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| Turbulence in the Solar Atmosphere and Solar Wind |
| The objective of this review article is to critically analyze turbulence and its role in the solar atmosphere and solar wind, as well as to provide a tutorial overview of topics worth clarification. Although turbulence is a ubiquitous phenomenon in the sun and its heliosphere, many open questions exist concerning the physical mechanisms of turbulence generation in solar environment. Also, the spatial and temporal evolution of the turbulence in the solar atmosphere and solar wind are still poorly understood. We limit the scope of this paper (leaving out the solar interior and convection zone) to the magnetized plasma that reaches from the photosphere and chromosphere upwards to the corona and inner heliosphere, and place particular emphasis on the magnetic field structures and fluctuations and their role in the dynamics and radiation of the coronal plasma. To attract the attention of scientists from both the fluid-dynamics and space-science communities we give in the first two sections a phenomenological overview of turbulence-related processes, in the context of solar and heliospheric physics and with emphasis on the photosphere-corona connection and the coupling between the solar corona and solar wind. We also discuss the basic tools and standard concepts for the empirical analysis and theoretical description of turbulence. The last two sections of this paper give a concise review of selected aspects of oscillations and waves in the solar atmosphere and related fluctuations in the solar wind. We conclude with some recommendations and suggest topics for future research. |
| Publication date: 26 Oct 2010 |
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| Evidence for ultra-fast outflows in radio-quiet AGNs. I. Detection and statistical incidence of Fe K-shell absorption lines |
Context: Blue-shifted Fe K absorption lines have been detected in recent years between 7 and 10 keV in the X-ray spectra of several radio-quiet AGNs. The derived blue-shifted velocities of the lines can often reach mildly relativistic values, up to 0.2-0.4c. These findings are important because they suggest the presence of a previously unknown massive and highly ionized absorbing material outflowing from their nuclei, possibly connected with accretion disk winds/outflows.
Aims: The scope of the present work is to statistically quantify the parameters and incidence of the blue-shifted Fe K absorption lines through a uniform analysis on a large sample of radio-quiet AGNs. This allows us to assess their global detection significance and to overcome any possible publication bias.
Methods: We performed a blind search for narrow absorption features at energies greater than 6.4 keV in a sample of 42 radio-quiet AGNs observed with XMM-Newton. A simple uniform model composed by an absorbed power-law plus Gaussian emission and absorption lines provided a good fit for all the data sets. We derived the absorption lines parameters and calculated their detailed detection significance making use of the classical F-test and extensive Monte Carlo simulations.
Results: We detect 36 narrow absorption lines on a total of 101 XMM-Newton EPIC pn observations. The number of absorption lines at rest-frame energies higher than 7 keV is 22.
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| Publication date: 20 Oct 2010 |
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| The origin of the hot gas in the galactic halo: confronting models with XMM-Newton observations |
| We compare the predictions of three physical models for the origin of the hot halo gas with the observed halo X-ray emission, derived from 26 high-latitude XMM-Newton observations of the soft X-ray background between l = 120° and l = 240°. These observations were chosen from a much larger set of observations as they are expected to be the least contaminated by solar wind charge exchange emission. We characterize the halo emission in the XMM-Newton band with a single-temperature plasma model. We find that the observed halo temperature is fairly constant across the sky (~(1.8-2.4) × 106 K), whereas the halo emission measure varies by an order of magnitude (~0.0005-0.006 cm-6 pc). When we compare our observations with the model predictions, we find that most of the hot gas observed with XMM-Newton does not reside in isolated extraplanar supernova (SN) remnants - this model predicts emission an order of magnitude too faint. A model of an SN-driven interstellar medium, including the flow of hot gas from the disk into the halo in a galactic fountain, gives good agreement with the observed 0.4-2.0 keV surface brightness. This model overpredicts the halo X-ray temperature by a factor of ~2, but there are a several possible explanations for this discrepancy. We therefore conclude that a major (possibly dominant) contributor to the halo X-ray emission observed with XMM-Newton is a fountain of hot gas driven into the halo by disk SNe. However, we cannot rule out the possibility that the extended hot halo of accreted material predicted by disk galaxy formation models also contributes to the emission. |
| Publication date: 15 Oct 2010 |
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| A Low-Magnetic-Field Soft Gamma Repeater |
| Soft gamma repeaters and anomalous x-ray pulsars form a rapidly increasing group of x-ray sources exhibiting sporadic emission of short bursts. They are believed to be magnetars - i.e., neutron stars powered by extreme magnetic fields, B ~ 1014 to 1015 Gauss. We report on a soft gamma repeater with low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of magnetars. X-ray observations show that its dipolar magnetic field cannot be greater than 7.5 x 1012 Gauss, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of B-field strengths, ages, and evolutionary stages than observed so far. |
| Publication date: 14 Oct 2010 |
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| A collision in 2009 as the origin of the debris trail of asteroid P/2010 A2 |
| The peculiar object P/2010 A2 was discovered in January 2010 and given a cometary designation because of the presence of a trail of material, although there was no central condensation or coma. The appearance of this object, in an asteroidal orbit (small eccentricity and inclination) in the inner main asteroid belt attracted attention as a potential new member of the recently recognized class of main-belt comets. If confirmed, this new object would expand the range in heliocentric distance over which main-belt comets are found. Here we report observations of P/2010 A2 by the Rosetta spacecraft. We conclude that the trail arose from a single event, rather than a period of cometary activity, in agreement with independent results. The trail is made up of relatively large particles of millimetre to centimetre size that remain close to the parent asteroid. The shape of the trail can be explained by an initial impact ejecting large clumps of debris that disintegrated and dispersed almost immediately. We determine that this was an asteroid collision that occurred around 10 February 2009. |
| Publication date: 13 Oct 2010 |
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| A recent disruption of the main-belt asteroid P/2010 A2 |
| Most inner main-belt asteroids are primitive rock and metal bodies in orbit about the Sun between Mars and Jupiter. Disruption, through high-velocity collisions or rotational spin-up, is believed to be the primary mechanism for the production and destruction of small asteroids and a contributor to dust in the Sun's zodiacal cloud, while analogous collisions around other stars feed dust to their debris disks. Unfortunately, direct evidence about the mechanism or rate of disruption is lacking, owing to the rarity of the events. Here we report observations of P/2010 A2, a previously unknown inner-belt asteroid with a peculiar, comet-like morphology. The data reveal a nucleus of diameter approximately 120 metres with an associated tail of millimetre-sized dust particles. We conclude that it is most probably the remnant of a recent asteroidal disruption in February/March 2009, evolving slowly under the action of solar radiation pressure, in agreement with independent work. |
| Publication date: 13 Oct 2010 |
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| Observations of slow electron holes at a magnetic reconnection site |
| We report in situ observations of high-frequency electrostatic waves in the vicinity of a reconnection site in the Earth's magnetotail. Two different types of waves are observed inside an ion-scale magnetic flux rope embedded in a reconnecting current sheet. Electron holes (weak double layers) produced by the Buneman instability are observed in the density minimum in the center of the flux rope. Higher frequency broadband electrostatic waves with frequencies extending up to fpe are driven by the electron beam and are observed in the denser part of the rope. Our observations demonstrate multiscale coupling during the reconnection: Electron-scale physics is induced by the dynamics of an ion-scale flux rope embedded in a yet larger-scale magnetic reconnection process. |
| Publication date: 12 Oct 2010 |
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| The effect of flares on total solar irradiance |
| Flares are powerful bursts of energy released by relatively poorly understood processes that take place in the atmospheres of stars. However, although solar flares, from our own Sun, are the most energetic events in the solar system, in comparison to the total output of the Sun they are barely noticeable. Consequently, the total amount of radiant energy they generate is not precisely known, and their potential contribution to variations in the total solar irradiance incident on the Earth has so far been overlooked. In this work, we identify a measurable signal from relatively moderate solar flares in total solar irradiance data. We find that the total energy radiated by flares exceeds by two orders of magnitude the flare energy radiated in the soft-X-ray domain only, indicating a major contribution in the visible domain. These results have implications for our understanding of solar-flare activity and the variability of our star. |
| Publication date: 12 Oct 2010 |
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| A European Roadmap for Exoplanets - prepared by the ESA-appointed Exoplanet Roadmap Advisory Team |
| The Exoplanet Roadmap Advisory Team (EPR-AT) was appointed by ESA with the purpose of
advising the Agency on the best scientific and technological roadmap to pursue in order to address
one of the most exciting goals in modern astrophysics: the characterization of terrestrial
exoplanets. |
| Publication date: 11 Oct 2010 |
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| Antiparallel magnetic reconnection rates at the Earth's magnetopause |
| Cluster observations at the Earth's high-latitude magnetopause are combined with magnetic field models to demonstrate that antiparallel reconnection was occurring at the magnetopause for an event on 3 December 2001. Over a 20 min period, the reconnection line passed over the spacecraft on two occasions. In between the encounters with the reconnection line, velocity cutoffs in the ion distributions are used to determine the distance to the reconnection site. These observations are consistent with an antiparallel reconnection line whose location relative to the spacecraft depends on the orientation of the interplanetary magnetic field. Using this knowledge of the reconnection site location and a previously developed, two-spacecraft method for computing the inflow velocity into the reconnection site, the reconnection rate (Vn/VA) is determined to be <0.08. The rate is consistent with fast reconnection and considerably higher than the reconnection rate for a component reconnection event that was determined using the same two-spacecraft method. |
| Publication date: 02 Oct 2010 |
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| CRISM Limb Observations of O2 Singlet Delta Nightglow in the Polar Winter Atmosphere of Mars |
| CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) near-IR spectroscopic imaging of the Mars atmospheric limb supports vertical profiling of aerosol (ice and dust) and gas [H2O, CO, CO2, O2(1_Delta_g)] constituents versus season (Ls), latitude, and (to a limited degree) longitude. These CRISM limb observations are obtained approximately every two months (15° Ls), over a full range of sunlighted latitudes for two MRO (Mars Reconnaissance Orbiter) orbits centered on equatorial longitudes of 100W and 300W. Daylight limb spectra indicate strong 1.27 µm atmospheric emission from the excited singlet delta of molecular oxygen, associated with photolysis of Mars atmospheric ozone. Limb observations extending to un-illuminated, polar night latitudes present a new source of O2(1_Delta_g) emission at higher altitudes (40-55 km), associated with three body recombination of atomic oxygen [O+O+CO2 -> O2(1_Delta_g) +CO2]. This nightglow requires strong poleward supply of atomic oxygen, produced from photolysis of CO2 at sunlighted latitudes and transported at high altitudes (above 70 km) into polar night altitudes of 40-60 km. CRISM limb observations indicate distinctive latitudinal and longitudinal distributions of this polar nightglow that evolve over the Feb-Aug 2010 (Ls=50-140°) period of observations for the southern winter. New observations include planned full orbit mapping (12 orbits) in August 2010 to characterize these spatial variations in more detail. Key comparisons with co-located MCS (Mars Climate Sounder) temperature and aerosol profile retrievals and LMD (Laboratoire Météoroligie Dynamique) GCM photochemical simulations provide new insights into poorly constrained meridional transport into polar winter latitudes on Mars. |
| Publication date: 01 Oct 2010 |
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| Three dimensional anisotropic k spectra of turbulence at subproton scales in the solar wind |
| We show the first three dimensional (3D) dispersion relations and k spectra of magnetic turbulence in the solar wind at subproton scales. We used the Cluster data with short separations and applied the k-filtering technique to the frequency range where the transition to subproton scales occurs. We show that the cascade is carried by highly oblique kinetic Alfvén waves with wplas below 0.1wci down to kperpRhoi ~ 2.
Each k spectrum in the direction perpendicular to Bo shows two scaling ranges separated by a breakpoint (in the interval [0.4, 1]kperpRhoi): a Kolmogorov scaling kperp-1.7 followed by a steeper scaling kperp-4.5. We conjecture that the turbulence undergoes a transition range, where part of the energy is dissipated into proton heating via Landau damping and the remaining energy cascades down to electron scales where electron Landau damping may predominate. |
| Publication date: 27 Sep 2010 |
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| Observing Supernova 1987A with the Refurbished Hubble Space Telescope |
| Observations with the Hubble Space Telescope (HST), conducted since 1990, now offer an unprecedented glimpse into fast astrophysical shocks in the young remnant of supernova 1987A. Comparing observations taken in 2010 with the use of the refurbished instruments on HST with data taken in 2004, just before the Space Telescope Imaging Spectrograph failed, we find that the Ly-a and H-a lines from shock emission continue to brighten, whereas their maximum velocities continue to decrease. We observe broad, blueshifted Ly-a, which we attribute to resonant scattering of photons emitted from hot spots on the equatorial ring. We also detect N v ll1239, 1243 angstrom line emission, but only to the red of Ly-a. The profiles of the N v lines differ markedly from that of H-a, suggesting that the N4+ ions are scattered and accelerated by turbulent electromagnetic fields that isotropize the ions in the collisionless shock. |
| Publication date: 24 Sep 2010 |
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| A&A Special: Pre-launch status of Planck mission |
This A&A issue features 13 articles describing the pre-flight status of the European Space Agency Planck mission, launched
together with the Herschel satellite on 14 May 2009. The Planck mission is designed to image the anisotropies of the cosmic
background radiation field over the whole sky, with unprecedented sensitivity and angular resolution, as well as a wide frequency
range. As a byproduct of that main goal, it will simultaneously address a wide range of galactic and extragalactic science. The
mission involves more than four hundred scientists, who are currently working on data processing, calibration, and data analysis. The satellite is scheduled to continuously acquire high-quality science data until the end of 2011. An early release of the compact source catalogue will be delivered in January 2011, together with a small set of science papers related to foreground astrophysical sources. The first major cosmology results will be delivered in December 2012. In this special feature, the telescope's optical system and the design, ground calibration, and performance of the Planck lowand
high-frequency instruments are described in detail. |
| Publication date: 15 Sep 2010 |
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| The large-scale magnetospheric electric field observed by Double Star TC-1 |
| The relationship between the average structure of the inner magnetospheric large-scale electric field and geomagnetic activity levels has been investigated by Double Star TC-1 data for radial distances between 4.5 Re and 12.5 Re and MLT between 18:00 h and 06:00 h from July to October in 2004 and 2005. The sunward component of the electric field decreases monotonically as radial distance increases and approaches zero as the distance off the Earth is greater than 10 Re. The dawn-dusk component is always duskward. It decreases at about 6 Re where the ring current is typically observed to be the strongest and shows strong asymmetry with respect to the magnetic local time. Surprisingly, the average electric field obtained from TC-1 for low activity is almost comparable to that observed during moderate activity, which is always duskward at the magnetotail (8 Re~12 Re). |
| Publication date: 03 Sep 2010 |
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| Warm water vapour in the sooty outflow from a luminous carbon star |
| The detection of circumstellar water vapour around the ageing carbon star IRC +10216 challenged the current understanding of chemistry in old stars, because water was predicted to be almost absent in carbon-rich stars. Several explanations for the water were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star, grain surface reactions, and photochemistry in the outer circumstellar envelope. With a single water line detected so far from this one carbon-rich evolved star, it is difficult to discriminate between the different mechanisms proposed. Here we report the detection of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC +10216 using the Herschel satellite. This includes some high-excitation lines with energies corresponding to ~1000 K, which can be explained only if water is present in the warm inner sooty region of the envelope. A plausible explanation for the warm water appears to be the penetration of ultraviolet photons deep into a clumpy circumstellar envelope. This mechanism also triggers the formation of other molecules, such as ammonia, whose observed abundances are much higher than hitherto predicted. |
| Publication date: 01 Sep 2010 |
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| CoRoT Reveals a Magnetic Activity Cycle in a Sun-Like Star |
| The 11-year activity cycle of the Sun is a consequence of a dynamo process occurring beneath its surface. We analyzed photometric data obtained by the CoRoT space mission, showing solarlike oscillations in the star HD49933, for signatures of stellar magnetic activity. Asteroseismic measurements of global changes in the oscillation frequencies and mode amplitudes reveal a modulation of at least 120 days, with the minimum frequency shift corresponding to maximum amplitude as in the Sun. These observations are evidence of a stellar magnetic activity cycle taking place beneath the surface of HD49933 and provide constraints for stellar dynamo models under conditions different from those of the Sun. |
| Publication date: 27 Aug 2010 |
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| Generation of whistler mode emissions in the inner magnetosphere: An event study |
| A careful analysis of the wave emissions for this event has shown that Cluster 4 passed through the wave source region. Simultaneous electron particle data from the PEACE instrument in the generation region indicated the presence of a mid-energy electron population (<100 s of eV) that had a highly anisotropic temperature distribution with the perpendicular temperature 10 times the parallel temperature. To understand this somewhat rare event in which the satellite passed directly through the wave generation region and in which a free energy source (i.e., temperature anisotropy) was readily identified, a linear theory and particle in cell simulation study has been carried out to elucidate the physics of the wave generation, wave-particle interactions, and energy redistribution. The theoretical results show that for this event the anisotropic electron distribution can linearly excite obliquely propagating whistler mode waves in the upper frequency band, i.e., above 0.5fce. Simulation results show that in addition to the upper band emissions, nonlinear wave-wave coupling excites waves in the lower frequency band, i.e., below 0.5fce. The instability saturates primarily by a decrease in the temperature anisotropy of the mid-energy electrons, but also by heating of the cold electron population. The resulting wave-particle interactions lead to the formation of a high-energy plateau on the parallel component of the warm electron velocity distribution. The theoretical results for the saturation time scale indicate that the observed anisotropic electron distribution must be refreshed in less than 0.1 s allowing the anisotropy to be detected by the electron particle instrument, which takes several seconds to produce a distribution. |
| Publication date: 21 Aug 2010 |
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| Island surfing mechanism of electron acceleration during magnetic reconnection |
| One of the key unresolved problems in the study of space plasmas is to explain the production of energetic electrons as magnetic field lines "reconnect" and release energy in an explosive manner. Recent observations suggest possible roles played by small-scale magnetic islands in the reconnection region, but their precise roles and the exact mechanism of electron energization have remained unclear. Here we show from two-dimensional particle-in-cell simulations that secondary islands generated in the reconnection region indeed produce energetic electrons. We found that when electrons are trapped inside the islands, they are energized continuously by the reconnection electric field prevalent in the reconnection diffusion region. Applications to observations in the Earth's magnetotail are briefly discussed. |
| Publication date: 21 Aug 2010 |
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| Cosmological Constraints from Strong Gravitational Lensing in Clusters of Galaxies |
| Current efforts in observational cosmology are focused on characterizing the mass-energy content of the universe. We present results from a geometric test based on strong lensing in galaxy clusters. Based on Hubble Space Telescope images and extensive ground-based spectroscopic follow-up of the massive galaxy cluster Abell 1689, we used a parametric model to simultaneously constrain the cluster mass distribution and dark energy equation of state. Combining our cosmological constraints with those from X-ray clusters and the Wilkinson Microwave Anisotropy Probe 5-year data gives Omegam = 0.25 ±0.05 and wx = -0.97 ±0.07, which are consistent with results from other methods. Inclusion of our method with all other available techniques brings down the current 2-sigma contours on the dark energy equation-of-state parameter wx by ~30%. |
| Publication date: 19 Aug 2010 |
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