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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Evidence for impulsive ion acceleration during the 0312 UT flare of 1980 June 7

One of the basic problems concerning the physics of solar flares is related to the process which is responsible for the acceleration of both electrons and ions. It has been proposed that the acceleration process proceeds in two different phases. The first phase results in the acceleration of electrons to energies in the range from 10 to 100 KeV. Electrons and ions with energies exceeding 30 MeV are finally produced during the second phase. Attention is given to the observational evidence which shows that these two phases, if in fact they are separate, must operate within seconds of each other, and that this process must be able to repeat itself producing pulses of electrons and ions approximately every 10 seconds. The observations were made with the Gamma Ray Spectrometer (GRS) on the SMM satellite during the 0312 UT flare on June 7, 1980.

Forrest, D. J.↗
NASA NTRS1993

Soft x ray properties of the Geminga pulsar

The ROSAT soft x ray spectrum and pulse profile of the Geminga pulsar are analyzed and interpreted in terms of thermal emission from the surface of the neutron star. The x ray spectrum appears to consist of two blackbody components with T(sub 1) = (5.2 +/- 1.0) x 10 (exp 5) K and T(sub 2) approximately 3 x 10(exp 6) K, respectively. The inferred ratio of surface areas, A(sub 2)/A(sub 1), is approximately 3 x 10(exp -5). Both components are highly modulated at the pulsar rotation period, but the harder x ray pulse is narrower, and leads the main (soft) x ray pulse by about 105 deg of phase. The soft x ray component is interpreted as photospheric cooling of much of the neutron star's surface area, while the small, hot region could be part of the much smaller polar cap heated by energetic particles flowing inward from the magnetospheric accelerator which is responsible for the production of Geminga's gamma rays. Geminga's gamma ray emission is consistent with outer-magnetosphere accelerator models for highly inclined dipoles. These predict the beaming of energetic gamma rays close enough to the star to give copious e(+/-) production in the stellar magnetic field and a large circumstellar pair density from pair inflow toward the surface. These pairs may quench radio emission, and also reflect most of the hard polar cap x rays back to the stellar surface by cyclotron resonance scattering. They are then reemitted from that much larger area at the lower temperature T(sub 1). The single-peaked nature of the x ray pulse and its energy-dependent phase suggest an off-center dipole geometry for the surface magnetic field. Under the assumption that the soft x ray emission comes from the full surface of a neutron star of radius R = 10 km, a distance estimate of (150-400) pc is derived. This range is consistent with the fit interstellar column density of (1.5 +/- 0.5) x 10(exp 20) cm(exp -2). Distances less than 150 pc are probably ruled out both by the lower limit on the column density, and also by the requirement that the Rayleigh-Jeans extrapolation of the soft x ray spectrum not exceed the observed blue flux of the faint optical counterpart. This distance estimate implies that Geminga's efficiency for converting spindown power into gamma-rays is near unity, and that there may be significant beaming of the gamma rays as well. These results tend to bolster the prospect that most of the unidentified high-energy gamma ray sources in the Galactic plane are pulsars, some of which may be radio quiet.

Halpern, J. P.↗
NASA NTRS1971

A method for determining the response of space shuttle to atmospheric turbulence. Volume 1: Space shuttle turbulence response

A computer program has been developed and demonstrated that can analyze the response of space shuttle to atmospheric turbulence. The method developed accounts for propellant slosh, gimballed engine, and stability augmentation system (SAS) coupling with the elastic vehicle. Statistical outputs are generated that relate vehicle loads and accelerations to level of random turbulence. For discrete turbulence descriptions, response time-histories are computed. Preliminary turbulence response analyses of space shuttle were conducted for one ascent and one booster flyback subsonic flight condition. The ascent case considered both symmetric and antisymmetric boundary conditions, while only the symmetric analysis was conducted for booster flyback. The three conditions were analyzed with SAS active and inactive, but apparent instabilities with SAS active (arising from improper gains) invalidated the results. Load and acceleration responses in all cases, however, were well within vehicle design limits.

Source record↗
NASA NTRS1978

Studies of acceleration processes in the corona using ion measurements on the solar probe mission

The energy spectra and composition of particles escaping from the Sun provide essential information on mechanisms responsible for their acceleration, and may also be used to characterize the regions where they are accelerated and confined and through which they propagate. The suprathermal energy range, which extends from solar wind energies (approximately 1 KeV) to about 1 MeV/nucleon, is of special interest to studies of nonthermal acceleration processes because a large fraction of particles is likely to be accelerated into this energy range. Data obtained from near earth observations of particles in the suprathermal energy range are reviewed. The necessary capabilities of an a ion composition experiment in the solar probe mission and the required ion measurements are discussed. A possible configuration of an instrument consisting of an electrostatic deflection system, modest post acceleration, and a time of flight versus energy system is described as well as its possible location on the spacecraft.

Gloeckler, G.↗
NASA NTRS2005

Particle acceleration magnetic field generation, and emission in Relativistic pair jets

Plasma waves and their associated instabilities (e.g., the Buneman instability, two-streaming instability, and the Weibel instability) are responsible for particle acceleration in relativistic pair jets. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic pair jet propagating through a pair plasma. Simulations show that the Weibel instability created in the collisionless shock accelerates particles perpendicular and parallel to the jet propagation direction. Simulation results show that this instability generates and amplifies highly nonuniform, small-scale magnetic fields, which contribute to the electron's transverse deflection behind the jet head. The "jitter' I radiation from deflected electrons can have different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants. The growth rate of the Weibel instability and the resulting particle acceleration depend on the magnetic field strength and orientation, and on the initial particle distribution function. In this presentation we explore some of the dependencies of the Weibel instability and resulting particle acceleration on the magnetic field strength and orientation, and the particle distribution function.

Nishikawa, K.-I.↗
NASA NTRS1989

Combined first- and second-order Fermi acceleration in cometary environments

Two-dimensional (one velocity space and one spatial dimension), time-dependent model calculations indicate that an interplay between velocity and spatial diffusion may be responsible for the acceleration of implanted heavy ions in the cometary preshock region. Velocity diffusion (second-order Fermi acceleration) accelerates the pickup ions to moderate energies, thus creating a seed population for the more efficient diffusive-compressive shock acceleration. Solar wind convection limits the time available for diffusive-compressive acceleration. Therefore, the resulting energy spectrum above the pickup energy is a combination of an exponential decrease at lower energies (up to about 100 keV) and a power law spectrum at higher energies. The calculated energy spectra constitute a surprisingly good fit to the published Vega and Giotto energy spectra above the pickup energy.

Gombosi, T. I.↗
NASA NTRS2001

Ocular counterrolling induced by centrifugation during orbital space flight

During the 1998 Neurolab mission (STS-90), four astronauts were exposed to interaural centripetal accelerations (Gy centrifugation) of 0.5 g and 1 g during rotation on a centrifuge, both on Earth and during orbital space flight. Subjects were oriented either left-ear out or right-ear out, facing or back to motion. Binocular eye movements were measured in three dimensions using a video technique. On Earth, tangential centrifugation that produces 1 g of interaural linear acceleration combines with gravity to tilt the gravitoinertial acceleration (GIA) vector 45 degrees in the roll plane relative to the head vertical, generating a summed vector of 1.4 g. Before flight, this elicited mean ocular counterrolling (OCR) of 5.7 degrees. Due to the relative absence of gravity during flight, there was no linear acceleration along the dorsoventral axis of the head. As a result, during in-flight centrifugation, gravitoinertial acceleration was strictly aligned with the centripetal acceleration along the interaural axis. There was a small but significant decrease (mean 10%) in the magnitude of OCR in space (5.1 degrees). The magnitude of OCR during postflight 1 g centrifugation was not significantly different from preflight OCR (5.9 degrees). Findings were similar for 0.5 g centrifugation, but the OCR magnitude was approximately 60% of that induced by centrifugation at 1 g. OCR during pre- and postflight static tilt was not significantly different and was always less than OCR elicited by centrifugation of Earth for an equivalent interaural linear acceleration. In contrast, there was no difference between the OCR generated by in-flight centrifugation and by static tilt on Earth at equivalent interaural linear accelerations. These data support the following conclusions: (1) OCR is generated predominantly in response to interaural linear acceleration; (2) the increased OCR during centrifugation on Earth is a response to the head dorsoventral 1 g linear acceleration component, which was absent in microgravity. The dorsoventral linear acceleration could have activated either the otoliths or body-tilt receptors that responded to the larger GIA magnitude (1.4 g), to generate the increased OCR during centrifugation on Earth. A striking finding was that magnitude of OCR was maintained throughout and after flight. This is in contrast to most previous postflight OCR studies, which have generally registered decreases in OCR. We postulate that intermittent exposure to artificial gravity, in the form of the centripetal acceleration experienced during centrifugation, acted as a countermeasure to deconditioning of this otolith-ocular orienting reflex during the 16-day mission.

Non-NASA Center↗
NASA NTRS1994

Particle/fluid simulations of an eruptive flare: Identifying the field-aligned currents responsible for the hard x-rays

While magnetohydrodynamics (MHD) can provide a reasonable description of the overall magnetic reconnection that is believed to drive flares, additional, and often separate processes have to be envoked to in order to explain the electron acceleration that is responsible for many of the observed flare emissions. A new model that incorporates the dynamic coronal current sheets, the reconnection site, and possible electron acceleration processes is developed through the use of two-dimensional particle and modified two-fluid simulations. The specific example of an eruptive flare driven by the coalescence of flux tubes supported by prescribed photospheric current elements is evaluated. It is shown that the electrons and ions have differential trajectories through the coronal current sheet which leads to the development of additonal plasma currents that flow around the surface of the current sheet. These surface currents are explicitly neglected in MHD but they are vital to the flare dynamics because they divert current from the coronal current sheet into the chromosphere, producing an effective resistivity that aids the development of fast reconnection. Because the surface currents are in the plane of the magnetic field, electrons in them experience strong acceleration and can account for the observed hard X-ray emissions. Model predictions are compared with observed time profiles of hard X-ray emissions and Doppler shifts seen in soft X-ray line emissions and are able to account for such features as (1) the asymmetry in the rise and decay time of the hard X-rays, (2) the apparent delay between the largest Doppler shifts and the hard X-ray peak, and (3) the relatively low intensity of the blue-shifted component. The use of particle and fluid simulations is important because it provides different, but complementary treatments of the electron acceleration, the global magnetic morphology, and the flare current system.

Winglee, R. M.↗
NASA NTRS2012

Development and Calibration of a System-Integrated Rotorcraft Finite Element Model for Impact Scenarios

Two full-scale crash tests of an MD-500 helicopter were conducted in 2009 and 2010 at NASA Langley's Landing and Impact Research Facility in support of NASA s Subsonic Rotary Wing Crashworthiness Project. The first crash test was conducted to evaluate the performance of an externally mounted composite deployable energy absorber (DEA) under combined impact conditions. In the second crash test, the energy absorber was removed to establish baseline loads that are regarded as severe but survivable. The presence of this energy absorbing device reduced the peak impact acceleration levels by a factor of three. Accelerations and kinematic data collected from the crash tests were compared to a system-integrated finite element model of the test article developed in parallel with the test program. In preparation for the full-scale crash test, a series of sub-scale and MD-500 mass simulator tests were conducted to evaluate the impact performances of various components and subsystems, including new crush tubes and the DEA blocks. Parameters defined for the system-integrated finite element model were determined from these tests. Results from 19 accelerometers placed throughout the airframe were compared to finite element model responses. The model developed for the purposes of predicting acceleration responses from the first crash test was inadequate when evaluating more severe conditions seen in the second crash test. A newly developed model calibration approach that includes uncertainty estimation, parameter sensitivity, impact shape orthogonality, and numerical optimization was used to calibrate model results for the full-scale crash test without the DEA. This combination of heuristic and quantitative methods identified modeling deficiencies, evaluated parameter importance, and proposed required model changes. The multidimensional calibration techniques presented here are particularly effective in identifying model adequacy. Acceleration results for the calibrated model were compared to test results and the original model results. There was a noticeable improvement in the pilot and copilot region, a slight improvement in the occupant model response, and an over-stiffening effect in the passenger region. One lesson learned was that this approach should be adopted early on, in combination with the building-block approaches that are customarily used, for model development and pretest predictions. Complete crash simulations with validated finite element models can be used to satisfy crash certification requirements, potentially reducing overall development costs.

Annett, Martin S.↗
NASA NTRS1973

Analytical study of the performance of a gust alleviation system for a STOL airplane

An analytical study has shown that a gust alleviation system for a STOL airplane in a cruise condition could reduce the root mean square of the normal acceleration of the airplane flying in random turbulence by as much as 50 percent. This alleviation is obtained by driving the flaps in response to normal acceleration and by moving the elevator in proportion to the commanded flap deflection angle and to a pitch-rate signal.

Oehman, W. I.↗
NASA NTRS1981

Photometer used for response time measurement

Photometer detects motion for measuring response speed and acceleration of servocontrol system. Instrument senses selected output movement shortly after operator activates hand-controlled input. Time delay is measured on X/T recorder and response calculated. With suitable motion targets, photometer measures any open- or closed-loop servoresponse and servorate or computer lag without system disturbance.

Da Silva, A. J.↗
NASA NTRS1998

Human Physiological Responses to Cycle Ergometer Leg Exercise During +Gz Acceleration

Spaceflight and bed-rest deconditioning decrease maximal oxygen uptake (aerobic power), strength, endurance capacity, and orthostatic tolerance. In addition to extensive use of muscular exercise conditioning as a countermeasure for the reduction in aerobic power (VO(sub 2max)), stimuli from some form of +Gz acceleration conditioning may be necessary to attenuate the orthostatic intolerance component of this deconditioning. Hypothesis: There will be no significant difference in the physiological responses (oxygen uptake, heart rate, ventilation, or respiratory exchange ratio) during supine exercise with moderate +Gz acceleration.

Chou, J. L.↗
NASA NTRS1998

The adequate stimulus for avian short latency vestibular responses to linear translation

Transient linear acceleration stimuli have been shown to elicit eighth nerve vestibular compound action potentials in birds and mammals. The present study was undertaken to better define the nature of the adequate stimulus for neurons generating the response in the chicken (Gallus domesticus). In particular, the study evaluated the question of whether the neurons studied are most sensitive to the maximum level of linear acceleration achieved or to the rate of change in acceleration (da/dt, or jerk). To do this, vestibular response thresholds were measured as a function of stimulus onset slope. Traditional computer signal averaging was used to record responses to pulsed linear acceleration stimuli. Stimulus onset slope was systematically varied. Acceleration thresholds decreased with increasing stimulus onset slope (decreasing stimulus rise time). When stimuli were expressed in units of jerk (g/ms), thresholds were virtually constant for all stimulus rise times. Moreover, stimuli having identical jerk magnitudes but widely varying peak acceleration levels produced virtually identical responses. Vestibular response thresholds, latencies and amplitudes appear to be determined strictly by stimulus jerk magnitudes. Stimulus attributes such as peak acceleration or rise time alone do not provide sufficient information to predict response parameter quantities. Indeed, the major response parameters were shown to be virtually independent of peak acceleration levels or rise time when these stimulus features were isolated and considered separately. It is concluded that the neurons generating short latency vestibular evoked potentials do so as "jerk encoders" in the chicken. Primary afferents classified as "irregular", and which traditionally fall into the broad category of "dynamic" or "phasic" neurons, would seem to be the most likely candidates for the neural generators of short latency vestibular compound action potentials.

NASA Discipline Neuroscience↗
NASA NTRS1992

Transverse ion acceleration by localized lower hybrid waves in the topside auroral ionosphere

Up to now, observations had been unable to show conclusively a one-to-one correspondence between perpendicular ion acceleration and a particular type of plasma wave within the O(+) source region below 2000 km. In this paper we demonstrate that intense (100-300 mV/m) lower hybrid waves are responsible for transversely accelerating H(+) and O(+) ions to characteristic energies of up to 6 eV. This wave-particle interaction takes place in thin filamentary density cavities oriented along geomagnetic field lines. The measurements we discuss were conducted in the nightside auroral zone at latitudes between 500 km and 1100 km. Our results are consistent with theories of lower hybrid wave condensation and collapse.

Vago, J. L.↗
NASA NTRS1999

Multi-Wavelength Analysis of the March 26, 1991: Solar Flare

We have engaged in detailed multi-wavelength analysis of the March 26, 1991, solar flare in order to develop a method of diagnostics of the physical processes responsible for the efficient acceleration of charged particles to high energies and also for diagnostics of the photospheric response to the injection of the accelerated particles. Consideration of this particular flare is of special interest because to date it is the only flare in which the gamma-ray emission with energies of 20-1000 MeV was registered throughout the entire development of the event in the optical, radio and soft x-ray bands. individual registration by the GAMMA-1 telescope of the energy and precise time for each registered gamma-ray photon allowed the maximum detailed investigation of the energy spectrum and flux evolution during the flare.

Kurt, V. G.↗
NASA NTRS2006

Particle Acceleration, Magnetic Field Generation and Emission from Relativistic Jets and Supernova Remnants

We performed numerical simulations of particle acceleration, magnetic field generation, and emission from shocks in order to understand the observed emission from relativistic jets and supernova remnants. The investigation involves the study of collisionless shocks, where the Weibel instability is responsible for particle acceleration as well as magnetic field generation. A 3-D relativistic particle-in-cell (RPIC) code has been used to investigate the shock processes in electron-positron plasmas. The evolution of theWeibe1 instability and its associated magnetic field generation and particle acceleration are studied with two different jet velocities (0 = 2,5 - slow, fast) corresponding to either outflows in supernova remnants or relativistic jets, such as those found in AGNs and microquasars. Slow jets have intrinsically different structures in both the generated magnetic fields and the accelerated particle spectrum. In particular, the jet head has a very weak magnetic field and the ambient electrons are strongly accelerated and dragged by the jet particles. The simulation results exhibit jitter radiation from inhomogeneous magnetic fields, generated by the Weibel instability, which has different spectral properties than standard synchrotron emission in a homogeneous magnetic field.

Nishikawa, K.-I.↗
NASA NTRS1984

Far-field brainstem responses evoked by vestibular and auditory stimuli exhibit increases in interpeak latency as brain temperature is decreased

The effect of decreasing of brain temperature on the brainstem auditory evoked response (BAER) in rats was investigated. Voltage pulses, applied to a piezoelectric crystal attached to the skull, were used to evoke stimuli in the auditory system by means of bone-conducted vibrations. The responses were recorded at 37 C and 34 C brain temperatures. The peaks of the BAER recorded at 34 C were delayed in comparison with the peaks from the 37 C wave, and the later peaks were more delayed than the earlier peaks. These results indicate that an increase in the interpeak latency occurs as the brain temperature is decreased. Preliminary experiments, in which responses to brief angular acceleration were used to measure the brainstem vestibular evoked response (BVER), have also indicated increases in the interpeak latency in response to the lowering of brain temperature.

Hoffman, L. F.↗