Engineering topics

Isenberg, P. A.

Publications and source records attributed to Isenberg, P. A..

At least 19 records

NASA NTRS2011

Transport of Solar Wind Fluctuations: A Two-Component Model

We present a new model for the transport of solar wind fluctuations which treats them as two interacting incompressible components: quasi-two-dimensional turbulence and a wave-like piece. Quantities solved for include the energy, cross helicity, and characteristic transverse length scale of each component, plus the proton temperature. The development of the model is outlined and numerical solutions are compared with spacecraft observations. Compared to previous single-component models, this new model incorporates a more physically realistic treatment of fluctuations induced by pickup ions and yields improved agreement with observed values of the correlation length, while maintaining good observational accord with the energy, cross helicity, and temperature.

Oughton, S.↗

NASA NTRS1995

Electron impact ionization rates for interstellar neutral H and He atoms near interplanetary shocks: Ulysses observations

During average solar wind flow conditions at 1 AU, ionization rates of interstellar neutrals that penetrate into the inner heliosphere are dominated by charge exchange with solar wind protons for H atoms, and by photoionization for He atoms. During occurrences of strong, coronal mass ejection (CME)-driven interplanetary shock waves near 1 AU, electron impact ionization can make substantial, if not dominating, contributions to interstellar neutral ionization rates in the regions downstream of the shocks. However, electron impact ionization is expected to be relatively less important with increasing heliocentric distance because of the decrease in electron temperature. Ulysses encountered many CME-driven shocks during its journey to and beyond Jupiter, and in addition, encountered a number of strong corotating interaction region (CIR) shocks. These shocks generally occur only beyond approximately 2 AU. Many of the CIR shocks were very strong rivalling the Earth's bow shock in electron heating. We have compared electron impact ionization rates calculated from electron velocity distributions measured downstream from CIR shocks using the Ulysses SWOOPS experiment to charge-exchange rates calculated from measured proton number fluxes and the photoionization rate estimated from an assumed solar photon spectrum typical of solar maximum conditions. We find that, although normally the ratio of electron-impact ionization rates to charge-exchange (for H) and to photoionization (for He) rates amounts to only about one and a few tens of percent, respectively, downstream of some of the stronger CIR shocks they amount to more than 10% and greater than 100%, respectively.

Feldman, W. C.↗

NASA NTRS1995

Effects of spatial transport and ambient wave intensity on the generation of MHD waves by interstellar pickup protons

Intense MHD waves generated by the isotropization of interstellar pickup protons were predicted by Lee and Ip (1987) to appear in the solar wind whenever pickup proton fluxes were high enough. However, in reality these waves have proved surprisingly difficult to identify, even in the presence of observed pickup protons. We investigate the wave excitation by isotropization from an initially broad pitch-angle distribution instead of the narrow ring-beam assumed by Lee and Ip. The pitch angle of a newly-ionized proton is given by theta(sub o), the angle between the magnetic field (averaged over a pickup proton gyroradius) and the solar wind flow at the time of ionization. Then, a broadened distribution results from spatial transport of pickup protons prior to isotropization from regions upstream along the field containing different values of theta(sub o). The value of theta(sub o) will vary as a result of the ambient long-wavelength fluctuations in the solar wind. Thus, the range of initial pitch-angles is directly related to the amplitude of these fluctuations within a length-scale determined by the isotropization time. We show that a broad initial pitch-angle distribution can significantly modify the intensity and shape of the pickup-proton-generated wave spectrum, and we derive a criterion for the presence of observable pickup-proton generated waves given the intensity of the ambient long wavelength fluctuations.

Isenberg, P. A.↗

NASA NTRS1994

Pickup protons and pressure-balanced structures: Voyager 2 observations in merged interaction regions near 35 AU

Five pressure-balanced structures, each with a scale of the order of a few hundredths of an astonomical unit (AU), were identified in two merged interaction regions (MIRs) near 35 AU in the Voyager 2 plasma and magnetic field data. They include a tangential discountinuity, simple and complex magnetic holes, slow correlated variations among the plasma and magnetic field parameters, and complex uncorrelated variations among the parameters. The changes in the magnetic pressure in these events are balanced by changes in the pressure of interstellar pickup protons. Thus the pickup protons probably play a major role in the dynamics of the MIRs. The solar wind proton and electron pressures are relatively unimportant in the MIRs at 35 AU and beyond. The region near 35 AU is transition region: the Sun is the source of the magnetic field, but the interstellar medium in source of pickups protons. Relative to the solar wind proton guyroadius, the thicknesses of the discontinuities and simple magnetic holes observed near 35 AU are at least an order of magnitude greater than those observed at 1 AU. However, the thicknesses of the tangential discontinuity and simple magnetic holes observed near 35 AU (in units of the pickup proton Larmor radius) are comparable to those observed at 1 AU (in units of the solar wind proton gyroradius). Thus the gyroradius of interstellar pickup protons controls the thickness of current sheets near 35 AU. We determine the interstellar pickup proton pressure in the PBSs. Using a model for the pickup proton temperature, we estimate that the average interstellar pickup proton pressure, temperature, and density in the MIRs at 35 AU are (0.53 +/- 0.14) x 10(exp -12) erg/cu cm, (5.8 +/- 0.4) x 10(exp 6) K and (7 +/- 2) x 10(exp -4)/cu cm.

Burlaga, L. F.↗

NASA NTRS1994

On the maximum energy release in flux-rope models of eruptive flares

We determine the photospheric boundary conditions which maximize the magnetic energy released by a loss of ideal-magnetohydrodynamic (MHD) equilibrium in two-dimensional flux-rope models. In these models a loss of equilibrium causes a transition of the flux rope to a lower magnetic energy state at a higher altitude. During the transition a vertical current sheet forms below the flux rope, and reconnection in this current sheet releases additional energy. Here we compute how much energy is released by the loss of equilibrium relative to the total energy release. When the flux-rope radius is small compared to its height, it is possible to obtain general solutions of the Grad-Shafranov equation for a wide range of boundary conditions. Variational principles can then be used to find the particular boundary condition which maximizes the magnetic energy released for a given class of conditions. We apply this procedure to a class of models known as cusp-type catastrophes, and we find that the maximum energy released by the loss of equilibrium is 20.8% of the total energy release for any model in this class. If the additional restriction is imposed that the photospheric magnetic field forms a simple arcade in the absence of coronal currents, then the maximum energy release reduces to 8.6%

Forbes, T. G.↗

NASA NTRS1991

A catastrophe mechanism for coronal mass ejections

The ideal-MHD equations are used to show that a coronal current filament can suddenly lose equilibrium if its magnetic energy exceeds a critical value. The loss of equilibrium in the configuration results from an imbalance between magnetic tension and compression, and this imbalance ejects the filament upwards. Near the critical value, the equilibrium configuration develops a vertical current sheet attached to the photosphere at the point directly below the filament. When equilibrium is lost, field lines anchored to the photosphere are stretched upwards, and the current sheet rapidly grows longer. Without reconnection in the current sheet, the filament travels only a short distance before reaching a new equilibrium, and the net magnetic energy released is less than 1 percent of the stored magnetic energy. However, with reconnection, the filament travels upwards indefinitely, and all of the stored energy is released.

Forbes, T. G.↗

NASA NTRS1990

Pressure-balanced structures between 1 AU and 24 AU and their implications for solar wind electrons and interstellar pickup ions

'Pressure-balanced structures' (PBSs) in the heliosphere are microscale phenomena with a characteristic length along the radial dimension of the order of 0.05 AU across which the sum of the thermal pressures and the magnetic pressure is constant. PBSs have been identified in the Voyager data from 1 AU to 24 AU. If the density of pickup ions were proportional to the proton density across a PBS, and if the electron pressure were negligible, then from measurements of the magnetic field and the proton density and temperature across pressure balanced structures a pickup ion density is derived which is significantly smaller than the density predicted by a model based on the same assumption. This suggests that there are processes which 'smear out' the pressure of the pickup ions so that it is nearly constant on the scale of a PBS. If the pressure of the pickup ions is uniform across a PBS, then one can determine the electron temperature from the pressure balance condition.

Burlaga, L. F.↗

NASA NTRS1986

Interaction of the solar wind with interstellar neutral hydrogen - Three-fluid model

It is commonly assumed in models of the solar wind-interstellar neutral hydrogen interaction that the ionized interstellar particles are quickly assimilated into the solar wind proton population and 'become indistinguishable' from the original solar wind. This assumption leads to the prediction that the solar wind proton temperature should increase with radius in the outer heliosphere. This temperature increase has not been observed. It is pointed out that assimilation of the interstellar particles to the point of indistinguishability takes place on the very long Coulomb collision time scale, and is not expected to occur within the heliosphere. Results are presented of a three-fluid model of the solar wind which consists of comoving thermal populations of protons of solar origin, protons produced by ionization of interstellar hydrogen, and electrons. The steady-state results yield a solar wind with a 'core' proton distribution which cools adiabatically, and a 'halo' of interstellar pickup protons which is maintained near 10 to the 7th K by the energy input of continued ionization and pickup. Such a distribution will not be observed to manifest the temperature increase at large heliocentric distances which is predicted from a one-fluid analysis. Further time-dependent calculations show a strong correlation between the densities of the solar wind and the interstellar pickup protons. It is suggested that the interstellar pickup population may be observable by the Voyager plasma instruments in low resolution mode during periods of high solar wind density and low solar wind temperature.

Isenberg, P. A.↗

NASA NTRS1986

On a difficulty with accelerating particles at slow-mode shocks

The available information on slow-mode shocks is briefly reviewed in order to determine whether these shocks are capable of accelerating particles to high energies, thus providing a possible mechanism for energetic particle production at reconnection sites. It is noted that in contrast to fast shocks, the evolutionary conditions at a steady slow shock require the upstream plasma to flow into the shock with a normal component slower than the intermediate speed. Consequently, the right-polarized waves will escape from a slow shock, and backstreaming particles scattered by these waves will not be convected back to the shock. There will be no effective compression, and further acceleration will not occur.

Isenberg, P. A.↗

NASA NTRS1985

The heating of the solar wind by the interstellar neutral gas

Solar wind heating by ionization and assimilation of the interstellar neutral wind is investigated using a numerical model of the interaction. The model is a time-dependent, one-dimensional, spherically symmetric, one-fluid code which includes mass, momentum, and energy sources due to the efficient incorporation of ionized interstellar hydrogen. Solar wind data at 1 AU from October 12, 1978 to February 25, 1980 were input to the inner boundary of the model and propagated out to 10 AU. Then, 52-day averages of proton temperature as functions of radius were produced for three values of the interstellar hydrogen density: 0.0, 0.03, and 0.1 per cu cm. It is concluded that, within the context of the model, the observations of solar wind proton temperatures at Voyager 1 and Pioneer 11 are consistent with heating of the solar wind by an interaction with inflowing interstellar neutral hydrogen. The density of this inflowing gas is near 0.03/cu cm.

Isenberg, P. A.↗

NASA NTRS1984

Resonant acceleration and heating of solar wind ions Anisotropy and dispersion

This paper is an extension of recent work by Isenberg and Hollweg dealing with the preferential acceleration and heating of solar wind heavy ions by a resonant cyclotron interaction with a turbulent spectrum of ion-cyclotron waves. Two major approximations of the previous work are eliminated: that of isotropic ion distributions, and that of nondispersive waves. The wave action flux of finite-amplitude Alfven waves in a multi-ion, differentially flowing, gyrotropic plasma is derived. This quantity is incorporated into the wave-driven solar wind model of Isenberg and Hollweg, which is then applied to gyrotropic protons and alpha particles. It is found that allowing for nonisotropic ions yields enhanced preferential acceleration of alpha particles. However, when dispersion is included in the resonant interaction, the preferential effects are again reduced. These more realistic calculations show that the observations of solar wind alpha particles cannot be explained by a resonant cyclotron interaction.

Isenberg, P. A.↗

NASA NTRS1984

The ion cyclotron dispersion relation in a proton-alpha solar wind

Solutions to the WKB warm plasma dispersion relation are investigated for the case of a proton-alpha plasma, to determine when parallel ion cyclotron waves can propagate at the alpha particle gyrofrequency. Examples are given of the behavior of the dispersion curves in this resonant regime, for several types of plasma states that are appropriate to the fast solar wind. It is found that while ion cyclotron waves can propagate at the alpha particle gyrofrequency for a bounded set of plasma parameters, use of the correct dispersion relation will probably make resonant cyclotron acceleration of solar wind plasma particles less feasible as a mechanism for the generation of the observed preferential effects.

Isenberg, P. A.↗

NASA NTRS1983

Acceleration of heavy ions in the solar wind

The preferential acceleration and heating of solar wind heavy ions by the resonant cyclotron interaction were studied. It is concluded that this interaction is incapable of producing the observed differential speeds for reasonable solar wind parameters.

Isenberg, P. A.↗

NASA NTRS1983

On the preferential acceleration and heating of solar wind heavy ions

It is pointed out that the properties of solar wind ions heavier than H(+) have presented a puzzle to theorists for some time. The most recent theoretical efforts to explain the observations are considered, taking into account studies conducted by Dusenbery and Hollweg (1981) and Marsch et al. (1982). The first model of a wave-driven, three-fluid, supersonic solar wind is considered. Details of the resonant wave-particle interaction are discussed and the relevant equations for the model are derived. An investigation is conducted of the effects of resonant cyclotron acceleration and heating by a spectrum of nondispersive ion cyclotron waves on the model. It is found that there are serious difficulties with the hypothesis that the resonant cyclotron interaction is responsible for the preferential heating and acceleration of heavy ions in the solar wind. The model presented here is unable to produce the required energization by this mechanism. However, this model should be viewed as an approximate one, subject to possible correction by more advanced calculations.

Isenberg, P. A.↗

NASA NTRS1982

Finite amplitude Alfven waves in a multi-ion plasma - Propagation, acceleration, and heating

An expression is derived for the wave action flux of finite-amplitude Alfven waves in a multi-ion plasma. The expression is valid in the presence of dissipative forces and permits an arbitrary angle between the average magnetic field and the wave vector. Applying the conservation of wave action and the first law of thermodynamics yields, for a multi-ion plasma, an expression for the spatial evolution of Alfven wave amplitude in the absence of dissipation. It also gives the relationship between the wave amplitude and the dissipative heating, as well as an expression for the acceleration of an ion species by finite-amplitude Alfven waves. It is pointed out that the acceleration comprises a nondissipative wave pressure that is identical to that derived previously under more restrictive conditions and a new term giving the acceleration that must accompany dissipative heating. The results are discussed in the context of the observations of heavy ions in the solar wind.

Isenberg, P. A.↗

NASA NTRS1982

On the preferential acceleration and heating of solar wind heavy ions

The feasibility of producing the observed velocities and temperatures of solar wind heavy ions by the resonant cyclotron interaction with left-polarized hydromagnetic waves was investigated. A "most favorable case" scenario in which the waves are parallel-propagating and dispersionless and the energy for the wave acceleration and heating is taken from saturated low-frequency Alfven waves via a cascade to higher frequencies, is incorporated into a numerical solar wind code and agreement with observation is tested. The resonant cyclotron interaction is shown to fail on at least three points, even in this most favorable case.

Isenberg, P. A.↗

NASA NTRS1981

On rotational forces in the solar wind

Solar rotational forces affecting the flow of minor ions in the solar wind are considered as corotating with the sun. Cold, noninteracting charged particles in the magnetic and gravitational fields of the sun rotate with the angular velocity of the sun, and calculations of lowest bulk order velocities show that differences in particle velocities decrease with increasing distance from the sun. A centrifugal potential in the corotating frame implies that ion motion is independent of protons, with velocities determined by the potential, which monotonically decreases without limit. The potential dominates the initial kinetic energy of the particles, and the equality of velocities within the potential is not due to interactions between particles as claimed by Mackenzie et al. (1979).

Hollweg, J. V.↗

NASA NTRS1979

Gradient and curvature drifts in magnetic fields with arbitrary spatial variation

It is shown that, for a magnetic field of arbitrary spatial variation, a nearly isotropic distribution of charged particles drifts with a velocity given by the usual first-order orbit theory drifts averaged over pitch angle. It is assumed that the near-isotropy brought about by scattering, but conclusions concerning drift are insensitive to the details of the scattering process. It is found that this drift velocity is correct even for arbitrarily large ratios of particle gyroradius to magnetic spatial scale, although this velocity must, like all drift effects, be viewed on a scale larger than a gyroradius. Hence for many astrophysical applications, such as cosmic rays, where anisotropies are small, the usual drift velocities provide a valid approximation to convective motions even if the magnetic field scales are very small.

Isenberg, P. A.↗