SEARCH · Engineering Papers
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
A one-dimensional model is considered in which an increasingly large electric field is induced by a rapidly evolving magnetic field. In the case of solar flares, energies are estimated to which protons and electrons may be directly accelerated by such an induced electric field.
Plasma current sheet permeability in coaxial accelerator determined from magnetic and electric field measurements
Beams of relativistic electrons and/or positrons leaving the surface of a strongly magnetized neutron star may give rise to gamma-ray bursts. The beams could be accelerated by strong, magnetically aligned electric fields that are produced by oscillations of the stellar surface. Here we investigate the particle acceleration in these electric fields, the resulting electron-positron pair cascade, and the gamma-ray emission. We find that beams of electrons and positrons moving parallel to the magnetic field are generated, with a reported differential energy distribution. These beams produce the bulk of the gamma-ray burst radiation below about 1 MeV by the resonant Compton scattering of thermal photons emitted from the stellar surface. The escaping synchrotron radiation from the cascade dominates the radiation spectrum above about 1 MeV.
Test particle calculations of the interaction of electrons and ions with electromagnetic ion cyclotron waves are examined. It is observed that field-aligned electrons can be accelerated parallel to the magnetic fields by the cyclotron waves and that the ions can be accelerated perpendicular to the magnetic field. The data reveal that the ions can be accelerated more effectively when the wave frequency is half (.5) the ion gyrofrequency.
Recent PIC simulations of relativistic electron-positron (electron-ion) jets injected into a stationary medium show that particle acceleration occurs at shocked regions. Simulations show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields and particle acceleration. These magnetic fields contribute to the electron's transverse deflection behind the shock. The "jitter" radiation from deflected electrons in turbulent magnetic fields has different properties than synchrotron radiation, which is calculated in a uniform magnetic field. This jitter radiation may be important for understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets in general, and supernova remnants. We will present detailed spectra for conditions relevant of various astrophysical sites of shock formation via the Weibel instability. In particular we will discuss the application to GRBs and SNRs.
Magnetic reconnection, a plasma process converting magnetic energy to particle kinetic energy, is often invoked to explain magnetic energy releases powering high-energy flares in astrophysical sources including pulsar wind nebulae and black hole jets. Reconnection is usually seen as the (essentially 2D) nonlinear evolution of the tearing instability disrupting a thin current sheet. To test how this process operates in 3D, we conduct a comprehensive particle-in-cell simulation study comparing 2D and 3D evolution of long, thin current sheets in moderately-magnetized, collisionless, relativistically-hot electron- positron plasma, and find dramatic differences. We first systematically characterize this process in 2D, where classic, hierarchical plasmoid-chain reconnection determines energy release, and explore a wide range of initial configurations, guide magnetic field strengths, and system sizes. We then show that 3D simulations of similar configurations exhibit a diversity of behaviours, including some where energy release is determined by the nonlinear relativistic drift-kink instability. Thus, 3D current-sheet evolution is not always fundamentally classical reconnection with perturbing 3D effects, but, rather, a complex interplay of multiple linear and nonlinear instabilities whose relative importance depends sensitively on the ambient plasma, minor configuration details, and even stochastic events. It often yields slower but longer-lasting and ultimately greater magnetic energy release than in 2D. Intriguingly, nonthermal particle acceleration is astonishingly robust, depending on the upstream magnetization and guide field, but otherwise yielding similar particle energy spectra in 2D and 3D. Though the variety of underlying current-sheet behaviours is interesting, the similarities in overall energy release and particle spectra may be more remarkable.
Gradual hard X-ray/radio bursts are characterized by their long duration, smooth time profile, time delays between peaks at different hard X-ray energies and microwaves, and radiation from extended sources in the low and middle corona. Their characteristic properties have been ascribed to the dynamic evolution of the accelerated electrons in coronal magnetic traps or to the separate acceleration of high-energy electrons in a 'second step' process. The information available so far was drawn from quality considerations of time profiles or even only from the common occurrence of emissions in different spectral ranges. This paper presents model computations of the temporal evolution of hard X-ray and microwave spectra, together with a qualitative discussion of radio lightcurves over a wide spectral range, and metric imaging observations. The basis hypothesis investigated is that the peculiar 'gradual' features can be related to the dynamical evolution of electrons injected over an extended time interval in a coronal trap, with electrons up to relativistic energies being injected simultaneously. The analyzed event (26 April. 1981) is particularly challenging to this hypothesis because of the long time delays between peaks at different X-ray energies and microwave frequencies. The observations are shown to be consistent with the hypothesis, provided that the electrons lose their energy by Coulomb collisions and possibly betatron deceleration. The access of the electrons to different coronal structures varies in the course of the event. The evolution and likely destabilization of part of the coronal plasma-magnetic field configuration is of crucial influence in determining the access to these structures and possibly the dynamical evolution of the trapped electrons through betatron deceleration in the late phase of the event.
Polar substorm particle event observed with magnetic field and trapped particle detectors on Explorer 26 during April 1965 worldwide magnetic storm
Mesoscale dynamics are a fundamental process in space physics, but fall within an observational gap of current and planned missions. Particularly in the solar wind, measurements at the mesoscales (100s RE to a few degrees heliographic longitude at 1 au) are crucial for understanding the connection between the corona and an observer anywhere within the heliosphere. Mesoscale dynamics may also be key to revealing the currently unresolved physics regulating particle acceleration and transport, magnetic field topology, and the causes of variability in the composition and acceleration of solar wind plasma. Studies using single-point observations do not allow for investigations into mesoscale solar wind dynamics and plasma variability, nor do they allow for the exploration of the sub-structuring of large-scale solar wind structures like coronal mass ejections (CMEs), co-rotating/stream interaction regions (CIR/SIRs), and the heliospheric plasma sheet. To address this fundamental gap in our knowledge of the heliosphere at these scales, the Interplanetary Mesoscale Observatory (InterMeso) concept employs a multi-point approach using four identical spacecraft in Earth-trailing orbits near 1 au. Varying drift speeds of the InterMeso spacecraft enable the mission to span a range of mesoscale separations in the solar wind, achieving significant and innovative science return. Simultaneous, longitudinally-separated measurements of structures co-rotating over the spacecraft also allow for disambiguation of spatiotemporal variability, tracking of the evolution of solar wind structures, and determination of how the transport of energetic particles is impacted by these variabilities.
Mesoscale dynamics are a fundamental process in space physics, but fall within an observational gap of current and planned missions. Particularly in the solar wind, measurements at the mesoscales (100s RE to a few degrees heliographic longitude at 1 au) are crucial for understanding the connection between the corona and an observer anywhere within the heliosphere. Mesoscale dynamics may also be key to revealing the currently unresolved physics regulating particle acceleration and transport, magnetic field topology, and the causes of variability in the composition and acceleration of solar wind plasma. Studies using single-point observations do not allow for investigations into mesoscale solar wind dynamics and plasma variability, nor do they allow for the exploration of the sub-structuring of large-scale solar wind structures like coronal mass ejections (CMEs), co-rotating/stream interaction regions (CIR/SIRs), and the heliospheric plasma sheet. To address this fundamental gap in our knowledge of the heliosphere at these scales, the Interplanetary Mesoscale Observatory (InterMeso) concept employs a multi-point approach using four identical spacecraft in Earth-trailing orbits near 1 au. Varying drift speeds of the InterMeso spacecraft enable the mission to span a range of mesoscale separations in the solar wind, achieving significant and innovative science return. Simultaneous, longitudinally-separated measurements of structures co-rotating over the spacecraft also allow for disambiguation of spatiotemporal variability, tracking of the evolution of solar wind structures, and determination of how the transport of energetic particles is impacted by these variabilities.
This grant was originally awarded for the flight of the AMICIST sounding rocket. However, upon launch failure of the PHAZE rocket, additional resources were placed in this grant to cover the launch of the PHAZE II rocket. AMICIST was successfully launched from the Poker Flat Range on February 24, 1995, and the PHAZE II was successfully launched also from the Poker Flat Range on February 10, 1997. The major objective of the AMICIST flight was to investigate the bursts of transverse ion acceleration occurring during aurora, commonly known as Lower Hybrid Solitary Structures, with the flight of two scientific payloads to unravel space from time effects. The data clearly showed that the structures of ion acceleration were a spatial phenomena having a scale size transverse to the local magnetic field of about 100 m. On the other hand, structures in the auroral electrons were generally observed on this flight to be temporal features that occurred at the same time at the two payloads separated by a few kilometers. The primary objective of the PHAZE rocket flight was to further study the temporal features of auroral electron precipitation having many electron detectors, some fixed in energy, that could study the distribution function of the auroral electrons on time scales of a fraction of a millisecond. A important paper on this topic has just been submitted for publication which uses the PHAZE data to show that the potential structure (electric field parallel to the magnetic field) that accelerates the auroral electrons within one Earth radius of the ground, is not static, but rather fluctuates with frequencies close to the local proton and hydrogen gyrofrequencies. The fluctuation appears to be an actual turning on and off of the electric field at these frequencies. When the potential is turned off, ambient electrons can enter the region and be accelerated along B when the potential is on creating field- aligned bursts which manifest themselves as flickering aurora seen from the ground. The on/off model can explain many of the dynamic features of the aurora which have been unexplained since man first looked up at the visual display. Much needs to be done yet to understand how ions couple into this process.
Hall-current variations, as a function of magnetic field strength, are measured in an ion accelerator, for axial electric discharges in radial magnetic fields
It has become increasingly clear in recent years that the available models describing the acceleration of high speed streams in solar coronal holes are inadequate. Thermal models do not result in winds with the proper thermal speed and density. In addition models which assume acceleration by Alfven waves, though promising, are not self-consistent when applied low in coronal holes where the acceleration of the wind takes place. An Alfven wave in the corona with a period of a few hundred seconds will have a wavelength of the order of 10 cm. This is of the order of the diameter of a coronal hole in the low corona, and therefore it is expected that the MHD waves are guided in some sense by the coronal hole in which they propagate. This basic idea led to the development of a leaky waveguide theory to describe MHD wave propagation in coronal holes. Recent efforts were concentrated in two areas: the development of numerical codes to properly describe the hydrodynamics of the flow, and more realistic models of the coronal hole properties such as density gradients and spherical divergence of the flux tube. Some of the important aspects of the theory are discussed, including the effect of density inhomogeneities and smooth transverse density gradients. In addition it is hoped that some early results from the numerical models will be presented. The acceleration of cool winds is a problem of interest in cool supergiants also. Some work was done on the development of wind models for these stars as well.
Discharge theory of solar flares - growth rate of flares, particle acceleration, topology of magnetic fields, and proton event prediction
A truly robust space exploration program will need to make use of in-situ resources as much as possible to make the endeavor affordable. Most space propulsion concepts are saddled with one fundamental burden; the propellant needed to produce momentum. The most advanced propulsion systems currently in use utilize electric and/or magnetic fields to accelerate ionized propellant. However, significant planetary exploration missions in the coming decades, such as the now canceled Jupiter Icy Moons Orbiter, are restricted by propellant mass and propulsion system lifetimes, using even the most optimistic projections of performance. These electric propulsion vehicles are inherently limited in flexibility at their final destination, due to propulsion system wear, propellant requirements, and the relatively low acceleration of the vehicle. A few concepts are able to utilize the environment around them to produce thrust: Solar or magnetic sails and, with certain restrictions, electrodynamic tethers. These concepts focus primarily on using the solar wind or ambient magnetic fields to generate thrust. Technically immature, quasi-propellantless alternatives lack either the sensitivity or the power to provide significant maneuvering. An additional resource to be considered is the ambient plasma and magnetic fields in solar and planetary magnetospheres. These environments, such as those around the Sun or Jupiter, have been shown to host a variety of plasma waves. Plasma wave propulsion takes advantage of an observed astrophysical and terrestrial phenomenon: Alfven waves. These are waves that propagate in the plasma and magnetic fields around and between planets and stars. The generation of Alfven waves in ambient magnetic and plasma fields to generate thrust is proposed as a truly propellantless propulsion system which may enable an entirely new matrix of exploration missions. Alfven waves are well known, transverse electromagnetic waves that propagate in magnetized plasmas at frequencies below the ion cyclotron frequency. They have been observed in both laboratory and astrophysical settings. On Earth, they are being investigated as a possible means for plasma heating, current drive, and momentum addition in magnetic confinement fusion systems. In addition, Alfven waves have been proposed as a mechanism for acceleration of the solar wind away from the sun.
Turbulence from eddies in plasma coaxial accelerator, turbulence instabilities at plasma magnetic field interfaces, and plasma intrusion into model magnetosphere
Pioneer Venus magnetic field and plasma wave data are examined in a particularly clear example of a plasma cloud above the Venus ionosphere. The magnetic configuration is suggestive of acceleration of the plasma cloud by magnetic tension. If the plasma is at rest at the subsolar point, it could be accelerated to approximately 90 km/sec by the observed stress at the location of the measurement. This far exceeds the escape velocity and suggests that plasma clouds do form a significant loss mechanism for the Venus ionosphere but does not necessarily indicate that the plasma cloud is detached from the ionosphere proper. The plasma cloud is accompanied by strong plasma wave activity and is significantly hotter than the ionospheric plasma encountered later on the same pass. A loss rate of the order of 2 x 10 to the 25th ions/sec is estimated during this event. The geometry suggested by these observations is one of a ridge of dense cold plasma starting in the subsolar regions and flowing over the poles of the planet. Thus, these plasma clouds may be the planetary analog of cometary tail rays.
A flare with several radio sources occurred on the solar limb at 2155 UT on 1989 June 20. It was observed by the Very Large Array (VLA) and the Owens Valley Radio Observatory (OVRO). The VLA data consisted of images at 1.4 GHz, while OVRO provided spectral and spatial information over the range 1-15 GHz. We develop a new gyrosynchrotron model to analyze the sources observed at flare peak. This model differs from many previous ones in that it contains spatial variations of both the magnetic field and accelerated particle density. It uses a new gyrosynchrotron approximation which is valid at very low harmonics of the gyrofrequency. For the first time we find that the cause of the change of microwave source size with frequency in the event studied was due primarily to the spatial variation of the accelerated electron density. This is contrary to the common assumption that the variation in size is due to a nonuniform magnetic field. We also investigate a polarized source which brightened later in the flare, finding that it could have been due to plasma radiation or gyrosynchrotron emission.