The concept of limiting magnitude and the performance of the Isaac Newton telescope for prime focus photography     

D. E. Blackwell  P. A. Ibbetson  C. A. Murray  N. M. Pratt 《Astrophysics and Space Science》1976,43(2):469-473

A quantitative definition of limiting magnitude for stellar photographs is proposed, together with a method of measurement employing a Racine prism. Limiting magnitudes inU, B andV for the Isaac Newton telescope at Herstmonceux derived from a total of 72 prime focus photographs are presented and the results correlated with the size of the photographed image. The limiting magnitude for an extrapolated value of the seeing of 1 isB=22^m.0, in good agreement with a value obtained by scaling the limiting magnitudes of the 200 inch telescope.  相似文献   

Synchrotron radio emission from Uranus and Neptune     

Lawrence D. Kavanagh 《Icarus》1975,25(1):166-170

A number of charged-particle radiation belt models for Uranus and Neptune are postulated, and the synchrotron emission spectrum for each is calculated over the frequency range 18.75 to 2400 MHz. Although no radio observations of these planets exist at frequencies below 1400MHz, available observations are used in conjunction with the synchrotron calculations to establish a rough upper limit to the size and strength of the planets' magnetic field strength and radiation belt intensity. The magnitude of synchrotron radio emission expected in many of the models is within the detection range of several existing ground-based telescopes, at decimetric and metric wavelengths.  相似文献   

Pioneer 10 ultraviolet photometer observations of the Jovian hydrogen torus: The angular distribution     

R.W. Carlson  D.L. Judge 《Icarus》1975,24(4):395-399

The Pioneer 10 ultraviolet photometer observations of the Jovian hydrogen torus are analyzed to obtain the angular distribution. The cloud is asymmetric about Io, where the atoms presumably originate, with the greater density occurring in the trailing portion. A simple model which assumes Jeans escape from the atmosphere of Io is developed and compared to the observations. The results suggest that the exospheric temperature is high (～3000 K) and that the ionization lifetime of the cloud atoms is ～1 × 10⁵ sec.  相似文献   

The atmosphere of Io from Pioneer 10 radio occultation measurements     

A.J. Kliore  G. Fjeldbo  B.L. Seidel  D.N. Sweetnam  T.T. Sesplaukis  P.M. Woiceshyn  S.I. Rasool 《Icarus》1975,24(4):407-410

The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S-band radio occultation measurements of its atmosphere. The dayside entry measurements revealed an ionosphere having a peak density of about 6 × 10⁴ elcm^?3 at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na⁺ and 495 K if N₂⁺ is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9 × 10³ elcm^?3 at an altitude of 50 km. The topside plasma temperature is 160 K for N₂^? and 131 K for Na⁺. If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 10¹¹?10¹² cm³, corresponding to a surface pressure of less than 10^?8 to 10^?9 bars. Two measurements of its radius were also obtained yielding a value of 1830 km for the entry and 192 km for the exit. The discrepancy between these values may indicate an ephemeris uncertainty of about 45 km. The two measurements yield an average radius of 1875 km, which is not in agreement with the results of the Beta Scorpii stellar occultation.  相似文献   

Observations of Jupiter at 26.3 MHz using a large array     

Michael D. Desch  Thomas D. Carr  Jorge Levy 《Icarus》1975,25(1):12-17

A 640 element phase-steerable dipole array has been used to make highly sensitive observations of the planet Jupiter during the 1973 apparition. The satellite Io is found to have very little influence at the low flux levels, whereas the definition of sources A and B appears to be relatively flux independent. A two-dimensional analysis of the data in the Jupiter-Io plane has revealed considerable source B activity at low intensities which is not influenced by Io.  相似文献   

Interpretation of faraday rotation in the galactic magnetic field     

D. Nissen  K. O. Thielheim 《Astrophysics and Space Science》1975,33(2):441-451

The autocorrelation function of Faraday rotation measures is discussed in terms of different types of galactic field configurations. The autocorrelation function evaluated from published data of 139 radio galaxies and quasars is found to resemble a form typical for a quasi-longitudinal field, whereas the autocorrelation function of 38 pulsars turns out to be of the form expected for a longitudinal field. These observations are interpreted with respect to the position of the solar system relative to the neutral sheet in a quasi-longitudinal field configuration.Rotation measures calculated theoretically using a mathematical formulation of the quasi-longitudinal field model are adapted to experimental data. The resulting polarity of the global field structure is discussed in connection with the original dipole-like configuration the magnetic momentum vector of which is found to have been antiparallel to the angular momentum vector of the Galaxy. The relation between the field strength and the density of electrons is found to be consistent with earlier results.  相似文献   

Relict magnesian calcite oolite and subsidence of the Amazon shelf     

JOHN D. MILLIMAN  HENYO T. BARRETTO 《Sedimentology》1975,22(1):137-145

Oolitic sediments occur over large portions of the outer continental shelf off northernmost Brazil and southern French Guiana. Carbon-14 dates, stable isotope values and analogies with similar deposits from other areas in the world indicate that the ooids were formed in shallow water (possibly protected hypersaline environments) during low stands of sea level. In contrast to both modern ooids and other relict ooids, however, the Amazon shelf ooids are composed entirely of magnesian calcite (12 mol % MgCO₃) and possess radial crystal microstructures. A comparison of water depths and ages of the Amazon ooids with the generally accepted eustatic sea level curve indicates that the Amazon shelf subsided more than 100 m 16-21 thousand years ago. Prior to and subsequent to the subsidence, however, the outer shelf remained relatively stable.  相似文献   

Observations of the structure and evolution of solar flares with a soft X-ray telescope     

J. A. Vorpahl  E. G. Gibson  P. B. Landecker  D. L. McKenzie  J. H. Underwood 《Solar physics》1975,45(1):199-216

132 soft X-ray flare events have been observed with The Aerospace Corporation/Marshall Space Flight Center S-056 X-ray telescope that was part of the ATM complement of instruments aboard Skylab. Analyses of these data are reported in this paper. The observations are summarized and a detailed discussion of the X-ray flare structures is presented. The data indicated that soft X-rays emitted by a flare come primarily from an intense well-defined core surrounded by a region of fainter, more diffuse emission. Loop structures are found to constitute a fundamental characteristic of flare cores and arcades of loops are found to play a more important role in the flare phenomena than previously thought. Size distributions of these core features are presented and a classification scheme describing the brightest flare X-ray features is proposed. The data show no correlations between the size of core features and: (1) the peak X-ray intensity, as indicated by detectors on the SOLRAD satellite; (2) the rise time of the X-ray flare event, or (3) the presence of a nonthermal X-ray component. An analysis of flare evolution indicates evidence for preliminary heating and energy release prior to the main phase of the flare. Core features are found to be remarkably stable and retain their shape throughout a flare. Most changes in the overall configuration seem to be the result of the appearance, disappearance or change in brightness of individual features, rather than the restructuring or re-orientation of these features. Brief comparisons with several theories are presented.  相似文献   

The sources of material comprising a mass ejection coronal transient     

E. Hildner  J. T. Gosling  R. T. Hansen  J. D. Bohlin 《Solar physics》1975,45(2):363-376

The origin of the material which is ejected during a white light coronal transient has not been determined heretofore. Study of a disturbance on 26 and 27 August 1973, during which a slowly ascending prominence and a more rapid accompanying coronal transient were simultaneously observed, helps to resolve this question. Prominence images obtained in Hα 6563 Å and in He II 304 Å are nearly identical. The mass ejection transient observed in white light (3700–7000 Å) appeared to be a loop about 1 R_⊙ higher than the top of the ascending prominence; it accelerated away from the prominence below it. These observations imply: (1) the bulk of the ejected material did not originate in the ascending prominence; (2) therefore, most of the material must have come from the low corona above the prominence, (and was at coronal temperatures during its outward passage); and (3) the total event - ascending prominence accompanied by coronal mass ejection - was far larger, more energetic, and longer lasting than would be inferred from the prominence observations alone. The transient of 26–27 August was slow and of atypical shape compared to other mass ejection transients, but we believe that these three conclusions apply to most, if not all, of the more than 60 loop-shaped coronal transients observed by the High Altitude Observatory's coronagraph during the nine-month flight of Skylab.  相似文献   

Small-scale inhomogeneities in the solar corona: Evidence from meter-λ radio bursts     

D. B. Melrose 《Solar physics》1975,43(1):79-86

A number of inconsistencies between simple theory and observations of solar radio bursts indicate that mode-mode coupling in the solar corona is much stronger than predicted. The inconsistencies include the absence of predicted reversal of the sense of polarization in a type 1 storm at CMP, and the anomalously weak polarization of type II and type III emission. The strong mode coupling could be explained in terms of small scale inhomogeneities (L _N? 100 km) throughout the relevant regions of the corona. The relevant regions are those with open magnetic field lines overlying active regions. It is suggested that the coronal plasma is confined to magnetically self-pinched sheets, and it is pointed out that another inconsistency, namely the anomalously small amount of Faraday variation in type III bursts, could be explained if the value of n _e B in the inter-sheet region were two orders of magnitude less than in the sheets.  相似文献