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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Observational test of hot flow anomaly formation by the interaction of a magnetic discontinuity with the bow shock

The formation of a hot flow anomaly (HFA) observed near the Earth's bow shock appears to be due to the interaction between the bow shock and an impinging discontinuity in the upstream plasma. Recent single-particle and 2D hybrid numerical studies have suggested that such an interaction will produce an HFA only if the motional electric field in the ambient plasma points toward the discontinuity, thereby focusing shock-reflected ions into it. We perform a test of this electric field orientation for a set of nine HFA events observed by the ISEE spacecraft and described previously in the literature. Under the assumption that the discontinuities were tangential, the predicted electric field orientation is found on at least one side of all nine observed HFAs (on the trailing edge of seven and the leading edge of five, and on both sides of three events). Further, there is evidence that asymmetries in the observed magnetic field signatures are related to the orientation of the motional electric field. The events in which the electric field points toward the discontinuity on both sides tend to be those with fairly symmetric flanking magnetic field enhancements.

Thomsen, M. F.↗
NASA NTRS1991

Shuffling foot points and magnetohydrodynamic discontinuities in the solar wind

ISEE three-field and plasma data are used to investigate the frequency of occurrence of isolated, large-amplitude rotational (RD) and tangential (TD) discontinuities in different types of solar wind flow. It is found that there are relatively more TDs in solar wind that originates in closed field regions and is ejected into interplanetary space by coronal transients than in the solar wind that originates in open field regions. The speed of the wind from open field regions is approximately linearly related to the number of RDs per hour; such a relation does not exist for the wind associated with coronal mass ejections. These results are consistent with the hypothesis that the convection-driven shuffling of magnetic foot points at the solar surface leads to TDs, magnetic reconnection, and heating of the corona on closed field lines, while in the open field regions the disturbances created by the shuffling are carried off by waves which contribute to the acceleration of the solar wind.

Neugebauer, M.↗
NASA NTRS1993

Structure of the dayside reconnection layer in resistive MHD and hybrid models

Numerical simulations were performed to investigate the structure of the reconnection layer at the dayside magnetopause. Two typical cases are examined in detail; both are asymmetric in magnetic field and plasma density. In case 1, the guide fields in the magnetosheath and in the magnetosphere are set at zero and thus the tangential magnetic fields on the two sides of the initial current sheet are exactly antiparallel. In case 2, the angle between the tangential magnetic fields on the two sides of the initial current sheet is 145 deg. The results obtained from a resistive MHD model and from a hybrid model are found to be different. In the MHD simulation of case 1, a 2-4 intermediate shock is found to bound the reconnection layer on the magnetosheath side, while an Alfven wave pulse bounds the reconnection layer on the magnetospheric side. In case 2, it is found that a time-dependent intermediate shock (TDIS) bounds the reconnection layer on the magnetosheath side, with a slow expansion wave propagating behind. With the MHD simulations, in the general case in which the tangential magnetic fields on the two sides of the initial current sheet are not exactly antiparallel, a rotational discontinuity across which the tangential magnetic field rotates, a large angle is found to bound the reconnection layer on the magnetosheath side.

Lin, Y.↗
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 NTRS1994

Landau damping of auroral hiss

Auroral hiss is observed to propagate over distances comparable to an Earth radius from its source in the auroral oval. The role of Landau damping is investigated for upward propagating auroral hiss. By using a ray tracing code and a simplified model of the distribution function, the effect of Landau damping is calculated for auroral hiss propagation through the environment around the auroral oval. Landau damping is found to be the likely mechanism for explaining some of the one-sided auroral hiss funnels observed by Dynamics Explorer 1. It is also found that Landau damping puts a lower limit on the wavelength of auroral hiss. Poleward of the auroral oval, Landau damping is found in a typical case to limit omega/k(sub parallel) to values of 3.4 x 10(exp 4) km/s or greater, corresponding to resonance energies of 3.2 keV or greater and wavelengths of 2 km or greater. For equatorward propagation, omega/k(sub parallel) is limited to values greater than 6.8 x 10(exp 4) km/s, corresponding to resonance energies greater than 13 keV and wavelengths greater than 3 km. Independent estimates based on measured ratios of the magnetic to electric field intensity also show that omega/k(sub parallel) corresponds to resonance energies greater than 1 keV and wavelengths greater than 1 km. These results lead to the difficulty that upgoing electron beams sufficiently energetic to directly generate auroral hiss of the inferred wavelength are not usually observed. A partial transmission mechanism utilizing density discontinuities oblique to the magnetic field is proposed for converting auroral hiss to wavelengths long enough to avoid damping of the wave over long distances. Numerous reflections of the wave in an upwardly flared density cavity could convert waves to significantly increased wavelengths and resonance velocities.

Morgan, D. D.↗
NASA NTRS2000

Visualization, Extraction and Quantification of Discontinuities in Compressible Flows

Scientific visualizations of two-dimensional compressible flow of a gas with discontinuities are presented. The numerical analogue to experimental techniques such as schlieren imaging, shadowgraphs, and interferograms are discussed. Edge detection techniques are utilized to identify the discontinuities. In particular, the zero crossing of the Laplacian of a field (usually density) is recommended for extracting the discontinuities. An algorithm to extract and quantify the discontinuities is presented. To illustrate the methods developed in the report, the example chosen is that of an unsteady interaction of a shock wave with a contact discontinuity.

Samtaney, Ravi↗
NASA NTRS2017

Evaluation of the Ozone Fields in NASA's MERRA-2 Reanalysis

The assimilated ozone product from the Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2), produced at NASAs Global Modeling and Assimilation Office (GMAO) is summarized. The reanalysis begins in 1980 with the use of retrieved partial-column ozone concentrations from a series of Solar Backscatter Ultraviolet Radiometer (SBUV) instruments on NASA and NOAA spacecraft. Beginning in October 2004, retrieved ozone profiles from the Microwave Limb Sounder (MLS) and total column ozone from the Ozone Monitoring Instrument (OMI) on NASAs EOS Aura satellite are assimilated. While this change in data streams does lead to a discontinuity in the assimilated ozone fields in MERRA-2, making it not useful for studies in decadal (secular) trends in ozone, this choice was made to prioritize demonstrating the value NASAs high-quality research data in the reanalysis context. The MERRA-2 ozone is compared with independent satellite and ozonesonde data, focusing on the representation of the spatial and temporal variability of stratospheric and upper-tropospheric ozone. The comparisons show agreement within 10 (standard deviation of the difference) between MERRA-2 profiles and independent satellite data in most of the stratosphere. The agreement improves after 2004, when EOS Aura data are assimilated. The standard deviation of the differences between the lower-stratospheric and upper-tropospheric MERRA-2 ozone and ozonesondes is 11.2 and 24.5, respectively, with correlations of 0.8 and above. This is indicative of a realistic representation of the UTLS ozone variability in MERRA-2. After 2004, the upper tropospheric ozone in MERRA-2 shows a low bias compared to the sondes, but the covariance with independent observations is improved compared to earlier years. Case studies demonstrate the integrity of MERRA-2 analyses in representing important features such as tropopause folds.

Ozone↗
NASA NTRS1994

A hybrid simulation of contact discontinuity

Contact discontinuities in a collisionless plasma are studied by hybrid simulations, in which ions are treated as particles and electrons are considered as a fluid. It is demonstrated that contact discontinuity with a stable density ramp can exist in cases with a finite electron temperature. An electron pressure gradient is present across the contact discontinuity, leading to the presence of a parallel electric field and hence field-aligned potential increase (Delta Phi (sub parallel)) in the transition region. By reflecting ions at the discontinuity, this parallel electric potential peak reduces the interpenetration between hot and cold ions and maintains a stable density ramp across the contact discontinuity. The ratio of the field-aligned electric potential energy to ion thermal energy, e(Delta) Phi(sub parallel)/kT(sub i), is found to be an increasing function of T(sub e)/T(sub i), where T(sub e) and T(sub i) are respectively the electron and ion temperature.

Wu, B. H.↗
DOE OSTI2024

A Fast Algebraic Multigrid Solver and Accurate Discretization for Highly Anisotropic Heat Flux I: Open Field Lines

We present a novel solver technique for the anisotropic heat flux equation, aimed at the high level of anisotropy seen in magnetic confinement fusion plasmas. Such problems pose two major challenges: (i) discretization accuracy and (ii) efficient implicit linear solvers. We simultaneously address each of these challenges by constructing a new finite element discretization with excellent accuracy properties, tailored to a novel solver approach based on algebraic multigrid (AMG) methods designed for advective operators. We pose the problem in a mixed formulation, introducing the directional temperature gradient as an auxiliary variable. The temperature and auxiliary fields are discretized in a scalar discontinuous Galerkin space with upwinding principles used for discretizations of advection. We demonstrate the proposed discretization’s superior accuracy over other discretizations of anisotropic heat flux, achieving error 1000x smaller for anisotropy ratio of 10 9 , for closed field lines. The block matrix system is reordered and solved in an approach where the two advection operators are inverted using AMG solvers based on approximate ideal restriction, which is particularly efficient for upwind discontinuous Galerkin discretizations of advection. To ensure that the advection operators are nonsingular, in this paper we restrict ourselves to considering open (acyclic) magnetic field lines for the linear solvers. We demonstrate fast convergence of the proposed iterative solver in highly anisotropic regimes where other diffusion-based AMG methods fail.

97 MATHEMATICS AND COMPUTING↗
NASA NTRS2007

Physics of the Dayside Magnetosphere: New Results From a Hybrid Kinetic Code

We use a global hybrid code kinetic model to demonstrate how kinetic processes at the bow shock and within the foreshock can dramatically modify the solar wind just before its interaction with the magnetosphere. During periods of steady radial interplanetary magnetic field (IMF) orientation, the foreshock fills with a diffuse population of suprathermal ions. The ions generate cavities marked by enhanced temperatures, depressed densities, and diminished magnetic field strengths that convect antisunward into the bow shock with the solar wind flow. Tangential discontinuities marked by inward-pointing electric fields and normals transverse to the Sun-Earth line generate hot flow anomalies marked by hot tenuous plasmas bounded by outward propagating shocks. When the motional electric field in the magnetosheath points inward towards the Earth, a solitary bow shock appears. For typical IMF orientations, the solitary shocks should appear at poorly sampled high latitudes, but for strongly northward or southward IMF orientations the solitary shocks should appear on the flanks of the magnetosphere. Although quasi-perpendicular, solitary shocks should be marked by turbulent magnetosheath flows, often directed towards the Sun-Earth line, and abrupt spike-like enhancements in the density and magnetic field strength at the shock. Finally,we show how flux transfer events generated between parallel subsolar reconnection lines are destroyed upon encountering the magnetopause at latitudes above the cusp.

Siebeck, D. G.↗
NASA NTRS1973

Observed properties of interplanetary rotational discontinuities.

In a prior study, discontinuities were identified as tangential or rotational principally on the basis of the existence or absence of a magnetic field component perpendicular to the plane of the discontinuity. The observed properties of the rotational discontinuities are described in this paper. The normals to the discontinuities were subjected to a variance analysis and did not exhibit a statistically significant anisotropy. These normals appear to be randomly oriented. Since rotational discontinuities are large-amplitude Alfven waves, the transverse perturbation in the magnetic field implies a corresponding jump in the solar wind velocity that is often only slightly less than the Alfven speed. The rotational discontinuities identified are all circularly polarized and transport vorticity and angular momentum.

Smith, E. J.↗
NASA NTRS1975

Evidence for magnetic field line reconnection in the solar wind

The basic scheme of Petschek's (1963) models for slow and fast magnetic reconnection in the solar wind is reviewed in order to determine what sort of results one would expect from satellite observations of possible reconnection processes. Data from HEOS 1 and OGO 5 are then analyzed, in which four observations of neutral sheet structures with large change of magnetic field direction were made. All four may be interpreted as indicating magnetic line reconnections rather than as D-sheets because they all show two distinct discontinuities which bound a lower magnetic field intensity region. The geometry of the magnetic field in the reconnection region appears to be similar to that described by Petschek. In one case the reconnection rate is in agreement with Petschek's prediction, but in the other three cases it is much larger than his limit. The reconnection rates observed do, however, agree with Sonnerup's (1972) limit.

Formisano, V.↗
NASA NTRS1999

Numerical and Analytical Solutions of Hypersonic Interactions Involving Surface Property Discontinuities

The local viscous-inviscid interaction field generated by a wall temperature jump on a flat plate in supersonic flow and on the windside of a Reusable Launch Vehicle in hypersonic flow is studied in detail by both a Navier-Stokes numerical code and an analytical triple-deck model. Treatment of the rapid heat transfer changes both upstream and downstream of the jump is included. Closed form relationships derived from the triple-deck theory are presented. The analytically predicted pressure and heating variations including upstream influence are found to be in generally good agreement with the Computational Fluid Dynamic (CFD) predictions. These analyses not only clarify the interactive physics involved but also are useful in preliminary design of thermal protection systems and as an insertable module to improve CFD code efficiency when applied to such small-scale interaction problems. The analyses only require conditions at the wall and boundary-layer edge which are easily extracted from a baseline, constant wall temperature, CFD solution.

Gnoffo, Peter A.↗
NASA NTRS1970

Hydromagnetic observations in the solar wind

Hydromagnetic fluid model of solar wind for interpreting discontinuities in plasma and magnetic field, interaction with atmosphere, heating mechanism and chemical composition

Burlaga, L. F.↗