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Johnson, M.

Publications and source records attributed to Johnson, M..

48 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Observations in the vicinity of substorm onset: Implications for the substrom process

Multi-instrument data sets from the ground and satellites at both low and high altitude have provided new results concerning substorm onset and its source region in the magnetosphere. Twenty-six out of 37 substorm onset events showed evidence of azimuthally spaced auroral forms (AAFs) prior to the explosive poleward motion associated with optical substorm onset. AAFs can span 8 hours of local time prior to onset and generally propagate eastward in the morning sector. Onset itself is, however, more localized spanning only about 1 hour local time. AAF onset occur during time periods when the solar wind pressure is relatively high. AAFs brighten in conjunction with substorm onset leading to the conclusion that they are a growth phase activity casually related to substorm onset. Precursor activity associated with these AAFs is also seen near geosynchronous orbit altitude and examples show the relationship between the various instrumental definitions of substorm onset. The implied mode number (30 to 135) derived from this work is inconsistent with cavity mode resonances but is consistent with a modified flute/ballooning instability which requires azimuthal pressure gradients. The extended source region and the distance to the open-closed field line region constrain reconnection theory and local mechanisms for substorm onset. It is demonstrated that multiple onset substorms can exist for which localized dipolarizations and the Pi 2 occur simultaneously with tail stretching existing elsewhere. These pseudobreakups can be initiated by auroral streamers which originate at the most poleward set of arc systems and drift to the more equatorward main UV oval. Observations are presented of these AAFs in conjunction with low- and high-altitutde particle and magnetic field data. These place the activations at the interface between dipolar and taillike field lines probably near the peak in the cross-tail current. These onsets are put in the context of a new scenario for substorm morphology which employs individual modules which operate independently or couple together. This allows particular substorm events to be more accurately described and investigated.

Elphinstone, R. D.↗
NASA NTRS1995

Coronal heating in flare stars: Resonant MHD absorption?

We report on the occurrence of periodicity in X-rays from certain flare stars. Statistically significant periods in the range 70-200 s are present in Einstein data for EQ Vir, Proxima Cen, and AD Leo. Periodicities are also present (although with lower statistical significance) in ROSAT data for AD Leo, and in EXOSAT data for EQ Vir and AD Leo. In UV Cet, we also find periodicity in Einstein data, but with lower statistical significance than for the above three stars: however, the UV Cet X-ray period is close to a period discovered by Gary et al. (1982) in microwaves in the close companion L726-8A. We argue that the properties of magnetic loops in flare stars are such that the variations in a single magnetic loop may be detectable in integrated flux. The period which we find in the X-ray flux from Proxima Cen is consistent with a global Alfven wave resonance in a large loop whose properties were derived some years ago by Haisch (1983) using an entirely different line of reasoning from what we use here. The periods, amplitudes, and durations of the variations which we have found in X-ray emission are consistent with the hypothesis that resonant absorption of MHD waves is occurring at certain times in coronal loops in the above four stars.

Mullan, D. J.↗
NASA NTRS1995

Two-dimensional structure of slow shocks

The two-dimensional (2-D) structure of switch-off slow magnetosonic shocks is investigated using an electromagnetic hybrid (fluid electrons, kinetic ions) code. It is shown that the basic physical processes occurring at 1-D slow shocks are also operative in 2-D. Specifically, the interaction between the upstream ions and those streaming away from the shock results in the excitation of Alfven/ions-cyclotron (AIC) waves. Depending on the plasma parameters, these waves may either stay in the upstream or convect back into the shock resulting in a non-steady shock behavior which prevents the formation of a trailing wave train. Despite this similarity, some slow shocks which are steady in 1-D are found to be non-steady in 2-D. Fourier analysis of the waves downstream of non-steady shocks identifies them as AIC, demonstrating that the waves remain on the same branch as they convect from upstream into the downstream region.

Omidi, N.↗
NASA NTRS1985

High pressure cosmochemistry applied to major planetary interiors: Experimental studies

The measurement of equilibria in binary fluid-solid systems in diamond anvil cells, represents a major advance of the art of high-pressure experimentation. Vibrational spectroscopy, direct visual observations, and X-ray diffraction crystallography of materials confined in externally heated cells are the primary experimental probes being used. Adiabats in these systems are being measured in order to constrain models of heat flow in these bodies and to detect phase transitions by thermal anomalies. Other studies are directed toward interpreting high pressure reactions in these systems that are suggested by shockwave measurements, and developing methods for reaching high temperatures and high pressures of planetary interest in diamond cells. The overall objective of this project is to determine the properties of the H2-He-H2O-HN3-CH4 system and related small-molecule systems that are needed to constrain theoretical models of the interiors of the major planets.

Nicol, M. F.↗
NASA NTRS1985

High Pressure Cosmochemistry of Major Planetary Interiors: Laboratory Studies of the Water-rich Region of the System Ammonia-water

The behavior of gas-ice mixtures in major planets at very high pressures was studied. Some relevant pressure-temperature-composition (P-T-X) regions of the hydrogen (H2)-helium (He)-water (H2O-ammonia (NH3)-methane (CH4) phase diagram were determined. The studies, and theoretical model, of the relevant phases, are needed to interpret the compositions of ice-gas systems at conditions of planetary interest. The compositions and structures of a multiphase, multicomponent system at very high pressures care characterized, and the goal is to characterize this system over a wide range of low and high temperatures. The NH3-H2O compositions that are relevant to planetary problems yet are easy to prepare were applied. The P-T surface of water was examined and the corresponding surface for NH3 was determined. The T-X diagram of ammonia-water at atmospheric pressure was studied and two water-rich phases were found, NH3-2H2O (ammonia dihydrate), which melts incongruently, and NH3.H2O (ammonia monohydrate), which is nonstoichiometric and melts at a higher temperature than the dihydrate. It is suggested that a P-T surface at approximately the monohydrate composition and the P-X surface at room temperature is determined.

Nicol, M.↗
NASA NTRS1984

High pressure cosmochemistry applied to major planetary interiors: Experimental studies

Progress is reported on a project to determine the properties and boundaries of high pressure phases of the H2-He-H2O-NH3-CH4 system that are needed to constrain theoretical models of the interiors of the major planets. This project is one of the first attempts to measure phase equilibria in binary fluid-solid systems in diamond anvil cells. Vibrational spectroscopy, direct visual observations, and X-ray diffraction crystallography of materials confined in externally heated cells are the primary experimental probes. Adiabats of these materials are also measured in order to constrain models of heat flow in these bodies and to detect phase transitions by thermal anomalies. Initial efforts involve the NH3-H2O binary. This system is especially relevant to models for surface reconstruction of the icy satellites of Jupiter and Saturn. Thermal analysis experiments were completed for the P-X space, p4GPa:0 or = 0.50, near room temperature. The cryostat, sample handling equipment, and optics needed to extend the optical P-T-X work below room temperature was completed.

Nicol, M. F.↗
NASA NTRS1983

High pressure cosmochemistry applied to major planetary interiors: Experimental studies

Progress is reported in the development of the P-T-X diagram for 0 less than or = X less than or = 0.50 and in the development of techniques for measuring adiabats of phases of NH3-H2O. The partial phase diagram is presented, investigations of the compositions of ammonia ices are described, and methods for obtaining the infrared spectra of ices are discussed.

Nicol, M. F.↗
NASA NTRS1983

High pressure cosmochemistry applied to major planetary interiors: Experimental studies

The infrared spectra of H2O ice VII and D2O ice VII were studied and techniques were developed for measuring adiabats of phases of NH3-H2O to 5 GPa. A mixing system for pressurized fluids was constructed in which liquid solutions of definite compositions can be prepared and loaded reliably into diamond cells in a project which seeks to determine the properties and boundaries of several high pressure phases of the H2-He-H2O-NH3-CH4 system. These data are needed to constrain theoretical models of the interiors of the major planets.

Nicol, M. F.↗
NASA NTRS1981

A survey of rich clusters of galaxies with HEAO 1. II

Results are presented of an X-ray survey of approximately 1900 Abell clusters of galaxies performed with the NRL large-area X-ray detectors on HEAO 1. The cluster luminosity function derived from the observations made at ecliptic longitudes between 80 and 180 deg and 260 and 360 deg is fit with both power law and exponential relations, and used to estimate a total contribution of Abell clusters to the volume emissivity of the diffuse X-ray background of between 3 and 10%. No strong correlation is found between X-ray luminosity and Bautz-Morgan class, richness class or 26-MHz radio power, although B-M type I and Abell richness class 3 appear more likely to be strong X-ray sources. A correlation is suggested between optical radius and X-ray luminosity.

Ulmer, M. P.↗
NASA NTRS1978

High-resolution observations of X-ray sources at the galactic center

A region about 1 deg of arc in radius and centered on the galactic nucleus has been surveyed during a rocket flight, using X-ray detectors equipped with 10-arcmin by 4-deg (FWHM) collimators. The extended source 4U 1743-29 has been resolved into at least four discrete sources having luminosities (2-10 KeV) between 4 by 10 to the 36th power and 3.5 by 10 to the 37th power erg/s.

Cruddace, R. G.↗
NASA NTRS1977

Overhead-handling, universal-positioning device

Lift-and-rotate mechanism can raise payload and reorient it while it is suspended in space. Mechanism is itself suspended from overhead lift point and is operated from single pendant/reel control panel. Lift capacity and size of mechanism is determined by application.

Johnson, M.↗