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Combi M.R.,University of Michigan | Fougere N.,University of Michigan | Makinen J.T.T.,Finnish Meteorological Institute | Bertaux J.-L.,University of Versailles | And 2 more authors.
Astrophysical Journal Letters | Year: 2014

The Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyα camera on the SOlar and Heliospheric Observer (SOHO) satellite observed the hydrogen coma of comet C/2012 S1 (ISON) for most of the last month of its activity from 2013 October 24 to November 24, ending just 4 days before perihelion and its final disruption. The water production rate of the comet was determined from these observations. SOHO has been operating in a halo orbit around the Earth-Sun L1 Lagrange point since its launch in late 1995. Most water vapor produced by comets is ultimately photodissociated into two H atoms and one O atom producing a huge hydrogen coma that is routinely observed in the daily SWAN images in comets of sufficient brightness. Water production rates were calculated from 22 images over most of the last month of the pre-perihelion apparition. The water production rate increased very slowly on average from October 24.9 until November 12.9, staying between 1.8 and 3.4 × 1028 s -1, after which it increased dramatically, reaching 1.6 to 2 × 1030 s-1 from November 21.6 to 23.6. It was not detected after perihelion on December 3.7 when it should have been visible. We examine the active surface area necessary to explain the water production rate and its variation and are able to place constraints on the physical size of the original nucleus necessary to account for the large amount of activity from November 12.9 and until just before perihelion. © 2014. The American Astronomical Society. All rights reserved.. Source


Bressac M.,ACRI ST | Bressac M.,French National Center for Scientific Research | Guieu C.,French National Center for Scientific Research
Global Biogeochemical Cycles | Year: 2013

Abiotic iron removal processes such as scavenging can significantly and rapidly modify iron distribution in the dissolved-colloidal-particulate continuum. Therefore, these processes could be considered, in addition to ligand complexation, as a major control on atmospheric iron dissolution in seawater. In this work, we investigated the seasonal abiotic processes occurring once dust deposited on surface seawater using a series of artificial seeding experiments (allowing us to take into consideration the settling of particles on a 1 m depth layer). Here, we demonstrate that atmospheric dissolved iron concentration ([DFe]) is driven by the processes governed by the dissolved organic matter (DOM) pool. Following artificial dust seeding, an order magnitude range increase in the [DFe] (12 - 181 nmol L-1) was observed depending on the season. Under high and fresh DOM conditions (spring and summer), the rapid formation of aggregates induced a negative feedback on the [DFe] through scavenging, while a fraction of the DFe was likely organically complexed. In contrast, in low-DOM surface waters (winter), aggregation was not observed, allowing a very large transient increase in [DFe] (181 nmol L-1) before being removed by adsorption onto settling particles. A key result of the findings is that depending on the age and quantity of DOM, the "lithogenic carbon pump" is likely a major pathway for organic carbon export. Modeling studies should therefore relate both atmospheric iron dissolution in seawater and the intensity of the subsequent biological response, to the age and quantity of DOM. Key Points Atmospheric iron dissolution is driven by the dissolved organic matter poolSeawater biogeochemical conditions are a key determinant of iron solubilityLithogenic carbon pump is likely a major pathway for organic carbon export. © 2013. American Geophysical Union. All Rights Reserved. Source


Combi M.R.,University of Michigan | Makinen J.T.T.,Finnish Meteorological Institute | Bertaux J.-L.,University of Versailles | Quemerais E.,University of Versailles | And 2 more authors.
Icarus | Year: 2013

The all-sky hydrogen Lyman-alpha camera, SWAN (Solar Wind Anisotropies), on the SOlar and Heliospheric Observatory (SOHO) satellite made observations of the hydrogen coma of Comet C/2009 P1 (Garradd) throughout its apparition from August 15, 2011 through April 6, 2012. SOHO has been operating in a halo orbit around the Earth-Sun L1 Lagrange point since its launch in late 1995. Most water vapor produced by the comet is ultimately photodissociated into two H atoms and one O atom producing a huge atomic hydrogen coma that is routinely observed in the daily full-sky SWAN images in comets of sufficient brightness. Water production rates were calculated from 117 images over 8months of the apparition yielding about 1 observation every 2days on the average. The activity during much of the pre-perihelion leg was much larger than the post-perihelion leg and varied rather irregularly, not following the more typical steadily increasing trend with decreasing heliocentric distance. It varied generally between 1 and 3×1029s-1, with a peak value of 4×1029s-1 on November 3, 2011, 50days before perihelion. The elevated production rate from the very large SWAN field-of-view compared with smaller aperture observations could be consistent with water production from icy particles rather than from gas sublimated directly from the nucleus. Scenarios for the nature and production of the icy grain source are explored. During the post-perihelion leg the water production rate decreased more uniformly and typically from 2×1029s-1 at perihelion, approximately as r-3.2, where r is the heliocentric distance. This is consistent with water sublimation from a reasonably constant total surface area. © 2013 Elsevier Inc. Source


Combi M.R.,University of Michigan | Bertaux J.-L.,University of Versailles | Quemerais E.,University of Versailles | Ferron S.,ACRI ST | Makinen J.T.T.,Finnish Meteorological Institute
Astrophysical Journal Letters | Year: 2011

Global water production rates were determined from the Lyα emission of hydrogen around comet 103P/Hartley 2, observed with the SWAN (Solar Wind Anisotropies) all-sky camera on the SOHO spacecraft from 2010 September 14 through December 12. This time period included the November 4 flyby by the EPOXI spacecraft. Water production was three times lower than during the 1997 apparition also measured by SWAN. In 2010, it increased by a factor of 2.5 within one day on September 30 with a similar corresponding drop between November 24 and 30. The total surface area of sublimating water within 20 days of perihelion was 0.5km2, about half of the mean cross section of the nucleus. Outside this period it was 0.2km2. The peak value was 90%, implying a significant water production by released nucleus icy fragments. © 2011. The American Astronomical Society. All rights reserved. Source


Welsh B.Y.,University of California at Berkeley | Lallement R.,University of Versailles | Vergely J.-L.,ACRI ST | Raimond S.,University of Versailles
Astronomy and Astrophysics | Year: 2010

Aims: We present new high resolution (R > 50 000) absorption measurements of the NaI doublet (5889-5895 Å) along 482 nearby sight-lines, in addition to 807 new measurements of the CaII K (3933 Å) absorption line. We have combined these new data with previously reported measurements to produce a catalog of absorptions towards a total of 1857 early-type stars located within 800 pc of the Sun. Using these data we have determined the approximate 3-dimensional spatial distribution of neutral and partly ionized interstellar gas density within a distance-cube of 300 pc from the Sun. Methods: All newly recorded spectra were analyzed by means of a multi-component line profile-fitting program, in most cases using simultaneous fits to the line doublets. Normalized absorption profiles were fitted by varying the velocity, doppler width and column density for all intervening interstellar clouds. The resulting total column densities were then used in conjunction with the Hipparcos distances of the target stars to construct inversion maps of the 3D spatial density distribution of the NaI and CaII bearing gas. Results: A plot of the equivalent width of NaI versus distance reveals a wall of neutral gas at ∼80 pc that can be associated with the boundary wall to the central rarefied Local Cavity region. In contrast, a similar plot for the equivalent width of CaII shows no sharply increasing absorption at 80 pc, but instead we observe a slowly increasing value of CaII equivalent width with increasing sight-line distance sampled. Low values for the volume density of NaI (n NaI < 10 -9 cm -3) are generally found within 50 pc of the Sun, whereas values in the range 10 -8 > n NaI > 10 -10 cm -3 are found for sight-lines with distance >300 pc. Both high and low values of the volume density of CaII (n CaII) are found for sight-lines <30 pc, dependent on whether local gas cloudlets are encountered. For distances >100 pc a value of n CaII ∼ 10 -9 cm -3 is typical for most sight-lines, indicating that the distribution of CaII bearing gas is fairly uniform throughout the general ISM. Our three maps of the 3D spatial distribution of local neutral NaI absorption extend and improve upon the accuracy of similar maps initially presented by Lallement et al. (2003, A&A, 411, 447), with many new neutral interstellar gas features (such as low neutral density gas tunnels) in the local interstellar medium now being revealed for the first time. The maps of the 3D distribution of partially ionized CaII gas are the first of their kind to be presented and exhibit many spatial similarities to those of their equivalent NaI absorption maps. A major finding from both sets of maps is that the low density Local Cavity region is surrounded by a highly fragmented wall of higher density NaI and CaII gas clouds. The appearance of this broken boundary may be linked to the purported explosive origin of the Local Cavity. Maps of the distribution of CaII gas density reveal the presence of many partially ionized low density cloudlets that reside within the Local Cavity, and their newly derived 3D spatial contours confirm previous observations of the local gas by Redfield & Linsky (2008, ApJ, 673, 283). Both the NaI and CaII maps suggest that the Local Cavity may contain several low density sub-cavities that are surrounded by thin filaments of neutral and/or partially ionized gas. However, further observations will be required to confirm the existence of a collection of cell-like interstellar cavities. The new maps also reveal several sight-lines where CaII absorption is high and the corresponding NaI absorption is low, and vice-versa. Such regions are probably influenced by the effects of the local stellar ionization field which can significantly affect the observed NaI/CaII column density ratio. Plots of this ratio as a function of distance for stars located near to the galactic plane show values in the range 0.1 to 1.0 for sight-lines with distances <80 pc. However, ratio values of between 0.5 and 20 are typical for more distant sight-lines. The highest values of the NaI/CaII ratio are found towards l ∼ 150° in the direction of the Taurus dark clouds, with ratio values in the narrower range of 0.1 to 5 being found in galactic quadrant 3. © 2010 ESO. Source

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