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At least 325 records · Page 18

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NASA NTRS1985

The interaction of a dipolar thunderstorm with its global electrical environment

The role of the thundercloud in the global electric circuit has been considered by many researchers. Thus, Holzer and Saxon (1952) have constructed a simple model of a bipolar thunderstorm. The global models considered provide insight into the atmospheric electric circuit but are restricted, both by various analytical mathematical representations and by computer size, to a grid of about five degrees in latitude and longitude. A need exists, therefore, for the development of a numerical regional model capable of resolving small-scale phenomena so that their coupling into the global-scale circuit can be examined. The construction of a two-dimensional quasi-static numerical model of atmospheric electricity is discussed. The model provides a basis for the calculation of the global electric field and current distribution produced by a single thunderstorm generator. In connection with the calculations, the thunderstorm was defined by a quasi-static current source function which generates a dipole charge configuration.

Tzur, I.↗

NASA NTRS1977

Longitudinal variations of very-low-frequency chorus activity in the magnetosphere - Evidence of excitation by electrical power transmission lines

The distribution of VLF chorus in the high-altitude magnetosphere as detected by the Ogo 3 satellite was investigated. Chorus occurrence frequency over 4,668 samples of 5 min of broadband data was computed for Northern Hemisphere bins of 10 deg by 10 deg in dipole invariant latitude and longitude. Peaks in activity were noted over Alaska, the eastern U.S. and Canada, western Europe, and western Siberia, and were shown not to be the result of biased sampling. It is suggested that power line radiation of only about 1 W or less at a given harmonic frequency may be sufficient to stimulate the observed chorus activity and that such power levels can be expected in industrial areas or from large distribution networks. Preliminary results show that chorus starting frequencies are highly correlated with 60 Hz harmonics in the American sector and with 50 Hz harmonics in the European sector.

Luette, J. P.↗

NASA NTRS2000

The Axisymmetric Pulsar Magnetosphere

We present the structure of the axisymmetric force-free magnetosphere of an aligned rotating magnetic dipole, in the case in which there exists a sufficiently large charge density (whose origin we do not question) to satisfy the ideal MHD condition, E (raised dot) B = 0, everywhere. The unique distribution of electric current along the open magnetic field lines which is required for the solution to be continuous and smooth is obtained numerically. We expect that our solution will be useful as the starting point for detailed studies of pulsar magnetospheres under more general conditions, namely when either the force-free and/or the ideal MHD condition E (raised dot) B = 0 are not valid in the entire magnetosphere. Based on our solution, we consider that the most likely positions of such an occurrence are the polar cap, the crossings of the zero space charge surface by open field lines, and the return current boundary, but not the light cylinder.

Contopoulos, J.↗

NASA NTRS2010

Plasma IMS Composition Measurements for Europa and the Other Galilean Moons

NASA and ESA are planning the joint Europa Jupiter System Mission (EJSM) to the Jupiter system with specific emphasis to Europa and Ganymede, respectively. The Japanese Space Agency is also planning an orbiter mission to explore Jupiter's magnetosphere and the Galilean satellites. For NASA's Jupiter Europa Orbiter (JEO) we are developing the 3D Ion Mass Spectrometer (IMS) with two main goals which can also be applied to the other Galilean moons, 1) measure the plasma interaction between Europa and Jupiter's magnetosphere and 2) infer the 4 pi surface composition to trace elemental and significant isotopic levels. The first goal supports the magnetometer (MAG) measurements, primarily directed at detection of Europa's sub-surface ocean, while the second gives information about transfer of material between the Galilean moons, and between the moon surfaces and subsurface layers putatively including oceans. The measurement of the interactions for all the Galilean moons can be used to trace the in situ ion measurements of pickup ions back to either Europa's or Ganymede's surface from the respectively orbiting spacecraft. The IMS instrument, being developed under NASA's Astrobiology Instrument Development Program, would maximally achieve plasma measurement requirements for JEO and EJSM while moving forward our knowledge of Jupiter system composition and source processes to far higher levels than previously envisaged. The composition of the global surfaces of Europa and Ganymede can be inferred from the measurement of ejected neutrals and pick-up ions using at minimum an in situ payload including MAG and IMS also fully capable of meeting Level 1 mission requirements for ocean detection and survey. Elemental and isotopic analysis of potentially extruded oceanic materials at the moon surfaces would further support the ocean objectives. These measurements should be made from a polar orbiting spacecraft about Europa or Ganymede at height 100 km. The ejecta produced by sputtering of the surfaces of Europa and Ganymede has been shown to be representative of the surface composition. Level 2 science on surface geology and composition can then be further enhanced by addition of the following: 3D Ion Neutral Mass Spectrometer (INNS), 3D plasma electron spectrometer (ELS), and hot plasma energetic particle instrument. The measurement approach is to alternate between times measuring pickup ions and times measuring plasma and magnetic field parameters along the spacecraft trajectory. By measuring the pickup ion energy, arrival direction and mass-per-charge, the ion can be traced back along the ejection trajectory to the approximate area of origin if the 3-D electric field and magnetic field are known. In situ observations of plasma flow velocities and vector magnetic fields can be used to determine the local convective electric field (E = -VXB) along the spacecraft trajectory. By combining this information with models of the magnetospheric interaction with Europa, one can generate 3D maps of the electric and magnetic field and compute the trajectories of the pickup ions back to the surface or exospheric points of origin. In the case of Ganymede there is the additional complexity of its own internal dipole magnetic field, while Io's volcanic activity introduces the complexity of a highly structured denser atmosphere. Callisto with its less globally extended exosphere will have a simpler interaction than for Europa (i.e., more like our moon). We will discuss these differences in light of the above proposed technique. Finally, the INNS observations and neutral exosphere models are needed to estimate production rates of pickup ions. The hot plasma measurements are needed to correct for sputtering rates which can be time dependent and electron plasma observations for electron impact ionization rates. Instrument characteristics, field-of-view requirements, modes of operation and effects of radiation on instrument functionality will be discussed.

Sittler, Edward↗

NASA NTRS2000

Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations

Galileo measured the magnetic field perturbations of Europa and Callisto, which are consistent with dipole fields created by temporal variations of the surrounding jovian magnetospheric field. These fields almost match those expected for perfectly conducting moons. Using a simple shell model, we analyze the implications of these observations for the electrical structure for the interiors of the moons. It is discovered that Europa and Callisto must possess areas where the conductivity exceeds 0.06 and 0.02 S/m at a depth of no more than 200 and 300 km below the surface, respectively. This conductivity is not attainable in ice or silicates, unless large temperature gradients can be maintained below the ice or the ice layer is at least partially molten. A cloud of pick-up ions or an ionosphere are probably insufficiently conductive. Global Earth-like oceans under the surface of both moons could explain the observations if they are at least a few kilometers thick.

Zimmer, Christophe↗

NASA NTRS1989

Analysis of lightning field changes during active Florida thunderstorms

A computer algorithm for deriving accurate values of lightning-caused changes in cloud electric fields under active storm conditions was developed and applied to data obtained during two thunderstorms from a network of ground-based electric field mills at the NASA Kennedy Space Center and the Cape Canaveral Air Force Station. The resulting field changes were analyzed using a least-squares optimization procedure and point-charge (Q) and point-dipole (P) models. The values and the time variations of the Q-model parameters under active storm conditions were found to be similar to those reported by Maier and Krider (1986) for small storms, when the computations were carried out with the same analysis criteria and comparable biases. The parameters of the P solutions were found to vary with time within the storm interval and from storm to storm.

Koshak, William J.↗

NASA NTRS1983

An upper bound on the radius of a highly electrically conducting lunar core

Parker's (1980) nonlinear inverse theory for the electromagnetic sounding problem is converted to a form suitable for analysis of lunar day-side transfer function data by: (1) transforming the solution in plane geometry to that in spherical geometry; and (2) transforming the theoretical lunar transfer function in the dipole limit to an apparent resistivity function. The theory is applied to the revised lunar transfer function data set of Hood et al. (1982), which extends in frequency from 10 to the -5th to 10 to the -3rd Hz. On the assumption that an iron-rich lunar core, whether molten or solid, can be represented by a perfect conductor at the minimum sampled frequency, an upper bound of 435 km on the maximum radius of such a core is calculated. This bound is somewhat larger than values of 360-375 km previously estimated from the same data set via forward model calculations because the prior work did not consider all possible mantle conductivity functions.

Hobbs, B. A.↗

NASA NTRS2012

Pair Cascades and Deathlines in Magnetic Fields with Offset Polar Caps

We present results of electron-positron pair cascade simulations in a dipole magnetic field whose polar cap is offset from the dipole axis. In such a field geometry, the polar cap is displaced a small fraction of the neutron star radius from the star symmetry axis and the field line radius of curvature is modified. Using the modified parallel electric field near the offset polar cap, we simulate pair cascades to determine the pair deathlines and pair multiplicities as a function of the offset. We find that the pair multiplicity can change dr;unatically with a modest offset, with a significant increase on one side of the polar cap. Lower pair deathlines allow a larger fraction of the pulsar population, that include old and millisecond pulsars, to produce cascades with high multiplicity. The results have some important implications for pulsar particle production, high-energy emission and cosmic-ray contribution.

Harding, Alice K.↗

NASA NTRS1975

Adiabatic Hamiltonian of charged particle motion in a dipole field

The Hamiltonian for a dipole field is developed, and the result is expressed by an analytic approximation accurate to within about 1%. This allows extension of results derived for equatorial particles to particles with arbitrary pitch angles; in particular, it makes available even in the presence of electric fields orthogonal to the magnetic field a function K that is preserved by the bounce-averaged motion. This function provides at once the equations of drift paths in (alpha, beta) or of their projections onto the equatorial plane; the derivation of a pacing function that times the progress of particles along such drift paths is also described.

Chen, A. J.↗

NASA NTRS2001

Detection of Sub-Surface Water on Mars by Controlled and Natural Source Electromagnetic Induction

Detection of subsurface liquid water on Mars is a leading scientific objective for Mars exploration in this decade. We describe electromagnetic induction (EM) methods that are both uniquely well suited for detection of subsurface liquid water on Mars and practical within the context of a Mars exploration program. EM induction methods are ideal for detection of more highly conducting (liquid water bearing) soils and rock beneath a more resistive overburden. A combined natural source and controlled source method offers an efficient and unambiguous characterization of the depth to liquid water and the extent of the aqueous region. The controlled source method employs an ac vertical dipole source (horizontal loop) to probe the depth to the conductor and a natural source method (gradient sounding) to characterize its conductivity-thickness product. These methods are proven in geophysical exploration and can be tailored to cope with any reasonable Mars crustal electrical conductivity. We describe a practical experiment and discuss experiment optimization to address the range of material properties likely encountered in the Mars crust.

Connerney, J. E. P.↗

NASA NTRS1983

Mapping electrostatic potentials from the ionosphere to the magnetosphere

Techniques for mapping observed ionospheric-potential distributions into the magnetosphere are discussed and illustrated using published Millstone Hill and Chatanika incoherent-scatter-radar data. It is shown that the mapping of a given field line to the equator is subject to strong diurnal and seasonal variations (attributed to the combination of internal and tail-current magnetic-field sources at auroral latitudes and the diurnal variation of solar declination in dipole coordinates) and longitude-dependent differences in ionospheric geometry. A mapping based on the tilt-dependent model of Olson and Pfitzer (1977) and using an empirical ionospheric-potential distribution derived from Chatanika plasma-drift measurements produces a relativity uniform magnetospheric electric field in the tail region. The field at 12 earth radii (Re) is found to be between 1 and 2 kV/Re; at the dawn-dusk meridian beyond the plasmasphere it is as high as 5 kV/Re. The plasmasphere is shown to have a dusk bulge in its equipotential structure and to be almost symmetric about the dawn-dusk meridian.

Sojka, J. J.↗

NASA NTRS1987

Resonant Alfven waves on auroral field lines

It is shown that resonant Alfven waves on dipole magnetic field geometry and plasma distributions suitable for auroral field lines can be conventiently treated in the theory of Mathieu functions. Resurgent interest in invoking large-scale Alfven waves to structure some elements of auroral electrodynamics calls for interpretation of measured perpendicular electric and magnetic disturbance fields in terms of Alfven waves. The ability to express the resonant eigenmodes in closed form in terms of Mathieu functions allows for convenient tests of the Alfven wave structuring hypothesis. Implications for current vector electric and magnetic disturbance measurements are discussed.

Chiu, Y. T.↗

NASA NTRS1995

Prediction of the interplanetary magnetic field strength

A new model of the coronal and interplanetary magnetic field can predict both the interplanetary magnetic field strength and its polarity from measurements of the photospheric magnetic field. The model includes the effects of the large-scale horizontal electric currents flowing in the inner corona, of the warped heliospheric current sheet in the upper corona, and of volume currents flowing in the region where the solar wind plasma totally controls the magnetic field. The model matches the MHD solution for a simple dipole test case better than earlier source surface and current sheet models. The strength and polarity of the radial interplanetary magnetic field component predicted for quiet time samples in each year from 1977 to 1986 agree with observations made near the Earth's orbit better than the hybrid MHD-source surface model (Wang and Sheeley, 1988). The results raise the question of whether coronal holes are the only solar source of the interplantary magnetic field in the solar wind. If some interplanetary flux originates outside coronal holes, the model can match the observed field using the accepted 1.8 saturation correction factor for lambda 5250 A magnetograph observations. Requiring open flux to come exclusively from coronal holes requires and additional factor of two.

Zhao, Xuepu↗

NASA NTRSDate not supplied

Phase-Free Orbital Element Model Designed to Enable Rapid Assessment of Eclipse and Radiation Profiles for Low-Thrust Spiral Transfers Around the Earth

The Gateway Power and Propulsion Element (PPE) will be the first low-thrust solar electric ion propulsion mission to transfer from a highly elliptical Earth-bound orbit to a southern near-rectilinear halo orbit (NRHO) at the Earth-Moon L2 point. Due to the low thrust nature of the transfer orbit, it is desirable to locate viable trajectories that minimize the time spent in the Van Allen radiation belts to reduce solar array degradation and keep radiation dosages below design limits. In addition to radiation, low thrust trajectories which spiral around the Earth will pass through at least one or more seasons of Earth eclipses. The solar electric propulsion system relies on sunlight to generate the power necessary to operate the ion thrusters. Therefore, it is advantageous to find trajectories which also minimize the amount of time spent in eclipse and avoid excessive battery draw-down periods. We present an analytical method which rapidly approximates low-thrust spiral trajectories, fit to high-fidelity simulated data and parameterized to allow for changes in vehicle thrust characteristics, that is post-processed to determine eclipse profiles and time spent in the belts using a novel approach that estimates the geometry of the belts using a first-order dipole approximation of the Earth’s magnetic field. This method can be used to find satisfactory launch dates and orbit orientations that can serve as initial guesses when optimizing such missions in high-fidelity software.

low thrust trajectory design↗

NASA NTRSDate not supplied

Phase-Free Orbital Element Model Designed to Enable Rapid Assessment of Eclipse and Radiation Profiles for Low-Thrust Spiral Transfers Around the Earth

low thrust trajectory design↗

NASA NTRS2000

Process of changing the refractive index of a composite containing a polymer and a compound having large dipole moment and polarizability and applications thereof

Fused ring bridge, ring locked dyes that form thermally stable photorfractive compositions. The fused ring bridge structures are .pi.-conjugated bonds in benzene-, naphthalene- or anthracene-derived fused ring systems that connect donor and acceptor groups. The donor and acceptor groups contribute to a high molecular dipole moment and linear polarizability anisotropy. The polarization characteristics of the dye molecules are stabilized since the bonds in the fused ring bridge are not susceptible to rotation, reducing the opportunity for photoisomerization. The dyes are compatible with polymeric compositions, including thermoplastics. The dyes are electrically neutral but have charge transport, electronic and orientational properties such that upon illumination of a composition containing the dye, the dye facilitates refractive index modulation and a photorefractive effect that can be utilized advantageously in numerous applications such as in optical quality devices and biological imaging.

Marder, Seth R.↗

NASA NTRS2002

Thermally stable molecules with large dipole moments and polarizabilities and applications thereof

Disclosed are fused ring bridge, ring-locked dyes that form thermally stable photorefractive compositions. The fused ring bridge structures are .pi.-conjugated bonds in benzene-, naphthalene- or anthracene-derived fused ring systems that connect donor and acceptor groups. The donor and acceptor groups contribute to a high molecular dipole moment and linear polarizability anisotropy. The polarization characteristics of the dye molecules are stabilized since the bonds in the fused ring bridge are not susceptible to rotation, reducing the opportunity for photoisomerization. The dyes are compatible with polymeric compositions, including thermoplastics. The dyes are electrically neutral but have charge transport, electronic and orientational properties such that upon illumination of a composition containing the dye, the dye facilitates refractive index modulation and a photorefractive effect that can be utilized advantageously in numerous applications such as in optical quality devices and biological imaging.

Marder, Seth R.↗

NASA NTRS2012

Pulsar Emission Geometry and Accelerating Field Strength

The high-quality Fermi LAT observations of gamma-ray pulsars have opened a new window to understanding the generation mechanisms of high-energy emission from these systems, The high statistics allow for careful modeling of the light curve features as well as for phase resolved spectral modeling. We modeled the LAT light curves of the Vela and CTA I pulsars with simulated high-energy light curves generated from geometrical representations of the outer gap and slot gap emission models. within the vacuum retarded dipole and force-free fields. A Markov Chain Monte Carlo maximum likelihood method was used to explore the phase space of the magnetic inclination angle, viewing angle. maximum emission radius, and gap width. We also used the measured spectral cutoff energies to estimate the accelerating parallel electric field dependence on radius. under the assumptions that the high-energy emission is dominated by curvature radiation and the geometry (radius of emission and minimum radius of curvature of the magnetic field lines) is determined by the best fitting light curves for each model. We find that light curves from the vacuum field more closely match the observed light curves and multiwavelength constraints, and that the calculated parallel electric field can place additional constraints on the emission geometry

DeCesar, Megan E.↗