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NASA NTRS1981

Surface waves on Saturn's magnetopause

Voyager 1 magnetometer data have shown that small-amplitude surface waves occurred on Saturn's dayside magnetopause, causing multiple inbound crossings of this boundary. These waves were travelling approximately parallel to Saturn's equatorial plane along the magnetopause ('tailward'), suggesting that they were driven by the rotation of Saturn's magnetosphere. Hydromagnetic waves (possibly slow mode) were observed in the adjacent magnetosheath.

Lepping, R. P.↗

NASA NTRS1982

Gravity and geoid anomalies of the Philippine Sea: Evidence on the depth of compensation for the negative residual water depth anomaly

A negative free-air gravity anomaly which occurs in the central part of the Philippine Sea was examined to determine the distribution and nature of possible regional mass excesses or deficiencies. Geoid anomalies from GEOS-3 observation were positive. A negative residual geoid anomaly consistent with the area of negative free-air gravity anomalies were found. Theoretical gravity-topography and geoid-topography admittance functions indicated that high density mantle at about 60 km dept could account for the magnitudes of the gravity and residual geoid anomaly and the 1 km residual water depth anomaly in the Philippine Sea. The negative residual depth anomaly may be compensated for by excess density in the uppermost mantle, but the residual geoid and regional free-air gravity anomalies and a slow surface wave velocity structure might result from low-density warm upper mantle material lying beneath the zone of high-density uppermost mantle. From a horizontal disk approximation, the depth of the low-density warm mantle was estimated to be on the order of 200 km.

Bowin, C.↗

NASA NTRS1982

Upper mantle heterogeneity: Comparisons of regions south of Australia with Philippine Basin

The nature of mass anomalies that occur beneath the regions of negative residual depth anomalies were identified. Residual geoid anomalies with negative residual depth anomalies are identified in the Philippine Basin (negative) and in the region south of Australia (positive and negative). In the latter region the geoid anomalies are eastward and the depth anomaly is northeast. It is suggested that the negative depth anomaly and the compensating mass excess in the uppermost mantle developed in the Eocene as the lithosphere of the west Philippine basin formed. Heating of the deeper upper mantle which causes slow surface wave velocities and negative gravity and geoid anomalies may be a younger phenomenon which is still in progress.

Source record↗

NASA NTRS1987

Structure of a slow mode shock observed in the plasma sheet boundary layer

A detailed study of the plasma parameters and magnetic field measured using instrumentation aboard ISEE 2 during the recovery phase of a substorm on Apr. 24, 1979, is presented. The purpose of this study was to determine the nature and structure of the lobe-plasma sheet boundary layer. The analysis shows that the lobeward edge of the boundary is consistent with a freestanding slow mode shock. This shock is followed by a slow compression wave containing a trailing ion cyclotron wave which also stands in the flow. Measurement of the cross-tail current in the shock and the current system within the wave gives a shock thickness and wavelength comparable to but larger than both the upstream ion inertial length and the downstream proton gyroradius. Equality of the shock thickness with the standing ion cyclotron wavelength suggests that the steepness of the front is limited by dispersion rather than current-driven anomalous resistivity.

Feldman, W. C.↗

NASA NTRS1989

Beam instability of the z mode in the solar wind

The instability of the slow extraordinary wave (z mode) caused by a tenuous population of energetic electrons, n(e), in the presence of a weak magnetic field and a cold plasma background, n(0), is investigated. The growth rate of the z mode was calculated within linear Vlasov theory and assuming that, for a weak magnetic field, the growth rate coincides with that of the electrostatic Langmuir wave, i.e., when the magnetic field is neglected for the cold background but retained for the energetic electrons. The analytical expression for the case of a thermal beam is deduced and numerically evaluated for some typical solar wind plasma conditions. The effects of the beam density, direction of propagation, and magnetic field on the observable polarization is discussed.

Krauss-Varban, D.↗

NASA NTRS1990

Heliocentric distance and temporal dependence of the interplanetary density-magnetic field magnitude correlation

The Helios, IMP 8, ISEE 3, ad Voyager 2 spacecraft are used to examine the solar cycle and heliocentric distance dependence of the correlation between density n and magnetic field magnitude B in the solar wind. Previous work had suggested that this correlation becomes progressively more negative with heliocentric distance out to 9.5 AU. Here it is shown that this evolution is not a solar cycle effect, and that the correlations become even more strongly negative at heliocentric distance larger than 9.5 AU. There is considerable variability in the distributions of the correlations at a given heliocentric distance, but this is not simply related to the solar cycle. Examination of the evolution of correlations between density and speed suggest that most of the structures responsible for evolution in the anticorrelation between n and B are not slow-mode waves, but rather pressure balance structures. The latter consist of both coherent structures such as tangential discontinuities and the more generally pervasive 'pseudosound' which may include the coherent structures as a subset.

Roberts, D. A.↗

NASA NTRS1991

On rotational discontinuities in both two-fluid and hybrid models

Rotational discontinuities are studied in a two-fluid model that includes finite ion inertia dispersion and in a hybrid model in which the full ion dynamics is retained while the electrons are treated as a massless fluid. It is shown that as in previous dissipative MHD studies, a rotational discontinuity is unstable in both models and evolves to a 2-3 intermediate shock, a slow rarefaction wave, and other waves. In addition, it is shown that the so-called Walen relation, which holds exactly for rotational discontinuities, can also be well satisfied by tran-Alfvenic intermediate shocks. Thus intermediate shocks can be candidates for those observed structures that satisfy the Walen relation.

Wu, C. C.↗

NASA NTRS1992

The role of intermediate shocks in magnetic reconnection

The present study examines the structure of discontinuity layers associated with magnetic reconnection by numerically solving the Riemann problem for the evolution of an initial current sheet which separates two plasma regions with antiparallel magnetic field components in the z direction and a common guide magnetic field in the y direction. In the presence of a nonzero normal component of the magnetic field, the initial current sheet evolves into a system of MHD discontinuities. For the initial current sheet with a zero guide field, steady intermediate shocks, slow shocks, slow expansion waves, or contact discontinuity are observed to develop. For the current sheet with a nonzero guide field, time-dependent intermediate shocks, instead of steady intermediate shocks, are observed to bound the reconnection layer.

Lin, Y.↗

NASA NTRS1990

The magnetopause

The magnetopause is the interface between the shocked solar wind in the magnetosheath and the geomagnetic field and plasma in the magnetosphere. This interface is far from simple because both sides of the interface contain magnetized plasma. As a result, there are boundary layers on both sides of the interface so the resulting structure is many-ion-gyroradii thick. There is also substructure which may be much less than an ion gyroradius in thickness. The structure of the magnetopause is also sensitive to the Mach number and beta of the plasma. When the beta is very high, the magnetopause resembles a slow mode wave. When the IMF is southward and the Mach number and/or beta is low the plasma is accelerated much as Dungey predicted. However, at other times reconnection seems to be less steady and perhaps patchy. Ropelike structures are seen which may be connected to the magnetosphere. These structures, which have been called FTEs, are still not fully understood.

Russell, C. T.↗

NASA NTRS1993

Stability of an inhomogeneous two-component plasma with magnetic viscosity

The stability of a semiinfinite quasi-neutral inhomogeneous plasma with magnetic viscosity has been discussed by using JWKB approximation in which the parameters are regarded slowing varying. Dispersion relation is obtained and discussed. It is found that the inhomogeneous system is unstable for all perturbations with k(y) = 0. A dispersion relation for homogeneous plasma is also obtained and discussed. It is shown that fast and slow-MHD waves propagate in the homogeneous plasma in the limit of almost perpendicular propagation under certain conditions.

Kumar, Nagendra↗

NASA NTRS1992

The stability of the near-Earth magnetotail against the ballooning instability

In an extension to a previous study, conditions for the ballooning instability in the magnetohydrodynamic approximation are derived. It is necessary for exciting the ballooning instability that the plasma pressure is higher where plasma is convected outward. However, the expansion of plasma, which results from the divergence of outward flows in the curved geometry of field lines, may overcompensate for the convective change in the plasma pressure. Conditions for the instability are derived from the equation describing the linear coupling between the Alfven and slow magnetosonic waves. The result indicates that excessive curvature stabilizes, rather than destabilizes, perturbations, as well as the field aligned current and the field aligned flow. That is the field line curvature should be smaller than the pressure gradient for exciting the ballooning instability. It is inferred that this condition is not satisfied during quiet times in the near Earth tail. The plasma sheet becomes more stable to the instability in accordance with the thinning of the plasma sheets during the growth phase of substorms, while in the region nearer to the Earth (the inner edge of the plasma sheet or the ring current region) the plasma pressure gradient, which is possibly localized during the growth phase, may drive the instability.

Ohtani, Shin-Ichi↗

NASA NTRS1995

Time-dependent simulation of oblique MHD cosmic-ray shocks using the two-fluid model

Using a new, second-order accurate numerical method we present dynamical simulations of oblique MHD cosmic-ray (CR)-modified plane shock evolution. Most of the calculations are done with a two-fluid model for diffusive shock acceleration, but we provide also comparisons between a typical shock computed that way against calculations carried out using the more complete, momentum-dependent, diffusion-advection equation. We also illustrate a test showing that these simulations evolve to dynamical equilibria consistent with previously published steady state analytic calculations for such shocks. In order to improve understanding of the dynamical role of magnetic fields in shocks modified by CR pressure we have explored for time asymptotic states the parameter space of upstream fast mode Mach number, M(sub f), and plasma beta. We compile the results into maps of dynamical steady state CR acceleration efficiency, epsilon(sub c). We have run simulations using constant, and nonisotropic, obliquity (and hence spatially) dependent forms of the diffusion coefficient kappa. Comparison of the results shows that while the final steady states achieved are the same in each case, the history of CR-MHD shocks can be strongly modified by variations in kappa and, therefore, in the acceleration timescale. Also, the coupling of CR and MHD in low beta, oblique shocks substantially influences the transient density spike that forms in strongly CR-modified shocks. We find that inside the density spike a MHD slow mode wave can be generated that eventually steepens into a shock. A strong layer develops within the density spike, driven by MHD stresses. We conjecture that currents in the shear layer could, in nonplanar flows, results in enhanced particle accretion through drift acceleration.

Frank, Adam↗

NASA NTRS1995

Numerical magetohydrodynamics in astronphysics: Algorithm and tests for one-dimensional flow`

We describe a numerical code to solve the equations for ideal magnetohydrodynamics (MHD). It is based on an explicit finite difference scheme on an Eulerian grid, called the total variation diminishing (TVD) scheme, which is a second-order-accurate extension of the Roe-type upwind scheme. We also describe a non-linear Riemann solver for ideal MHD, which includes rarefractions as well as shocks. The numerical code and the Riemann solver have been used to test each other. Extensive tests encompassing all the possible ideal MHD structures with planar symmetries (i.e., one-dimensional flows) are presented. These include those for which the field structure is two dimensional (i.e., those flows often called '1 + 1/2 dimensional') as well as those for which the magnetic field plane rotates (i.e., those flows often called '1 + 1/2 + 1/2 dimensional'). Results indicate that the code can resolve strong fast, slow, and magnetosonic shocks within two to four cells, but more cells are required if shocks become weak. With proper steepening, we could resolve rotational discontinuities within three to five cells. However, without successful implementation of steepening, contact discontinuities are resolved with approximately 10 cells and tangential discountinuities are resolved with approximately 15 cells. Out tests confirm that slow compound structures with tow-dimensional magnetic fields are composed of intermediate shocks (so-called 2-4 intermediate shocks) followed by slow rarefaction waves. Finally, tests demostrate that in two-dimensional magnetohydrodynamics, fast compound structures, which are composed of intermediate shocks (so-called 1-3 intermediate shocks) preceeded by fast rarefaction waves, are also possible.

Ryu, Dongsu↗

NASA NTRS1994

Anisotropic Alfven-ballooning modes in Earth's magnetosphere

We have carried out a theoretical analysis of the stability and parallel structure of coupled shear Alfven and slow magnetosonic waves in Earth's inner magnetopause (i.e., at equatorial distances between about five and ten Earth radii) including effects of finite anisotropic Grad-Shafranov equation yields an approximate self-consistent magnetohydrodynamic (MHD) equilibrium. This MHD equilibrium is used in the numerical solution of a set of eigenmode equations which describe the field line eigenfrequency, linear stability, and parallel eigenmode structure. We call these modes anisotropic Alfven-ballooning modes. The main results are: (1) The field line eigenfrequency can be significantly lowered by finite pressure effects. (2) The parallel mode structure of the transverse wave components is fairly insensitive to changes in the plasma pressure, but the compressional magnetic component can become highly peaked near the magnetic equator as a result of increased pressure, especially when P(sub perpendicular to) is greater than P(sub parallel) (here P(sub perpendicular to) and P(sub parallel) are the perpendicular and parallel plasma pressure). (3) For the isotropic (P(sub parallel) = P(sub perpendicular to) = P) case ballooning instability can occur when the ratio of the plasma presure to the magnetic pressure, beta = P/(B squared/8 pi), exceeds a critical value beta(sup B)(sub 0) is approximately equal to 3.5 at the equator. (4) Compared to the isotropic case the critical beta value is lowered by anisotropy, either due to decreased field line bending stabilization when P(sub parallel) is greater than P(sub perpendicular to) or due to increased ballooning-mirror destabilization when P(sub perpendicular to) is greater than P(sub parallel). (5) We use a beta-delta stability diagram to display the regions of instability with respect to the equatorial values of the parameters bar beta and delta, where bar beta = (1/3)(beta(sub parallel) + 2 beta(sub perpendicular to)) is an average beta value and delta = 1 - P(sub parallel)/P(sub perpendicular to) is a measure of the plasma anisotropy. The diagram is divided into regions corresponding to the firehose, mirror and ballooning instabilities. It appears that observed values of the plasma pressure are below the critical value for the isotropic ballooning instability but it may be possible to approach a ballooning-mirror instability when P(sub perpendicular to)/P(sub parallel) is greater than or approximately 2.

Chan, Anthony A.↗

NASA NTRS1993

Does the ballooning instability trigger substorms in the near-Earth magnetotail?

The stability of the near-Earth magnetotail against ballooning (or configurational) instability is examined in the framework of the MHD approximation. It is emphasized that a change in plasma pressure induced by a meriodional electric field drift delta u(sub n) is an important factor that determines the stability. We have to consider two ways in which plasma pressure changes, that is, a convective change -delta u(sub n) grad(P(sub 0)), where P(sub 0) is background plasma pressure, and plasma expansion/compression -P(sub 0) dive (delta u(sub n)). Since delta u(sub n) is perpendicular to the magnetic field and its magnitude is inversely proportional to the magnetic field strength, delta u(sub n) diverges/converges in usual tail magnetic field configurations. For the instability, the convective change must overwhelm the effects of the plasma expansion/compression. However, near the equator in the near-Earth tail, the latter may overcompensate for the former. We describe the ballooning instability in terms of a coupling between the Alfven and slow magnetosonic waves in an inhomogeneous plasma and derive instability conditions. The result shows that the excessive curvature stabilizes, rather than destabilizes, perturbations. It is also found that the field-aligned flow stabilizes perturbations, as well as the field-aligned current. We infer that under quiet conditions, the plasma pressure gradient in the near-Earth tail is not sharp enough to trigger the instability. The plasma sheet is expected to become more stable during the substorm growth phase because of an increase in the field line curvature associated with the plasma sheet thinning. In the region closer to the Earth, including the ring current, the plasma pressure gradient may be localized in a limited range of the radial distance during the growth phase. However, recently reported plasma and magnetic field parameters before substorm onsets do not provide very convincing evidence that the ballooning instability is the triggering mechanism of substorms.

Ohtani, Shin-Ichi↗

NASA NTRS2010

Concerning the Perturbations Generated by Flux Transfer Events

The interaction of the solar wind with the Earth's magnetosphere is often highly unsteady. Bursts of magnetic reconnection at multiple locations on the dayside equatorial magnetopause generate flux transfer events or FTEs: twisted ropes of interconnected magnetosheath and magnetospheric magnetic field lines. Once formed, the events move antisunward, displacing and perturbing the ambient media. This talk explores the perturbations predicted by both global numerical simulations and analytical models. Results from global hybrid code simulation confirm the predictions of analytical models indicating that the events generate standing forward slow mode waves as their speeds relative to the magnetosheath flow approach the Alfven velocity. Geometric considerations lead to the conclusion that events generated by component reconnection on the dayside magnetopause move poleward and exhibit strong signatures during intervals of southward IMF orientation, but move towards the flanks and exhibit weak signatures during intervals of northward IMF orientation. Changing event orientations and magnetosheath/magnetospheric magnetic field orientation can enhance the amplitudes of events reaching the flanks. Although the orientations of events on the flanks inferred from multispacecraft timing techniques are consistent with the predictions of the component reconnection model, occurrence patterns versus latitude and IMF orientation require an explanation in terms of both the component and antiparallel reconnection models.

Sibeck, David G.↗

NASA NTRS2014

Size and Shape of the Distant Magnetotail

We employ a global magnetohydrodynamic model to study the effects of the interplanetary magnetic field (IMF) strength and direction upon the cross-section of the magnetotail at lunar distances. The anisotropic pressure of draped magnetosheath magnetic field lines and the inclusion of a reconnection-generated standing slow mode wave fan bounded by a rotational discontinuity within the definition of the magnetotail result in cross-sections elongated in the direction parallel to the component of the IMF in the plane perpendicular to the Sun-Earth line. Tilted cross-tail plasma sheets separate the northern and southern lobes within these cross-sections. Greater fast mode speeds perpendicular than parallel to the draped magnetos heath magnetic field lines result in greater distances to the bow shock in the direction perpendicular than parallel to the component of the IMF in the plane transverse to the Sun-Earth line. The magnetotail cross-section responds rapidly to reconnected magnetic field lines requires no more than the magnetosheath convection time to appear at any distance downstream, and further adjustments of the cross-section in response to the anisotropic pressures of the draped magnetic field lines require no more than 10-20 minutes. Consequently for typical ecliptic IMF orientations and strengths, the magnetotail cross-section is oblate while the bow shock is prolate.

lunar↗

NASA NTRS2017

Unravelling the Components of a Multi-Thermal Coronal Loop Using Magnetohydrodynamic Seismology

Coronal loops, constituting the basic building blocks of the active Sun, serve as primary targets to help understand the mechanisms responsible for maintaining multi-million Kelvin temperatures in the solar and stellar coronae. Despite significant advances in observations and theory, our knowledge on the fundamental properties of these structures is limited. Here, we present unprecedented observations of accelerating slow magnetoacoustic waves along a coronal loop that show differential propagation speeds in two distinct temperature channels, revealing the multi-stranded and multithermal nature of the loop. Utilizing the observed speeds and employing nonlinear force free magnetic field extrapolations, we derive the actual temperature variation along the loop in both channels, and thus are able to resolve two individual components of the multithermal loop for the first time. The obtained positive temperature gradients indicate uniform heating along the loop, rather than isolated foot point heating.

Prasad, S. Krishna↗