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NASA NTRS1987

Ionospheric convection driven by NBZ currents

Computer simulations of Birkeland currents and electric fields in the polar ionosphere during periods of northward IMF were conducted. When the IMF z component is northward, an additional current system, called the NBZ current system, is present in the polar cap. These simulations show the effect of the addition of NBZ currents on ionospheric convection, particularly in the polar cap. When the total current in the NBZ system is roughly 25 to 50 percent of the net region 1 and 2 currents, convection in the central portion of the polar cap reverses direction and turns sunward. This creates a pattern of four-cell convection with two small cells located in the polar cap, rotating in an opposite direction from the larger cells. When the Birkeland currents are fixed (constant current source), the electric field is reduced in regions of relatively high conductivity, which affects the pattern of ionospheric convection. Day-night asymmetries in conductivity change convection in such a way that the two polar-cap cells are located within the large dusk cell. When ionospheric convection is fixed (constant voltage source), Birkeland currents are increased in regions of relatively high conductivity. Ionospheric currents, which flow horizontally to close the Birkeland currents, are changed appreciably by the NBZ current system. The principal effect is an increase in ionospheric current in the polar cap.

Rasmussen, C. E.↗

NASA NTRS1992

Large shield volcanos on Venus: The effect of neutral buoyancy zone development on evolution and altitude distribution

The Magellan mission to Venus has emphasized the importance of volcanism in shaping the surface of the planet. Volcanic plains make up 80 percent of the terrain and hundreds of regions of localized eruptions have been identified. Large volcanos, defined as edifices with diameters greater than 100 km, are the sites of some of the most voluminous eruptions. Head et al. have identified 158 of these structures. Their spatial distribution is neither random nor arranged in linear chains as on the Earth; large volcanos on Venus are concentrated in two large, near-equatorial clusters that are also the site of many other forms of volcanic activity. The set of conditions that must be met on Venus that controls the change from widespread, distributed volcanism to focused, shield-building volcanism is not well understood. Future studies of transitional features will help to address this problem. It is likely, however, that the formation and evolution of a neutral buoyancy zone (NBZ) plays an important role in both determining the style of the volcanism and the development of the volcanic feature once it has begun to erupt. Head and Wilson have suggested that the high surface pressure on Venus may inhibit volatile exsolution, which may influence the density distribution of the upper crust and hence control the nature and location of a NBZ. The extreme variations in pressure with elevation may result in significantly different characteristics of such a NBZ at different locations on the planet. In order to test these ideas regarding the importance of NBZ development in the evolution of a large shield and to determine the style of volcanism, three large volcanos that occur at different basal elevations were examined and the distribution of large volcanos as a function of altitude was determined.

Keddie, S.↗

NASA NTRS1994

Height and altitude distribution of large volcanoes on Venus

Magellan data reveal 156 large volcanoes on Venus (greater than 100 km in diameter) which range in height from 300 m to 5.55 km, with an average height of 1.42 km. On the basis of theory it has been predicted that the development of neutral buoyancy zones (NBZ) on Venus and thus the resulting volcanic deposits are strongly influenced by the altitude-controlled variations in surface pressure. The distribution and height of these large volcanoes as a function of altitude was examined to begin to test these predications. Although large volcanoes are relatively uniformly distributed in altitude, there may be a slight paucity of volcanoes at the lowest elevations and a slight surplus at mid-altitudes. In addition, it is observed that the volcanoes at the highest altitudes tend to be the tallest. The observed distributions at low-mid altitudes is consistent with the prediction of NBZ theory. The high altitude distribution and heights, however, emphasize the necessity of considering other factors, such as tectonic setting, edifice age, magma supply, and thermal gradient, in describing the location and development of large volcanoes on Venus.

Keddie, S. T.↗

NASA NTRS2012

A High-resolution Model of Field-aligned Currents Through Empirical Orthogonal Functions Analysis (MFACE)

Ten years of CHAMP magnetic field measurements are integrated into MFACE, a model of field-aligned currents (FACs) using empirical orthogonal functions (EOFs). EOF1 gives the basic Region-1/Region-2 pattern varying mainly with the interplanetary magnetic field Bz component. EOF2 captures separately the cusp current signature and By-related variability. Compared to existing models, MFACE yields significantly better spatial resolution, reproduces typically observed FAC thickness and intensity, improves on the magnetic local time (MLT) distribution, and gives the seasonal dependence of FAC latitudes and the NBZ current signature. MFACE further reveals systematic dependences on By, including 1) Region-1/Region-2 topology modifications around noon; 2) imbalance between upward and downward maximum current density; 3) MLT location of the Harang discontinuity. Furthermore, our procedure allows quantifying response times of FACs to solar wind driving at the bow shock nose: we obtain 20 minutes and 35-40 minutes lags for the FAC density and latitude, respectively.

CHAMP↗

NASA NTRS1990

Plasma sheet polytropic index as inferred from the FPE measurements

It is shown here that a four-cell convection pattern imposed on the magnetosphere by the solar wind during northward IMF conditions can be distorted into a wrapped two-cell convection pattern which is similar to the empirical distorted two-cell convection patterns deduced by Heppner and Maynard (1987). The convection distortions are caused by the anisotropic reflections of Alfven waves due to the anisotropic and nonuniform ionospheric conductance. The distributions of the resulting field-aligned currents in the model are characteristically consistent with observed current distributions, including the NBZ currents during northward IMF.

Zhu, X. M.↗

NASA NTRS1991

On the influence of the magnetization of a modal solar wind on a laboratory magnetosphere

The interaction of a magnetized plasma beam with a stationary dipole field, analogous to the interaction of the solar wind with the earth's magnetosphere, is explored in a laboratory experiment. Experimental parameters are chosen to scale qualitatively similar to the parameters in the earth's magnetosphere. It is found that the magnetization of the laboratory 'solar wind', generated by injecting a plasma across a preexisting magnetic field, requires a certain minimum magnetic field strength. Differences between the resulting magnetospheres for northward and southward 'solar wind' or 'interplanetary' magnetic fields (IMF) are demonstrated by global pictures and by magnetic field measurements above the north polar region. These measurements show patterns of the variation of the transverse field component which are similar to those found by satellite measurements above the earth. This indicates the presence of similar field-aligned current systems. Particularly, the presence (for northward IMF) and absence (for southward IMF) of the pattern attributed to the 'NBZ' (northward Bz) current system are demonstrated.

Rahman, H. U.↗

NASA NTRS1993

The distribution of large volcanoes on Venus as a function of height and altitude

Theory predicts that the slower cooling of lava flows on Venus should result in lava flows that are typically 20 percent longer than their terrestrial counterparts and that the development of neutral buoyancy zones (NBZ) on Venus may be strongly influenced by altitude-controlled variations in surface pressure. Observations that support these predictions would include relatively low heights for Venus volcanoes, and an increase in both the number and development of large edifices with increasing basal altitude. The results of an analysis of the height and altitude distribution of 123 large (diameter greater than 100 km) volcanoes made using Magellan image and altimetry data are presented and these results are used to begin to test the predications of the above theories.

Keddie, S. T.↗

NASA NTRS1995

The Earth's magnetosphere is 165 R(sub E) long: Self-consistent currents, convection, magnetospheric structure, and processes for northward interplanetary magnetic field

The subject of this paper is a self-consistent, magnetohydrodynamic numerical realization for the Earth's magnetosphere which is in a quasi-steady dynamic equilibrium for a due northward interplanetary magnetic field (IMF). Although a few hours of steady northward IMF are required for this asymptotic state to be set up, it should still be of considerable theoretical interest because it constitutes a 'ground state' for the solar wind-magnetosphere interaction. Moreover, particular features of this ground state magnetosphere should be observable even under less extreme solar wind conditions. Certain characteristics of this magnetosphere, namely, NBZ Birkeland currents, four-cell ionospheric convection, a relatively weak cross-polar potential, and a prominent flow boundary layer, are widely expected. Other characteristics, such as no open tail lobes, no Earth-connected magnetic flux beyond 155 R(sub E) downstream, magnetic merging in a closed topology at the cusps, and a 'tadpole' shaped magnetospheric boundary, might not be expected. In this paper, we will present the evidence for this unusual but interesting magnetospheric equilibrium. We will also discuss our present understanding of this singular state.

Fedder, J. A.↗