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At least 271 records · Page 15

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NASA NTRS1990

On the formation and expansion of H II regions

The evolution of H II regions in spherical clouds with small, constant-density cores and power-law density distributions r exp -w outside the core is described analytically. It is found that there is a critical exponent above which the cloud becomes completely ionized. Its value in the formation phase depends on the initial conditions, but it has a well-defined value w(crit) = 3/2 during the expansion phase. For w less than w(crit), the radius of the H II region grows at a given rate, while neutral mass accumulates in the interphase between the ionization and shock fronts. For w = w(crit), the fronts move together without mass accumulation. Cases with w greater than w(crit) lead to the champagne phase: once the cloud is fully ionized, the expansion becomes supersonic. For self-gravitating disks without magnetic fields, the main features include a new 'variable-size' stage. The initial shape of the H II region has a critical point beyond which the disk becomes completely ionized.

Franco, Jose↗

NASA NTRS1991

Observations of forbidden Si II (35 microns) and Si I (25 microns) in Orion - Evidence of a wind shock near IRc2

Forbidden Si II and Si I line emission from Orion's BN-KL was measured using a cryogenic grating spectrometer aboard NASA's Kuiper Airborne Observatory. It is believed that the bulk of the forbidden Si II emission in Orion originates in photodissociated gas at the interface between the H II region and its parent molecular cloud. There is, however, a twofold enhancement in forbidden Si II emission near IRc2, which is attributed to fast dissociative J-shock where the wind from IRc2 impact slower moving material. Model fits suggest a silicon gas-phase depletion near ITc2 of 0.3-1.0 relative to solar. The spatial distribution of the forbidden Si II emission has a centralized peak.

Haas, Michael R.↗

NASA NTRS1999

Scanning Mode Shock Position Sensor Invented and Demonstrated

A flow-visualizing system with a scanning optical beam offers greater advantages than the conventional approaches. In addition to a higher signal-to-noise ratio and lower source power, the scanning permits visualization of weak phenomena such as the scattering and diffraction of light on shocks. Scanning beam flow-visualization techniques were evaluated at the NASA Lewis Research Center for shock position sensing. In an effort to eliminate moving parts, a spectral scanning technique was invented and demonstrated.

Adamovsky, Grigory↗

NASA NTRS1985

A piston-driven shock in the solar corona

A solar flare that occurred on the west limb at 20:38 UT on Mar. 25, 1981 generated a massive, rapidly expanding optical coronal transient, which moved outward with an approximately constant velocity of 800 km/s. An associated magnetohydrodynamic shock traveled out ahead of the transient with a velocity estimated to be approximately 1000 km/s. The optical and radio data on the transient and shock fit well with general theories concerning piston-driven shocks and with current MHD models for propagation of such shocks through the solar corona.

Maxwell, A.↗

NASA NTRS1987

The coronal mass ejection of July 6, 1980 - A candidate for interpretation as a coronal shock wave

The July 6, 1980 looplike coronal mass ejection observed with the Solar Maximum Mission coronagraph is described and analyzed. Two observations are given special consideration: (1) the sides of the loop moved laterally while they were visible and did not form stationary bright columns, or 'legs', in the late stage, and (2) raylike features initially outside the mass ejection loop were not pushed aside or bent until the laterally moving loop sides passed over them. These observations, as well as the results of quantitative analysis of the motion of the bright loop, are consistent with the identification of the bright loop as a shock front. The loop top moved outward faster (at about 1000 km/s) than most mass ejections studied previously, and possibly faster than the Alfven speed in the corona, while the loop sides moved laterally at a speed of about 800 km/s and showed no evidence of deceleration. The excess mass in the bright loop is estimated to be less than about 2 x 10 to the 14th g, well below the value found to be typical of coronal mass ejections.

Sime, D. G.↗

NASA NTRSDate not supplied

Global Magnetic Reconnection with Weakly Shocked Solar Wind During Geomagnetic Storms

The solar wind and magnetic field emanating from the sun typically reaches Earth moving at a super-Alfvénic velocity (multiple times the local Alfvén wave speed). This leads to the formation of a bow shock upstream of the planet where the plasma is slowed and heated and the interplanetary magnetic field is compressed before it interacts with Earth's magnetic field. However, a number of recent geomagnetic storm events have been associated with sustained intervals of nearly or indeed fully sub-Alfvénic flows. A weakly- or un-shocked magnetosheath changes the conditions for magnetic reconnection at the magnetopause and subsequently the storm-time evolution of the magnetosphere. We perform Multiscale Atmosphere-Geospace Environment (MAGE) simulations of three different geomagnetic storms each containing a period of nearly or fully sub-Alfvénic solar wind driving and examine the properties of global reconnection. The structure of the night-side magnetosphere and polar cusps are also examined. The dynamical evolution of the global magnetosphere during strong driving represents a key science target for the NASA Heliophysics System Observatory, particularly Magnetospheric Multiscale (MMS) targeting magnetotail/magnetopause reconnection and Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) observing ion dispersion in the cusps.

Brandon Burkholder↗

NASA NTRS1993

Hybrid simulations of the effects of interstellar pickup hydrogen on the solar wind termination shock

Hybrid (kinetic ions/fluid electrons) plasma simulations are used to study the effects of a population of energetic interstellar pickup hydrogen ions on the solar wind termination shock. The pickup hydrogen is treated as a second ion species in the simulations, and thus the effects of the pick-ups on the shock, as well as the effects of the shock on the pickups, are treated in a fully self-consistent manner. For quasi-perpendicular shocks with 10-20 percent pickup hydrogen the pickup ions manifest themselves in a small foot ahead of the shock ramp caused by pickup ion reflection. For oblique shocks with smaller angles between the field and the shock normal, a large fraction of the pickup ions are reflected and move back upstream where they excite large amplitude magnetosonic waves which steepen into shocklets. These backstreaming pickup ions may provide advance warning of a spacecraft encounter with the termination shock.

Liewer, P. C.↗

NASA NTRS1974

On the source of flare-ejecta responsible for geomagnetic storms

It is shown that magnetic bottles as the sources of moving metric type 4 bursts are not responsible for the development of geomagnetic storms, despite the fact that shock waves producing type 2 bursts are the sources of the interplanetary shock waves, which produce SSC's on the geomagnetic field. These magnetic bottles, in general, tend to move in the solar envelope with the speed of several hundred Km/sec at most, which is much slower than that of the motion of type 2 radio sources.

Sakurai, K.↗

NASA NTRS1981

Interaction of comets with the interplanetary medium

The interaction of solar radiation and the solar wind with a comet as it moves around the sun is discussed. The overall structure of the comet-solar wind interaction, which is believed to give rise to a tangential discontinuity surface excluding the solar wind from the cometary ionosphere, a collisionless bow shock serving to decelerate and divert the solar wind, and a weak outer shock, is considered. The variable nature of the interaction as the comet moves around the sun and as solar wind conditions vary is then examined. It is shown that the cometary neutrals play a dominant role in standing off the solar wind, particularly under quiet conditions, making the cometary ionosphere in general highly incompressible. Conditions are found under which cometary neutrals cannot penetrate the ionopause to interact with the solar wind, and the inner shock becomes a hybrid ion-neutral shock. Finally, consideration of the direct interaction of the solar wind and solar ultraviolet radiation with the cometary nucleus indicates that electrostatic charging of the surface is capable of causing transport and blow-off of cometary dust at large heliocentric distances.

Mendis, D. A.↗

NASA NTRS1987

Field line draping about fast coronal mass ejecta - A source of strong out-of-the-ecliptic interplanetary magnetic fields

Fast coronal mass ejecta interact strongly with the ambient interplanetary plasma and magnetic field into which they propagate. A shock forms in front of an ejection, and the slower moving ambient plasma ahead is accelerated and deflected from its path. It is argued that such flow accelerations and deflections of the ambient plasma must produce a draping of the ambient interplanetary magnetic field about the ejected material similar to that which occurs in the magnetosheath surrounding the earth's magnetosphere. The draping pattern should depend upon the overall size and shape of the ejection, its speed relative to the ambient plasma ahead, the orientation of the ambient magnetic field, and the position where the shocked plasma is sampled. At some locations upstream from an ejection draping leads to an enhancement of the out-of-the-ecliptic field component B(Z) at the expense of the ecliptic components. It is suggested that draping plays an important role in producing intervals of strong and prolonged negative B(Z) in the ecliptic plane at 1 AU, and thus may be an important factor in stimulating geomagnetic activity.

Gosling, J. T.↗

NASA NTRS1982

Particle acceleration by coronal and interplanetary shock waves

Utilizing many years of observation from deep space and near-earth spacecraft a theoretical understanding has evolved on how ions and electrons are accelerated in interplanetary shock waves. This understanding is now being applied to solar flare-induced shock waves propagating through the solar atmosphere. Such solar flare phenomena as gamma-ray line and neutron emissions, interplanetary energetic electron and ion events, and Type II and moving Type IV radio bursts appear understandable in terms of particle acceleration in shock waves.

Pesses, M. E.↗

DOE OSTI2023

Pinwheel Experiment Confirmatory Shot 1 Pre-Shot Report

The purpose of this experimental series is to validate the reactive burn model in the PAGOSA hydrodynamic code. It will also help validate the EOS and Strength models for the aluminum and tantalum cylinders. C4 high explosive (HE) is used to drive an aluminum cylinder into an aluminum sandwiched PBX-9501. The follow-on experiment will drive the aluminum cylinder into a tantalum sandwiched PBX-9501. The experiment will provide time of arrival (TOA) data for the burn front in the HE through the metal cylinders using Fiber Bragg Gratings (FBG). Photon Doppler Velocimetry (PDV) will provide shock wave TOA and the velocity of the inward moving cylinder. The PAGOSA code will be used to predict TOA and cylinder wall velocity using PDV and tracers.

42 ENGINEERING↗

DOE OSTI2023

Pinwheel Experiment Tantalum Shot 1 Pre-Shot Report

The purpose of this experimental series is to validate the reactive burn model in the PAGOSA hydrodynamic code. It will also help validate the EOS and Strength models for the aluminum and tantalum cylinders. C4 high explosive (HE) is used to drive an aluminum cylinder into a tantalum sandwiched PBX‐9501. The experiment will provide time of arrival (TOA) data for the burn front in the HE through the metal cylinders using Fiber Bragg Gratings (FBG). Photon Doppler Velocimetry (PDV) will provide shock wave TOA and the velocity of the inward moving cylinder. The PAGOSA code will be used to predict TOA and cylinder wall velocity using PDV and tracers.

36 MATERIALS SCIENCE↗

NASA NTRS1974

Envelope dynamics of iron-core supernova models

Wilson (1971) has found that the neutrino transport mechanism is unable to generate a supernova explosion in stars with collapsing iron cores. The present work uses Wilson's analysis to investigate the behavior of the overlying potentially explosive layers which Wilson omitted. The outer boundary of the core of Wilson's models moves in such a manner as to deliver a shock to the base of the envelope. We have numerically followed the progress of such shocks into the envelope of a realistic model obtained from evolutionary calculations. We find that only shocks so strong as to be inconsistent with our treatment are capable of ejecting material. For reasonable shocks the nuclear burning does not proceed rapidly at densities below 1,000 kg/cu cm, and the nuclear energy released is less than the shock energy in all models that come near to ejecting matter.

Barkat, Z.↗

NASA NTRS1975

An extremely carbon enhanced 'nova' model

Expanding upon the authors' earlier work, Hoyle and Clayton (1974) have suggested that a thermonuclear runaway in a white dwarf envelope that consists of equal numbers of protons and C-12 nuclei will produce the s or r-process elements. The present work studies such a runaway and finds that these initial conditions cause a 'super' nova outburst that does not result in any s or r-process nucleosynthesis. However, the model is very interesting in that it reaches peak temperatures of 1.6 billion K and peak burning rates exceeding 2 times 10 to the 23rd erg/gm/sec. A shock forms and ejects 10 to the 29th grams moving with speeds up to 60,000 km/sec. The peak bolometric magnitude is -21.2 and this model will also produce a gamma-ray burst.

Starrfield, S. G.↗

NASA NTRS1978

Radiative shock dynamics. II - Hydrogen continua

The interaction between radiation and a shock wave propagating through a stellar atmosphere is investigated. Departures from local thermodynamic equilibrium (LTE) are permitted in the first two levels of a 10-level hydrogen atom; levels 3-10 are in LTE. A piston moving at constant velocity into the bottom of the atmosphere drives a shock wave. This shock produces precursor radiation that diffuses through the gas well ahead of the shock and causes a mild luminosity flash in the emergent Balmer and free-free radiation when it reaches the surface. The precursor wave deposits a large amount of radiative energy in the outer layers of the atmosphere, initiating a radiation-induced pressure wave. The process of energy transfer from the radiation field to the compression wave is similar to the Eddington valve mechanism which drives stellar pulsations. Material is accelerated outward by the radiation-induced wave; eventually it free-falls inward, hits the quasistationary atmosphere, and forms an accretion shock. The piston driven shock is weakened by radiative energy losses. When it reaches the surface, the shock is invisible in the continuum radiation.

Klein, R. I.↗

NASA NTRS1984

Collisionless dissipation in quasi-perpendicular shocks

Microscopic dissipation processes in quasi-perpendicular shocks are studied by two-dimensional plasma simulations in which electrons and ions are treated as particles moving in self-consistent electric and magnetic fields. Cross-field currents induce substantial turbulence at the shock front reducing the reflected ion fraction, increasing the bulk ion temperature behind the shock, doubling the average magnetic ramp thickness, and enhancing the upstream field aligned electron heat flow. The short scale length magnetic fluctuations observed in the bow shock are probably associated with this turbulence.

Forslund, D. W.↗

NASA NTRS1986

Kelvin-Helmholtz instability at the magnetopause boundary

An MHD model is defined for the convective Kelvin-Helmholtz instability (CKHI) which satellite data indicates produces vortices in the near-earth region of the plasma sheet through turbulence caused by shear flow. The MHD equations are solved as an initial value problem, which reveals that the CKHI growth rate is the same as that of the periodic KHI (PKHI). However, the CKHI is not as limited in the range of amplitude as is the PKHI, which experiences stability in nonlinear growth because of the periodic characteristics of the growth. A perpendicularly moving CKHI can produce a large vortex flow and shocks off the vortex boundary. The satellite data indicate that the vortices are in the size range of 10-20 earth radii. Additional effects can include the expulsion of magnetic flux and the formation of the boundary layer. The success of the model up to the point of formation of the boundary layer is taken as an incentive for further model development using more realistic magnetospheric data.

Wu, C. C.↗