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| Transient Pc3 wave activity generated by a hot flow anomaly: Cluster, Rosetta, and ground-based observations |
| Pc3 pulsations are observed in the magnetosphere with wave periods of 10-45 s. Two distinct populations have been observed; one exhibits a frequency dependence on the solar wind magnetic field strength, whereas the other does not. The first population is explained in terms of a model where the bow shock reflects ions which generate upstream foreshock ULF waves. These waves are convected through the shock to the dayside magnetopause and thus to the magnetosphere. The source of the second population is not well understood. In this paper we examine the generation of a transient patch of Pc3 wave activity due to a hot flow anomaly (HFA) using a unique spacecraft conjunction that occurred during the first Earth flyby of the Rosetta spacecraft. Cluster, upstream of the bow shock and close to the Sun-Earth line observed an HFA. At this time Rosetta was nearing closest approach and together with ground magnetometer stations, observed a transient interval of Pc3 wave activity. Analysis also shows that the Pc3 waves occurred in the absence of a ULF wavefield just upstream of the bow shock. This result shows that HFAs can be a source of Pc3 wave activity, and may explain in part the origin of the second population of Pc3 waves. It also demonstrates in new detail the manner in which kinetic physics at the bow shock, driven by structure in the solar wind, can influence magnetospheric dynamics. |
| Publication date: 27 Aug 2011 |
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| The origin of the Martian moons revisited |
| The origin of the Martian moons, Phobos and Deimos, is still an open issue: either they are asteroids captured by Mars or they formed in situ from a circum-Mars debris disk. The capture scenario mainly relies on the remote-sensing observations of their surfaces, which suggest that the moon material is similar to outer-belt asteroid material. This scenario, however, requires high tidal dissipation rates inside the moons to account for their current orbits around Mars. Although the in situ formation scenarios have not been studied in great details, no observational constraints argue against them. Little attention has been paid to the internal structure of the moons, yet it is pertinent for explaining their origin. The low density of the moons indicates that their interior contains significant amounts of porous material and/or water ice. The porous content is estimated to be in the range of 3060% of the volume for both moons. This high porosity enhances the tidal dissipation rate but not sufficiently to meet the requirement of the capture scenario. On the other hand, a large porosity is a natural consequence of re-accretion of debris at Mars' orbit, thus providing support to the in situ formation scenarios. The low density also allows for abundant water ice inside the moons, which might significantly increase the tidal dissipation rate in their interiors, possibly to a sufficient level for the capture scenario. Precise measurements of the rotation and gravity field of the moons are needed to tightly constrain their internal structure in order to help answering the question of the origin. |
| Publication date: 25 Aug 2011 |
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| Nonaxisymmetric Anisotropy of Solar Wind Turbulence |
| A key prediction of turbulence theories is frame-invariance, and in magnetohydrodynamic (MHD) turbulence, axisymmetry of fluctuations with respect to the background magnetic field. Paradoxically the power in fluctuations in the turbulent solar wind are observed to be ordered with respect to the bulk macroscopic flow as well as the background magnetic field. Here, nonaxisymmetry across the inertial and dissipation ranges is quantified using in situ observations from Cluster. The observed inertial range nonaxisymmetry is reproduced by a "fly through" sampling of a direct numerical simulation of MHD turbulence. Furthermore, fly through sampling of a linear superposition of transverse waves with axisymmetric fluctuations generates the trend in nonaxisymmetry with power spectral exponent. The observed nonaxisymmetric anisotropy may thus simply arise as a sampling effect related to Taylor's hypothesis and is not related to the plasma dynamics itself. |
| Publication date: 23 Aug 2011 |
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| Detection of Emerging Sunspot Regions in the Solar Interior |
| Sunspots are regions where strong magnetic fields emerge from the solar interior and where major eruptive events occur. These energetic events can cause power outages, interrupt telecommunication and navigation services, and pose hazards to astronauts. We detected subsurface signatures of emerging sunspot regions before they appeared on the solar disc. Strong acoustic travel-time anomalies of an order of 12 to 16 seconds were detected as deep as 65,000 kilometers. These anomalies were associated with magnetic structures that emerged with an average speed of 0.3 to 0.6 kilometer per second and caused high peaks in the photospheric magnetic flux rate 1 to 2 days after the detection of the anomalies. Thus, synoptic imaging of subsurface magnetic activity may allow anticipation of large sunspot regions before they become visible, improving space weather forecast. |
| Publication date: 19 Aug 2011 |
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| Magnetic signatures of a tenuous atmosphere at Dione |
| We present Cassini magnetic field observations from the only two close flybys (16DI and 129DI) of Saturn's icy satellite Dione which have been carried out so far. Data from 16DI show a weak field perturbation in the upstream region, indicative of a tenuous atmosphere around the satellite. By applying an analytical model of the perturbations caused by subalfvénic atmosphere-magnetosphere interactions, we demonstrate that an atmospheric column density of approximately 1x1017 m-2 would be able to sustain the observed field signature. Magnetic field data from 16DI also contain hints that Dione's gas envelope might possess a slight asymmetry between the Saturn-facing and the Saturn-averted hemisphere. The detection of a thin atmosphere at Dione might be correlated to the occurrence of a transient radiation belt near the moon's L-shell at the time of the 16DI flyby, as reported by Roussos et al. (2008b). On the other hand, magnetic field observations from the subsequent downstream encounter 129DI show no clear evidence of an atmosphere, probably due to the flyby trajectory being unsuitable for the detection of the associated perturbations. |
| Publication date: 12 Aug 2011 |
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| Comparative Planetology: Venus-Earth-Mars |
Comparative planetology has long been a field of general interest but with a fairly small number of scientists actively involved. During the last decade, until recently, there has been no significant growth, possibly much due to lack of new data from Venus; perhaps the most obvious planet to compare with the Earth. Availability of ample data of high quality is of paramount importance for proper comparisons. With the arrival of Venus Express at Venus in March 2006 a new impulse to the field has been injected. Venus Express addresses a large number of topics relevant to comparative planetology; in particular in the field of atmospheric dynamics and chemistry, clouds and atmospheresolar wind interaction.
Mars has been the subject of significant interest and many space missions in the recent years. Being smaller and cooler and in several aspects more evolved, Mars is still a planet of great interest for comparison with the Earth on the other end of the parameter space.
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| Publication date: 02 Aug 2011 |
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| Reflection in Seyfert Galaxies and the Unified Model of AGN |
We present a deep study of the average hard X-ray spectra of Seyfert galaxies. We aim to test the unified model of active galactic nuclei, and constrain differences and similarities between different classes of objects. We analyzed all public INTEGRAL IBIS/ISGRI data available on all the 165 Seyfert galaxies detected at z < 0.2. Our final sample consists of 44 Seyfert 1s, 29 Seyfert 1.5s, 78 Seyfert 2s, and 14 narrow-line Seyfert 1s. For each subsample, we stacked all the images, and derived their average hard X-ray spectra in the 17-250 keV energy range. We performed a detailed spectral analysis using both a model-independent and a model-dependent approach. All classes of Seyfert galaxies show on average the same nuclear continuum, as foreseen by the zeroth order unified model, with a cutoff energy of EC >~ 200 keV, and a photon index of Gamma~1.8. The average optical depth of the Comptonizing medium is consistent for the different classes (tau~0.8). Compton-thin Seyfert 2s show a reflection component stronger than Seyfert 1s and Seyfert 1.5s. Most of this reflection is due to mildly obscured (1023 cm-2 <= NH < 1024 cm-2) Seyfert2s, which have a significantly stronger reflection component (R = 2.2+4.5-1.1) than Seyfert 1s (R <= 0.4), Seyfert 1.5s (R <= 0.4) and lightly obscured (NH < 1023 cm-2) Seyfert 2s (R <= 0.5). This cannot be explained easily by the unified model. The absorber/reflector in mildly obscured Seyfert 2s might cover a large fraction of the X-ray source, and contain clumps of Compton-thick material. The large reflection found in the spectrum of mildly obscured Seyfert 2s reduces the amount of Compton-thick objects needed to explain the peak of the cosmic X-ray background. Our results are consistent with the fraction of Compton-thick sources being ~10 percent.
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| Publication date: 02 Aug 2011 |
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| Atmospheric angular momentum variations of Earth, Mars and Venus at seasonal time scales |
| Atmospheric angular momentum variations of a planet are associated with the global atmospheric mass redistribution and the wind variability. The exchange of angular momentum between the fluid layers and the solid planet is the main cause for the variations of the planetary rotation at seasonal time scales. In the present study, we investigate the angular momentum variations of the Earth, Mars and Venus, using geodetic observations, output of state-of-the-art global circulation models as well as assimilated data. We discuss the similarities and differences in angular momentum variations, planetary rotation and angular momentum exchange for the three terrestrial planets. We show that the atmospheric angular momentum variations for Mars and Earth are mainly annual and semi-annual whereas they are expected to be "diurnal" on Venus. The wind terms have the largest contributions to the LOD changes of the Earth and Venus whereas the matter term is dominant on Mars due to the CO2 sublimation/condensation. The corresponding LOD variations (DLOD) have similar amplitudes on Mars and Earth but are much larger on Venus, though more difficult to observe. |
| Publication date: 01 Aug 2011 |
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| Herschel measurements of molecular oxygen in Orion |
| We report observations of three rotational transitions of molecular oxygen (O2) in emission from the H2 Peak 1 position of vibrationally excited molecular hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using HIFI on the Herschel Space Observatory, having velocities of 11 km s-1 to 12 km s-1 and widths of 3 km s-1. The beam-averaged column density is N(O2) = 6.5×1016 cm-2, and assuming that the source has an equal beam filling factor for all transitions (beam widths 44, 28, and 19'), the relative line intensities imply a kinetic temperature between 65 K and 120 K. The fractional abundance of O2 relative to H2 is 0.3-7.3×10-6. The unusual velocity suggests an association with a ~5' diameter source, denoted Peak A, the Western Clump, or MF4. The mass of this source is ~10 MSun and the dust temperature is >= 150 K. Our preferred explanation of the enhanced O2 abundance is that dust grains in this region are sufficiently warm (T >= 100 K) to desorb water ice and thus keep a significant fraction of elemental oxygen in the gas phase, with a significant fraction as O2. For this small source, the line ratios require a temperature >= 180 K. The inferred O2 column density ~5×1018 cm-2 can be produced in Peak A, having N(H2) ~4×1024 cm-2. An alternative mechanism is a low-velocity (10 to 15 km s-1) C-shock, which can produce N(O2) up to 1017 cm-2. |
| Publication date: 01 Aug 2011 |
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| Super-Alfvénic Propagation of Substorm Reconnection Signature and Poynting Flux |
The propagation of reconnection signatures and their associated energy are examined using kinetic particle-in-cell simulations and Cluster satellite observations. It is found that the quadrupolar out-of-plane magnetic field near the separatrices is associated with a kinetic Alfvén wave. For magnetotail parameters, the parallel propagation of this wave is super-Alfvénic (V||<~1500-5500 km/s) and generates substantial Poynting flux (S~10-5-10-4 W/m2) consistent with Cluster observations of magnetic reconnection. This Poynting flux substantially exceeds that due to frozen-in ion bulk outflows and is sufficient to generate white light aurora in Earth's ionosphere. |
| Publication date: 01 Aug 2011 |
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