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Niemann, H. B.

Publications and source records attributed to Niemann, H. B..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Wave-Like Perturbations Observed at Low Altitudes by the Pioneer Venus Orbiter Neutral Mass Spectrometer During Orbiter Entry

Wave-like perturbations have been observed in the nightside neutral density data acquired for He, N, O, N2 and CO2 by the Pioneer Venus Orbiter Neutral Mass Spectrometer (ONMS) during entry in late 1992. The data cover an altitude range of 133-200 km from 0.5-4.5 hours local solar time and occur at medium solar activity (F(sub 10.7) = 120) The perturbations, with an effective wavelength along the orbit of about 100 to 600 km, have similar amplitudes for the various species and helium is out of phase with respect to the heavier mass species. The measurements are comparable to those observed in 1978-80 at solar maximum activity (F(sub 10.7) = 200) above 145 km. Between 133 and 160 km the rms amplitudes grow with altitude at a rate of about (1.6, 2.1, 2.7, 4.4) x 10(exp -3)/ km for N, O, N2, and CO2 respectively. The average rms amplitudes above 145 km of 0.08 for N and O and 0.1 for N2 are comparable in magnitude to those observed in the earlier 1978-80 data of 0.06, 0.08 and 0.095 respectively. CO2 is an exception for which the entry value is 0.17 compared to 0.09 earlier. By combining the two overlapping data sets there is a suggestion that the CO2 amplitudes grow in value with altitude up to about 140-170 km and then decrease in amplitude. Like the earlier data, the entry data are consistent with the interpretation that the neutral density perturbations are due to gravity waves propagating upward from the lower thermosphere. Earlier data sampled a higher altitude region where dissipation is important in limiting and ultimately reversing the growth of gravity waves while the lower altitude entry data probed the region where growth of these waves can be observed.

Kasprzak, W. T.↗
NASA NTRS1992

Galileo Probe Mass Spectrometer experiment

The chemical and isotopic composition of the Jupiter atmosphere's constituents, including their vertical variations, will be measured by the Galileo Probe Mass Spectrometer instrument through in situ sampling; batch sampling will also be undertaken for noble gas composition and isotopic ratio determinations. The instrument's gas-sampling system is connected to a quadrupole mass analyzer for molecular weight analysis. Threshold values are lowered through sample enrichment by a factor of 100-500 for stable hydrocarbons and by a factor of 10 for noble gases. The instrument follows a sampling sequence of 8192 steps, at a rate of 2 steps/sec.

Niemann, H. B.↗
NASA NTRS1992

Evidence for enhanced dynamic flow in ionospheric holes from the Pioneer Venus Orbiter Neutral Mass Spectrometer

Ion mode measurements made by the Pioneer Venus Orbiter Neutral Mass Spectrometer (ONMS) for two ionospheric holes on orbit numbers 530 and 531 were used to find evidence for enhanced dynamic flow in ionospheric holes. The analysis of the spin modulation has provided measurements of one component of the ion drift in the hole regions. It was found that, inside the holes, the He(+)/O(+) ratio is enhanced relative to that occurring outside the holes. The in drift direction in the ecliptic plane for the hole regions was found to be consistent with downward ion flow (i.e., toward the planet) and, for one orbit examined without a hole, with upward ion flow at the approach of ionopause. The ONMS measurements in the hole and near the ionopause suggest that both areas are regions of enhanced dynamic flow.

Kasprzak, W. T.↗
NASA NTRS1991

Superthermal over 36-eV ions observed in the near-tail region of Venus by the Pioneer Venus Orbiter neutral mass spectrometer

The PVO neutral mass spectrometer has measured the over 36-eV ions in the 1300-3700 km altitude range for solar zenith angles greater than 120 deg. The composition is mainly O(+), but He(+), N(+), NO(+), and O2(+) have been identified. The average O(+) flux is about 100,000/sq cm/s, but higher fluxes from 10 to the 6th to 10 to the 8th/sq cm/s are observed about 10 percent of the time. The directions of the apparent O(+) flow in the ecliptic plane show predominantly tailward components with a smaller number of nontailward components. The over 36-eV O(+) escape flux in the ionotail is estimated to be about 100,000/sq cm/s. The O(+) flux data show a factor of 2.5 increase from solar minimum to maximum, implying a photoionization source for these ions. The composition of the superthermal ions in the ionotail suggests that their source is most likely the high-altitude nightside ionosphere. Transport of superthermal O(+) across the terminator to the nightside has been observed.

Kasprzak, W. T.↗
NASA NTRS1991

On the response of exospheric temperature on Venus to solar wind conditions

Exospheric temperatures, T(ex), inferred from in situ measurements of neutral densities by the Pioneer Venus Orbiter have been analyzed to examine any possible dependence on solar wind dynamic pressure, P(sw). No linear relationship was found between P(sw) and T(ex). However, on one occasion when P(sw) was unusually high for a prolonged period, an unmistakable temperature enhancement was observed. Minor enhancement of T(ex) were also observed in few other cases, with sustained but moderately high P(sw).

Kar, J.↗
NASA NTRS1990

Response of Venus exospheric temperature measured by neutral mass spectrometer to solar EUV flux measured by Langmuir probe on the Pioneer Venus orbiter

The photoelectron current from the Pioneer Venus Langmuir probe has provided measurements of the total flux of solar EUV photons at Venus since 1979. The neutral oxygen scale height measured by the orbiter neutral mass spectrometer has permitted the exospheric temperature to be derived during the same mission. In this paper, the EUV observations are used to examine the response of exospheric temperature to changes in solar activity, primarily those related to solar rotation. It is found that the dayside exospheric temperature quite faithfully tracks variations in the EUV flux. Comparison is also made with the earth-based solar activity index F10.7 adjusted to the position of Venus. This index varied from 142 to 249 flux units during the period of measurements. The exospheric temperature is better correlated with EUV flux than with the 10.7-cm solar radio flux.

Mahajan, K. K.↗
NASA NTRS1988

Fast O(sup +) ion flow observed around Venus at low altitudes

The Pioneer Venus Orbiter Neutral Mass Spectrometer (ONMS) has observed fast O(+) ions with an energy exceeding 40 eV in the spacecraft reference frame. The orbit of the spacecraft is nearly polar with periapsis near the equator. The ONMS is mounted at an angle to the spin axis which, in turn, is perpendicular to the ecliptic plane. From the spin modulated data the direction of the ion flow in that plane can be determined. Data from the first 11 diurnal cycles (orbits 1 to 2475) are vector averaged in order to display the general flow pattern. Plots of the averaged data are presented. On the dayside and near the terminators, where fast O(+) is observed near the ionopause, the directions are more or less parallel to the planet's surface with evidence of an asymmetry about the Sun-Venus line. On the nightside below 2000 km and near the equator there is a preferred dawn to dusk direction while at higher altitudes (lower solar zenith angles and higher latitudes) the flow direction is more antisunward. The averaged flux for this time period is 8x10 to the 5th/sq cm/s with a maximum of 5x10 to the 8th.

Kasprzak, W. T.↗
NASA NTRS1988

Wavelike perturbations observed in the neutral thermosphere of Venus

Wavelike perturbations are evident in the neutral thermosphere measurements of He, N, O, N2, and CO2 by the Pioneer Venus orbiter neutral mass spectrometer. In the wavelength range from 100 to 600 km, the amplitudes of the various species are comparable in magnitude, with CO2 having the largest amplitude, and with helium out of phase with respect to the heavier species. On the dayside, the small-scale variations of CO2 and N2 increase with increasing altitude from 170 to 210 km, with CO2 having the largest slope. Simple and complex wave structures are observed, including wave trains and pulselike events. There is more activity during nighttime than during daytime, and larger values of the activity occur in the vicinity of the predawn and postdusk terminator sectors. Below 160 km, the standard deviation of the CO2 density on the nightside is 12 percent, about 3 times the daytime value. The data are consistent with the interpretation that the density perturbation are due to gravity waves propagating upward from the lower thermosphere.

Kasprzak, W. T.↗
NASA NTRS1987

Observations of energetic ions on the nightside of Venus

Ions with energy greater than 40 eV have been measured on the nightside of Venus by the Pioneer Venus orbiter neutral mass spectrometer (ONMS). For periapsis altitudes beyond 1500 km, they are confined to solar zenith angles greater than 120 deg. The nightside energetic ion composition at 2000 km resembles the nightside thermal ion composition occurring at 1000 km with the dominant species being O(+). Postflight laboratory calibrations performed on the ONMS backup unit have been used to estimate the energetic O(+) ambient flux and density. The direction from which the energetic ions appear to be coming can be determined because the ONMS is mounted at an angle with respect to the spin axis of the spacecraft. Both tailward and antitailward components of the apparent ion flow are exhibited when projected into the ecliptic plane. The major component of the apparent ion flow is observed perpendicular to the ecliptic plane.

Kasprzak, W. T.↗
NASA NTRS1985

CO2 impact ionization-driven plasma instability observed by Pioneer Venus Orbiter at Periapsis

Observations of enhanced ac electric field noise about Pioneer Venus periapsis are shown to be related to spacecraft-generated impact ionization of the ambient CO2. The frequency of the electric field noise is found to peak in the vicinity of the CO2(+) ion plasma frequency and to closely follow the form of the neutral CO2 density profile. When the electric field noise in all channels is normalized by the square root of the CO2 number density, the ratio is constant. Since the impact electron density measured by the Pioneer Venus Langmuir probe, is observed to scale directly with the neutral CO2, the growth of the electric field amplitude is found to be linear in time with a growth rate proportional to the CO2(+) ion plasma frequency. On the basis of these results the impact ionization-driven instability is shown to be the ion acoustic instability. Implications for the lack of observations by Pioneer Venus of reflected-O(+)-driven instabilities, as have been proposed for the space shuttle, are discussed.

Curtis, S. A.↗
NASA NTRS1985

Space Science and Applications

The exploitation of the present tethered system and the need for repeat missions; multiple payloads placed along the tether; the extension of the present altitude limit to below 130 km; the development of free-flying tether satellites for studying both the Earth's atmosphere and ionosphere and also for studying the plantes; and a subtether were discussed. Areas of research were identified: aeronomy studies, gravity and magnetic potential mission, advanced sensors for geodynamics, and mapping and remote sensing.

Hudson, R.↗
NASA NTRS1985

On the diurnal variations in the temperature and composition - A three-dimensional model with superrotation

An improved (Mayr et al., 1980) three-dimensional multiconstituent spectral model of the Venus thermosphere dynamics is presented, which describes the diurnal variations in the atmospheric composition (in terms of O, CO, and CO2 densities obtained by ONMS and OIMS, temperature, and wind fields on the basis of simplified theoretical interpretations of data obtained by the Pioneer Venus mission. The improved model accounts for nonlinear processes, includes higher order tidal elements, and describes the major gases in self-consistent form. Also presented is a self-consistent, nonlinear solution from a two-dimensional quasi-axisymmetric spectral model which describes the four-day superrotation in the lower atmosphere.

Mayr, H. G.↗
NASA NTRS1985

In situ results on the variation of neutral atmospheric hydrogen at Venus

The concentrations of neutral hydrogen, n(H), in the Venus atmosphere were derived from an ion and neutral charge exchange relationship involving O(+), H(+), O, and CO2, which were measured by OIMS and ONMS on board the Pioneer Venus for the period 1979-1980 covering three diurnal cycles of Venus. There was a persisting dawn bulge in the diurnal distribution of n(H), which peaked at levels near 5 x 10 to the 7th/cu cm at altitudes below 165 km. Large day-to-day variations of up to a factor of 5 in n(H) were frequently encountered, in addition to some local time variations in the bulge location. Although the appreciable short-term variability in n(H) makes precise assessment of the interannual variations difficult, no distinct evidence for interannual variation in n(H) was found. These results were confirmed by the facts that no significant interannual differences in n(H3) measured directly by ONMS were detected, and that only small (not more than 10 percent, decline) variations in the solar EUV flux were found in the 1979-1980 period.

Taylor, H. A., Jr.↗
NASA NTRS1985

Empirical model of the composition of the Venus ionosphere Repeatable characteristics and key features not modeled

In-situ measurements of positive ion composition of the ionosphere of Venus are combined in an empirical model which is a key element for the Venus International Reference Atmosphere (VIRA) model. The ion data are obtained from the Pioneer Venus Orbiter Ion Mass Spectrometer (OIMS) which obtained daily measurements beginning in December 1978 and extending to July 1980 when the uncontrolled rise of satellite periapsis height precluded further measurements in the main body of the ionosphere. For this period, measurements of 12 ion species are sorted into altitude and local time bins with altitude extending from 150 to 1000 km. The model results exhibit the appreciable nightside ionosphere found at Venus, the dominance of atomic oxygen ions in the dayside upper ionosphere and the increase in prominence of atomic oxygen and deuterium ions on the nightside. Short term variations, such as the abrupt changes observed in the ionopause, cannot be represented in the model.

Taylor, H. A., Jr.↗
NASA NTRS1983

Global empirical model of the Venus thermosphere

Direct measurements of neutral CO2, O, CO, N2, He, and N densities from the Pioneer Venus Orbiter Neutral Mass Spectrometer are described in terms of a spherical harmonic representation (latitude and local time coordinates) of exospheric temperature and number densities at 150 km, using modified Bates temperature profiles. The exospheric temperatures are determined from the altitude variations of atomic oxygen. A global average temperature of 228 K is derived with a first harmonic variation of 5%. The altitude profiles are extended downward to 100 km by using empirical formulas to provide a transition through the turbopause region (simulating the effect of eddy diffusion and vertical flows) and matching entry probe density data. The model reflects the observed variations of temperature and density with the 10.7 cm radio flux index. For a given change in flux at the planet, the exospheric temperature on Venus changes by only 10% of the change seen in the terrestrial thermosphere.

Hedin, A. E.↗
NASA NTRS1982

Observations of energetic ions near the Venus ionopause

Ions (primarily O/+/) with spacecraft rest frame energies greater than 40 eV have been observed by the Pioneer Venus Neutral Mass Spectrometer. The signature occurs in about 13% of the 700 orbits examined, primarily near the ionopause and at all solar zenith angles. The energetic ions coincide in location with superthermal ions observed by the Ion Mass Spectrometer and more rarely occur in some of the plasma clouds observed by the Electron Temperature Probe. These observations in conjunction with measurements by the Plasma Wave Instrument near the ionopause suggest that the ions are accelerated out of ionospheric plasma by the shocked solar wind through plasma wave-particle interactions.

Kasprzak, W. T.↗
NASA NTRS1982

Observed composition of the ionosphere of Venus - Implications for the ionization peak and the maintenance of the nightside ionosphere

Across the nightside of Venus, daily measurements from the PV Orbiter Ion Mass Spectrometer often indicate an ionosphere of relatively abundant concentration, with a composition characteristic of the dayside ionosphere. Such conditions are interspersed by other days on which the ionosphere appears to largely 'disappear' down to about 200 km, with ion concentrations at lower heights also much reduced. These characteristics, coupled with observations of strong day to night flows of O(+) in the upper ionosphere, support arguments that ion transport from the dayside is important for the maintenance of the nightside ionosphere. In the range 140-160 km, strong concentrations of O2(+) and NO(+) indicate that the ionization peak is at times composed of at least two prominent ion species. Nightside concentrations of O2(+) and NO(+) as large as 100,000 and 10,000/cu cm, respectively, appear to require sources in addition to that provided by transport. The most probable sources are considered briefly, and no satisfactory explanation is yet found for the observed NO(+) concentrations.

Taylor, H. A., Jr.↗