li et al. (1988) semikinetic plasma model and was used to study the interaction of the Saturnian ionosphere and ring plasma. The model includes the gravitational magnetic mirror and centripetal and ambipolar electric forces, and the effect of the mixing of two plasma populations. The results obtained indicate that the density, temperature, and composition of plasma near the rings changing in the direction from the inner C ring to the outer A ring, due to the fact that the predominant source of plasma changes from the ionosphere to the rings. The model results also suggest that the outflow of hydrogen from the ionosphere to the rings may be shut off for field lines passing through the outer B and A ring, due to the ambipolar electric field set up by the warm ring plasma trapped near the ring plane by the centipetal force. In these regions, there will be a net flux of O(+) ions from the rings to the ionosphere.

Wilson, G. R.↗
NASA NTRS1990

Planning for coordinated space and ground-based ionospheric modification experiments

The planning and conduction of coordinated space and ground-based ionospheric modification experiments are discussed. The purpose of these experiments is to discuss: (1) the nonlinear VLF wave interaction with the ionospheric plasmas; and (2) the nonlinear propagation of VLF waves in the HF-modified ionosphere. It is expected that the HF-induced ionospheric density striations can render the nonlinear mode conversion of VLF waved into lower hybrid waves. Lower hybrid waves can also be excited parametrically by the VLF waves in the absence of the density striations if the VLF waves are intense enough. Laboratory experiments are planned for crosschecking the results obtained from the field experiments.

Lee, M. C.↗
NASA NTRS1991

The structure of the Venus ionosphere

The morphology and temporal variability of the Venus ionosphere are characterized, reviewing the results of recent theoretical investigations, observations, and in situ measurements, especially by the Pioneer Venus Orbiter (PVO). Consideration is given to the Pioneer mission and orbit evolution, early radio occultation profiles of ionospheric N(e), the mean structure and thermal balance of the ionosphere, the ion composition and its dawn-dusk asymmetry, the small-scale spatial structure on the nightside, latitudinal and seasonal variations, solar-cycle effects, suprathermal electrons and superthermal ions, and the global configuration and stability of the ionopause. Data from a single PVO passage through the ionotail are discussed in detail, examining the implications for ion escape and the solar-cycle and short-term variability. The differences among the terrestrial, Martian, and Venusian ionospheres are outlined; the PVO data base is described; and the sources of measurement error are indicated.

Brace, L. H.↗
NASA NTRS1991

Magnetization of the ionospheres of Venus and Mars - Results from radio occultation measurements

Remote sensing radio occultation measurements are used here to study magnetization of the ionospheres of Venus and Mars. For Venus, the measurements yield results on frequency of occurrence of magnetization during solar maximum that are similar to those obtained from Pioneer Venus in situ magnetic field measurements. During solar minimum, magnetization of the Venus ionosphere is more pervasive than at solar maximum. Magnetization extends to higher solar zenith angles and appears stronger than at solar maximum. These results confirm that during solar minimum the high solar wind dynamic pressure state is more prevalent at Venus because the ionospheric plasma pressure is weaker than at solar maximum. Comparison of a large number of electron density profiles of Mars with those of Venus shows an absence of the ledge and disturbed topside plasma observed in the Venus profiles. These results do not constitute evidence against magnetization of the ionosphere of Mars.

Woo, Richard↗
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 NTRS1992

Comparisons of peak ionosphere pressures at Mars and Venus with incident solar wind dynamic pressure

Radio occultation measurements of electron density profiles from Mariner 6 and 7, the Mariner 9 extended mission, and the U.S. Viking orbiters, together with model ion and electron temperature profiles, are used to derive thermal pressure profiles in the Mars ionosphere. The comparison of the Mars peak ionosphere pressure with the incident solar and dynamic pressure suggests that at solar maximum the Mars ionosphere, like that of Venus, should generally be sufficient to balance the incident solar wind pressure. At solar minimum, when the ionosphere is weakest and the solar wind dynamic pressure is highest, only the peak pressures at high solar zenith angles (SZAs) at Mars appear to be strong enough to balance the incident solar wind pressure. This is similar to the situation at Venus at solar minimum.

Zhang, M. H. G.↗
NASA NTRS1992

The ionospheric effects of a weak intrinsic magnetic field at Mars

An improved model of the Martian ionosphere which allows the magnetic field to have any direction in the horizontal plane is presented, as well as results of calculations for several different intrinsic magnetic field strengths and directions. When the solar wind dynamic pressure exceeds the Martian ionospheric thermal pressure, the plasma motion is weakly downward throughout the ionosphere for the case of no intrinsic magnetic field, but when the intrinsic and induced fields are in opposite directions, the plasma flow tends to converge toward the current sheet. As a consequence of this convergence, the plasma density is somewhat enhanced near the current sheet, which is located near an altitude of 170 km. The ionosphere above an altitude of about 190 km is not significantly affected by the existence of an intrinsic field as weak as 60 nT.

Shinagawa, H.↗
NASA NTRS1992

Venus and Mars: Atmospheres, ionospheres, and solar wind interactions; Proceedings of the Chapman Conference, Balatonfured, Hungary, June 4-8, 1990

The present conference discusses the evolution and chemistry-surface interactions of the Venus and Mars atmospheres, the properties of thermospheric gravity waves on earth, Venus, and Mars, far-UV remote sensing of Mars and Venus, in situ results for the Venus ionosphere, radio occultation data for the Mars and Venus ionospheres, and solar-wind interactions with Mars. Also discussed are evidence for waves in the Venus and Mars atmospheres, an interpretation of the large-scale ionospheric magnetic fields of the Venus and Mars daysides' ionospheric plasma, a computer model of solar wind interaction with Venus and Mars, and solar-wind effects on the atmospheric evolution of Venus and Mars.

Luhmann, Janet G.↗
NASA NTRS1992

Diodelike response of high-latitude plasma in magnetosphere-ionosphere coupling in the presence of field-aligned currents

The dynamic processes in the plasma along high-latitude field lines plays an important role in ionosphere-magnetosphere coupling process. A time-dependent, large-scale simulation of these dynamics parallel to the geomagnetic field lines from the ionosphere well into the magnetosphere is created. The plasma consists of hot e(-) and H(+) of magnetospheric origin and low-energy e(-), H(+), and O(+) of ionospheric origin. Including multiple electron species, a major improvement to the model, made it possible for the first time to simulate the upward current region properly and to dynamically simulate the diodelike response of the field-line plasma to the parallel currents coupling the ionosphere and magnetosphere. It is shown that return currents flow with small resistance, while upward currents produce kilovolt-sized potential drops along the field, as concluded from satellite observations. The kilovolt potential drops are due to the effect of the converging magnetic field on the high-energy magnetospheric electrons.

Mitchell, H. G., Jr.↗
NASA NTRS1992

Thunderstorm coupling to the magnetosphere and associated ionospheric effects

This project deals with the coupling of electromagnetic energy released during a thunderstorm to the magnetosphere and the ionosphere. Both the effects of an individual lightning event as well the aggregate of all the lightning events during a thunderstorm are considered. Energy in the very low frequency (VLF) band can play a variety of roles in the magnetospheric and ionospheric physics: generation of plasmaspheric hiss believed to be responsible for the slot region in the radiation belts, generation of lower hybrid waves that can heat ions in the auroral and subauroral regions, precipitation of energetic electrons, ionospheric heating etc. While these phenomena have been identified, and characterized to some extent, the influence and role of thunderstorm energy on the magnetosphere and ionosphere at a global scale is not known. Only recently, simultaneous high resolution (temporal and spatial) data sets from ground based lightning detectors and space and ground based VLF detectors have become available, and thus it has become possible to raise a question of the kind mentioned above and try to answer it quantitatively. Work on the correlation between individual lightning discharges in a thunderstorm as detected by the lightning network and the whistlers observed on the DE-1 satellite continued during this period. Results are summarized.

Inan, Umran S.↗
NASA NTRS1993

Upper limits to the nightside ionosphere of Mars

The nightside ionosphere of Mars could be produced by electron precipitation or by plasma transport from the dayside, by analogy to the Venus, but few measurements are available. We report here model calculations of upper limits to the nightside ion densities on Mars that would be produced by both mechanisms. For the auroral model, we have adopted the downward traveling portions of the electron spectra measured by the HARP instrument on the Soviet Phobos spacecraft in the Martian plasma sheet and in the magnetotail lobes. For the plasma transport case, we have imposed on a model of the nightside thermosphere, downward fluxes of O(+), C(+), N(+), NO(+) and O2(+) that are near the maximum upward fluxes that can be sustained by the dayside ionosphere. The computed electron density peaks are in the range (1.3 - 1.9) x 10 exp 4/cu cm at altitudes of 159 to 179 kin. The major ion for all the models is O2(+), but significant differences in the composition of the minor ions are found for the ionospheres produced by auroral precipitation and by plasma transport. The calculations reported here provide a guide to the data that should be acquired during a future aeronomy mission to Mars, in order to determine the sources of the nightside ionosphere.

Fox, J. L.↗
NASA NTRS1993

The response of the high-latitude dayside ionosphere to an abrupt northward transition in the IMF

We examine the response of the high-latitude ionosphere in the prenoon sector to a northward turning of the IMF. The event was observed in the 11-13 UT interval on June 1, 1987, in the course of a multiday SUNDIAL campaign. The transition in the IMF was observed by the IMP-8 satellite which was located upstream of the earth at a distance of 36 Re. The ionospheric response in the 70-80 deg invariant latitude interval was monitored by two radars. The preexisting plasma convection observed by the radars exhibited large velocities (500-1000 m/s) and stable longterm trends, consistent with the inertial rotation of the convection pattern expected of the conditions then prevailing, B(z) less than 0, B(y) greater than 0. The plasma flow rapidly abated in response to the IMF transition. The electron density measurements made by radar in the meridional plane showed that the ionosphere had been rich in structure with the active deposition of ionization by particle precipitation. Subsequently it resembled an inactive, unstructured mid-latitude configuration. There was a dramatic decrease in the amount of backscatter observed by the HF radar. We analyze the times of transition in the various data sets and show that the ionosphere began to show the effects of the IMF transition about 2 min after its probable arrival at the magnetopause boundary.

Ruohoniemi, J. M.↗
NASA NTRS1994

Modeling of the magnetosphere-ionosphere-atmosphere system

During the period covered by this report we continued to investigate basic processes in magnetosphere-ionosphere coupling and to develop algorithms for analyzing ultraviolet, visible, and X-ray images that will be acquired on the POLAR spacecraft. An image analysis procedure combines several numerical models to obtain a self-consistent picture of the ionosphere and thermosphere. Our ionosphere-aurora model was extended by the inclusion of the magnetic mirror force and a self-consistent parallel electric field in the ionosphere. The electron transport model which forms a critical part of the aurora model was evaluated by comparison with laboratory measurements and auroral observations. We determined the uncertainties that arise from our limited knowledge of cross sections and energy degradation functions for electron neutral interaction.

Source record↗
NASA NTRS1994

Magnetospheric-ionospheric Poynting flux

Over the past three years of funding SRI, in collaboration with the University of Texas at Dallas, has been involved in determining the total electromagnetic energy flux into the upper atmosphere from DE-B electric and magnetic field measurements and modeling the electromagnetic energy flux at high latitudes, taking into account the coupled magnetosphere-ionosphere system. This effort has been very successful in establishing the DC Poynting flux as a fundamental quantity in describing the coupling of electromagnetic energy between the magnetosphere and ionosphere. The DE-B satellite electric and magnetic field measurements were carefully scrutinized to provide, for the first time, a large data set of DC, field-aligned, Poynting flux measurement. Investigations describing the field-aligned Poynting flux observations from DE-B orbits under specific geomagnetic conditions and from many orbits were conducted to provide a statistical average of the Poynting flux distribution over the polar cap. The theoretical modeling effort has provided insight into the observations by formulating the connection between Poynting's theorem and the electromagnetic energy conversion processes that occur in the ionosphere. Modeling and evaluation of these processes has helped interpret the satellite observations of the DC Poynting flux and improved our understanding of the coupling between the ionosphere and magnetosphere.

Thayer, Jeffrey P.↗
NASA NTRS1994

Ionospheric footprint of magnetosheathlike particle precipitation observed by an incoherent scatter radar

We have examined Sondrestrom incoherent scatter radar observations of ionospheric plasma density and temperature distributions and measurements of F region ion drifts that were made during a prenoon pass of the Defense Meteorological Satellite Program (DMSP)-F7 satellite through the radar field of view. The spacecraft traversed a region of intense electron precipitation with a characteristic energy below approximately 200 eV. Particles with such low characteristic energies are believed to be directly or indirectly of magnetosheath origin. The precipitation region had a width about 2 deg invariant latitude and covered the low-latitude boundary layer (LLBL), the cusp, and the equatorward section of the plasma mantle (PM). The corotating radar observed a patch of enhanced electron density and elevated electron temperature in the F2 region between about 10.5 and 12 magnetic local time in the same invariant latitude range where DMSP-F7 detected the soft-electron flux. The ion drift pattern, also obtained by radar, shows that it is unlikely that the plasma patch was produced by solar radiation and advected into the radar field of view. We suggest that the radar observed modifications of the ionospheric plasma distribution, which resulted from direct entry of magnetosheath electrons into the magnetosphere and down to ionospheric altitudes. Model calculations of the ionospheric response to the observed electron precipitation support our interpretation. The spectral characteristics of the electron flux in the LLBL, cusp, and equatorward section of the PM were in this case too similar to allow to distinguish between them by using incoherent scatter radar measurements only.

Watermann, Jurgen↗
NASA NTRS1994

Response of the thermosphere and ionosphere to geomagnetic storms

Four numerical simulations have been performed, at equinox, using a coupled thermosphere-ionosphere model, to illustrate the response of the upper atmosphere to geomagnetic storms. The storms are characterized by an increase in magnetospheric energy input at high latitude for a 12-hour period; each storm commences at a different universal time (UT). The initial response at high latitude is that Joule heating raises the temperature of the upper thermosphere and ion drag drives high-velocity neutral winds. The heat source drives a global wind surge, from both polar regions, which propagates to low latitudes and into the opposite hemisphere. The surge has the character of a large-scale gravity wave with a phase speed of about 600 m/s. Behind the surge a global circulation of magnitude 100 m/s is established at middle latitudes, indicating that the wave and the onset of global circulation are manifestations of the same phenomena. A dominant feature of the response is the penetration of the surge into the opposite hemisphere where it drives poleward winds for a few hours. The global wind surge has a preference for the night sector and for the longitude of the magnetic pole and therefore depends on the UT start time of the storm. A second phase of the meridional circulation develops after the wave interaction but is also restricted, in this case by the buildup of zonal winds via the Coriolis interaction. Conservation of angular momentum may limit the buildup of zonal wind in extreme cases. The divergent wind field drives upwelling and composition change on both height and pressure surfaces. The composition bulge responds to both the background and the storm-induced horizontal winds; it does not simply rotate with Earth. During the storm the disturbance wind modulates the location of the bulge; during the recovery the background winds induce a diurnal variation in its position. Equatorward winds in sunlight produce positive ionospheric changes during the main driving phase of the storm. Negative ionospheric phases are caused by increases of molecular nitrogen in regions of sunlight, the strength of which depends on longitude and the local time of the sector during the storm input. Regions of positive phase in the ionosphere persist in the recovery period due to decreases in mean molecular mass in regions of previous downwelling. Ion density changes, expressed as a ratio of disturbed to quiet values, exhibit a diurnal variation that is driven by the location of the composition bulge; this variation explains the ac component of the local time variation of the observed negative storm phase.

Fuller-Rowell, T. J.↗