The secular variation of cometary magnitude

This paper calculates the mean variation in absolute magnitude per perihelion passage, ΔH₁₀, for short-period comets from the data of Vsekhsvyatskii and finds a value of 0.30 ± 0.06. Other mechanisms used for estimating cometary decay are reviewed and it is concluded that a more probable value for ΔH₁₀ is about 0.002. Reasons for the discrepancy between these two values are given.     

LONG-TERM SOIL AND VEGETATION RECOVERY IN FIVE SEMIARID MONTANA GHOST TOWNS*

Five semiarid Montana ghost towns abandoned for more than 45 years were studied to understand better the nature of soil and vegetation recovery following severe human impacts. Discriminant analysis was used to interpret and classify variation among land-use intensity groups. Recovery at the five towns was strongly linked to the degree of the initial soil disturbance, vegetation type, and precipitation. Recovery of the vegetation to ambient conditions was far from complete in all but one town.     

Geochemistry of polymict ureilite EET83309, and a partially-disruptive impact model for ureilite origin

Abstract— For most elements, polymict ureilite EET83309 shows no significant compositional difference from other ureilites, including ordinary (“monomict”) ureilites. Polymict ureilites appear to be mixtures of a wide variety of ordinary ureilites, with little dilution by “foreign” extra-ureilitic materials. Thus, they apparently were mixed (i.e., the ureilites in general formed) on a very small number of parent bodies. In one respect, polymict ureilites do stand out. Along with the only other polymict ureilite that has been analyzed for REE (Nilpena), EET83309 has much higher concentrations of light-middle REE than most ordinary ureilites. Despite these relative enrichments in LREE, polymict ureilites are nearly devoid of basaltic (Al-rich) material. A basaltic component should have formed along with (and presumably above) the ultramafic ureilites, in any closed-system differentiation of an originally chondritic asteroid. This scarcity of complementary basaltic materials may be an important clue to ureilite origins. We suggest that ureilites originated as paracumulates (mushy, cumulate-like, partial melt residues) deep within a primordially-heated asteroid or asteroids. While still largely molten, the asteroid was severely disrupted, and most of its external basaltic portion was permanently blown away, by impact of a large, C-rich projectile. This partially-disruptive impact tended to permeate the paracumulates with C-rich, noble-gas-rich, and ¹⁶O-rich magma derived mainly from shock-melting of the projectile. After reaccumulation and cooling, the resultant mixtures of cumulus mafic silicates with essentially “foreign” C-matrix became “monomict” ureilites. Further small impacts produced polymict ureilites as components of a newly-developed, basalt-poor megaregolith. The consistently moderate pyroxene/olivine ratios of the ureilites are as expected for partial melt residues, but not for cumulate (sensu stricto) rocks. The final projectile/target mixing ratio tended to be greatest among the more magnesian and pyroxene-rich portions of the paracumulate, because these portions were lowest in density, and thus concentrated toward the upper surface of the paracumulate layer. As a result, ureilites show correlations among C, Δ¹⁷O, and silicate-core mg. This model appears to reconcile many paradoxical aspects of ureilite composition (primitive, near-chondritic, except depleted in basalt, diverse Δ¹⁷O) and petrography (igneous, cumulate-like).     

Electron acceleration in solar flares

For the period September 1978 to December 1982 we have identified 55 solar flare particle events for which our instruments on board the ISEE-3 (ICE) spacecraft detected electrons above 10 MeV. Combining our data with those from the ULEWAT spectrometer (MPI Garching and University of Maryland) electron spectra in the range from 0.1 to 100 MeV were obtained. The observed spectral shapes can be divided into two classes. The spectra of the one class can be fit by a single power law in rigidity over the entire observed range. The spectra of the other class deviate from a power law, instead exhibiting a steepening at low rigidities and a flattening at high rigidities. Events with power-law spectra are associated with impulsive (<1 hr duration) soft X-ray emission, whereas events with hardening spectra are associated with long-duration (<1 hr) soft X-ray emission. The characteristics of long-duration events are consistent with diffusive shock acceleration taking place high in the corona. Electron spectra of short-duration flares are well reproduced by the distribution functions derived from a model assuming simultaneous second-order Fermi acceleration and Coulomb losses operating in closed flare loops.     

Beta-dispersion of low-frequency Rossby waves

An investigation is made into the dispersion of oceanic internal Rossby waves at annual and semi-annual frequencies. Turning of the group velocity vector due to latitudinal variations in the radius of deformation cannot be neglected, particularly in basins as large as the Pacific. This turning allows disturbances to propagate from high lattitudes into the equatorial zone and distorts the solutions in the western part of the basin. For no mean flow, and a coastline aligned north-south, an almost exact focus of wave energy is found very close to the equator at a distance of just under $\text{πc/4ω}$ from the eastern boundary, where $\text{c}$ is the eigenspeed of a high-frequency internal wave mode, and ω is the angular frequency of the low-frequency wave being studied. The focus depends on a long meridional wavelength excited at the coast, and a frequency small compared with $\text{c/a}$, where $\text{a}$ is the radius of the Earth. For the lowest baroclinic mode and waves of annual period, this distance is about 12 000 km. Equivalence of the ray theory and the theory of equatorial meridional modes is demonstrated for the simple cases where the latter applies.The effects of mean currents and irregular coastlines are examined. Barotropic mean currents may change the turning latitude and ray shapes, inducing critical layers and enhancing reflection. Baroclinic mean currents are seen to affect the rays by simply changing the speed in proportion to the depth of the thermocline. As long as the mean currents are geostrophically balanced, no “effective beta” term from variations in the thermocline depth appears, in contrast to the topographic Rossby wave problem.     

Hydrodynamical models of presolar nebula formation

Theories of solar system formation often presuppose the existence of the protosun and an accompanying preplanetary nebula. Numerical three-dimensional calculations are presented which demonstrate the possibility of formation of a co-orbital, triple protostellar system, which is unstable to decay to a binary and an ejected single star. The calculations are used to construct a plausible scenario for presolar nebula formation based on a hierarchy of collapse and fragmentation. While this sequence is unlikely to produce many single stars, it remains a possible sequence for the formation of the presolar nebula.