Rotational state of the nucleus of Comet 9P/Tempel 1: Results from Hubble Space Telescope observations in 2004

The nucleus of Comet 9P/Tempel 1 was first observed with the Hubble Space Telescope (HST) in December 1997 [Lamy, P., Toth, I., A'Hearn, M.F., Weaver, H., Weissman, P.R., 2001. Icarus 154, 337-344], but the temporal coverage was insufficient to determine its rotational period. Because the success of the Deep Impact mission was critically dependent on understanding the rotational state and approximate shape and size of the nucleus, we extensively re-observed 9P/Tempel 1, this time with the Advanced Camera for Surveys (HST/ACS), from May 7.9 to 9.5, 2004 (UT). At the mid-point of the observing window, the comet was 3.52 AU from the Sun, 4.03 AU from the Earth, and at a solar phase angle of 13.3°. The program was comprised of 18 separate visits, each one corresponding to an HST orbit filled with 3 ACS exposures of either 800 or 857 s duration with the F606W broadband filter. These very deep exposures revealed a star-like object, without any apparent coma. The light curve, defined by 49 data points, is characterized by a mean apparent V magnitude of 21.8 and an amplitude of 0.5 mag, indicating that we were viewing the varying cross-section of a rotating, elongated body. The periodicity was analyzed with seven different techniques yielding a rotational period in the range 39.40 to 43.00 h, and a mean value of 41.27±1.85 h (1σ). Using an albedo pV=0.04 and a linear phase law with a coefficient , we determined an effective radius of 3.01 km; a possible prolate spheroid solution has semi-axes a=3.71 km, b=2.36 km and a minimum axial ratio a/b∼1.57. By comparing the light curves obtained in 1997 and in 2004, we were able to constrain the phase function of the nucleus. Finally, an upper limit of Afρ<0.04 cm is set based on the non-detection of the coma.     

Photometric calibration of the LASCO-C2 coronagraph for Solar System objects

We present a photometric calibration of the SOHO/LASCO-C2 coronagraph appropriate to Solar System objects based on the extensive analysis of all stars down to magnitude V=8 which transited its field-of-view during the past nine years of operation (1996-2004). An automatic procedure was developed to analyze some 143,000 images, and to detect, locate and measure those stars. Aperture photometry was performed using three different aperture sizes and the zero points of the photometric transformations between the LASCO-C2 magnitudes for its three filters (orange, blue and red) and the standard V magnitudes were determined after introducing a correction for the color of the stars. The calibration coefficients for the surface photometry of extended sources were then derived from the zero points. An analysis of their temporal evolution indicates a slight decrease of the sensitivity of LASCO-C2 at a rate of ∼0.7% per year.     

Estimating the Size of Hale-Bopp's Nucleus

A variety of independent methods have been used to estimate the size of the nucleus of comet Hale-Bopp. Several groups have analyzed optical and infrared images of the comet and claim to detect the signature of the nucleus, despite the presence of a strong coma. A detection of the nucleus was also claimed during mm- and cm-wave observations of Hale-Bopp shortly before perihelion. A team of observers detected the occultation of a star by the nucleus of Hale-Bopp in October 1996. The maximum observed gas production rate of the comet near perihelion can be used to place a lower limit on the size of the nucleus. This paper critically reviews the many different methods used to constrain the size of Hale-Bopp's nucleus. All of the techniques are affected by systematic errors that can be difficult to quantify precisely. Nevertheless, the available evidence strongly suggests that the nucleus of Hale-Bopp has an effective radius of at least 15 km and is probably in the range 20–35 km. Thus, the prodigious gas and production rates from this comet are naturally explained by its unusually large size. This revised version was published online in July 2006 with corrections to the Cover Date.     

Lasco Observations of Disconnected Magnetic Structures Out to Beyond 28 Solar Radii During Coronal Mass Ejections

Two coronal mass ejections have been well observed by the LASCO coronagraphs to move out into the interplanetary medium as disconnected plasmoids. The first, on July 28, 1996, left the Sun above the west limb around 18:00 UT. As it moved out, a bright V-shaped structure was visible in the C2 coronagraph which moved into the field-of-view of C3 and could be observed out to beyond 28 solar radii. The derived average velocity in the plane of the sky was 110 ± 5 km s^-1 out to 5 solar radii, and above 15 solar radii the velocity was 269 ± 10 km s^-1. Thus there is evidence of some acceleration around 6 solar radii. The second event occurred on November 5, 1996 and left the west limb around 04:00 UT. The event had an average velocity in the plane of the sky of ∼54 km s^-1 below 4 R⊙, and it accelerated rapidly around 5 R⊙ up to 310 ± 10 km s^-1. In both events the rising plasmoid is connected back to the Sun by a straight, bright ray, which is probably a signature of a neutral sheet. In the November event there is evidence for multiple plasmoid ejections. The acceleration of the plasmoids around a projected altitude of 5 solar radii is probably a manifestation of the source surface of the solar wind. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1004994214697     

SOHO/LASCO observation of an outburst of Comet 2P/Encke at its 2000 perihelion passage

Comet 2P/Encke was observed with the SOHO/LASCO C2 and C3 coronagraphs over a time interval of 11 days, starting 4 days before its September 2000 perihelion passage and through several broadband visible filters. The lightcurve reveals an outburst which started 4.9 days after perihelion, with the brightness of the coma increasing by 1.5 mag in just a few hours and progressively decreasing thereafter, probably going back to its original state in about 9 days. The color information indicates that an approximately solar color continuum was detected, implying that the observed signals were dominated by solar light scattered off submillimetric dust grains. We propose that the rapid migration of the subsolar point over the southern hemisphere during the perihelion passage activates one or several new active regions enriched in submillimetric grains, with the observed outburst corresponding to the initial blow-off of their mantle. This scenario is consistent with other observations and implies that the south polar region of the nucleus of 2P/Encke has very distinct properties.     

North Atlantic control on precipitation pattern in the eastern Mediterranean/Black Sea region during the last glacial

Based on proxy records from western Black Sea cores, we provide a comprehensive study of climate change during the last glacial maximum and late-glacial period in the Black Sea region. For the first time we present a record of relative changes in precipitation for NW Anatolia based on variations in the terrigenous supply expressed as detrital carbonate concentration. The good correspondence between reconstructed rainfall intensity in NW Anatolia and past western Mediterranean sea surface temperatures (SSTs) implies that during the glacial period the precipitation variability was controlled, like today, by Mediterranean cyclonic disturbances. Periods of reduced precipitation correlate well with low SSTs in the Mediterranean related to Heinrich events H1 and H2. Stable oxygen isotopes and lithological and mineralogical data point to a significant modification in the dominant freshwater/sediment source concomitant to the meltwater inflow after 16.4 cal ka BP. This change implies intensification of the northern sediment source and, with other records from the Mediterranean region, consistently suggests a reorganization of the atmospheric circulation pattern affecting the hydrology of the European continent. The early deglacial northward retreat of both atmospheric and oceanic polar fronts was responsible for the warming in the Mediterranean region, leading simultaneously to more humid conditions in central and northern Europe.     

Climate variability at the southern boundaries of the Namib (southwestern Africa) and Atacama (northern Chile) coastal deserts during the last 120,000 yr

In this study, we present grain-size distributions of the terrigenous fraction of two deep-sea sediment cores from the SE Atlantic (offshore Namibia) and from the SE Pacific (offshore northern Chile), which we ‘unmix’ into subpopulations and which are interpreted as coarse eolian dust, fine eolian dust, and fluvial mud. The downcore ratios of the proportions of eolian dust and fluvial mud subsequently represent paleocontinental aridity records of southwestern Africa and northern Chile for the last 120,000 yr. The two records show a relatively wet Last Glacial Maximum (LGM) compared to a relatively dry Holocene, but different orbital variability on longer time scales. Generally, the northern Chilean aridity record shows higher-frequency changes, which are closely related to precessional variation in solar insolation, compared to the southwestern African aridity record, which shows a remarkable resemblance to the global ice-volume record. We relate the changes in continental aridity in southwestern Africa and northern Chile to changes in the latitudinal position of the moisture-bearing Southern Westerlies, potentially driven by the sea-ice extent around Antarctica and overprinted by tropical forcing in the equatorial Pacific Ocean.     

The nucleus of comet C/1983 H1 IRAS-Araki-Alcock

We present a synthetic analysis of all available infrared (2-20 μm) and radio (1.3-6.1 cm) observations of comet C/1983 H1 IRAS-Araki-Alcock performed during its close approach to Earth in May 1983. We implement a model based on a spherical nucleus with a macroscopic mosaic of small and numerous active and inactive regions, and take into account the strong phase effect in the calculations of the thermal flux (often neglected in past interpretations). The orientation of the spin axis is assumed to be that determined by Sekanina [1988. Astron. J. 95, 1876-1894]. Additional constraints coming from visible photometry, measurements of the water production rate and the temporal variations of the cometary activity are introduced. We derive an equivalent nucleus radius of 3.4±0.5 km, consistent with a geometric albedo of 0.04 ±0.01 and a phase coefficient in the visible, and an active fraction of 2.9 ±1.9%. Although the nucleus is probably elongated as found in the past (Sekanina, 1988), we show that the relevant measurements were likely contaminated by the contribution of a variable coma.     

The size and thermal properties of the nucleus of Comet 22P/Kopff

We detected the nucleus of Comet 22P/Kopff at 4.87 AU from the Sun with the two IRS peak-up cameras of the Spitzer Space Telescope on April 19, 2007. Using the thermal model of [Groussin, O., and 15 colleagues, 2007. Icarus 187, 16-25], we derive a nucleus size of 1.89±0.16 km, in agreement with [Lamy, P., Toth, I., Jorda, L., Groussin, O., A'Hearn, M.F., Weaver, H.A., 2002. Icarus 156, 442-455], and a thermal inertia .     

Three-Dimensional Kinematics of Coronal Mass Ejections from STEREO/SECCHI-COR2 Observations in 2007 – 2008

We present a new method to perform the three dimensional characterization of coronal mass ejections (CMEs) using stereoscopic images obtained with the STEREO/SECCHI-COR2 coronagraphs. Two approaches are proposed, and each associated algorithm gives the trajectory of the CME and its kinematical properties (velocity and acceleration profiles) intended for space weather forecast. The first approach is based on forward modeling appropriate to the reconstruction of surfaces in an optically thin medium, and performs a local approximation of the observed CME by a hemispherical shell, thus tracking the leading edge of the event. The second approach is based on tracking the center of gravity of the radiance of the CME in the images. More than 16?000 blind tests have been performed to assess the performance of each algorithm. For that purpose, we used three distinct libraries of simulated images of CMEs that correspond to three CME models: hemispherical shell, flux rope, and cloud-like. The two methods are applied to a set of CMEs observed in 2007 and 2008 by the SECCHI-COR2 coronagraphs, and when possible, our results are compared to those already published. The determinations of the direction of propagation and of the velocity are generally found in good agreement.