Image Stabilization System for Hinode (Solar-B) Solar Optical Telescope

The Hinode Solar Optical Telescope (SOT) is the first space-borne visible-light telescope that enables us to observe magnetic-field dynamics in the solar lower atmosphere with 0.2 – 0.3 arcsec spatial resolution under extremely stable (seeing-free) conditions. To achieve precise measurements of the polarization with diffraction-limited images, stable pointing of the telescope (<0.09 arcsec, 3σ) is required for solar images exposed on the focal plane CCD detectors. SOT has an image stabilization system that uses image displacements calculated from correlation tracking of solar granules to control a piezo-driven tip-tilt mirror. The system minimizes the motions of images for frequencies lower than 14 Hz while the satellite and telescope structural design damps microvibration in higher frequency ranges. It has been confirmed from the data taken on orbit that the remaining jitter is less than 0.03 arcsec (3σ) on the Sun. This excellent performance makes a major contribution to successful precise polarimetric measurements with 0.2 – 0.3 arcsec resolution. K. Kobayashi now at NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA.     

Meteoroid ablation models

The fate of entering meteoroids in atmosphere is determined by their size, velocity and substance properties. Material from ablation of small-sized meteors (roughly R≤0.01–1 cm) is mostly deposited between 120 and 80 km altitudes. Larger bodies (up to meter sizes) penetrate deeper into the atmosphere (down to 20 km altitude). Meteoroids of cometary origin typically have higher termination altitude due to substance properties and higher entry velocity. Fast meteoroids (V>30–40 km/s) may lose a part of their material at higher altitudes due to sputtering. Local flow regime realized around the falling body determines the heat transfer and mass loss processes. Classic approach to meteor interaction with atmosphere allows describing two limiting cases: – large meteoroid at relatively low altitude, where shock wave is formed (hydrodynamical models); – small meteoroid/or high altitudes – free molecule regime of interaction, which assumes no collisions between evaporated meteoroid particles. These evaporated particles form initial train, which then spreads into an ambient air due to diffusion. Ablation models should make it possible to describe physical conditions that occur around meteor body. Several self-consistent hydrodynamical models are developed, but similar models for transition and free molecule regimes are still under study. This paper reviews existing ablation models and discusses model boundaries.     

A New Model for the Separation of Meteoroid Fragments in the Atmosphere

This work is devoted to modeling of the transverse scattering of meteoroid fragments in the atmosphere by adopting supersonic gas dynamics around a system of bodies. Artem’eva and Shuvalov (1996, Shock Waves, 367) and Zhdan et al. (2004, Dokl. Phys., 315–317) found that the transverse force decreases with the increase of the distance between fragments, that is, fragments do not separate in a transverse direction under the action of constant repulsion force. This work on the decreasing transverse force uses the values of the transverse force coefficient by Zhdan et al. (2004, Dokl. Phys., 315–317) obtained from numerical modeling for spheres in a supersonic flow to derive the analytical solution of the dynamic equation for a fragment. The new model of layer-by-layer scattering of meteoroid fragments moving as a system of bodies is constructed on the basis of the analytical solutions derived in this work and the numerical data by Zhdan et al. (2005, Dokl. Phys., 514–518).     

Ionization structure of the atmospheres and line profiles in the spectra of Wolf-Rayet stars

The ionization structure of the atmospheres of Wolf-Rayet (WR) and WC stars is studied. The stellar atmospheres were assumed to consist of helium, hydrogen, and carbon. Profiles of the C III l 5696 line are calculated, both for a spherically symmetric atmosphere with a density that decreases monotonically outward and for an atmosphere containing a dense condensation (inhomogeneity). The dependence of line profiles on the parameters of the inhomogeneity is investigated. It is shown that profiles of the C III λ 5696 line calculated assuming no inhomogeneities in the atmosphere are too weak, whereas assuming the existence of inhomogeneities enables one to reconcile the observed and calculated profiles. An equation is obtained relating the mass of an inhomogeneity to the flux in the detail of the total profile of the CIII λ 5696 line formed by that inhomogeneity. This equation is used to construct a stochastic cloud model of the atmosphere of a WR star, consisting of a large number of inhomogeneities in a homogeneous, spherically symmetric stellar wind. In the proposed model, the formation of inhomogeneities was treated as a random process. It is shown that in this model it is possible both to obtain an average line profile corresponding to the observed one and to reproduce the amplitude and overall pattern of variability of profiles in the spectra of Wolf-Rayet stars. Translated from Astrofizika, Vol. 42, No. 3, pp. 373–398, July–September, 1999.     

Direct and Indirect Thermospheric Heating Sources for Solar Cycles 21–23

Solar variability is often cast in terms of radiative emission and the associated long-term climate response; however, growing societal reliance on technology is creating more interest in day-to-day solar variability. This variability is associated with both solar radiative and solar wind emissions. In this paper we explore the combined effects of radiative and solar wind fluctuations at Earth. The fluctuations in radiative and geomagnetic power create an extended interval of solar maximum for the upper atmosphere. We use a trio of empirical models to estimate, over the last three solar cycles, the relative contributions of solar extreme ultraviolet (UV) power, Joule power, and particle kinetic power to the Earth’s upper atmosphere energy budget. Daily power values are derived from three source models. The SOLAR2000 solar irradiance specification model provides estimates of the daily extreme and far UV solar power input. Geomagnetic power is derived from a combination of satellite-estimated particle precipitation power and an empirical model of Joule power from hemispherically integrated estimates of high-latitude energy deposition. During the interval 1975 to 2003, the average daily contributions were: particles – 36 GW, Joule – 95 GW and solar – 464 GW for a total of 595 GW. Solar wind-driven geomagnetic power provided 22% of the total global upper atmospheric energy. In the top 15 power events, geomagnetic power contributed two-thirds of the total power budget. In each of these events, Joule power alone exceeded solar power. With rising activity, Joule power becomes the most variable element of solar upper atmosphere interactions.     

A semi-analytic algorithm for constructing lower dimensional elliptic tori in planetary systems

We adapt the Kolmogorov’s normalization algorithm (which is the key element of the original proof scheme of the KAM theorem) to the construction of a suitable normal form related to an invariant elliptic torus. As a byproduct, our procedure can also provide some analytic expansions of the motions on elliptic tori. By extensively using algebraic manipulations on a computer, we explicitly apply our method to a planar four-body model not too different with respect to the real Sun–Jupiter–Saturn–Uranus system. The frequency analysis method allows us to check that our location of the initial conditions on an invariant elliptic torus is really accurate.     

A one-dimensional numerical model of H<Subscript>2</Subscript>O cloud formation in the Martian atmosphere

A one-dimensional numerical model with a size distribution of aerosol particles in Martian atmosphere is developed. The model incorporates detailed microphysics and turbulent transport. Dust particles suspended in the Martian atmosphere play a role of cloud condensation nuclei. Diurnal cycle of condensational processes is obtained on the basis of GCM temperature profiles. An effective radius of ice particles is 1–2 μm near the lower boundary of cloud layer and 0.2–0.3 μm at the altitude of 50–60 km. These results are consistent with solar infrared occultations by SPICAM experiment on Mars-Express. Near-surface fogs may form under specific conditions. The connections of condensational processes and cloud macroscopic parameters on microphysical properties of aerosol particles are main focus of this paper. In particular, the dependence on variations of cloud condensation nuclei contact parameter is analyzed, taking into account new experimental data of adsorption properties of minerals at low temperatures.     

Photometric study of the unusual binary system vsx j052807.9 + 725606

Results are reported from a three color (B, V, and R) photometric study of the recently discovered, unusual binary system VSX J052807.9 + 725606 = USNO-B1.0 1629–0064825. This system is extremely similar to the system V361 Lyr, which had previously been regarded as unique. We confirm the strong asymmetry of the phase curve and find that its amplitude depends on wavelength. This is interpreted using a model of a “direct impact” of the accretion flow with the atmosphere of the accreting component and the formation of a “hot spot.” The color temperatures are determined. The characteristics of the hot spot are estimated. We have also calculated new ephemerides for VSX J052807.9 + 725606.     

A review of Titan’s atmospheric phenomena

Saturn’s satellite Titan is a particularly interesting body in our solar system. It is the only satellite with a dense atmosphere, which is primarily made of nitrogen and methane. It harbours an intricate photochemistry, that populates the atmosphere with aerosols, but that should deplete irreversibly the methane. The observation that methane is not depleted led to the study of Titan’s methane cycle, starting with its atmospheric part. The features that inhabit Titan’s atmosphere can last for timescales varying from year to day. For instance, the reversal of the north–south asymmetry is linked to the 16-year seasonal cycle. Diurnal phenomena have also been observed, like a stratospheric haze enhancement or a possible tropospheric drizzle. Furthermore, clouds have been reported on Titan since 1993. From these first detections and up to now, with the recent inputs from the Cassini–Huygens mission, clouds have displayed a large range of shapes, altitudes, and natures, from the flocky tropospheric clouds at the south pole to the stratiform ones in the northern stratosphere. It is still difficult to compose a clear picture of the physical processes governing these phenomena, even though of lot of different means of observation (spectroscopy, imaging) are available now. We propose here an overview of the phenomena reported between 1993 and 2008 in the low atmosphere of Titan, with indications on the location, altitude, and their characteristics in order to give a perspective of our up-to-date understanding of Titan’s meteorological manifestations. We shall focus mainly on direct imaging observations, from both space- and ground-based facilities. All of these observations, published in more than 30 different refereed papers to date, allow us to build a precise chronology of Titan’s atmospheric changes (including the north–south asymmetry, diurnal and seasonal effects, etc). Since the interpretation is at an early stage, we only briefly mention some of the current theories regarding the features’ nature.     

ALMA as the ideal probe of the solar chromosphere

The very nature of the solar chromosphere, its structuring and dynamics, remains far from being properly understood, in spite of intensive research. Here we point out the potential of chromospheric observations at millimeter wavelengths to resolve this long-standing problem. Computations carried out with a sophisticated dynamic model of the solar chromosphere due to Carlsson and Stein demonstrate that millimeter emission is extremely sensitive to dynamic processes in the chromosphere and the appropriate wavelengths to look for dynamic signatures are in the range 0.8–5.0 mm. The model also suggests that high resolution observations at mm wavelengths, as will be provided by ALMA, will have the unique property of reacting to both the hot and the cool gas, and thus will have the potential of distinguishing between rival models of the solar atmosphere. Thus, initial results obtained from the observations of the quiet Sun at 3.5 mm with the BIMA array (resolution of 12″) reveal significant oscillations with amplitudes of 50–150 K and frequencies of 1.5–8 mHz with a tendency toward short-period oscillations in internetwork and longer periods in network regions. However higher spatial resolution, such as that provided by ALMA, is required for a clean separation between the features within the solar atmosphere and for an adequate comparison with the output of the comprehensive dynamic simulations.     

Time-domain demodulation of RHESSI light curves

This paper presents an algorithm to decompose the modulated RHESSI light curves into periodic functions and a smooth function, representing the true (demodulated) time profile of an impulsive source. The decomposition is achieved by optimizing a trade-off between the Poisson likelihood, a smoothness constraint, and conditions on the average grid transmission and the (modulating or non-modulating) background. The algorithm, which operates on the level of count rates and does not require imaging information, is verified by numerical simulations and applied to some early RHESSI data, where – as a preliminary result – several impulsive features on time scales < 4 s may have been identified.     

RHESSI Observation of Atmospheric Gamma Rays from Impact of Solar Energetic Particles on 21 April 2002

The RHESSI high-resolution spectrometer detected γ-ray lines and continuum emitted by the Earth's atmosphere during impact of solar energetic particles in the south polar region from 16:00–17:00 UT on 21 April 2002. The particle intensity at the time of the observation was a factor of 10–100 weaker than previous events when gamma-rays were detected by other instruments. This is the first high-resolution observation of atmospheric gamma-ray lines produced by solar energetic particles. De-excitation lines were resolved that, in part, come from ¹⁴N at 728, 1635, 2313, 3890, and 5106 keV, and the ¹²C spallation product at ∼ 4439 keV. Other unresolved lines were also detected. We provide best-fit line energies and widths and compare these with moderate resolution measurements by SMM of lines from an SEP event and with high-resolution measurements made by HEAO 3 of lines excited by cosmic rays. We use line ratios to estimate the spectrum of solar energetic particles that impacted the atmosphere. The 21 April spectrum was significantly harder than that measured by SMM during the 20 October 1989 shock event; it is comparable to that measured by Yohkoh on 15 July 2000. This is consistent with measurements of 10–50 MeV protons made in space at the time of the γ-ray observations.     

Orbit of an Astrometric Binary System

We present a new method to solve the problem of initial orbit determination of any binary system. This method is mainly based on the material available for an observer, for example relative positions at a given time of the couple in the “plane of sky”, namely the tangent plane to the celestial sphere at the position of the primary component. The problem of orbit determination is solved by splitting in successive stages in order to decorrelate the parameters of each other as much as possible. On one hand, the geometric problem is solved using the first Kepler’s law from a single observing run and, on the other hand, dynamical parameters are then inferred from the fit of the Kepler’s equation. At last, the final stage consists in determining the main physical parameters involved in the secular evolution of the system, that is the spin axis and the J₂ parameter of the primary if we assume that it is a quasi-spherical body. As a matter of fact there is no need to make too restrictive initial assumptions (such as circular orbit or zero eccentricity) and initial guesses of parameters required by a non-linear least-squares Levenberg–Marquardt algorithm are finally obtained after each stage. Such a protocol is very useful to study systems like binary asteroids for which all of the parameters should be considered a priori as unknowns. As an example of application, we used our method to estimate the set of the Pluto–Charon system parameters from observations collected in the literature since 1980.     

Simulating H2O Maser Emission in the Mass Outflows from Evolved Stars

The combination of a time-dependent spherically symmetric hydrodynamic model of stellar atmosphere pulsation and a radiation transport code, which incorporates maser saturation theory, enabled us to synthesise maps and spectra of H₂O maser emission from the circumstellar envelopes of long period variable stars. The synthetic maps and spectra compare favourably with observed 22, 321 and 325 GHz H₂O maser emission. As is observed in H₂O maser regions the peak emission occurs between 3–8 stellar radii from the star. The calculated H₂O maser regions are in conditions of n_H2 = 10⁶ − 10⁸ cm⁻³, assuming a fractional abundance of 10⁻⁴; kinetic temperatures of 550–3000 K; dust ensemble temperatures of 500–1200 K and an accelerating velocity field. The IR radiation field is explicitly included in the radiation transport model, incorporating the latest absorption efficiency data for silicates from Draine. We reproduce the features seen in high angular resolution MERLIN spectral line datacubes. This shows that a mass outflow model which extends the photosphere using pulsations and incorporates radiation pressure on silicate based dust particles can produce the observed data on small (10-mas) angular scales. This revised version was published online in September 2006 with corrections to the Cover Date.     

Orbit Determination of Binary Asteroids

In addition to the detection of an asteroid moon or a binary asteroid, the knowledge of the satellite’s true orbit is of high importance to derive fundamental physical parameters of the binary system such as its mass and to shed light on its possible formation history and dynamical evolution (prograde/retrograde orbit, large/small eccentricity or inclination, etc.). A new methodology for preliminary orbit determination of binary asteroids – and visual binaries in general – is proposed. It is based on Thiele–Innes method combined with a ‘trial and error’ Monte-Carlo technique. This method provides the full set of solutions (bundle of orbits, with the 7 orbital elements) even for a reduced number of observations. The mass is a direct by-product of this orbit determination, from which one can next infer the bulk-density and porosity. In addition to the bundle of orbits, the method provides the marginal probability densities of the foreseen parameters. Such error analysis – since it avoids linear approximation – can be of importance for the prediction of the satellite’s position in the plane-of-sky during future stellar occultations or subsequent observations, but also for the analysis of the orbit’s secular evolution. After briefly describing the method, we present the algorithm and its application to some practical cases, with particular emphasis on asteroids binaries and applications on orbital evolution.     

The Restricted Hill Four-Body Problem with Applications to the Earth–Moon–Sun System

Starting from the four-body problem a generalization of both the restricted three-body problem and the Hill three-body problem is derived. The model is time periodic and contains two parameters: the mass ratio ν of the restricted three-body problem and the period parameter m of the Hill Variation orbit. In the proper coordinate frames the restricted three-body problem is recovered as m → 0 and the classical Hill three-body problem is recovered as ν → 0. This model also predicts motions described by earlier researchers using specific models of the Earth–Moon–Sun system. An application of the current model to the motion of a spacecraft in the Sun perturbed Earth–Moon system is made using Hill's Variation orbit for the motion of the Earth–Moon system. The model is general enough to apply to the motion of an infinitesimal mass under the influence of any two primaries which orbit a larger mass. Using the model, numerical investigations of the structure of motions around the geometric position of the triangular Lagrange points are performed. Values of the parameter ν range in the neighborhood of the Earth–Moon value as the parameter m increases from 0 to 0.195 at which point the Hill Variation orbit becomes unstable. Two families of planar periodic orbits are studied in detail as the parameters m and ν vary. These families contain stable and unstable members in the plane and all have the out-of-plane stability. The stable and unstable manifolds of the unstable periodic orbits are computed and found to be trapped in a geometric area of phase space over long periods of time for ranges of the parameter values including the Earth–Moon–Sun system. This model is derived from the general four-body problem by rigorous application of the Hill and restricted approximations. The validity of the Hill approximation is discussed in light of the actual geometry of the Earth–Moon–Sun system. This revised version was published online in July 2006 with corrections to the Cover Date.     

Beam Mode Expansion of Corrugated Conical Horns with Phase Correcting Lens: Application to Radioastronomy Receivers

A classical radioastronomy receiver is fed with a corrugated horn and an independent lens, both placed in a cryostat to lower the noise temperature. The beam is focused and directed using a combination of elliptical and plane mirrors. This paper proposes modifying the initial feeding system by placing the lens onto the horn aperture, thereby allowing a size reduction of the horn and lens, and a simplification of their mechanical design. The profiled lens is shaped to correct the phase error on the horn aperture. A quasi-optical model of the horn-plus-lens system has been developed using a Beam Mode Expansion (BME). Results using both a hyperbolic-planar lens and a spherical-elliptical lens, as well as results obtained by using Geometrical Optics (GO) with a Kirchoff–Huygens integration to get the far-field pattern, have been compared with measurements. As a direct application, a full focusing system for the new 40-m radiotelescope at the “Centro Astronómico de Yebes” is presented for the 22, 30 and 45 GHz bands. This paper has developed a QO model for a corrugated conical horn with a phase-correcting lens. This revised version was published online in July 2006 with corrections to the Cover Date.     

Conservation laws for multilevel transfer problems

We discuss the question whether the way of finding the conservation laws based on variational formalism is applicable to the multilevel problems of radiative transfer in a homogeneous atmosphere. For expository reasons, the simplest one-dimensional model case is considered. For the special three-level problem treated in the paper the Lagrangian approach allows one to derive not only the H- and K-integrals, but also the nonlinear integral which is an analog of the Q-integrals previously obtained for the classical transfer problems. It is shown that, in general, the constraints imposed by the variational principle on the symmetry properties of the transfer equations are too stringent to be satisfied. Translated from Astrofizika, Vol. 42, No. 2, pp. 235–252, April–June, 1999.     

A New Method for Polar Field Interpolation

The photospheric magnetic field in the Sun’s polar region is not well observed compared to the low-latitude regions. Data are periodically missing due to the Sun’s tilt angle, and the noise level is high due to the projection effect on the line-of-sight (LOS) measurement. However, the large-scale characteristics of the polar magnetic field data are known to be important for global modeling. This report describes a new method for interpolating the photospheric field in polar regions that has been tested on MDI synoptic maps (1996 – 2009). This technique, based on a two-dimensional spatial/temporal interpolation and a simple version of the flux transport model, uses a multi-year series of well-observed, smoothed north (south) pole observations from each September (March) to interpolate for missing pixels at any time of interest. It is refined by using a spatial smoothing scheme to seamlessly incorporate this filled-in data into the original observation starting from lower latitudes. For recent observations, an extrapolated polar field correction is required. Scaling the average flux density from the prior observations of slightly lower latitudes is found to be a good proxy of the future polar field. This new method has several advantages over some existing methods. It is demonstrated to improve the results of global models such as the Wang–Sheeley–Arge (WSA) model and MHD simulation, especially during the sunspot minimum phase.     

Optimal Baffle Design in a Cassegrain Telescope

An exact analytical procedure is described for the optimum design of baffles for any Cassegrain telescope with conicoid type mirrors. Here, `optimum' means the least obstruction coefficient given perfect blocking of direct light by baffles. The corresponding algorithm is based on the ray-tracing formulas in a two-mirror telescope with arbitrary position of the aperture stop. The optimal configuration of baffles is unique. The dependence of the obstruction coefficient upon the telescope characteristics is studied. The optical system can be designed in such a way that the obstruction is equal to a predefined value. As examples, the Hubble Space Telescope and the Ritchey–Chretien system with the obstruction coefficient 0.25 are considered. This revised version was published online in July 2006 with corrections to the Cover Date.