Relationship between the ionospheric conductance, field aligned current, and magnetopause geometry: Global MHD simulations

The role of the ionospheric conductance in the solar wind-magnetosphere coupling is studied using global MHD simulations. The simulations with varying conductance and a constant solar wind input show that the field-aligned currents, whose magnitude depends on the ionospheric conductance, affect the size of the magnetopause at the flanks by increasing the local magnetic pressure and thus altering the surface equilibrium at the magnetopause. A current system that generates the magnetic stresses required to account for the location and geometrical structure of the magnetosphere observed in the simulations is proposed.     

Direct measurement of the size, shape, and pole of 511 Davida with Keck AO in a single night

Using the high-quality data set of 165 images taken at 11 epochs over the 5.13 h rotation of the large C-type Asteroid 511 Davida, we find the dimensions of its triaxial ellipsoid model to be 357±2×294±2×231±50 km. The images were acquired with the adaptive optics system on the 10 m Keck II telescope on December 27, 2002. The a and b diameters are much better determined than previously estimated from speckle interferometry and indirect measurements, and our mean diameter, (abc)^1/3=289±21 km, is 19% below previous estimates. We find the pole to lie within 2° of [RA=295°; Dec=0°] or in Ecliptic coordinates [λ=297°; β=+21°], a significant improvement to the pole direction. Otherwise, previous determinations of the axial ratios agree with our new results. These observations illustrate that our technique of finding the dimensions and pole of an asteroid from its changing projected size and shape is very powerful because it can be done in essentially one night as opposed to decades of lightcurves. Average departures of 3% (5 km) of the asteroid's mean radius from a smooth outline are detected, with at least two local positive-relief features and at least one flat facet showing approximately 15 km deviations from the reference best-fit ellipsoid. The facet is reminiscent of large global-scale craters on Asteroid 253 Mathilde (also a C-type) when seen edge-on in close-up images from the NEAR mission flyby. We show that giant craters (up to 150 km diameter, the size of the largest facets seen on Davida) can be expected from the impactor size distribution, without likelihood of catastrophic disruption of Davida.     

ROOSEVELT COUNTY 027: A LOW-SHOCK UREILITE WITH INTERSTITIAL SILICATES AND HIGH NOBLE GAS CONCENTRATIONS

The lightly-shocked ureilite RC027 was found in Roosevelt County, New Mexico in 1984. In terms of petrography, texture, mineral compositions, bulk chemical composition, and oxygen isotopic composition it is a typical ureilite. It contains ～75% olivine (Fo 79.4) and 25% pigeonite (mg 81.3, Wo 8.0), with intergranular graphite and (Fe, Ni) metal. It also contains less than 1% of fine-grained, interstitial silicate material, which had not previously been recognized in any ureilite. This material is an assemblage of low-Ca pyroxene (Wo 3.5–9, mg 87–93), augite (Wo 24–36, mg 90–98), glass (typically ～95% SiO₂, 4% Al₂O₃, 0.5% Na₂O), and crystalline SiO₂. This material has an igneous texture, indicating that it crystallized from an interstitial liquid. Low-Ca pyroxene compositions indicate that the interstitial liquid was not in equilibrium with core pigeonite and olivine and cannot have been either an evolved intercumulus liquid or a low-degree partial melt. It may contain a component of shock-melted olivine and pigeonite, although petrographic evidence indicates that it could not have been an in situ shock melt. One sample of RC027 has a V-shaped rare earth element pattern, typical of ureilites. Another is depleted in light rare earth elements (LREE), similar to acid-treated samples of ureilites, which suggests that LREE in ureilites are contained in an inhomogeneously-distributed phase. RC027 shows the strongest olivine preferred-orientation yet observed in a ureilite. Its fabric is characteristic of fabrics formed by tabular minerals in a fluid laminar flow regime and is unlike those formed by syntectonic recrystallization and plastic flow. The elemental and isotopic compositions of noble gases in RC027 are typical of previously analyzed ureilites. This result indicates that there is no correlation of noble gas content with degree of shock in ureilites, and thus suggests that the gases were present in the ureilite material before shock. Cosmogenic He and Ne contents indicate cosmic ray exposure ages of 1.7 and 1.9 Myr, respectively. Thus, RC027 is not paired with Kenna (a ureilite also found in Roosevelt County), which has an exposure age of ～33 Myr.     

Thomson Thick X-Ray Absorption in a Broad Absorption Line Quasar, PG 0946+301

We present results of new ASCA observations of the low-luminosity active galactic nucleus (LLAGN) NGC 4579 obtained on 1998 December 18 and 28, and we report on the detection of variability of an iron K emission line. The X-ray luminosities in the 2-10 keV band for the two observations are nearly identical (LX approximately 2x1041 ergs s(-1)), but they are approximately 35% larger than that measured in 1995 July by Terashima et al. An Fe K emission line is detected at 6.39+/-0.09 keV (source rest frame), which is lower than the line energy 6.73+0.13-0.12 keV in the 1995 observation. If we fit the Fe lines with a blend of two Gaussians centered at 6.39 and 6.73 keV, the intensity of the 6.7 keV line decreases, while the intensity of the 6.4 keV line increases, within an interval of 3.5 yr. This variability rules out thermal plasmas in the host galaxy as the origin of the ionized Fe line in this LLAGN. The detection and variability of the 6.4 keV line indicates that cold matter subtends a large solid angle viewed from the nucleus and that it is located within approximately 1 pc from the nucleus. It could be identified with an optically thick standard accretion disk. If this is the case, a standard accretion disk is present at the Eddington ratio of Lbol/LEdd approximately 2x10-3. A broad disk-line profile is not clearly seen, and the structure of the innermost part of accretion disk remains unclear.     

Ureilitic breccias: clues to the petrologic structure and impact disruption of the ureilite parent asteroid

The majority of the 143 ureilite meteorites are monomict (unbrecciated) ultramafic rocks, which represent the mantle (olivine+low-Ca pyroxene residues and less abundant cumulates) of a partially melted (25–30%), carbon-rich asteroid 125 km in radius. Accumulated petrologic and geochemical studies of these meteorites have led to a picture of a ureilite parent body (UPB) that was stratified in mg#, pyroxene abundance and pyroxene type, due to the pressure dependence of carbon redox control, and which preserved a pre-magmatic heterogeneity in Δ¹⁷O. The absence, however, of ureilitic crustal rocks (i.e. basalts) in the meteorite record, leads to significant gaps in our knowledge of the geologic history of the UPB.

Ureilitic breccias provide considerable information that cannot be obtained from the monomict samples, and help to fill in those gaps. Fourteen ureilites are polymict breccias (at least three of which contain solar wind gases) that formed in a regolith. They contain a variety of clast types representing indigenous ureilitic lithologies not known among the monomict samples, as well as several types of non-indigenous impactor materials. In addition, one ureilite (FRO 93008) is a dimict breccia, consisting of two ultramafic lithologies that could not have formed in close proximity on the UPB.

Several feldspathic lithologies representing melts complementary to the monomict ureilite residues or cumulates have been recognized in polymict ureilites. From these lithologies we infer that melt extraction on the UPB was a rapid, fractional process in which trace element and oxygen isotopic equilibrium was not achieved. The majority of melts that reached the surface erupted explosively (due to high contents of CO/CO₂) and were lost into space. Thus, it is likely that the UPB never had an extensive basaltic crust. Melts generated at the shallowest depths and late fractionates, in which carbon had largely been consumed by reduction, were the most likely to have been preserved. Our sample of the UPB is limited to depths equivalent to 100 bars pressure or less, but minor augite-bearing feldspathic lithologies and related cumulates may represent melts derived from deeper.

In addition, we infer that the UPB was catastrophically disrupted, while still hot, by an impacting projectile. Meter-sized ejecta from this impact reaccreted into one or more daughter bodies, on which the brecciated ureilites formed. Ureilite meteorites are derived from these offspring, rather than from the UPB. The remnant of the original UPB may consist largely of olivine plus augite, and thus not resemble the majority of ureilites.     

Location of segregated ice in frost-susceptible soil

X-ray photography has been used to locate the position of a growing ice lens in soil under laboratory conditions in order to establish the temperature of the actively growing face by means of its position in the thermal gradient field. It can be shown that in a general way the phase change temperatures are predictable from the Clapeyron equation. The structure of the ice phase also appears to follow the predictions of the heave rate equation proposed by Penner and Ueda (1978). When heaving occurs at low overburden pressures, there is a tendency for the ice lens to be very discrete and essentially soil free; at higher pressures and under similar thermal gradients it tends to develop in a more diffuse band and over a much wider temperature range.     

Phosphoran pyroxene and olivine in silicate inclusions in natural iron-carbon alloy,Disko Island,Greenland

A survey of literature data on the P contents of olivine, pyroxene, and garnet in mantle xenoliths and the bulk P contents of mantle xenoliths shows that available data is ambiguous as to whether mantle P is primarily contained in silicates or in phosphates, even though phosphates are rarely observed in mantle xenoliths. Garnet probably contains more of the mantle's P than olivine or pyroxene. Bulk P contents of mantle xenoliths vary significantly. Estimation of the primitive mantle P content from xenoliths requires interpretation of the partial melting and contamination histories of the xenoliths.     

Aerosol radiative properties in the semiarid Western United States

Characterization of aerosol optical properties, such as aerosol optical depth, Angstrom exponent, and volume size distribution at the semiarid site of Tombstone Arizona (31°23′N, 110°05′W, 1408 m) will be presented for one annual cycle. In this region, extensive observations of selected optical parameters such as aerosol optical depth (AOD) have been made in the past and reported on in the literature. Less is known about other optical characteristics that are important in climate modeling and remote sensing. New observational techniques and inversion methods allow for the expansion of the earlier information. Observations have been taken with a state of the art sun photometer for a 1-year period and their analysis will be presented here. Monthly mean AODs at 500 nm were found to be in the range of 0.03–0.12; the monthly mean Angstrom exponent ranged from 0.9 to 1.6, being higher in spring and summer and lower in late fall and winter. Volume size distributions exhibit clear dominance of smaller particles, with a gradual increase in size from winter to spring and into summer. Annual variation of the radii of the smaller and the larger particles ranged between 0.05–0.4 and 4–8 μm, respectively. Radiance measurements at 940 nm were used to estimate precipitable water. The retrieved values compared within limits of uncertainty with independently derived estimates from the National Center for Environmental Prediction (NCEP) regional weather forecast model. An interesting outcome from this study was the consistency found in aerosol optical depths as observed in this study and those derived about two decades ago.     

Keck Observations of Solar System Objects: Perspectives for Extremely Large Telescopes

From differential tracking techniques, required for appulse observations of KBOs with Laser Guide Star Adaptive Optics (LGSAO), to developing methods for collecting spectra at the precise moment of a predicted impact, each Solar System observation conducted on a large telescope presents a unique set of challenges. We present operational details and some key science results from our science program, adaptive optics observations of main belt asteroids and near earth objects; as well as the technical and operational details of several Keck Solar System observations conducted by other teams: the impact of Shoemaker-Levy 9 on Jupiter, volcanoes on Io, the Deep Impact mission to Comet 9P/Tempel 1, and recent observations of Pluto’s moons Nix and Hydra. For each of these observations, we draw from our Keck experience to predict what challenges may lie ahead when similar observations are conducted on next generation telescopes.