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NASA NTRS1983

Backstreaming ions from oblique earth bow shocks

One-dimensional, hybrid (particle ions, fluid electrons) numerical simulations of oblique shock structure are used to analyze the mechanism by which solar wind ions can be reflected off the earth's bow shock front and escape upstream. It is shown that no backstreaming ions are produced in the simulation over 50-90 degrees of theta(nB), the angle between the interplanetary magnetic field and the shock normal. Backstreaming ions are emitted for theta(nB) of 45 degrees or less, with characteristics in reasonable agreement with those of observed reflected ion streams. The simulated shock structure becomes rather turbulent as theta(nB) decreases from 45 to 30 degrees. Analysis of the backstreaming ion trajectories shows that all backstreaming ions are originally specularly reflected. The oblique shock structure and effects such as whistler formation and ion heating are discussed.

Leroy, M. M.↗

NASA NTRS1985

Ion and electron heating at collisionless shocks near the critical Mach number

The present study has the objective to document more fully a somewhat surprising heating pattern for 10 of the bow shock crossings investigated by Russell et al. (1982). A description of the detailed evolution of the actual distribution functions across the subcritical shocks is also provided. It is pointed out that the shocks examined are all very near the theoretical one-fluid critical Mach number. None of the shocks has a significant magnetic foot or overshoot. It is argued that the observed heating is not due to reflected ions but rather to physical processes characteristic of subcritical shocks.

Thomsen, M. F.↗

NASA NTRS1985

A time-dependent theoretical model of the polar wind Preliminary results

The coupled, time-dependent continuity, momentum, and energy equations of a two-ion O(+) and H(+) quasi-neutral plasma were solved in order to extend understanding of polar wind behavior. This numerical code allows studies of the time dependent behavior of polar wind-type flows into and out of the ionosphere. Initial studies indicate that the typical time constants for electron and ion temperature changes are of the order of minutes and tens of minutes, respectively. The response time of the minor high altitude ion O(+) is less than an hour, whereas that of the major ion, H(+), is many hours. The initial test runs also demonstrate the fact that temporary supersonic flows of both O(+) and H(+) are possible, especially in the presence of significant ion heating.

Gombosi, T. I.↗

NASA NTRS1985

Spacecraft-induced plasma energization and its role in flow phenomena

Plasma instabilities induced by orbiting vehicles can cause many important phenomena ranging from electron and ion heating and suprathermal electron tail energization, to enhanced ionization and optical emissions. We outline the basic collective processes leading to plasma energization near plasma sheaths and in regions of neutral gas streaming through plasma, and discuss the role of the induced collective effects in producing the optical emission spectra.

Papadopoulos, K.↗

NASA NTRS1985

ISEE observations of the magnetopause - Reconnection and the energy balance

The total energy balance for two events with the objective of obtaining check on the interpretation in terms of reconnection is examined. To within experimental uncertainties, the plasma and magnetic field data are consistent with reconnection. An enthalpy increase comparable to the kinetic energy increase occurs in the magnetopause. Thus substantial dissipation is present in the rotational discontinuity. An ion heat flow associated with a beam of reflected magnetosheath particles carried away some 20 percent of the total converted electromagnetic energy.

Paschmann, G.↗

NASA NTRS1987

Advances in magnetospheric physics by numerical simulations - A critical analysis by an outsider

This is a selective, somewhat editorialized assessment, based on a literature survey, of computer simulation as it exists today in magnetospheric physics. Both large (MHD) and small scale dynamical simulations are described and considered from the perspective of what they are trying to do and with what success. Several specific problem areas where simulations are being carried out are called for commentary: global magnetospheric structure, magnetotail and magnetopause reconnection, Kelvin-Helmholtz instability, hydrodynamic expansion of ionospheric ions, electric double layers, heavy ion heating, and auroral kilometric radiation. It is concluded that simulation is a necessary tool for understanding magnetospheric physics and that significant progress has been made in simulation development. However, results should be evaluated knowing that many factors, some real and physical, others structural, may contribute to such output. A plea is made for greater intercalibration among different simulators working in parallel areas, so that facts can be distinguished from artifacts.

Birmingham, Thomas J.↗

NASA NTRS1988

Simulations of active ion injection experiments on ARCS 3

The ARCS 3 sounding rocket experiment used two argon guns oriented parallel and perpendicular to the geomagnetic field. The parameters of this experiment have been used in a series of particle simulations in order to interpret ion beam interaction with the multispecies ionospheric plasma. Along with the lower hybrid mode between the electron and the hydrogen gyrofrequencies, ion-ion hybrid modes between the hydrogen and helium gyrofrequencies and between the helium and oxygen gyrofrequencies are identified in the simulations. Besides these cold plasma modes, warm Bernstein modes of the respective ion species are identified. The parallel argon beam injections excite lower hybrid waves perpendicular to the magnetic field because of the finite thermal spread of the parallel beam. Perpendicular ion heating of background hydrogen, helium, and oxygen is comparable in the simulations. Comparisons are made with the wave and particle data from the ARCS 3 rocket experiment.

Hudson, M. K.↗

NASA NTRS1988

Ion equation of state in quasi-parallel shocks - A simulation result

Ion equation of state in the quasi-parallel collisionless shock is deduced from simulation results. The simulations were performed for theta(bn) = 10 deg, beta = 0.5 and M sub A in the range from 1.2 to 8, where M sub A is the Alfven Mach number, beta is the upstream ratio of plasma pressure to magnetic pressure, and theta(bn) is the angle between the shock normal and the upstream magnetic field. The equation of state can be approximated by a power law with different exponents in the upstream and downstream sides of the shock transition region. The exponent in the upstream side of the transition region is much greater than the adiabatic value of 5/3 and increases with M sub A. The exponent in the downstream side of the transition region is slightly less than 5/3. The results show that ion heating in the quasi-parallel shock is highly nonadiabatic with a large increase in entropy and in temperature ratio in the upstream side of the transition region, while the heating is highly isentropic with a large increase in temperature difference across the principal density jump in the downstream side of the transition region.

Mandt, M. E.↗

NASA NTRS1988

Transverse ion energization and low-frequency plasma waves in the mid-altitude auroral zone - A case study

Evidence of transverse ion energization at altitudes of several earth radii in the auroral zone was reexamined using several hundred hours of high-sensitivity and high-resolution plasma data obtained by the Dynamics Explorer 1 satellite. The data on particle environment encountered at midaltitudes in the auroral zone disclosed rapid variations in the values of total density, thermal structure, and composition of the plasma in the interval measured; the modes of low-frequency plasma waves also varied rapidly. It was not possible to unambiguously identify in these data particle and wave signature of local transverse ion energization; however, many intervals were found where local transverse ion heating was consistent with the observations.

Peterson, W. K.↗

NASA NTRS1991

Interaction of a finite-length ion beam with a background plasma - Reflected ions at the quasi-parallel bow shock

The coupling of a finite-length, field-aligned, ion beam with a uniform background plasma is investigated using one-dimensional hybrid computer simulations. The finite-length beam is used to study the interaction between the incident solar wind and ions reflected from the earth's quasi-parallel bow shock, where the reflection process may vary with time. The coupling between the reflected ions and the solar wind is relevant to ion heating at the bow shock and possibly to the formation of hot, flow anomalies and re-formation of the shock itself. Consistent with linear theory, the waves which dominate the interaction are the electromagnetic right-hand polarized resonant and nonresonant modes. However, in addition to the instability growth rates, the length of time that the waves are in contact with the beam is also an important factor in determining which wave mode will dominate the interaction. It is found that interaction will result in strong coupling, where a significant fraction of the available free energy is converted into thermal energy in a short time, provided the beam is sufficiently dense or sufficiently long.

Onsager, T. G.↗

NASA NTRS1993

The thermal structure of the magnetized solar transition region

The detailed thermal structure of the magnetized solar transition region, as measured by its differential emission measure DEM(T), is unknown. Proposals have been made that envision a significant lower-temperature contribution to the energy balance from cross-field (ion) heat flux. In this paper, we describe a self-consistent 2D MHD simulation (including the full effects of anisotropic thermal conduction) of a conceptual model due to Athay (1990). We display the detailed irregular thermal and magnetic structure of the transition region and demonstrate that the predicted DEM agrees with observations, particularly in the T less than 10 exp 5 K regime where previous theories had difficulty.

Mok, Y.↗

NASA NTRS1993

The differential emission measure of nested hot and cool magnetic loops

The detailed thermal structure of the magnetized solar transition region, as measured by its differential emission measure (DEM(T)), is poorly known. Building on the fact that the solar surface is strongly magnetized and thereby structured, proposals have been made that envision a significant lower-temperature contribution to the energy balance from (ion) heat flux across an arcade of different temperature loops. In this paper, we describe a self-consistent 2D MHD simulation, which includes the full thermal effects of parallel stability and anisotropic conduction, of a nested-loop model of the thermal and magnetic structure of the transition region. We then demonstrate that the predicted DEM agrees with observations in the conceptually elusive T less than 10 exp 5 K regime.

Van Hoven, G.↗

NASA NTRS1992

Energy dissipation in substorms

The energy dissipated by substorms manifested in several ways is discussed: the Joule dissipation in the ionosphere; the energization of the ring current by the injection of plasma sheet particles; auroral election and ion acceleration; plasmoid ejection; and plasma sheet ion heating during the recovery phase. For each of these energy dissipation mechanisms, a 'rule of thumb' formula is given, and a typical dissipation rate and total energy expenditure is estimated. The total energy dissipated as Joule heat (approximately) 2 x 10(exp 15) is found about twice the ring current injection term, and may be even larger if small scale effects are included. The energy expended in auroral electron precipitation, on the other hand, is smaller than the Joule heating by a factor of five. The energy expended in refilling and heating the plasma sheets is estimated to be approximately 5 x 10(exp 14)J, while the energy lost due to plasmoid ejection is between (approximately) (10 exp 13)(exp 14)J.

Weiss, Loretta A.↗

NASA NTRS1994

Ionospheric response to the sustained high geomagnetic activity during the March '89 great storm

A simulation was conducted to model the high-latitude ionospheric to the sustaied level of high geomagnetic activity for the great magnetic storm period of March 13-14, 1989. The geomagnetic and solar activity indices and the Defense Meterological Satellite Program (DMSP) F8 and F9 satellite data for particle precipitation and high-latitude convection were used as inputs to a time-dependent ionospheric model (TDIM). The results of the TDIM were compared to both DMSP plasma density data and ground-based total electron content (TEC) measurements for the great storm period as well as with earlier storm observations. The comparisons show that the overall structure of the high-latitude ionosphere was dominated by an increased convection speed within the polar cap that led to increased ion temperatures. In turn, this enhanced the NO(+) density, raised the atomic-to-molecular ion transition height to over 300 km, decreased N(sub m)F(sub 2), increased h(sub m)F(sub 2), and in places either increased n(sub e) at 800 km or slightly decreased it. The morphology of the ionosphere under these extreme conditions was considerably different than that modeled for less distributed intervals. These differences included the character of the dayside tongue of ionization that no longer extended deep into the polar cap. Instead, as a result of the ion heating and consequent reduction in N(sub m)F(sub 2), a large polar hole occupied much of the polar region. This polar hole extended beyond the auroral oval and merged with the night sector midatitude trough. The limitaions associated with the applicability of the TDIM to the geomagnetic conditions present on March 13 and 14 are discussed. The primary limitations of the TDIM derive from the limited temporal resolution of the model input parameters and the lack of suitably dynamic thermospheric specification for the great storm conditions. These limitations leads to midlatitude ionospheric storm phases that do no follow those observed.

Sojka, J. J.↗

NASA NTRS1994

Modulational instability of lower hybrid waves at the magnetopause

The role of lower bybrid waesat the magnetopause is reexamined. It is found that for the maximum observed wave power, the lower hybrid waves are unstable to a modulational instability on the magnetosheath side of the magnetopause. The modulational instabitlity leads to localized field structures oriented predominantly along the magnetic field. Such patchy lower hybrid turbulence has been observed by some spacecraft. As a result of the large T(sub i)/T(sub e) ratio, the waves saturate by ion heating; as a result, unlike other settings (e.g. comets, critical ionization phenomena) energetic electrons are not expected. The stochasitc electron transport in the presence of such turbulence is analyzed and results in strongly anistropic electron diffusion, with the dominant direction across the magnetic field. The diffusion rate exceeds significantly that expected from quasi-linear considerations and, for magnetospause parameters, also exceeds the rate discussed by Sonnerup (1980).

Shapiro, V. D.↗

NASA NTRS1995

Diffusion at the magnetopause: Hybrid simulations

Electromagnetic wave generation and resulting cross-field diffusion of plasma are considered at a tangential discontinuity, which characterizes the magnetopause for northward interplanetary magnetic field. Two-dimensional hybrid (particle ions, massless fluid electrons) simulations, in which the tangential discontinuity is generated self-consistently via a stream-stream interaction, are used to show that wave growth occurs when the ambient magnetic field is predominantly perpendicular to the direction of the density gradient. Low-frequency (much less than ion gyrofrequency) waves, with amplitudes delta B/B less than or equal to 0.2 and anticorrelated density fluctuations delta n/n less than or equal to 0.6, are generated at the discontinuity, resulting in cross-field diffusion that is comparable to the Bohm rate. Both the fluctuation level and the lack of ion heating in the calculations are consistent with observations at the magnetopause. The magnitude of the diffusion is considered in the presence of numerical effects and in the context of the inferred diffusion rate at the magnetopause. The relation of the low-frequency waves and their consequences to faster growing, short-wavelength waves due to the lower hybrid drift instability is also addressed. The overall conclusion of this initial study is that diffusion due to low frequency waves is not likely to be a major effect at the magnetopause.

Winske, D.↗

NASA NTRS1995

A dynamic model of the three-fluid solar wind: Equilibrium structures and fluxes

We report results from a dynamic approach to equilibrium for a three fluid solar-wind model in spherical 1D. We advance the mass, momentum and energy transport equations for protons, alpha particles and electrons, with full interspecies coulomb coupling and the simplifications recommended by Burgi (1992). They include no ion heat conduction and an ambipolar electric field in place of the negligible electron momentum coupling. At the inner boundary (the solar surface) we specify the inflow velocity, mass flux, and temperature for each species, and at the outer boundary at large radius we allow the fluids to flow freely from the computational domain. Starting from an initial wind model (e.g.. single fluid) we evolve to a three-fluid steady state. We will describe comparisons with Burgi's static equilibrium results. and examine the stability of the discontinuities which appear in his derived parameter variations.

Ruden, S. P.↗

NASA NTRS2000

Investigating the Role of the Earth's Ionosphere in Space Weather: Modeling and Observations of High-Latitude Ionospheric Outflows

We will present what we believe to be the first reported observations of up- and downflows of topside ionospheric thermal plasmas from multiple near-simultaneous tracks through the high-latitude topside ionosphere. From several Southern polar passes, it has been possible to construct plots of field-aligned flows of 0+ observed by the Thermal Ion Dynamics Experiment(TIDE) on the POLAR spacecraft near 5000 km altitude together with vertical ion flow observations from one or more DMSP spacecraft near 800 km altitude. We also will present simultaneous observations of POLAR auroral LTVI images with field-aligned flows. We will further show new fluid-kinetic simulations of auroral ionospheric flows in the altitude range 120 km to several R-E as subjected to the synergistic auroral effects of soft electron precipitation, transverse wave-driven ion heating, and hot plasma-driven electric potentials.

Horwitz, J. L.↗