Adaptive optics for large telescopes

The performance of large telescopes is determined both by their angular resolution and by their collection area. It is, therefore, important to achieve as high an angular resolution as possible by site selection, by avoiding image deterioration by the telescope and its environment, and by real time image restoration by adaptive optics. We summarize the principles of adaptive optics, their predicted performance and the current programs underway to implement adaptive optics for astronomical purposes.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.     

Pancake cyclones and anti-cyclones in the ionosphere

The formation of pancake cyclones and anticyclones in the E-region of the Earth's ionosphere is considered. It is shown that the vortices can be maintained by the neutral winds or by chemical reactions including the energy release caused by the triple collisions of atomic oxygen with neutrals. It is found that the variations of the magnetic fields induced by the vortices are not localized and decrease slowly far from the vortex core. They can be easily detected by ground based magnetometers or by facilities on board the low-orbiting satellites.     

Books and Multimedia Reviews

Book reviewed in this article: Shoemaker by Levy: The Man who Made an Impact by David H. Levy Impact Stratigraphy: The Italian Record by Alessandro Montanari and Christian Koeberl The Odessa Meteorite Craters and Their Geological Implications by Glen L. Evans and Charles E. Mear Mapping and Naming the Moon: A History of Lunar Cartography and Nomenclature by Ewen A. Whitaker Impacts and the Early Earth edited by Iain Gilmour and Christian Koeberl     

The light curve analysis of MM Herculis

The photoelectric light curves of MM Her obtained in 1983 and 1984 by Evren (1985, 1987) were analyzed by two different methods. Firsly, the effects of the wave-like distortions on the observations were removed from the observed magnitudes by obtaining its mathematical expression. The remaining light curves were analyzed by using Wood's approach. Later, the light curves of the same years were treated by the method of Wilson-Devinney and distortions seen in the light curves were thought to be explained by locating the spots on the surface of the cooler component. The results obtained by two different approaches are in good agreement.     

On reverberation and cross-correlation estimates of the size of the broad-line region in active galactic nuclei

It is known that the dependence of the emission-line luminosity of a typical cloud in the active galactic nuclei (AGN) broad-line regions (BLRs) upon the incident flux of ionizing continuum can be non-linear. We study how this non-linearity can be taken into account in estimating the size of the BLR by means of the 'reverberation' methods. We show that the BLR size estimates obtained by cross-correlation of emission-line and continuum light curves can be much (up to an order of magnitude) less than the values obtained by reverberation modelling. This is demonstrated by means of numerical cross-correlation and reverberation experiments with model continuum flares and emission-line transfer functions and by means of practical reverberation modelling of the observed optical spectral variability of NGC 4151. The time behaviour of NGC 4151 in the Hα and Hβ lines is modelled on the basis of the observational data by Kaspi et al. and the theoretical BLR model by Shevchenko. The values of the BLR parameters are estimated that allow to judge on the size and physical characteristics of the BLR. The small size of the BLR, as determined by the cross-correlation method from the data of Kaspi et al., is shown to be an artefact of this method. So, the hypothesis that the BLR size varies in time is not necessitated by the observational data.     

Nonresonant generation of MHD disturbances during supernova explosions in an inhomogeneous interstellar medium

We analyze the nonresonant generation of large-scale magnetic inhomogeneities near a shock front by accelerated particles. The MHD disturbances are generated by the electric current excited by relativistic particles in the preshock medium in the presence of weak large-scale density inhomogeneities. The MHD modes considered can be amplified by other resonant and nonresonant mechanisms related to the presence of relativistic particles. We estimate the magnetic fields and the energies to which charged particles can be accelerated in different phases of the interstellar medium by taking into account the random magnetic fields generated by the mechanism considered.     

Determination and analysis of coronal hole radio spectra

Solar radio maps obtained by our group and others over a wide wavelength range (millimeter to meter) and over a considerable time span (1973–1978) have allowed us to compute the radio spectrum of an average coronal hole, i.e., the brightness temperature inside a coronal hole normalized by the brightness temperature of the quiet Sun outside the coronal hole measured at several different radio wavelengths. This radio spectrum can be used to obtain the changes of the quiet Sun atmosphere inside coronal holes and also as an additional check for coronal hole profiles obtained by other methods. Using a standard solar atmosphere and a computer program which included ray tracing, we have tried to reproduce the observed radio spectrum by computing brightness temperatures at many different wavelengths for a long series of modifications in the electron density, neutral particle density and temperature profiles of the standard solar atmosphere. This analysis indicates that inside an average coronal hole the following changes occur: the upper chromosphere expands by about 20% and its electron density and temperature decrease by about 10%. The transition zone experiences the largest change, expanding by a factor of about 6, its electron density decreases by a similar factor, and its temperature decreases by about 50%. Finally in the corona the electron density decreases by about 20% and the temperature by about 15%.     

Book and Multimedia Reviews

Book reviewed in this article: Encyclopedia of Mineral Names by William H. Blackburn and William H. Dennen. Radiogenic Isotope Geology by A. P. Dickin Evolution of Hydrothermal Ecosystems on Earth (and Mars?) edited by Gregory R. Bock and Jamie A. Goode The Strange Case of Mrs. Hudson's Cat: And Other Science Mysteries Solved by Sherlock Holmes by Colin Bruce Astrophysical Implications of the Laboratory Study of PreSolar Materials edited by Thomas Bernatowicz and Ernst Zinner     

A new approach of the adding method for the computations of emergent radiation of an inhomogeneous plane-parallel planetary atmosphere

The procedure of computing the intensity and the polarization parameters of radiation diffusely reflected and transmitted by an inhomogeneous, plane-parallel planetary atmosphere is discussed with the aid of the adding method. If the atmosphere is simulated by a number of homogeneous sublayers (aerosols and ozone may be included), the matrices of radiation diffusely reflected and transmitted by the atmosphere can be expressed in terms of these matrices of sublayers by using only a couple of iterative equations with the polarity effect of radiation. This procedure is to be extended to the model atmosphere bounded by the surface reflector with a quite arbitrary phase matrix.     

Book and Multimedia Reviews

Book reviewed in this article: Blind Watchers of the Sky: The People and Ideas that Shaped Our View of the Universe by Rocky Kolb Mining the Sky: Untold Riches from the Asteroids, Comets, and Planets by John S. Lewis The Sun as a Star by Roger J. Tayler Shadow of a Star: The Neutrino Story of Supernova 1987A by Alfred K. Mann. W. H. Freeman Worlds Unnumbered: The Search for Extrasolar Planets by Donald Goldsmith     

On the perturbations of the five outer planets by the four inner ones

The expressions given by Clemence were checked by the comparison of numerical values of their second differentials with the numerical values of the perturbing forces. Agreement was good in most cases, save that the use of the second differentials unduly magnified some terms rightly neglected by Clemence. In the appendix the work of Clemence is compared with that of Carpenter and the differences are found to be all smaller than the contributions from the secular variation of the eccentricity, which are so small that they were neglected by Clemence. The differences can be removed by considering the secular variation of the eccentricity and by making a small adjustment in the integration constants.     

Sunspot Observations Made by Hallaschka During the Dalton Minimum

We present and analyse the sunspot observations performed by Franz I.C. Hallaschka in 1814 and 1816. These solar observations were carried out during the so-called Dalton minimum, around the maximum phase of Solar Cycle 6. These records are very valuable because they allow us to complete observational gaps in the collection of sunspot group numbers, improving the coverage for this epoch. We have analysed and compared the observations made by Hallaschka with the records made by other contemporary observers. Unfortunately, the analysis of the sunspot areas and positions showed that they are too inaccurate for scientific use. We conclude, however, that the sunspot counts made by Hallaschka are similar to those made by other astronomers of that time. The observations by Hallaschka confirm a low level of solar activity during the Dalton minimum.     

Books and Multimedia Reviews

Book reviewed in this article: Fire in the Sky: Comets and Meteors, the Decisive Centuries, in British Art and Science by Roberta J. M. Olson and Jay M. Pasachoff Planetary Materials edited by J. J. Papike Destiny or Chance: Our Solar System and Its Place in the Cosmos by Stuart Ross Taylor Properties of Earth and Planetary Materials at High Pressure and Temperature edited by Murli H. Manghnami and Takehiko Yagi Night Comes to the Cretaceous: Dinosaur Extinction and the Transformation of Modern Geology by James Lawrence Powell     

Books and Multimedia Reviews

Book reviewed in this article: Encyclopedia of the solar system edited by P. R. Weissman, L-A. McFadden and T. V. Johnson. Worlds Without End: The Exploration of Planets Known and Unknown by John S. Lewis. Impact: Ground Zero (CD-ROM). The Babylonian Theory of the Planets by Noel M. Swerdlow. Einstein's Mirror by Tony Hey and Patrick Walters. Stars by James B. Kaler.     

Distribution of iron on Vesta

We have completed a mapping study of 7.6 MeV gamma rays produced by neutron capture by Fe at the surface of the main belt asteroid 4 Vesta as measured by the bismuth germanate scintillator of the Gamma Ray and Neutron Detector (GRaND) on the Dawn spacecraft. The procedures used to determine Fe counting rates are presented, along with a global map, constituting the necessary initial step to quantify Fe abundances. While the final calibration of orbital data to absolute concentrations has not been determined, the range of fully corrected Fe counting rates is compared with that of Fe in howardites. We find that the global distribution of corrected Fe counting rates is generally consistent with mineralogy and composition determined independently by other instruments on the Dawn spacecraft, including measurements of pyroxene absorption bands by the Visible and Infrared Spectrometer and Framing Camera, and an index of diogenitic materials provided by neutron absorption measurements by GRaND. In addition, there is a distinctive low Fe region in the western hemisphere that was not reported by reflectance or optical observations, possibly indicating the presence of a cumulate eucrite component in Vesta's regolith.     

Comments on the next generation of ground-based solar telescopes

The development of telescope capabilities tends to go in spurts. These are triggered by the availability of new techniques in optics, mechanics and/or instrumentation. So has nighttime telescope technology developed since the construction in the nineteen-forties of the 5-m Hale telescope, first by the introduction in the sixties of high efficiency electronic detectors, followed recently by the production of large 8- to 10-m mirrors and now by the implementation of adaptive optics. In solar astronomy, major steps were the introduction of the coronagraph by Lyot in the nineteen-thirties and the vacuum telescope concept by Dunn in the sixties. In the last thirty years, telescope developments in solar astronomy have relied primarily on improved instrumental capabilities. As in nighttime astronomy, these instruments and their detectors are reaching their limits set by the quantum nature of light and the telescope diffraction. Larger telescopes are needed to increase sensitivity and angular resolution of the observations. In this paper, I will review recent efforts to increase substantially the telescope capabilities themselves. I will emphasize the concept of a large all-wavelength, coronagraphic telescope (CLEAR) which is presently being developed.Dedicated to Cornelis de Jager     

Books and Multimedia Review

Book reviewed in this article: Turn Right at Orion: Travels Through the Cosmos by Mitchell Begelman. Satellites of the Outer Planets: Worlds in their Own Right, Second Edition by David A. Rothery. Our Worlds: The Magnetism and Thrill of Planetary Exploration by S. Alan Stern. A Dictionary of Earth Sciences, Second Edition by Ailsa Allaby and Michael Allaby.     

Book and Multimedia Reviews

Book reviewed in this article: The Three Big Bangs: Comet Crashes, Exploding Stars, and the Creation of the Universe by Philip M. Dauber and Richard A. Muller Stars and Their Spectra: An Introduction to the Spectral Sequence by James B. Kaler Impact!: The Threat of Comets and Asteroids by Gerrit L. Verschuur Cosmic Clouds: Birth, Death, and Recycling in the Galaxy by James B. Kaler     

类Crab脉冲星的三维磁层外隙模型

利用三维脉冲星磁层模型研究了磁层外隙的几何结构,首先用自治模型确定“外隙”的垂直尺度,在该模型中外隙尺度受回流的外隙流（带有隙加速的带电粒子发射的曲率光子）加热极帽而产生的热光子的碰撞而成对生成所限,外隙的横向尺度也受本地对生成所限,在脉冲星的磁层中,原则上有两个拓扑分离的外隙,允许同时进入和流出粒子,不过,流入粒子流产生的辐射形态受隙了本地对生成和恒星附近的磁对生成的严格制约,根据外隙及其本地结构的三维模型计算了类Crab脉冲星的辐射和相位分解谱。