Automated Processing of LASCO Coronal Images: Spurious Point-Source-Filtering and Missing-Blocks Correction

We report on automated procedures for correcting the images of the LASCO coronagraph for i) spurious quasi-point-sources such as the impacts of cosmic rays, stars, and planets, and ii) the absence of signal due to transmission errors or dropouts, which results in blocks of missing information in the images. Correcting for these undesirable artifacts is mandatory for all quantitative works on the solar corona that require data inversion and/or long series of images, for instance. The nonlinear filtering of spike noise or point-like objects is based on mathematical morphology and implements the procedure opening by morphological reconstruction. However, a simple opening filter is applied whenever the fractional area of corrupted pixels exceeds 50 % of the original image. We describe different strategies for reconstructing the missing information blocks. In general, it is possible to implement the method of averaged neighbors using the two images obtained immediately before and after the corrupted image. For the other cases, and in particular when missing blocks overlapped in three images, we developed an original procedure of weighted interpolation along radial profiles from the center of the Sun that intercept the missing block(s). This procedure is also adequate for the saturated images of bright planets (such as Venus) that bleed along the neighboring pixels. Missing blocks in polarized images may generally be reconstructed using the associated unpolarized image of the same format. But in the case of overlapping missing blocks, we implemented our procedure of weighted interpolation. All tests performed on numerous LASCO-C2 images at various periods of solar activity (i.e. varying complexity of the structure of the corona) demonstrate the excellent performance of these new procedures, with results vastly superior to the methods implemented so far in the pipeline-processing of the LASCO images.     

Three-Dimensional Electron Density from Tomographic Analysis of LASCO-C2 Images of the K-Corona Total Brightness

We present the first quantitative three-dimensional (3D) tomographic reconstructions of electron density from coronagraph measurements of the K-corona’s total brightness (B) made by LASCO-C2 on SOHO. This is possible because new calibrations of the LASCO-C2 images in both polarized brightness (pB) and B have now been made for the entire mission. The B and pB reconstructions are compared, and the differences are explained in terms of line of sight weighting functions in Thomson scattering. We conclude that the LASCO-C2 B archive, which is vastly larger than the pB archive, will be a very valuable resource for determining the 3D electron density throughout the SOHO mission which started taking data in 1996.     

On the size distribution and physical properties of interplanetary dust grains

This paper synthesizes information on the size distribution and physical properties of interplanetary dust grains obtained from analyses of lunar microcraters performed until 1979. The different aspects of these analyses (counting methods, simulation, calibrations) are summarized and a large amount of data is collected and discussed in order to clarify past contradictions. The number of small microcraters (D_c < 5 μm) is found to be higher than previously derived and the ratio P/D_c (depth to crater diameter) to depend upon their sizes. All results converge to a two-component dust population: Population 1 consists principally of large grains (d > 2 μm) with density typical of silicates while Population 2 consists of small grains (d < 2 μm) with higher density typical of iron, with a minor component of silicates. The conclusion appears to be further supported by spatial measurements and collection experiments. Fluffy grains of very low density (0.3 g/cm³) are probably not present to a large extent.     

Meteoroid Streams Associated to Comets 9P/Tempel 1 and 67P/Churyumov-Gerasimenko

The meteoroid streams associated to short-period comets 9P/Tempel 1 (the target of the Deep Impact mission)^. and 67P/Churyumov-Gerasimenko (the target of the Rosetta mission) are studied. Their structure is overwhelmingly under the control of Jupiter and repeated relatively close encounters cause a reversal of the direction of the spatial distribution of the stream relative to the comet* an initial stream trailing the comet as usually seen eventually collapses, becomes a new stream leading the comet and even splits into several components. Although these two comets do not produce meteor showers on Earth, this above feature shows that meteor storms can occur several years before the perihelion passage of a parent body.     

Collisional processes among interplanetary dust grains: An unlikely origin for the β meteoroids

The question of the collisional production of the β meteoroids is reexamined incorporating recent experimental results (A. Fugiwara, G. Kamimoto, A. Tsukamoto, 1977, Icarus31, 277–288). The collisional model yields a flux of fragments supported by the conservation of mass flux which does not account by far for the observed flux of submicron grains. Particles larger than about 100 μm will be destroyed by collisions inside 1 AU, well before they can get near the Sun. The existence of two independent populations of interplanetary dust grains as proposed by L. B. Le Sergeant and Ph. L. Lamy (1978, Nature266, 822–824; 1980, Icarus43, 350–372) appears reinforced. It is proposed that the bulk of submicron grains does not necessarily travel in hyperbolic orbits and that β meteoroids may be a phenomenon—possibly transitory—of limited importance.     

Depositional history of the Helgoland mud area,German Bight,North Sea

The Helgoland mud area in the German Bight is one of the very few sediment depocenters in the North Sea. Despite the shallowness of the setting (<30 m water depth), its topmost sediments provide a continuous and high-resolution record allowing the reconstruction of regional paleoenvironmental conditions for the time since ~400 a.d. The record reveals a marked shift in sedimentation around 1250 a.d., when average sedimentation rates drop from >13 to ~1.6 mm/year. Among a number of major environmental changes in this region during the Middle Ages, the disintegration of the island of Helgoland appears to be the most likely factor which caused the very high sedimentation rates prior to 1250 a.d. According to historical maps, Helgoland used to be substantially bigger at around 800 a.d. than today. After the shift in sedimentation, a continuous and highly resolved paleoenvironmental record reflects natural events, such as regional storm-flood activity, as well as human impacts at work at local to global scales, on sedimentation in the Helgoland mud area.     

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.     

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.     

Radiation forces on small particles in the solar system

We present a new and more accurate expression for the radiation pressure and Poynting-Robertson drag forces; it is more complete than previous ones, which considered only perfectly absorbing particles or artificial scattering laws. Using a simple heuristic derivation, the equation of motion for a particle of mass m and geometrical cross section A, moving with velocity v through a radiation field of energy flux density S, is found to be (to terms of order $\text{v}\text{c}$)

$\text{m}\text{v}\text{?}\text{= (}\text{SA}\text{c}\text{)Q}{}_{\mathrm{<mtext>pr</mtext>}}\text{[(1 ?}\text{r}\text{?}\text{c}\text{)}\text{S}\text{?}\text{?}\text{v}\text{c}\text{]}$

, where ? is a unit vector in the direction of the incident radiation, $\text{r}\text{?}$ is the particle's radial velocity, and c is the speed of light; the radiation pressure efficiency factor Q_pr ≡ Q_abs + Q_sca(1 ? 〈cos α〉), where Q_abs and Q_sca are the efficiency factors for absorption and scattering, and 〈cos α〉 accounts for the asymmetry of the scattered radiation. This result is confirmed by a new formal derivation applying special relativistic transformations for the incoming and outgoing energy and momentum as seen in the particle and solar frames of reference. Q_pr is evaluated from Mie theory for small spherical particles with measured optical properties, irradiated by the actual solar spectrum. Of the eight materials studied, only for iron, magnetite , and graphite grains does the radiation pressure force exceed gravity and then just for sizes around 0.1 μm; very small particles are not easily blown out of the solar system nor are they rapidly dragged into the Sun by the Poynting-Robertson effect. The solar wind counterpart of the Poynting-Robertson drag may be effective, however, for these particles. The orbital consequences of these radiation forces-including ejection from the solar system by relatively small radiation pressures-and of the Poynting-Robertson drag are considered both for heliocentric and planetocentric orbiting particles. We discuss the coupling between the dynamics of particles and their sizes (which diminish due to sputtering and sublimation). A qualitative derivation is given for the differential Doppler effect, which occurs because the light received by an orbiting particle is slightly red-shifted by the solar rotation velocity when coming from the eastern hemisphere of the Sun but blue-shifted when from the western hemisphere; the ratio of this force to the Poynting-Robertson force is $\text{(}\text{R}{}_{\mathrm{\odot }}\text{r}\text{)}{}^2\text{[(}\text{w}{}_{\mathrm{\odot }}\text{n}\text{) ? 1]}$, where R_⊙ and w_⊙ are the solar radius and spin rate, and n is the particle's mean motion. The Yarkovsky effect, caused by the asymmetry in the reradiated thermal emission of a rotating body, is also developed relying on new physical arguments. Throughout the paper, representative calculations use the physical and orbital properties of interplanetary dust, as known from various recent measurements.