W. C. Knudsen

John Wiley & Sons (United States)

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K. L. Miller

Winthrop University

K. Spenner

Fraunhofer Institute for Physical Measurement Techniques

R. C. Whitten

Search for Extraterrestrial Intelligence

L. H. Brace

University of Michigan–Ann Arbor

C. T. Russell

University of California, Los Angeles

V. Novák

Czech Academy of Sciences, Institute of Physics

G. W. Sharp

University of New Mexico

F. L. Scarf

Redondo Optics (United States)

J. R. Spreiter

Stanford University

A. F. Nagy

University of Michigan–Ann Arbor

Cooperative Institutions

John Wiley & Sons (United States)

107

Hudson Institute

Johns Hopkins University

Johns Hopkins Medicine

Sidney Kimmel Comprehensive Cancer Center

Goddard Space Flight Center

University of Michigan–Ann Arbor

Nara Medical University

Ames Research Center

University of California, Los Angeles

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Evidence for ion transport and molecular ion dominance in the Venus ionotail

Journal of Geophysical Research Atmospheres (1994)

D. S. Intriligator L. H. Brace P. A. Cloutier J. M. Grebowsky R. E. Hartle

We present analyses from the five Pioneer Venus Orbiter plasma experiments and the plasma wave experiment when a patch of plasma with enhanced densities was encountered in the near‐Venus ionotail during atmospheric entry at an altitude of ∼ 1100 km in the nightside ionosphere. Our analyses of the thermal and superthermal ion measurements in this plasma feature provides the first evidence that at times molecular ions in the 28‐32 amu mass range are dominant over atomic mass species thus yielding evidence for a transport mechanism that reaches into the lower ionosphere. Analysis of plasma analyzer (OPA) observations at this time indicates the presence of ions measured in the rest frame of the spacecraft at ∼27 and 37 volt energy per unit charge steps. In the rest frame of the planet these superthermal ions are flowing from the dawn direction at speeds (assuming they are O 2 + ) of ∼8 km/s and with a flow component downward (perpendicular to the ecliptic plane) at speeds of ∼2 km/s. OPA analyses also determine the ion number flux, energy, flow angles, and angular distributions. Plasma wave bursts appear to indicate that plasma density decreases within and on the equatorward edge of the patch of enhanced plasma densities are associated with ion acoustic waves and relative ion streaming.

Orbiter

Polar wind

Bow wave

10.1029/94ja01341

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Citations (4)

Pioneer Venus observations of plasma and field structure in the near wake of Venus

Journal of Geophysical Research Atmospheres (1982)

J. G. Luhmann C. T. Russell L. H. Brace H. A. Taylor W. C. Knudsen

Ionospheric plasma density depletions or ‘holes’ are observed by the Pioneer Venus orbiter in association with radial magnetic fields in the near wake of Venus. This report presents examples of the collected observations of these unexpected features of the Venus nightside ionosphere obtained by the Langmuir probe, magnetometer, ion mass spectrometer, retarding potential analyzer, plasma analyzer, and electric field experiments. The connection between plasma density depletions and temperature changes, changes in ion composition, plasma wave emissions, and magnetic fields with a substantial radial component is illustrated. Mechanisms that may be responsible for the formation and maintenance of holes are suggested.

Atmosphere of Venus

Orbiter

Langmuir Probe

10.1029/ja087ia11p09205

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Citations (35)

The Venus transterminator ion flux at solar maximum

Journal of Geophysical Research Atmospheres (1992)

W. C. Knudsen K. L. Miller

The median Venus transterminator ion flux nv(h) derived from Pioneer Venus retarding potential analyzer measurements of the ion velocity and ion density during a period of solar maximum is presented as a function of altitude h from 150 km to the solar maximum median ionopause altitude of 800 km. This flux is integrated with altitude to provide the integrated total ion flux S(h) crossing the terminator plane as a function of altitude. S(h) is nearly a linear function of h , reflecting the nearly constant value of nv with altitude. The value of S at 800 km altitude, called F , is 5×10 26 s −1 , a value equal to the total nightside hemispheric molecular ion recombination rate L calculated previously. The loss of ions down the wake of Venus is smaller than F or L by at least a factor of 5. Thus the equality of F and L is evidence that the dominant Venus nightside ionization source at solar maximum is transterminator ion transport.

Atmosphere of Venus

10.1029/92ja01460

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Citations (23)

Neutral atmosphere wave generation by the equatorial electrojet

Journal of Geophysical Research Atmospheres (1969)

W. C. Knudsen

Equatorial electrojet

10.1029/ja074i016p04191

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Citations (15)

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

Journal of Geophysical Research Atmospheres (1980)

W. C. Knudsen K. Spenner K. L. Miller V. Novák

Vector velocities of O + ions in the Venus ionosphere are reported for the dawn and dusk terminator regions. The velocity vectors are generally directed antisunward and radially inward toward the planet with magnitudes ranging from approximately 1 to 8 km/s. The velocity generally increases with altitude. The velocity may increase to still larger values just below the ionopause, but experimental limitations prevent measuring such an increase. The Alfvén and ion Mach numbers are generally greater than 1. The estimated O + flux across the terminator is equal within a factor of 2 to the total ion recombination rate on the nightside and is evidently a factor of 10 larger than that which could be exiting down the planet wake. The higher velocity flow near the ionopause is hypothesized to converge on the antisolar axis and, after thermalization, to descend into the ionosphere. The kinetic energy and O + flux in this high‐velocity stream are sufficient to maintain, respectively, the high ion temperature and peak ion concentration, respectively, measured at solar zenith angles in excess of 150°. The nightside ionospheric variability is hypothesized to result from temporal and spatial variations in the ionospheric terminator and wake flow fields. Ion transport contributes substantially and possibly predominantly to the maintenance of the nightside ionosphere.

Terminator (solar)

Solar zenith angle

Zenith

10.1029/ja085ia13p07803

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Citations (179)

Nightward ion flow in the Venus ionosphere: Implications of momentum balance

Geophysical Research Letters (1984)

R. C. Elphic H. G. Mayr R. F. Theis L. H. Brace K. L. Miller

Using global empirical models of Venus ionospheric conditions, we solve for the plasma flow field consistent with the horizontal momentum equation in both viscid and inviscid forms. We find that plasma viscosity is negligible except at low altitudes and that the observed plasma flows are consistent with the inviscid solution above 300 km but are larger than the calculated flows at lower altitudes. This is probably due to downward momentum advection. The strong vertical shear in the calculated and observed flows just beyond the terminator may produce turbulence there, manifested as the observed transterminator waves in plasma density and magnetic field. We discuss the implications of ionospheric superrotation.

Inviscid flow

Momentum (technical analysis)

Atmosphere of Venus

10.1029/gl011i010p01007

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Citations (28)

Model calculations of the dayside ionosphere of Venus: Energetics

Journal of Geophysical Research Atmospheres (1980)

T. E. Cravens T. I. Gombosi J. U. Kozyra A. F. Nagy L. H. Brace

A model of the energy balance of the dayside ionosphere of Venus is presented. Calculations of the dayside electron and ion temperature profiles are carried out and compared with data from experiments on the Pioneer Venus orbiter. The coupled heat conduction equations for electrons and ions are solved for several values of the solar zenith angle. It is shown that thermal conductivities are inhibited by the presence of a horizontal magnetic field. A realistic model of the magnetic field that includes fluctuations is employed in deriving an appropriate expression for the thermal conductivity. The contributions of photoelectrons, ion chemistry, Joule heating, and solar wind heating to the energy balance of the ionosphere are considered.

Atmosphere of Venus

Solar zenith angle

Zenith

Orbiter

10.1029/ja085ia13p07778

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Citations (108)

Tropical ultraviolet nightglow from oxygen ion-ion neutralization

Journal of Geophysical Research Atmospheres (1970)

W. C. Knudsen

The reaction O− + O+ → O(5S or ³S) + O(³P) + KE may be responsible for the recently observed tropical ultraviolet nightglow composed principally of the 1304 and 1356 A lines of atomic oxygen. The O− concentration needed to obtain the observed emission intensity is less than 10−3 of the electron concentration. Emission intensity from this reaction in a steady state is controlled by the production rate of O− which, in turn, is probably controlled by the height-integrated product of atomic oxygen and electron concentration. The quiescent intensity of the UV emission computed for this mechanism is of the order of 100 rayleighs. Some of the consequences of the mechanism are outlined and are compared with the consequences of a radiative recombination mechanism.

Airglow

Atomic oxygen

Ultraviolet

Intensity

Emission intensity

Luminous intensity

10.1029/ja075i019p03862

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Citations (47)

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

Journal of Geophysical Research Atmospheres (1980)

K. L. Miller W. C. Knudsen K. Spenner R. C. Whitten V. Novák

Data from the first year of operation of the Pioneer Venus orbiter retarding potential analyzer are presented, showing the variation with solar zenith angle of median values of ion and electron temperatures and density. Ion density is found to decrease by about an order of magnitude from dayside to nightside. Median ion temperatures above 300‐km altitude are essentially constant with solar zenith angle for solar zenith angles less than 150° and have values near 2300°K at the ionopause. Near the antisolar point, the ion temperature rises to 5000°K. At altitudes below 300 km the median ion temperature is nearly constant with solar zenith angle throughout the dayside but increases with solar zenith angle on the nightside. Ion temperatures on the dayside are nearly constant and are larger than the neutral temperature in the altitude range between 175 and 225 km. A strong heat source near 160 km is required to model the observations. Joule heating is suggested as the probable source. The median electron temperature at a given height is independent of both solar zenith angle and ionopause height. The electron temperatures suggest a constant heat flux into the electron gas at the ionopause. The flux may be provided by dissipation of energy by whistler mode plasma waves at the ionopause and/or conduction of heat from the ionosheath through the mantle.

Solar zenith angle

Zenith

Atmosphere of Venus

Electron temperature

10.1029/ja085ia13p07759

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Citations (128)

Standing Waves of Third Sound in He II Films

Physical Review Letters (1965)

W. C. Knudsen

Standing wave

Second sound

Sound wave

10.1103/physrevlett.15.379

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Citations (2)