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At least 235 records · Page 13

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NASA NTRS1982

Reverse draping of magnetic field lines in the boundary layer

An unexpected orientation of field lines has been found by ISEE satellite measurements of the earth magnetosphere boundary layer, perhaps indicating a reverse draping. Three-dimensional plasma flow measurements by ISEE 1 and 2 show the tailward flow in the boundary layer to have an equatorward component which suggests high-latitude plasma entry. Simultaneous magnetic field measurements with the two satellites during a boundary crossing sequence yielded data on the boundary layer electric currents and the boundary layer interface with plasma sheet and magnetosheath. The currents are found to be largely field-aligned, with intensities in the 0.03-0.06 microampere/sq m range.

Hones, E. W., Jr.↗

NASA NTRS1981

Formation of auroral arcs by plasma sheet processes

It is noted that, in the distant plasma sheet, it is likely that curvature drift is the most important source of drift parallel to the electric field, leading to what is commonly called Fermi acceleration of the particles. The energization mechanism here is proportional to the neutral sheet current density. It is a form of field-aligned acceleration, with rapid lowering of mirror points, caused by the transverse electric field in the plasma sheet. It is noted that the process will work for both negative and positive particles. A filamentation of the neutral current sheet is postulated. Here, the maximum energization by curvature drift and the accompanying intense precipitation will form an auroral band or arc along the sheet of magnetic field lines that maps out to the local enhancement of the crosstail current, explaining inverted V events.

Heikkila, W. J.↗

NASA NTRS1979

Energetic protons in the Jovian magnetosphere

The time histories, angular distributions and energy spectra of energetic protons were measured over an energy range extending from 0.2 - 20 MeV for the four passes of Pioneers 10 and 11 through the Jovian magnetosphere. Azimuthal asymmetries appear to dominate with time variations also contributing to the very complex topology. On the inbound P-10 pass the expected corotation anisotropy was not observed in the outer magnetosphere supporting the probable existence of a planetary wind in this region. Near the dawn meredian particle streaming away from the planet begins at about 15 RJ. On both the P-10 inbound and P-11 outbound passes, there are regions where only partial corotation is achieved. In the mid-magnetosphere, field-aligned streaming away from the near-equatorial current sheet region is the most prominent feature. At mid-latitudes in the subsolar regime, the streaming pattern is more chaotic and its magnitude is smaller. Qualitative discussions are presented for a number of possible mechanisms which could produce this streaming.

Mcdonald, F. B.↗

NASA NTRS1985

Auroral arc formation - Kinetic and MHD effects

Recent observations by the Dynamics Explorer satellites indicate that auroral field lines are not equipotentials. The field-aligned 'conductivity' is the same for up and downgoing currents and of the order of several 10 to the -8th to several 10 to the -9th/ohms sq m. This is not in agreement with models based on the resistance provided by the magnetic mirror force alone. It is suggested that the resistivity is due to damping of kinetic shear-Alfven wave packets. This damping may be nonlinear in nature or Landau damping. The properties of kinetic Alfven waves are reviewed.

Goertz, C. K.↗

NASA NTRS1978

Topside sounder observations of equatorial bubbles

Large scale regions of depleted equatorial ionospheric plasma, called equatorial bubbles, are investigated using topside sounder data. The sounder's unique remote measuring capability enables the magnetic field-aligned nature of the bubbles to be investigated. A search of all available Alouette 2 and ISIS 1 ionograms during nighttime perigee passes near the magnetic equator has revealed a variety of echo signatures associated with bubbles. In addition to a sudden drop in electron density, these signatures usually include in situ spread F and ducted traces. The ducted traces have been used to determine the electron density distribution and to infer changes in ion composition along the magnetic field line within the duct associated with the bubble. In some cases it can be determined that the bubble is asymmetric with respect to the magnetic equator. Even though such features require 3 dimensional models for their explanation, the great field-aligned extent of the bubbles (relative to their cross section) suggests that current theories, which ignore variations along the magnetic field, are still applicable.

Dyson, P. L.↗

NASA NTRS1981

Mechanism of parallel electric fields inferred from observations

Data from various experiments of the Atmosphere Explorer satellites are analyzed to test their consistency with the model of single-particle linear acceleration through a static parallel potential drop. Theoretical current/voltage and energy flux/voltage relations are applied to the data to predict the densities of source current carriers above the acceleration region for upward Birkeland currents. Comparison of these model predictions with simultaneous electron data indicates that the observed field-aligned potential drops are consistent with direct acceleration in the absence of anomalous resistivity.

Yeh, H.-C.↗

NASA NTRS1985

A new source of suprathermal O(+) ions near the dayside polar cap boundary

A large number of data on suprathermal O(+) ions taken during the retarding ion mass spectrometer (RIMS) experiment aboard the DE 1 satellite are surveyed. Examples are found of low-energy, upflowing O(+) which are consistent with one or more of the proposed ionospheric escape mechanisms. These include transversely accelerated O(+) ions, indicating low-altitude transverse acceleration, and O(+) field-aligned flows which indicate low-altitude parallel acceleration by either ambipolar or current-driven electric fields. However, by far the most common pitch angle distribution of escaping O(+) is found to be a new type of O(+) flow event which provides evidence for both perpendicular and parallel ion acceleration below the satellite and is found exclusively in the lower latitudes of the dayside polar cap. All species of ions are observed to move upward during these events, with an upward heat flux.

Lockwood, M.↗

NASA NTRS1985

Direct measurements of severe spacecraft charging in auroral ionosphere

Questions are addressed concerning how large space structures in polar orbit will interact with auroral environments. Because spacecraft charging at ionospheric attitudes does not seriously threaten the operation of today's relatively small polar satellites the subject of environment interactions has not received the widespread attention given to it at geostationary altitude. As a matter of economics it is desirable to apply as much as possible of what was learned about spacecraft interactions at geostationary orbit to low Earth orbits. The environment at auroral latitudes in the ionosphere differs from that encountered at geostationary altitude in at least two major aspects. (1) There is a large reservoir of high-density, cold plasma which tends to mitigate charging effects by providing a large source of charged particles from which neutralizing currents maybe drawn. Significant wake effects behind large structures will introduce new problems with differential charging. (2) Between the magnetic equator and the ionosphere, auroral electrons frequently undergo field-aligned accelerations of several kilovolts. In such environments, fluxes of energetic protons are usually below the levels of instrumentation sensitivity.

Burke, W. J.↗

NASA NTRS1986

Effects of anomalous transport on the potentials of discrete auroral arcs

The evolution of U-shaped double layers in discrete auroral arcs occurs much faster than the global current in the arc flux tube can be redistributed. For asymptotic stability of the arc on MHD time scales, this implies that local transport mechanisms are required to charge the nearly field-aligned flanks of the equipotential structure over an extended region along the magnetic field. The diversion of the uniform upstream current provides a constraint which limits the double-layer potential.

Smith, R. A.↗

NASA NTRS1985

Electrostatic ion cyclotron waves in a plasma with an ion beam and counterstreaming bulk electrons - Waves in the zero-frequency band

A common feature of the auroral plasma in the region above field-aligned (parallel) potential drops are electrostatic hydrogen cyclotron (EHC) waves. The present paper has the objective to show that wave excitation in the zero-frequency band can occur when the ion beams and the current-carrying bulk electrons counterstream. The instability mechanism involves the Landau interaction of the slow (negative energy) ion-beam-cyclotron waves with the drifting electrons and also with the target (background) ions. Only the latter resonant interaction between the beam and the target ions was considered by Okuda and Nishikawa (1984). In this study, it is shown that an electron drift makes an additional unstable contribution to the waves in the zero-frequency band, including those discussed by Okuda and Nishikawa.

Singh, N.↗

DOE OSTI2026

Fast solvers for tokamak fluid models with PETSc

Multigrid (MG) is widely recognized as a highly effective solver for the model problem, the Laplacian, but textbook MG fails on most problems of interest. MG methods have been applied to complex, real-world applications with careful consideration of the physical model and discretization. In this work we develop the first step in applying MG methods to science and engineering relevant magnetohydrodynamics (MHD) tokamak models in the M3D-C1 (https://m3dc1.pppl.gov) fusion energy science code. The semi-implicit time integrator in M3D-C1 is composed of many linear solves. The implicit advance of the momentum equation is the most challenging and is the focus of this work. The current production solver in M3D-C1 is a block Jacobi (BJ) preconditioner within a Krylov solver, where blocks group degrees of freedom on planes of constant toroidal coordinate. BJ convergence degrades as the number of planes increases due to the spectral properties of the matrix preconditioned with BJ. The partially magnetic field-aligned, regular toroidal grid structure in M3D-C1 is amenable to semi-coarsening geometric MG in the toroidal direction. This paper develops such a solver and demonstrates competitive performance on a runaway electron model of a SPARC (https://cfs.energy/technology/sparc) disruption, and superior robustness on a stellarator model on which the BJ solver fails to converge.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2026

Comparison between Alcator C-Mod ICRF experiments and 3D full wave simulations

Reliable modeling of ion cyclotron range of frequencies (ICRF) antenna performance is essential for interpreting present experiments and guiding the design of future reactors. In this work, a 3D model of the Alcator C-Mod field-aligned antenna is implemented in the Petra-M finite-element framework [S. Shiraiwa et al 2023 Nucl. Fusion 63 026024] and benchmarked against experimental results. Four experimental cases are examined. First, the simulated rectified sheath potentials on the antenna limiters are compared with measurements from a power tapering experiment. Second, proof-of-principle far-field sheath simulations are performed. In scenarios with low single-pass absorption, simulations predict enhanced sheath potentials on a distant poloidal limiter in the far field of the antenna, consistent with experimental observations. Third, the simulated antenna loading during edge localized modes follows experimental trends and appears to be dominated by the density gradient at the pedestal. Finally, Petra-M predicts the unintended excitation of high-$k$ ∥ modes during monopole phasing operation, in agreement with experimental evidence of poor wave coupling and accessibility to the plasma core. Overall, reasonable agreement is found between the simulations and experiments. At the same time, areas of imperfect agreement are identified. These provide important guidance on the limits of the current state-of-the-art modeling, which should be kept in mind when using it as a predictive tool for future reactors.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

NASA NTRS1973

Experimental test of magnetospheric models.

The barium cloud injection deep into the magnetosphere of Sept. 21, 1971, has revealed that the quiet-time near-equatorial nightside magnetic field is significantly different from that predicted by existing quite-time models. The influence of currents flowing on or near the geomagnetic equator gives the field a more tail-like structure at a distance as close as 5 earth radii. Several models have been tried out to obtain a best fit to the experimental observations of the shape of the field-aligned ion cloud. Although this method does not lead to a unique solution, the most acceptable option emerging from this analysis is a quiet-time tilted image-dipole model with a Williams-Mead tail field and a disclike ring current on the geomagnetic equator.

Barish, F. D.↗

NASA NTRS1975

The L = 6.6 Oosik barium plasma injection experiment and magnetic storm of March 7, 1972

A high-explosive shaped charge vaporizing a hollow conical liner of Ba metal and producing a fast field-aligned jet of plasma was detonated at high altitude, during a quiescent phase of a magnetic storm initiated by an ssc 10 hrs prior to the experiment, in an attempt to trace out and observe the dynamics of an auroral field line in the magnetosphere. Observations offer evidence for an upward Birkeland current sheet at the poleward edge of the auroral spiral of 8 x 10 to the minus second power A/m. Unusual features of the substorm leave open the possibility that the plasma injection may have triggered it.

Wescott, E. M.↗

NASA NTRS1976

Plasma injection at synchronous orbit and spatial and temporal auroral morphology

Analysis of ATS 5 particle spectrograms and simultaneous meridian-scanning photometer data obtained at the base of the ATS field line has revealed systematic relationships between temporal and spatial auroral morphology and particle-injection events. The details of these relationships depend on previous injection events, the local time of the observatory with respect to current-injection events, and the extent and magnitude of current injections. A persistent zone of weak aurora is related to steady plasma-sheet drizzle, and the equatorward edge of this aurora delineates field lines threading the inner edge of the plasma sheet. Auroras associated with injection events superimpose on this preexisting pattern, with the lowest latitude of new precipitation depending on local time. In general, plasma energization is not required between the equatorial plane and the ionosphere to account for observed auroral intensities. The data indicate occasional occurrence of enhanced loss-cone fluxes, suggesting low-altitude field-aligned acceleration. A summary figure shows how various characteristic auroral-precipitation regions are related to the shape, extent, and orientation of the substorm injection boundary and to the subsequent drift paths of injected plasma.

Eather, R. H.↗

NASA NTRS1978

Interplanetary magnetic holes: Theory

Magnetic holes in the interplanetary medium are explained as stationary, non-propagating, equilibrium structures in which there are field-aligned enhancements of the plasma density and/or temperature. Magnetic anti-holes are considered to be associated with depressions in the plasma pressure. In this model, the observed changes in the magnetic field intensity and direction are due to diamagnetic currents that are carried by ions which drift in a sheath as the result of gradients in the magnetic field and in the plasma pressure within the sheath. The thickness of the sheaths considered is approximately a few ion Larmor radii. An electric field is normal to the magnetic field in the sheath. Solutions of Vlasov's equation and Maxwell's equations are presented which account for several types of magnetic holes, including null-sheets, that were observed.

Burlaga, L. F.↗

NASA NTRS1978

Interplanetary magnetic holes - Theory

Magnetic holes in the interplanetary medium are explained as stationary nonpropagating equilibrium structures in which there are field-aligned enhancements of the plasma density and/or temperature. Magnetic antiholes are considered to be associated with depressions in the plasma pressure. In this model the observed changes in the magnetic field intensity and direction are due to diamagnetic currents that are carried by ions which drift in a sheath as the result of gradients in the magnetic field and in the plasma pressure within the sheath. The thickness of the sheaths that we consider is approximately a few ion Larmor radii. An electric field is normal to the magnetic field in the sheath. Solutions of Vlasov's equation and Maxwell's equations are presented which account for several types of magnetic holes, including 'null sheets,' that have been observed.

Burlaga, L. F.↗

NASA NTRS1982

Interplanetary magnetic field control of high-latitude activity on July 29, 1977

Multisatellite particle and magnetic field data for the substorms of July 29, 1977, show auroral-like activity above 80 deg invariant latitude during the recovery period. The movement of auroral zone activity to high latitudes followed the substorm sequence, at which time the inferred interplanetary magnetic field (IMF) was strongly northward. Electron energy spectra indicative of a field-aligned potential drop, and the absence of supporting precipitating ions, are found at latitudes greater than 80 deg. The north-south symmetry of these observations suggests that the events are on closed field lines. It is noted the very strong northward IMF connected to the sunward tilted geomagnetic dipole field plays a role in the driving of strong Birkeland and ionospheric current systems in the northern polar regions, while eliminating them from the southern polar regions.

Zanetti, L. J.↗