Engineering Papers⌕ Search

Engineering topics

Hill, T. W.

Publications and source records attributed to Hill, T. W..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Mapping of the solar wind electric field on the earth's polar caps

A quantitative model of a magnetically interconnected (open) magnetosphere is developed as a perturbation of Voight's closed magnetosphere model with a given magnetopause shape. The model is shown to give a reasonable representation of the open magnetosphere configuration and is especially useful in testing the magnetospheric consequences of various merging theories. The convection pattern obtained for southward IMF is in reasonable agreement with observations, especially in the manifestation of a convection 'throat' near noon. For northward IMF, this region of constricted flow is rotated in local time in the direction of the IMF, also in accordance with observations.

Toffoletto, F. R.↗
NASA NTRS1988

Motion of charged particles in planetary magnetospheres with nonelectromagnetic forces

Expressions are derived for the mirror point, the bounce period, the second adiabatic invariant, and the bounce-averaged azimuthal drift velocity as functions of equatorial pitch angle for a charged particle in a dipole magnetic field in the presence of centrifugal, gravitational, and Coriolis forces. These expressions are evaluated numerically, and the results are displayed graphically. The average azimuthal drift speed for a flux tube containing a thermal equilibrium plasma distribution is also evaluated.

Huang, T. S.↗
NASA NTRS1986

Quantitative model of magnetic coupling between solar wind and magnetosphere

Preliminary results are presented of a quantitative three-dimensional model of an open steady-state magnetosphere configuration incorporating a normal-component distribution corresponding to the subsolar merging-line hypothesis. The distribution of the normal magnetic-field component at the magnetopause is used as input and is used to calculate an interconnection magnetic field that links the internal and external fields. The interconnected field is then used to map the solar-wind electric field onto the polar cap. The resulting polar-cap flow patterns are found to be in agreement with observations.

Toffoletto, F. R.↗
NASA NTRS1985

Jovian ionospheric conductivity and magnetospheric plasma outflow - Voyager 1

Using magnetic field and plasma data from the Voyager 1 inbound encounter, local values are derived for the mass outflow rate and the radial bulk velocity in the dayside Jovian magnetosphere, and for the height-integrated Pedersen conductivity of the Jovian ionosphere. These values, accurate within a factor of 2, are compatible with previous order-of-magnitude estimates of these quantities. The results suggest that the shape of the plasma sheet is more complicated than previously suspected. A search for evidence of enhanced plasma outflow in the active sector was inconclusive.

Hairston, M. R.↗
NASA NTRS1985

Remote sensing of the magnetic moment of Uranus - Predictions for Voyager

The current understanding of the power transfer mechanisms by which power is supplied to a planet's magnetosphere by the kinetic energy of planetary spin and the energy flux of the impinging solar wind is applied to the case of Uranus, in order to predict the detectability of radio and auroral emissions by the planetary radio astronomy (PRA) and UV spectrometer (UVS) instruments of the Voyager spacecraft. The power available for the two energy transfer phenomena cited is a function of Uranus' magnetic moment, which is presently derived for each power source as a function of the date of first detection of radio emissions by the PRA or auroral emissions by the UVS.

Hill, T. W.↗
NASA NTRS1984

Do the Satellites of Uranus Control Its Magnetosphere?

The importance of the satellites of Uranus as sources of magnetospheric plasma were investigated. It is found that neither an Io like plasma source nor a Titan like source is likely at Uranus. The likely presence of a heavy ion plasma torus maintained by charged particle sputtering of the icy satellites is examined. Sputtering of Saturn's icy satellites is considered an important source of heavy ion (oxygen) plasma in Saturn's inner magnetosphere. A major unresolved question is whether this sputtering process does depend on the preexistence of magnetospheric heavy ions derived from another source, Titian.

Cheng, A. F.↗
NASA NTRS1984

Rotationally-induced Birkeland current systems

Rotational effects which are negligible in earth's magnetosphere become dominant in the magnetospheres of the outer planets where they give rise to Birkeland current circuits coupling the ionospheric and magnetospheric motions. The centrifugal force of corotation produces an azimuthal ring current in the magnetosphere while the Coriolis force produces a current parallel to the plasma flow. The acceleration current also becomes significant when deviations from strict corotation are appreciable. In general, none of these currents are divergence-free, and closure is provided by ionospheric conduction currents via connecting Birkeland currents. Thus the ionospheric conductivity regulates magnetospheric motions much as it does in the terrestrial case. The effects of such currents have been clearly observed in the magnetospheres of Jupiter and Saturn. Birkeland current circuits also transmit planetary angular momentum to external sinks such as conducting satellites (e.g., Io), plasma production sites (e.g., the Io torus), and/or the surrounding solar wind (as proposed for Uranus).

Hill, T. W.↗
NASA NTRS1983

Longitudinal asymmetry of the Io plasma torus

Remote observations of the Io plasma torus have revealed a persistent longitudinal asymmetry in the brightness of optical S(+) emission lines but not of ultraviolet S(2+) emission lines. The S(+) asymmetry (with respect to the corotating System III coordinates) has been attributed to an asymmetric plasma source; the question then arises, why do the higher ionization states of sulfur not reflect a similar asymmetry? The explanation proposed here is based on the fact that torus ions do not quite corotate at the System III rate because of mass loading. If the resulting System III drift period of the ions is intermediate between the ionization lifetimes of S(+) and S(2+), then the asymmetry of the source will be obscured by longitudinal drift for the relatively long-lived multiply-ionized sulfur ions, but not for short-lived S(+).

Hill, T. W.↗
NASA NTRS1983

Aurora on Uranus - A Faraday disc dynamo mechanism

A mechanism is proposed whereby the solar wind flowing past the magnetosphere of Uranus causes a Faraday disk dynamo topology to be established and power to be extracted from the kinetic energy of rotation of Uranus. An immediate consequence of this dynamo is the generation of Birkeland currents that flow in and out of the sunlit polar cap with the accompanying production of polar aurora. The power extracted from planetary rotation is calculated as a function of planetary dipole magnetic moment and the ionospheric conductivity of Uranus. For plausible values of ionospheric conductivity, the observed auroral power requires a magnetic moment corresponding to a surface equatorial field of the order of 4 Gauss, slightly larger than the value 1.8 Gauss given by the empirical 'magnetic Bodes law'.

Hill, T. W.↗
NASA NTRS1983

The magnetosphere of Uranus - Plasma sources, convection, and field configuration

It is suggested by qualitative considerations based on analogy with earth, Jupiter, and Saturn that the magnetosphere of Uranus may lack a plasma source able to produce significant internal currents, internal convection, and associated effects. A class of approximately self-consistent quantitative magnetohydrostatic equilibrium configurations for the case of a pole-on magnetosphere with variable plasma parameters is presently constructed in order to test this hypothesis by means of forthcoming Voyager measurements. The configurations that can be computed for the geometries of the magnetic field and of the tail current sheet, for a given distribution of plasma pressure, have a single, funnel-shaped polar cusp pointing into the solar wind and a cylindrical tail plasma sheet whose currents close within the tail, rather than on the tail magnetopause. Interconnection of interplanetary and magnetospheric fields yields a highly asymmetric tail-field configuration.

Voigt, G.-H.↗
NASA NTRS1983

Magnetospheric models

Of the planetary magnetospheres which have been explored, Jupiter's is by far the largest. It is a magnetosphere largely dominated by rotational effects. As such, it offers unique insight to the study of inaccessible pulsar magnetospheres. The present investigation is concerned with theoretical concepts which are believed to be consistent with available observations. It has been found that Io, the innermost of the Galilean satellites, is the principal source of plasma for the Jovian magnetosphere. The Io source is considered along with the solar-wind source, the ionospheric source, other satellite sources, and variations of the temperature and the content of the Io plasma torus with time. The rotation of Jupiter as the dominant source of energy for magnetospheric phenomena is discussed along with aspects of the Io-Jupiter interaction. Attention is also given to particle acceleration, and spin periodicity.

Hill, T. W.↗
NASA NTRS1982

Departure from corotation of the Io plasma torus - Local plasma production

The departure of the Jovian magnetosphere from rigid corotation is adequately explained by outward plasma transport at distances where L is greater than approximately 10. The departure of 5% observed in the Io plasma torus, however, is too large to be accounted for simply by plasma transport. Local plasma production is proposed as the main factor determining the corotation lag in the torus. The outward pick-up current provided by ionization of neutral atoms is calculated and related to the current produced in the ionosphere by the corotation lag. This leads to an expression giving the corotation lag of the torus as a function of radial distance. Charge transfer is found to be an important process, allowing the majority of the torus mass to be ejected from the magnetosphere in a neutral state. Thus, the mass loading rate is found to be several times that inferred from examination of the corotation lag associated with outward plasma transport.

Pontius, D. H., Jr.↗
NASA NTRS1982

Some consequences of corotating magnetospheric convection

A comparison is conducted of the expected signature of the inflow region of the proposed corotating convection pattern with relevant magnetic field and plasma flow observations made by Pioneers 10 and 11 and Voyagers 1 and 2 in the Jovian magnetosphere. A region of net plasma inflow would be characterized by superrotation and a negative radial current in the equatorial plane within a characteristic distance L. It is found that no value of L exists that is consistent with both plasma and magnetic field observations. Hence it is concluded that at the time of these flybys, such a simple large-scale convection pattern did not dominate the plasma transport in the outer magnetosphere, although the existence of such a pattern in the inner magnetosphere is not ruled out. A more detailed test is proposed to determine whether or not a superposition of corotating convection and radial diffusion is consistent with the observations.

Hill, T. W.↗
NASA NTRS1982

Highlights of theoretical progress related to the International Magnetospheric Study

U.S. theoretical research efforts have addressed three areas within the International Magnetospheric Study. The first, solar wind/magnetosphere interaction, is presently concerned with the suggestion that magnetic merging may predominantly occur near the polar cusps rather than near the subsolar point. Mechanisms have been proposed for noncollisional diffusion of solar wind plasma across the closed magnetopause entailed by such a phenomenon. The second area considers the importance to magnetotail dynamics of a continuous source of solar wind plasma, and of sporadic plasma loss associated with an unsteady convection cycle. In the third area, the electrodynamic magnetosphere/ionosphere interaction, an advanced state has been reached in the understanding of the relevant physics, with respect both to coupling in the subauroral region and the large scale structure of auroral zone electric fields parallel, and perpendicular to, the magnetic field.

Hill, T. W.↗
NASA NTRS1981

Dependence of polar cap potential drop on interplanetary parameters

The convection potential drop across the polar cap is computed from data obtained on high-inclination low-altitude satellites. Potential measurements are correlated with various combinations of parameters measured simultaneously in the upstream solar wind. Most of the potential drop is successfully predicted by merging theory, although a significant background potential drop of 35 kV does not depend on IMF parameters and is attributed to a process other than merging. Results indicate that small values of the IMF are amplified by a factor of 5-10 at the dayside magnetopause, which, when taken into account, improves correlations between IMF parameters and polar cap potential drop. Potential drop is better correlated with IMF parameters than with geomagnetic indices, due to nonlinear response of the magnetosphere affecting geomagnetic activity indices.

Reiff, P. H.↗
NASA NTRS1981

Mechanism of parallel electric fields inferred from observations

Data from various experiments of the Atmosphere Explorer satellites are analyzed to test their consistency with the model of single-particle linear acceleration through a static parallel potential drop. Theoretical current/voltage and energy flux/voltage relations are applied to the data to predict the densities of source current carriers above the acceleration region for upward Birkeland currents. Comparison of these model predictions with simultaneous electron data indicates that the observed field-aligned potential drops are consistent with direct acceleration in the absence of anomalous resistivity.

Yeh, H.-C.↗