Space Sciences

Lower Plate Structure and Upper Plate Deformational Segmentation at the Sunda-Banda Arc Transition, Indonesia

M.C. , K.Anne , K.Heidrun , P.Lars , L.E. , F.E. , S.Alexey , D.Y.


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The Sunda‐Banda arc transition at the eastern termination of the Sunda margin (Indonesia) represents a unique natural laboratory to study the effects of lower plate variability on upper plate deformational segmentation. Neighboring margin segments display a high degree of structural diversity of the incoming plate (transition from an oceanic to a continental lower plate, presence/absence of an oceanic plateau, variability of subducting seafloor morphology) as well as a wide range of corresponding fore‐arc structures, including a large sedimentary basin and an accretionary prism/outer arc high of variable size and shape. Here, we present results of a combined analysis of seismic wide‐angle refraction, multichannel streamer and gravity data recorded in two trench normal corridors located offshore the islands of Lombok (116°E) and Sumba (119°E). On the incoming plate, the results reveal a 8.6–9.0 km thick oceanic crust, which is progressively faulted and altered when approaching the trench, where upper mantle velocities are reduced to ∼7.5 km/s. The outer arc high, located between the trench and the fore‐arc basin, is characterized by sedimentary‐type velocities (Vp < 5.5 km/s) down to the top of the subducting slab (∼13 km depth). The oceanic slab can be traced over 70–100 km distance beneath the fore arc. A shallow serpentinized mantle wedge at ∼16 km depth offshore Lombok is absent offshore Sumba, where our models reveal the transition to the collisional regime farther to the east and to the Sumba block in the north. Our results allow a detailed view into the complex structure of both the deeper and shallower portions of the eastern Sunda margin

Morpho-Acoustic Variability of Cold Seeps on the Continental Slope Offshore Nicaragua: Result of Fluid Flow Interaction With Sedimentary Processes

B.Dietmar , S.Heiko , K.Ingo , W.Reimer


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Based on multibeam bathymetry, high-resolution deep-towed sidescan sonar and Chirp subbottom profiling 32 cold seep sites, already identified in Sahling et al. (2008a), have been studied in an approximately 1000 km2 large area ranging from 800 to 2600 m water depth along the middle slope of the active continental margin offshore Nicaragua. Ground truthing is available from towed camera surveys and coring on seven of the structures. The seeps occur in different settings on the slope: upslope and along the headwall of large submarine slides, as isolated eroded massifs, and forming linear ridges between deeply incised canyons. The seep sites show a wide range regarding their size and morphology, their backscatter intensity patterns, their structure in subbottom profiles, and their fluid venting activity inferred from seafloor observations. Surface extension of the seep sites ranges from less than 200 to more than 1500 m in diameter, and relief height varies between no relief and 180 m. Indications of extruded materials such as mud flows are not observed in the area of the seep sites. Instead the seeps are characterized by high proportions of authigenic carbonates. The carbonates occur as crusts, detritus, or single layers embedded in the seafloor sediments. They appear as high backscatter intensities on sidescan sonar images. On some seep sites living vent fauna indicative of active seepage is observed, but gas bubbles have not been observed. To explain the high morphological variability of the features, we propose a generic model including the interaction of several processes: (1) episodic fluid venting and associated authigenic carbonate formation; (2) background sedimentation and subsidence; (3) linear erosion along canyons and denudation on the slope surface

Two distinct energetic electron populations of different origin in the earth's magnetotail: a cluster case study

T.A. , Q.Zong , M.I. , E.T.


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Energetic electrons travelling along and perpendicular to the magnetic field lines have been observed in the magnetotail at L~17:00 and 22:00 MLT during the recovery phase of a storm-time substorm on 7 October 2002. Three-dimensional electron distributions of the full unit sphere obtained from the IES/RAPID sensor system demonstrated a rather complicated and random behavior of the energetic electrons. Occasionally these electrons were appearing to travel parallel, perpendicular, or in both directions, relative to the magnetic field direction, forming in this way bi-directional, perpendicular-peaked, and mixed distributions. The electron enhancements occurred while the Cluster spacecraft were on closed field lines in the central plasma sheet approaching the neutral sheet from the northern tail lobe. Magnetic field and energetic particle measurements have been used from geosynchronous and Cluster satellites, in order to describe the general context of the event and then give a possible interpretation regarding the occurrence of the electron anisotropies observed by the IES/RAPID spectrometer on board Cluster. According to geosynchronous measurements an electron dispersionless ejection is very well correlated with a dipolar re-configuration of the magnetic field. The latter fact supports the idea that electrons and, in general, particle ejections at geosynchronous altitude are directly related to electric fields arising from field dipolarization caused by current disruption. Also, having as a main objective the understanding of the way 3-D electron distributions are formed, we have analyzed electron energy spectra along and perpendicular to the magnetic field direction, demonstrating the fact that the electron population consists of two distinct components acting independently and in a random manner relative to each other. This leads to the conclusion that these two electron populations along and perpendicular to the field are generated at different remote locations at different rates. The main conclusion of the present paper is that the perpendicular-peaked electron enhancements observed by Cluster are produced in a remote location duskward of the satellite location, due to the longitudinal and tailward expansion of a current disruption region, and subsequently transported to the Cluster location by means of curvature drift. On the other hand, bi-directional electrons are believed to be generated in the vicinity of the neutral sheet or around an X-type region, as suggested by a plethora of previous studies. Finally, in the Discussion section, we make an attempt to present in a more thorough way the substorm model developed by Vogiatzis et al. , which is intimately related to the importance of X-line formation for the initiation of a substorm.

H2 emission from non-stationary magnetized bow shocks

L.Tram , P.Lesaffre , S.Cabrit , A.Gusdorf , P.Nhung


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When a fast moving star or a protostellar jet hits an interstellar cloud, the surrounding gas gets heated and illuminated: a bow shock is born that delineates the wake of the impact. In such a process, the new molecules that are formed and excited in the gas phase become accessible to observations. In this paper, we revisit models of H 2 emission in these bow shocks. We approximate the bow shock by a statistical distribution of planar shocks computed with a magnetized shock model. We improve on previous works by considering arbitrary bow shapes, a finite irradiation field and by including the age effect of non-stationary C-type shocks on the excitation diagram and line profiles of H 2. We also examine the dependence of the line profiles on the shock velocity and on the viewing angle: we suggest that spectrally resolved observations may greatly help to probe the dynamics inside the bow shock. For reasonable bow shapes, our analysis shows that low-velocity shocks largely contribute to H 2 excitation diagram. This can result in an observational bias towards low velocities when planar shocks are used to interpret H 2 emission from an unresolved bow. We also report a large magnetization bias when the velocity of the planar model is set independently. Our 3D models reproduce excitation diagrams in BHR 71 and Orion bow shocks better than previous 1D models. Our 3D model is also able to reproduce the shape and width of the broad H 2 10S line profile in an Orion bow shock.

Transient rift opening in response to multiple dike injections in the manda hararo rift (afar, ethiopia) imaged by time-dependent elastic inversionof interferometric synthetic aperture radar data

A.Socquet , E.Jacques , J.De , R.Grandin , M.Doin , G.C.


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Interferometric synthetic aperture radar data spanning the time intervals separating thirteen dike intrusions in the Manda Hararo-Dabbahu rift from 2005 to 2009 show that transient deformation occurs in the inter-diking period. This deformation can be explained by the presence of seven inflating or deflating pressure sources. By combining the data acquired on four different InSAR tracks, through time-dependent elastic models, we are able to track these deformation modes with a time resolution smaller than 1 month. Sustained deflation of a deep magma reservoir at Dabbahu in the 6 months following the main rifting event of 2005, and slow decelerating post-eruptive re-inflation of two shallow magma reservoirs below Dabbahu and Gabho volcanoes, are monitored. A deflation signal of deep origin on the neighboring rift system is also detected, possibly caused by outflow of material from a preexisting reservoir into the deep plate boundary. In contrast, rapidly evolving deformation is observed at the center of the Manda Hararo rift segment. Transient deformation events are monitored in the weeks/months following the diking events, with pulses of localized rift opening after the dike intrusions, followed by an exponential-like decay of opening rate. This signal may be associated with the replenishment of the central magma reservoir involved in feeding the 2005-2009 dikes. Alternatively, the predominantly horizontal mode of deformation suggests an interaction between the response of the lithosphere to tectonic strain accumulation, and the process of hydraulic connectivity within the central magma plumbing system.

Galactic Cosmic RayInduced Radiation Dose on Terrestrial Exoplanets

G.Jean-Mathias , A.Dimitra , H.B.


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International audienceThis past decade has seen tremendous advancements in the study of extrasolar planets. Observations are now made with increasing sophistication from both ground- and space-based instruments, and exoplanets are characterized with increasing precision. There is a class of particularly interesting exoplanets that reside in the habitable zone, which is defined as the area around a star where the planet is capable of supporting liquid water on its surface. Planetary systems around M dwarfs are considered to be prime candidates to search for life beyond the Solar System. Such planets are likely to be tidally locked and have close-in habitable zones. Theoretical calculations also suggest that close-in exoplanets are more likely to have weaker planetary magnetic fields, especially in the case of super-Earths. Such exoplanets are subjected to a high flux of galactic cosmic rays (GCRs) due to their weak magnetic moments. GCRs are energetic particles of astrophysical origin that strike the planetary atmosphere and produce secondary particles, including muons, which are highly penetrating. Some of these particles reach the planetary surface and contribute to the radiation dose. Along with the magnetic field, another factor governing the radiation dose is the depth of the planetary atmosphere. The higher the depth of the planetary atmosphere, the lower the flux of secondary particles will be on the surface. If the secondary particles are energetic enough, and their flux is sufficiently high, the radiation from muons can also impact the subsurface regions, such as in the case of Mars. If the radiation dose is too high, the chances of sustaining a long-term biosphere on the planet are very low. We have examined the dependence of the GCR-induced radiation dose on the strength of the planetary magnetic field and its atmospheric depth, and found that the latter is the decisive factor for the protection of a planetary biosphere. Key Words: RadiationRadiation physicsHabitabilityHabitable zonePlanetary atmospheres

Probing of medium-scale traveling ionospheric disturbances using hf-induced scatter targets

M.T. , M.C. , N.F. , T.D. , V.A. , I.V. , V.L. , V.P. , L.M. , Y.M. , V.L. , A.V. , S.B. , A.V. , M.G. , V.G.


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Experimental results from the Troms and Sura heating experiments at high and mid-latitudes are examined. It is shown that the combination of HF-induced target and bi-static HF Doppler radio scatter observations is a profitable method for probing medium-scale traveling ionospheric disturbances at high and mid-latitudes. HF ionospheric modification experiments provide a way of producing the HF-induced scatter target in a controlled manner at altitudes where the sensitivity to TIDs is highest. Bi-static HF Doppler radio scatter observations were carried out on the London-Troms-St. Petersburg path in the course of a Troms heating experiment on 16 November 2004 when the pump wave was reflected from an auroral Es-layer. During Sura heating experiments on 19 and 20 August 2004, when the HF pump wave was reflected from the F2 ionospheric layer, multi-position bi-static HF Doppler radio scatter observations were simultaneously performed at three reception points including St. Petersburg, Kharkov, and Rostov-on-Don. Ray tracing and Doppler shift simulations were made for all experiments. A computational technique has been developed allowing the reconstruction of the TID phase velocities from multi-position bi-static HF Doppler scatters. Parameters of medium-scale TIDs were found. In all experiments they were observed in the evening and pre-midnight hours. TIDs in the auroral E-region with periods of about 23 min were traveling southward at speeds of 210 m/s. TIDs in the mid-latitudinal F-region with periods from 20 to 45 min travelled at speeds between 40 and 150 m/s. During quiet magnetic conditions the waves were traveling in the north-east direction. In disturbed conditions the waves were moving in the south-west direction with higher speeds as compared with quiet conditions. Possible sources for the atmospheric gravity waves at middle and high latitudes are discussed.

Transport modelling of a pyro-convection event in alaska

A.Stohl , M.Fromm , C.Forster , R.Servranckx , R.Damoah , N.Spichtinger , E.W. , I.A. , P.James , M.Shulski


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Summer 2004 saw severe forest fires in Alaska and the Yukon Territory that were mostly triggered by lightning strikes. The area burned in the year 2004 was the highest on record to date in Alaska. Pollutant emissions from the fires lead to violation of federal standards for air quality in Fairbanks. This paper studies deep convection events that occurred in the burning regions at the end of June 2004. The convection was likely enhanced by the strong forest fire activity and penetrated into the lower stratosphere, up to about 3 km above the tropopause. Emissions from the fires did not only perturb the UT/LS locally, but also regionally. POAM data at the approximate location of Edmonton show that the UT/LS aerosol extinction was enhanced by a factor of 4 relative to unperturbed conditions. Simulations with the particle dispersion model FLEXPART with the deep convective transport scheme turned on showed transport of forest fire emissions into the stratosphere, in qualitatively good agreement with the enhancements seen in the POAM data. A corresponding simulation with the deep convection scheme turned off did not result in such deep vertical transport. Lidar measurements at Wisconsin on 30 June also show the presence of substantial aerosol loading in the UT/LS, up to about 13 km. In fact, the FLEXPART results suggest that this aerosol plume originated from the Yukon Territory on 25 June.
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