Publication archive

Publication archive

We present a multi-wavelength study of the Magellanic Stream (MS), a massive gaseous structure in the Local Group that is believed to represent material stripped from the Magellanic Clouds. We use ultraviolet, optical and radio data obtained with HST/COS, VLT/UVES, FUSE, GASS, and ATCA to study metal abundances and physical conditions in the Stream toward the quasar Fairall 9. Line absorption in the MS from a large number of metal ions and from molecular hydrogen is detected in up to seven absorption components, indicating the presence of multi-phase gas. From the analysis of unsaturated S II absorption, in combination with a detailed photoionization model, we obtain a surprisingly high alpha abundance in the Stream toward Fairall 9 of [S/H] = -0.30 ± 0.04 (0.50 solar). This value is five times higher than what is found along other MS sightlines based on similar COS/UVES data sets. In contrast, the measured nitrogen abundance is found to be substantially lower ([N/H] = -1.15 ± 0.06), implying a very low [N/alpha] ratio of -0.85 dex. The substantial differences in the chemical composition of MS toward Fairall 9 compared to other sightlines point toward a complex enrichment history of the Stream. We favor a scenario, in which the gas toward Fairall 9 was locally enriched with alpha elements by massive stars and then was separated from the Magellanic Clouds before the delayed nitrogen enrichment from intermediate-mass stars could set in. Our results support (but do not require) the idea that there is a metal-enriched filament in the Stream toward Fairall 9 that originates in the LMC.
Published: 02 July 2013
SWAP images from PROBA2 taken at 174 Å in the Fe ix/x lines are compared with simultaneous slitless flash spectra obtained during the solar total eclipse of 11 July 2010. Myriad faint low-excitation emission lines together with the He i and He ii Paschen alpha chromospheric lines are recorded on eclipse spectra where regions of limb prominences are obtained with space-borne imagers. We analyzed a deep flash spectrum obtained by summing 80 individual spectra to evaluate the intensity modulations of the continuum. Intensity deficits are observed and measured at the prominences boundaries in both eclipse and SWAP images. The prominence cavities interpreted as a relative depression of plasma density, produced inside the corona surrounding the prominences, and some intense heating occurring in these regions, are discussed. Photometric measurements are shown at different scales and different, spectrally narrow, intervals for both the prominences and the coronal background.
Published: 02 July 2013
The Magellanic Stream (MS) is a massive and extended tail of multi-phase gas stripped out of the Magellanic Clouds and interacting with the Galactic halo. In this first paper of an ongoing program to study the Stream in absorption, we present a chemical abundance analysis based on HST/COS and VLT/UVES spectra of four active galactic nuclei (RBS 144, NGC 7714, PHL 2525, and HE 0056-3622) lying behind the MS. Two of these sightlines yield good MS metallicity measurements: toward RBS 144 we measure a low MS metallicity of [S/H] = [SII/HI] = -1.13 ± 0.16 while toward NGC 7714 we measure [O/H] = [OI/HI] = -1.24 ± 0.20. Taken together with the published MS metallicity toward NGC 7469, these measurements indicate a uniform abundance of ~0.1 solar along the main body of the Stream. This provides strong support to a scenario in which most of the Stream was tidally stripped from the SMC ~ 1.5-2.5 Gyr ago (a time at which the SMC had a metallicity of ~0.1 solar), as predicted by several N-body simulations. However, in Paper II of this series, we report a much higher metallicity (S/H = 0.5 solar) in the inner Stream toward Fairall 9, a direction sampling a filament of the MS that Nidever et al. claim can be traced kinematically to the Large Magellanic Cloud, not the Small Magellanic Cloud. This shows that the bifurcation of the Stream is evident in its metal enrichment, as well as its spatial extent and kinematics. Finally we measure a similar low metallicity [O/H] = [OI/HI] = -1.03 ± 0.18 in the vLSR = 150 km/s cloud toward HE 0056-3622, which belongs to a population of anomalous velocity clouds near the south Galactic pole. This suggests these clouds are associated with the Stream or more distant structures (possibly the Sculptor Group, which lies in this direction at the same velocity), rather than tracing foreground Galactic material.
Published: 02 July 2013
The Large Yield Radiometer (LYRA) is an XUV-EUV-MUV (soft X-ray to mid-ultraviolet) solar radiometer onboard the European Space Agency Project for On-Board Autonomy 2 (PROBA2) mission, which was launched in November 2009. LYRA acquires solar-irradiance measurements at a high cadence (nominally 20 Hz) in four broad spectral channels, from soft X-ray to MUV, which have been chosen for their relevance to solar physics, space weather, and aeronomy. We briefly review the design of the instrument, give an overview of the data products distributed through the instrument website, and describe how the data are calibrated. We also briefly present a summary of the main fields of research currently under investigation by the LYRA consortium.
Published: 02 July 2013
We analyze the variability of the spectral solar irradiance during the period from 7 January 2010 until 20 January 2010 as measured by the Herzberg channel (190-222 nm) of the Large Yield RAdiometer (LYRA) onboard PROBA2. In this period of time, observations by the LYRA nominal unit experienced degradation and the signal produced by the Herzberg channel frequently jumped from one level to another. Both factors significantly complicate the analysis. We present the algorithm that allowed us to extract the solar variability from the LYRA data and compare the results with SORCE/SOLSTICE measurements and with modeling based on the Code for the Solar Irradiance (COSI).
Published: 02 July 2013
The Sun Watcher using Active Pixel system detector and Image Processing (SWAP) onboard the PRoject for OnBoard Autonomy-2 (PROBA2) spacecraft provides images of the solar corona in EUV channel centered at 174 Å. These data, together with the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO), are used to study the dynamics of coronal bright points. The evolution of the magnetic polarities and associated changes in morphology are studied using magnetograms and multi-wavelength imaging. The morphology of the bright points seen in low-resolution SWAP images and high-resolution AIA images show different structures, whereas the intensity variations with time show similar trends in both SWAP 174 Å and AIA 171 Å channels. We observe that bright points are seen in EUV channels corresponding to a magnetic flux of the order of 1018 Mx. We find that there exists a good correlation between total emission from the bright point in several UV-EUV channels and total unsigned photospheric magnetic flux above certain thresholds. The bright points also show periodic brightenings, and we have attempted to find the oscillation periods in bright points and their connection to magnetic-flux changes. The observed periods are generally long (10-25 minutes) and there is an indication that the intensity oscillations may be generated by repeated magnetic reconnection.
Published: 02 July 2013
We analyze the light curves of the recent solar eclipses measured by the Herzberg channel (200-220 nm) of the Large Yield RAdiometer (LYRA) onboard Project for OnBoard Autonomy (PROBA2). The measurements allow us to accurately retrieve the center-to-limb variations (CLV) of the solar brightness. The formation height of the radiation depends on the observing angle, so the examination of the CLV provide information about a broad range of heights in the solar atmosphere. We employ the 1D NLTE radiative transfer COde for Solar Irradiance (COSI) to model the measured light curves and corresponding CLV dependencies. The modeling is used to test and constrain the existing 1D models of the solar atmosphere, e.g. the temperature structure of the photosphere and the treatment of the pseudo-continuum opacities in the Herzberg continuum range. We show that COSI can accurately reproduce not only the irradiance from the entire solar disk, but also the measured CLV. Hence it can be used as a reliable tool for modeling the variability of the spectral solar irradiance.
Published: 02 July 2013
We analyze multi-wavelength and multi-viewpoint observations of a large-scale event viewed on 7 April 2011, originating from an active-region complex. The activity leads to a white-light jet being formed in the outer corona. The topology and evolution of the coronal structures were imaged in high resolution using the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). In addition, large field-of-view images of the corona were obtained using the Sun Watcher using Active Pixel System detector and Image Processing (SWAP) telescope onboard the PRoject for Onboard Autonomy (PROBA2) microsatellite, providing evidence for the connectivity of the coronal structures with outer coronal features that were imaged with the Large Angle Spectrometric Coronagraph (LASCO) C2 on the Solar and Heliospheric Observatory (SOHO). The data sets reveal an Eiffel-tower type jet configuration extending into a narrow jet in the outer corona. The event starts from the growth of a dark area in the central part of the structure. The darkening was also observed in projection on the disk by the Solar TErrestrial RElations Observatory-Ahead (STEREO-A) spacecraft from a different point of view. We assume that the dark volume in the corona descends from a coronal cavity of a flux rope that moved up higher in the corona but still failed to erupt. The quadrupolar magnetic configuration corresponds to a saddle-like shape of the dark volume and provides a possibility for the plasma to escape along the open field lines into the outer corona, forming the white-light jet.
Published: 02 July 2013
The identification of solar-wind sources is an important question in solar physics. The existing solar-wind models (e.g., the Wang-Sheeley-Arge model) provide the approximate locations of the solar wind sources based on magnetic field extrapolations. It has been suggested recently that plasma outflows observed at the edges of active regions may be a source of the slow solar wind. To explore this we analyze an isolated active region (AR) adjacent to small coronal hole (CH) in July/August 2009. On 1 August, Hinode/EUV Imaging Spectrometer observations showed two compact outflow regions in the corona. Coronal rays were observed above the active-region coronal hole (ARCH) region on the eastern limb on 31 July by STEREO-A/EUVI and at the western limb on 7 August by CORONAS-Photon/TESIS telescopes. In both cases the coronal rays were co-aligned with open magnetic-field lines given by the potential field source surface model, which expanded into the streamer. The solar-wind parameters measured by STEREO-B, ACE, Wind, and STEREO-A confirmed the identification of the ARCH as a source region of the slow solar wind. The results of the study support the suggestion that coronal rays can represent signatures of outflows from ARs propagating in the inner corona along open field lines into the heliosphere.
Published: 02 July 2013
We investigate the interaction of three consecutive large-scale coronal waves with a polar coronal hole, simultaneously observed on-disk by the Solar TErrestrial Relations Observatory (STEREO)-A spacecraft and on the limb by the PRoject for On-Board Autonomy 2 (PROBA2) spacecraft on 27 January 2011. All three extreme ultraviolet (EUV) waves originate from the same active region, NOAA 11149, positioned at N30E15 in the STEREO-A field of view and on the limb in PROBA2. For the three primary EUV waves, we derive starting velocities in the range of approximately 310 km/s for the weakest up to approximately 500 km/s for the strongest event. Each large-scale wave is reflected at the border of the extended coronal hole at the southern polar region. The average velocities of the reflected waves are found to be smaller than the mean velocities of their associated direct waves. However, the kinematical study also reveals that in each case the ending velocity of the primary wave matches the initial velocity of the reflected wave. In all three events, the primary and reflected waves obey the Huygens-Fresnel principle, as the incident angle with approximately 10° to the normal is of the same magnitude as the angle of reflection. The correlation between the speed and the strength of the primary EUV waves, the homologous appearance of both the primary and the reflected waves, and in particular the EUV wave reflections themselves suggest that the observed EUV transients are indeed nonlinear large-amplitude MHD waves.
Published: 02 July 2013
Observations of the early rise and propagation phases of solar eruptive prominences can provide clues about the forces acting on them through the behavior of their acceleration with height. We have analyzed such an event, observed on 13 April 2010 by SWAP on PROBA2 and EUVI on STEREO. A feature at the top of the erupting prominence was identified and tracked in images from the three spacecraft. The triangulation technique was used to derive the true direction of propagation of this feature. The reconstructed points were fitted with two mathematical models: i) a power-law polynomial function and ii) a cubic smoothing spline, in order to derive the accelerations. The first model is characterized by five degrees of freedom while the second one is characterized by ten degrees of freedom. The results show that the acceleration increases smoothly, and it is continuously increasing with height. We conclude that the prominence is not accelerated immediately by local reconnection, but rather is swept away as part of a large-scale relaxation of the coronal magnetic field.
Published: 02 July 2013
The PROBA2 Science Centre (P2SC) is a small-scale science operations centre supporting the Sun observation instruments onboard PROBA2: the EUV imager Sun Watcher using APS detectors and image Processing (SWAP) and Large-Yield Radiometer (LYRA). PROBA2 is one of ESA's small, low-cost Projects for Onboard Autonomy (PROBA) and part of ESA's In-Orbit Technology Demonstration Programme. The P2SC is hosted at the Royal Observatory of Belgium, co-located with both Principal Investigator teams. The P2SC tasks cover science planning, instrument commanding, instrument monitoring, data processing, support of outreach activities, and distribution of science data products. PROBA missions aim for a high degree of autonomy at mission and system level, including the science operations centre. The autonomy and flexibility of the P2SC is reached by a set of web-based interfaces allowing the operators as well as the instrument teams to monitor quasi-continuously the status of the operations, allowing a quick reaction to solar events. In addition, several new concepts are implemented at instrument, spacecraft, and ground-segment levels allowing a high degree of flexibility in the operations of the instruments. This article explains the key concepts of the P2SC, emphasising the automation and the flexibility achieved in the commanding as well as the data-processing chain.
Published: 02 July 2013
The Sun Watcher with Active Pixels and Image Processing (SWAP) is an EUV solar telescope onboard ESA's Project for Onboard Autonomy 2 (PROBA2) mission launched on 2 November 2009. SWAP has a spectral bandpass centered on 17.4 nm and provides images of the low solar corona over a 54×54 arcmin field-of-view with 3.2 arcsec pixels and an imaging cadence of about two minutes. SWAP is designed to monitor all space-weather-relevant events and features in the low solar corona. Given the limited resources of the PROBA2 microsatellite, the SWAP telescope is designed with various innovative technologies, including an off-axis optical design and a CMOS-APS detector. This article provides reference documentation for users of the SWAP image data.
Published: 02 July 2013
The Sun Watcher with Active Pixel System detector and Image Processing (SWAP) telescope was launched on 2 November 2009 onboard the ESA PROBA2 technological mission and has acquired images of the solar corona every one to two minutes for more than two years. The most important technological developments included in SWAP are a radiation-resistant CMOS-APS detector and a novel onboard data-prioritization scheme. Although such detectors have been used previously in space, they have never been used for long-term scientific observations on orbit. Thus SWAP requires a careful calibration to guarantee the science return of the instrument. Since launch we have regularly monitored the evolution of SWAP's detector response in-flight to characterize both its performance and degradation over the course of the mission. These measurements are also used to reduce detector noise in calibrated images (by subtracting dark-current). Because accurate measurements of detector dark-current require large telescope off-points, we also monitored straylight levels in the instrument to ensure that these calibration measurements are not contaminated by residual signal from the Sun. Here we present the results of these tests and examine the variation of instrumental response and noise as a function of both time and temperature throughout the mission.
Published: 02 July 2013
The heliosphere represents a uniquely accessible domain of space, where fundamental physical processes common to solar, astrophysical and laboratory plasmas can be studied under conditions impossible to reproduce on Earth and unfeasible to observe from astronomical distances. Solar Orbiter, the first mission of ESA's Cosmic Vision 2015 - 2025 programme, will address the central question of heliophysics: How does the Sun create and control the heliosphere? In this paper, we present the scientific goals of the mission and provide an overview of the mission implementation.
Published: 02 June 2013
This booklet is a compilation of the White Papers received from the scientific community in response to ESA's Call for Science Themes for the L2 and L3 Missions, issued in March 2013. On 3-4 September 2013, a meeting will be held at the Institut Océanographique de Paris, France, at which a number of spokespersons will introduce and advocate their proposed science themes. Further details of this meeting are available here. The pdf file, which can be downloaded from the link below, comprises 587 pages and is approximately 130 Mb in size.
Published: 06 July 2013
We discuss the state of the assembly of the Hubble Sequence in the mix of bright galaxies at redshift 1.4 < z <= 2.5 with a large sample of 1,671 galaxies down to HAB~26, selected from the HST/ACS and WFC3 images of the GOODS-South field obtained as part of the GOODS and CANDELS observations. We investigate the relationship between the star formation properties and morphology using various parametric diagnostics, such as the Sersic light profile, Gini (G), M20, Concentration (C), Asymmetry (A) and multiplicity parameters. Our sample clearly separates into massive, red and passive galaxies versus less massive, blue and star forming ones, and this dichotomy correlates very well with the galaxies' morphological properties. Star-forming galaxies show a broad variety of morphological features, including clumpy structures and bulges mixed with faint low surface brightness features, generally characterized by disky-type light profiles. Passively evolving galaxies, on the other hand, very often have compact light distribution and morphology typical of today's spheroidal systems. We also find that artificially redshifted local galaxies have a similar distribution with z~2 galaxies in a G-M20 plane. Visual inspection between the rest-frame optical and UV images show that there is a generally weak morphological k-correction for galaxies at z~2, but the comparison with non-parametric measures show that galaxies in the rest-frame UV are somewhat clumpier than rest-frame optical. Similar general trends are observed in the local universe among massive galaxies, suggesting that the backbone of the Hubble sequence was already in place at z~2.
Published: 03 July 2013
Plumes, forming at the plasmapause and released outwards, constitute a well-established mode for plasmaspheric material release to the Earth's magnetosphere. They are associated to active periods and the related electric field change. In 1992, Lemaire and Shunk proposed the existence of an additional mode for plasmaspheric material release to the Earth's magnetosphere: a plasmaspheric wind, steadily transporting cold plasmaspheric plasma outwards across the geomagnetic field lines, even during prolonged periods of quiet geomagnetic conditions. This has been proposed on a theoretical basis. Direct detection of this wind has, however, eluded observation in the past. Analysis of ion measurements, acquired in the outer plasmasphere by the CIS experiment onboard the four Cluster spacecraft, provide now an experimental confirmation of the plasmaspheric wind. This wind has been systematically detected in the outer plasmasphere during quiet and moderately active conditions, and calculations show that it could provide a substantial contribution to the magnetospheric plasma populations outside the Earth's plasmasphere. Similar winds should also exist on other planets, or astrophysical objects, quickly rotating and having an atmosphere and a magnetic field.
Published: 03 July 2013
The mechanism that produces energetic electrons during magnetic reconnection is poorly understood. This is a fundamental process responsible for stellar flares, substorms, and disruptions in fusion experiments. Observations in the solar chromosphere and the Earth's magnetosphere indicate significant electron acceleration during reconnection, whereas in the solar wind, energetic electrons are absent. Here we show that energetic electron acceleration is caused by unsteady reconnection. In the Earth's magnetosphere and the solar chromosphere, reconnection is unsteady, so energetic electrons are produced; in the solar wind, reconnection is steady, so energetic electrons are absent. The acceleration mechanism is quasi-adiabatic: betatron and Fermi acceleration in outflow jets are two processes contributing to electron energization during unsteady reconnection. The localized betatron acceleration in the outflow is responsible for at least half of the energy gain for the peak observed fluxes.
Published: 26 June 2013
eLISA is a space mission designed to measure gravitational radiation over a frequency range of 0.1-100 mHz (European Space Agency LISA Assessment Study Report 2011). It uses laser interferometry to measure changes of order 10 pm per square root Hertz in the separation of inertial test masses housed in spacecraft separated by 1 million km. LISA Pathfinder (LPF) is a technology demonstrator mission that will test the key eLISA technologies of inertial test masses monitored by laser interferometry in a drag-free spacecraft. The optical bench that provides the interferometry for LPF must meet a number of stringent requirements: the optical path must be stable at the few per square root Hertz level; it must direct the optical beams onto the inertial masses with an accuracy of better than ±25 µm, and it must be robust enough not only to survive launch vibrations but to achieve full performance after launch. In this paper we describe the construction and testing of the flight optical bench for LISA Pathfinder that meets all the design requirements.
Published: 26 June 2013
23-Sep-2019 08:54 UT

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