Engineering topics

Wolf, R. A.

Publications and source records attributed to Wolf, R. A..

At least 37 records · Page 2

NASA NTRS1988

Penetrating of high-latitude-electric-field effects to low latitudes during SUNDIAL 1984

Electric-field-penetration events have been identified using F-region vertical-drift measurements obtained in the October 6-13, 1984 period by the Jicamarcan incoherent-backscatter radar and corresponding h-prime F measurements from ionosondes at Fortaleza, Cachoeira Paulista, and Dakar. Predictions made using the Rice Convection Model for the pattern, strength, and duration of the low-latitude electric field occurring in response to an increasing high-latitude convection agree with observations. The observed 1-2 h duration of the low-latitude response to decreased convection can be explained by the fossil-wind theory of Richmond (1983).

Spiro, R. W.↗

NASA NTRS1987

Effect of a localized minimum in equatorial field strength on resistive tearing instability in the geomagnetotail

A two-dimensional, resistive-MHD computer code is used to investigate the spontaneous reconnection of magnetotaillike configurations. The initial conditions adopted in the simulations are of two types: (1) in which the equatorial normal magnetic field component B(ze) declines monotonically down the tail, and (2) in which B(ze) exhibits a deep minimum in the near-earth plasma sheet. To represent the case where the earthward convection stops before the X line forms, zero-flow boundary conditions are imposed at the edges of the computational box. The initial configurations are in equilibrium and table within ideal MHD. The dynamic evolution of the system starts after the resistivity is turned on. The main results of these simulations basically support the neutral-line model of substorms and confirm Birn's (1980) computer studies. Spontaneous formation of an X-type neutral point and a single O-type plasmoid with strong tailward flow on the tailward side of the X point is found. The time interval from the turning on of the resistivity to the formation of a plasmoid is much shorter in the case where there is an initial deep minimum. A simple analytic calculation is also carried out to demonstrate why the configuration with a deep minimum is more susceptible to the development of the neutral point.

Hau, L.-N.↗

NASA NTRS1986

Theoretical comments on the nature of the plasmapause

The traditional theoretical interpretations of the observed plasmapause are compared, namely, the plasmapause as: (1) The boundary between closed flux tubes that have been in the inner magnetosphere for several days and those that have recently drifted in from the magnetotail or (2) the last closed electric equipotential. Although the two interpretations become equivalent in the case where the electric-field pattern is steady for several days, interpretation 1 seems theoretically more secure for typical magnetospheric conditions, due to the essentially time-dependent nature of the magnetospheric electric field. The results of old theoretical studies of the effects of time variations in the electric-field pattern on the shape of the plasmapause are reviewed briefly. The formulation of the present version of the Rice Convection Model is also reviewed. Preliminary results of recent computations of quiet-time electric fields, carried out with this model, are presented and discussed. Quiet-time thermospheric winds are found to have only minor effects on drift paths of magnetospheric particles.

Wolf, R. A.↗

NASA NTRS1985

On the configuration of the polar cusps in earth's magnetosphere

The interaction between the solar wind and the earth's vacuum dipole field leads to the formation of a discontinuity called the magnetopause. In the standard picture, the magnetopause confines the magnetic field in such a manner that the polar cusp field lines originate from high latitudes in the dayside ionosphere and end at the two magnetic neutral points. Wu (1983, 1984) has questioned this standard picture of the polar cusp. MHD simulations indicate the existence of a current sheet above the polar cusp region, called 'the cusp current sheet' by Wu. Wu (1983) concluded that the difference between his cusp configuration and the standard picture is due to the fact that his geometry results from a plasma model, whereas the standard picture is based on a vacuum concept. In the present investigation, Wu's conclusion is questioned, and it is demonstrated that the standard cusp configuration is not restricted to the vacuum magnetosphere.

Voigt, G.-H.↗

NASA NTRS1985

Comparison of polar cap potential drops estimated from solar wind and ground magnetometer data - CDAW 6

It is pointed out that the maximum electrostatic potential difference across the polar cap, Phi, is a fundamental measure of the coupling between the solar wind and the earth's magnetosphere/ionosphere sytem. During the Coordinated Data Analysis Workshop (CDAW) 6 intervals, no suitably instrumented spacecraft was in an appropriate orbit to determine the polar-cap potential drop directly. However, two recently developed independent techniques make it possible to estimate the polar-cap potential drop for times when direct spacecraft data are not available. The present investigation is concerned with a comparison of cross-polar-cap potential drop estimates calculated for the two CDAW 6 intervals on the basis of these two techniques. In the case of one interval, the agreement between the potential drops and Joule heating rates is relatively good. In the second interval, however, the agreement is not very good. Explanations for this discrepancy are discussed.

Reiff, P. H.↗

NASA NTRS1985

Particle behavior in the magnetosphere

The Rice Convection Model deals with large-scale processes in the earth's inner and middle magnetosphere, including coupling to the ionosphere. Starting from appropriate initial and boundary conditions, the model computes the following physical parameters: ionospheric electric fields and currents; magnetospheric particle distributions, electric fields, and electric currents; and magnetic-field-aligned (Birkeland) currents connecting the two regions. This paper evaluates work on the model, with emphasis on the assumptions made, the basic equations, and the numerical methods. The theoretical basis of the model is compared and contrasted with standard magnetohydrodynamics. The limitations imposed by the major assumptions are discussed. Model inputs and boundary conditions are listed, and the methods of specifying them discussed. Some physical conclusions and insights that have been gained from the model are listed and described very briefly. References are given to published discussions of the major points of physics.

Wolf, R. A.↗

NASA NTRS1984

Ionosphere-magnetosphere coupling and convection

The following international Magnetospheric Study quantitative models of observed ionosphere-magnetosphere events are reviewed: (1) a theoretical model of convection; (2) algorithms for deducing ionospheric current and electric-field patterns from sets of ground magnetograms and ionospheric conductivity information; and (3) empirical models of ionospheric conductances and polar cap potential drop. Research into magnetic-field-aligned electric fields is reviewed, particularly magnetic-mirror effects and double layers.

Wolf, R. A.↗

NASA NTRS1984

Electrodynamics of convection in the inner magnetosphere

During the past ten years, substantial progress has been made in the development of quantitative models of convection in the magnetosphere and of the electrodynamic processes that couple that magnetosphere and ionosphere. Using a computational scheme first proposed by Vasyliunas, the convection models under consideration separate the three-dimensional problem of convection in the inner magnetosphere/ionosphere into a pair of two-dimensional problems coupled by Birkeland currents flowing between the two regions. The logic, development, and major results of the inner magnetosphere convection model are reviewed with emphasis on ionospheric and magnetospheric currents. A major theoretical result of the models has been the clarification of the relationship between the region 1/region 2 picture of field-aligned currents and the older partial ring current/tail current interruption picture of substorm dynamics.

Spiro, R. W.↗

NASA NTRS1984

Region one Birkeland currents connecting to sunward convecting flux tubes

On the basis of Birkeland current observations, analytic stability arguments demonstrate that there may be a sector within the plasma sheet in the night side magnetosphere where plasma pressures are reduced relative to the surrounding regions. Physical arguments, in addition to this analysis, imply that a depleted region near the center of the magnetotail would be stable against the interchange instability, thereby generating currents of the same sense as the region-1 currents. The depleted region is presently simulated by means of computer experiments based on the Rice Convection Model. Results indicate that flux tube content gradients across the tailward boundary can cause region-1 currents to flow along sunward-convecting flux tubes, in general agreement with satellite data.

Karty, J. L.↗

NASA NTRS1983

Inferring electric fields and currents from ground magnetometer data - A test with theoretically derived inputs

Advanced techniques considered by Kamide et al. (1981) seem to have the potential for providing observation-based high time resolution pictures of the global ionospheric current and electric field patterns for interesting events. However, a reliance on the proposed magnetogram-inversion schemes for the deduction of global ionospheric current and electric field patterns requires proof that reliable results are obtained. 'Theoretical' tests of the accuracy of the magnetogram inversion schemes have, therefore, been considered. The present investigation is concerned with a test, involving the developed KRM algorithm and the Rice Convection Model (RCM). The test was successful in the sense that there was overall agreement between electric fields and currents calculated by the RCM and KRM schemes.

Wolf, R. A.↗

NASA NTRS1983

Study plasma interactions in the auroral ionosphere

Analyzed data from rocket flight, 29.007UE is presented. In a discrete electron arc the measured upward moving electrons are well accounted for by secondaries produced in collisional scattering of the measured downcoming electrons. No collective mechanisms need to invoke. The low energy downcoming electrons are accounted for by thermal plasma accelerated through a potential drop of a few kV that specularly reflects upward-moving lower energy electrons. No low altitude collective effects need to invoke in the arc. Simultaneous measurements of electric field by double probes on 29.007 and the Chatanika Radar allow one to infer that there are upward drifting ions above the discrete electron arc, and there is a westward neutral wind in the discrete arc. Two rocket payloads were built to investigate plasma effects in the pulsating aurora.

Anderson, H. R.↗

NASA NTRS1983

The quasi-static (slow-flow) region of the magnetosphere

In the present treatment of the theory of the earth's inner and middle magnetosphere, attention is given to the region earthward and equatorward of the magnetopause and its associated boundary layers in the case of the magnetosphere's dayside, while with regard to the night side, the part of the inner plasma sheet which lies on closed magnetic field lines that experience only those flows that are slow (by comparison to the fast mode speed) are considered. The theoretically determined boundaries of the region discussed are all magnetic field-aligned, since the theory treats each magnetic field line and the particles on it as a discrete entity due to the ease with which the particles and electric currents flow along the magnetic field.

Wolf, R. A.↗

NASA NTRS1983

The role of the auroral ionosphere in magnetospheric substorms

It is pointed out that the energy which is released during a magnetospheric substorm has large and obvious effects on the earth's auroral ionosphere. The present investigation is concerned with the question of the effect of the ionosphere on substorm phenomena which occur near the magnetospheric equatorial plane. Plasma transfer between ionosphere and magnetosphere are discussed along with general theoretical considerations regarding magnetospheric currents, plasma sheet dynamics, and ring current injection. Attention is given to questions concerning the occurrence of substorms, ion circulation in the magnetospheric convection cycle, time-dependent plasma ejection from plasma-sheet flux tubes, and a schematic of the magnetospheric-convection current circuit.

Wolf, R. A.↗

NASA NTRS1982

Overview of the IMS July 29, 1977, magnetic storm analysis

The physical characteristics and temporal development of a significant IMS magnetospheric event - the sudden commencement and multiple substorms of July 29, 1977 - are reviewed. It is pointed out that the ring current showed a maximum at 0600 UT and a major perturbation at 1230 UT, corresponding to the last substorm. The computerized coordinated data analysis workshop (CDAW 2) conducted in October 1979 is described. Attention is given to the solar wind conditions and magnetospheric response.

Manka, R. H.↗

NASA NTRS1982

Computer simulation of inner magnetospheric dynamics for the magnetic storm of July 29, 1977

The Rice University convection model is applied to the early main phase of the July 29, 1977 magnetic storm through a computer implementation that self-consistently calculates electric fields, currents, and plasma distributions and velocities in the inner magnetosphere/ionosphere system. On the basis of solar wind parameters and AL index as inputs, the model predicts the injection of plasma sheet plasma to form a substantial storm time ring current whose total predicted strength agrees with the observed Dst index. The possibility that the magnetic field may be sufficiently inflated to make 60 deg field lines extend to the outer magnetosphere is examined. In the model, distortion of the plasma sheet inner edge by magnetospheric compression associated with the sudden commencement temporarily disrupts the normal Birkeland current pattern. Normal Birkeland currents and shielding reassert themselves in about an hour.

Wolf, R. A.↗

NASA NTRS1982

Theoretical magnetograms based on quantitative simulation of a magnetospheric substorm

Substorm currents derived from the Rice University computer simulation of the September 19, 1976 substorm event are used to compute theoretical magnetograms as a function of universal time for various stations, integrating the Biot-Savart law over a maze of about 2700 wires and bands that carry the ring, Birkeland and horizontal ionospheric currents. A comparison of theoretical results with corresponding observations leads to a claim of general agreement, especially for stations at high and middle magnetic latitudes. Model results suggest that the ground magnetic field perturbations arise from complicated combinations of different kinds of currents, and that magnetic field disturbances due to different but related currents cancel each other out despite the inapplicability of Fukushima's (1973) theorem. It is also found that the dawn-dusk asymmetry in the horizontal magnetic field disturbance component at low latitudes is due to a net downward Birkeland current at noon, a net upward current at midnight, and, generally, antisunward-flowing electrojets.

Chen, C.-K.↗

NASA NTRS1982

Modeling of high-latitude currents in a substorm

The currents and electric fields in the area covered by the poleward set of field-aligned currents are computed by means of a quantitative model, in which the high-latitude band considered carries most of the poleward electrojet, and lies poleward of the region covering the inner magnetosphere and corresponding ionosphere. The Birkeland current and aurorally enhanced conductivity are assumed to be uniformly distributed across the band. The time-dependent conductivity model used is based on electron fluxes and mean energies measured from the S3-2 satellite. Joule heating of the upper atmosphere was found to be about 2 x 10 to the 11th W during the substorm period. Model values for the strength of the electrojet and the amount of Joule heating agree to within about 20% with results based on a simple Cowling conductivity band.

Karty, J. L.↗

NASA NTRS1981

Quantitative simulation of a magnetospheric substorm. I - Model logic and overview

Results of a comprehensive computer simulation of the behavior of the earth's inner magnetosphere during a substorm-type event are reported. It is pointed out that the computer model self-consistently computes electric fields, currents, and plasma distributions and velocities in the inner-magnetosphere/ionosphere system; parallel electric fields and ionospheric neutral winds, however, are not included. The basic equations of the model are derived, and the inputs are described. An overview of the results is also given. The first appendix contains derivations of general, useful laws of bounce-averaged gradient, curvature, and E x B drifts in a plasma with isotropic pitch angle distribution. The second appendix gives a description of the numerical method used in the simulation.

Harel, M.↗