Engineering topics
Publications and source records attributed to Brandt, J. C..
Present models of the plasma tails of comets are described. The interaction of the solar wind with ions from the cometary atmosphere is discussed, and the phenomenon of magnetic reconnection observed in plasma tails is explained. The accomplishments of the ICE mission to the Comet Giacobini-Zinner are summarized, and the tasks and expected contributions from upcoming Soviet, European, and Japanese missions to Comet Halley are addressed.
Aspects of the International Cometary Explorer (ICE) flight to the comet Giacobini-Zinner (GZ) are discussed. The most important experiments to be performed by ICE are reviewed, and the orbital parameters of GZ are described. The dust characteristics of GZ that pose a hazard to the spacecraft are addressed, and the ICE targeting strategy toward the comet is discussed. Requested ground-based coverage of GZ is indicated, and the complementarity of the GZ coverage with that given to the Halley mission is shown.
The use of comet observations to characterize three-dimensional solar-wind structure is discussed, including the plasma-tail orientations, the Lyman-alpha emission contours from the hydrogen cloud, and periodic disconnection of the plasma tail. Models based on observations of plasma-tail orientations produce solar-wind radial speeds in the 400-420 km/s range, and azimuthal speeds of 5-7 km/s. Studies of disconnection events indicate that the sector structure can extend to solar latitudes of greater than 45 deg for much of a solar cycle, and that the tilt angles could be quite high.
Theoretical line profiles are presented that demonstrate why 100,000 spectral resolution and high S/N are needed to derive accurate column densities from spectral lines that lie close to the flat part of the curve of growth and may contain multiple velocity components. The aim of the Space Telescope high-resolution-spectrograph observing program is to obtain column densities in the hydrogen and deuterium Lyman alpha lines along a variety of lines of sight within and extending beyond the local cloudlet, in which the sun is located near an edge. The broad chromospheric Lyman alpha emission lines of late-type stars are used as background sources against which to measure the interstellar absorption features. Profiles of interstellar absorption features in Mg II and Fe II lines will be obtained to derive the broadening parameter and/or identify possible multiple velocity components in the lines of sight.
The International Cometary Explorer (ICE), scheduled to pass the tail of Comet Giacobini-Zinner on September 11, 1985, was expected to address first-order questions bout the interaction between solar wind and comets. Since ICE was not initially intended for comet studies, it did not have sun-sensitive imaging instruments designed to image the comet nucleus. In effect, ICE was free to pass through the tail of the comet. The ICE spacecraft, a 16-faceted cylindrical drum just over five ft high with a spin axis normal to the ecliptic plane, was designed to encounter the comet at a distance of 0.47 AU from the earth. Scientific instruments on board the spacecraft include: (1) the electron plasma experiment, (2) the vector helium magnetometer, (3) the plasma wave experiment, (4) the radio waves experiment, (5) the plasma ion experiment, (6) the low-energy cosmic ray experiment, and (7) the energetic protons experiment. Targeting error was expected as a result of solar-wind induced plasma tail wagging. Comet interception was planned to be at a distance of 10,000 km from the nucleus.
The primary objectives of the International Cometary Explorer (ICE) mission is to provide in situ data on the interaction between solar wind and the atmosphere of the P/Giacobini-Zinner comet (G/Z), making measurements of particles, fields, and waves while passing through the cometary tail of G/Z on September 11, 1985. Following the G/Z tail intercept, the ICE measurements will complement the later upstream measurements obtained by the Comet Halley probe. The major ICE payload includes a vector helium magnetometer, the plasma-wave experiment, the radio-wave experiment, the plasma-electron experiment, and the plasma ion experiment. Other experiments are intended to measure energetic protons, X-rays, low energy to high energy cosmic rays, cosmic ray electrons, and gamma-ray bursts. The ICE measurements of G/Z will be supplemented with ground-based measurements. Schematic diagrams are included.
The International Cometary Explorer (ICE) spacecraft passed through the plasma tail of Comet Giacobini-Zinner (G/Z) on Sept. 11, 1985, and made in situ measurements of particles, fields, and waves. The scientific results indicate that Alfven's magnetic-field capture and draping model is correct; that the comet/solar-wind interaction produces energetic ions (probably by the pick-up process), and that the bow wave, as seen on the flanks, is not a shock but an extended interaction region. It is concluded that the ICE spacecraft survived the encounter virtually unscathed and made field and particle measurements upstream of Comet Halley, actually detecting the comet via plasma wave and energetic particle measurements.
An overview of tail phenomena is presented based on worldwide submissions to the Large-Scale Phenomena Discipline Specialist Team of the International Halley Watch. Examples of tail phenomena and science are presented along with estimates of total expected yield from the Network. The archive of this material will clearly be very valuable for studying the solar-wind/comet interaction during the 1985-1986 apparition of Halley's Comet.
Some of the laboratory measured performance parameters of the HRS are reported. Then a description is given of three aspects of performance of particular importance to astronomers: (1) the capability of detecting very weak features against a continuum, (2) the capability of producing reliable line profiles, and (3) the capability of assigning accurate wavelengths to spectral features. Specific technical descriptions of the performance of the HRS detectors (Eck and Beaver, this volume), gratings (Bottema et al., this volume), and control and data-handling system (Becker, this volume) are reported separately.
Solar wind plasma data from the ISEE-3 and Helios 2 spacecraft were examined to explain a uniquely rapid 10 deg turning of the plasma tail of comet Bradfield 1979L on 1980 February 6. It was suggested that the tail position angle change occurred in response to a solar wind velocity shear across which the polar component changed by approx. 50 km s-1. The present activity was caused by noncorotating, disturbed plasma flows probably associated with an Importance 1B solar flare.
The hypersonic pressure balance model of flaring in cometary plasma tails of Ershkovich et al. (1982) has been modified to include the effects of magnetohydrodynamic (MHD) instabilities occurring along the ionopause in the outer-tail regions. The effect of instability is to mix the solar-wind and comet-tail plasmas, increasing the tail magnetic field strength above that calculated from magnetic flux conservation. The earlier model assumed the ionopause to be a tangential discontinuity surface (flux conserving) at all distances, with the result that the magnetic field approached zero in the outer regions of strongly flaring tails. The present model is more realistic and is in better agreement with measurements of cometary plasma tail widths and flaring angles. This agreement leads to an important conclusion that the magnetic flux is not conserved in distant comet tails.
Obsrvations and theoretical investigations of the Gum nebula (GN) since about 1971 are reviewed. Direct observations of the GN, the Vela X supernova remnant (SNR), the Vela pulsar, and other stars in or near the GN are discussed with those of related phenomena such as the radio loops and known SNRs; the emphasis is on studies of the interstellar absorption lines, the evidence for hot gas in the GN, and the extended diffuse emission. The four basic models proposed for the GN are considered: a fossil Stromgren sphere, an old SNR, an H II region, or a superbubble. The GN physical parameters predicted by each model are listed in a table and compared. A minimum explanation which attributes the 36 x 36-deg filamentary structure and the 125-pc radius structure to the action of the stellar winds from Zeta Pup and Gamma-2 Vel (and perhaps the effect of a Vel X supernova explosion 20,000 years ago) is found most appropriate, at least until the questions of the net expansion rate of the GN (about 20 km/sec or about zero?) and the existence of the diffuse emission beyond the filamentary structure are resolved by observations.
The High Resolution Spectrograph is one of five scientific instruments which will be part of the Space Telescope observatory. The spectrograph was designed to take advantage of the imaging and pointing capabilities of the telescope to obtain ultraviolet spectra with spectral resolution comparable to large, ground-based coudeinstruments. Some of the results of the ground-based testing program will be described, along with applications of these properties to future science programs.
Solar-wind plasma data from the ISEE-3 and Helios 2 spacecraft were examined in order to explain a uniquely rapid 10 deg turning of the plasma tail of comet Bradfield 1979l on 1980 February 6. An earlier study conducted before the availability of in situ solar-wind data (Brandt et al., 1980) suggested that the tail position angle change occurred in response to a solar-wind velocity shear across the polar component changed by approximately 50 km/s. The present contribution confirms this result and further suggests that the comet-tail activity was caused by non-corotating, disturbed plasma flows probably associated with an Importance 1B solar flare.
Assuming that hypersonic pressure balance with the solar wind governs the shape of plasma tails, it is found that the gas pressure of tail ions and the magnetic field strength at the flanks of the ionopause control the flaring state. The gas pressure exhibits the larger effect: for constant pressures above a certain critical value, the tail flares essentially without limit, while for smaller values the tail flares only near the head (becoming cylindrical at greater distances). The influence of the magnetic field is that the tail flares to larger distances the higher the field strength at the flanks of the ionopause. The observed variability in flaring (and the implied differences in gas pressure and magnetic field) are throught to be the result of changes in the position and shape of the sunward cometary ionopause. Insertion of reasonable comet and solar wind parameters into the pressure balance equations is found to give good agreement with the observations.
Three significant Spacelab facilities which are being developed, or planned by NASA will use the 1-2 week observational capability for attached payloads on Shuttle to carry out advanced astronomical observations in the ultraviolet visible, and infrared spectral regions during the decade of the 1980s. The three facilities are expected to evolve in capability over several Shuttle flights with a final objective of protracted observational capability from a permanent facility in space with periodic servicing and/or retrieval. The three facilities are the Office of Space Science package (OSS-3/7), the Solar Optical Telescope (SOT), and the Shuttle Infrared Telescope Facility (SIRTF). The scientific and technical objectives of these astronomical facilities include both their current and projected capabilities, their use of the manned Shuttle, and their eventual disposition to a long-term facility. International aspects of the missions, including General Investigator programs, are also being planned.
Progress in contemporary astronomy and astrophysics is shown to depend on complementary investigations with sensitive telescopes operating in several wavelength regions, some of which can be on the Earth's surface and others of which must be in space.
The High Resoltuion Spectrograph (HRS) in conjunction with the Space Telescope (ST), extends ultraviolet astronomical spectroscopy to higher spectra, spatial, and time resolutions than previously achieved, as well as to fainter and more distant celestial objects. Other significant advances inherent in the instrument are high photometric accuracy and efficient operation via exposure meter control and real time rejection of bad data. These capabilities are provided to accomplish the scientific programs of the HRS investigation definition team, which concern the interstellar medium, stellar winds, and evolutionary aspects of stellar atmosphere studies; the determination of chemical abundances relevant to stellar evolution; the investigation of quasars and Seyfert galaxy nuclei; and the analysis of the atmospheres of solar system objects, including comets.