Applying essentially non‐oscillatory interpolation to controlled‐source electromagnetic modelling*

Modelling and inversion of controlled‐source electromagnetic (CSEM) fields requires accurate interpolation of modelled results near strong resistivity contrasts. There, simple linear interpolation may produce large errors, whereas higher‐order interpolation may lead to oscillatory behaviour in the interpolated result. We propose to use the essentially non‐oscillatory, piecewise polynomial interpolation scheme designed for piecewise smooth functions that contains discontinuities in the function itself or in its first or higher derivatives. The scheme uses a non‐linear adaptive algorithm to select a set of interpolation points that represent the smoothest part of the function among the sets of neighbouring points. We present numerical examples to demonstrate the usefulness of the scheme. The first example shows that the essentially non‐oscillatory interpolation (ENO) scheme better captures an isolated discontinuity. In the second example, we consider the case of sampling the electric field computed by a finite‐volume CSEM code at a receiver location. In this example, the ENO interpolation performs quite well. However, the overall error is dominated by the discretization error. The other examples consider the comparison between sampling with essentially non‐oscillatory interpolation and existing interpolation schemes. In these examples, essentially non‐oscillatory interpolation provides more accurate results than standard interpolation, especially near discontinuities.     

The biogeochemical sulfur cycle in the marine boundary layer over the Northeast Pacific Ocean

The major components of the marine boundary layer biogeochemical sulfur cycle were measured simultaneously onshore and off the coast of Washington State, U.S.A. during May 1987. Seawater dimethylsulfide (DMS) concentrations on the continental shelf were strongly influenced by coastal upwelling. Concentration further offshore were typical of summer values (2.2 nmol/L) at this latitude. Although seawater DMS concentrations were high on the biologically productive continental shelf (2–12 nmol/L), this region had no measurable effect on atmospheric DMS concentrations. Atmospheric DMS concentrations (0.1–12 nmol/m³), however, were extremely dependent upon wind speed and boundary layer height. Although there appeared to be an appreciable input of non-sea-salt sulfate to the marine boundary layer from the free troposphere, the local flux of DMS from the ocean to the atmosphere was sufficient to balance the remainder of the sulfur budget.     

The gravitational field of topographic-isostatic masses and the hypothesis of mass condensation II-the topographic-isostatic geoid

In Part I we focussed on a convergent representation of the gravitational potential generated bytopographic masses on top of the equipotential surface atMean Sea Level, thegeoid, and by those masses which compensate topography. Topographic masses have also been condensated, namely represented by a single layer. Part II extends the computation of the gravitational field of topographic-isostatic masses by a detailed analysis of itsforce field in terms ofvector-spherical harmonic functions. In addition, the discontinuous mass-condensated topographic gravitational force vector (head force) is given. Once we identify theMoho discontinuity asone interface of isostatically compensated topographical masses, we have computed the topographic potential and the gravitational potential which is generated by isostatically compensated masses atMean Sea Level, the geoid, and illustrated by various figures of geoidal undulations. In comparison to a data oriented global geoid computation ofJ. Engels (1991) the conclusion can be made that the assumption of aconstant crustal mass density, the basic condition for isostatic modeling, does not apply. Insteaddensity variations in the crust, e.g. betweenoceanic and continental crust densities, have to be introduced in order to match the global real geoid and its topographic-isostatic model. The performed analysis documents that thestandard isostatic models based upon aconstant crustal density areunreal.     

Infrasonic detection of a Leonid bolide: 1998 November 17

Abstract— During the early morning hours of the night of the peak of the annual Leonid meteor shower on 1998 November 17, a bright fireball (approximately ?12 to ?14 visual magnitude at 100 km in the zenith) was observed over northern New Mexico with visual sightings as far away from Los Alamos as Albuquerque (～150 km to the south of Los Alamos), including direct persistent trail observations at the U. S. A. F. Starfire Optical Range (SOR), which is also near Albuqerque. This event did not produce any sonic boom reports, presumably because of its high altitude. It was also detected locally by an infrared radiometer at Sandia National Laboratory and by an intensified charge-coupled device (CCD) camera located in Placitas, New Mexico. Subsequent investigations of the data from the six infrasound arrays used by Los Alamos National Laboratory (LANL) and operated for the Department of Energy as a part of the Comprehensive Test Ban Treaty (CTBT) Research and Development program for the International Monitoring System (IMS) showed the presence of an infrasonic signal from the proper direction at the correct time for this bolide from two of our six arrays (both located in Los Alamos). The infrasound recordings (i.e., the wave amplitude and period data) indicated that an explosion occurred in the atmosphere at a source height of ～93.5 km (with respect to sea level) or ～90 km with respect to the altitude of Los Alamos, having its origins slightly to the north and west of Los Alamos. Purely geometric solutions from the ground observers reports combined with direct measurements from the CCD camera at Placitas produced a source height of 91 ± 7 km. The signal characteristics analyzed from 0.5 to 3.0 Hz include a total duration of about 3–4 s for a source directed from Los Alamos toward 353.6 ± 0.4° measured from true north at a maximum elevation arrival angle of ～72.7°. The latter was deduced on the basis of the observed signal trace velocities (for the part of the recording with the highest cross-correlation) and ranged from a constant value of about 920–1150 m/s (depending on the window length used in the analysis) for a ray trajectory along a direct refractive path between the source and the Los Alamos arrays. The dominant signal frequency at maximum amplitude at Los Alamos was ～0.71 Hz. These highly correlated signals had a peak to peak, maximum amplitude of ～2.1 microbars (0.21 Pa). Using several methods that incorporate various observed signal characteristics, total distance traveled, etc., our analysis indicates that the bolide probably had a source energy of ～1.14 t (TNT equivalent) or 4.77 × 10⁹ J. This is ～14.1× smaller than the source energy estimate made using the infrasonic, empirical source energy relationship for low-altitude stationary point sources developed in the 1960s by the Air Force Technical Applications Center (AFTAC), Patrick Air Force Base, Florida. This relation was originally developed, however, for much larger source energies and at much longer ranges.     

Distribution of hydrate on Europa: Further evidence for sulfuric acid hydrate

Sulfuric acid hydrate has been proposed as an important species on Europa's surface, the acid being produced by radiolysis of surficial sulfur compounds. We investigated the spectral properties of disordered and crystalline forms of sulfuric acid and suggest that the hydration properties of Europa's hypothesized sulfuric acid lie between two end members: liquid sulfuric acid and its higher crystalline hydrates. The spectra of these end members are similar except for spectral shifts at the band edges. We measured the optical constants of sulfuric acid octahydrate and used these with simple radiative transfer calculations to fit Europa spectra obtained by Galileo's Near Infrared Mapping Spectrometer (NIMS). The global distribution of the hydrate that we associate here with hydrated sulfuric acid shows a strong trailing-side enhancement with a maximum fractional hydrate abundance of 90% by volume, corresponding to a sulfur atom to water molecule ratio of 10%. The hydrate concentration spatially correlates with the ultraviolet and visible absorption of the surface and with the sulfur dioxide concentration. The asymmetric global distribution is consistent with Iogenic plasma ion implantation as the source of the sulfur, possibly modified by electron irradiation and sputtering effects. The variegated distribution also correlates with geologic forms. A high spatial resolution image shows resolved lineae with less hydrate appearing within the lineae than in nearby crustal material. The low concentration of hydrated material in these lineae argues against their conveying sulfurous material to the surface from the putative ocean.     

Effect of polychromatic X‐ray microtomography imaging on the amino acid content of the Murchison CM chondrite

X‐ray microcomputed tomography (μCT) is a useful means of characterizing cosmochemical samples such as meteorites or robotically returned samples. However, there are occasional concerns that the use of μCT may be detrimental to the organic components of a chondrite. Small organic compounds such as amino acids comprise up to ~10% of the total solvent extractable carbon in CM carbonaceous chondrites. We irradiated three samples of the Murchison CM carbonaceous chondrite under conditions akin to and harsher than those typically used during typical benchtop X‐ray μCT imaging experiments to determine if detectable changes in the amino acid abundance and distribution relative to a nonexposed Murchison control sample occurred. After subjecting three meteorite samples to ionizing radiation dosages between ~300 Gray (Gy) and 3 kGy with bremstrahlung X‐rays, we analyzed the amino acid content of each sample. Within sampling and analytical errors, we cannot discern differences in the amino acid abundances and amino acid enantiomeric ratios when comparing the control samples (nonexposed Murchison) and the irradiated samples. We conclude that a polychromatic X‐ray μCT experiment does not alter the abundances of amino acids to a degree greater than how well those abundances are measured with our techniques and therefore any damage to amino acids is minimal.     

Variations in exospheric density at heights near 1100km, derived from satellite orbits

In an earlier paper, values of exospheric density were obtained from the orbit of Echo 2 for the years 1964–1965. The results indicated a semi-annual variation in density by a factor of between 2 and 3, considerably larger than predicted by existing atmospheric models.

These studies have now been extended to the beginning of 1967, using both Echo 2 and Calsphere 1, to show how the density is responding to increasing solar activity. Variations in density during 1964 have been analysed in more detail. The long-term variation associated with the solar cycle and the short-term variations associated with magnetic and solar disturbances agree with the variations expected on the basis of current models. The semi-annual variation is persisting to higher levels of solar activity, and although its amplitude is diminishing the factor of variation was still 1.6 in 1966.     

Energetic protons from the solar flare of March 24, 1966

Ten to 100 meV protons from the solar flare of March 24, 1966 were observed on the University of California scintillation counter on OGO-I. The short rise and decay times observed in the count rates of the 32 channels of pulse-height analysis show that scattering of the protons by the interplanetary field was much less important in this event than in previously observed proton flares. A diffusion theory in which D = M _r is found to be inadequate to account for the time behavior of the count rates of this event. Small fluctuations of the otherwise smooth decay phase may be due to flare protons reflected from the back of a shock front, which passed the earth on March 23.