Engineering topics

Miller, K. L.

Publications and source records attributed to Miller, K. L..

At least 37 records · Page 2

NASA NTRS1982

A new concept of plasma motion and planetary magenetic field for Venus

It is shown that the magnetohydrodynamic conditions of the Venus ionosphere near the terminator favor convection of a magnetic field rather than diffusion. Consequently, any planetary magnetic field which Venus may possess will be strongly affected by the global antisunward flow of the ionosphere which has been revealed by the Pioneer-Venus retarding potential analyzer. The magnetic flux from an internal magnetic field will accumulate in the night hemisphere. Details of the structure and dynamics of such accumulations depend on particular details of the magnetic field source and the time-dependent plasma flow pattern, but a simple interpretation of observational data yields a magnetic dipole moment of 7 x 10 to the 20th cu cm directed along the planet spin vector.

Knudsen, W. C.↗

NASA NTRS1982

Improved Venus ionopause altitude calculation and comparison with measurement

Improved approximations are incorporated into the inviscid fluid method originated by Spreiter et al (1970) in calculations of the altitude of the Venus ionopause. The altitude calculations are then compared with median altitudes measured by the Pioneer Venus retarding potential analyzer. The calculated ionopause shape is found to closely approximate the measured meridian shape in the solar zenith angle (SZA) range of zero-135 deg. The use of improved ionospheric pressure field ionosheath pressure and Prandtl-Meyer expansion approximations lowers the terminator ionopause altitude to approximately half that obtained with the usual Spreiter approximations. It is also determined that the calculated dawn ionopause is about 300 km higher than the dusk ionopause, and that both ionopauses are close to their respective measured meridian values. It is concluded that median ionopause altitude within the SZA angle range may be calculated without inclusion of a viscous interaction in the theory.

Knudsen, W. C.↗

NASA NTRS1981

On the maintenance of the Venus nightside ionosphere - Electron precipitation and plasma transport

The relative contributions of electron precipitation and transport of dayside plasma to the maintenance of the Venus nightside ionosphere during the long Venusian night are investigated based on simultaneous Pioneer Venus Orbiter Retarding Potential Analyzer measurements of suprathermal electron fluxes and plasma densities. In about 20 orbits, the nightside integral electron flux of electrons with energies between 5 and 45 eV is observed to be relatively constant in time and altitude, while plasma density is observed to vary by a factor of 10 or more with no correlation with the electron flux. Ionization rates and ion density height profiles are computed for O(+) and O2(+) as a function of magnetic dip angle based on a typical electron spectrum, or a downward flux of O(+) ions. Comparison of the computed profiles with the measured median O(+) and O2(+) density profiles reveals that the measured profiles can only be reproduced by a downward flux of O(+) equal to about 10 to the 8th/sq cm per sec; suprathermal electron energy distributions produce O2(+) and O(+) levels only about half and one tenth those usually observed, respectively. It is thus concluded that transport of O(+) ions from the dayside Venus ionosphere is responsible for approximately 75% of the typical nightside ionization, with variations in O(+) transport mechanism responsible for most of the observed nightside density variations. The remaining ionization is attributed to suprathermal electrons, which contribute principally to the O2(+) peak.

Spenner, K.↗

NASA NTRS1981

Anti-solar acceleration of ionospheric plasma across the Venus terminator

It is demonstrated that the horizontal ionospheric particle pressure gradient across the Venus terminator is the principal body force accelerating the plasm to the observed anti-solar velocity. The large scale horizontal electromagnetic body force is typically an order of magnitude smaller than the particle pressure gradient. The viscous body forces above 300 km - drag or acceleration - are also an order of magnitude smaller than the pressure gradient body force. In the immediate vicinity of the ionopause where the ionospheric plasma density decreases below 1000 ions/cu cm and the magnetic field strength increases, the electromagnetic body force may become significant. The electromagnetic body force may also be significant in the nightside ionosphere. The bulk of the ionospheric flow momentum is not derived from the ionosheath momentum.

Knudsen, W. C.↗

NASA NTRS1981

Midlatitude sporadic-E layers

Rocket borne probes and incoherent scatter radar were demonstrated to be effective methods of studying the structure of midlatitude sporadic E layers. Layers are formed when metal ions are converged vertically in a wind shear to produce a local enhancement of electron density. Rocket and radar observations show that the layers may occassionally have complex structure produced by an unstable wind shear. The partial transparency to radio waves of sporadic E layers is shown to be due to localized regions of high electron density.

Smith, L. G.↗

NASA NTRS1980

Observation of the Venus mantle, the boundary region between solar wind and ionosphere

For three orbit paths of the Pioneer Venus orbiter the interaction between the solar wind and the Venusian ionosphere has been studied. Results of the retarding potential analyzer and the magnetometer are described for the boundary region between the solar wind and the planetary ionosphere. These are the first measurements that show that a transition region exists between the two plasmas of different origin. The observed magnetic field and current system producing it appear strong enough to stop the solar wind flow in front of the ionosphere and to separate the shocked solar wind from the ionosphere. The transition region between the ionosheath and the ionosphere is called the 'mantle'. The observed mantle electron energy spectra close to the ionopause show ionospheric character. With increasing height the number of electrons that have ionospheric energies decreases, and the number of electrons that have solar wind energies gradually increases toward the ionosheath boundary, where only solar wind energy spectra are observed. The mantle surrounds the frontside of the ionosphere and extends probably more than eight Venus radii downstream.

Spenner, K.↗

NASA NTRS1980

Suprathermal electron energy distribution within the dayside Venus ionosphere

The suprathermal electron energy distribution for the dayside ionosphere has been derived from data returned by the Pioneer-Venus orbiter retarding potential analyzer. The shape and magnitude of the spectrum are consistent with the assumption that solar EUV radiation is the only significant source. The magnitude of the spectrum and its variation with altitude suggest that significant vertical transport occurs, with the electrons being lost through the ionopause. In turn, significant vertical transport suggests that the effective vertical electron heat conductivity may be comparable to the field-free value. The heat input to the thermal electron gas from the measured suprathermal electron flux is too small by a factor of at least five to maintain the observed electron temperature profile if the electron thermal conductivity is assumed to be close to the field-free value. It is thus inferred that most of the heat is supplied by the solar wind.

Knudsen, W. C.↗

NASA NTRS1980

Solar zenith angle dependence of ionospheric ion and electron temperatures and density on Venus

The measurements taken during the first year of the Pioneer Venus orbiter retarding potential analyzer indicate the changes of ion and electron temperatures with solar zenith angles. The ion density decreases by an order of magnitude from dayside to nightside; median ion temperatures above 300 km are constant with the solar zenith angle below 150 deg and reach 2300 K at the ionopause. The ion temperatures below 300 km are almost constant with solar zenith angles during the dayside, but increase with the angles on the nightside. The electron temperatures suggest a constant heat flux into the electron gas at the ionopause which may be supplied by dissipation of energy by the whistler mode plasma waves at the ionopause and/or conduction of heat from the ionosheath through the mantle.

Miller, K. L.↗

NASA NTRS1980

Transport of ionospheric O/+/ ions across the Venus terminator and implications

Data from the Pioneer Venus orbiter retarding potential analyzer demonstrate that the velocity of ionospheric O(+) ions in the vicinity of the terminator is directed antisunward and radially inward toward the planet in the range of 1-8 km/s. The velocity tends to increase with altitude, and may increase to still larger values just below the ionopause. The Alfven Mach number of the flow is generally greater than 1, indicating that the flow is controlled more by inertial forces than by magnetic forces. The ion Mach number is also greater than one. The flux of O(+) ions across the terminator planetwise is estimated to be equal to the integrated recombination rate of O2(+) on the night side within a factor of 2. Ion transport contributes substantially, and possibly predominantly, to the maintenance of the night side ionosphere.

Knudsen, W. C.↗

NASA NTRS1980

Ion energetics in the Venus nightside ionosphere

Consideration is given to the energetics of the ion gas flowing across the terminator into the Venus nightside ionosphere. Expressions are derived for the transport time of the ion gas (through 1 radian in solar zenith angle), the heat transfer time from the hot electron gas to the ions of an amount equal to the ion thermal energy), and the time required for vertical heat conduction to remove the internal energy of the ion column above a reference altitude, and it is shown that the time constant for transport is an order of magnitude smaller than the electron heat transfer time and comparable to the conduction time, and thus the ion gas is not a vertical conductive steady state. The conversion of bulk flow ion kinetic energy into heat is suggested as the mechanism responsible for the maintenance of the nightside ion temperatures at their observed values. It is thus concluded that the flow of the ion gas is quasi-adiabatic, and that steady-state, vertical, one dimensional energy balance models must be used with caution in the Venus ionosphere.

Knudsen, W. C.↗

NASA NTRS1980

Sporadic-E layers and unstable wind shears

Electron density profiles of sporadic-E layers have been observed with good height resolution using rocket-borne probes. These generally show a simple shape consistent with the effect of a linear wind shear acting on metallic ions. Occasionally more complex shapes have been recorded, including double peaks and, on one occasion, a nearly rectangular profile. A direct method of obtaining the wind profile from the concentration profile of metallic ions has been developed. The metallic ion concentration profile itself is obtained from the electron density profile. Both procedures derive from the steady-state continuity equation. For linear wind shears it is found that the maximum value of the shear is about 50 m/s/km which corresponds to a Richardson number of 1/4. Layers of complex shape are associated with non-linear wind shears in which the maximum shear considerably exceeds this value. It is concluded that the complex profiles of sporadic-E layers can be interpreted as an effect of unstable wind shears.

Smith, L. G.↗

NASA NTRS1979

Thermal structure and energy influx to the day- and nightside Venus ionosphere

The thermal structure of and energy influx to the dayside and nightside Venus ionosphere are discussed, based on data obtained by the Pioneer Venus spacecraft. Where the dayside ionosphere appears relatively constant in total ion concentration, ion and electron temperatures and ion composition below the ionopause, retarding potential analyzer measurements confirm the presence of strong spatial and temporal variations in nightside ion concentration. Ion temperatures on the nightside above 225 km are found to be larger by a factor of two or more than on the dayside, and comparable to electron temperatures on the day and night sides. Models of dayside and nightside ion temperatures consistent with observations require the heat input of 0.003 erg/sq cm sec and 0.005 erg/sq cm sec, respectively, to the ion gas to raise the ion temperature above that of the neutral gas, presumably by means of Joule heating.

Knudsen, W. C.↗

NASA NTRS1979

Thermal structure and major ion composition of the Venus ionosphere - First RPA results from Venus orbiter

Pioneer Venus in situ measurements of thermal plasma quantities were obtained by a retarding potential analyzer. Evidence for significant solar wind heating of the ionosphere and indications that the ionosphere is close to diffusive equilibrium are reported. Information on ionopause height, the ionospheric particle pressures at the ionopause, and the measured ratio of ionospheric scale height to ionopause ratio is presented.

Knudsen, W. C.↗

NASA NTRS1978

First measurements of mesospheric vertical velocities by VHF radar at temperate latitudes

A program of ground-based VHF measurements of turbulence and of wave and tidal motions in the mesosphere has been initiated by the University of Illinois using a high-power radar facility located at Urbana, Illinois. Observations show scattering occurring intermittently during the daytime throughout the mesosphere. Vertical velocities exhibit the presence of gravity waves with dominant periods near 10 min and amplitudes ranging from less than 1 m/s to approximately 5 m/s.

Miller, K. L.↗

NASA NTRS1978

Incoherent scatter radar observations of irregular structure in mid-latitude sporadic E layers

The basic experiments used phase-coded pulses to record electron density profiles with a resolution of 600 m in range and 300 m in horizontal extent, while scanning in azimuth. Data from incoherent scatter radar were compared with simultaneous ionosonde observations. Observations of sporadic E layers by incoherent scatter radar were discussed in terms of the effects of the neutral wind system acting on metallic ions. Several features were noted in the data, which support the wind shear mechanism of layer formation. The sporadic E layers often contained a pronounced small-scale structure, especially at times when partially transparent echoes were observed by the ionosonde. Under specific conditions, the ions in a meteor trail can be converged by a shear in the neutral wind into a relatively small irregularity at the center of a sporadic E layer.

Miller, K. L.↗

NASA NTRS1977

Reflection of radio waves by sporadic-E layers

A full-wave analysis of the reflection coefficient is developed and applied to electron-density profiles of midlatitude sporadic-E layers observed by rocket-borne probes. It is shown that partial reflection from the large electron-density gradients at the upper and lower boundaries of sporadic-E layers does not account for the partial transparency observed by ionosondes.

Miller, K. L.↗

NASA NTRS1976

Midlatitude sporadic-E layers

The partially transparent echo from midlatitude sporadic E layers was recorded by ionosondes between the blanketing frequency and the maximum frequency. The theory that the midlatitude sporadic E layers are not uniform in the horizontal plane but contain localized regions of high electron density was evaluated using data obtained by incoherent scatter radar and found to provide a satisfactory explanation. The main features of midlatitude sporadic E layers are consistent with the convergence of metallic ions as described by the wind shear theory applied to gravity waves and tides. The interference of gravity waves with other gravity waves and tides can be recognized in the altitudes of occurrence and the structure of the layers. Small scale horizontal irregularities are attributed in some cases to critical level effects and in others to fluid instabilities. The convergence of a meteor trail can, under some circumstances, account for localized enhancement of the electron density in the layer.

Miller, K. L.↗

NASA NTRS1975

Horizontal structure of midlatitude sporadic-E layers observed by incoherent scatter radar

The investigation reported is concerned with a model considered by Whitehead (1972). The partial transparency of the sporadic-E layer observed on certain occasions is attributed to regions of high electron density embedded in the layer. Observations obtained with an incoherent scatter radar facility are presented. Taking into account all factors, it is concluded that the partial transparency of sporadic-E layers, on the occasions of these observations, are explained by the Whitehead model.

Miller, K. L.↗