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NASA NTRS1994

Global observations and modeling of the ionosphere, thermosphere and mesosphere

The Imaging Spectrometric Observatory (ISO) flown on the Atmospheric Laboratory for Applications and Science (ATLAS) 1 mission between 24 Mar 1992 and 2 Apr 1992, acquired a database designed to study several outstanding problems in the ionosphere, thermosphere and mesosphere. In this paper we discuss the goals and preliminary results from three of these studies. To support these studies, the ISO acquired a database of: (1) emissions for the retrieval of neutral and ion densities to test global models of the ionosphere and thermosphere; (2) emissions for the retrieval of mesospheric composition of major and minor constituents needed to test models of the oxygen-hydrogen photochemistry, (3) emissions of the bands of the metastable states of O2, and O(S-1) produced by three-body recombination of O in the mesosphere.

Torr, D. G.↗
NASA NTRS1994

Ion transport and loss in the Earth's quiet ring current. 2: Diffusion and magnetosphere-ionosphere coupling

We have studied the transport and loss of H(+), He(+), and He(++) ions in the Earth's quiet time ring current (1 to 300 keV/e, 3 to 7 R(sub E), Kp less than 2+, absolute value of Dst less than 11, 70 to 110 degs pitchangles, all LT) comparing the standard radial diffusion model developed for the higher-energy radiation belt particles with measurements of the lower energy ring current ions in a previous paper. Large deviations of that model, which fit only 50% of the data to within a factor of 10, suggested that another transport mechanism is operating in the ring current. Here we derive a modified diffusion coefficient corrected for electric field effects on ring current energy ions that fit nearly 80% of the data to within a factor of 2. Thus we infer that electric field fluctuations from the low-latitude to midlatitude ionosphere (ionospheric dynamo) dominated the ring current transport, rather than high-latitude or solar wind fluctuations. Much of the remaining deviation may arise from convective electric field transport of the E less than 30 keV particles. Since convection effects cannot be correctly treated with this azimuthally symmetric model, we defer treatment of the lowest-energy ions to a another paper. We give chi(exp 2) contours for the best fit, showing the dependence of the fit upon the internal/external spectral power of the predicted electric and magnetic field fluctuations.

Sheldon, R. B.↗
NASA NTRS1994

Langmuir turbulence in the auroral ionosphere 1: Linear theory

Intense bursts of Langmuir waves with electric fields of 50 to 500 mV / m have been frequently observed at altitudes greater than 500 km in the auroral ionosphere. These bursts are driven by 20 eV to 4 keV field-aligned electrons, which are embedded in an approximately isotropic nonthermal tail of scattered electrons. The Langmuir bursts are often observed at altitudes where the ionosphere is moderately magnetized (OMEGA (sub e) approximately equals omega (sub pe)). Both the moderate magnetization and the scattered electrons have a major influence on the linear dispersion and damping of Langmuir waves. In particular, the linear dispersion is topologically different depending on whether the magnetic field is subcritical (OMEGA (sub e) less than omega (sub pe)) or supercritical (OMEGA (sub e) greater than omega (sub pe)). The correct dispersion and damping can account for the observed polarization of the Langmuir waves, which is very nearly parallel to the geomagnetic field. Inferred properties of the linear instability driven by the field-aligned electrons are discussed. The linear dispersion and damping derived here provide the basis for a nonlinear turbulence study described in a companion paper (Newman et al., this issue).

Newman, D. L.↗
NASA NTRS1994

Interhemispheric asymmetry of the high-latitude ionospheric convection pattern

The assimilative mapping of ionospheric electrodynamics technique has been used to derive the large-scale high-latitude ionospheric convection patterns simultaneously in both northern and southern hemispheres during the period of January 27 to 29, 1992. When the interplanetary magnetic field (IMF) B(sub z) component is negative, the convection patterns in the southern hemisphere are basically the mirror images of those in the northern hemisphere. The total cross-polar cap potential drops in the two hemispheres are similar. When B(sub z) is positive and absolute value of B(sub y) greater than B(sub z), the convection configurations are mainly determined by B(sub y) and they may appear as normal 'two-cell' patterns in both hemispheres much as one would expect under southward IMF conditions. However, there is a significant difference in the cross-polar-cap potential drop between the two hemispheres, with the potential drop in the southern (summer) hemisphere over 50% larger than that in the northern (winter) hemisphere. As the ratio of absolute value of B(sub y)/B(sub z) decreases (less than one), the convection configuration in the two hemispheres may be significantly different, with reverse convection in the southern hemisphere and weak but disturbed convection in the northern hemisphere. By comparing the convection patterns with the corresponding spectrograms of precipitating particles, we interpret the convection patterns in terms of the concept of merging cells, lobe cells, and viscous cells. Estimates of the ' merging cell' potential drops, that is, the potential ascribed to the opening of the dayside field lines, are usually comparable between the two hemispheres, as they should be. The 'lobe cell' provides a potential between 8.5 and 26 kV and can differ greatly between hemispheres, as predicted. Lobe cells can be significant even for southward IMF, if absolute value of B(sub y) greater than the absolute value of B(sub z). To estimate the potential drop of the 'viscous cells,' we assume that the low-latitude boundary layer is on closed field lines. We find that this potential drop varies from case to case, with a typical value of 10 kV. If the source of these cells is truly a viscous interaction at the flank of the magnetopause, the process is likely spatially and temporally varying rather than steady state.

Lu, G.↗
NASA NTRS1994

Electric currents in the subsolar region of the Venus lower ionosphere

The ion and electron momentum equations, along with Ampere's law, are solved for the ion and electron drift velocities and the electric field in the subsolar Venus ionosphere, assuming a partially ionized gas and a single ion species having the ion mean mass. All collision terms among the ions, electrons and neutral particles are retained in the equations. A general expression for the evolution of the magnetic field is derived and compared with earlier expressions. Subsolar region data in the altitude range 150-300 km from the Pioneer Venus Orbiter are used to calculate altitude profiles of the components of the current due to the electric field, gradients of pressure, and gravity. Altitude profiles of the ion and electron velocities as well as the electric field, electrodynamic heating, and the energy density are determined. Only orbits having a complete set of measured plasma temperatures and densities, neutral densities, and magnetic field were considered for analysis; the results are shown only for orbit 202. The vertical velocity at altitudes above 220 km is upgoing for orbit 202. This result is consistent with observations of molecular ions at high altitudes and of plasma flow to the nightside, both of which require upward velocity of ions from the dayside ionosphere. Above about 230 km the momentum equations are extremely sensitive to the altitude profiles of density, temperature, and magnetic field.

Cole, K. D.↗
NASA NTRS1994

A thermosphere-ionosphere-mesosphere-electrodynamic general circulation model (time-GCM): Equinox solar cycle minimum simulations (30-500 km)

A new simulation model of the mesosphere, thermosphere, and ionosphere with coupled electrodynamics has been developed and used to calculate the global circulation, temperature and compositional structure between 30-500 km for equinox, solar cycle minimum, geomagnetic quiet conditions. The model incorporates all of the features of the National Center for Atmospheric Research (NCAR) thermosphere-ionosphere- electrodynamics general circulation model (TIE-GCM) but the lower boundary has been extended downward from 97 to 30 km (10 mb) and it includes the physical and chemical processes appropriate for the mesosphere and upper stratosphere. The first simulation used Rayleigh friction to represent gravity wave drag in the middle atmosphere and although it was able to close the mesospheric jets it severely damped the diurnal tide. Reduced Rayleigh friction allowed the tide to penetrate to thermospheric heights but did not close the jets. A gravity wave parameterization developed by Fritts and Lu (1993) allows both features to exist simultaneously with the structure of tides and mean flow dependent upon the strength of the gravity wave source. The model calculates a changing dynamic structure with the mean flow and diurnal tide dominant in the mesosphere, the in-situ generated semi-diurnal tide dominating the lower thermosphere and an in-situ generated diurnal tide in the upper thermosphere. The results also show considerable interaction between dynamics and composition, especially atomic oxygen between 85 and 120 km.

Roble, R. G.↗
NASA NTRS1994

Spatial distribution of plasma wave activity in the nightside ionosphere of Venus

In this study we use 14 years of Pioneer Venus Orbiter Electric Field Detector (OEFD) data to define the characteristics of (VLF) burst activity in the nightside ionosphere of Venus. Our statistical results show that there are essentially four types of VLF signals. The first type of signal is only observed in the 100 Hz channel and not in any of the higher frequency channels (730 Hz, 5.4 kHz or 30 kHz). Occurrence of these waves is controlled by the magnetic field with a weaker dependence on electron density. The occurrence of these waves is controlled by the magnetic field with a weaker dependence on electron density. The occurrence rate decreases with increasing altitude to a height of 600 km. For higher altitudes beyond 600 km the occurrence rate remains roughly constant. The statistics of these signals are what one would expect for whistler mode waves from a subionospheric source. The second type of signal is broadline wave activity appearing below 300 km in the low altitude ionosphere. These signals often occur in all four channels of the OEFD. These signals are also thought to come from a subionospheric source. The third type of signal appearing near the edge of the planetary optical shadow. They are probably ion acoustic waves generated by a current driven instability associated with plasma clouds in the wake. The fourth type of signal is a narrow band wave. It occurs in either of the two high frequency channels in the high altitude tail region, and is attributed to locally generated Langmuir waves. In addition, we also observe spacecraft interference noise in both the 100 and 730 Hz channels. These signals mainly occur near the edge of the planetary optical shadow and have an inbound and outbound asymmetry in activity.

Ho, C.-M.↗
NASA NTRS1994

Auroral ionospheric signatures of the plasma sheet boundary layer in the evening sector

We report on particles and fields observed during Defense Meteorological Satellite Program (DMSP) F9 and DE 2 crossings of the polar cap/auroral oval boundary in the evening magnetic local time (MLT) sector. Season-dependent, latitudinally narrow regions of rapid, eastward plasma flows were encountered by DMSP near the poleward boundary of auroral electron precipitation. Ten DE 2 orbits exhibiting electric field spikes that drive these plasma flows were chosen for detailed analysis. The boundary region is characterized by pairs of oppositely-directed, field-aligned current sheets. The more poleward of the two current sheets is directed into the ionosphere. Within this downward current sheet, precipitating electrons either had average energies of a few hundred eV or were below polar rain flux levels. Near the transition to upward currents, DE 2 generally detected intense fluxes of accelerated electrons and weak fluxes of ions, both with average energies between 5 and 12 keV. In two instances, precipitating ions with energies greater than 5 keV spanned both current sheets. Comparisons with satellite measurements at higher altitudes suggest that the particles and fields originated in the magnetotail inside the distant reconnection region and propagated to Earth through the plasma sheet boundary layer. Auroral electrons are accelerated by parallel electric fields produced by the different pitch angle distributions of protons and electrons in this layer interacting with the near-Earth magnetic mirror. Electric field spikes driving rapid plasma flows along the poleward boundaries of intense, keV electron precipitation represent ionospheric responses to the field-aligned currents and conductivity gradients. The generation of field-aligned currents in the boundary layer may be understood qualitatively as resulting from the different rates of earthward drift for electrons and protons in the magnetotail's current sheet.

Burke, W. J.↗
NASA NTRS1994

Ionospheric projections of magnetospheric regions under low and high solar wind pressure conditions

The projection of magnetospheric regions into the dayside ionosphere as determined by particle precipitation characteristics was studied for dependence on solar wind parameters. It was found that the solar wind kinetic pressure p dramatically affected the map of magnetospheric projections. Under the constraint that p greater than or equal 4 nPa(yielding (p) = 5.9 nPa), the area of the cusp (magnetic latitude times magnetic local time extent) was 4.83 degree-hours; whereas under the constraint that p greater than or equal 2 nPa (yielding (p) = 1.5 nPa), the cusp area was only 1.01 degree-hours. The ionospheric footprint of the low-latitude boundary layer was similarly affected. Various possible correlations of p with other solar wind variables, including n, v, and absolute value of B(sub z), proved unable to account for the pressure effect. Because one criterion for identifying the cusp is high fluxes, the effect of nv was investigated with particular care, both in examples and statistically. Again, p itself had by far the most striking effect. Thus we concluded that some physical mechanism is needed to account for the pressure effect. One possibility is that increased direct solar wind plasma penetration of the magnetopause occurs under high-p conditions in the manner suggested by various proponents of impulsive penetration models. An alternative, which we find promising, is that, regardless of the original interplanetary magnetic field (IMF) strength, a high-p solar wind leads to a large compression factor for the magnetosheath field, which is the field actually in contact with the magnetosphere. From this latter viewpoint, the chief effect of high particle pressure is simply to enhance the effectiveness of the interaction of the IMF with the magnetosphere.

Newell, Patrick T.↗
NASA NTRS1994

Cone structure and focusing of VLF and LF electromagnetic waves at high altitudes in the ionosphere

The frequency and angle dependencies of the electric field radiated by an electric dipole E = E(sub 0) cos omega(t) are studied through numerical calculations of absolute value of E in the VLF and LF frequency bands where F is less than or equal 0.02 to 0.05 f(sub b) in a model ionosphere over an altitude region of 800-6000 km where the wave frequency and electron gyrofrequency varies between F approximately 4-500 kHz and f(sub b) is approximately equal (1.1 to 0.2) MHz respectively. It is found that the amplitudes of the electric field have large maxima in four regions: close to the direction of the Earth magnetic field line B(sub 0) (it is called the axis field E(sub 0), in the Storey E(sub St), reversed Storey E(sub RevSt), and resonance E(sub Res) cones. The maximal values of E(sub 0), E(sub Res), and E(sub RevSt) are the most pronounced close to the lower hybrid frequency, F approximately F(sub L). The flux of the electric field is concentrated in very narrow regions, with the apex angles of the cones Delta-B is approximately (0.1-1) deg. The enhancement and focusing of the electric field increases with altitude starting at Z greater than 800 km. At Z greater than or equal to 1000 up to 6000 km, the relative value of absolute value of E, in comparison with its value at Z = 800 km is about (10(exp 2) to 10(exp 4)) times larger. Thus the flux of VLF and LF electromagnetic waves generated at high altitudes in the Earth's ionosphere are trapped into very narrow conical beams similar to laser beams.

Alpert, Ya. L.↗
NASA NTRS1993

Nonlinear unstable auroral-arc driven thermospheric winds in an ionosphere-magnetosphere coupled model

The nonlinear evolution of thermospheric winds in an ionosphere-magnetosphere coupled model has been studied for the first time for a dynamic unstable auroral-arc environment. We treat the problem using a multi-layer, quasi-three-dimensional model which averages in altitude the thermospheric dynamics over each layer. For the upper thermosphere, we find that (1) the thermosphere can respond to the ionospheric Kelvin-Helmholtz (KH) instability on temporal scales on the order of an hour, depending on ambient conditions, and on spatial scales of tens to hundreds of kilometers, (2) strong thermospheric meridional and zonal vortical flows with embedded nonlinear jet-like structures can be generated by the ionospheric/magnetospheric KH instability and (3) neutral thermospheric winds, vortices, and associated power spectra develop in a distinctly different manner in the presence of magnetospheric coupling effects. Comparison with recent observations is made.

Keskinen, M. J.↗
NASA NTRS1993

The magnetic state of the lower ionosphere during Pioneer Venus entry phase

During the entry phase of the Pioneer Venus Orbiter (PVO), defined as that period at the end of mission in 1992 when the periapsis fell below 185 km, the magnetometer made repeated measurements throughout the post midnight ionosphere until about 0430 LT. In this region the magnetic field is generally stronger at comparable altitudes than it was earlier at times of higher solar activity. This increase combined with a decrease in electron density causes the ratio of the magnetic pressure to thermal pressure to approach unity at altitudes above 200 km, whereas it was much lower than unity at these altitudes during solar maximum. From 160-200 km the magnetic field pressure exceeds that of the ionospheric plasma quite unlike the usual conditions seen at the beginning of the mission. At lowest altitudes below 150 km, however, the field becomes weaker and hence no evidence for a planetary magnetic field is found.

Russell, C. T.↗
NASA NTRS1993

Evidence for day-to-night ion transport at low solar activity in the Venus pre-dawn ionosphere

Periapsis of the Pioneer Venus (PV) spacecraft dropped below 180 km on August 28, 1992 near midnight, and 42 orbits of low altitude data at moderately low solar activity in the pre-dawn sector were obtained before contact was lost to the spacecraft in October, 1992. Through a combination of analysis of data from the PV orbiter ion mass spectrometer (OIMS) and modeling, we consider here what can be learned about the relative importance of plasma transport from the dayside and electron precipitation in maintaining the nightside ionosphere during the re-entry period. In particular, we examine here the atomic ion density profiles. We compute the average peak density of O(+) as a function of solar zenith angle and determine what fluxes of atomic ions or precipitating electrons would be necessary to produce those values. We then compare model calculations of the ion densities to those observed during the re-entry period. We find that the low solar activity nightside ionosphere shows evidence of significant day-to-night plasma transport.

Brannon, J. F.↗
NASA NTRS1993

Theory of kilometer-size density waves in the nightside Venus ionosphere

Quasi-sinusoidal density waves were frequently observed during the end of the Pioneer Venus Orbiter (PVO) mission when the orbiter was at low periapsis. These waves occur at altitudes approx. 145 - 155 km and have wavelengths approx. 1 km. It is suggested that a radial, ambipolar electric field E(sub 0), directed downward, is established in the Venus ionosphere during electron pressure enhancements above approx. 160 km. This field generates an electron E x B drift V(sub E); the ions move radially and do not E x B drift because they are unmagnetized (i.e, nu(sub in) much greater than Omega(sub i)). This drift is shown to drive a collisional drift wave instability for sufficiently large values of V(sub E), nominally, V(sub E) greater than nu(sub i) where nu(sub i) is the ion thermal velocity. For parameters typical of the nightside Venus ionosphere, this instability generates plasma fluctuations with wavelengths approx. 1 km, consistent with observations.

Huba, J. D.↗
NASA NTRS1993

Observation of intense wave bursts at very low altitudes within the Venus nightside ionosphere

Intense ELF (100 Hz) bursts were detected by the Pioneer Venus (PV) Orbiter plasma wave instrument during the final operations of the spacecraft prior to atmospheric entry. These bursts were detected at approx. 130 km altitude around 0400 local time. The wave activity lasted for several tens of seconds. Furthermore the bursts were not symmetric about periapsis, unlike instrument noise caused by neutral impacts on the spacecraft. The bursts had a vertical attenuation scale height of the order 1 km, consistent with that expected for whistler-mode waves propagating through a collisional ionosphere. Since the decay of the signals appears to be due to attenuation, the source must persist for several tens of seconds. The wave bursts could therefore be the signature of electromagnetic radiation entering the bottomside ionosphere from several distant sources, as would be expected if lightning were a relatively persistent phenomenon within the Venus atmosphere.

Strangeway, R. J.↗
NASA NTRS1994

Ground-based studies of ionospheric convection associated with substorm expansion

The instantaneous patterns of electric fields and currents in the high-latitude ionosphere are deduced by combining satellite and radar measurements of the ionospheric drift velocity, along with ground-based magnetometer observations for October 25, 1981. The period under study was characterized by a relatively stable southward interplanetary magnetic field (IMF), so that the obtained electric field patterns do reflect, in general, the state of sustained and enhanced plasma convection in the magnetosphere. During one of the satellite passes, however, an intense westward electrojet caused by a substorm intruded into the satellite (DE2) and radar (Chatanika, Alaska) field of view in the premidnight sector, providing a unique opportunity to differentiate the enhanced convection and substorm expansion fields. The distributions of the calculated electric potential for the expansion and maximum phases of the substorm show the first clear evidence of the coexistence of two physically different systems in the global convection pattern. The changes in the convection pattern during the substorm indicate that the large-scale potential distributions are indeed of general two-cell patterns representing the southward IMF status, but the night-morning cell has two positive peaks, one in the midnight sector and the other in the late morning hours, corresponding to the substorm expansion and the convection enhancement, respectively.

Kamide, Y.↗
NASA NTRS1994

Observations of ionospheric electric fields above atmospheric weather systems

We report on the observations of a number of quasi-dc electric field events associated with large-scale atmospheric weather formations. The observations were made by the electric field experiment onboard the San Marco D satellite, operational in an equatorial orbit from May to December 1988. Several theoretical studies suggest that electric fields generated by thunderstorms are present at high altitudes in the ionosphere. In spite of such favorable predictions, weather-related events are not often observed since they are relatively weak. We shall report here on a set of likely E field candidates for atmospheric-ionospheric causality, these being observed over the Indonesian Basin, northern South America, and the west coast of Africa; all known sites of atmospheric activity. As we shall demonstrate, individual events often be traced to specific active weather features. For example, a number of events were associated with spacecraft passages near Hurricane Joan in mid-October 1988. As a statistical set, the events appear to coincide with the most active regions of atmospheric weather.

Farrell, W. M.↗
NASA NTRS1995

Satellite observations of zonal electric fields near sunrise in the equatorial ionosphere

We report here on a number of examples of anomalous enhancements of eastward electric fields near sunrise in the equatorial ionospheric F-region. These examples were selected from the data base of the equatorial satellite, San Marco D (1988), which measured ionospheric electric fields during a period of solar minimum. The eastward electric fields reported correspond to vertical plasma drifts. The examples studied here are similar in signature and polarity to the pre-reversal electric field enhancements seen near sunset from ground-based radar systems. The morphology of these sunrise events, which are observed on about 14% of the morning-side satellite passes, are studied as a function of local zonal velocity, magnetic activity, geographic longitude and altitude. The nine events studied occur at locations where the zonal plasma flow is generally measured to be eastward, but reducing as a function of local time and at satellite longitudes where the magnetic declination has the opposite polarity as the declination of the sunrise terminator.

Aggson, T. L.↗