Engineering topics

Kennel, C. F.

Publications and source records attributed to Kennel, C. F..

At least 37 records · Page 2

NASA NTRS1991

Lightning and plasma wave observations from the Galileo flyby of Venus

Durig the Galileo flyby of Venus the plasma wave instrument was used to search for impulsive radio signals from lightning and to investigate locally generated plasma waves. A total of nine events were detected in the frequency range from 100 kilohertz to 5.6 megahertz. Although the signals are weak, lightning is the only known source of these signals. Near the bow shock two types of locally generated plasma waves were observed, low-frequency electromagnetic waves from about 5 to 50 hertz and electron plasma oscillation at about 45 kilohertz. The plasma oscillations have considerable fine structure, possibly because of the formation of soliton-like wave packets.

Gurnett, D. A.↗

NASA NTRS1991

Alfven shock trains

The Cohen-Kulsrud-Burgers equation (CKB) is used to consider the nonlinear evolution of resistive, quasi-parallel Alfven waves subject to a long-wavelength, plane-polarized, monochromatic instability. The instability saturates by nonlinear steepening, which proceeds until the periodic waveform develops an interior scale length comparable to the dissipation length; a fast or an intermediate shock then forms. The result is a periodic train of Alfven shocks of one or the other type. For propagation strictly parallel to the magnetic field, there will be two shocks per instability wavelength. Numerical integration of the time-dependent CKB equation shows that an initial, small-amplitude growing wave asymptotes to a stable, periodic stationary wave whose analytic solution specifies how the type of shock embedded in the shock train, and the amplitude and speed of the shock train, depend on the strength and phase of the instability. Waveforms observed upstream of the earth's bowshock and cometary shocks resemble those calculated here.

Malkov, M. A.↗

NASA NTRS1990

Chaos in driven Alfven systems

The chaos in a one-dimensional system, which would be nonlinear stationary Alfven waves in the absence of an external driver, is characterized. The evolution equations are numerically integrated for the transverse wave magnetic field amplitude and phase using the derivative nonlinear Schroedinger equation (DNLS), including resistive wave damping and a long-wavelength monochromatic, circularly polarized driver. A Poincare map analysis shows that, for the nondissipative (Hamiltonian) case, the solutions near the phase space (soliton) separatrices of this system become chaotic as the driver amplitude increases, and 'strong' chaos appears when the driver amplitude is large. The dissipative system exhibits a wealth of dynamical behavior, including quasiperiodic orbits, period-doubling bifurcations leading to chaos, sudden transitions to chaos, and several types of strange attractors.

Hada, T.↗

NASA NTRS1990

Comparison of plasma wave measurements in the bow shocks at Earth, Jupiter, Saturn, Uranus and Neptune

Plasma wave measurements from the Voyager 2 crossing of Neptune's bow shock are presented and compared with measurements from the bow shocks of Earth, Jupiter, Saturn, and Uranus. The wave amplitudes above 0.01fp, when normalized to the solar wind ion thermal energy density at each planet, are significantly higher at the outer planets than at Earth. Despite the differences in amplitude, the shock spectra of all the planets can be fitted to curves of similar form in this frequency range. The total normalized electric field energy densities exhibit an exponential dependence on ion thermal Mach number, Magnetosheath wave energies are comparable at all of the planets when normalized to the downstream plasma pressure.

Moses, S. L.↗

NASA NTRS1990

Ion precipitation from the inner plasma sheet due to stochastic diffusion

Plasma sheet ions do not conserve their first adiabatic invariant when the magnetic field is appreciably tail-like. They do conserve a different adiabatic invariant but only to linear, rather than exponential, accuracy in the appropriate small parameter. Thus significant stochastic diffusion can occur for particles crossing the separatrix dividing the segments of orbits on which the particles cross and do not cross the tail midplane. Such ions can escape the plasma sheet and precipitate into the atmosphere. Stochastic scattering is strongest from those field lines where the ion's Larmor period in the normal component of the neutral sheet magnetic field approximately equals its bounce period. By comparing the rates of stochastic ion loss and convection in the tail, it is possible to estimate the location and thickness of the inner edge of the ion plasma sheet created by stochastic ion loss. Ions of different masses precipitate into the atmosphere at slightly different locations. Since wave particle interactions are not needed, this precipitation will always occur and should be particularly evident during quiet geomagnetic conditions, when it is less likely to be masked by other precipitation mechanisms.

Zelenyi, L.↗

NASA NTRS1990

Structure and evolution of small-amplitude intermediate shock waves

A simplified set of equations is derived that approximates the magnetohydrodynamic (MHD) Navier-Stokes equations for weakly nonlinear disturbances whose speeds are close to the MHD intermediate speed. Its shock structure solutions are then examined. The fast and slow shock solutions are uniquely specified by their Rankine-Hugoniot relations. However, the intermediate shock solutions, which are not unique, are characterized by the integral through the shock of the noncoplanar component of the magnetic field. For situations in which this integral is conserved, the Riemann problem is well defined and predicts the evolution of intermediate shocks. This analysis is substantiated by numerical computations.

Kennel, C. F.↗

NASA NTRS1989

Electrostatic waves in the bow shock at Uranus

Electrostatic emissions measured by the Voyager 2 plasma wave detector (PWS) during the inbound crossing of the Uranian bow shock are shown to differ in some aspects from the waves measured during bow shock crossings at Jupiter and Saturn. The wave amplitudes in the foot of the bow shock at Uranus are in general much lower than those detected at the other outer planets due to the unusually enhanced solar wind ion temperature during the crossing. This reduces the effectiveness of wave-particle interactions in heating the incoming electrons. Strong wave emissions are observed in the shock ramp that possibly arise from currents producing a Buneman mode instability. Plasma instrument (PLS) and magnetometer (MAG) measurements reveal a complicated shock structure reminiscent of computer simulations of high-Mach number shocks when the effects of anomalous resistivity are reduced, and are consistent with high ion temperatures restricting the growth of electrostatic waves.

Moses, S. L.↗

NASA NTRS1989

MHD intermediate shock discontinuities. I - Rankine-Hugoniot conditions

Recent numerical investigations have focused attention once more on the role of intermediate shocks in MHD. Four types of intermediate shock are identified using a graphical representation of the MHD Rankine-Hugoniot conditions. This same representation can be used to exhibit the close relationship of intermediate shocks to switch-on shocks and rotational discontinuities. The conditions under which intermediate discontinuities can be found are elucidated. The variations in velocity, pressure, entropy and magnetic-field jumps with upstream parameters in intermediate shocks are exhibited graphically. The evolutionary arguments traditionally advanced against intermediate shocks may fail because the equations of classical MHD are not strictly hyperbolic.

Kennel, C. F.↗

NASA NTRS1989

Dynamics of Mars' magnetosphere

If Mars has a small intrinsic magnetic moment, Mars' magnetosphere could vary on time scales of a few minutes due to reconnection with the solar wind magnetic field. The day-side magnetopause will be one or two reflected-ion Larmor radii from the bow shock. Substorms will have scale-times of about six minutes. Mars' high ionospheric conductance will virtually stop polar cap convection, and create a magnetic 'topological crisis' unless convecting magnetic flux finds a dissipative way to return to the day-side. The strong magnetic shear induced by magnetospheric convection above the ionosphere could be tearing unstable. The magnetic field might diffusively 'percolate' through the tearing layer. This shearing also draws field aligned currents from the ionosphere which could inject few KeV heavy ionospheric ions into the magnetotail.

Kennel, C. F.↗

NASA NTRS1989

Stationary nonlinear Alfven waves and solitons

Stationary solutions of the derivative nonlinear Schroedinger equation are discussed and classified by using a pseudopotential formulation. The solutions consist of a rich family of nonlinear Alfven waves and solitons with parallel and oblique propagation directions. Expressions for the envelope and the phase of nonlinear waves with periodic envelope modulation, and 'hyperbolic' and 'algebraic' solitons are given. The propagation angle for the slightly modulated elliptic, periodic waves and for oblique solitons is evaluated.

Hada, T.↗

NASA NTRS1988

Switch-on shocks

It is argued that parallel propagating Alfven waves steepen to second order in the wave amplitude, rather than first order, as is the case for oblique MHD waves. An analytical solution for the structure of weak switch-on shocks is derived. The Rankine-Hugoniot relations for finite amplitude switch-on shocks are obtained in a compact form. The differential equation for resistive switch-on shocks is solved, and the resistive critical Mach number is obtained. The differential equation for resistive switch-on shocks with finite ion inertia is identical to the one obtained in the pure resistive case. It is argued that efficient electron heat conduction can significantly extend the range of upstream parameters for which switch-on shocks are expected.

Kennel, C. F.↗

NASA NTRS1988

Generation, saturation, and convection of electrostatic waves in Jupiter's shock foot

In this paper, a model is developed for the analysis of the electrostatic waves produced in the shock foot at Jupiter. It is shown that an ion beam instability involving the ions reflected at the shock ramp and the incoming solar-wind electrons produces waves at the observed frequencies and that saturation via orbit diffusion limits the waves to amplitudes near to what is observed. Results from a two-dimensional model of the reflected beam in the foot indicate that the waves propagate against the solar wind away from the shock ramp and are amplified up to their saturation amplitudes. The saturation results, combined with the electron temperature profile due to wave-particle interactions predicted by quasi-linear theory, reproduce a wave amplitude profile for the shock foot that is in reasonable agreement with the observations.

Moses, S. L.↗

NASA NTRS1988

Nonlinear, dispersive, elliptically polarized Alfven wavaes

The derivative nonlinear Schroedinger (DNLS) equation is derived by an efficient means that employs Lagrangian variables. An expression for the stationary wave solutions of the DNLS that contains vanishing and nonvanishing and modulated and nonmodulated boundary conditions as subcases is then obtained. The solitary wave solutions for elliptically polarized quasiparallel Alfven waves in the magnetohydrodynamic limit (nonvanishing, unmodulated boundary conditions) are obtained. These converge to the Korteweg-de Vries and the modified Korteweg-de Vries solitons obtained previously for oblique propagation, but are more general. It is shown that there are no envelope solitary waves if the point at infinity is unstable to the modulational instability. The periodic solutions of the DNLS are characterized.

Kennel, C. F.↗

NASA NTRS1988

A search for lower hybrid drift turbulence in slow shocks

The slow shocks which have been observed in the distant geomagnetic tail by ISEE 3 do not exhibit the large-amplitude magnetic field rotations which are theoretically expected. A possible explanation is that the rotation trailing wave train is resistively damped by strong lower hybrid drift turbulence excited in the shock front. Magnetic wave spectra for three slow shocks are constructed from ISEE 3 dc magnetometer and search coil experiments. The observed magnetic amplitudes near the lower hybrid frequency are below the level at which anomalous resistance would damp the slow shock magnetic rotations.

Coroniti, F. V.↗

NASA NTRS1988

Wave particle interactions in the foot of the Saturnian bow shock

Voyager 1 plasma wave data show that the quasi-perpendicular, supercritical bow shock of Saturn exhibits the same plasma wave phenomenology observed near Jupiter. Using a quasi-linear model developed for the Jovian bow shock, it is shown that plasma waves at Saturn can generate a significant portion of the totalelectron temperature jump measured across the shock. In this respect, Saturn's bow shock more closely resembles Jupiter's than the earth's, where plasma waves contribute negligibly to the total electron temperature jump.

Moses, S. L.↗

NASA NTRS1987

Plasma wave measurements in the magnetosphere of Uranus

As Voyager 2 traversed the magnetosphere of Uranus, the plasma-wave instrument detected very significant phenomena related to local wave-particle interactions, radio emissions, and dust impacts. Here the region between the inbound and outbound bow shock traversals (covering a distance of more than 250 Uranus radii) is considered, and it is demonstrated that intense plasma-wave activity developed only in the inner magnetosphere (r less than 12 Uranus radii); this result is similar to that found at Saturn, but it is in marked contrast with the Jupiter case, where very strong wave activity was detected out to distances of 250 Jupiter radii. The Uranus plasma-wave observations in the inner magnetosphere are compared with corresponding results from the Jupiter and Saturn encounters, and it is shown that the Uranus wave measurements are unique in several significant ways. These new aspects include: (1) the detection of a marked inbound-outbound asymmetry and (2) the detection of whistler-mode waves that yield the strongest wave-particle interactions found in outer planet magnetospheres.

Scarf, F. L.↗

NASA NTRS1987

Whistler mode emissions in the Uranian radiation belts

Voyager 2 detected intense whistler mode emissions and fluxes of energetic electrons during the outbound pass through the region of auroral L shells. The observed energetic (E greater than 22 keV) electron distribution, a model warm (E less than 27.5 keV) electron distribution, and the cold plasma density profile deduced by Kurth et al. (1987) are used to calculate the ray path-integrated spatial amplification of whistlers which arrive at Voyager 2 from the magnetic equator. By matching the calculated amplification and the relative gains at different frequencies deduced from the observed whistler power spectrum, the pitch angle anisotropy parameters of the electron distributions are determined to within a fairly narrow range of values. The estimated bounce average pitch angle diffusion coefficient indicates that electrons are on strong diffusion over a wide range in energies. The electron precipitation energy flux is sufficient to produce the observed auroral light emissions.

Coroniti, F. V.↗

NASA NTRS1987

When will a pulsar in supernova 1987a be seen?

The means by which a pulsar might be detected in the remnant of supernova 1987a in the Large Magellanic Cloud is examined. One possibility is that the slower-than-radioactive decay typically seen in the type II light curves is itself the sign of powering by the underlying pulsar, with the decline representing not the spinning down of the pulsar but rather the declining nebular opacity that would allow increasing amounts of the energy to escape as gamma rays. The test of this hypothesis (if the supernova conforms to type II expectations) would be to look for the 'missing' energy in the form of those gamma rays that escape from the remnant instead of powering it.

Michel, F. Curtis↗