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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Investigation of plasma instabilities in the polar cusp

This technical paper describes the occurrence of magnetosonic waves in the dayside and nightside polar magnetosphere and the roles that these waves can play in the heating of ions in the ionosphere. Plasma waves near the auroral kilometric radiation (AKR) source regions is studied, using wave data obtained from both the Galileo and DE-1. During the Galileo encounter with the Earth in 1992, very intense auroral kilometric radiation was observed. The high spectral resolution obtained by the wide-band receiver of the plasma wave instrument on board the spacecraft often show distinctive, dispersive, arc-like features each extending over a period of several seconds. Similar features were also observed by De-1. These features may be due to an intrinsic velocity and/or to a source simulation by different wave modes. A reply to the comment by Orlowski and Russell on the earlier paper 'Electron Beam Excitation of Upstream Waves in the Whistler Frequency Mode Frequency Range' is also included.

Wong, H. K.↗
NASA NTRS2016

Mixing the Solar Wind Proton and Electron Scales: Effects of Electron Temperature Anisotropy on the Oblique Proton Firehose Instability

The double adiabatic expansion of the nearly collisionless solar wind plasma creates conditions for the firehose instability to develop and efficiently prevent the further increase of the plasma temperature in the direction parallel to the interplanetary magnetic field. The conditions imposed by the firehose instability have been extensively studied using idealized approaches that ignore the mutual effects of electrons and protons. Recently, more realistic approaches have been proposed that take into account the interplay between electrons and protons, unveiling new regimes of the parallel oscillatory modes. However, for oblique wave propagation the instability develops distinct branches that grow much faster and may therefore be more efficient than the parallel firehose instability in constraining the temperature anisotropy of the plasma particles. This paper reports for the first time on the effects of electron plasma properties on the oblique proton firehose (PFH) instability and provides a comprehensive vision of the entire unstable wave-vector spectrum, unifying the proton and the smaller electron scales. The plasma β and temperature anisotropy regimes considered here are specific for the solar wind and magnetospheric conditions, and enable the electrons and protons to interact via the excited electromagnetic fluctuations. For the selected parameters, simultaneous electron and PFH instabilities can be observed with a dispersion spectrum of the electron firehose (EFH) extending toward the proton scales. Growth rates of the PFH instability are markedly boosted by the anisotropic electrons, especially in the oblique direction where the EFH growth rates are orders of magnitude higher.

plasmas↗
NASA NTRS1985

Anomalous transport by Kelvin-Helmholtz instabilities

A simulation of magnetohydrodynamic Kelvin-Helmholtz instabilities has been performed for parallel and transverse configurations, modeling high latitude (or downstream flanks) and dayside low latitude magnetospheric boundaries. In the parallel configuration, a super-Alfvenic and transsonic shear flow develops into small eddies, which strongly compresses, twists, and hence amplifies the magnetic field by the dynamo action with an amplification factor of M(A)/2. In the nonlinear stage, however large the initial Alfven mach number M(A) may be, the magnetic field amplified and twisted by the hydromagnetic flow vortices reacts back upon the flow evolution, and the flow vortices cascade into smaller structures. In the transverse configuration, the instability leads to the formation of a fast shock discontinuity from an initially subfast shear flow. Anomalous tangential stress by the instability in the transverse configuration reaches 1 percent of the magnetosheath momentum flux, but for the parallel configuration, the anomalous transport is 2-3 times larger than the anomalous transport in the transverse configuration. The anomalous transport for both configurations satisfies the requirement of the viscous-like interaction at the magnetospheric boundary.

Miura, A.↗
NASA NTRS1981

Global simulation of auroral arcs

Numerical simulation of global formation of auroral arcs is carried out for a three-dimensional coupled ionosphere-magnetosphere system. With the presence of the stationary convection electric field, the field-aligned current driven by the polarization and drift of the ionospheric conductivity enhancement is shown to be responsible for the feedback instability of the shear Alfven wave. The simulation result demonstrates the dynamic process of auroral arc formation, where an ionospheric perturbation nearly resonant with the magnetospheric Alfven wave (toroidal mode) can eventually grow into auroral arcs. Many of the important characteristics of quiet auroral arcs are demonstrated, and it is shown that the overall distribution of auroras depends critically on the stationary state of the auroral oval characterized by the large-scale Birkeland current, the electric field, and the electron density distribution.

Miura, A.↗
NASA NTRS1992

Viscously driven plasma flows in the deep geomagnetic tail

An analysis, based on the principles of stress balance in a 1-dimensional current sheet is presented, which considers the problem of closed magnetic flux transport into the deep tail by a 'viscous'-like interaction between the solar wind and the magnetosphere. The analysis is illustrated with an example of ISEE-3 data showing strong tailward plasma sheet flows on apparently closed field lines in the deep tail. Apart from narrow regions adjacent to the magnetopause, these flows are not driven by the scattering of magnetosheath plasma into the magnetosphere. The fraction of the magnetosheath momentum flux needed to be anomalously transferred into the plasma sheet to drive the flows is estimated. In the example this is 6 percent. No previously suggested mechanism (e.g., the Kelvin-Helmholtz instability) has been shown capable of providing anomalous momentum transport of this magnitude. The current understanding of the 'viscous' interaction between the solar wind and magnetosphere is thus insufficient to explain these observations.

Owen, C. J.↗
NASA NTRS1976

Accretion onto magnetized neutron stars - Normal mode analysis of the interchange instability at the magnetopause

Results are reported for a linearized hydromagnetic stability analysis of the magnetopause of an accreting neutron star. The magnetosphere is assumed to be slowly rotating, and the plasma just outside the magnetopause is assumed to be weakly magnetized. The plasma layer is assumed to be bounded above by a shock wave and to be thin compared with the radius of the magnetosphere. Under these circumstances, the growing modes are shown to be localized in the direction parallel to the zero-order magnetic field, but the structure of the modes is still similar to the flute mode. An expression for the growth rate at each magnetic latitude is obtained in terms of the magnitude of the gravitational acceleration normal to the surface, the azimuthal mode number, the radius of the magnetosphere, the height of the shock above the magnetopause, and the effective Atwood number which embodies the stabilizing effects of favorable curvature and magnetic tension. The effective Atwood number is calculated, and the stabilizing effects of viscosity and aligned flow parallel to the magnetopause are discussed.

Arons, J.↗
NASA NTRS1975

Investigation of electron dynamics in the magnetosphere with electron beams injected from sounding rockets

The Electron Echo experiments are described which were conducted with three sounding rockets to study the basic mechanisms by which electrons precipitate from the geomagnetic field to produce auroral X-rays during periods of geomagnetic activity. Echo I (1970) injected electron pulses at low latitude and successfully measured the returning pulses from the conjugate region. Echo II (1972) was launched from high latitude, studied the interaction of electron beams with background radiation as well as beam motion near the rocket, and obtained evidence for a beam plasma instability. Echo III (1974) detected a series of conjugate echoes during the presence of a strong convective field in the magnetosphere and conducted an unsuccessful search for beam bunching at the local plasma frequency. It is shown that the electric field measurement in the ionosphere, using incoherent backscatter radar and detectors on the rocket, was transferred to the equatorial plane as though field lines were equipotentials.

Winckler, J. R.↗
NASA NTRS1993

Nonlinear unstable auroral-arc driven thermospheric winds in an ionosphere-magnetosphere coupled model

The nonlinear evolution of thermospheric winds in an ionosphere-magnetosphere coupled model has been studied for the first time for a dynamic unstable auroral-arc environment. We treat the problem using a multi-layer, quasi-three-dimensional model which averages in altitude the thermospheric dynamics over each layer. For the upper thermosphere, we find that (1) the thermosphere can respond to the ionospheric Kelvin-Helmholtz (KH) instability on temporal scales on the order of an hour, depending on ambient conditions, and on spatial scales of tens to hundreds of kilometers, (2) strong thermospheric meridional and zonal vortical flows with embedded nonlinear jet-like structures can be generated by the ionospheric/magnetospheric KH instability and (3) neutral thermospheric winds, vortices, and associated power spectra develop in a distinctly different manner in the presence of magnetospheric coupling effects. Comparison with recent observations is made.

Keskinen, M. J.↗
DOE OSTI2025

Statistical Survey of Magnetic Flux Integral Quantities in Saturn's Magnetosphere

Magnetic flux integral quantities (e.g., flux tube entropy, flux tube content) are conserved quantities under the frozen-in assumption. The change of these quantities often indicates the violation of the frozen-in condition (e.g., interchange instability). In this study, we combine the Cassini CAPS and CHEMS moments data with a steady-state magnetic field model (i.e., the Caudal model) to estimate the flux tube mass and flux tube entropy in Saturn's magnetosphere. Our statistical survey found that the flux tube mass rapidly decreases with radial distance away from Saturn in the inner magnetosphere and roughly levels out in the middle magnetosphere. This indicates that the radial transport processes could occur via a double-reconnection process in the inner magnetosphere, while the radial transport processes appear to be more advective in the middle magnetosphere. Notice that Saturn's magnetosphere is stabilized by a radially increasing profile of flux tube entropy and destabilized by a radially decreasing profile of flux tube content. In this study, we also estimate the expected penetration location by using the flux tube interchange stability formalism developed by Southwood and Kivelson (1987, https://doi.org/10.1029/ja092ia01p00109). The results show that flux tube entropy can play a crucial role in braking the injections, while the flux tube content has a relatively smaller influence on the injected flux tube, being consistent with our previous case study by Wing et al. (2022, https://doi.org/10.3847/1538-4357/ac85b2).

Ma, Xuanye [Embry-Riddle Aeronautical University, ↗
NASA NTRS1976

Plasma processes in space

Elementary microscopic interactions in plasmas are described. The importance of plasma physics in space studies is illustrated by examining several phenomena which cannot be explained satisfactorily by MHD theory. These include kinetic instabilities, plasma turbulence in the bow shock, magnetic turbulence near the moon, VLF emissions in the magnetosphere, planetary and solar radio emissions, and interaction of planetary and cometary plasmas with the solar wind. Evidence for the existence of anomalous transport processes in terrestrial and planetary magnetospheres is presented.

Wu, C. S.↗
NASA NTRS1990

Perpendicular heating of electrons by upper hybrid waves generated by a ring distribution

Satellite observations of electron conical distributions with enhanced fluxes just outside the loss cone suggest that telectrons have been heated perpendicularly to the magnetic field in the mid-altitude polar magnetosphere. To understand electron conical distributions, plasma simulations are conducted to examine an upper hybrid wave instability of a ring electron distribution perpendicular to the magnetic field in a cold electron background. The simulations indicate that both the cold and ring distributions are heated perpendicularly during the saturation stage. From the plasma data, a ring distribution can be identified as a trapped distribution function with an enhancement near 90-deg pitch angle in the phase space density plot. It is suggested that the ring distribution might provide an additional free energy source for generating upper hybrid waves associated with electron conical events.

Lin, C. S.↗
NASA NTRS1992

On radial oscillations in viscous accretion discs surrounding neutron stars

Radial oscillations resulting from axisymmetric perturbations in viscous accretion disks surrounding neutron stars in X-ray binary systems have been investigated. Within the framework of the alpha-viscosity model a series of hydrodynamic calculations demonstrates that the oscillations are global for alpha of about 1. On the other hand, for alpha of 0.4 or less, the oscillations are local and confined to the disk boundaries. If viscous stresses acting in the radial direction are included, however, it is found that the disk can be stabilized. The application of such instabilities in accretion disks, without reference to the boundary layer region between the neutron star (or magnetosphere) and the inner edge of the disk, to the phenomenology of quasi-periodic oscillations is brought into question.

Chen, Xingming↗
NASA NTRS2022

Hybrid Kinetic Model of the Interaction Between the Dense Plasma Clouds and Magnetospheric Plasma on Large Time and Spatial Scales, and Comparison With MMS Observations

We present a new simulation results of the cloud dynamics in the ambient magnetospheric plasma on the large time and spatial scales. It was assumed that these impulsive structures observed by the MMS spacecraft originally were created because of the reconnection at the magnetopause. Our new 3-D hybrid kinetic modeling on the large time and spatial scales captures several of these processes: an excitation of the electromagnetic waves (whistler and shear-Alfvén waves) and plasma instabilities (mirror and flute); a formation of shock waves, and collapsing diamagnetic cavity; particle acceleration. A strong overshoot in plasma density profile was observed in the modeling and MMS observation at the interface between the cloud and magnetospheric plasma. The cloud expansion into ambient magnetospheric plasma causes the flute waves connected with excitation of the Rayleigh-Taylor instability observed at the overshoot in plasma density profile across the external magnetic field. The modeling demonstrates a formation of the whistler waves at the initial stage which propagate in the external magnetic field direction. At the later stage, a formation of shear-Alfvén waves was observed

A. S. Lipatov↗
NASA NTRS1999

Kelvin Helmholtz Instability at the Equatorial Magnetotail Boundary: MHD Simulation and Comparison with Geotail Observations

On March 24, 1995 the Geotail spacecraft observed large fluctuations of the magnetic field and plasma properties in the Low Latitude Boundary Layer (LLBL) about 15 R(sub E) tailward of the dusk meridian. Although the magnetospheric and the magnetosheath field were strongly northward, the B(sub z) component showed strong short duration fluctuations in which B(sub z) could even reach negative values. We have used two-dimensional magnetohydrodynamic simulations with magnetospheric and magnetosheath input parameters specifically chosen for this. Geotail event to identify the processes which cause the observed boundary properties. It is shown that these fluctuations can be explained by the Kelvin-Helmholtz instability if the k vector of the instability has a component along the magnetic field direction. The simulation results show many of the characteristic properties of the Geotail observations. In particular, the quasi-periodic strong fluctuations are well explained by satellite crossings through the Kelvin-Helmholtz vortices. It is illustrated how the interior structure of the Kelvin-Helmholtz vortices leads to the rapid fluctuations in the Geotail observations. Our results suggest an average Kelvin-Helmholtz wavelength of about 5 R(sub E) with a vortex size of close to 2 R(sub E) for an average repetition time of 2.5 minutes. The growth time for these waves implies a source region of about 10 to 16 R(sub E) upstream from the location of the Geotail spacecraft (i.e., near the dusk meridian). The results also indicate a considerable mass transport of magnetosheath material into the magnetosphere by magnetic reconnection in the Kelvin-Helmholtz vortices.

Fairfield, Donald H.↗
NASA NTRS1988

Nature and the nonlinear evolution of electrostatic waves associated with the AMPTE solar wind releases

The nonlinear evolution of the electrostatic waves observed during the AMPTE (Active Magnetosphere Particle Tracer Explorers) solar wind releases is investigated. Previous linear studies indicate two distinct sets of instabilities may be responsible for the generation. One set consists of ion-acoustic type instabilities which are insensitive to the presence of a background magnetic field, while the other group corresponds to the modified two stream instabilities and requires the solar wind flow to be across the ambient magnetic field. To establish which set of instabilities is more viable a detailed linear Vlasov theory has been conducted by numerically solving the full electromagnetic dispersion relation. In addition both the plasma wave as well as the magnetic field measurements by the IRM (Ion Release Module) spacecraft were used to correlate the frequency and the power of the observed waves with the magnitude and the direction of the solar wind magnetic field. The results of these analyses indicate that the ion-acoustic type instabilities have growth rates that are an order of magnitude or more larger than those of the modified two stream instabilities. Results show that both the solar wind protons and the released ions may be heated and accelerated in the directions oblique to the solar wind flow velocity.

Omidi, N.↗
NASA NTRS1995

Dawn-dusk asymmetries in the low-latitude boundary layer arising from the Kelvin-Helmholtz instability: A particle simulation

Along the low-latitude boundary layer (LLBL), the Kelvin-Helmholtz (K-H) instability can provide a means for injection of solar wind plasma across closed field lines of the magnetosphere. A fully electromagnetic, two-dimensional (three-velocity) particle code is used to investigate dawn-dusk asymmetries that can arise from velocity differences due to gradient drifts and electric field gradients, and ion acceleration across the narrow field transition layers in the dawn and dusk flanks. The model includes the dawn and dusk sides of the LLBL simultaneously in a slab geometry, incorporating seperate populations of ions and electrons for the magnetosphere and the magnetosheath plasmas. We report several effects: (1) asymmetries in the observed morphology of the turbulent structures, with familiar fluidlike vortex formation on the duskside, but tongues of magnetosheath plasma penetrating into the magnetosphere on the dawnside; (2) the formation of discrete current layers, characterized by strong currents and sharp gradients in the magnetic field, and discrete charge layers, having net charges and constant, weaker currents; (3) increasing asymmetry in dawn/dusk behavior with a decrease of initial currents; (3) increasing asymmetry in dawn/dusk behavior with a decrease of initial boundary layer width; and (4) enhancement of the dawn-to-dusk electric field as magnetosheath particles and momenta are transported across the magnetopause.

Wilber, M.↗
NASA NTRS1985

Coupling of newborn ions to the solar wind by electromagnetic instabilities and their interaction with the bow shock

The process by which the solar wind assimilates newly ionized atoms is important for understanding the presence of planetary or interstellar helium in the solar wind, the dynamics of the Active Magnetospheric Particle Tracer Explorers (AMPTE) lithium releases in front of the earth's bow shock, and the formation of cometary tails. In this paper is examined how newborn ions can be coupled to the solar wind in the direction parallel to the magnetic field by means of electromagnetic instabilities driven by the distribution of newborn ions. The linear properties of three instabilities are analyzed and compared with numerical solutions of the linear dispersion equation, while their nonlinear behavior is followed by means of computer simulation to obtain the characteristic time for the pickup process. With a primary emphasis on the AMPTE lithiuim releases, various degrees of realism are introduced into the calculations to model the upstream conditions and the intersection of the lithium with the bow shock. It is shown that a time-dependent shock model is needed to correctly reproduce the amount of lithium which is transmitted through the shock and that the resulting lithium ion distribution is still likely to be subject to the same type of instabilities in the magnetosheath. Applications of these results to comets, in particular the artificial comet expected to be generated by the AMPTE barium release in the magnetosheath, is also briefly discussed.

Winske, D.↗
NASA NTRS1974

Barium cloud evolution and striation formation in the magnetospheric release on September 21, 1971

The joint NASA-Max Planck Institute Barium Ion Cloud (BIC) Experiment on September 21, 1971 involved the release of 1.7 kg of neutral barium at an altitude of 31,500 km at a latitude of 6.93 deg N. and a longitude of 74.40 deg W. A theoretical model describing the barium neutral cloud expansion and the ion cloud formation is developed. The mechanism of formation of the striational features observed in the release is also discussed. Two candidate instabilities, which may contribute to striation formation, are examined. The drift instability stemming from the outwardly directed drag force exerted on the ions by the outstreaming neutrals is rejected on the grounds that the ion density is too low during the collision-dominated phase of the cloud expansion to support this kind of instability. The joint action of Rayleigh-Taylor and flute instabilities plausibly accounts for the observed striational structure. This same mechanism may well be operative at times of sudden injection of plasma into the inner magnetosphere during geomagnetic storms and may thus contribute to the formation of field-alined inhomogeneities which serve as whistler ducts.

Adamson, D.↗