Engineering topics
Publications and source records attributed to Huber, M. C. E..
The solar ultraviolet measurement of emitted radiation (SUMER) observational program provides continuous observations of the solar disk and the low corona off-limb since 24 January 1996. The achievements of the observational program to date are reviewed. Examples of results from various researchers are presented and discussed. The SUMER instrument and its performance onboard the Solar and Heliospheric Observatory (SOHO) are summarized.
The observations of the ultraviolet coronagraph spectrometer (UVCS), operating onboard the Solar and Heliospheric Observatory (SOHO) spacecraft, are discussed. The purpose of the UVCS is the study of the quiescent coronal streamer and the slow solar wind. The observations started in January 1996. Polarized radiance data in the visible continuum were obtained. Some characteristics of the coronal streamer from the UVCS recorded data are discussed. A model for the source of the slow solar wind in the inner corona is proposed.
The electron temperature is a fundamental physical parameter of the coronal plasma. Currently, there are no direct measurements of this quantity in the extended corona. Observations with the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the upcoming Solar and Heliospheric Observatory (SOHO) mission can provide the most direct determination of the electron kinetic temperature (or, more precisely, the electron velocity distribution along the line of sight). This measurement is based on the observation of the Thomson-scattered Lyman alpha (Ly-alpha) profile. This observation is made particularly challenging by the fact that the integrated intensity of the electron-scattered Ly-alpha line is about 10(exp 3) times fainter than that of the resonantly-scattered Ly-alpha component. In addition, the former is distributed across 50 A (FWHM), unlike the latter that is concentrated in 1 A. These facts impose stringent requirements on the stray-light rejection properties of the coronagraph/spectrometer, and in particular on the requirements for the grating. We make use of laboratory measurements of the UVCS Ly-alpha grating stray-light, and of simulated electron-scattered Ly-alpha profiles to estimate the expected confidence levels of electron temperature determination. Models of different structures typical of the corona (e.g., streamers, coronal holes) are used for this parameter study.
The ultraviolet coronal spectrometer (UVCS)/SOHO instrument will provide spectroscopic determinations of plasma parameters describing the evolution of coronal conditions at the site of coronal mass ejections (CME's). Prior to a CME event, UVCS can provide an empirical model of the coronal region where the event originates. The model would include values for the proton density, temperature, and outflow velocity; the electron density and temperature; and minor ion densities, temperatures, and outflow velocities. This information would be determined for heliocentric heights from about 1.5 to 3.5 of the solar radius with a subset of the parameters determined up to heights of 12 of the solar radius. During the event, UVCS would repeatedly sample one or more heights between 1.5 and 3.5 of the solar radius with a time resolution of about 1 min. The goal would be to determine the mass and energy content of the CME as it moves outward. After the event, the conditions in the region would be determined as before. Information about the instrument optical specifications and sensitivity is provided.
The SUMER (Solar Ultraviolet Measurement of Emitted Radiation) instrumentation that will be mounted on the SOHO (Solar and Heliospheric Observatory) spacecraft is in development. It has some capability of improving the solar angular resolution and the spectral resolution already obtained in the far UV to the extreme UV, corresponding to the temperature range between 10,000 and a few 1,000,000 K. Some insights into the SUMER spectrometer, developed to study the dynamics and to infer temperatures and densities of the low corona and the chromosphere-corona transition zone in using the 50 to 160 nm wavelength range, are given. The SUMER scientific goals and the techniques used are outlined. The instrumentation and the expected performances are described. The way the observations can be conducted is emphasized and the operation of SUMER in coordination with other SOHO instrumentations and in cooperation with ground based observations is explained.
The experiment Solar Ultraviolet Measurements of Emitted Radiation (SUMER) is designed for the investigations of plasma flow characteristics, turbulence and wave motions, plasma densities and temperatures, structures and events associated with solar magnetic activity in the chromosphere, the transition zone and the corona. Specifically, SUMER will measure profiles and intensities of Extreme Ultraviolet (EUV) lines emitted in the solar atmosphere ranging from the upper chromosphere to the lower corona; determine line broadenings, spectral positions and Doppler shifts with high accuracy, provide stigmatic images of selected areas of the Sun in the EUV with high spatial, temporal and spectral resolution and obtain full images of the Sun and the inner corona in selectable EUV lines, corresponding to a temperature from 10,000 to more than 1,800,000 K.
The objectives were to address currently unanswered fundamental questions concerning the fine scale structure of the chromosphere, transition region, and corona. The unique characteristics of the spectroheliometer was used in combination with plasma diagnostic techniques to study the temperature, density, and velocity structures of specific features in the solar outer atmosphere. A unified understanding was sought of the interplay between the time dependent geometry of the magnetic field structure and the associated flows of mass and energy, the key to which lies in the smallest spatial scales that are unobservable with current EUV instruments. Toroidal diffraction gratings were fabricated and tested by a new technique using an elastically deformable substrate. The toroidal diffraction gratings was procured and tested to be used for the evaluation of the Multi-Anode Microchannel Array (MAMA) detector systems for the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) and UV Coronagraph Spectrometer (UVCS) instruments on the SOHO mission.
The SUMER (solar ultraviolet measurements of emitted radiation) experiment is described. It will study flows, turbulent motions, waves, temperatures and densities of the plasma in the upper atmosphere of the Sun. Structures and events associated with solar magnetic activity will be observed on various spatial and temporal scales. This will contribute to the understanding of coronal heating processes and the solar wind expansion. The instrument will take images of the Sun in EUV (extreme ultra violet) light with high resolution in space, wavelength and time. The spatial resolution and spectral resolving power of the instrument are described. Spectral shifts can be determined with subpixel accuracy. The wavelength range extends from 500 to 1600 angstroms. The integration time can be as short as one second. Line profiles, shifts and broadenings are studied. Ratios of temperature and density sensitive EUV emission lines are established.
The design and test results of a high-efficiency EUV imaging spectrometer employing a concave toroidal grating which is illuminated at normal incidence in a Rowland circle mounting and has only one reflecting surface are discussed. Photographic tests and initial photoelectric tests with a two-dimensional pulse-counting detector system demonstrate the excellent image quality of the grating at wavelengths near 600 A. Applications of the spectrometer for the study of nonsolar objects and solar regions (including the chromosphere, transition region, and corona) are considered.
A high-efficiency, extreme ultraviolet (EUV) imaging spectrometer has been constructed and tested. The spectrometer employs a concave toroidal grating illuminated at normal incidence in a Rowland circle mounting and has only one reflecting surface. The toroidal grating has been fabricated by a new technique employing an elastically deformable submaster grating which is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The fixed toroidal grating used in the spectrometer is then replicated from this surface. Photographic tests and initial photoelectric tests with a two-dimensional, pulse-counting detector system have verified the image quality of the toroidal grating at wavelengths near 600 A. The basic designs of two instruments employing the spectrometer for astrophysical investigations in space are described, namely, a high-resolution EUV spectroheliometer for studies of the solar chromosphere, transition region, and corona; and an EUV spectroscopic telescope for studies of nonsolar objects.
Measurements of the oscillator strengths of the ultraviolet lines of neutral nickel obtained by the use of the combined hook and absorption technique are reported. A total of 221 transitions in the range 1964-4094 A was measured for nickel atoms from a high-temperature graphite furnace (2000-2500 K) using a continuum background source, a Mach-Zehnder interferometer and a 3-m Czerny-Turner spectrograph. Hook and absorption measurements are presented, and radiative lifetimes are derived from log gf values. Comparison of the present values with previous results indicates only those of Bell et al. (1966) and Lennard et al. (1975) to consistently agree with the data presented, although the reliability laser-excitation technique of lifetime measurement is supported over that of Hanle methods.
The design of a stigmatic spectroheliometer for photometric studies of dynamic phenomena in the solar atmosphere at extreme ultraviolet (EUV) wavelengths is described. The normal-incidence spectrometer requires only one reflective surface, and is equipped with a series of exit slits and associated one-dimensional detector arrays that are mounted at the secondary (vertical) foci of the concave diffraction grating. It is shown that such a spectrometer mounted at the focus of an off-axis paraboloid telescope mirror of the size employed in the EUV spectroheliometer flown on Skylab could record monochromatic images of a 2 x 2 (arcmin) sq field-of-view with a spatial resolution element of 1 x 1 (arcsec) sq in a time of 4 s, 24 s, or 4 min, depending on whether the region studied is flaring, active, or quiet. The resulting spectroheliograms would have an average photometric precision of 10% and a spectral purity of 0.1 A.
Measurements of 148 oscillator strengths of neutral chromium transitions were made on Cr vapor in a high-temperature furnace by the hook and absorption methods for strong and weak lines, respectively. With the aid of a 5D-z 5F0 multiplet, the product of the oscillator strengths of the lines of this multiplet with the column densities of their respective lower levels could be determined, and by using estimated oscillator strengths for these lines, all data could be put on a common relative scale. This scale was altered so that the results were matched with relative emission intensities. Results are compared with other authors' results. A correction to the hook method constant was also determined.
The extreme UV spectroheliometer flown on ATM observed within the 280-1340 A spectral range at seven wavelengths simultaneously, with a spatial resolution element 5 x 5 sq arcsec field of view in 5.5 min. The instrument could also obtain spectral scans over the same wavelength range with a spectral resolution of 1.6 A in 3.8 min. The instrument is described in detail with attention given to the telescope, the spectrometer, and the detection system.
This paper describes the derivation of the preflight photometric calibration of the UV spectrometer on Skylab. The calibration of the orbiting instrument through cross-comparison with two rocket instruments is discussed in assessing the observed changes in response to quiet solar regions during the mission. Formulas are presented for the determination of the instrument sensitivity, and an uncertainty of plus or minus 35% is assigned over most of the 296-1340-A wavelength range.
Precision measurements of the refractivities of H2, He, O2, CO, and Kr were made in the wavelength range 168-288 nm. By using a 1.2-m-long test cell and by keeping the test gas at accurately determined conditions near atmospheric pressure and room temperature, accuracies (90% confidence limit) were achieved for the absolute refractivities that ranged from plus or minus 0.1% to plus or minus 1.0% depending upon the gas and wavelength range. For a given gas, the ratio of refractivities at any two wavelengths has a smaller uncertainty. For H2, CO, and O2, results are for wavelengths shorter than those of previous measurements and, for He and Kr, the uncertainties are less than those of other measurements. For He refractivities agree with the theoretical ones, but in the case of H2 results are about 1% larger than the theoretical values. At the upper end of the wavelength range studied, the data are in agreement with previous measurements on H2, CO, and Kr. For O2 results indicate that the hitherto available data are too large by amounts ranging from 0.8% to 10%.
Extreme-ultraviolet (EUV) observations of two polar transient features ('macrospicules') are described. These features appear to be caused by jets of chromospheric material that shoot upward to a height of 35,000 km above the limb and then fall back into the chromosphere, reaching terminal velocities of about 140 km/s. On the basis of a model developed from the EUV measurements, it is found that the energy required to produce each event is about 3 by 10 to the 26th power ergs, about two orders of magnitude more than that required to produce an ordinary spicule. This indicates that macrospicules may be an important factor in the energy balance of the chromosphere and corona.
The Harvard College Observatory photoelectric spectroheliometer on the Apollo Telescope Mount operated correctly in orbit from May 29, 1973 to February 7, 1974. During this period, many thousands of spatial and spectral scans at extreme ultraviolet wavelengths were recorded during observations of a variety of solar features. The construction and modes of operation of the instrument are outlined, and the principal scientific results from a preliminary analysis of the data are described.