Engineering topics

Knudsen, William C.

Publications and source records attributed to Knudsen, William C..

9 records

NASA NTRS1993

Pioneer Venus data analysis for the retarding potential analyzer

This report describes the data analysis and archiving activities, analysis results, and instrument performance of the orbiter retarding potential analyzer (ORPA) flown on the Pioneer Venus Orbiter spacecraft during the period, Aug. 1, 1988 to Sept. 30, 1993. During this period, the periapsis altitude of the Orbiter spacecraft descended slowly from 1900 km altitude, at which altitude the spacecraft was outside the Venus ionosphere, to approximately 130 km altitude in Oct. 1992 at which time communication with the spacecraft ceased as a result of entry of the spacecraft into the Venus atmosphere. The quantity of ORPA data returned during this reporting period was greatly reduced over that recovered in the previous years of the mission because of the reduced power capability of the spacecraft, loss of half of the onboard data storage, and partial failure of the ORPA. Despite the reduction in available data, especially ionospheric data, important scientific discoveries resulted from this extended period of the Pioneer Venus mission. The most significant discovery was that of a strong solar cycle change in the size of the dayside ionosphere and the resulting shutoff of flow of dayside ions into the nightside hemisphere. The large, topside O+ F2 ionospheric layer observed during the first three years of the Pioneer Venus mission, a period of solar cycle maximum activity, is absent during the solar cycle minimum activity period.

Knudsen, William C.↗

NASA NTRS1992

Finite grid radius and thickness effects on retarding potential analyzer measured suprathermal electron density and temperature

The effect of finite grid radius and thickness on the electron current measured by planar retarding potential analyzers (RPAs) is analyzed numerically. Depending on the plasma environment, the current is significantly reduced below that which is calculated using a theoretical equation derived for an idealized RPA having grids with infinite radius and vanishingly small thickness. A correction factor to the idealized theoretical equation is derived for the Pioneer Venus (PV) orbiter RPA (ORPA) for electron gasses consisting of one or more components obeying Maxwell statistics. The error in density and temperature of Maxwellian electron distributions previously derived from ORPA data using the theoretical expression for the idealized ORPA is evaluated by comparing the densities and temperatures derived from a sample of PV ORPA data using the theoretical expression with and without the correction factor.

Knudsen, William C.↗

NASA NTRS1992

The Venus ionosphere from in situ measurements

The present characterization of the Venus ionosphere gives attention to features observed by the Pioneer Venus orbiter in situ ionospheric particle measuring instrument over an entire cycle of solar activity. A striking change is noted in the typical ionospheric morphology from solar maximum to minimum. The principal source of ionization in the dayside ionosphere during both solar maximum and minimum is solar EUV radiation, while that in the nightside is a downward flux of O(+) ions that originate in the dayside ionosphere and are transported across the nightside hemisphere by a supersonic ion wind. The dayside ionosphere contains fluxes of suprathermal electrons whose energy spectra are consistent with fluxes originating in EUV photo production.

Knudsen, William C.↗

NASA NTRS1990

Role of hot oxygen in Venusian ionospheric ion energetics and supersonic antisunward flow

The column heating rate of the dayside Venus ionospheric ion gas resulting from transfer of energy from the hot oxygen component of the neutral atmosphere is estimated and found equal to that which, when inserted into ionospheric models at the ionopause, raises the calculated temperature to measured values. The transfer of energy is effected through resonant charge exchange between the relatively cold ionospheric O(+) ions and the hot oxygen neutrals. The hot oxygen density in the nightside hemisphere does not appear to play a significant role in the nightside ion energetics. The hot oxygen neutral gas flowing across the terminator from its dayside source to its nightside sink will exchange momentum with the antisunward flowing ionospheric gas. Although the flow rate of hot oxygen can be estimated only crudely, the estimated rate of deposition and absorption is comparable to that produced by the plasma pressure and should be included in numerical studies of the terminator ionospheric wind.

Knudsen, William C.↗

NASA NTRS1989

The Venus precursor - An environmental effect on the Pioneer Venus spacecraft

Data obtained by the Pioneer Venus Orbiter's retarding potential analyzer during two orbits with periapsis in the subpolar region of Venus are presented. Two electron populations are observed: a low-temperature population that is interpreted to be an electron sheath population trapped in the positive spacecraft potential well, and a high-temperature population representing the ambient solar wind electron population. The sheath population's increase and decrease by a factor of 5, as the spacecraft approaches and recedes from a position near periapsis (independently of the bow shock's position relative to the spacecraft), is interpreted as being due to an increase and decrease in the density of cold electrons trapped in the spacecraft potential well.

Knudsen, William C.↗

NASA NTRS1988

Solar cycle changes in the morphology of the Venus ionosphere

Altitude profiles of median plasma density are presented for the central nightside Venus ionosphere for the solar cycle maximum (SCmax) and minimum (SCmin), constructed from Pioneer Venus retarding-potential-analyzer total ion density data. The curves demonstrate that, between the altitudes of 200 and 2000 km, the nightside ionospheric density during SCmin was smaller, by a factor of 10, than it was during SCmax. This observation, along with the Venera radio occultation measurements at SCmin, support the contention that the Venus SCmin ionosphere is typically confined to an altitude below 250 km at all solar zenith angles (SZAs), with possible exception of an SZA interval near the terminator, and is composed primarily of O2(+).

Knudsen, William C.↗

NASA NTRS1987

Solar cycle changes in the ionization sources of the nightside Venus ionosphere

Using new and previous experimental evidence, it is argued that the height-integrated flux of O(+) ions transported from the dayside Venus ionosphere into the nightside ionosphere is strongly reduced (shut off) at solar cycle minimum (SCmin) and that the electron impact source becomes the predominant nightside ionization source during this phase of the solar cycle. The height-integrated transterminator O(+) flux, which is probably the dominant nightside ionization source at solar cycle maximum (SCmax), is shut off at SCmin by compression of the dayside ionopause to low altitude. It is suggested that the ionopause is depressed because the dayside ionospheric kinetic pressure field is reduced by a factor of approximately 3 from its SCmas value and that the typical SCmin solar wind dynamic pressure, which is not less than the typical SCmax value, cannot be balanced by the ionospheric pressure field at altitudes above approximately 250 km.

Knudsen, William C.↗

NASA NTRS1987

Frequency functions of Venus nightside ion densities

The variability of the total and major ion densities in the nightside Venus ionosphere are studied by deriving an experimental frequency function for these quantities. The frequency function N(i) derived for the total ion density is found to be insensitive to division of the parent set into two approximately equal subsets representing dawn and dusk hemispheres, an upper and lower altitude interval, or two solar zenith angle intervals. The N(i) function is reasonably well peaked near the median value, and 78 percent of the normalized values fall within a factor of 2.8 of the median value. The frequency of occurrence decreases more rapidly on the high side of the median than on the low side. The frequency functions for the constituent ions are similar to that for N(i).

Knudsen, William C.↗

NASA NTRS1986

Median density altitude profiles of the major ions in the central nightside Venus ionosphere

Altitude profiles of median values for total ion density, constituent ion densities, and the molecular ion group of the central nightside Venus ionosphere are presently derived from the Pioneer Venus retarding potential analyzer's measurements during the first five seasonal passages of the spacecraft through the nightside hemisphere. The altitude profiles for total ion density, constituent ions and the molecular ion group are compared with previously predicted profiles derived from numerical models in which the ionization source was O(+) ions transported from the dayside ionosphere and electron impact-produced ionization. The electron impact-produced profiles for O(+) is 2 orders of magnitude smaller than that observed.

Knudsen, William C.↗