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At least 919 records · Page 51
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NASA NTRS1992

Instrument requirements for imaging the magnetosphere in extreme-ultraviolet and energetic neutral atoms derived from computer-simulated images

Scientific requirements for He(+)(304 A) and energetic neutral atom (ENA) magnetospheric imaging, as well as the derived instrumental requirements are presented. Both ENA imaging of the hot plasma and EUV imaging of the cold plasma are highlighted. The question of the accuracy with which physically significant parameters can be extracted from actual images using computerized algorithms is addressed. An example of an ENA image analyzed by means of the Powell minimization algorithm is given. Automated unfolding of global magnetospheric images is also discussed. A Mercator projection of a model INA image is shown.

Roelof, Edmond C.↗
NASA NTRS1993

Reconstruction of low-dimensional magnetospheric dynamics by singular spectrum analysis

Singular spectrum analysis was used to study global magnetospheric dynamics using time series auroral electrojet (AE) data. The calculated correlation dimension of approximately 2.5 confirms the low dimensionality of earlier results. This technique indicated that global magnetospheric dynamics can be described by three variables whose dynamical features were obtained from the AE data.

Sharma, A. S.↗
NASA NTRS1993

Particle tracing in the magnetosphere - New algorithms and results

A fast and efficient method is employed to trace charged particles through realistic magnetospheric electric and magnetic fields, greatly reducing computer simulation times. The method is applicable for particles having arbitrary charge, energy, or pitch angle but which conserve the first two adiabatic invariants. An efficient method is used to classify drift orbits, which greatly facilitates identification of the last closed drift path or other drift boundaries. The time-independent evolution of the bounce-averaged phase space density along convective drift orbits is calculated. These three tools can be used to quantitatively describe convective evolution of the particle distribution from the tail, an essential step in understanding the production of unstable distributions in the magnetosphere.

Sheldon, R. B.↗
NASA NTRS1993

Mechanism for the acceleration and ejection of dust grains from Jupiter's magnetosphere

The Ulysses mission detected quasi-periodic streams of high-velocity submicron-sized dust particles during its encounter with Jupiter. It is shown here how the dust events could result from the acceleration and subsequent ejection of small grains by Jupiter's magnetosphere. Dust grains entering the plasma environment of the magnetosphere become charged, with the result that their motion is then determined by both electromagnetic and gravitational forces. This process is modeled, and it is found that only those particles in a certain size range gain sufficient energy to escape the Jovian system. Moreover, if Io is assumed to be the source of the dust grains, its location in geographic and geomagnetic coordinates determines the exit direction of the escaping particles, providing a possible explanation for the observed periodicities. The calculated mass and velocity range of the escaping dust gains are consistent with the Ulysses findings.

Horanyi, M.↗
NASA NTRS1993

Plasma dynamics driven by finite-width current filament and KV potential drops in ionosphere-magnetosphere coupling

We have simulated the plasma dynamics of a quasi-two-dimensional current filament in ionosphere-magnetosphere coupling. The simulation consists of a set of one-dimensional flux tube simulations with different imposed time-dependent, field-aligned currents. The dynamical interactions of the individual current filaments with the ionospheric and magnetospheric plasma generate kV field-aligned potential drops along the field lines, and the side-by-side display exhibits the evolution of the implied potential structure in the horizontal direction. The primary cross-field motion produced by the horizontal E-field (ExB drift) is perpendicular to both of the significant spatial directions, and is thus ignorable in this geometry. The effects of other cross-field drict processes are discussed. The simulation thus provides insight into the dynamical evolution of 2D potential structures driven by an imposed finite-width field-aligned current profile. Spatial and temporal variation of other plasma parameters (temperatures, density, etc.) that are determined primarily by parallel transport effects are also displayed.

Ganguli, Supriya B.↗
NASA NTRS1993

Mapping and energization in the magnetotail. I - Magnetospheric boundaries

The definition and mapping of the principal observed magnetospheric boundaries between the ionospheric and the equatorial plane are considered. All field tracing is done using the Tsyganenko (1989) or T89 magnetospheric model. Some of the model limitations are described. Particular attention is given to a search for signatures of the magnetotail subregions that may be observable by low- or middle-altitude satellites.

Kaufmann, Richard L.↗
NASA NTRS1993

Origin and maintenance of the oxygen torus in Saturn's magnetosphere

Observations of thermal ions in Saturn's inner magnetosphere suggest distributed local sources rather than diffusive mass loading from a source located further out. We suggest that the plasma is produced and maintained mainly by 'self-sputtering' of E ring dust. Sputtered particles are 'picked up' by the planetary magnetospheric field and accelerated to corotation energies (of the order of 8 eV/amu). The sputter yield for oxygen on ice at, for example, 120 eV is about 5, which implies that an avalanche of self-sputtering occurs. The plasma density is built up until it is balanced by local losses, presumably pitch angle scattering into the loss cone and absorption in the planet's ionosphere. The plasma density determines the distribution of dust in the E ring through plasma drag. Thus a feedback mechanism between the plasma and the E ring dust is established. The model accounts for the principal plasma observations and simultaneously the radial optical depth profile of the E ring.

Morfill, G. E.↗
NASA NTRS1992

The Harang discontinuity and magnetospheric forecasting

This paper describes the physics of the Harang discontinuity and its application to forecasting of the magnetospheric environment. Gradient/curvature depletion of energetic ions from earthward drifting flux tubes results in upward Birkeland currents. Ionospheric closure rotates the dawn-dusk convection electric field resulting in the Harang reversal. This physical mechanism has been incorporated into the Rice Convection Model (RCM) and can serve as a fundamental part of the forecasting of the magnetospheric system. The inclusion of this physics within the RCM results in a physical picture of the Harang discontinuity which did not previously exist. Model results are close to observations and represent a significant improvement in predicted electric field and Birkeland current patterns.

Erickson, G. M.↗
NASA NTRS1993

LEICA - A low energy ion composition analyzer for the study of solar and magnetospheric heavy ions

The SAMPEX LEICA instrument is designed to measure about 0.5-5 MeV/nucleon solar and magnetospheric ions over the range from He to Ni. The instrument is a time-of-flight mass spectrometer which measures particle time-of-flight over an about 0.5 m path, and the residual energy deposited in an array of Si solid state detectors. Large area microchannel plates are used, resulting in a large geometrical factor for the instrument (0.6 sq cm sr) which is essential for accurate compositional measurements in small solar flares, and in studies of precipitating magnetospheric heavy ions.

Mason, Glenn M.↗
NASA NTRS1994

Analytic model of aurorally coupled magnetospheric and ionospheric electrostatic potentials

This paper describes modest but significant improvements on earlier studies of electrostatic potential structure in the auroral region using the adiabatic auroral arc model. This model has crucial nonlinearities (connected, for example. with aurorally produced ionization) which have hampered analysis; earlier work has either been linear, which I will show is a poor approximation or, if nonlinear, either numerical or too specialized to study parametric dependencies. With certain simplifying assumptions I find new analytic nonlinear solutions fully exhibiting the parametric dependence of potentials on magnetospheric (e.g.. cross-tail potential) and ionospheric (e.g., recombination rate) parameters. No purely phenomenological parameters are introduced. The results are in reasonable agreement with observed average auroral potential drops, inverted-V scale sizes, and dissipation rates. The dissipation rate is quite comparable to tail energization and transport rates and should have a major effect on tail and magnetospheric dynamics. This paper gives various relations between the cross-tail potential and auroral parameters (e.g., total parallel currents and potential drops) which can be studied with existing data sets.

Cornwall, J. M.↗
NASA NTRS1993

Analytic model of aurorally coupled magnetospheric and ionospheric electrostatic potentials

This paper describes modest but significant improvements on earlier studies of electrostatic potential structure in the auroral region, using the adiabatic auroral arc model. With certain simplifying assumptions, new analytic nonlinear solutions fully exhibiting the parametric dependence of potentials on magnetospheric (e.g., cross-tail potential) and ionospheric (e.g., recombination rate) parameters are found. No purely phenomenological parameters are introduced. The results are in reasonable agreement with observed average auroral potential drops, inverted-V scale sizes, and dissipation rates. The dissipation rate is quite comparable to tail energization and transport rates and should have a major effect on tail and magnetospheric dynamics. Various relations between the cross-tail potential and auroral parameters (e.g., total parallel currents and potential drops) are given which can be studied with existing data sets.

Cornwall, John M.↗
NASA NTRS1993

On the cause of thin current sheets in the near-Earth magnetotail and their possible significance for magnetospheric substorms

The formation of thin current sheets in the near-Earth magnetotail during substorm growth phases is addressed in terms of a simple model. An appropriate part of the unperturbed magnetotail is represented by a plane sheet model. Perturbations are applied to the upper and to the left boundaries, representing the magnetopause and the near-Earth tail boundary, where the perturbation at the latter models the interaction between the tail and the inner magnetosphere. Treating the perturbation as ideal (dissipation-free), we found that singular current sheets develop in the midplane of the tail. The analytical results are explored numerically. Using realistic dimensions of the domain considered, the influence of the earthward boundary on current sheet formation dominates. It is argued that current sheet formation of this type plays an important role in the processes associated with the onset of magnetospheric substorms.

Schindler, K.↗
NASA NTRS1992

Inferences concerning the magnetospheric source region for auroral breakup

Inferences concerning the magnetospheric source region for auroral arcs obtained from particle measurements on polar orbiting satellites are presented and contrasted with other ideas. An argument that the magnetospheric source region for auroral arc breakup and substorm initiation is along Boundary Plasma Sheet (BPS) magnetic field lines is given. This source region lies beyond a distinct central plasma sheet region and sufficiently far from the Earth that energetic ion motion violates the guiding center approximation (is chaotic). The source region is not constrained to any particular range of distances from the Earth, and substorm initiation may be possible over a wide range of distances from near synchronous orbit to the distant tail. It is also argued that the layer of low energy electrons and velocity dispersed ion beams observed at low altitudes on Aureol 3 is not a different region from the region of auroral arcs. Both comprise the BPS. The two regions occasionally appear distinct at low altitudes because of the effects of arc field aligned potential drops on precipitating particles.

Lyons, L. R.↗
NASA NTRS1994

Particle acceleration and transport in the tail and at the front side of the magnetosphere, task 1 and 2

The work under this grant involved studies of: (1) the acceleration and heating of ions in the course of magnetospheric substorms and the spatial distributions of the ion populations in the magnetotail; and (2) the comparison in in-situ acceleration at the bow shock and the leakage of energetic particles from the magnetosphere as source of energetic ions upstream of the Earth's bow shock.

Kistler, Lynn M.↗
NASA NTRS1995

Modeling of the coupled magnetospheric and neutral wind dynamos

Work at SRI involved modeling the exchange of electromagnetic energy between the ionosphere and magnetosphere to help interpret the DE-B Poynting flux observations. To describe the electrical properties of the high-latitude ionosphere, we constructed a numerical model, from the framework provided by the Vector Spherical Harmonic (VSH) model, that determines the ionospheric currents, conductivities, and electric fields including both magnetospheric inputs and neutral wind dynamo effects. This model development grew from the earlier question of whether an electrical energy source in the ionosphere was capable of providing an upward Poynting flux. The model solves the steady-state neutral wind dynamo equations and the Poynting flux equation to provide insight into the electrodynamic role of the neutral winds. The modeling effort to determine the high-latitude energy flux has been able to reproduce many of the large-scale features observed in the Poynting flux measurements made by DE-2. Because the Poynting flux measurement is an integrated result of energy flux into or out of the ionosphere, we investigated the ionospheric properties that may contribute to the observed flux of energy measured by the spacecraft. During steady state the electromagnetic energy flux, or DC Poynting flux, is equal to the Joule heating rate and the mechanical energy transfer rate in the high-latitude ionosphere. Although the Joule heating rate acts as an energy sink, transforming electromagnetic energy into thermal or internal energy of the gas, the mechanical energy transfer rate may be either a sink or source of electromagnetic energy. In the steady state, it is only the mechanical energy transfer rate that can generate electromagnetic energy and result in a DC Poynating flux that is directed out of the ionosphere.

Thayer, J. P.↗
NASA NTRS1994

Radio sounding of the magnetosphere from a lunar-based VLF array

Using a lunar-based active radio transmitter and receiver system operating in the 'free space' wave modes, we can obtain much information on the structures and dynamics of remote magnetospheric plasma regions in a way similar to ionosondes. Powerful, narrow-band electromagnetic pulses can be transmitted over a wide frequency range (from 10 kHz to 1 MHz). The signals would be refracted and reflected off magnetospheric structures such as the plasmapause, plasmasheet, magnetopause, and the high and low latitude boundary layers. With a series of long dipole antennas, ranging in size from 400 m to 20 km with an output voltage ranging from 6 kV to less than 0.2 kV, a target plasma region at up to 100 R(sub E) can be explored. We illustrate this remote sensing technique by using the plasmasphere as a remote target, and modeling the propagations of the sounder transmitted and received pulses by ray tracing calculations.

Green, James L.↗
NASA NTRS1994

Null fields in the outer Jovian magnetosphere: Ulysses observations

This paper reports on a magnetic field phenomenon, hereafter referred to as null fields, which were discovered during the inbound pass of the recent flyby of Jupiter by the Ulysses spacecraft. These null fields which were observed in the outer dayside magnetosphere are characterised by brief but sharp decreases of the field magnitude to values less than 1 nT. The nulls are distinguished from the current sheet signatures characteristic of the middle magnetosphere by the fact that the field does not reverse across the event. A field configuration is suggested that accounts for the observed features of the events.

Haynes, P. L.↗
NASA NTRS1994

Magnetospheric accretion models for T Tauri stars. 1: Balmer line profiles without rotation

We argue that the strong emission lines of T Tauri stars are generally produced in infalling envelopes. Simple models of infall constrained to a dipolar magnetic field geometry explain many peculiarities of observed line profiles that are difficult, if not impossible, to reproduce with wind models. Radiative transfer effects explain why certain lines can appear quite symmetric while other lines simultaneously exhibit inverse P Cygni profiles, without recourse to complicated velocity fields. The success of the infall models in accounting for qualitative features of observed line profiles supports the proposal that stellar magnetospheres disrupt disk accretion in T Tauri stars, that true boundary layers are not usually present in T Tauri stars, and that the observed 'blue veiling' emission arises from the base of the magnetospheric accretion column.

Hartmann, Lee↗