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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

The Heliosphere as Resonant Cavity

If a disturbance in the supersonic solar wind reaches the heliospheric shock, a number of events ensue. First, the shock itself responds with inward or outward motion. Secondly, the disturbance propagates outward through the heliosheath as a sound or magnetoacoustic wave; eventually it reaches the heliopause and is (partially) reflected back toward the termination shock. The reflected wave can return to the shock, affecting the shock's motion, and be reflected yet again. The repetition of these processes can produce a 'ringing' in the heliosheath. This suggests that it may be useful to regard the heliosheath as a resonant acoustic cavity with inner and outer boundaries at the termination shock and heliopause, respectively. To evaluate this concept we have developed a simple model of small-amplitude resonant oscillations in an outwardly flowing gas, with appropriate boundary conditions (shock on the interior, tangential discontinuity on the exterior boundary). The fundamental mode of oscillation has a period of order T approx. 2D/C, where C is the speed of sound in the heliosheath and D is the distance between the two boundaries. Typical numerical models of the heliosphere give C approx. 200-500 km/s and D approx. 20 - 100 AU, giving T approx. 0.5 - 2.5 years. Hence we suggest that motions of the heliosheath and termination shock will occur with time scales of the order of a year, and are the consequence of the resonant nature of the heliospheric cavity rather than the history of variation at the Sun and/or in the solar wind. In particular, we suggest that the motion of the termination shock may be unrelated to solar variations over the time scale of the sunspot cycle.

Bames, Aaron↗
NASA NTRS1995

The interaction of a magnetized solar wind with the VLISM

The interaction of a magnetized solar wind with magnetized interstellar flow is modelled in two dimensions. The VLISM magnetic field and flow velocity are assumed to be parallel to each other and perpendicular to the sun's magnetic axis, contrast to a recent study by Washimi. The more realistic orientation of the axis and VLISM flow requires a less realistic heliospheric field model in two dimensions. The Parker spiral is replaced by a poloidal field which is tangent to the termination shock, and whose magnitude is consistent with estimates of the heliospheric field at termination shock distances. Of particular interest is the effect of the heliospheric field on the heliospheric flow beyond the termination shock, and on the structure and location of the bow shock

Brackbill, J. U.↗
NASA NTRS1992

Constraints on the acceleration of anomalous cosmic rays

The recent determination of an upper limit of 0.2 pc on the distance to the source of the anomalous cosmic rays, and the implied upper limit on their age imposes a significant observational constraint on possible acceleration mechanisms. Adiabatic cooling in the expanding wind makes the constraints significantly more stringent. In the light of these constraints, the rate of second-order Fermi acceleration is much too low to play a significant role. Diffusive shock acceleration, even at the strong termination shock of the solar wind, is sufficient only if one discards the standard expression for the maximum acceleration rate based on the Bohm limit. Only diffusive acceleration at the quasi-perpendicular solar wind termination shock has a high enough acceleration rate to be the source of the anomalous component.

Jokipii, J. R.↗
NASA NTRS1993

Large-scale density structures in the outer heliosphere

The Plasma Science experiment on the Voyager 2 spacecraft has measured the solar wind density from 1 to 38 AU. Over this distance, the solar wind density decreases as the inverse square of the heliocentric distance. However, there are large variations in this density at a given radius. Such changes in density are the dominant cause of changes in the solar wind ram pressure in the outer heliosphere and can cause large perturbations in the location of the termination shock of the solar wind. Following a simple model suggested by Suess, we study the non-equilibrium, dynamic location of the termination shock as it responds to these pressure changes. The results of this study suggest that the termination shock is rarely if ever at its equilibrium distance and may depart from that distance by as much as 50 AU at times.

Belcher, J. W.↗
NASA NTRS1990

Ion acceleration to cosmic ray energies

The acceleration and transport environment of the outer heliosphere is described schematically. Acceleration occurs where the divergence of the solar-wind flow is negative, that is at shocks, and where second-order Fermi acceleration is possible in the solar-wind turbulence. Acceleration at the solar-wind termination shock is presented by reviewing the spherically-symmetric calculation of Webb et al. (1985). Reacceleration of galactic cosmic rays at the termination shock is not expected to be important in modifying the cosmic ray spectrum, but acceleration of ions injected at the shock up to energies not greater than 300 MeV/charge is expected to occur and to create the anomalous cosmic ray component. Acceleration of energetic particles by solar wind turbulence is expected to play almost no role in the outer heliosphere. The one exception is the energization of interstellar pickup ions beyond the threshold for acceleration at the quasi-perpendicular termination shock.

Lee, Martin A.↗
NASA NTRS1995

Cosmic rays and the boundaries of the heliosphere

Anomalous cosmic rays (ACRs) are interstellar neutrals that drift into the heliosphere, become singly ionized, and are convected to the termination shock of the solar wind, where they are thought to be accelerated to hundreds of MeV. Because their effective origin is at the termination shock, studies of their gradients and spectral shape can reveal important clues about the shock's location, its strength, and the source flux of ACRs. Recently, such studies have predicted that one or more of the Voyager and Pioneer spacecraft may cross the termination shock in the next few years. In addition, there have been studies of galactic cosmic rays that shed new light on the location of the modulation boundary of these particles, which may be the heliopause region. In this talk, we will review these observations and the information they provide about the boundaries of the heliosphere.

Cummings, A. C.↗
NASA NTRS1992

A model for the radio brightness of the supernova remnant 1987A

The recently observed rise in radio flux from SN1987A is earlier than predicted in models of the interaction with the observed line-emitting gas. The observed size of the radio source is roughly consistent with the expected position of the SN shock wave and the position of the termination shock of the blue supergiant progenitor wind. The radio increase is attributed to the interaction of the SN shock with this density jump. The required efficiency of electron acceleration and magnetic field amplification is larger than usually occurs in SNe and SNR. Possible reasons for the enhanced radio emission are particle acceleration by the weak reflected shock that travels through the interaction region, and the action of the Richtmyer-Meshkov instability near the initial termination-shock density jump. The action of these mechanisms is time-limited; the prediction of this model is that the radio flux should begin to decline within the next year.

Chevalier, Roger A.↗
DOE OSTI2022

A Model of Double Coronal Hard X-Ray Sources in Solar Flares

A number of double coronal X-ray sources have been observed during solar flares by RHESSI, where the two sources reside at different sides of the inferred reconnection site. However, where and how these X-ray-emitting electrons are accelerated remains unclear. Here we present the first model of the double coronal hard X-ray (HXR) sources, where electrons are accelerated by a pair of termination shocks driven by bidirectional fast reconnection outflows. We model the acceleration and transport of electrons in the flare region by numerically solving the Parker transport equation using velocity and magnetic fields from the macroscopic magnetohydrodynamic simulation of a flux rope eruption. We show that electrons can be efficiently accelerated by the termination shocks and high-energy electrons mainly concentrate around the two shocks. The synthetic HXR emission images display two distinct sources extending to >100 keV below and above the reconnection region, with the upper source much fainter than the lower one. The HXR energy spectra of the two coronal sources show similar spectral slopes, consistent with the observations. Our simulation results suggest that the flare termination shock can be a promising particle acceleration mechanism in explaining the double-source nonthermal emissions in solar flares.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1993

Supersonic vortex breakdown over a delta wing in transonic flow

The effects of freestream Mach number and angle of attack on the leading-edge vortex breakdown due to the terminating shock on a 65-degree, sharp-edged, cropped delta wing are investigated computationally, using the time-accurate solution of the laminar unsteady compressible full Navier-Stokes equations with the implicit upwind flux-difference splitting, finite-volume scheme. A fine O-H grid consisting of 125 x 85 x 84 points in the wrap-around, normal, and axial directions, respectively, is used for all the flow cases. Keeping the Reynolds number fixed at 3.23 x 10 exp 6, the Mach number is varied from 0.85 to 0.9 and the angle of attack is varied from 20 to 24 deg. The results show that, at 20-deg angle of attack, the increase of the Mach number from 0.85 to 0.9 results in moving the location of the terminating shock downstream. The results also show that, at 0.85 Mach number, the increase of the angle of attack from 20 to 24 deg results in moving the location of the terminating shock upstream. The results are in good agreement with the experimental data.

Kandil, Hamdy A.↗
NASA NTRS1993

The low-frequency interplanetary radiation

Observations of the low-frequency interplanetary radiation have continued, although no major activity has begun since mid-1991. Theoretical work has been completed which suggests that the long-speculated source at the termination shock is feasible and could at least qualitatively account for several aspects of the observations, although some difficulties exist. The Ulysses encounter with Jupiter occurred in February 1992 and has renewed discussion of a Jovian source for the emissions. We reconsider the arguments used previously to rule out Jupiter as a source and conclude that the existence of a relatively high-density boundary to the heliosphere renders most of these arguments ineffective in eliminating Jupiter as a source. We conclude, therefore, that Jupiter must be considered as likely a candidate source as the termination shock for the low-frequency emissions and, given that Jupiter is a known source of emissions in the few-kHz range, perhaps Jupiter is the more likely candidate. Even with a Jovian source, however, the low-frequency emissions continue to be useful sounders of the heliosphere which can provide information on the size of the heliospheric cavity. No conclusive evidence is available which identifies either Jupiter or the termination shock as the source.

Kurth, W. S.↗
NASA NTRS1986

The anomalous component, its variation with latitude and related aspects of modulation

Consideration is given to the origin of the anomalous cosmic ray component as well as to the mechanism and location for the acceleration of these particles. The following three theories for anomalous component acceleration are discussed in detail: (1) transit-time damping, (2) the termination shock over the solar poles, and (3) the termination shock near the equatorial plane. It is noted that observations from Ulysses should provide important constraints on the likely locations for the acceleration, and that acceleration of interstellar H, along with the He, N, O, and Ne could have serious consequences for possible acceleration mechanisms.

Fisk, L. A.↗
NASA NTRS1995

Recent observations and analyses of heliospheric 2-3 kHz radio emissions

Since the discovery of the 2-3 kHz heliospheric radio emissions in the Voyager data over twelve years ago, there have been two major events, the first in 1983-84 and the second in 1992-93, as well as several minor events. Strong evidence now exists that these radio emissions are generated by interactions that take place in the outer regions of the heliosphere, most likely in response to strong interplanetary shocks and associated disturbances propagating outward from the Sun. Just where these interactions take place is still a subject of controversy. The three main possibilities are at or near the termination shock, between the termination shock and the heliopause and at or near the heliopause. In this paper we will review the present state of knowledge concerning the 2-3 kHz radio emissions, including the most recent data from Voyagers 1 and 2. These observations will then be compared to models that have been proposed to explain the origin of the radio emissions. Depending on the model, various estimates can be made concerning the plasma densities in the region where the radio emission is produced, and the distance to the source. The constraints provided by these models on the size and structure of the heliosphere are discussed.

Gurnett, D. A.↗
NASA NTRS1990

The heliopause

A Pioneer or Voyager spacecraft will soon pass through one of the last major frontiers in the solar system - the heliospheric terminal shock. Some unknown, but perhaps small, distance beyond the terminal shock is the heliopause, marking the final boundary between solar-wind and Galactic plasmas and the final goal of these spacecraft. This occasion offers an opportunity to obtain a wealth of information on the properties of the Galaxy, the interstellar medium, and the large-scale interactions of that medium with stellar winds. Several tentative remote detections of the shock place it just beyond the present spacecraft locations. However, uncertainties in the physical processes and parameters that determine the location of the boundary lead to equal uncertainties in predicting which of the spacecraft will reach it first.

Suess, Steven T.↗
NASA NTRS1994

Relativistic particle acceleration in plerions

We discuss recent research on the structure and particle acceleration properties of relativistic shock waves in which the magnetic field is transverse to the flow direction in the upstream medium, and whose composition is either pure electrons and positrons or primarily electrons and positrons with an admixture of heavy ions. Particle-in-cell simulation techniques as well as analytic theory have been used to show that such shocks in pure pair plasmas are fully thermalized -- the downstream particle spectra are relativistic Maxwellians at the temperature expected from the jump conditions. On the other hand, shocks containing heavy ions which are a minority constituent by number but which carry most of the energy density in the upstream medium do put approximately 20% of the flow energy into a nonthermal population of pairs downstream, whose distribution in energy space is N(E) varies as E(exp -2), where N(E)dE is the number of particles with energy between E and E+dE. The mechanism of thermalization and particle acceleration is found to be synchrotron maser activity in the shock front, stimulated by the quasi-coherent gyration of the whole particle population as the plasma flowing into the shock reflects from the magnetic field in the shock front. The synchrotron maser modes radiated by the heavy ions are absorbed by the pairs at their (relativistic) cyclotron frequencies, allowing the maximum energy achievable by the pairs to be gamma(sub +/-)m(sub +/-)c squared = m(sub i)c squared gamma(sub 1)/Z(sub i), where gamma(sub 1) is the Lorentz factor of the upstream flow and Z(sub i) is the atomic number of the ions. The shock's spatial structure is shown to contain a series of 'overshoots' in the magnetic field, regions where the gyrating heavy ions compress the magnetic field to levels in excess of the eventual downstream value. This shock model is applied to an interpretation of the structure of the inner regions of the Crab Nebula, in particular to the 'wisps,' surface brightness enhancements near the pulsar. We argue that these surface brightness enhancements are the regions of magnetic overshoot, which appear brighter because the small Larmor radius pairs are compressed and radiate more efficiently in the regions of more intense magnetic field. This interpretation suggests that the structure of the shock terminating the pulsar's wind in the Crab Nebula is spatially resolved, and allows one to measure gamma(sub 1) and a number of other properties of the pulsar's wind. We also discuss applications of the shock theory to the termination shocks of the winds from rotation-powered pulsars embedded in compact binaries. We show that this model adequately accounts for (and indeed predicted) the recently discovered X-ray flux from PSR 1957+20, and we discuss several other applications to other examples of these systems.

Arons, Jonathan↗
NASA NTRS1994

Gasdynamic models of the solar wind/interstellar medium interaction

The interaction between the solar wind and the interstellar medium is modeled self-consistently using numerical solutions of the time-dependent gasdynamic equations in spherical and cylindrical coordinates. For the results presented here it is assumed that the solar system moves through the surrounding medium with a supersonic velocity. After an initial (nonequilibrium) state has been specified, the numerical solution follows the evolution in time until the interaction relaxes to a dynamic equilibrium. As would be expected, the solutions show the formation of a bow shock upstream of the traveling solar system to deflect the interstellar plasma around the cavity created by the solar wind. A terminiation shock also forms to slow and compress the solar wind plasma. For the simulation in spherical coordinates, the downstream portion of the termination shock reaches equilibrium more than three times further from the Sun than the equilibrium distance to the termination shock on the upstream side.

Steinolfson, R. S.↗
NASA NTRS1992

Relativistic magnetosonic shock waves in synchrotron sources - Shock structure and nonthermal acceleration of positrons

The theoretical properties of relativistic, transverse, magnetosonic collisionless shock waves in electron-positron-heavy ion plasmas of relevance to astrophysical sources of synchrotron radiation are investigated. Both 1D electromagnetic particle-in-cell simulations and quasi-linear theory are used to examine the spatial and kinetic structure of these nonlinear flows. A new process of shock acceleration of nonthermal positrons, in which the gyrating reflected heavy ions dissipate their energy in the form of collectively emitted, left-handed magnetosonic waves which are resonantly absorbed by the positrons immediately behind the ion reflection region, is described. Applications of the results to the termination shocks of pulsar winds and to the termination shocks of jets emanating from the AGN are outlined.

Hoshino, Masahiro↗
NASA NTRS1989

Recurrent solar wind structures in the outer heliosphere

The paper presents recent work on evolution of recurrent solar wind structures in the outer heliosphere. Corotating shocks, corotating interaction regions, and merged interaction regions are studied, and an MHD simulation model in which the jump conditions at all shock crossings satisfy the Rankine-Hugoniot solution is examined. Simulation results which describe the evolution of idealized recurrent solar wind structures between 14 AU and the termination shock are reported which show that merged interaction regions belonging to two to four consecutive solar rotations coalesce with each other between 30 AU and the termination shock to reshape the heliospheric structures. In the outer heliosphere, forward and reverse interplanetary shocks which continuously heat the solar wind plasma are investigated.

Whang, Y. C.↗
NASA NTRS1985

Cosmic ray acceleration by stellar wind. Simulation for heliosphere

The solar wind deceleration by the interstellar medium may result in the existence of the solar wind terminal shock. In this case a certain fraction of thermal particles after being heated at the shock would obtain enough energy to be injected to the regular acceleration process. An analytical solution for the spectrum in the frame of a simplified model that includes particle acceleration at the shock front and adiabatic cooling inside the stellar wind cavity has been derived. It is shown that the acceleration of the solar wind particles at the solar wind terminal shock is capable of providing the total flux, spectrum and radial gradients of the low-energy protons close to one observed in the interplanetary space.

Petukhov, S. I.↗