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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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Generation of slow-mode waves in front of the dayside magnetopause

Slow-mode waves have been observed to appear frequently in front of the dayside magnetopause. It is found based on two-dimensional global magnetohydrodynamic (MHD) simulations that slow-mode waves are generated through the interaction between the bow shock and various MHD waves (fast-mode, Alfven-mode, or slow-mode waves) in the upstream solar wind. The generated slow-mode waves stay in front of the magnetopause for a long time (over 15 minutes) before the wave energy is convected away tailward. Since various waves are often present in the solar wind, this mechanism may lead to the frequent appearance of slow-mode waves in front of the magnetopause.

Yan, M.↗
NASA NTRS2024

Exploring Standing and Reflected Slow-mode Waves in Flaring Coronal Loops: A Parametric Study Using 2.5D MHD Modeling

Abstract Recent observations of reflected propagating and standing slow-mode waves in hot flaring coronal loops have spurred our investigation into their underlying excitation and damping mechanisms. To understand these processes, we conduct 2.5D magnetohydrodynamic (MHD) simulations using an arcade active region model that includes a hot and dense loop. Our simulations allow for in-depth parametric investigations complementing and expanding our previous 3D MHD modeling results. We excite these waves using a large-amplitude, flow pulse applied at one footpoint of the loop in two distinct models as motivated by observations from the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). The first model (Model 1) incorporates classical compressive viscosity coefficient, while the second model (Model 2) adopts a 10-times enhanced viscosity coefficient. We obtain the following major results: (1) Our 2.5D MHD simulations reinforce previous conclusions derived from 1D and 3D MHD models that significantly enhanced viscosity is crucial for the rapid excitation of standing slow waves with damping times consistent with observations by Wang et al. (2015). (2)We uncover that nonlinearity in Model 1 delays the conversion of a reflected propagating wave into a standing wave. In contrast, Model 2 exhibits a much weak influence of nonlinearity on the excitation time of standing waves, thanks to the suppression of these effects by enhanced viscosity. (3) Our results reveal that the transverse temperature structure holds more influence on wave behavior than the density structure. In Model 1, increased loop temperature contrast significantly enhances wave trapping within the structure, mitigating the impact of temperature-dependent viscous damping. Conversely, in Model 2, the impact of temperature structure on wave behavior weakens in comparison to the effect of viscosity. (4) Model 1 displays evident nonlinear coupling to the fast and kink magnetoacoustic waves and pronounced wave leakage into the corona. Model 2 exhibits significantly weaker effects in this regard. Analyzing three observed wave events by SDO/AIA aligns with Model 2 predictions, providing further support for the substantial viscosity increase. Our 2.5D study unravels the complex interplay of wave-flow phenomena and nonlinear processes in coronal loops, extending our previous 1D modeling results to incorporate more realistic loop geometry. This provides insights into scenarios where 3D effects may be neglected, thereby enhancing our understanding of the intricate dynamics of the solar corona.

Tongjiang Wang↗
NASA NTRS2007

Slow Mode Waves in the Heliospheric Plasma Sheet

We report the results of a search for waves/turbulence in the Heliospheric Plasma Sheet (HPS) surrounding the Heliospheric Current Sheet (HCS). The HPS is treated as a distinctive heliospheric structure distinguished by relatively high Beta, slow speed plasma. The data used in the investigation are from a previously published study of the thicknesses of the HPS and HCS that were obtained in January to May 2004 when Ulysses was near aphelion at 5 AU. The advantage of using these data is that the HPS is thicker at large radial distances and the spacecraft spends longer intervals inside the plasma sheet. From the study of the magnetic field and solar wind velocity components, we conclude that, if Alfven waves are present, they are weak and are dominated by variations in the field magnitude, B, and solar wind density, NP, that are anti-correlated.

heliospheric magnetic field↗
NASA NTRS2000

The Interaction of Solar wind Discontinuities with the Earth's Bow Shock

Funding from NASA Grant No. NAG54679 was received in three installments. The first year's installment amounted to only one month of salary support and was used to prepare survey plots. The second year's installment allowed us to complete two research papers concerning the interaction of solar wind discontinuities with the Earth's bow shock. In the first (published) paper, we reported that the discontinuities launch slow mode waves into the magnetosheath and the slow mode waves always propagate antisunward through the flank magnetosheath. Because the sunward/antisunward sense of the magnetosheath magnetic field reverses across local noon, so does the (north/south or east/west) sense of the velocity fluctuations associated with the waves. Wind, Geotail, and IMP-8 observations were used for this study. In the second study, we used Wind and Interball-1 observations to demonstrate that pressure pulses in the magnetosheath occur in pairs and that they bound pressure cavities and/or brief intervals of outward magnetopause motion. This paper is now in press. Funding from the third year's installment has been used to investigate the two aspects of the foreshock. Two manuscripts are now in preparation for submission to the Journal of Geophysical Research. The first reports that waves within the foreshock account for many instances of poor correlations between two solar wind monitors. Remaining cases of poor correlation occur during intervals of nearly constant IMF orientations and magnetic field strengths. While the former category pose a significant difficulty for space weather forecasts, the latter do not. The second study surveys IMP-8 observations of the foreshock. We find that diamagnetic cavities are common, particularly during periods of high solar wind velocity and low solar wind density. Plasma densities, temperatures, and magnetic field strengths fall during intervals of enhanced energetic particle fluxes. The cavities are bounded by regions of decelerated solar wind plasma and enhanced densities and magnetic field strengths.

Sibeck, David G.↗
NASA NTRS1994

MHD simulations of the magnetosheath

Ideal magnetohydrodynamic (MHD) simulations of the dayside magnetosheath are reported. The calculations show a depletion layer near the magnetopause and a new stagnation flow structure. The stagnation flow appears to be a generalization of the stagnation line for ideal MHD. For Mach numbers typical of the solar wind at the Earth, there is no evidence of the slow mode waves recently observed, but for low Mach numbers prominent slow mode waves are seen in the magnetosheath.

Lyon, J. G.↗
NASA NTRS1992

Waves in the inner magnetosheath - A case study

We study the waves within, and upstream of, the density enhancements in front of the magnetopause using magnetic field and plasma measurements from ISEE 1 and 2. The waves upstream of the density enhancements are most likely to be mirror modes and are convected with the magnetosheath flow. Within the density enhancements there is additional wave power at lower frequencies. These low frequency waves appear to be slow modes propagating sunward and quasi-standing in the flow. These observations lend support to the contention that the outer edge of the density enhancement is a slow mode wave front which marks the greatest distance that slow modes can propagate upstream into the solar wind.

Song, P.↗
NASA NTRS1987

Resonance absorption of magnetohydrodynamic surface waves Physical discussion

It is shown how the phenomenon of MHD surface wave resonance absorption can be described in simple terms, both physically and mathematically, by applying the 'thin flux tube equations' to the finite-thickness transition layer which supports the surface wave. The thin flux tubes support incompressible slow-mode waves that are driven by fluctuations in the total pressure which exist due to the presence of the surface wave. It is shown that the equations for the slow-mode waves can be reduced to a simple equation, equivalent to a driven harmonic oscillator. Certain field lines within the transition layer are equivalent to a harmonic oscillator driven at resonance, and neighboring field lines are effectively driven at resonance as long as a given condition is satisfied. Thus, a layer which secularly extracts energy from the surface wave develops. The analysis indicates that nonlinear effects may destroy the resonance which is crucial to the whole effect.

Hollweg, Joseph V.↗
NASA NTRS1993

Wave properties near the subsolar magnetopause - Pc 3-4 energy coupling for northward interplanetary magnetic field

Strong slow mode waves in the Pc 3-4 frequency range are found in the magnetosheath close to the magnetopause. We have studied these waves at one of the ISEE subsolar magnetopause crossings using the magnetic field, electric field, and plasma measurements. We use the pressure balance at the magnetopause to calibrate the Fast Plasma Experiment data versus the magnetometer data. When we perform such a calibration and renormalization, we find that the slow mode structures are not in pressure balance and small scale fluctuations in the total pressure still remain in the Pc 3-4 range. Energy in the total pressure fluctuations can be transmitted through the magnetopause by boundary motions. The Poynting flux calculated from the electric and magnetic field measurements suggests that a net Poynting flux is transmitted into the magnetopause. The two independent measurements show a similar energy transmission coefficient. The transmitted energy flux is about 18 percent of the magnetic energy flux of the waves in the magnetosheath. Part of this transmitted energy is lost in the sheath transition layer before it enters the closed field line region. The waves reaching the boundary layer decay rapidly. Little wave power is transmitted into the magnetosphere.

Song, P.↗
NASA NTRS1990

Concave-outward slow shocks in coronal mass ejections

Formation of slow shocks in a simplified model corona consisting of closed magnetic field lines near the coronal base with overlying open magnetic field lines is analyzed. An increase in the magnetic field strength in the closed region is utilized to drive the corona outward, generating slow shocks for a suitable selection of parameters. The study demonstrates that concave-upward slow shocks can be formed in a magnetic environment, and provides evidence that nonlinear manifestations of all three wave modes (slow, intermediate, and fast) can occur as the corona adjusts from ambient conditions to those produced by the driver. The results show the existence of nonlinear disturbances due to all three wave modes in the coronal response to the expanding driver magnetic fields.

Steinolfson, R. S.↗
NASA NTRS2007

Sheaths: A Comparison of Magnetospheric, ICME, and Heliospheric Sheaths

When a supersonic flow encounters an obstacles, shocks form to divert the flow around the obstacle. The region between the shock and the obstacle is the sheath, where the supersonic flow is compressed, heated, decelerated, and deflected. Supersonic flows, obstacles, and thus sheaths are observed on many scales throughout the Universe. We compare three examples seen in the heliosphere, illustrating the interaction of the solar wind with obstacles of three very different scales lengths. Magnetosheaths form behind planetary bow shocks on scales ranging from tens to 100 planetary radii. ICME sheath form behind shocks driven by solar disturbances on scale lengths of a few to tens of AU. The heliosheath forms behind the termination shock due to the obstacle presented by the interstellar medium on scale lengths of tens to a hundred AU. Despite this range in scales some common features have been observed. Magnetic holes, possibly due to mirror mode waves, have been observed in all three of these sheaths. Plasma depletion layers are observed in planetary and ICME sheaths. Other features observed in some sheaths are wave activity (ion cyclotron, plasma), energetic particles, transmission of Alfven waves/shocks, tangential discontinuities turbulence behind quasi-parallel shocks, standing slow mode waves, and reconnection on the obstacle boundary. We compare these sheath regions, discussing similarities and differences and how these may relate to the scale lengths of these regions.

Sibeck, D. G.↗
NASA NTRS1984

Plasma wave spectra near slow mode shocks in the distant magnetotail

Recently Feldman et al. (1984) used ISEE-3 data to analyze variations in charged particle and magnetic field characteristics near plasma sheet boundaries in the distant magnetotail, and they demonstrated that several of the crossings had the properties of slow mode shocks. ISEE-3 plasma wave measurements at two of these crossings are presently discussed, and it is shown that all conventional characteristics of fast collisionless shock spectra are present. Upstream (lobe side) electron plasma oscillations are detected in association with electron heat flux enhancements, and pronounced spectral peaks at lower frequencies are found within the shock layer.

Scarf, F. L.↗
NASA NTRS1991

A study of slow-mode structures in the dayside magnetosheath

Recent observations indicate that a region of enhanced plasma pressure and decreased magnetic field intensity frequently occurs in front of the plasma depletion layer at the dayside magnetopause (Song et al., 1990). This inverse relationship is characteristic of a slow-mode wave. This phenomenon was simulated with a two-dimensional incompressible MHD simulation code. When a normal component of the interplanetary magnetic field is present (Bx not equal to 0), the total magnetic field intensity tends to decrease in front of the depletion layer due to the bending of the magnetic field lines, and the plasma pressure is enhanced in this region. On the other hand, when Bx = 0, this slow-mode structure is not present in the simulation and only the plasma depletion layer is observed.

Lee, L. C.↗
DOE OSTI2024

Data for "Propagation of EMIC Waves from Shabansky Orbits in the Dayside Magnetosphere"

Full-wave simulations of electromagnetic ion cyclotron waves in a non-dipole compressed magnetic field have been conducted using the Petra-M simulation code. The manuscript investigates the effect of misalignment of the Faraday screen and electron density in the scrape-off layer on the excitation and propagation of fast helicon and slow mode waves.

Magnetosphere: outer↗
NASA NTRS1990

Lower solar chromosphere-corona transition region. II - Wave pressure effects for a specific form of the heating function

Lower transition region models with a balance between mechanical heating and radiative losses are expanded to include wave pressure effects. The models are used to study the simple damping length form of the heating function. The results are compared to the results obtained by Woods et al. (1990) for solutions in the lower transition region. The results suggest that a mixture of fast-mode and slow-mode waves may provide the appropriate heating mechanism in the lower transition region, with the decline in effective vertical wave speed caused by the refraction and eventual total reflection of the fast-mode wave resulting from the decreasing atmospheric density.

Woods, D. Tod↗
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 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 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 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.↗