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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.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Simultaneous imaging and spectral observations in microwaves and hard X-rays of the impulsive phase of a solar limb flare

Observations of the impulsive phase of a solar flare at microwave wavelengths and in hard X-rays are used to deduce the strength of the magnetic field and the number of energetic electrons producing the burst. The microwave observations, using the VLA at 6 cm, had spatial resolution of 8 x 8 arcsec, close to the resolution of the Hard X-ray Imaging Spectrometer on SMM which also imaged this flare. The Hard X-ray Burst Spectrometer determined the spectrum of the burst in the range 25-512 keV, and several patrol telescopes recorded the microwave time profile at frequencies from 2.8 to 19.6 GHz. The combined data show that the derived number of microwave-emitting electrons is at least three orders of magnitude fewer than the number of thick target electrons producing the hard X-rays. It is proposed that the fast electrons are highly beamed and radiate gyrosynchrotron emission less efficiently than isotropically distributed electrons.

Schmahl, E. J.↗
NASA NTRS1994

Electron time dispersion

Observations of electrons during a sounding rocket flight from Sondrestromfjord, Greenland, in 1985 showed numerous instances of electron time-of-flight dispersion, that is, a coherent structure in which fast electrons arrived before slow electrons. The instrumentation measured many pitch angles simultaneously with high time resolution, providing a detailed view of the temporal evolution of the electron signature at rocket altitudes which was indicative of an impulsive injection of electrons over a range of energies. A model utilizing an extended, quasi-static (static on a timescale of 1 s), parallel electric field was developed to investigate a possible mechanism for the injection of these electrons. The model is found to fit the data well only when a background is added which cannot be self-consistently generated by the model, although other parameters used in the model are in general agreement with previous reports. This leads to the conclusion that electron dispersion is more probably generated by a wave acceleration mechanism.

Kletzing, C. A.↗
NASA NTRS1980

Diffusion length measurements of thin GaAs solar cells by means of energetic electrons

A calculation of the short circuit current density (j sub sc) of a thin GaAs solar cell induced by fast electrons is presented. It is shown that in spite of the disparity in thickness between the N-type portion of the junction and the P-type portion of the junction, the measurement of the bulk diffusion length L sub p of the N-type part of the junction is seriously hampered due to the presence of a sizable contribution to the j sub sc from the P-type region of the junction. Corrections of up to 50% had to be made in order to interpret the data correctly. Since these corrections were not amenable to direct measurements it is concluded that the electron beam method for the determination of the bulk minority carrier diffusion length, which works so well for Si solar cells, is a poor method when applied to thin GaAs cells.

Vonross, O.↗
NASA NTRS1975

Feasibility of coherent X-ray production by X-ray pumping

It is suggested that coherent soft X rays can be produced by inverting the electron population in a suitable target, such as Li, through irradiation with X rays generated by fast electrons traversing an electromagnetic field (as in a storage ring). Conditions to be satisfied by target and radiation parameters are stated, and examples given.

Csonka, P. L.↗
NASA NTRS2011

STEREO Observations of Solar Energetic Particles

We report on observations of Solar Energetic Particle (SEP) events as observed by instruments on the STEREO Ahead and Behind spacecraft and on the ACE spacecraft. We will show observations of an electron event observed by the STEREO Ahead spacecraft on June 12, 2010 located at W74 essentially simultaneously with electrons seen at STEREO Behind at E70. Some similar events observed by Helios were ascribed to fast electron propagation in longitude close to the sun. We will look for independent verification of this possibility. We will also show observations of what appears to be a single proton event with very similar time-history profiles at both of the STEREO spacecraft at a similar wide separation. This is unexpected. We will attempt to understand all of these events in terms of corresponding CME and radio burst observations.

vonRosenvinge, Tycho↗
NASA NTRS1968

Penetration of electrons and associated bremsstrahlung through aluminum targets

This paper contains a brief description of Monte Carlo programs designed to calculate the transport of fast electrons and associated bremsstrahlung through extended media. Two applications are discussed: (1) transmission of electrons through plane-parallel targets, and (2) emergence of secondary bremsstrahlung from such targets. It is shown that the predicted results are in reasonably good agreement with recent experiments for electron beams with energies up to 8 MeV incident normally on aluminum targets. Extensive new calculated data for transmission and thick-target bremsstrahlung production are presented for aluminum targets exposed to an isotropic electron flux.

M. J. Berger↗
NASA NTRS1974

Feasibility of coherent X-ray production by X-ray pumping

It is suggested that coherent X-rays can be produced by inverting the electron population in a suitable target, such as Li, through irradiation with X-rays generated by fast electrons traversing an electromagnetic field (as in a storage ring). Conditions to be satisfied by target and radiation parameters are stated, and examples given.

Csonka, P. L.↗
NASA NTRS1982

Solar flare energetics

A review is presented regarding the current knowledge of the energetics of solar flares. Recent observations by the Solar Maximum Mission and by balloon-borne instrumentation indicate that the flare hard X-ray emission arises from nonthermal bremsstrahlung - the collisions of fast electrons into a cold ambient medium (Ee much greater than kT). Under this interpretation, most of the energy released for many flares is initially contained in the energetic electrons. These electrons can produce most of the observed flare phenomena via interactions with the solar atmosphere. In large flares a shock wave may result from explosive heating of the solar atmosphere by these electrons. This shock wave can accelerate nuclei to relativistic energies. It is argued that recent SMM observations of fast gamma-ray bursts are consistent with this picture of shock acceleration of nuclei.

Lin, R. P.↗
NASA NTRS2000

Electrostatic Discharge Induced in Packaging by Space Radiation

Radiation belts around Planets have sufficient high energy electron flux to penetrate spacecraft skins and statically charge insulators inside the electronic boxes. For example, geosynchronous-orbit Earth spacecraft require 100 mils aluminum shielding to sufficiently attenuate the fast electron flux. Electrons are stopped and accumulate slowly in the insulated materials to produce strong electric fields. Typically the field strength achieves a threshold for occasional spontaneous discharge in the insulating material. The field strength remains high yet pulsing is infrequent. Charge can leak off if the insulator is sufficiently leaky. The conductivity of insulators is usually controlled by mobile ions such as H and OH in ground service. In space the mobile ions are eventually out-gassed. The resistivity of several insulators is known to increase over three decades after exposure to vacuum for several months. Insulators in space were seen to pulse more frequently as they aged.

Frederickson, A. R.↗
NASA NTRS1990

Kinetics of metastable atoms and non-Maxwellian electrons in two-temperature plasmas

Numerical and analytical solutions of the electron Boltzmann equation in two-temperature steady-state helium plasma are studied in a broad range of conditions T(a) = 5,000-20,000 K, T(e) = 10,000-20,000 K; N(a) = 10 to the 10th - 10 to the 18th per cu cm. The WKB analytical solution is found to be satisfactory in most situations. The deviation of the electron distribution from Maxwellian and a possibility of raising of the tail of the distribution in presence of sources of fast electrons is also discussed.

Kunc, J. A.↗
NASA NTRS1988

Fine structure of microwave spike bursts and associated cross-field energy transport

The characteristics of the maser emission from a driven system where energetic electrons continue to flow through the source region is investigated using electronic particle simulations. It is shown that, under appropriate conditions, the maser can efficiently radiate a significant portion of the energy of the fast electrons in a very short time. The radiation is emitted in pulses even though the flow of electrons through the system is at a constant rate. The mission of these pulses is proposed as the source of the fine structure. Under other conditions the dominant maser emission changes from fundamental x-mode to either fundamental z-mode or to electrostatic upper hybrid or Bernstein modes. The bulk of the emission from the maser instability cannot propagate across field lines in this regime, and hence strong local plasma heating is expected, with little energy transport across the magnetic field lines.

Winglee, R. M.↗
NASA NTRS1984

Observational evidence for chromospheric footpoint penetration of nonthermal electrons during two well-observed flares

Recent advances have enabled simultaneous H-alpha and X-ray observations with substantially improved spatial, spectral, and temporal resolution. In this paper a study is conducted of two events observed as part of a coordinated observing program between the Solar Maximum Mission and Sacramento Peak Observatory: the flares of 1456 UT, 7 May 1980 and 1522 UT, 24 June 1980. Using recently developed physical models of static flare chromospheres, and corresponding theoretical H-alpha line profiles, effects of intense nonthermal electron heating can be distinguished from those of high conduction and pressure from the overlying flare corona. Both flares show the signature of intense chromospheric heating by fast electrons, temporally correlated with X-ray light curves at E greater than 27keV, and spatially associated with X-ray emission sites at E greater than 16 keV. Interpreting the H-alpha line profile observations using the theoretical H-alpha line profiles, values of the thick-target input power contained in nonthermal electrons that are observationally indistinguishable (within a factor of 2-3) from those inferred from the X-ray data are inferred. Although these events are small, the energy flux values are large: of order 10 to the 11th ergs per sq cm per s above 20 keV.

Canfield, R. C.↗
NASA NTRS1974

Non-relativistic solar electrons

Summary of both the direct spacecraft observations of nonrelativistic solar electrons, and observations of the X-ray and radio emission generated by these particles at the sun and in the interplanetary medium. These observations bear on three physical processes basic to energetic particle phenomena: (1) the acceleration of particles in tenuous plasmas; (2) the propagation of energetic charged particles in a disordered magnetic field, and (3) the interaction of energetic charged particles with tenuous plasmas to produce electromagnetic radiation. Because these electrons are frequently accelerated and emitted by the sun, mostly in small and relatively simple flares, it is possible to define a detailed physical picture of these processes. In many small solar flares nonrelativistic electrons accelerated during flash phase constitute the bulk of the total flare energy. Thus the basic flare mechanism in these flares essentially converts the available flare energy into fast electrons. Nonrelativistic electrons exhibit a wide variety of propagation modes in the interplanetary medium, ranging from diffusive to essentially scatter-free. This variability in the propagation may be explained in terms of the distribution of interplanetary magnetic field fluctuations.

Lin, R. P.↗
NASA NTRS1975

What we have learned from the magnetosphere

Three significant discoveries resulting from recent studies of earth's magnetosphere are discussed: (1) the central role of magnetic-field-line reconnection in determining the topology, geometry, and dynamics of the magnetosphere; (2) the realization that auroral arcs result from coupling of solar-wind stresses to the ionosphere and neutral atmosphere; and (3) the role of plasma turbulence in affecting both the microscopic state and macroscopic observables of the magnetospheric plasma. Magnetic-field-line reconnection is described in terms of Dungey's (1961) model of the magnetosphere, a simple picture of magnetic-substorm evolution is outlined, and questions concerning the reconnection rate are considered. The production of aurorae by field-aligned electron beams is examined, and an analogy is made between earth's magnetosphere and that of a pulsar with aligned dipole and spin axes. Plasma-turbulence problems are reviewed which concern anomalously fast electron and proton losses from the Van Allen belts, whistler-electron interactions, and anomalous resistance in auroral arcs. The Pioneer 10 encounter with Jupiter's magnetosphere is briefly noted.

Kennel, C. F.↗
NASA NTRS1985

Temporal evolution of an energetic electron population in an inhomogeneous medium: Application to solar hard X-ray bursts

Energetic electrons accelerated during solar flares can be studied through the hard X-ray emission they produce when interacting with the solar ambient atmosphere. In the case of the non thermal hard X-ray emission, the instanteous X-ray flux emitted at one point of the atmosphere is related to the instantaneous fast electron spectrum at that point. A hard X-ray source model then requires the understanding of the evolution in space and time of the fast particle distribution. The physical processes involved here are energy losses due to Coulomb collisions and pitch angle scattering due to both collisions and magnetic field gradients.

Vilmer, N.↗
NASA NTRS1973

Energetic particles as a source of flare energy

The production of solar flares by the reservoir of energy generated by sunspot magnetic fields is discussed. Electrons play an important part in the flare process since high energy electrons are responsible for X-ray and radio emission. The energy required in the form of protons trapped in sunspot magnetic fields to produce large solar flares is analyzed. A trigger mechanism is described where the particles can be rapidly dumped into the chromosphere when fast electrons are produced by Coulomb collisions and through pion production by the highest energy protons.

Elliot, H.↗
NASA NTRS1982

Radio wave heating of the corona and electron precipitation during flares

Electron-cyclotron masers, excited while energy release is occurring in a flaring magnetic loop, are likely to generate extremely intense radiation at decimeter wavelengths. The energy in the radiation can be comparable with that in the electrons associated with hard X-ray bursts, i.e., a significant fraction of the total energy in the flare. Essentially all of the radio energy is likely to be reabsorbed by gyroresonance absorption, either near the emitting region or at some distance away in neighboring loops. Enhanced diffusion of fast electrons caused by the maser can lead to precipitation at the maximum possible rate, and hence account for hard X-ray emission from the footpoints of the magnetic loops.

Melrose, D. B.↗
NASA NTRS1994

Evolution of a localized Langmuir packet in the solar wind and on auroral field lines

Langmuir emissions in space are reported to be clumpy and intermittent. The high-frequency wave power appears concentrated in spatial packets, whether amidst the solar wind or on auroral field lines. Due to the plasma motion relative to the spacecraft, determining the source for the wave free energy in the three-dimensional electron distribution function has always been difficult, since the unstable features pass by the detector in presumably too short time to be measured. The range of unstable phase velocities and growth rates have generally been estimated rather than determined by unequivocal measurements. The analysis of wave-particle interactions in a space environment has taken recently a new turn with the development of wave correlators on board rockets and satellites. Such instruments seek to identify correlations between the phase of the wave-field and the fluxes of energetic particles. The data interpretation is complex, however, it must be backed by a detailed theoretical understanding of the wave-particle interaction, including the phase relation for inhomogeneous packets. To this end Langmuir packets interacting with fast electrons can be studied in the appropriate regime by means of particle-in-cell simulations, provided that one succeeds in reducing the level of the fluctuations, enhancing the signal-to-noise ratio, and incorporating the appropriate boundary conditions. The first results of such simulations are presented here as a test and expansion of previous analysis.

Roth, I.↗