Royal Observatory of Belgium

Brussels, Belgium

Royal Observatory of Belgium

Brussels, Belgium
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Lobel A.,Royal Observatory of Belgium
Canadian Journal of Physics | Year: 2011

Herein we develop a new method to determine oscillator strength values of atomic absorption lines with state-of-the-art detailed spectral synthesis calculations of the optical spectrum of the Sun and of standard spectral reference stars. We update the log(gf) values of 911 neutral lines observed in the KPNO-FTS flux spectrum of the Sun and high-resolution echelle spectra (R = 80 000) of Procyon (F5 IV-V) and ε Eri (K2 V) observed with large signal-to-noise (S/N) ratios of ∼2000 using the new Mercator-Hermes spectrograph at La Palma Observatory (Spain). We find for 483 Fe i, 85 Ni i, and 51 Si i absorption lines in the sample a systematic overestimation of the literature log(gf) values with central line depths below 15%. We employ a curve-of-growth analysis technique to test the accuracy of the new oscillator strength values and compare calculated equivalent line widths to the Moore, Minnaert, and Houtgast atlas of the Sun. The online SpectroWeb database at interactively displays the observed and synthetic spectra and provides the new log(gf) values together with important atomic line data. The graphical database is under development for stellar reference spectra of every spectral sub-class observed with large spectral resolution and S/N ratios. © 2011 Published by NRC Research Press.

Lefevre L.,Royal Observatory of Belgium | Clette F.,Royal Observatory of Belgium
Solar Physics | Year: 2014

In view of the construction of new sunspot-based activity indices and proxies, we conducted a comprehensive survey of all existing catalogs providing detailed parameters of photospheric features over long time intervals. Although there are a fair number of such catalogs, a global evaluation showed that they suffer from multiple limitations: finite or fragmented time coverage, limited temporal overlap between catalogs, and, more importantly, a mismatch in contents and conventions. Starting from the existing material, we demonstrate how the information from parallel catalogs can be merged to form a much more comprehensive record of sunspots and sunspot groups. To do this, we use the uniquely detailed Debrecen Photoheliographic Data (DPD), which is already a composite of several ground-based observatories and of SOHO data, and the USAF/Mount Wilson catalog from the Solar Observing Optical Network (SOON). We also outline our cross-identification method, which was needed to match the non-overlapping solar active-region nomenclature. This proved to be the most critical and subtle step when working with multiple catalogs. This effort, focused here first on the last two solar cycles, should lead to a better central database that collects all available sunspot group parameters to address future solar-cycle studies beyond the traditional sunspot-index time series [R i]. © 2012 Springer Science+Business Media Dordrecht.

Lapenta G.,Catholic University of Leuven | Bettarini L.,Royal Observatory of Belgium
Geophysical Research Letters | Year: 2011

We report a 3D magnetohydrodynamics simulation that studies the formation of dipolarization fronts during magnetotail reconnection. The crucial new feature uncovered in the present 3D simulation is that the process of reconnection produces flux ropes developing within the reconnection region. These flux ropes are unstable to the kink mode and introduce a spontaneous structure in the dawn-dusk direction. The dipolarization fronts forming downstream of reconnection are strongly affected by the kinking ropes. At the fronts, a density gradient is present with opposite direction to that of the acceleration field and leads to an interchange instability. We present evidence for a causal link where the perturbations of the kinking flux ropes with their natural and well defined scales drive and select the scales for the interchange mode in the dipolarization fronts. The results of the simulation are validated against measured structures observed by the THEMIS mission. Copyright 2011 by the American Geophysical Union.

Beuthe M.,Royal Observatory of Belgium
Icarus | Year: 2010

Contraction, expansion and despinning have been common in the past evolution of Solar System bodies. These processes deform the lithosphere until it breaks along faults. Their characteristic tectonic patterns have thus been sought for on all planets and large satellites with an ancient surface. While the search for despinning tectonics has not been conclusive, there is good observational evidence on several bodies for the global faulting pattern associated with contraction or expansion, though the pattern is seldom isotropic as predicted. The cause of the non-random orientation of the faults has been attributed either to regional stresses or to the combined action of contraction/expansion with another deformation (despinning, tidal deformation, reorientation). Another cause of the mismatch may be the neglect of the lithospheric thinning at the equator or at the poles due either to latitudinal variation in solar insolation or to localized tidal dissipation. Using thin elastic shells with variable thickness, I show that the equatorial thinning of the lithosphere transforms the homogeneous and isotropic fault pattern caused by contraction/expansion into a pattern of faults striking east-west, preferably formed in the equatorial region. By contrast, lithospheric thickness variations only weakly affect the despinning faulting pattern consisting of equatorial strike-slip faults and polar normal faults. If contraction is added to despinning, the despinning pattern first shifts to thrust faults striking north-south and then to thrust faults striking east-west. If the lithosphere is thinner at the poles, the tectonic pattern caused by contraction/expansion consists of faults striking north/south. I start by predicting the main characteristics of the stress pattern with symmetry arguments. I further prove that the solutions for contraction and despinning are dual if the inverse elastic thickness is limited to harmonic degree two, making it easy to determine fault orientation for combined contraction and despinning. I give two methods for solving the equations of elasticity, one numerical and the other semi-analytical. The latter method yields explicit formulas for stresses as expansions in Legendre polynomials about the solution for constant shell thickness. Though I only discuss the cases of a lithosphere thinner at the equator or at the poles, the method is applicable for any latitudinal variation of the lithospheric thickness. On Iapetus, contraction or expansion on a lithosphere thinner at the equator explains the location and orientation of the equatorial ridge. On Mercury, the combination of contraction and despinning makes possible the existence of zonal provinces of thrust faults differing in orientation (north-south or east-west), which may be relevant to the orientation of lobate scarps. © 2010 Elsevier Inc.

Beuthe M.,Royal Observatory of Belgium
Icarus | Year: 2013

In a body periodically strained by tides, heating produced by viscous friction is far from homogeneous. The spatial distribution of tidal heating depends in a complicated way on the tidal potential and on the internal structure of the body. I show here that the distribution of the dissipated power within a spherically stratified body is a linear combination of three angular functions. These angular functions depend only on the tidal potential whereas the radial weights are specified by the internal structure of the body. The 3D problem of predicting spatial patterns of dissipation at all radii is thus reduced to the 1D problem of computing weight functions. I compute spatial patterns in various toy models without assuming a specific rheology: a viscoelastic thin shell stratified in conductive and convective layers, an incompressible homogeneous body and a two-layer model of uniform density with a liquid or rigid core. For a body in synchronous rotation undergoing eccentricity tides, dissipation in a mantle surrounding a liquid core is highest at the poles. Within a soft layer (or asthenosphere) in contact with a more rigid layer, the same tides generate maximum heating in the equatorial region with a significant degree-four structure if the soft layer is thin. The asthenosphere can be a layer of partial melting in the upper mantle or, very differently, an icy layer in contact with a silicate mantle or solid core. Tidal heating patterns are thus of three main types: mantle dissipation (with the icy shell above an ocean as a particular case), dissipation in a thin soft layer and dissipation in a thick soft layer. Finally, I show that the toy models predict well patterns of dissipation in Europa, Titan and Io. The formalism described in this paper applies to dissipation within solid layers of planets and satellites for which internal spherical symmetry and viscoelastic linear rheology are good approximations. © 2012 Elsevier Inc.

Parenti S.,Royal Observatory of Belgium
Living Reviews in Solar Physics | Year: 2014

Solar prominences are one of the most common features of the solar atmosphere. They are found in the corona but they are one hundred times cooler and denser than the coronal material, indicating that they are thermally and pressure isolated from the surrounding environment. Because of these properties they appear at the limb as bright features when observed in the optical or the EUV cool lines. On the disk they appear darker than their background, indicating the presence of a plasma absorption process (in this case they are called filaments). Prominence plasma is embedded in a magnetic environment that lies above magnetic inversion lines, denoted a filament channel. This paper aims at providing the reader with the main elements that characterize these peculiar structures, the prominences and their environment, as deduced from observations. The aim is also to point out and discuss open questions on prominence existence, stability and disappearance. The review starts with a general introduction of these features and the instruments used for their observation. Section 2 presents the large scale properties, including filament morphology, thermodynamical parameters, magnetic fields, and the properties of the surrounding coronal cavity, all in stable conditions. Section 3 is dedicated to small-scale observational properties, from both the morphological and dynamical points of view. Section 4 introduces observational aspects during prominence formation, while Section 5 reviews the sources of instability leading to prominence disappearance or eruption. Conclusions and perspectives are given in Section 6.

News Article | October 7, 2016

A Saturnian satellite joins the club of moons with oceans. A subsurface sea might hide beneath the icy crust of Dione, a moon of Saturn, researchers report online September 28 in Geophysical Research Letters. That puts Dione in good company alongside Enceladus (another moon of Saturn), several moons of Jupiter and possibly even Pluto. Dione’s ocean is about 100 kilometers below the surface and is roughly 65 kilometers deep, Mikael Beuthe, a planetary scientist at the Royal Observatory of Belgium in Brussels, and colleagues report. They inferred the ocean’s presence from measurements of Dione’s gravity made by the Cassini spacecraft, which has been in orbit around Saturn since 2004.

There's a subsurface ocean deep within Dione, one of Saturn's moons, says data from the Cassini mission. In a study published in the journal Geophysical Research Letters, researchers from the Royal Observatory of Belgium detailed that, for gravity data acquired from recent flybys carried out by the Cassini spacecraft to make sense, Dione will have to have a crust floating atop an ocean situated 62 miles from the moon's surface and wrapping around a massive rocky core. It has been established that Saturn's other moons, Enceladus and Titan, have oceans beneath their icy crusts and this new study is suggesting Dione could also be an ocean world. "A future orbiter hopping around Saturn's moons could test this prediction," said Antony Trinh, a co-author for the study. Ocean worlds are icy planets or moons with subsurface oceans. So far, there are three orbiting Jupiter, three for Saturn, and Pluto also holds the possibility of being an ocean world. According to the researchers, the subsurface ocean in Dione has probably existed as long as the moon so it offers a long-lived zone habitable for microbial life. Attilio Rivoldini, also a co-athor for the study, adds that contact between Dione's ocean and rocky core is crucial, pointing out that rock-water interactions offer not only key nutrients but also an energy source, which are both needed to start life. In August, NASA revealed that the Cassini spacecraft was able to detect a canyon network flooded with liquid hydrocarbons for the first time in Titan. The discovery was based on data gathered by the mission during a flyby in 2013. During this event, Cassini's radar instruments were focused on channels branching out from the Ligeia Mare, the large northern sea on Titan. These branching channels appeared dark on radar images, similar to how seas rich in methane on Titan looked, and this led scientists to believe that they contain liquid as well. There was the chance at first that the dark material could just be saturated sediment but further analysis using Cassini's radar showed that scientists are indeed looking at fluid-filled channels. In July, another study revealed that a chemical trail has been discovered on Titan, indicating the high plausibility that the Saturn moon could harbor alien life. in fact, it's possible that prebiotic conditions may already be in place, thanks to the hydrogen cyanide produced when sunlight hits Titan's dense and toxic atmosphere. As an organic chemical, hydrogen cyanide can react with itself or with other molecules, forming long chains called polymers in the process. © 2017 Tech Times, All rights reserved. Do not reproduce without permission.

News Article | October 12, 2016

Like its neighbours Titan and Enceladus, Saturn's moon Dione may harbour an ocean beneath its icy surface. Mikael Beuthe and his colleagues at the Royal Observatory of Belgium in Brussels studied data collected from Enceladus and Dione by NASA's Cassini spacecraft. They looked for small changes in the moons' gravity and shape that can reveal layers of buried liquid. Data modelling suggested that Dione has a 65-kilometre-deep global ocean hidden beneath some 100 kilometres of ice. Those waters are a possible habitat for extraterrestrial microbes, should they exist.

News Article | September 12, 2016

The two satellites together are called Proba-3, set for launch in late 2019. Through precise formation flying, one will cast a shadow across the second to open up an unimpeded view of the inner area of the 'corona', which is a million times fainter than the blindingly brilliant solar disc. "When I first heard of the idea I said 'Wow! That's just what we need'," said Andrei Zhukov of the Royal Observatory of Belgium, serving as Principal Investigator for Proba-3's solar instrument. "The best way to observe the corona from the ground is during a solar eclipse, although we still have to cope with stray light – we cannot correct for the influence of Earth's atmosphere. "The next best method is by using 'coronagraphs' to create an articifical eclipse, either on ground telescopes or inside Sun-watching satellites such as SOHO and Stereo. "The problem is that stray light bending around the edge of the occulting disc limits our view of the most important inner portion of the corona. SOHO's coronagraph, for instance, can observe no closer in than 1.1 Sun-diameters. Others can see closer, but with strong stray light making detailed observation impossible. "With Proba-3 we aim to see extremely close to the solar surface in visible light, by flying the occulter and coronagraph on separate satellites some 150 m apart. "This should give us a ringside seat on the most interesting segment of the corona, where a lot of interesting physics is going on, where the solar wind is born and 'coronal mass ejections' originate – gigantic solar eruptions with the potential to affect our terrestrial infrastructure." While the Sun's surface is a comparatively cool 6000ºC, the corona averages a sizzling million degrees. The mystery is how energy travels from the cool Sun to the hot corona, in apparent defiance of the laws of thermodynamics. "By mapping the fine structure of the inner corona for a prolonged time – we are targeting around six hours – our hope is that we gain insight into the kind of energy flows that are taking place," notes Dr Zhukov. "Our standard observing mode will be once per minute, but we could speed that up to a few seconds within a selected field of view, for instance when tracing the rapid evolution of a mass ejection. "The ultimate goal is to be able to solve the physics of space weather, in order to forecast coronal mass ejections, which are known to have dramatic effects on terrestrial electricity grids and other infrastructure." Proba-3 is first and foremost a technology demonstration, exploring the potential of precise formation flying in orbit, but achieving meaningful scientific results will also help to prove its approach works. Proba-3's pair of satellites will be in a highly elliptical orbit around Earth, performing formation flying manoeuvres as well as scientific studies of the solar corona. The occulter satellite will have solar panels on its Sun-facing side. Credit: ESA - P. Carril, 2013

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