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At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Global Validation of Single-Station Schumann Resonance Lightning Location

Global measurements of large, optically bright lightning events from the Optical Transient Detector (OTD) satellite are used to validate estimates of lightning location from single-station Schumann resonance (SR) data. Bearing estimates are obtained through conventional magnetic direction-finding techniques, while source range is estimated from the range-dependent impedance spectrum of individual SR transients. An analysis of 40 such transients suggests that single-station techniques can locate lightning globally with an accuracy of 1-2mm. This is confirmed by further validation at close ranges from flashes detected by the National Lightning Detection-Network (NLDN). Observations with both OTD and SR systems may be useful for globally locating lightning with necessary, if not sufficient, characteristics to trigger mesospheric sprites.

Boccippio, Dennis↗
NASA NTRS2006

Ultra-fine-scale filamentary structures in the Outer Corona and the Solar Magnetic Field

Filamentary structures following magnetic field lines pervade the Sun's atmosphere and offer us insight into the solar magnetic field. Radio propagation measurements have shown that the smallest filamentary structures in the solar corona are more than 2 orders of magnitude finer than those seen in solar imaging. Here we use radio Doppler measurements to characterize their transverse density gradient and determine their finest scale in the outer corona at 20-30 R(circled dot operator), where open magnetic fields prevail. Filamentary structures overly active regions have the steepest gradient and finest scale, while those overlying coronal holes have the shallowest gradient and least finest scale. Their organization by the underlying corona implies that these subresolution structures extend radially from the entire Sun, confirming that they trace the coronal magnetic field responsible for the radial expansion of the solar wind. That they are rooted all over the Sun elucidates the association between the magnetic field of the photosphere and that of the corona, as revealed by the similarity between the power spectra of the photospheric field and the coronal density fluctuations. This association along with the persistence of filamentary structures far from the Sun demonstrate that subresolution magnetic fields must play an important role not only in magnetic coupling of the photosphere and corona, but also in coronal heating and solar wind acceleration through the process of small-scale magnetic reconnection. They also explain why current widely used theoretical models that extrapolate photospheric magnetic fields into the corona do not predict the correct source of the solar wind.

Sun : Corona↗
NASA NTRS2010

A Multi-Spacecraft View of Solar-Energetic-Particle Onsets in the 1977 November 22 Event

We examine the onset timing of solar energetic particles (SEPs) in the large ground-level event (GLE) of 1977 November 22 as observed from six spacecraft at four distinct solar longitudes. In most cases, it was possible to use velocity dispersion of the energetic protons to fix the solar particle release (SPR) time and the path-length traveled by the initial particle burst from each solar longitude. We find that the SPR times do depend upon solar longitude, being earliest for spacecraft that are magnetically well-connected to the source region, and later for longitudes on the flanks of the outward driven shock wave. The earliest SPR time occurs well after peak photon emission from the associated Ha flare. These observations are consistent with conclusions derived from single-longitude observations of different GLE events.

Reames, Donald V.↗
NASA NTRS2011

Nanoflare Evidence from Analysis of the X-Ray Variability of an Active Region Observed with Hinode/XRT

The investigation of the heating mechanisms of the confined coronal plasma is still under intense debate. It is widely believed that the energy source for coronal heating is the magnetic energy stored in the solar corona. An unsolved problem is how this magnetic energy is converted into thermal energy of the confined coronal plasma. As Parker proposed in 1988 rapid pulses called nanoflares are among the best candidate mechanisms of magnetic energy release. Nowadays a challenging problem is to obtain evidence that such nanoflares are really at work. If small energy discharges (nanoflares) contribute in some way to coronal heating, they could be too small and frequent to be resolved as independent events. In this case, we would need to search for indirect evidence. The idea of this work is that, if the solar corona emission is sustained by repeated nanoflares, locally the X-ray emission may not be entirely constant but may show variations around the mean intensity. So the nanoflares may leave their signature on the light curves. Many authors (Shimizu & Tsuneta 1997; Vekstein & Katsukawa 2000; Katsukawa & Tsuneta 2001; Katsukawa 2003; Sakamoto et al. 2008) pointed out that a detailed analysis of intensity fluctuations of the coronal X-ray emission could give us information on these smallest flares. Following this hint we use this approach for the first time on Hinode data, searching, with statistical analysis, for small but systematic variability in noisy background light curves and their link to coronal heating models.

Terzo, S.↗
NASA NTRS2012

The Gravity and Extreme Magnetism Small Explorer Mission (GEMS)

Polarization is an inherently geometric quantity and provides information on source geometry inaccessible via spectroscopy or timing. To date, there have been reliable detections of X-ray polarization from only one object outside the solar system (the Crab nebula). Recent development of photoelectric polarimetry makes it possible to perform sensitive X-ray polarimetry with a modest mission. GEMS was selected by NASA to be the 13th Small Explorer mission with launch planned for 2014. GEMS will be approximately 100 times more sensitive than any previously flown X-ray polarimeter and will provide useful polarization measurements for dozens of cosmic X-ray sources. GEMS will lead to new insights into the nature of accreting black holes, magnetized neutron stars, and supernova remnants. In this talk I will review the science motivation for GEMS and describe the status of the mission implementation.

Kallman, Tim↗
NASA NTRSDate not supplied

Surface Temperature and the Compositional Effects of Electron Bombardment on Dione

Saturn’s icy moons are composed of mostly water ice with minor “contaminants,” such as CO2 ice, and a dark component thought to be organics, hematite and/or metallic Fe [1]. The space weathering process of electron bombardment is expected to be particularly important on the surfaces of Saturn’s inner moons (Mimas, Tethys, Dione, and Rhea), as they orbit within the extended plasma sheet of Saturn’s magnetosphere. Terrains exhibiting thermal anomalies (i.e., colder temperatures in the day and warmer temperatures at night than surrounding areas) correspond to regions of high energy electron bombardment [2; 3]. Energetic electrons impact the surfaces, fracturing ice particles into sub-micron sized grains, and this process becomes more effective for increasing particle energies [4]. In addition to alteration of ices, the dark material may also be modified or generated within icy surfaces by interaction with solar UV radiation, cosmic rays, or by particles trapped in Saturn’s magnetic field [5]. CO2 could be sourced from irradiation of dark organic material [7]. Many of these alterations should be observable in Visible and Infrared Mapping Spectrometer (VIMS) spectra [8]. We used surface temperatures derived from Cassini’s Composite Infrared Spectrometer (CIRS) to help isolate areas where space weathering due to electron bombardment is more predominant and investigated the subtle changes in VIMS spectra using machine learning techniques. We present our results here for Dione. On ocean worlds, space weathering products would be present along with any potential indications of life. We will need to understand the manifestation of space weathering in the data to separate the two processes.

Cindy Lee Young↗
DOE OSTI2024

Phase I Final Repoert

Explore the source record for details and available documents.

gyrotron, fusion, electron cyclotron heating, plas↗
DOE OSTI2020

Anisotropic Electron Fluid Closure Validated by in Situ Spacecraft Observations in the far Exhaust of Guide-field Reconnection

Anisotropic electron heating predictions of the equations of state of Le et al. (2009); https://doi.org/10.1103/PhysRevLett.102.085001 (“the EoS”) are applied to reconnection in the Earth's magnetosheath. The model is applicable to open systems where electrons are streaming along magnetic field lines into the reconnection region, sourced by fixed external reservoirs of plasma ambient to the reconnection region. While spacecraft observations have previously shown the EoS to hold in the region near the X-line, we find that for an event observed far downstream (~100 d i ) from the X-line both inflows and the exhaust follow the predictions of the EoS. Furthermore, the model underlying the EoS is extended to include additional perpendicular heating terms relevant to the considered event undergoing active magnetic compression with local electron trapping. Finally, the agreement between the spacecraft observations and the EoS emphasizes the roles of the ambient plasma sources and the dynamics of trapped and passing electrons in setting and controlling the anisotropic electron heating at large scale in naturally occurring plasma configurations.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1995

Structure of magnetic field in the solar wind

This work is concerned with empirical data on magnetic field in the solar wind in frame of a concept of dissipative solar wind, developed in papers (Solar Wind 7 Conf., Pergamon Press, 1992, 165 and 1992 STEP/5th COSPAR Coll. Pergamon Press, 1994, 117; 235; 803). Interplanetary magnetic fields should be classified with respect to their origin. It is very important for all the theoretical problems from the necessity to specify correctly boundary and initial conditions: the magnetic field must be sewed with its source. One should select the field, connected directly with the Sun (stretched out from it), and the field of moving electric currents. It occured central in discussion about the velocity of Alfven waves, probably warming up the solar wind, relative to the Sun, the magnetic field and solar wind plasma. The selection problem corresponds to an inverse problem and obviously has no single solution. The dissipative model of the solar wind introduce the slipping and leakage of plasma relative to magnetic field. There are no 'interplanetary current sheets' in it. But temporal fluctuations from the filamentation of electric currents play the key role. As a whole, the new concept requires the re-interpretation of main objects in the interplanetary magnetic field.

Chertkov, A. D.↗
DOE OSTI2022

Bayesian optimization to design a novel x-ray shaping device

In radiation therapy, x-ray dose must be precisely sculpted to the tumor, while simultaneously avoiding surrounding organs at risk. This requires modulation of x-ray intensity in space and/or time. Typically, this is achieved using a multi leaf collimator (MLC) - a complex mechatronic device comprising over one hundred individually powered tungsten ‘leaves’ that move in or out of the radiation field as required. Here, an all-electronic x-ray collimation concept with no moving parts is presented, termed “SPHINX”: Scanning Pencil-beam High-speed Intensity-modulated X-ray source. SPHINX utilizes a spatially distributed bremsstrahlung target and collimator array in conjunction with magnetic scanning of a high energy electron beam to generate a plurality of small x-ray “beamlets.” A simulation framework was developed in Topas Monte Carlo incorporating a phase space electron source, transport through user defined magnetic fields, bremsstrahlung x-ray production, transport through a SPHINX collimator, and dose in water. This framework was completely parametric, meaning a simulation could be built and run for any supplied geometric parameters. This functionality was coupled with Bayesian optimization to find the best parameter set based on an objective function which included terms to maximize dose rate for a user defined beamlet width while constraining inter-channel cross talk and electron contamination. Designs for beamlet widths of 5, 7, and 10 mm 2 were generated. Each optimization was run for 300 iterations and took approximately 40 h on a 24-core computer. For the optimized 7-mm model, a simulation of all beamlets in water was carried out including a linear scanning magnet calibration simulation. Finally, a back-of-envelope dose rate formalism was developed and used to estimate dose rate under various conditions. The optimized 5–, 7–, and 10-mm models had beamlet widths of 5.1 , 7.2 , and 10.1 mm 2 and dose rates of 3574, 6351, and 10 015 Gy/C, respectively. The reduction in dose rate for smaller beamlet widths is a result of both increased collimation and source occlusion. For the simulation of all beamlets in water, the scanning magnet calibration reduced the offset between the collimator channels and beam centroids from 2.9 ±1.9 mm to 0.01 ±0.03 mm. A slight reduction in dose rate of approximately 2% per degree of scanning angle was observed. Based on a back-of-envelope dose rate formalism, SPHINX in conjunction with next-generation linear accelerators has the potential to achieve substantially higher dose rates than conventional MLC-based delivery, with delivery of an intensity modulated 100 x 100 mm 2 field achievable in 0.9 to 10.6 s depending on the beamlet widths used. Bayesian optimization was coupled with Monte Carlo modeling to generate SPHINX geometries for various beamlet widths. A complete Monte Carlo simulation for one of these designs was developed, including electron beam transport of all beamlets through scanning magnets, x-ray production and collimation, and dose in water. These results demonstrate that SPHINX is a promising candidate for sculpting radiation dose with no moving parts, and has the potential to vastly improve both the speed and robustness of radiotherapy delivery. A multi-beam SPHINX system may be a candidate for delivering magavoltage FLASH RT in humans.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
NASA NTRS1988

Global Geospace Science Programme

The Global Geospace Science (GGS) Program, an element of the international Solar Terrestrial Physics Program dedicated to the study of the global plasma dynamics of the solar-terrestrial environment, is discussed. Past research on the injection of solar wind ions into the magnetosphere and on the detection of ions in the terrestrial ring current of both solar wind and ionospheric origins is reviewed, showing its relevance for the GGS program. Research on the interplanetary magnetic field, the auroral electrojet, the outer magnetosphere, the geomagnetic tail, the ionospheric electric field and the related electron precipitation is also addressed. The results demonstrate that the solar wind and the ionsophere both contribute to the magnetospheric particle population. Unanswered questions regarding hot plasma sources, transport processes, energy storage in the magnetic field, and energization of plasmas are discussed. The relevant mission strategy, instrumentation, theory and modeling, and data collection are addressed.

Parks, George↗
NASA NTRS1989

Radio emission from the magnetic equator of Uranus

The major observational characteristics of the smooth, narrow bandwidth component of Uranus' radio emissions are well described by sources radiating near the local electron gyrofrequency, confined to the magnetic equatorial plane and encircling the planet at radial distances of approximately 2 to 3 R(U). The most intense emission appears to be generated in association with the epsilon ring at 2.0 R(U) radial distance. A cold electron density of less than or equal to 4/cu cm are inferred in this region.

Kaiser, M. L.↗
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.↗
NASA NTRS1976

Electron transport and the magnetic field in the Crab Nebula

Observational data obtained at radio, optical, and X-ray frequencies on the size of the emission region in the Crab Nebula are used to examine various energy-transport models. It is shown that the data cannot be fitted by diffusion models, even when the energy-injection region is extended. A more favorable comparison is made with an extended source model in which bulk motion is the transport mechanism. Assuming a point source of electrons and a uniform nebular magnetic field, a perpendicular magnetic-field component of less than 0.0008 gauss is determined. Combining this with the lower limit obtained from gamma-ray observations, a perpendicular field component of between 0.0005 and 0.0008 gauss is obtained. A break in the size dependence is predicted at about 12 microns and 45 keV. Above 45 keV, the size should decrease as the inverse square root of proton energy.

Wolff, R. S.↗
NASA NTRS1985

Ray tracing of Z-mode emissions from source regions in the high-altitude auroral zone

Two-dimensional ray tracing of Z-mode radiation from sources in the auroral zone has been performed. The Z-mode waves are assumed to be excited by the cyclotron maser mechanism at frequencies near the electron gyrofrequency and at wave normal angles near 90 deg. The emission is found to propagate primarily perpendicular to the magnetic field line (horizontal propagation) for all sources considered, and thus not to substantially lower altitudes. The frequency bandwidth estimated from the ray paths is much less than that observed by DE-1. It is suggested that the wave source needs to be in the region of field-aligned current in order for the cyclotron maser mechanism to explain the broad frequency bandwidth of Z-mode emissions.

Menietti, J. D.↗
NASA NTRS1972

Jupiter's decametric rotation period and the source-A emission beam

Determinations of the average decametric rotation period for the planet Jupiter are discussed. Mean rotation periods for a twelve year span are plotted as a function of the mean epochs. Two lesser peaks in the longitude histograms were resolved in addition to the main peak. The three peaks, which are believed to result from emission beams oriented in different directions relative to the corotating magnetic field of the planet, are designated Sources A, B, and C in decreasing order of prominence at frequencies in the vicinity of 18 MHz.

Carr, T. D.↗
NASA NTRS2011

X-Ray Polarimetry with GEMS

The polarization properties of cosmic X-ray sources are still largely unexplored. The Gravity and Extreme Magnetism SMEX (GEMS) will carry out the first sensitive X-ray polarization survey of a wide range of sources including; accreting compact objects (black holes and neutron stars), AGN, supernova remnants, magnetars and rotation-powered pulsars. GEMS employs grazing-incidence foil mirrors and novel time-projection chamber (TPC) polarimeters leveraging the photoelectric effect to achieve high polarization sensitivity in the 2 - 10 keV band. I will provide an update of the project status, illustrate the expected performance with several science examples, and provide a brief overview of the data analysis challenges

Strohmayer, Tod↗