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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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Ion Sources and Acceleration Mechanisms Inferred from Local Distribution Functions

This study investigates the sources of the ions up the complex and nonisotropic H(+) velocity distribution functions observed by the Geotail spacecraft on May 23, 1995, in the near-Earth magnetotail region and recently reported by Frank et al. [1996]. A distribution function observed by Geotail at -10 R(sub E) downtail is used as input for the large scale kinetic (LSK) technique to follow the trajectories of approximately 90,000 H(+) ions backward in time. Time-dependent magnetic and electric fields are taken from a global magnetohydrodynamic (MHD) simulation of the magnetosphere and its interactions with appropriate solar wind and IMF conditions. The ion population described by the Geotail distribution function was found to consist of a mixture of particles originating from three distinct sources: the ionosphere, the low latitude boundary layer (LLBL), and the high latitude plasma mantle. Ionospheric particles had direct access along field lines to Geotail, and LLBL ions convected adiabatically to the Geotail location. Plasma mantle ions, on the other hand, exhibited two distinct types of behavior. Most near-Earth mantle ions reached Geotail on adiabatic orbits, while distant mantle ions interacted with the current sheet tailward of Geotail and had mostly nonadiabatic orbits. Ions from the ionosphere, the LLBL, and the near-Earth mantle were directly responsible for the well-separated, low energy structures easily discernible in the observed and modeled distribution functions. Distant mantle ions formed the higher energy portion of the Geotail distribution. Thus, we have been successful in extracting useful information about particle sources, their relative contribution to the measured distribution and the acceleration processes that affected particle transport during this time.

Ashour-Abdalla, M.↗
NASA NTRS1981

A satellite investigation of energy flux and inferred potential drop in auroral electron energy spectra

The relationship between auroral electron energy flux and the inferred accelerating potential drop for accelerated Maxwellian distributions is investigated on the basis of Atmospheric Explorer D spectral measurements. An analytical approximation for the total downward energy flux carried by an isotropic Maxwellian electron population accelerated by a field-aligned electrostatic potential drop is derived which is valid for values of the electron energy/characteristic accelerated Maxwellian distribution energy which are less than the difference between the ratio of the magnetic field strengths at the altitude of observation and the altitude of potential drop, and unity. Data from the Low Energy Electron Experiment on board AE D obtained on both the dayside and the nightside during periods of significant inverted-V type electron precipitation shows that the 455 energy spectra considered, 160 of them, obtained between 60 and 85 deg invariant latitude, could be fit to accelerated Maxwellian distributions. The 160 Maxwellian spectra are then shown to be in agreement with the predictions of the accelerated Maxwellian model. Finally, analysis of individual spectra suggests that the altitude of the inferred potential drop is at a maximum near the center of the inverted-V structures.

Menietti, J. D.↗
NASA NTRS1981

A satellite investigation of energy flux and inferred potential drop in auroral electron energy spectra

Because predicted relationship (epsilon directly varies with V squared) between auroral electron energy flux (epsilon) and the inferred acceleration potential drop (V) for accelerated Maxwellian distributions was favorably tested by other using sounding rocket data for the limiting case of eVE 1 (where Ec is the characteristic energy of the accelerated Maxwellian distribution) and for a single inverted-V observed by the Injun 5 satellite, data from Atmosphere D were used to extend these studies over the range .2 eV/Ec 5 and for a wide range of latitudes and local times on both the nightside and the dayside. Results show good agreement with the full accelerated Maxwellian model. An analytical approximation to the electron energy flux was derived which better describes the data over the range .2 eV/Ec approximated 3. Analyses of individual energy spectra at small and large pitch angles through well-defined inverted-V structures suggest that the altitude of the inferred potential drop maximizes near the center of the inverted-V's.

Meniett, J. D.↗
NASA NTRS1986

Electron acceleration in flares inferred from radio and hard X-ray emissions

Properties of electron acceleration in flares, especially the density structure in the acceleration region, are deduced from a correlation study between decimetric type III, spike, and hard X-ray (HXR) bursts. The high association rate found (71 percent) strongly suggests that spikes also originate from energetic electrons. Spikes and type III bursts have been found to be easily identified by their different polarizations. The two types of emission generally do not overlap in frequency. A reliable lower limit to the density is derived from the starting frequency of type III and U bursts. The spike emission very likely yields an upper limit. The density inhomogeneity in the acceleration region spans more than one order of magnitude and is more than one order of magnitude larger in the associated type U sources. A peak-to-peak correlation does not always exist between type III, spike and HXR bursts. This discrepancy can be interpreted in terms of the different source conditions and propagation properties. Whereas spikes need special conditions to become visible, type III and peaks of HXR may be the product of many elementary accelerations.

Benz, A. O.↗
NASA NTRS1989

Hypersonic rarefied-flow aerodynamics inferred from Shuttle Orbiter acceleration measurements

Data obtained from multiple flights of sensitive accelerometers on the Space Shuttle Orbiter during reentry have been used to develop an improved aerodynamic model for the Orbiter normal- and axial-force coefficients in hypersonic rarefied flow. The lack of simultaneous atmospheric density measurements was overcome in part by using the ratio of normal-to-axial acceleration, in which density cancels, as a constraint. Differences between the preflight model and the flight-acceleration-derived model in the continuum regime are attributed primarily to real gas effects. New insights are gained into the variation of the force coefficients in the transition between the continuum regime and free molecule flow.

Blanchard, R. C.↗
NASA NTRS2008

Inferring the Energy Distribution of Accelerated Electrons in Solar Flares from X-ray Observations

Knowledge of the energy distribution of electrons accelerated in solar flares is important for constraining possible acceleration mechanisms and for understanding the relationships between flare X-ray sources, radio sources, and particles observed in space. Solar flare hard X-rays are primarily emitted from dense, thick-target regions in the lower atmosphere, but the electrons are understood to be accelerated higher in the corona. Various processes can distort the X-ray spectrum or the energy distribution of electrons before they reach the thick-target region. After briefly reviewing the processes that affect the X-ray spectrum and the electron distribution, I will describe recent results from a study of flare spectra from RHESSI to determine the importance of these processes in inferring the energy distribution of accelerated electrons.

Holman, Gordon D.↗
NASA NTRS2011

Heliocentric Distance of Coronal Mass Ejections at the Time of Energetic Particle Release: Revisiting the Ground Level Enhancement Events of Solar Cycle 23

Using the kinematics of coronal mass ejections (CMEs), onset time of soft X-ray flares, and the finite size of the pre-eruption CME structure, we derive the heliocentric distan~e at which the energetic particles during the ground level enhancement (GLE) events of Solar Cycle 23. We find that the GLE particles are released when the CMEs reach an average heliocentric distance of approx.3.25 solar radii (Rs). From this we infer that the shocks accelerating the particles are located at similar heights. Type II radio burst observations indicate that the CMEs are at much lower distances (average approx.1.4 Rs) when the CME-driven shock first forms. The shock seems to travel approx.1.8 Rs over a period of approox.30 min on the average before releasing the GLE particles. In deriving these results, we made three assumptions that have observational support: (i) the CME lift off occurs from an initial distance of about 1.25 Rs; (ii) the flare onset and CME onset are one and the same because these are two different manifestations of the same eruption; and (iii) the CME has positive acceleration from the onset to the first appearance in the coronagraphic field of view (2.5 to 6 Rs). Observations of coronal cavities in eclipse pictures and in coronagraphic images justify the assumption (i). The close relationship between the flare reconnection magnetic flux and the azimuthal flux of interplanetary magnetic clouds justify assumption (ii) consistent with the standard model (CSHKP) of solar eruption. Coronagraphic observations made close to the solar surface indicate a large positive acceleration of CMEs to a heliocentric distance of approx.3 Rs before they start slowing down due to the drag force. The inferred acceleration (approx.1.5 km/s/s) is consistent with reported values in the literature.

Gopalswamy, Natchimuthuk↗
NASA NTRS1994

An estimate of the momentum deposition in the lower thermosphere by the observed diurnal tide

This paper reports a calculation of the acceleration of the zonal mean flow induced by dissipating tides in the equatorial lower thermosphere. Estimates of the gravest symmetric gravitiational Hough mode (1,1) of the migrating diurnal tide are obtained from monthly composites of global winds observed by the Upper Atmosphere Research Satellite (UARS) High Resolution Doppler Imager (HRDI). Using the principles of classical tidal theory, the tidal momentum flux divergence is computed for a series of monthly mean (1,1) fields from January 1992 to May 1993. The contribution to the mean flow by the leading mode of the migrating tide ranges between -5 and -20 (easterly) m/s/day in the equatorial lower thermosphere. A semiannual variation is noted in the tidal amplitudes and the inferred tidal accelerations. These variations are consistent with observed trends in the zonal mean flow of the lower thermosphere.

Lieberman, Ruth S.↗
NASA NTRS1994

Latitudinal variation of speed and mass flux in the acceleration region of the solar wind inferred from spectral broadening measurements

Spectral broadening measurements conducted at S-band (13-cm wavelength) during solar minimum conditions in the heliocentric distance range of 3-8 R(sub O) by Mariner 4, Pioneer 10, Mariner 10, Helios 1, Helios 2, and Viking have been combined to reveal a factor of 2.6 reduction in bandwidth from equator to pole. Since spectral broadening bandwidth depends on electron density fluctuation and solar wind speed, and latitudinal variation of the former is available from coherence bandwidth measurements, the remote sensing spectral broadening measurements provide the first determination of the latitudinal variation of solar wind speed in the acceleration region. When combined with electron density measurements deduced from white-light coronagraphs, this result also leads to the first determination of the latitudinal variation of mass flux in the acceleration region. From equator to pole, solar wind speed increases by a factor of 2.2, while mass flux decreases by a factor of 2.3. These results are consistent with measurements of solar wind speed by multi-station intensity scintillation measurements, as well as measurements of mass flux inferred from Lyman alpha observations, both of which pertain to the solar wind beyond 0.5 AU. The spectral broadening observations, therefore, strengthen earlier conclusions about the latitudinal variation of solar wind speed and mass flux, and reinforce current solar coronal models and their implications for solar wind acceleration and solar wind modeling.

Woo, Richard↗
NASA NTRS1993

On the High- and Low- Altitude Limits of the Auroral Electric Field Region

Using measurements from the High Altitude Plasma Instrument (HAPI) on the Dynamics-Explorer 1 (DE-1) spacecraft and the Low Altitude Plasma Instrument (LAPI) on Dynamics Explorer 2 (DE 2), we investigate both die high altitude and low altitude extents of the auroral acceleration region. To infer the high altitude limit, we searched the HAPI data base for evidence of upward-directed auroral electric fields located above the spacecraft when the HAPI spacecraft is above 9000 km altitude. We find that such acceleration is common when DE-1 flies through die auroral oval at an altitude of 9,000-11,000 km. At altitudes above 11,000 km, the fraction of the orbits with evidence of at least a 1000 V potential drop above the spacecraft falls, becoming essentially zero above an altitude of 15,000 km. Above that altitude, small (100 V) potential drops are frequently observed, but only rarely are approx. 1 kV potentials observed, typically associated with polar cap or 'theta' arcs or westward traveling surges. To investigate the low-altitude limit of the auroral acceleration region, we use conjunctions of DE 1 and DE 2 along auroral field lines and match the upgoing fluxes of ionospheric ions observed by DE 2 with the flux of accelerated upgoing ions observed at DE 1. Calculating the ionospheric scale height from the ion and electron temperatures and assuming that the parallel flow velocity is independent of height above 800 km, we calculate the altitude at which the upwelling ionospheric ions are effectively completely lost to upward acceleration. The initial lowest-altitude acceleration process could be either a perpendicular acceleration or a parallel electric field, but it must be sufficient to give the entire distribution escape energy. We find that in the two cases studied, near the region of peak auroral potential drop the altitude of this acceleration was around 1700 km (near the O/H neutral crossover altitude), but was significantly higher (approx. 2000 km) near the edges of the arc, where the potential was lower. The composition of the upgoing ion beam was consistent with these heights, being predominately H(+) near the edges and O(+) near the peak.

Reiff, P. H.↗
NASA NTRS2001

Vestibular compensation and orientation during locomotion

Body, head, and eye movements were studied in three dimensions while walking and turning to determine the role of the vestibular system in directing gaze and maintaining spatial orientation. The body, head, and eyes were represented as three-dimensional coordinate frames, and the movement of these frames was related to a trajectory frame that described the motion of the body on a terrestrial plane. The axis-angle of the body, head, and eye rotation were then compared to the axis-angle of the rotation of the gravitoinertial acceleration (GIA). We inferred the role of the vestibular system during locomotion and the contributions of the VCR and VOR by examining the interrelationship between these coordinate frames. Straight walking induced head and eye rotations in a compensatory manner to the linear accelerations, maintaining head pointing and gaze along the direction of forward motion. Turning generated a combination of compensation and orientation responses. The head leads and steers the turn while the eyes compensate to maintain stable horizontal gaze in space. Saccades shift horizontal gaze as the turn is executed. The head pitches, as during straight walking. It also rolls so that the head tends to align with the orientation of the GIA. Head orientation changes anticipate orientation changes of the GIA. Eye orientation follows the changes in GIA orientation so that the net orientation gaze is closer to the orientation of the GIA. The study indicates that the vestibular system utilizes compensatory and orienting mechanisms to stabilize spatial orientation and gaze during walking and turning.

Non-NASA Center↗
NASA NTRS1992

Observations of VHF emissions from 50-mA electron beam injections in the ionosphere that are associated with beam-induced discharges

Results are presented of observations of strong VHF plasma waves with amplitudes in excess of 0.1 mV/m (Hz)1/2 associated with 50 mA electron beam injections in the ionosphere. Data from three swept-frequency receivers carried on two daughter payloads are analyzed to determine the emission spectra of the electron beam for various energies and currents. These results were obtained from the rocket-borne experiment SCEX 3, NASA flight 39.002 UE, launched February 1, 1990. The accelerator payload also carried photometers, which measured luminosity at wavelengths of 391.4 and 380.5 nm. Several times during electron gun activity the measured luminosity increased much faster than proportional to the beam current. This nonlinear increase is evidence that a discharge is occurring in the vicinity of the accelerator payload. It is inferred from the uniform distribution of the luminosity around the accelerator payload that the observed discharge extends significantly outside the beam cylinder.

Goerke, R. T.↗
NASA NTRS2004

Measuring Gravitation Using Polarization Spectroscopy

A proposed method of measuring gravitational acceleration would involve the application of polarization spectroscopy to an ultracold, vertically moving cloud of atoms (an atomic fountain). A related proposed method involving measurements of absorption of light pulses like those used in conventional atomic interferometry would yield an estimate of the number of atoms participating in the interferometric interaction. The basis of the first-mentioned proposed method is that the rotation of polarization of light is affected by the acceleration of atoms along the path of propagation of the light. The rotation of polarization is associated with a phase shift: When an atom moving in a laboratory reference interacts with an electromagnetic wave, the energy levels of the atom are Doppler-shifted, relative to where they would be if the atom were stationary. The Doppler shift gives rise to changes in the detuning of the light from the corresponding atomic transitions. This detuning, in turn, causes the electromagnetic wave to undergo a phase shift that can be measured by conventional means. One would infer the gravitational acceleration and/or the gradient of the gravitational acceleration from the phase measurements.

Matsko, Andrey↗
NASA NTRS1993

Latitudinal Variation of Solar Wind Speed and Mass Flux in the Acceleration Region of the Solar Wind during Solar Minimum Inferred from Spectral Broadening measurements

In this paper, we use an aggregate of S-band 2.3 GHz (13 cm) spectral broadening observations conducted during solar minimum conditions by the Mariner 4, Pioneer 10, Mariner 10, Helios 1 & 2 and Viking spacecraft to infer the first measurements of the latitudinal variation of solar wind speed and mass flux in the acceleration region of the solar wind at 3-8 R(sub o).

latitudinal variation of solar wind mass flux in a↗
NASA NTRS1988

Collisionless shock formation and the prompt acceleration of solar flare ions

The formation mechanisms of collisionless shocks in solar flare plasmas are investigated. The priamry flare energy release is assumed to arise in the coronal portion of a flare loop as many small regions or 'hot spots' where the plasma beta locally exceeds unity. One dimensional hybrid numerical simulations show that the expansion of these 'hot spots' in a direction either perpendicular or oblique to the ambient magnetic field gives rise to collisionless shocks in a few Omega(i), where Omega(i) is the local ion cyclotron frequency. For solar parameters, this is less than 1 second. The local shocks are then subsequently able to accelerate particles to 10 MeV in less than 1 second by a combined drift-diffusive process. The formation mechanism may also give rise to energetic ions of 100 keV in the shock vicinity. The presence of these energetic ions is due either to ion heating or ion beam instabilities and they may act as a seed population for further acceleration. The prompt acceleration of ions inferred from the Gamma Ray Spectrometer on the Solar Maximum Mission can thus be explained by this mechanism.

Cargill, P. J.↗
NASA NTRS1990

Energetic particle abundances in solar electron events

The results of a comprehensive search of the ISEE 3 energetic particle data for solar electron events with associated increases in elements with atomic number Z = 6 or greater are reported. A sample of 90 such events was obtained. The events support earlier evidence of a bimodal distribution in Fe/O or, more clearly, in Fe/C. Most of the electron events belong to the group that is Fe-rich in comparison with the coronal abundance. The Fe-rich events are frequently also He-3-rich and are associated with type III and type V radio bursts and impulsive solar flares. Fe-poor events are associated with type IV bursts and with interplanetary shocks. With some exceptions, event-to-event enhancements in the heavier elements vary smoothly with Z and with Fe/C. In fact, these variations extend across the full range of events despite inferred differences in acceleration mechanism. The origin of source material in all events appears to be coronal and not photospheric.

Reames, D. V.↗
NASA NTRS1994

The source of Jovian auroral hiss observed by Voyager 1

Observations of auroral hiss obtained from the Voyager 1 encounter with Jupiter have been reanalyzed. The Jovian auroral hiss was observed near the inner boundary of the warm Io torus and has a low-frequency cutoff caused by propagation near the resonance cone. A simple ray tracing procedure using an offset tilted dipole of the Jovian magnetic field is used to determine possible source locations. The results obtained are consistent with two sources located symmetrically with respect to the centrifugal equator along an L shell (L approximately = 5.59) that is coincident with the boundary between the hot and cold regions of the Io torus and is located just inward of the ribbon feature observed from Earth. The distance of the sources from the centrifugal equator is approximately 0.58 +/- 0.01 R(sub J). Based on the similarity to terrestrial auroral hiss, the Jovian is auroral hiss is believed to be generated by beams of low energy (approximately tens to thousands of eV) electrons. The low-frequency cutoff of the auroral hiss suggests that the electrons are accelerated near the inferred source region, possibly by parallel electric fields similar to those existing in the terrestrial auroral regions. A field-aligned current is inferred to exist at L shells just inward of the plasma ribbon. A possible mechanism for driving this current is discussed.

Morgan, D. D.↗