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All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

Voyager Observations of Magnetic Fields and Cosmic Rays in the Heliosheath

The major features of the profile of >70 MeV/nuc cosmic ray intensity (CRI) observed by Voyager 1 (V1) in the heliosheath from 2005.8 - 2010.24 are described by the empirical "CR-B" relation as the cumulative effect of variations of the magnetic field strength B. The CRI profile observed by Voyager 2 (V2) from 2008.60 to 2010.28 in the heliosheath is also described by the CR-B relation. On a smaller scale, of the order of a hundred days, a sequence of 3 CRI decreases observed by V1 during 2006 was interpreted as the effect of a propagating interplanetary shock first interacting with the termination shock, then moving past V1, and finally reflecting from the heliopause and propagating back to V1. Our observations show that the second CRI decrease in this sequence began during the passage of a "Global Merged Interaction Region" (GMIR), approx. 40 days after the arrival of the GMIR and its possible shock. The first and third CRI decreases in the sequence were associated with local enhancements of B. The magnetic field observations associated with the second sequence of 3 cosmic ray intensity decreases observed by V1 in 2007/2008 are more difficult to reconcile with the scenario of Webber et al. and the CR-B relation. The discrepancy might indicate the importance of latitudinal effects.

Burlaga, L. F.↗
NASA NTRS2011

Voyager Observations of Magnetic Fields and Cosmic Rays in the Heliosheath

The major features of the profile of greater than 70 MeV/nuc cosmic ray intensity (CRI) observed by Voyager 1 (VI) in the heliosheath from 2005.8-2010.24 are described by the empirical "CR-B" relation as the cumulative effect of variations of the magnetic field strength B. The CRI profile observed by Voyager 2 (V2) from 2008.60 to 2010.28 in the heliosheath is also described by the CR-B relation. On a smaller scale, of the order of a hundred days, a sequence on CRI decreases observed by V 1 during 2006 was interpreted as the effect of a propagating interplanetary shock first interacting with the termination shock, then moving past V1, and finally reflecting from the heliopause and propagating back to V1. Our observations show that the second CRI decrease in this sequence began during the passage of a "Global Merged Interaction Region" (GMIR), ~40 days after the arrival of the GMIR and its possible shock. The first and third CRI decreases in the sequence were associated with local enhancements of B. The magnetic field observations associated with the second sequence of 3 cosmic ray intensity decreases observed by V 1 in 2007/2008 are more difficult to reconcile with the scenario of Webber et al. (2009) and the CR-B relation. The discrepancy might indicate the importance of latitudinal effects

Burlaga, L. F.↗
DOE OSTI2024

Evolving Outer Heliosphere: Tracking Solar Wind Transients from 1 au to the VLISM with IBEX and Voyager 1

Abstract Interstellar Boundary Explorer (IBEX) observations of energetic neutral atom (ENA) fluxes from the heliosphere have greatly enriched our understanding of the interaction of the solar wind (SW) with the local interstellar medium (LISM). However, there has been recent controversy surrounding the inability of most ENA models to produce as high an intensity of ∼0.5–6 keV ENAs as IBEX observes at 1 au, especially as a function of time. In our previous study (E. J. Zirnstein et al.), we introduced a new model that utilizes a data-driven magnetohydrodynamic simulation of the SW–LISM interaction to propagate pickup ions through the heliosheath (HS) after they are nonadiabatically heated at the heliospheric termination shock. E. J. Zirnstein et al. only simulated and analyzed IBEX observations from the direction of Voyager 2. In this study, we expand our model to include fluxes from the direction of Voyager 1, as well as in the low-latitude part (middle) of the ribbon (10° below the ecliptic plane). We show that the model results at Voyager 1 are consistent with E. J. Zirnstein et al.’s results at Voyager 2 in terms of a secondary ENA source contribution of ≲20% from both directions. Our results in the middle of the ribbon also reproduce the data, when including a time-dependent secondary ENA source. Finally, we demonstrate with our simulation that three large pressure waves likely merged in the VLISM and were observed by Voyager 1 as “pf2,” while at least one of the wave’s effects in the HS was observed by IBEX as a brief enhancement in ENA flux in early 2016.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1995

The distance to the heliospheric VLF emission region

Two major episodes of heliospheric VLF emissions near 3 kHz have been observed by the Voyager spacecraft in 1983/84 and 1992/3. This higher-frequency component is apparently triggered by solar wind transients with sufficiently large spatial extents and energies to continue to propagate as shocks in the heliosheath. Entrainment of previously unshocked material and changed flow conditions in the heliosheath both tend to slow the shock propagation. The shock evolution is not self-similar. Rather, it is intermediate to two blast-wave similarity solutions in the moving solar wind frame. In one solution the shock moves as time to the 2/3 power and in the other as time to the 4/5 power. Using these models, the shock/Forbush decrease observed at Voyager 2 in September, 1991 and the turn-on of the 1992 emission is consistent with an emission region distance of approximately 130 AU (assuming no additional slowing of the shock in the heliosheath). If the termination shock was at approximately 70 AU when the transient shock collided with it, the true distance to the source region was probably closer to approximately 115 AU.

Mcnutt, R. L., Jr.↗
NASA NTRS1995

The Distance to the Heliospheric VLF Emission Region

Two major episodes of heliospheric VLF emissions near 3 kHz have been observed by the Voyager spacecraft in 1983-1984 and 1992-1993. This higher-frequency component is apparently triggered by solar wind transients with sufficiently large spatial extents and energies to continue to propagate as shocks in the heliosheath. Entrainment of previously unshocked material and changed flow conditions in the heliosheath both tend to slow the shock propagation. The shock evolution is not self-similar. Rather, it is intermediate to two blast-wave similarity solutions in the moving solar wind frame. In one solution the shock moves as time to the 2/3 power and in the other as time to the 4/5 power. Using these models, the shock/Forbush decrease observed at Voyager 2 in September, 1991 and the turn-on of the 1992 emission is consistent with an emission region distance of approx. 130 AU (assuming no additional slowing of the shock in the heliosheath). If the termination shock was at approx. 70 AU when the transient shock collided with it, the true distance to the source region was probably closer to approx. 115 AU.

McNutt, R. L., Jr.↗
NASA NTRS1999

High Energy Observations of X-Ray Binaries and Gamma-Ray Blazars

The report discusses the CGRO observation of X-ray binary systems and studies of gamma-ray blasars. Numerous authors have suggested mechanisms for particle acceleration within X-Ray Binary (XRB) systems. Among the acceleration mechanisms that have been applied are pulsar acceleration, shock acceleration at an accretion shock front, shock acceleration at a pulsar wind termination shock, plasma turbulence excited by the accretion flow, and a number of electrodynamic mechanisms. There are therefore many mechanisms which are capable of generating very energetic particles in the XRB environment. If the reports of TeV/PeV gamma-ray generation in XRBs are correct, then one can show that the accelerated particles must be hadrons and that the most likely gamma-ray production mechanism is the decay of collisionally-produced (or photoproduced) neutral pions. At these ultra-high energies, the emission is so strongly beamed that the target conditions are constrained by the requirement that the column depth be large enough to efficiently generate gamma-rays, but not so large that the gamma-rays are absorbed. These constraints naturally lead to models that explain the periodic, narrow duty-cycle pulses observed at TeV/PeV energies as arising from interactions with, either, the atmosphere of the binary companion, an accretion column, or an accretion disk. The production of these TeV/PeV gamma-rays by the decay of pions from "leading isobars" must also be accompanied by a more isotropic emission component in the EGRET energy band from the decay of slower pions (i.e. the "pionization" component). Since the attenuation of 35 MeV-1 GeV photons by photon-photon pair production is not likely to be significant in most XRBs, the TeV/PeV reports therefore strongly suggest sporadic emission in the EGRET energy band. One of the key unresolved issues for understanding AGN is the relationship between XBLs and RBLs.To test the "reunification" hypothesis, authors conducted a multiwavelength campaign during Cycle 7 of the CGRO observing program. The analysis of the cycle 4 through cycle 6 CGRO measurements was completed. Two previously unknown EGRET sources was discovered.

Vestrand, W. Thomas↗
NASA NTRS1993

(abstract) 3D Electromagnetic Plasma Particle Simulations

A 3D electromagnetic plasma particle-in-cell code has been developed using the General Concurrent PIC algorithm. The GCPIC algorithm uses a domain decomposition to divide the computation among the processors. Particles must be exchanged between processors as they move. The efficiencies for 1-, 2-, and 3-dimensional partitions of the three dimensional domain are compared, and the algorithm is found to be very efficient even when a large fraction (e.g., 30%) of the particles must be exchanged at every time step. This PIC code has been used to perform simulations of a variety of space plasma physics problems. Results of three applications will be discussed: 1) plasma disturbances induced by moving conducting bodies in a magnetized plasma; 2) plasma plume interactions; and 3) solar wind termination shock.

electromagnetic plasma particles simulations 3D al↗
NASA NTRS1995

Mechanism for generation of 2-3 kHz radiation in the outer heliosphere

The question of how low-frequency non-thermal radio emissions at the boundary of the heliosphere might be generated is considered. The mechanism consists of two steps. First, the beam of energetic electrons generates a high level of electrostatic Langmuir plasma waves. Second, electromagnetic radiation results from the non-linear interaction between Langmuir waves. Intensity of radio emissions at 2 to 3 kHz detected by the Voyager plasma wave instrument in the outer heliosphere can be explained provided that the electron beams generating Langmuir waves exist also in the postshock plasma due to secondary shocks in the compressed solar wind beyond the termination shock. Modification of the heliospheric shocks by the cosmic ray pressure is also taken into account. The field strengths of Langmuir waves required to generate the second harmonic emissions are of 50 to 100 microvolts per meter. These waves may be observed in situ by Voyager 1 and 2 in the near future.

Macek, W. M.↗
NASA NTRS2005

Planar Laser-Induced Iodine Fluorescence Measurements in Rarefied Hypersonic Flow

A planar laser-induced fluorescence (PLIF) technique is discussed and applied to measurement of time-averaged values of velocity and temperature in an I(sub 2)-seeded N(sub 2) hypersonic free jet facility. Using this technique, a low temperature, non-reacting, hypersonic flow over a simplified model of a reaction control system (RCS) was investigated. Data are presented of rarefied Mach 12 flow over a sharp leading edge flat plate at zero incidence, both with and without an interacting jet issuing from a nozzle built into the plate. The velocity profile in the boundary layer on the plate was resolved. The slip velocity along the plate, extrapolated from the velocity profile data, varied from nearly 100% down to 10% of the freestream value. These measurements are compared with results of a DSMC solution. The velocity variation along the centerline of a jet issuing from the plate was measured and found to match closely with the correlation of Ashkenas and Sherman. The velocity variation in the oblique shock terminating the jet was resolved sufficiently to measure the shock wave thickness.

Cecil, Eric↗
NASA NTRS2021

MMS Observations of Energized He+ Pickup Ions at Quasiperpendicular Shocks

Accelerated He+ PUIs, downstream of quasi-perpendicular shocks, are studied as 10a function of the fast mode Mach number (Mf) and shock obliquity (𝜃Bn).We analyze quasi-perpendicular shocks with Mach numbers in the range [1, 7] observed by the Magnetospheric MultiScale (MMS) mission, and compare upstream and downstream He+ velocity distribution functions (VDFs). For each shock event, we characterize the upstream PUI distribution and derive reduced 1D velocity distributions for the selected upstream and downstream intervals. We also compare the upstream to downstream ratio of spectral indices, computed from the He+ perpendicular distributions, to Mf and 𝜃Bn. We find a positive correlation between this spectral index ratio and Mf, which suggests that perpendicular energization of He+ PUIs is enhanced as the shock becomes stronger. These results inform modeling efforts of PUIs and shock acceleration processes, particularly those taking place at the termination shock.

Michael J. Starkey↗
NASA NTRS1994

Axisymmetric modeling of cometary mass loading on an adaptively refined grid: MHD results

The first results of an axisymmetric magnetohydrodynamic (MHD) model of the interaction of an expanding cometary atmosphere with the solar wind are presented. The model assumes that far upstream the plasma flow lines are parallel to the magnetic field vector. The effects of mass loading and ion-neutral friction are taken into account by the governing equations, whcih are solved on an adaptively refined unstructured grid using a Monotone Upstream Centered Schemes for Conservative Laws (MUSCL)-type numerical technique. The combination of the adaptive refinement with the MUSCL-scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the near nucleus of the comet. The main findingsare the following: (1) A shock is formed approximately = 0.45 Mkm upstream of the comet (its location is controlled by the sonic and Alfvenic Mach numbers of the ambient solar wind flow and by the cometary mass addition rate). (2) A contact surface is formed approximately = 5,600 km upstream of the nucleus separating an outward expanding cometary ionosphere from the nearly stagnating solar wind flow. The location of the contact surface is controlled by the upstream flow conditions, the mass loading rate and the ion-neutral drag. The contact surface is also the boundary of the diamagnetic cavity. (3) A closed inner shock terminates the supersonic expansion of the cometary ionosphere. This inner shock is closer to the nucleus on dayside than on the nightside.

Gombosi, Tamas I.↗
NASA NTRS1995

Contact and pressure balance structures in two-fluid cosmic-ray hydrodynamics

The role of cosmic-ray-modified contact discontinuities and pressure balance structures in two-fluid cosmic-ray hydrodynamics in one Cartesian space dimension are investigated by means of analytic and numerical solution examples, as well as by weakly nonlinear asymptotics. The fundamental wave modes of the two-fluid cosmic-ray hydrodynamic equations in the long-wavelength limit consist of the backward and forward propagating cosmic-ray-modified sound waves, with sound speed dependent on both the cosmic-ray and thermal gas pressures; the contact discontinuity; and a pressure balance mode in which the sum ofthe cosmic ray and thermal gas pressure perturbations is zero. The pressure balance mode, like the contact discontinuity is advected with the background flow. The interaction of the pressure balance mode with the contact discontinuity is investigated by means of the method of multiple scales. The thermal gas and cosmic-ray pressure perturbations satisfy a linear diffusion equation, and entropy perturbations arising from nonisentropic initial conditions for the thermal gas are frozen into the fluid. The contact discontinuity and pressure balance eigenmodes both admit nonzero perturbations in the thermal gas, whereas the cosmic-ray-modified sound waves are isentropic. The total entropy perturbation is shared between the contact discontinuity and pressure balance eigenmodes, and examples are given in which there is a transfer of entropy between the two modes. In particular, N-wave type density disturbances are obtained which arise as a result of the entropy transfer between the two modes. A weakly nonlinear geometric optics perturbation expansion is used to study the long timescale evolution of the short-wavelength entropy wave and the thermal gas sound waves in a slowly varying, large-scale background flow. The weakly nonlinear geometric optics expansion is also used to generalize previous studies of squeezing instability for short-wavelength sound waves in the two fluid model, by including a weakly nonlinear wave steepening term that leads to shock formation, as well as the effect of long time and space dependence of the background flow. Implications of cosmic-ray-modified pressure balance structures and contact discontinuities in models of the interaction of traveling interplanetary shocks and compression and rarefraction waves with the solar wind termination shock are briefly discussed.

Webb, G. M.↗
NASA NTRS1995

Reflection and transmission of GMIR shock at the heliopause and their relation to the 2- and 3-kHz radio emissions

We use Voyager 2 plasma and magnetic field data together with a one-fluid MHD model to study the interactions of the 1991 Global Merged Interaction Region (GMIR) shock with the heliopause. The 1991 GMIR is an extraordinarily large global solar wind structure in radial, longitudinal and latitudinal extents. It has a strong shock at the leading edge. After its penetration through the termination shock, the GMIR shock first propagates through the subsonic solar wind, then interacts with the heliopause. The interaction produces a transmitted shock propagating outward in the interstellar medium, and a reflected shock propagating backward in the subsonic solar wind. We identify the reflected shock and the transmitted shock as the possible source of the radio noise detected at Voyagers. The plasma frequency behind the reflected and the transmitted shock can be, respectively, responsible for the 2- and 3-kHz radio emissions. The two bands of radio noise are emitted from sources on both sides of the heliopause starting at about the same time. If the emission is generated by f(sub p)-radiation then the heliopause is located at R approximately 130 AU. If the emission is generated by 2f(sub p)-radiation the n R approximately 150 AU. Because the relative speed of the interstellar plasma with respect to the sun appears to be sub-Alfvenic, it is very unlikely there is a fast-bow shock of the heliosphere.

Whang, Y. C.↗
NASA NTRS1958

Investigation of Inlet Control Parameters for an External-internal-compression Inlet from Mach 2.1 to 3.0

Investigation of the control parameters of an external-internal compression inlet indicates that the cowl-lip shock provides a signal to position the spike and to start the inlet over a Mach number range from 2.1 to 3.0. Use of a single fixed probe position to control the spike over the range of conditions resulted in a 3.7-count loss in total-pressure recovery at Mach 3.0 and 0 deg angle of attack. Three separate shock-sensing-probe positions were required to set the spike for peak recovery from Mach 2.1 to 3.0 and angles of attack from 0 deg to 6 deg. When the inlet was unstarted, an erroneous signal was obtained from the normal-shock control through most of the starting cycle that prevented the inlet from starting. Therefore, it was necessary to over-ride the normal-shock control signal and not allow the control to position the terminal shock until the spike was positioned.

POSITIONING EQUIPMENT↗
NASA NTRS1988

The effects of compressibility on dynamic stall

In this paper, typical computational predictions and experimental measurements of compressible flow past an airfoil at dynamic stall conditions are studied and compared to develop an insight into the effect of compressibility on dynamic stall. The dependency of the critical Mach number on airfoil leading edge curvature, camber, and angle of attack is investigated. Evidence is presented to show that a local region of supersonic flow occurs on an oscillating airfoil, even for a freestream Mach number as low as 0.2, if the boundary layer remains attached and the angle of attack is sufficiently high; that a shock terminates this local supersonic bubble; and that the vorticity that this shock generates grows rapidly and becomes very unstable as the angle of attack increases beyond the value at which the maximum local flow speed first exceeds the speed of sound. It is suggested that these shock-induced effects compete with the dynamic viscous effects occurring in the boundary layer in determining the onset of separation, which can lead to premature dynamic stall and can significantly reduce the maximum dynamic lift that can otherwise be obtained.

Fung, K.-Y.↗
NASA NTRS1987

Numerical simulation of three-dimensional supersonic inlet flow fields

Supersonic inlet flows with mixed external-internal compressions of an axisymmetric inlet model were computed using a combined implicit-explicit (Beam-Warming-Steger/MacCormack) method for solving the three-dimensional unsteady, compressible Navier-Stokes equations in conservation form. Numerical calculations were made of various flows typically found in supersonic inlets such as shock-wave intersections, flow spillage around the cowl lip, shock-wave/boundary-layer interactions, control of shock-induced flow separation by means of boundary layer bleed, internal normal (terminal) shocks, and the effects of flow incidence. Computed results were compared with available wind tunnel data.

Kawamura, T.↗
NASA NTRS1973

Dynamic response of Mach 2.5 axisymmetric inlet with 40 percent supersonic internal area contraction

Results of experimental tests conducted on a supersonic, mixed compression, axisymmetric inlet are presented. The inlet is designed for operation at Mach 2.5 with a turbofan engine (TF-30). The inlet was terminated with either a choked-orifice plate or a long pipe with variable area choked exit plug. Frequency responses were obtained for selected static pressures in the diffuser. These pressures were selected as potential control signals for terminal shock control. Frequency responses were obtained for the Mach 2 and 2.5 conditions for different terminations. Responses also were obtained with and without cowl bleed. Internal disturbances were produced by sinusoidally varying the inlet overboard bypass doors at frequencies out to 100 hertz.

Baumbick, R. J.↗
NASA NTRS1982

The acceleration of particles on the sun and in the heliosphere

Within the heliosphere, the sun, the interplanetary medium, and at least four planetary magnetospheres contain acceleration sites which produce significant fluxes of energetic particles. The detailed study of the acceleration processes which produce the different energetic particle populations in the heliosphere is of great importance for cosmic-ray astrophysics. Shock acceleration has been recognized as one of the dominant mechanisms for providing energetic particles. Because of their unique importance to cosmic rays, the present investigation is concerned with shock acceleration processes within the heliosphere. The mechanism by which particles gain energy through their interaction with shocks is briefly reviewed. Attention is given to particle acceleration in the region of the earth's bow shock, interplanetary acceleration processes, solar flare acceleration processes, and particle acceleration at the solar wind termination shock.

Mcdonald, F. B.↗