Engineering topics

Roberts, B.

Publications and source records attributed to Roberts, B..

32 records · Page 2

NASA NTRS1986

Theoretical and Experimental Beam Plasma Physics (TEBPP)

The theoretical and experimental beam plasma physics (TEBPP) consists of a package of five instruments to measure electric and magnetic fields, plasma density and temperature, neutral density, photometric emissions, and energetic particle spectra during firings of the particle injector (SEPAC) electron beam. The package is developed on a maneuverable boom (or RMS) and is used to measure beam characteristics and induced perturbations field ( 10 m) and mid field ( 10 m to 100 m) along the electron beam. The TEBPP package will be designed to investigate induced oscillations and induced electromagnetic mode waves, neutral and ion density and temperature effects, and beam characteristics as a function of axial distance.

Roberts, B.↗

NASA NTRS1986

Recoverable Plasma Diagnostics Package (RPDP)

The recoverable plasma diagnostics package (RPDP) is an ejectable and recoverable satellite with flight and ground support systems so that it can be utilized in three modes: attached to an remote manipulator system; tethered; or as a subsatellite. The satellite is well instrumented with particle and field diagnostic as well as optical sensors to: investigate the dynamics of the natural environment or ejected perturbations from particle beams; measure the characteristics and propagation of electrostatic and electromagnetic waves; study wave particle interactions; and study natural properties of the magnetosphere, ionosphere, and upper atmosphere.

Roberts, B.↗

NASA NTRS1986

Electrodynamic tether

The electrodynamic tether consists of a satellite deployed to a distance of 20 km by an electrically conducting tether. The space station hardware consists of a 12 meter deployment boom, satellite cradle, tether reel and motor, and other tether support systems. The electrodynamic tether will be used to perform a variety of wave experiments by exciting a wide spectrum of low frequency waves in the ionospheric plasma. The system can also be used to artificially generate and study field aligned currents and associated plasma effects. Hydromagnetic waves generated by the passage of the system through the space plasma are of particular interest in space plasma are of particular interest in space plasma research.

Roberts, B.↗

NASA NTRS1986

Imaging Spectrometric Observatory (ISO)

The imaging spectrometric observatory (ISO) is discussed. The objectives of this instrument are to measure the spectral signatures of a large range of minor constituents, metastable, and excited species of both atomic and molecular ions, and neutrals in the atmosphere (from the stratosphere to the upper thermosphere). The instrument is composed of five identical spectrometers, each restricted to a given spectral range between 20 and 1200 nanometers designed for high speed operation as an imaging device. Each module is an imaging scanning spectrometer with coincident 0.5 x 0.007 degree field of view.

Roberts, B.↗

NASA NTRS1986

Atmospheric Emission Photometric Imaging (AEPI)

The atmospheric emission photometric imaging (AEPI) consists of a dual channel, low light level video system with a filter wheel to isolate the emissions of interest, mounted on a stabilized, two axis gimbal system for pointing and control. The objectives are to produce images of various atmospheric emissions to: investigate ionospheric transport processes; observe induced emissions from artificial particle injection; measure electron impact cross sections of atmospheric species; study natural aurora at high spatial and temporal resolutions and in the ultraviolet.

Roberts, B.↗

NASA NTRS1986

Wide Angle Michelson Doppler Imaging Interferometer (WAMDII)

The wide angle Michelson Doppler imaging interferometer (WAMDII) is a specialized type of optical Michelson interferometer working at sufficiently long path difference to measure Doppler shifts and to infer Doppler line widths of naturally occurring upper atmospheric Gaussian line emissions. The instrument is intended to measure vertical profiles of atmospheric winds and temperatures within the altitude range of 85 km to 300 km. The WAMDII consists of a Michelson interferometer followed by a camera lens and an 85 x 106 charge coupled device photodiode array. Narrow band filters in a filter wheel are used to isolate individual line emissions and the lens forms an image of the emitting region on the charge coupled device array.

Roberts, B.↗

NASA NTRS1986

Vehicle Charging And Potential (VCAP)

The vehicle charging and potential (VCAP) payload includes a small electron accelerator capable of operating in a pulsed mode with firing pulses ranging from 600 nanoseconds to 107 seconds (100 milliamps at 1000 volts), a spherical retarding potential analyzer - Langmuir probe, and charge current probes. This instrumentation will support studies of beam plasma interactions and the electrical charging of the spacecraft. Active experiments may also be performed to investigate the fundamental processes of artificial aurora and ionospheric perturbations. In addition, by firing the beam up the geomagnetic field lines of force (away from the Earth) investigations of parallel electric field may be performed.

Roberts, B.↗

NASA NTRS1986

Impulsively generated fast coronal pulsations

Rapid oscillations in the corona are discussed from a theoretical standpoint, developing some previous work on ducted, fast magnetoacoustic waves in an inhomogeneous medium. In the theory, impulsively (e.g., flare) generated mhd (magnetohydrodynamic) waves are ducted by regions of low Alfven speed (high density) such as coronal loops. Wave propagation in such ducts is strongly dispersive and closely akin to the behavior of Love waves in seismology, Pekeris waves in oceanography and guided waves in fiber optics. Such flare-generated magnetoacoustic waves possess distinctive temporal signatures consisting of periodic, quasi-periodic and decay phases. The quasi-periodic phase possesses the strongest amplitudes and the shortest time scales. Time scales are typically of the order of a second for inhomogeneities (coronal loop width) of 1000 km and Alfven speeds of 1000/kms, and pulse duration times are of tens of seconds. Quasi-periodic signatures have been observed in radio wavelengths for over a decade and more recently by SMM. It is hoped that the theoretical ideas outlined may be successfully related to these observations and thus aid the interpretation of oscillatory signatures recorded by SMM. Such signatures may also provide a diagnostic of coronal conditions. New aspects of the ducted mhd waves, for example their behavior in smoothly varying as opposed to tube-like inhomogeneities, are currently under investigation. The theory is not restricted to loops but applied equally to open field regions.

Edwin, P. M.↗

NASA NTRS1984

Surface solitary waves and solitons

The solar atmosphere and solar wind are magnetically structured. The structuring can include tangential discontinuities, which can support surface waves. Such waves can be dispersive. This means that dispersion and nonlinearity can balance in such a way that solitary waves (or solitons) can result. This general point is illustrated by a two-dimensional nonlinear analysis which explicitly demonstrates the presence of long-wavelength solitary waves propagating on tangential discontinuities. If the waves are only weakly nonlinear, then they obey the Korteweg-de Vries equation and are true solitons.

Hollweg, J. V.↗

NASA NTRS1983

Wave propagation in intense flux tubes

The nature of nonadiabatic wave propagation in a slender magnetic flux tube is explored. The results of the theory are compared with the observations of Giovanelli et al. (1978), and found to be in general agreement. Those observations, of tubes in the photosphere-chromosphere, show outwardly propagating waves, with periods of 300 s, which take some 19 s to propagate from one level of line formation to another level higher in the atmosphere. In sharp contrast to this, is the time of 7 s for a similar disturbance outside the tube to propagate between the same two levels of line formation, estimated to be some 600 km apart in the field-free atmosphere. It is argued that the sharply contrasting propagation times for the tube and its environment is principally due to the elasticity of the tube and its subsequent propensity for propagation. A nonadiabatic disturbance may be essentially propagating within the tube but essentially nonpropagating outside, with considerably slower phase speeds thus arising inside the tube. The theory suggests that the observed disturbances are nonadiabatic, acoustic-gravity waves channelled along a magnetic flux tube and modulated by external pressure variations.

Roberts, B.↗

NASA NTRS1981

Bound oscillations on thin magnetic flux tubes - Convective instability and umbral oscillations

The possibility that 'tube waves' can be trapped on slender solar magnetic flux tubes is investigated. For rigid isothermal flux tubes, it is found that the flux tube geometry can by itself lead to waves which are trapped on the part of the tube that expands with height. Some geometries lead to trapped modes with eigenperiods near 180 s, if parameters appropriate to sunspot umbrae are chosen. It is possible that the umbral oscillations are a manifestation of such trapped waves, if sunspot umbrae consist of an assembly of slender flux tubes, as in the spaghetti model of Parker (1979). For flux tubes which have a constant ratio of Alfven speed to sound speed, it is found that it is primarily the variation of temperature with height which determines whether trapped waves can exist. Certain temperature profiles lead to disturbances for which omega squared is less than zero, corresponding to convective instability or Rayleigh-Taylor instability.

Hollweg, J. V.↗

NASA NTRS1980

The thermal statics of coronal loops

The thermal statics of constant pressure coronal loops is discussed, with particular emphasis on non-equilibrium and scaling relations. An analytical solution showing explicitly the occurrence of non-equilibrium in radiation dominated loops is presented. In addition, the general scaling law for hot loops is given. However, in view of the uncertainties in the coronal heating function and the observational determined loop parameters, it is suggested that scaling laws are currently of limited value.

Roberts, B.↗

NASA NTRS1976

Intense magnetic fields and umbral dots

Intense magnetic fields (of one or two kilogauss) situated in a background of weak field and umbral dots (regions of weak field) situated in a background of intense field (the sunspot) have both received considerable, though largely unrelated, discussion in the recent literature. It is suggested that the two phenomena may in fact be less disparate than hitherto presumed. Applying a simple (hydrostatic) model calculation to both phenomena and assuming temperature differences to be constant with height, the depth of each of these structures can be estimated.

Roberts, B.↗

NASA NTRS1976

Overstability and cooling in sunspots

The role played by overstable Alfven modes in magnetic structures such as sunspots is considered in detail for a column of magnetic field. It is demonstrated explicitly that overstable Alfven waves cool the interior of the magnetic column. It is suggested that these waves account for the cooling in sunspot umbrae, and therefore, in concurrence with Parker, we conclude that a sunspot is a region of enhanced heat transport. The calculations indicate that sunspots have small regions at normal photospheric brightness, and we tentatively suggest that these regions are umbral dots. We also suggest that cooling by overstable Alfven waves may explain the existence of the intense small magnetic flux tubes that constitute the general solar magnetic field.

Roberts, B.↗