Low Resolution Mode Research Articles

Interaction of the solar wind with interstellar neutral hydrogen: Three‐fluid model

It is commonly assumed in models of the solar wind‐interstellar neutral hydrogen interaction that the ionized interstellar particles are quickly assimilated into the solar wind proton population and “become indistinguishable” from the original solar wind. This assumption leads to the prediction that the solar wind proton temperature should increase with radius in the outer heliosphere. This temperature increase has not been observed. We point out that assimilation of the interstellar particles to the point of indistinguishability takes place on the very long Coulomb collision time scale and is not expected to occur within the heliosphere. We present results of a three‐fluid model of the solar wind which consists of comoving thermal populations of protons of solar origin, protons produced by ionization of interstellar hydrogen, and electrons. The steady‐state results yield a solar wind with a “core” proton distribution which cools adiabatically, and a “halo” of interstellar pickup protons which is maintained near 107 °K by the energy input of continued ionization and pickup. Such a distribution will not be observed to manifest the temperature increase at large heliocentric distances which is predicted from a one‐fluid analysis. Further time‐dependent calculations show a strong correlation between the densities of the solar wind and the interstellar pickup protons. We suggest that the interstellar pickup population may be observable by the Voyager plasma instruments in low resolution mode during periods of high solar wind density and low solar wind temperature.

Design and evaluation of a positron emission tomograph: HEADTOME III.

A high performance positron emission tomograph, HEADTOME III, with 480 bismuth germanate oxide detectors (13.4 X 25.0 X 40.0 mm), arranged in three rings of 750 mm diameter, with independent shadow masks and septa, is described. Image plane resolution at the center of the field of view (FOV) is 7.9 and 6.5 mm full width at half maximum (FWHM) in low resolution (LR) and high resolution (HR) mode, respectively. Axial resolution is 13.1, 11.3, and 9.1 mm FWHM at the center of FOV for direct planes of low quantitation (LQ), cross planes of LQ, and high quantitation (HQ) mode, respectively. Sensitivities evaluated by true events for a 68Ga 20 cm diameter cyclindrical pool were 34.5 (56.7), 16.4 (27.5), 19.4 and 9.5 kcps (muCi/ml)-1 with the direct (cross) planes of LQLR, direct (cross) planes of LQHR, and direct planes of HQLR and HQHR, respectively. The fraction of scattered coincidence in a cold spot phantom after software correction is 2.5% in LRLQ mode. Count rate linearity after software correction is within 1% up to 50 X 10(3) true events per second per plane with LRLQ mode.

Photometric calibration and first results obtained with the UFT experiment on board the astron satellite

The UV telescope of 30 cm diameter equipped with a three channel scanner spectrometer aboard the highly eccentric orbit space station “ASTRON” is described Absolute calibration of the whole device is performed using spectra of standard stars, of which absolute values were obtained by TDI or IUE experiments. Several photometric controls have been made on the stability of the equipment after one year of operation which demonstrate reliability of some first results obtained by the UFT. The sensitivity curve of the spectrophotometric system has been observed with a reproducibility of 2 σ = 10% or better. The repeated observation of one star (139 Tau) indicates that the overall performance exhibits the same quality after one year in orbit. The low resolution mode ( Δ λ = 28 A ̊ ) leads to a powerful new investigation of faint extended sources.

Space Telescope observations of aurorae on the giant planets

For the distant giant planets, Uranus and Neptune, the observation of aurorae may be the best astronomical technique for the detection of planetary magnetic fields, with implications for the structure and composition of their interiors. Aurorae may be detected by emssion of H I Ly α (1216 Å) and by H 2 bands near 1600 Å. The latter are important for very faint aurorae because there is essentially no planetary, interplanetary or geocoronal scattering of sunlight to contaminate the signal. For Uranus, present IUE results suggest the presence of a strong aurora at Ly α, but the background and instrument noise levels are very high compared to the apparent signal. At 1600 Å, the IUE instrument noise renders the H 2 emission bands on Uranus marginal at best. No aurora has yet been observed on Neptune. For Jupiter, where the existence and general characteristics of the magnetic field are well established, there is disagreement between ground-based infrared and space-borne ultraviolet observations of the location of the aurorae. For all four giant planets, Space Telescope can improve upon the quality of current optical observations. For spectroscopy, the low resolution mode of the High Resolution Spectrograph (HRS) is particularly well suited to auroral observations because of its spectral range, adequate resolution and high sensitivity. For ultraviolet imaging through appropriate filters, the ST spatial resolution, expected to be of order 5 hundredths of an arc second, is also well suited to determine the spatial properties of the aurorae.

An improved approach for measurement of regional cerebral rate constants in the deoxyglucose method with positron emission tomography.

A new experimental approach was designed to measure regional rate constants for [18F]2-fluoro-2-D-deoxyglucose in the three-compartment model. A programmed infusion was used to keep the plasma tracer concentration constant for the first 20 min. Positron emission tomography images of the brain were taken every minute for 20 min in a low-resolution mode, then every 15-20 min for 2-3 h in a medium-resolution mode. Two simplified operational equations for the calculation of the regional rate constants were derived that incorporated the contribution of the vascular compartment to tissue activity. The first equation was applied to data collected during the initial 20 min (when the concentration of plasma tracer was constant) to estimate the values of k1*, k2*, k3*, and the fraction of the vascular compartment. The second equation was applied to data collected during the whole experimental period to find the value of k4* and to provide a better estimate of k3*. The regional rate constants measured experimentally in three dogs and a brain tumor patient agreed well with those in the literature. This method permits estimation of the local CMRglu under pathological conditions using regionally measured rate constants and provides new information on the pathophysiological meaning of the rate constants.

Performance Figures and Images from the Therascan 3128 Positron Emission Tomograph

The THERASCAN 3128 system has been designed to study cerebral function using positron emission tomography (PET) methods. Two rings of bismuth germanate (BGO) detectors define three image planes per scan. For the low resolution mode image resolution is 19 mm × 19 mm × 20 mm (axial) and the direct slice sensitivity for true coincidences is 88 kcps/(?Ci. cm-3). In the high resolution mode image resolution is 12 mm × 12 mm × 12 mm with a sensitivity of 33 kcps/(?Ci. cm-3). Deconvolution methods used to remove scattered events have been verified by repeated imaging of a phantom designed to provide a wide variation in contrast with time. Clinical images from 18F-2-fluorodeoxyglucose (FDG) and 15O studies are presented as practical examples of system performance and to illustrate the useful information obtained from coronal/sagittal sectioning of closely spaced transverse images.

The H-R diagram of O-type subdwarfs

A spectroscopic survey by Drilling (1983) of non-emission OB+ stars in the Case-Hamburg survey and its extension to b = ±30° for 1 = ±60° has turned up 12 hot, relatively bright O-type subdwarfs (V<13). Drilling's survey covers more than 13 times the area of the south galactic polar survey of Slettebak and Brundage (1971), but has revealed only the hottest and/or hydrogen-poor objects because only the weakest-lined 0-and B-type stars were observed. We have observed all of these stars in the large-aperture, low-resolution mode of the IUE satellite. A full description of their spectral appearance is given elsewhere (Schönberner and Drilling, 1984). The spectra were corrected for interstellar absorption using the Seaton (1979) reddening law to nullify the 2200 å interstellar feature, and in all cases EB-V < 0.25.

PETT VI

We designed and built a positron emission transverse tomograph (PETT VI), designed specifically for fast dynamic studies in the human brain, and for cardiac studies in experimental animals. The scintillation detectors incorporated into this device are fitted with cesium fluoride crystals. Cesium fluoride was selected for this purpose because its short fluorescence decay allows the use of a short coincidence resolving time with a concomitant reduction of unwanted random coincidences. PETT VI utilizes four rings of 72 detectors simultaneously yielding seven tomographic sections. The system can be operated in either a low or high resolution mode with intrinsic geometrical resolutions in the plane of section of 7.1 to 11.7 mm full width at half maximum (FWHM), for a slice thickness with a resolution at the center of 13.9 mm FWHM. The maximum sensitivity of the system for seven slices in the low resolution mode is 322,000 cps/microCi/cc in a 20 cm diameter phantom. The contribution of random coincidences before subtraction in PETT VI was found to be approximately 14% of the counts in the phantom image with a source of approximately 3.5 mCi of a positron emitting radionuclide dispersed in a 20 cm diameter tissue equivalent phantom with a concentration of 1 microCi/cc. The short coincidence resolving time of the system permits rapid data acquisition for attenuation corrections and clinical dynamic studies with data acquisition times of less than a minute.

Performance Study of PETT VI, a Positron Computed Tomograph with 288 Cesium Fluoride Detectors

The first positron computed tomography system with cesium fluoride scintillation detectors, PETT VI, has been developed. The system provides 7 slice images with 4 detector rings (57 cm). Performances of the system are discussed and clarified based on experimental data. Adoption of CsF detectors decrease the random coincidence rate by achieving a short coincidence timing resolution. The timing resolution of a pair of detector heads is 1.5 nsec FWHM and 3.0 nsec FWTM. After a preliminary timing alignment of 288 detectors, the coincidence window width (2τ) of 11 nsec or wiider has yielded maximum coincidence sensitivity, and operation at 5.9 nsec has given 91% of maximum. The full sensitivity for a cyclindrical uniform phantom (20 cm dia × 13 cm) is 354 kcps/microCi/cc/7 slices in a low resolution mode (intrinsic resolution at center is 11.7 mm FWHM). In a high resolution mode (7.1 mm), it is 31% of the full sensitivity. Coincidence rates ratio, [random/(true + scattered)], in the low resolution mode, is 0.16 α with 5.9 nsec window width inside the phantom images reconstructed without a random correction, where α is activity density (microCi/cc). The ratio is 1/3 of that obtained when operating at 20 nsec. Scattered coincidence fraction at the center of the phantom images, without the random correction process, is 9% of the [true + scattered].

Ultraviolet observations of RR Telescopii

The ultraviolet spectrum of RR Tel was extensively studied with the IUE satellite since 1978 in both the high and low resolution modes. A comprehensive study of these observations was made by Penston et al.(l98l) who measured more than 400 emission lines. As it is clearly shown in figure 1, the UV spectrum of this symbiotic star is very rich in emission lines. Like in the optical spectrum (Thackeray 1977), the UV spectrum presents emission lines of ions belonging to a wide range of ionization energies, from neutral up to four and five times ionized species (OV, MgV, CaVI, etc.). Permitted, intercombination and forbidden transitions were found which may allow a diagnosis of the physical conditions of the emitting regions. Electron temperatures of 1.2-1.9 104°K and densities of 106-108 cm-3 were derived by Penston et al.

The Magellan project

Magellan is one of the future space projects being studied by the European Space Agency. The aim is to provide high resolution (λ/Δλ ≥ 2.5 × 104) spectra in the far and extreme UV (between 500 and 1550 Å) of faint galactic and extragalactic objects (V ≤ 16m). The instrument consists of a mechanical collimator, a concave holographic grating and a bidimensional photon-counting detector. A low resolution mode (λ/Δλ ≥ 103) will provide spectra of objects as faint as 18m.5. Magellan is planned as an observatory, operated in real time, and allowing interaction with the observer.

A review of the properties of the symbiotic star CH Cygni

The spectrum of CH Cygni usually has the appearance of a normal M6 III star, but at intervals of several years, it has phases of activity during which it shows the characteristics of a symbiotic star, that is, a blue continuum extending shortward of the Balmer discontinuity, and strong emission lines of He I, Fe II, [Fe II] , [S II], [0 1] , [0 III]; the Balmer lines and the H and K lines of Ca II present emission wings. Outbursts were observed: 1) in September 1963; the star returned to its normal phase by August 1965 (Deutsch, 1964); 2) in June 1967 (Deutsch, 1967); the spectrum was observed by Faraggiana and Hack (1971) until December 1970, when the activity phase was over; 3) another outburst was observed in August 1977 (Morris, 1977; Fehrenbach, 1977), and at the time of our last spectroscopic observations (June 1981) it was still going on. During this last period of activity several observations were made with IUE in both the low and high resolution modes, from 1175 A to 3100 A.

The abundance of carbon and magnesium in the Orion Nebula

The Orion nebula has been observed in two positions with IUE (International Ultraviolet Explorer) in the low-resolution mode (approx.7 A) and in the spectral range 1150--3200 A. Emission lines of C II), C III), (O II), and He I have been measured and used to determine what is probably the first reliable abundance of carbon in H II regions. The logarithmic total abundance of carbon is found to be 8.4 close to the solar value. In contrast with the situation in the planetary nebula of similar excitation, IC 418, where the resonance Mg II lambda2800 line is observed to be relatively strong, in the Orion nebula the lambda2800 line is not detectable. an upper limit for the magnesium abundance of the order of 10 times smaller than in the Sun is suggested.

Polar clearing in the Venus clouds observed from the Pioneer Orbiter

PIONEER Venus 1 was put into a 24-h orbit around the planet on 4 December 1978. Since then, it has made remote sensing observations of the clouds and the overlying atmosphere at IR, visible and UV wavelengths. The IR instrument includes a channel at a wavelength of 11.5 µm, which is used to measure the effective temperature of the cloud tops. Carbon dioxide, and all of the known constituents of the gaseous atmosphere, are highly transparent at this wavelength. Earth-based telescopic observations at similar wavelengths1–3 have generally shown fairly uniform temperatures of the order of 240 K across the face of Venus. The accepted interpretation has been that the cloud cover on Venus not only covers the whole planet, but is also extremely uniform. The early Pioneer observations confirmed this general picture, for the equatorial and mid-latitudes which comprise most of the surface area of the planet4. Interesting and meteorologically important structure with diurnal, seasonal and random components is found in the Pioneer measurements, as it was in earlier ground-based and spacecraft observations, but this structure has an amplitude of 10 K or less1–6. However, these subtle contrasts which dominate thermal maps of lower latitudes change to dramatic structure near the pole. Even the Earth-based observers, who must view the polar regions on Venus at very oblique angles, have reported cold bands surrounding one or other of the poles at various times2 and occasional polar ‘hot spots’1. The Pioneer orbit passes almost directly over the pole (inclination = 105°) and so provides the first good views of this interesting region. The IR instrument was operated only in its low spatial resolution mode for the first part of the mission. Results from those observations4 showed a wave-shaped collar of high, cold cloud surrounding the north pole, with warmer cloud temperatures polewards. Close to the pole itself, the observed temperature is the highest anywhere on the planet, not only at the cloud top but also in the overlying atmosphere4. We report here the first high data rate, high spatial resolution observations of the polar region. These were obtained on the tenth orbit of Venus on 15 December 1978.

The Ultraviolet Spectrum of Beta Lyrae

AbstractSelected regions of the ultraviolet spectrum of Beta Lyrae were observed in 1973 and 1974 at the epochs of the two eclipses and of the two quadratures with the Copernicus Princeton University spectrometer. The results were published in the Astrophysical Journal (Hack et al. 1975, 1976). Here we summarize the results of a third series of observations with the same instrument covering the whole period of 12.93 days. The following spectral regions were observed in June 1975 during 13 consecutive days: λ 1036–1060; 1300–1326; 1398–1416; 2050–2098; 2580–2632; 2777–2812 in the low resolution mode; λ 1172–1177 and 2795–2799 in the high resolution mode. The main results are the following: 1) The radial velocity curve for λ 1175 C III is almost 180° out of phase with the orbital velocity curve of the primary star and the mean velocity is - 240 km s−1; K = 70 km s−1. This suggests that λ 1175 C III is formed in an expanding envelope associated with the secondary. The mass ratio can be determined: m2 /m1 = 2.7 −1.1/+0.2 .2) The continuum variation confirms the results obtained with 0A0-2. At λ 2100 we observe the light of a continuum indicating that the temperature of the body eclipsing the primary star is lower than that of the primary and of about 9000 K. The light curves at λ 1900, 2100 can be explained with the contribution of Fe III emission in the circumbinary plasma. On the contrary, the depth of the secondary minimum at λ 1311–1326 and at λ 1410 cannot be explained with the Si III and Si IV emissions. A possible explanation is the presence of a hot spot in the region of the eclipsing body (probably a disk surrounding the companion) where the stream from the primary impinges on the disk. The hot spot can also explain the behavior of the infrared light curves observed by Jameson and Longmore (1976).

Copernicus spectra of β Lyrae

WE have observed the far ultraviolet spectrum of β Lyrae using the Princeton spectrometer, on the Copernicus satellite1. The principal data were scans in the low resolution mode (0.2 A in the (U2) region 1,000–1,500 A and 0.4 A in the (V2) region 2,000–2,900 A) at phases nearly coincident with the two light minima. Shorter scans (1,100–1,300 A) were obtained at the two quadrature phases. All observations were made in August and September 1973 but not in the same orbital cycle.

Ion Storage in Three-Dimensional, Rotationally Symmetric, Quadrupole Fields. II. A Sensitive Mass Spectrometer

A mass spectrometer has been constructed in which ions are trapped in a three-dimensional, rotationally symmetric, quadrupole field. Ions of a specific charge to mass ratio are stored and then periodically drawn out of the trapping field into an electron multiplier. The ion trapping feature permits integration of the ion current over long periods of time. Electrodynamic ion trapping also enables a long effective path length to be attained in a small size device. Ions have been stored for several days using a low-resolution mode. Experimental data are presented concerning storage time, resolution, and sensitivity. Factors affecting the peak shape include storage time, resolution, and bias voltage between the end-caps. The theoretical basis for the trapping of charged particles in a three-dimensional, quadrupole field is discussed in a companion paper.