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NASA NTRS1982

Magsat scalar anomaly distribution - The global perspective

It is established that geographic coincidences exist between high-altitude Magsat scalar anomalies and major geologic and tectonic structures, with oceanic abyssal plains overlain by negative anomalies agreeing well in spatial extent and position and submarine platforms lying beneath positive scalar anomalies. In addition, geographic coincidence is found in the continents between many high-latitude positive anomalies and shields and cratons in North America, Eurasia and Australia. While these correlations are qualitative, they serve to identify regions for detailed study. The global distribution of anomalies provides a basis for comparative study which will be enhanced when reduced-to-pole versions of the Magsat data become available.

Frey, H.↗

NASA NTRS1982

Reverse draping of magnetic field lines in the boundary layer

An unexpected orientation of field lines has been found by ISEE satellite measurements of the earth magnetosphere boundary layer, perhaps indicating a reverse draping. Three-dimensional plasma flow measurements by ISEE 1 and 2 show the tailward flow in the boundary layer to have an equatorward component which suggests high-latitude plasma entry. Simultaneous magnetic field measurements with the two satellites during a boundary crossing sequence yielded data on the boundary layer electric currents and the boundary layer interface with plasma sheet and magnetosheath. The currents are found to be largely field-aligned, with intensities in the 0.03-0.06 microampere/sq m range.

Hones, E. W., Jr.↗

NASA NTRS1982

Present wind activity on Mars - Relation to large latitudinally zoned sediment deposits

The relation of present Martian winds to large latitudinally zoned sediment deposits has been investigated using global wind streak data and mapping of large sand and dust deposits. Dune sand deposits occur primarily in three latitude belts: north polar (74-85 degrees North), low latitude (5 degrees North-20 degrees South), and south polar (40-80 degrees South). Comparison with wind streak data shows the high-latitude dunes to be in areas of seasonally reversing winds. The present winds can form latitudinal dune belts from a variety of initial dune distributions, including uniform distribution and a polar source. The presence of dune sand within the polar layered deposits, the erosional state of the deposits, and the present surface wind flow away from the poles indicate that both polar dune concentrations have been derived from erosion of the layered deposits. The low-latitude dunes are topographically confined in canyons and craters; they are probably subject to long-term reversal of orientations with climate cycles.

Thomas, P.↗

NASA NTRS1982

On the distribution of interstellar gas in the galactic halo

New and existing observations of 21-cm emission lines toward 10 distant, high-latitude OB stars are combined with existing observations of interstellar Lyman-alpha absorption lines, in order to determine the ratio, N21/N-alpha, of the two different column densities of H I. This ratio, which is related to the fraction of the cool, neutral gas in the halo that lies beyond each star, decreases smoothly to about unity with increasing distance from the galactic plane. The column density of neutral gas beyond about 1 kpc can be as much as one-third of the total above the plane, but only relatively small amounts of such gas lie more than 2 kpc from the plane. The distances to, and the possible birthplaces of, these Population I stars in the halo are discussed.

Hobbs, L. M.↗

NASA NTRS1983

Auroral hiss, Z mode radiation, and auroral kilometric radiation in the polar magnetosphere - DE 1 observations

The polar-orbiting DE 1 spacecraft has provided the first measurements of high-latitude auroral phenomena. Three types of plasma-wave emissions were observed: auroral hiss, Z-mode radiation, and auroral kilometric radiation. Whistler mode auroral hiss emissions were observed on virtually every pass over the auroral zone. The shape of the auroral hiss frequency-time spectrum is explained by a whistler mode propagation effect if the radiation is emitted from a spatially localized source below the spacecraft. Broadband Z emissions have been observed in the low-density region over the auroral zone and polar cap. The auroral hiss may be distinguished from the Z-mode radiation by the sharp upper cutoff of the whistler mode at the local electron plasma frequency. Auroral kilometric radiation usually occurs at frequencies above electron gyrofrequency, indicating that this radiation is propagating in the free-space R-X mode.

Gurnett, D. A.↗

NASA NTRS1982

A stratospheric chemical instability

The equations which determine partitioning of Cl(x) in steady state have multiple (three) solutions under conditions which might arise in the high-latitude winter stratosphere. Two of these solutions are stable, one is unstable, to infinitesimal perturbations. The relative stability of solutions is examined by subjecting the system to finite perturbations. The more stable solution is found to eliminate the less stable when semi-infinite volumes of the two solutions are placed in contact. The high-ClO, low NO2 solution is more stable under most conditions. Transitions from less to more stable states are slow in winter but may occur more rapidly when the seasonal variation of insolation is taken into account.

Fox, J. L.↗

NASA NTRS1983

Location and source of ionospheric high latitude troughs

The global extent of the high-latitude troughs from altitudes between approximately 400 and 1100 km are explored using ion composition measurements from the satellite OGO 6. The trough locations are compared with prominent magnetosphere-ionosphere coupling signatures in order to understand the source and controlling mechanisms for these plasma depletions. It is found that, on the average, the troughs at all local times are in the vicinity of the auroral oval and move equatorward in response to increasing magnetic activity. The average trough location is compared with the average polar cap boundary as defined by the convection electric field reversal and the electron trapping boundary, as well as with the maximum horizontal magnetic disturbance associated with the large-scale field-aligned currents. It is concluded that the troughs are mainly the result of enhanced chemical O(+) losses in regions having high convection velocities.

Grebowsky, J. M.↗

NASA NTRS1983

Comparison of model high-lititude electron densities with Millstone Hill observations

The predictions of a high-latitude ionospheric model are compared with the diurnal variations of plasma convection velocities and electron densities observed at Millstone Hill on a geomagnetically moderately active day near equinox. The observed convection pattern was consistent with a two-cell, asymmetric pattern with enhanced plasma flow in the dusk sector, with flow speeds reaching 1.5 km/s. In the dusk strong convection cell, the falloff of the magnetospheric potential with latitude was proportional to the inverse of the sine of colatitude to the fourth power. On the dayside, a region of high density occurred at 500 km in the 1000-1900 LT sector. The nocturnal midlatitude trough was deepest and widest and reached its most equatorward position in the morning sector. The model, which is based on average auroral precipitation fluxes, can describe the gross features of the enhanced densities in the auroral zone.

Sojka, J. J.↗

NASA NTRS1983

Interstellar absorption lines in the spectrum of 3C 273

Absorption lines near the rest wavelengths of the Si IV doublet (wavelengths 1393.76, 1402.77) with equivalent widths 533 and 235 mA are reported in the spectrum of QSO 3C 273. The profiles show the nature of hot gas along a high-latitude (b = 64 deg) line of sight through our entire halo. The lines have full width at half-maxima of 100 km/s, compared to an instrumental resolution of 35 km/s, are centered at zero velocity, and show no evidence for components with equivalent widths greater than 50 mA at an absolute value of v greater than 75 km/s. The absence of high peculiar velocities is consistent with published spectra of halo gas over shorter pathlengths. The implications of this result are discussed, with particular reference to the possible connection between galactic halos and QSO absorption lines, for which C IV and Si IV usually have a broader velocity range.

York, D. G.↗

NASA NTRS1983

Calculated stormtime variations in plasmaspheric thermal ion composition

Model calculations describing stormtime variations in the earth's dayside plasmasphere are used to examine variations in ion composition. The model storm is initiated by high-latitude thermospheric heating that generates meridional winds that carry neutral species, momentum, and energy equatorward. The thermosphere acts on the plasmasphere through collisional transfer of momentum and through chemical reactions between neutral species and ions. Over latitudes near the region of thermospheric heating, the thermosphere-plasmasphere coupling processes cause enhancement in the density of oxygen ions while protons are being lost. Meanwhile, densities of oxygen ions and protons near the equator are increasing together, almost in phase. The largest enhancements in ion density develop at latitudes near 45 deg invariant for both oxygen and hydrogen.

Miller, N. J.↗

NASA NTRS1983

The high latitude circulation and temperature structure of the thermosphere near solstice

NCAR thermospheric-general-circulation-model (TGCM) computations of solar-maximum thermospheric neutral-gas temperature and circulation around the December solstice are presented and discussed. The TGCM uses a 5 x 5-deg grid and 24 constant-pressure layers, corresponding to altitudes of about 97-500 km. The results are mapped as electron-density contours, polar plots, cylindrical equidistant projections, meridional cross sections, and F-region polar plots comparing the TGCM predictions with DE-2 satellite observations. The significant differences between summer and winter high-latitude F-region winds are attributed to the ion drag momentum associated with magnetospheric convection. The TGCM wind predictions follow the same pattern as the satellite measurements but are too small; possible model corrections are considered.

Roble, R. G.↗

NASA NTRS1984

Fourier analysis of polar cap electric field and current distributions

A theoretical study of high-latitude electric fields and currents, using analytic Fourier analysis methods, is conducted. A two-dimensional planar model of the ionosphere with an enhanced conductivity auroral belt and field-aligned currents at the edges is employed. Two separate topics are treated. A field-aligned current element near the cusp region of the polar cap is included to investigate the modifications to the convection pattern by the east-west component of the interplanetary magnetic field. It is shown that a sizable one-cell structure is induced near the cusp which diverts equipotential contours to the dawnside or duskside, depending on the sign of the cusp current. This produces characteristic dawn-dusk asymmetries to the electric field that have been previously observed over the polar cap. The second topic is concerned with the electric field configuration obtained in the limit of perfect shielding, where the field is totally excluded equatorward of the auroral oval. When realistic field-aligned current distributions are used, the result is to produce severely distorted, crescent-shaped equipotential contours over the cap. Exact, analytic formulae applicable to this case are also provided.

Barbosa, D. D.↗

NASA NTRS1984

A statistical study of F region ion temperatures at high latitudes based on Atmosphere Explorer C data

The ion-energy balance in the high-latitude F region was studied using 40,000 simultaneous ion-temperature and drift measurements on Atmosphere Explorer C. The large number of measurements revealed how the mean ion temperature, and the average deviation from that mean, change as a function of ion drift speed during solar-minimum conditions. It was found that, in spite of an apparent large scatter in the data, the measurements could always be reasonably explained in terms of fluctuations in the neutral dynamical properties, to which the ion energy is coupled. The results also provide evidence for increased neutral drifts as the ion E x B drift increases. Evidence for systematic ion-drag effects can also be found, on the average, for dc electric fields stronger than 60 mV/m.

St.-Maurice, J.-P.↗

NASA NTRS1983

The terrestrial ionosphere

The theory relating to the basic physics governing the behavior of the terrestrial ionosphere is reviewed. The review covers the coupling of the ionosphere to both the neutral atmosphere and magnetosphere, the creation and transport of ionization in the ionosphere, and the ionospheric thermal structure. The review also covers the variation of the ionosphere with altitude, latitude, longitude, universal time, season, solar cycle, and geomagnetic activity. In addition, some unique ionospheric features are discussed, such as the polar ionization hole, the main electron density trough, the ion temperature hot spots, the high-latitude ionization tongue, the equatorial fountain, Appleton's peaks, and the polar wind.

Schunk, R. W.↗

NASA NTRS1984

Stratospheric ozone changes during the first year of SBUV observations

Synoptic analyses of solar backscattered ultraviolet (SBUV) ozone data have been performed on a daily basis for the first year of Nimbus 7 operation, November 1978 through October 1979, at the standard pressure levels: 30, 10, 5, 2, 1, and 0.4 mbar. By combining these data with the daily NMC meteorological analyses of temperature and geostrophic wind at the same levels, elements of the ozone-temperature relationship as well as the horizontal eddy transports are examined. In particular, the existence of an upper stratospheric high-latitude ozone minimum in winter is discussed, and the eddy fluxes are found to play a significant role in this feature, mainly associated with stratospheric warming pulses. At the same time, monthly average quantities are examined, and standing eddy components are related.

Nagatani, R. M.↗

NASA NTRS1984

Estimation of electric fields and current from ground-based magnetometer data

Recent advances in numerical algorithms for estimating ionospheric electric fields and currents from groundbased magnetometer data are reviewed and evaluated. Tests of the adequacy of one such algorithm in reproducing large-scale patterns of electrodynamic parameters in the high-latitude ionosphere have yielded generally positive results, at least for some simple cases. Some encouraging advances in producing realistic conductivity models, which are a critical input, are pointed out. When the algorithms are applied to extensive data sets, such as the ones from meridian chain magnetometer networks during the IMS, together with refined conductivity models, unique information on instantaneous electric field and current patterns can be obtained. Examples of electric potentials, ionospheric currents, field-aligned currents, and Joule heating distributions derived from ground magnetic data are presented. Possible directions for future improvements are also pointed out.

Kamide, Y.↗

NASA NTRS1984

Magnetospheric dynamo processes

Three processes are examined whereby an effective electromotive force and energy input arise in circuits of magnetospheric currents, even in the absence of time-varying magnetic fields. The first involves currents on 'open' field lines, linking the ionosphere with the solar wind, and it underscores the role of polarization currents. The second may exist on the current filament observed in the vicinity of Jupiter's satellite Io. The third may operate along the high-latitude boundary of the earth's magnetic tail, from where it pumps energy into the plasma sheet.

Stern, D. P.↗

NASA NTRS1984

Effect of hot electrons on the polar wind

A semikinetic model is used to describe the steady state collisionless flow of H(+), O(+), and electrons along diverging geomagnetic field lines in the high-latitude topside ionosphere. The effect that hot electron populations have on the polar wind is emphasized. Several such populations are considered, including the polar rain, polar showers, and polar squall. Hot electron densities and temperatures are calculated from the characteristic energy and flux measurements. The results indicate that the hot/cold electron temperature ratio varies from 10 to 10,000 and that the hot/cold electron density ratio varies from 0.001 to 0.1 at the baropause. For higher hot electron temperatures and a greater percentage of hot electrons, there is a discontinuity in the kinetic solution, which indicates the presence of a sharp transition corresponding to a contact surface between the hot and cold electrons. Along this surface, a double-layer potential barrier exists which reflects the cold ionospheric electrons and prevents their penetrations to higher altitudes.

Barakat, A. R.↗