Engineering topics

Toffoletto, F. R.

Publications and source records attributed to Toffoletto, F. R..

10 records

NASA NTRS1996

Self-consistent modeling of inner magnetospheric convection

The initial results of a model of inner magnetospheric convection are presented. The model employs the Rice convection model with a magnetic field computed with the constraint of magnetostatic equilibrium. The approach computes equilibria from a magnetofriction code which is a modified version of the Hesse-Birn equilibrium code adopted for use in the inner magnetosphere. The code uses the pressure distribution computed from the Rice convection model to update the magnetic field. The algorithm used to compute the inner magnetospheric equilibria is outlined, and the coupling of the equilibrium code with the convection model is described.

Toffoletto, F. R.↗

NASA NTRS1994

Magnetic mapping and Birkeland currents in the Toffoletto-Hill and Tsyganenko magnetosphere models

We investigate the mapping of magnetic flux tubes between the high-latitude ionosphere and the equatorial magnetosphere, using the Toffoletto and Hill (1989) theoretical model and the Tsyganenko (1987, 1989) empirical models. In all models, strong distortion of flux tubes occurs near the magnetopause and in the distant tail. These effects are illustrated for various interplanetary magnetic field directions in the Toffoletto and Hill (1989) model and for various Kp values in the Tsyganenko (1987, 1989) model. We calculate the Birkeland current that is actually present in each model (from Ampere's law), as well as the Birkeland current that would be implied if the model were in magnetostatic equilibrium with isotropic plasma pressure (from the Vasyliunas equation). Comparison of the actual and implied Birkeland currents indicates that no physically significant Birkeland current is included in any of the models tested. Each model contains a 'noise' component of Birkeland current that is, however, much smaller in magnitude than the 'real' Birkeland currents that remain to be included.

Ding, Cheng↗

NASA NTRS1994

Solution of the Chapman-Ferraro problem with an arbitrary magnetopause

We present a global model of the magnetic field of the magnetosphere that includes the effects of the Chapman-Ferraro currents at the magnetopause. In contrast to ealier models, the magnetopause shape is arbitrary, thus allowing the use of more realistic geometries. The internal magnetospheric field model of Hilmer and Voigt (1993), is completely shielded within the magnetopause by solving the Laplace equation with Neumann boundary conditions using a finite difference method on a non-orthogonal, curvilinear grid. The resulting model magnetosphere is perfectly closed although the method can also be applied with more general boundary conditions, to generate a set of open models based on the approach of Toffoletto and Hill (1989, 1993). The purpose of this paper is to demonstrate the feasibility of a purely numerical approach to solving the Chapman-Ferraro problem with arbitrary magnetopause shape and boundary conditions.

Toffoletto, F. R.↗

NASA NTRS1993

A nonsingular model of the open magnetosphere

We present a modified version of the Toffoletto and Hill (1989) open magnetosphere model that incorporates a tail-like interconection field with a discontinuity 10 represent the slow-mode expansion fan that defines the high-latitude tail magnetopause. (The interconnection field is defined as the perturbation on an initially closed magnetosphere model to make it open.) The expansion fan controls the open field line region in the tail, and the intersection of the fan with the tail current sheet is, by design, the x line. The new interconnection field allows greater control of the tail field structure; in particular, it enables us to eliminate the nightside mapping singularity that occurs in previous models when the interplanetary magnetic field is nonsouthward. Also, in contrast to earlier models, the far tail x line extends farther downstream on the flanks than in the center of the tail, consistent with observations.

Toffoletto, F. R.↗

NASA NTRS1991

Opening the cusp

This paper discusses the magnetic field topology (determined by the superposition of dipole, image, and uniform fields) for mapping the cusp to the ionosphere. The model results are compared to both new and published observations and are then used to map the footprint of a flux transfer event caused by a time variation in the merging rate. It is shown that the cusp geometry distorts the field lines mapped from the magnetopause to yield footprints with dawn and dusk protrusions into the region of closed magnetic flux.

Crooker, N. U.↗

NASA NTRS1990

A tangent subsolar merging line

A global magnetospheric model is described with a single subsolar merging line whose position is determined neither locally by the relative orientations and strengths of the merging fields nor globally by the orientation of a separator line (the governing parameters of most previous models) but by the condition of tangential contact between the external field and the magnetopause. Attention is given to the case of finite leakage and partial penetration, which distort the IMF into a draped pattern, thus changing the locus of tangency to the sphere. In the special case where the penetrating IMF flux is balanced by an equal amount of earth flux leakage, the tangent merging line bisects the angle between the IMF and earth's northward subsolar field. This result is identical to the local merging line model result for merging fields with equal magnitude. Here, a global flux balance condition replaces the local equal magnitude condition.

Crooker, N. U.↗

NASA NTRS1990

Magnetic field configuration of the theta aurora

A magnetic configuration of the open magnetosphere is described which is conducive to the formation of the theta aurora when the IMF has a significant northward component. A magnetic field topology and polar cap configuration, derived from a quantitative model of the open magnetosphere that incorporates Crooker's antiparallel merging hypothesis, are presented. Under this hypothesis, when the IMF has a northward component, the dayside merging line bifurcates, leaving a large fraction of the subsolar magnetopause untouched by the merging process. The polar cap, defined by tracing magnetic field lines that connect from the solar wind to the earth, is similarly bifurcated, leaving a sun-aligned stagnation region that is not magnetically connected to the solar wind and may plausibly be associated with the sun-aligned 'bar' of the theta aurora. The model provides testable predictions with regard to the position of this 'convection gap' in both Northern and Southern Hemispheres as functions of IMF direction.

Toffoletto, F. R.↗

NASA NTRS1990

A model of FTE footprints in the polar cap

The present investigation of the mapping of flux-transfer events (FTEs) onto the polar cap using the Toffoletto and Hill (1989) open version of Voigt's (1981) closed magnetic field model assumes that the magnetic flux associated with FTEs crosses the magnetopause through small regions of large normal components, whence these small flux tubes proceed to map to the polar cap ionosphere. It is found that while the footprint of a circular hole in the magnetopause of an otherwise closed magnetosphere becomes progressively distorted as the hole moves from the day to the night side, a similar region of enhanced open flux in an otherwise open magnetosphere retains an approximately circular footprint. Such regions, moving through the magnetosphere at a slower rate than the background, would generate the dipolar-electric field and current structures predicted by Southwood (1987).

Toffoletto, F. R.↗

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 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.↗