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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Transport of Mars atmospheric water into high northern latitudes during a polar warming

Several numerical experiments were conducted with a simplified tracer transport model in order to attempt to examine the poleward transport of Mars atmospheric water during a polar warming like that which occurred during the winter solstice dust storm of 1977. The flow for the transport experiments was taken from numerical simulations with a nonlinear beta-plane dynamical model. Previous studies with this model have demonstrated that a polar warming having essential characteristics like those observed during the 1977 dust storm can be produced by a planetary wave mechanism analogous to that responsible for terrestrial sudden stratospheric warmings. Several numerical experiments intended to simulate water transport in the absence of any condensation were carried out. These experiments indicate that the flow during a polar warming can transport very substantial amounts of water to high northern latitudes, given that the water does not condense and fall out before reaching the polar region.

Barnes, J. R.↗
NASA NTRS1989

Global behavior of the height/seasonal structure of tides between 40 deg and 60 deg latitude

The radars utilized are meteor (2), medium frequency (2) and the new low frequency (1) systems: analysis techniques were exhaustively studied internally and comparatively and are not thought to affect the results. Emphasis is placed upon the new height-time contours of 24-, 12-h tidal amplitudes and phases, which best display height and seasonal structures; where possible high resolution (10 d) is used (Saskatoon), but all stations provide monthly mean resolution. At these latitudes the diurnal tide is generally smaller than the semidiurnal, and displays more variability. However, there is a tendency for vertical wavelengths and amplitudes to be larger during summer months. On occasions in winter and fall, wavelengths may be less than 50 km. The dominant semidiurnal tide shows significant regular season structure; wavelengths are generally small (about 50 km) in winter, large in summer (equal to or greater than 100 km), and these states are separated by rapid equinoctial transitions. There is some evidence for less regularity toward 40 deg. Coupling with mean winds is apparent. Data from earlier ATMAP campaigns are mentioned, and reasons for their inadequacies presented.

Manson, A. H.↗
NASA NTRS1988

Stratospheric ozone and temperature responses to short-term solar ultraviolet variations - Reproducibility of low-latitude response measurements

Two independent 22-month time intervals of NIMBUS 7 solar backscattered UV (SBUV) ozone and stratospheric and mesospheric sounder (SAMS) temperature measurements for the upper stratosphere at low latitudes are analyzed to calculate mean responses to observed changes in solar ultraviolet spectral irradiance occurring on the time scale of the solar rotation period. Average cross-correlation functions of both SBUV ozone and SAMS temperature versus the solar 205 nm flux are in substantial agreement for these two intervals. Linear regression methods are applied to estimate response amplitudes or sensitivities. The derived sensitivities and phase lags relative to the 205 nm flux are also in approximate agreement for the two separate intervals although the temperature response measurements exhibit larger deviations. These results support the validity of previously reported measurements on the 27-day time scale, and impose firmer constraints on proposed theoretical models for the response of the stratosphere to solar UV forcing on both short and long time scales.

Hood, L. L.↗
NASA NTRS1988

An analysis of the response of the thermospheric normal modes to temporally varying convection at high latitudes

The response of the high-latitude thermosphere to temporal variations in the forcing due to the plasma convection pattern is investigated using a normal modes model that includes realistic temperature and electron density profiles as well as the effects of viscosity, heat conduction, and Lorentz forcing. The calculations show that the rotational wind component is only slightly modified when the plasma rotation is varied, while the divergent wind component responds more strongly to variations of the rotation rate, with the effect being greatest in the E region. A variation of the rotation rate then results in a greater magnitude and phase variation with the height of the divergent wind component in the E region, indicating that vertically propagating gravity waves are of importance in that height range.

Clark, M. A.↗
NASA NTRS1988

Low latitude thermospheric meridional winds between 250 and 450 km altitude - AE-E satellite data

In situ measurements from the Atmosphere Explorer-E satellite are used to obtain the daily variations of the meridional wind at + or - 18 deg latitude for summer and winter between 1977 and 1979. Data are presented for altitudes around 270, 350, and 440 km. Two features showing significant seasonal variation are found: a late afternoon maximum of the poleward wind occurring in winter at 1800 LT and a nighttime maximum in the equatorward wind with the summer equatorward wind abating earlier and more rapidly than the winter wind. The characteristics of these seasonal variations are discussed in detail.

Herrero, F. A.↗
NASA NTRS1989

Self-consistent model of the low-latitude boundary layer

A simple, two-dimensional, steady state, viscous model of the dawnside and duskside low-latitude boundary layer is presented. The model described boundary layer currents, magnetic fields, and plasma flow in a self-consistent way. Coupling to the ionosphere via field-aligned currents and associated field-aligned potential drops governed by a simple conductance law are included in the model. The model equations are given and the results from the model are compared with observations.

Phan, T. D.↗
NASA NTRS1989

A faint, dust-enshrouded carbon star at high galactic latitude

During the initial phase of a program to search for substellar candidates at high galactic in the IRAS Serendipitous Survey Catalog, one source, SSC 08546+1732, was found to have no optical couterpart on the Palomar Observatory Sky Survey plates. Ground-based positional, photometric, and spectroscopic observations have identified this source as a heavily dust-enshrouded carbon star, similar to those found in the Galactic plane. The high latitude and relative faintness of this source imply that it lies at a distance of 20-50 kpc, and is 10-30 kpc out of the plane of the Galaxy. SSC 08546+1732 and one other distant obscured carbon star found in the IRAS Point Source Catalog (Low 1987; Beichman et al. 1989) represent the first examples of such objects found in the Galactic halo. These stars may either have evolved from Population I precursors ejected from the disk, or they may be the first obscured Population II halo carbon stars to be observed. A survey for other distant enshrouded carbon stars in the IRAS Faint Source Cattalog should help to elucidate the nature of this new population.

Cutri, Roc M.↗
NASA NTRS1989

Pioneer and Voyager observations of the solar wind at large heliocentric distances and latitudes

Data obtained from the electrostatic analyzers aboard the Pioneer 10 and 11 spacecraft and from the Faraday cup aboard Voyager 2 were used to study spatial gradients in the distant solar wind. Prior to mid-1985, both spacecraft observed nearly identical solar wind structures. After day 150 of 1985, the velocity structure at Voyager 2 became flatter, and the Voyager 2 velocities were smaller than those observed by Pioneer 11. It is suggested that these changes in the solar wind at low latitudes may be related to a change which occurred in the coronal hole structure in early 1985.

Gazis, P. R.↗
NASA NTRS1988

Electron density distributions in the high-latitude magnetosphere

Electron density profiles were constructed to study the plasma density depletions in the nightside auroral zone and the density variations with increasing altitude in the polar cap, using electric field spectrum measurements from the plasma wave instrument on DE-1. Sharply defined regions of depleted plasma densities were commonly observed on nightside auroral field lines, in which electron densities were strongly depleted in relation to the adjacent plasmaspheric and polar densities, forming a low-density cavity at about 70 deg invariant latitude. A correlation was found between low auroral plasma densities, upflowing ion distributions, and an energetic precipitating electron population, indicating that electron density depletions in the nightside auroral zone are directly associated with auroral acceleration processes.

Persoon, Ann M.↗
NASA NTRS1989

Monte Carlo computations of F-region incoherent radar spectra at high latitudes and the use of a simple method for non-Maxwellian spectral calculations

Monte Carlo simulations of ion velocity distributions in the high-latitude F region have been performed in order to improve the calculation of incoherent radar spectra in the auroral ionosphere. The results confirm that when the ion temperature becomes large due to frictional heating in the presence of collisions with the neutral background constituent, F region spectra evolve from a normal double hump, to a triple hump, to a spectrum with a single maximum. An empirical approach is developed to overcome the inadequacy of the Maxwellian assumption for the case of radar aspect angles of between 30 and 70 deg.

Kikuchi, K.↗
NASA NTRS1989

Low-latitude zonal and vertical ion drifts seen by DE 2

Horizontal and vertical ion drift data from the DE 2 spacecraft have been used to determine average zonal and vertical plasma flow (electric field) characteristics in the +/- 26-deg dip latitude region during a time of high solar activity. The 'average data' local time profile for an apex height bin centered at 400 km indicates westward plasma flow from 0600 to 1900 solar local time ((SLT) with a maximum westward velocity of 80 m/s in the early afternoon. There is a sharp change to eastward flow at approximately 1900 hours with an early evening peak of 170 m/s. A secondary nighttime maximum exists at 0430 SLT preceeding the reversal to westward flow. This profile is in good agreement with Jicamarca, Peru, radar measurements made under similar solar maximum conditions. Haramonic analysis indicates a net superrotation which is strongest at lower apex altitudes. The diurnal term is dominant, but higher order terms through the quatradiurnal are significant.

Coley, W. R.↗
NASA NTRS1989

Comment on 'Observations of Low-Latitude Electron Precipitation' by R. Lieu, J. Watermann, K. Wilhelm, J. J. Quenby, and W. I. Axford

Observations made by an electron spectrometer aboard Spacelab 1 and presented by Lieu et al. (1988) are examined critically. The precipitation of electrons in the energy range of 0.1-12.5 keV was measured on December 6 and 7, 1983. Data for 16 passes near 240 km altitude, between + and - 30 deg geographic latitude, outside the South Atlantic Anomaly were included. It is argued that there is no geophysical confirmation of the large electron fluxes reported by Lieu et al. In their response, Lieu et al. discuss the sampling bias in the Spacelab 1 data and the magnetic shielding deficiencies of the calibration facility below about 500 eV.

Rassoul, H. K.↗
NASA NTRS1989

Double-peaked sodium layers at high latitudes

Na lidar observations indicate that at high latitudes in summer the neutral Na layer frequently attains a double-peaked structure. The main layer with a maximum near 90 km altitude is supplemented by a secondary, narrow layer near 95 km altitude. Results are presented concerning secondary sodium layers. It appears likely that the formation of secondary Na layers observed frequently above the lidar site is not solely a 'sodium phenomenon', but part of a more comprehensive layering process for metal atoms and ions. Na(+)/Na density ratios close to 0.5 near the peaks of both the main and secondary layers are derived.

Von Zahn, U.↗
NASA NTRS1989

Solar activity influence on climatic variations of stratosphere and mesosphere in mid-latitudes

The direct modulation of temperature of the mid-latitude mesosphere by the solar-cycle EUV variation, which leads to greater heat input at higher solar activity, is well established. Middle atmosphere temperature modulation by the solar cycle is independently confirmed by the variation of reflection heights of low frequency radio waves in the lower ionosphere, which are regularly monitored over about 30 years. As explained elsewhere in detail, these reflection heights depend on the geometric altitude of a certain isobaric surface (near 80 k), and on the solar ionizing Lyman-alpha radiation flux. Knowing the solar cycle variation of Lyman-alpha how much the measured reflection heights would be lowered with the transition from solar minimum to maximum can be calculated, if the vertical baric structure of the neutral atmosphere would remain unchanged. An discrepancy between expected and observed height change must be explained by an uplifting of the isobaric level from solar minimum to maximum, caused by the temperature rise in the mesosphere. By integrating the solar cycle temperature changes over the height region of the middle atmosphere, and assuming that the lower boundary (tropopause) has no solar cycle variation, the magnitude of this uplifting can be estimated. It is given for the Lidar-derived and for the rocket-measured temperature variations. Comparison suggests that the real amplitude of the solar cycle temperature variation in the mesosphere is underestimated when using the rocket data, but probably overestimated with the Lidar data.

Taubenheim, J.↗
NASA NTRS1989

The influence of IMF on the lower ionosphere plasma in high and middle latitudes

As shown by ground-based absorption measurements, the lower ionospheric plasma is markedly controlled by the structure of the IMF. Whereas in high auroral and subauroral latitudes this effect is very pronounced, in midlatitudes its influence is less important. A comparison of these results with satellite data of the IMF and the solar wind speed confirms the important role of these components, not only during special events but also for the normal state of the ionospheric D region plasma.

Bremer, J.↗
NASA NTRS1989

Post-flare effects in the lower ionosphere of middle latitudes

Beginning in the 1960s, records were made of noise from the region around the Polar Star on 29 MHz (Krivsky and Tlamicha, 1960) at the Ondrejov Observatory near Prague. Since the aerial characteristic was not too narrow, radio bursts were received of solar origin (of flares) at the noise level, SCNA effects (sudden cosmic noise absorption) at the time of intensive flare X-emission and in some rare cases, after large proton flares, small absorption effects of a few hours duration (Krivsky, 1969). These post-flare absorption effects in cosmic noise are evidently analogous with PCA effects (polar cap absorption) and are connected with ionospheric absorption of radio cosmic noise, caused by fast particles of subcosmic radiation. The recording of long term absorption effects after large particle flares at European midlatitudes was reported at the beginning of the 1960s. It was then usual to record radio cosmic noise with riometers at frequencies of about 18 MHz in the polar or subpolar regions in an effort to record PCA effects of subcosmic radiation (Hakura, 1968). An attempt was made to record the complex of emissions mentioned as well as the effects in a new frequency range (30 MHz), which did not agree with the ideas of the contemporaneous representatives of the Ionospheric Department of the Geophysical Institute in Prague. In recent years radio cosmic noise has been recorded at the Upice Observatory. These long term after flare effects of cosmic radio noise absorption (AF-CNA) at middle latitudes are reported to the geophysical and ionospheric community for the first time.

Krivsky, L.↗
NASA NTRS1989

Atmospheric density remote sensing of mesosphere and thermosphere to be used for spacecraft design by adopting VHF radar and HF Doppler sounder at low latitude west Pacific site during winter time

Simultaneous observations of VHF radar and HF Doppler array systems located at Chung Li (Taiwan) are used to observe three-dimensional wind speeds and gravity waves. The density perturbations are determined at different altitudes of the mesosphere and thermosphere during weak convective motions of the cold front in the winter. The present observations are believed to be valuable for space projects dealing with the low-latitude atmosphere.

Hung, R. J.↗
NASA NTRS1988

A mathematical model of the middle and high latitude ionosphere

A time-dependent three-dimensional model of the middle and high latitude ionosphere is described. The density distributions of six ion species NO(+), O(2+), N(2+), O(+), N(+), He(+), and the electron and ion temperatures are obtained from a numerical solution of the appropriate continuity, momentum, and energy equations. The equations are solved as a function of height for an inclined magnetic field at E and F region altitudes. The three-dimensional nature of the model is obtained by following flux tubes of plasma as they convect or corotate through a moving neutral atmosphere. The model takes account of field-aligned diffusion, cross-field electrodynamic drifts, thermospheric winds, polar wind escape, energy-dependent chemical reactions, neutral composition changes, ion production due to solar EUV radiation and auroral precipitation, thermal conduction, diffusion-thermal heat flow and local heating and cooling processes. The model also takes account of the offset between the geomagnetic and geographic poles.

Schunk, R. W.↗