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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

Inner Plasma Structure of the Low-Latitude Reconnection Layer

We report a clear transition through a reconnection layer at the low-latitude magnetopause which shows a complete traversal across all reconnected field lines during northwestward interplanetary magnetic field (IMF) conditions. The associated plasma populations confirm details of the electron and ion mixing and the time history and acceleration through the current layer. This case has low magnetic shear with a strong guide field and the reconnection layer contains a single density depletion layer on the magnetosheath side which we suggest results from nearly field-aligned magnetosheath flows. Within the reconnection boundary layer, there are two plasma boundaries, close to the inferred separatrices on the magnetosphere and magnetosheath sides (Ssp and Ssh) and two boundaries associated with the Alfvén waves (or Rotational Discontinuities, RDsp and RDsh). The data are consistent with these being launched from the reconnection site and the plasma distributions are well ordered and suggestive of the time elapsed since reconnection of the field lines observed. In each sub-layer between the boundaries the plasma distribution is different and is centered around the current sheet, responsible for magnetosheath acceleration. We show evidence for a velocity dispersion effect in the electron anisotropy that is consistent with the time elapsed since reconnection. In addition, new evidence is presented for the occurrence of partial reflection of magnetosheath electrons at the magnetopause current layer.

magnetosheath↗
NASA NTRS1985

Unstable current systems and plasma instabilities in astrophysics; Proceedings of the 107th Symposium, University of Maryland, College Park, August 8-11, 1983

Among the topics discussed are: magnetic field reconnection in cosmic plasmas; energy dissipation mechanisms in the solar corona; and the acceleration of runaway electrons and Joule heating in solar flares. Consideration is also given to: the nonlinear evolution of the resistive tearing mode; anomalous transport in current sheets; equilibrium and instability in extragalactic jets; and magnetic field reconnection in differentially rotating accretion disks. Among additional topics discussed are: the creation of high energy electron tails by lower hybrid waves and its connection with type-II and type-III bursts; beam current systems in solar flares; and the spatio-temporal features of microwave emissions of active regions and flares.

Kundu, M. R.↗
NASA NTRS2019

Mechanism of Reconnection on Kinetic Scales Based on Magnetospheric Multiscale Mission Observations

We examine the role that ions and electrons play in reconnection using observations from the Magnetospheric Multiscale (MMS) mission on kinetic ion and electron scales, which are much shorter than magnetohydrodynamic scales. This study reports observations with unprecedented high resolution that MMS provides for magnetic field (7.8 ms) and plasma (30 ms for electrons and 150 ms for ions). We analyze and compare approaches to the magnetopause in 2016 November, to the electron diffusion region in the magnetotail in 2017 July followed by a current sheet crossing in 2018 July. Besides magnetic field reversals, changes in the direction of the flow velocity, and ion and electron heating, MMS observed large fluctuations in the electron flow speeds in the magnetotail. As expected from numerical simulations, we have verified that when the field lines and plasma become decoupled a large reconnecting electric field related to the Hall current (1–10 mV/m) is responsible for fast reconnection in the ion diffusion region. Although inertial accelerating forces remain moderate (1–2 mV/m), the electric fields resulting from the divergence of the full electron pressure tensor provide the main contribution to the generalized Ohm’s law at the neutral sheet (as large as 200 mV/m). In our view, this illustrates that when ions decouple electron physics dominates. The results obtained on kinetic scales may be useful for better understanding the physical mechanisms governing reconnection processes in various magnetized laboratory and space plasmas.

W. M. Macek↗
NASA NTRS2015

The Role of the Hall Effect in Global Structure and Dynamics of Planetary Magnetospheres: Ganymede as a Case Study

We present high-resolution Hall MHD simulations of Ganymede's magnetosphere demonstrating that Hall electric fields in ion-scale magnetic reconnection layers have significant global effects not captured in resistive MHD simulations. Consistent with local kinetic simulations of magnetic reconnection, our global simulations show the development of intense field-aligned currents along the magnetic separatrices. These currents extend all the way down to the moon's surface, where they may contribute to Ganymede's aurora. Within the magnetopause and magnetotail current sheets, Hall J x B forces accelerate ions to the local Alfven speed in the out-of-plane direction, producing a global system of ion drift belts that circulates Jovian magnetospheric plasma throughout Ganymede's magnetosphere. We discuss some observable consequences of these Hall-induced currents and ion drifts: the appearance of a sub-Jovian 'double magnetopause' structure, an Alfvenic ion jet extending across the upstream magnetopause, and an asymmetric pattern of magnetopause Kelvin-Helmholtz waves.

Magnetic Reconnection↗
NASA NTRS1980

The dynamic expansion and contraction of the Jovian plasma sheet

Observations suggesting the sequential expansion and compression of the Jovian plasma sheet are reported. Plasma flow in the vicinity of Jupiter was monitored by the four modulated-grid Faraday cups on board each of the Voyager spacecraft at times of closest Jupiter approach. Sensor measurements reveal the flow of magnetospheric plasma to be directed away from the equatorial current sheet near local noon and to be directed towards the sheet in the dusk to midnight sector. The observed flow patterns are interpreted in terms of short-time-scale perturbations of magnetic flux tubes due to the compression of the dayside magnetosphere by the solar wind. It is noted that such a dynamic motion is quite different from what would be expected of slower, quasi-static equilibrium plasma sheet expansion and contraction.

Belcher, J. W.↗
NASA NTRS1990

Magnetohydrodynamic modeling of the solar corona

The ideal and resistive magnetohydrodynamic (MHD) model is used to examine the dynamics and structure of the solar corona. When the coronal magnetic field is deformed by photospheric flow it can evolve to states that become unstable to ideal MHD modes. The nonlinear evolution of these instabilities can lead to the generation of current sheets, field line reconnection, and energy release. The disruption of an arcade field and the kinking of coronal loops is described. The braiding of the large-scale coronal field by convective photospheric motions develops fine-scale structure in the magnetic field and leads to the development of intense current filaments. The resistive dissipation of these currents can provide an efficient coronal heating mechanism.

Mikic, Zoran↗
NASA NTRS1994

Hybird state of the tail mangetic configuration during steady convection events

Previous observations have shown that during periods of steady magnetospheric convection (SMC) a large amount of magnetic flux crosses the plasma sheet (corresponding to approximately 10 deg wide auroral oval at the nightside) and that the magnetic configuration in the midtail is relaxed (the curent sheet is thick and contains enhanced B(sub Z). These signatures are typical for the substorm recovery phase. Using near-geostationary magnetic field data, magnetic field modeling and a noval diagostic technique (isotropic boundary algorithm), we show that in the near-Earth tail the magnetic confirguration is very stretched during the SMC events. This stretching is caused by an intense, thin westward current. Because of the srongly depressed B(sub Z), there is a large radial gradient in the near-tail magetic field. These signatures have been peviously associated only with the substorm growth phase. Our results indicate that during the SMC periods the magnetic configuration is very peculiar, with co-existing thin near-Earth current sheet and thick midtail plasma sheet. The deep local minimum of the equatorial B(sub Z) that devleops at R approximately 12 R(sub E) is consistent with steady, adiabatic, Earthward convection in the midtail. These results impose contraints on the existing substorm theories, and call for an explanation of how such a stressed configuration can persist for such a long time without tail current disruptions that occur at the end of a substorm growth phase.

Sergeev, V. A.↗
DOE OSTI2025

Observation of a Knotted Electron Diffusion Region in Earth's Magnetotail Reconnection

Magnetic reconnection is a fundamental plasma process that alters the magnetic field topology and releases magnetic energy. Most numerical simulations and spacecraft observations assume a two‐dimensional diffusion region, with the electron diffusion region (EDR) embedded in the same plane as the ion diffusion region (IDR) and a uniform guide field throughout. Using observations from Magnetospheric Multiscale mission, we report a non‐coplanar, knotted EDR in Earth's magnetotail current sheet. The reconnection plane of the knotted EDR deviates by approximately 38° from that of the IDR, with the guide field exhibiting both a 38° directional shift and a twofold increase in amplitude. Moreover, the Hall magnetic field is bipolar in the EDR but quadrupolar in the IDR, indicating different Hall current structures at electron and ion scales. These observations highlight the importance of three‐dimensional effects and illustrate the complexity of multiscale coupling between the EDR and IDR during reconnection studies.

diffusion region↗
NASA NTRS1984

Externally driven magnetic reconnection

A model is presented in which externaly driven reconnection is simulated by solving the MHD equations in an initially plane current sheet. Magnetic reconnection in the earth's magnetotail is widely believed to be the direct cause of magnetospheric substorms. Both 2-D and 3-D versions of the model have been developed. It is postulated that connection in the tail is triggered by a local compression of the plasma sheet which results from an invasion of the solar wind into the magnetotail. Thus, the simulation is started by introducing flow from the lobes normal to the plasma sheet. When resistivity is generated in a local region of the neutral sheet, reconnection develops and magnetic energy is converted into plasma bulk flow. Although the driven reconnection model is highly simplified, it can aid in understanding many features of substorms in the tail; in particular, results show that rapid flows both earthward and tailward of the neutral line and the nightside substorm current system are natural consequences of driven magnetic reconnection.

Walker, R. J.↗
NASA NTRS1990

Observational signatures of nonlinear magnetotail particle dynamics

It has been predicted that the nonlinear particle dynamics in the magnetotail leads to a class of resonance structures in the quiet-time ion distribution functions in the central plasma sheet (CPS). These structures exhibit a scaling law of H to the 1/4 power. The first identification of such a scaling law in quiet-time CPS distribution functions obtained aboard ISEE 1 is reported. A method is proposed which uses the observed resonance structures to determine the quiet-time current sheet thickness based on a measurement of the distribution function and magnetic field obtained by one satellite.

Chen, James↗
NASA NTRS1988

Substorm variations in the magnitude of the magnetic field - AMPTE/CCE observations

Using energetic-particle data taken in the near-earth tail by the AMPTE/Charge Composition Explorer (CCE) satellite, 167 ion injection events, that were essentially dispersionless over a 25-285 keV energy range, were identified, and the variations in the total magnetic field strength over the course of these events were examined in order to determine the dependence of the magnetic field strength on dipole latitude. Results indicate that, during periods of substorm activity, the latitudinal position of the current sheet varied significantly within the 32-deg wedge centered on the dipole equator traversed by CCE. Results also suggest that, even in the near-earth magnetotail out to 8.8 R(E) (CCE apogee), the local field measurements are a better guide to the determination of satellite's position relative to the current shield during a substorm, than is the magnetic latitude.

Lopez, R. E.↗
NASA NTRS2022

Drill-Site Selection for Cosmogenic-Nuclide Exposure Dating of the Bed of the Greenland Ice Sheet

Direct observations of the size of the Greenland Ice Sheet during Quaternary interglaciations are sparse yet valuable for testing numerical models of ice-sheet history and sea level contribution. Recent measurements of cosmogenic nuclides in bedrock from beneath the Greenland Ice Sheet collected during past deep-drilling campaigns reveal that the ice sheet was significantly smaller, and perhaps largely absent, sometime during the past 1.1 million years. These discoveries from decades-old basal samples motivate new, targeted sampling for cosmogenic-nuclide analysis beneath the ice sheet. Current drills available for retrieving bed material from the US Ice Drilling Program require < 700 m ice thickness and a frozen bed, while quartz-bearing bedrock lithologies are required for measuring a large suite of cosmogenic nuclides. We find that these and other requirements yield only ∼ 3.4 % of the Greenland Ice Sheet bed as a suitable drilling target using presently available technology. Additional factors related to scientific questions of interest are the following: which areas of the present ice sheet are the most sensitive to warming, where would a retreating ice sheet expose bare ground rather than leave a remnant ice cap, and which areas are most likely to remain frozen bedded throughout glacial cycles and thus best preserve cosmogenic nuclides? Here we identify locations beneath the Greenland Ice Sheet that are best suited for potential future drilling and analysis. These include sites bordering Inglefield Land in northwestern Greenland, near Victoria Fjord and Mylius-Erichsen Land in northern Greenland, and inland from the alpine topography along the ice margin in eastern and northeastern Greenland. Results from cosmogenic-nuclide analysis in new sub-ice bedrock cores from these areas would help to constrain dimensions of the Greenland Ice Sheet in the past.

Greenland↗
NASA NTRS1982

The fine structure of the front side magnetopause during two successive crossings

The magnetopause crossings of November 10, 1977, which are considered to be representative of a class of structures, are investigated, and a procedure is developed to derive approximate surface currents near the magnetopause from ISEE 1 magnetic field data by assuming a constant speed of the magnetopause along its normal for each crossing of the magnetopause layer. Changes in the magnetic field indicate that current sheets exist on each side of the magnetopause layer, which is characterized by an irregular magnetic field and a magnetosheath-like energy distribution. No boundary layer plasma is observed on the magnetospheric field lines inside the magnetopause, and electromagnetic energy is found to be dissipated at the two edges of the magnetopause layer where the current layers are observed.

Formisano, V.↗
NASA NTRS1996

Estimates of magnetic flux, and energy balance in the plasma sheet during substorm expansion

The energy and magnetic flux budgets of the magnetotail plasma sheet during substorm expansion are investigated. The possible mechanisms that change the energy content of the closed field line region which contains all the major dissipation mechanisms of relevance during substorms, are considered. The compression of the plasma sheet mechanism and the diffusion mechanism are considered and excluded. It is concluded that the magnetic reconnection mechanism can accomplish the required transport. Data-based empirical magnetic field models are used to investigate the magnetic flux transport required to account for the observed magnetic field dipolarizations in the inner magnetosphere. It is found that the magnetic flux permeating the current sheet is typically insufficient to supply the required magnetic flux. It is concluded that no major substorm-type magnetospheric reconfiguration is possible in the absence of magnetic reconnection.

Hesse, Michael↗
NASA NTRS2007

Design of a Low-Energy FARAD Thruster

The design of an electrodeless thruster that relies on a pulsed, rf-assisted discharge and electromagnetic acceleration using an inductive coil is presented. The thruster design is optimized using known performance,scaling parameters, and experimentally-determined design rules, with design targets for discharge energy, plasma exhaust velocity; and thrust efficiency of 100 J/pulse, 25 km/s, and 50%, respectively. Propellant is injected using a high-speed gas valve and preionized by a pulsed-RF signal supplied by a vector inversion generator, allowing for current sheet formation at lower discharge voltages and energies relative to pulsed inductive accelerators that do not employ preionization. The acceleration coil is designed to possess an inductance of at least 700 nH while the target stray (non-coil) inductance in the circuit is 70 nH. A Bernardes and Merryman pulsed power train or a pulse compression power train provide current to the acceleration coil and solid-state components are used to switch both powertrains.

Polzin, K. A.↗
DOE OSTI2022

Do chaotic field lines cause fast reconnection in coronal loops?

Over the past decade, Boozer has argued that three-dimensional (3D) magnetic reconnection fundamentally differs from two-dimensional reconnection due to the fact that the separation between any pair of neighboring field lines almost always increases exponentially over distance in a 3D magnetic field. According to Boozer, this feature makes 3D field-line mapping chaotic and exponentially sensitive to small non-ideal effects; consequently, 3D reconnection can occur without intense current sheets. We test Boozer's theory via ideal and resistive reduced magnetohydrodynamic simulations of the Boozer–Elder coronal loop model driven by sub-Alfvénic footpoint motions [A. H. Boozer and T. Elder, Phys. Plasmas 28, 062303 (2021)]. In this work, our simulation results significantly differ from their predictions. The ideal simulation shows that Boozer and Elder under-predict the intensity of current density due to missing terms in their reduced model equations. Furthermore, resistive simulations of varying Lundquist numbers show that the maximal current density scales linearly rather than logarithmically with the Lundquist number.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1978

Investigation of a staged plasma-focus apparatus

A new staged plasma-focus geometry combining two Mather-type plasma-focus guns was constructed, and the current-sheet dynamics were investigated. The production of simultaneous pairs of plasma foci was achieved. The intensities of X-ray and fusion-neutron emission were measured and found to agree with the scaling law for a plasma focus. Advantages of this new geometry include the possibility of using plasma-focus type pinches in multiple arrays at power levels beyond the validity regime of the current scaling law for a single gun.

Lee, J. H.↗
NASA NTRS1976

Flare build-up study; Proceedings of the Workshop, Falmouth, Mass., September 8-11, 1975

The papers deal with problems which might be common to solar flares and earth's magnetosphere, problems associated with the initial phases of the flare phenomenon, the acceleration processes that seem to occur in flares and the magnetosphere, as well as the buildup and storage of flare energy in magnetic-field structures. Topics include the active role of magnetic fields in providing flare energy, current-sheet models of solar flares, the role of plasma turbulence in flare development, similarities and differences between magnetospheric substorms and solar flares, observations of magnetic merging in earth's magnetotail during magnetospheric substorms, evidence for magnetic-energy storage in coronal active regions, the possible role of transition-zone instabilities in preflare energy buildup, and flare energy storage and deposition. Other papers discuss energy release through the interaction of coronal magnetic fields, photospheric electric currents as a source of flare energy, magnetic-energy buildup in the solar atmosphere, magnetic and velocity fields in an active region, flare onset at meter wavelengths, laboratory experiments on field-line reconnection, key problems in auroral flare processes, and the solar-physics Shuttle/Spacelab program. Individual items are announced in this issue.

Svestka, Z.↗