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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
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

Empirical high-latitude electric field models

The present analysis of electric field measurements from the Dynamics Explorer 2 satellite, which extends previous empirical models, emcompasses much data from polar crossings entering and exiting the high latitudes in all magnetic local time zones. The goal is to represent the typical distributions of convective electric fields with a minimum number of characteristic patterns. Significant large-scale revisions of the OGO 6 dawn-dusk measurement models are made. The deformations of the two-cell patterns lead to sunward convection in dayside polar regions, while maintaining the integrity of the nightside convection pattern.

Heppner, J. P.↗
NASA NTRS1987

Velocity fields in a low-latitude coronal hole - Results from the Solar Maximum Mission

The Solar Maximum Mission (SMM) satellite has been used to observe Doppler signatures in C IV in a low-latitude coronal hole as it crossed the central meridian (1985 February 2-8). Scatter plots of C IV emission intensity versus velocity do not show the pronounced positive correlation which has been reported in other regions on the sun. These data suggest that the coronal hole may control the gross velocity field in the solar atmosphere at the level where C IV is formed. Some localized regions of upflow coincide with EUV bright points in the coronal hole.

Mullan, D. J.↗
NASA NTRS1987

A survey of formaldehyde in high Galactic latitudes

The discovery of formaldehyde (H2CO) in four out of 15 CO-clouds observed in high Galactic latitudes is reported. For one of these a velocity-integrated formaldehyde map is presented. The clouds are clearly connected to CO clouds described by de Vries et al. (1986), to Lynds bright and dark nebulae (Lynds, 1963), and to the Galactic infrared cirrus (Low et al., 1984). The distribution of CO and H2CO clouds is compared. The depths of the l(10-11)H2CO lines show no correlation to the integrated J = l-0(C-12)O line intensities.

Heithausen, A.↗
NASA NTRS1987

Hemispheric differences in the morphology of the high latitude ionosphere measured at about 500 km

Ambient electron density measurements made by the radio frequency capacitance probe on Ariel 4 have been analyzed in order to study the structures present at about 500 km altitude at latitudes poleward of + or - 50 deg Lambda during the 1971 Northern Hemisphere winter (December solstice) and the 1972 Southern Hemisphere winter (June solstice); data obtained on 94 days centered on each solstice have been used. The general morphology of, and the extreme densities within, the midlatitude electron density trough, the polar cap depletions, and the electron density enhancements associated with the cusp-auroral zone were determined statistically. Analyses of data acquired during quiet magnetic conditions show that the Northern and Southern Hemisphere winter ionospheres were significantly different; in particular, the Southern Hemisphere densities were lower than those in the Northern Hemisphere. The maximum electron densities observed in the Northern Hemisphere occurred in a magnetic-local-time range symmetrical about the 02-14 MLT meridian, whereas in the Southern Hemisphere the maxima were symmetrical with respect to the midnight-noon magnetic meridian.

Tulunay, Y. K.↗
NASA NTRS1987

An extended study of the low-latitude boundary layer on the dawn and dusk flanks of the magnetosphere

The characteristics and structure of the low-latitude boundary layer (LLBL) have been studied for 66 ISEE 1 passes through the LLBL region. The dawn and dusk LLBL are on closed magnetic field lines for northward magnetosheath and/or IMF (M/IMF), and are on both closed and open field lines for southward M/IMF. For southward M/IMF, the regions of open LLBL field lines lie adjacent to the magnetopause and outside the closed LLBL. The LLBL is thicker (thinner) for northward (southward) M/IMF. With distance away from the subsolar magnetosphere, the LLBL becomes thicker for northward M/IMF and more variable in thickness for southward M/IMF. No dependence of LLBL thickness or electric field on geomagnetic activity is seen in these data. The LLBL electric field is a few millivolts per meter with a apparent upper limit of about 10 mV/m. The field captures magnetospherically drifting particles and propels them tailward.

Mitchell, D. G.↗
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

Electrodynamics of the high latitude middle atmosphere

Atmospheric electrodynamics is reviewed. The discovery of apparent large (V/m) electric fields within the mesosphere invites the possibility for this region to be electrically active. Observations of the V/m field were made at high latitudes even under active conditions, but always below heights where significant enhancements in electrical conductivity are found to occur. Two measurements at Andoya (Norway) show anticorrelation of horizontal field directions with wind directions, suggesting a mechanism which involves mechanical separation of charged aerosols. Reported evidence for such aerosols makes this concept more viable. Noctilucent clouds and mesospheric turbulence, and their influence on the local electrical environment are mentioned.

Goldberg, R. A.↗
NASA NTRS1988

IRAS galaxies at low galactic and high supergalactic latitudes

Based on IR colors, 371 IRAS point sources with absolute value of b = 2-16 deg (b = galactic latitude) were selected for study at H I 21 cm as potential galaxies: 25 percent (93) of these sources are galaxies with redshifts less than 8000 km/s based on H I spectra. Most of the detected galaxies are at redshifts between 2000 and 7500 km/s. Fifty-five of these lie in an area about 40 deg away from the junction of the Pisces-Perseus and Lynx-Ursa Major superclusters, and show a distribution of systemic velocities very similar to these superclusters. There is an enhanced density of galaxies near 6000 km/s with galactic longitude l about 40 deg and b about 0 deg. The two regions surveyed, centered at l about 192 deg and l about 54 deg, lie outside the plane of the Local Supercluster and do not show any preference for galaxies with low velocity widths.

Dow, M. W.↗
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

High-latitude dayside electric fields and currents during strong northward interplanetary magnetic field - Observations and model simulation

On July 23, 1983 the IMF turned strongly northward, becoming about 22 nT for several hours. Using a combined data set of ionospheric convection measurements made by the Sondre Stromfjord incoherent scatter radar and convection inferred from Greenland magnetometer measurements, the onset of the reconfiguration of the high-latitude ionospheric currents is found to occur about 3 min after the northward IMF encounters the magnetopause. The large-scale reconfiguration of currents, however, appears to evolve over a period of about 22 min. These observations and the results of numerical simulations indicate that the dayside polar-cap electric field observed during strong northward IMF is produced by a direct electrical current coupling with the solar wind.

Clauer, C. Robert↗
NASA NTRS1988

Average low-latitude meridional electric fields from DE 2 during solar maximum

The average meridional electric field (zonal ion flow) patterns in the region between +/- 30 deg magnetic latitude were obtained by analyzing the electric field data taken by the Vector Electric Field Instrument aboard the DE 2 spacecraft during solar maximum conditions. The average data set displayed a rapid increase of the downward meridional electric field with local time near 1800 MLT; there was a secondary nighttime maximum of this electric field component, observed post midnight, with the crossover to upward electric fields (westward ion flow) occurring between 0400 and 0500 MLT. A sharp return to near zero was observed between 1200 and 1300. The zonal ion drift was found to be approximately symmetric with respect to the geomagnetic equator. These results are compared with results of the DE-2 neutral wind measurements and with results obtained from radar measurements.

Maynard, N. C.↗
NASA NTRS1988

DE/ISIS conjunction comparisons of high-latitude electron density features

This paper presents a comparison between the ISIS-1 and -2 topside sounder measurements of electron number density, N(e), with the in situ ion and N(e) measurements by the Langmuir probe aboard the Dynamics Explorer 2 (DE 2) during four high-latitude ISIS/DE magnetic field-aligned conjunctions. The ISIS-derived N(e) values, even at the greatest distance from the sounder, were found to agree with the Langmuir probe measurements to within about 30 percent over a density range of more than two decades on three of the four comparisons; the fourth comparison which included data with strong N(e) irregularities, showed a difference of 60 percent.

Hoegy, Walter R.↗
NASA NTRS1988

Color-magnitude diagrams for six metal-rich, low-latitude globular clusters

Colors and magnitudes for stars on CCD frames for six metal-rich, low-latitude, previously unstudied globular clusters and one well-studied, metal-rich cluster (47 Tuc) have been derived and color-magnitude diagrams have been constructed. The photometry for stars in 47 Tuc are in good agreement with previous studies, while the V magnitudes of the horizontal-branch stars in the six program clusters do not agree with estimates based on secondary methods. The distances to these clusters are different from prior estimates. Redding values are derived for each program cluster. The horizontal branches of the program clusters all appear to lie entirely redwards of the red edge of the instability strip, as is normal for their metallicities.

Armandroff, Taft E.↗
NASA NTRS1988

The high-latitude F supergiant IRAS 18095 + 2704 - A proto-planetary nebula

The paper reports the discovery of a new high-latitude F supergiant, IRAS 18095 + 2704, which shows a large excess in the far-infrared. Ground-based observations have identified it as a V = 10.4 mag F3 Ib star which displays light and velocity variability. Comparison with the post-asymptotic giant branch (AGB) evolution model of Volk and Kwok suggests that it is an excellent candidate for a protoplanetary nebula. Model fitting of the spectrum of 18095 + 2704 from 0.35 to 100 microns suggests that it evolved from the AGB approximately 265 yr ago and had a mass-loss rate of 0.00003 solar mass/yr at the end of the AGB.

Hrivnak, Bruce J.↗
NASA NTRS1988

Birkeland currents and charged particles in the high-latitude prenoon region - A new interpretation

The relationship between Birkeland currents and charged particles in the ionosphere was investigated for the period between the late morning through early afternoon, using data on simultaneous conjugate measurements of magnetic fields and charged particles at low altitude in the high-latitude prenoon sector, acquired on November 1, 1984 by four spacecraft, IMP 8, AMPTE CCE, DMSP F7, and HILAT. The results of these observations indicate that, for both northward and southward IMF, the traditional 'cusp Birkeland currents' lie poleward of the most intense sheathlike particle precipitation, and it is likely that these currents map to the plasma mantle and are associated with divergent flow of ionospheric plasma near noon. It is suggested that the traditional 'cusp' current system might be more appropriately named the 'mantle' Birkeland current system.

Bythrow, P. F.↗
NASA NTRS1988

Simulation of the transport of halogen species from the equatorial and mid-latitude stratosphere to the polar stratosphere in a two-dimensional model

The bulk of O sub 3 destruction in the Antarctic stratosphere takes place in the lower stratosphere between 15 and 25 km. Both O sub 3 and the halogen reservoir species have their origins in the higher altitude region (20 to 30 km) in the equatorial and mid-latitude stratosphere. Using the Caltech-JPL two-dimensional residual circulation model, researchers investigate the growth of stratospheric halogen due to the increase of CFCl sub 3 and CF sub 2 Cl sub 2.

Yung, Yuk L.↗
NASA NTRS1988

A regional perspective on the palynofloral response to K-T boundary event(s) with emphasis on variations imposed by the effects of sedimentary facies and latitude

Palynological studies deal with fossil reproductive bodies that were produced by fully functioning plants, whereas most faunal studies are based on death assemblages. Therefore, changes in pollen and spore assemblages cannot be used directly as evidence of catastrophic mass killings but only to indicate changes in ecological conditions. The impact of the Cretaceous-Tertiary boundary event on terrestrial plant communities is illustrated by the degree, rate and selectivity of change. As in most classical palynological studies, the degree of change is expressed in terms of relative abundance and changes in species diversity. It is recognized that sampling interval and continuity of the rock record within individual sections can affect the percieved rate of change. Even taking these factors into account, a gradual change in relative abundance and multiple levels of apparent extinctions, associated with the interval bounding the K-T boundary, can be demonstrated. Climatic change, which locally exceeds the tolerance of individual species, and the possible loss of a group of pollinating agents are examined as possible explanations for the selectivity of apparent extinctions and/or locally truncated occurrences. The aspects of change are demonstrated with data from four different K-T boundary localities in Western Canada between paleolatitudes 60 and 75 deg north. Together, the four localities discussed allow changes imposed by latitude and differences in the depositional environment be isolated from the boundary event itself which is reflected by the truncated ranges of several species throughout the region of study. What must be recognized is that variations in the response of vegetation to the K-T boundary event(s) occurred throughout the Western Interior basin.

Sweet, A. R.↗