Solar rotation, 1966–1978

Photospheric and chromospheric spectroscopic Doppler rotation rates for the full solar disk are analyzed for the period July, 1966 to July, 1978. An approximately linear secular increase of the equatorial rate of 3.7% for these 12 years is found (in confirmation of Howard, 1976). The high latitude rates above 65 ° appear to vary with a peak-to-peak amplitude of 8%, or more, phased to the sunspot cycle such that the most rapid rotation occurs at, or following, solar maximum. The chromosphere, as indicated by H, has continued to rotate on the average 3% faster than the photosphere agreeing with past observations. Sources of error are discussed and evaluated.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

3D triangulation of a Sun-grazing comet

The bright Kreutz Comet C/2007 L3 (SOHO) entered the fields of view of the twin Solar Terrestrial Relations Observatory (STEREO) COR1 telescopes on 7–8 June 2007. The 12° separation between the two spacecraft at the time afforded the opportunity to derive the position of the comet's tail in three-dimensional space using direct triangulation. The track of the comet's orbit is compared against more traditional orbital calculations using observations from the STEREO COR2 telescopes, and from the Large Angle and Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO). The shape of the comet's tail shows that it is composed of dust particles released when the comet was between 18 and 22 solar radii, with no significant dust production after that. The comet did not survive perihelion passage, but a rare faint remnant of the comet tail persisted for several hours after the break-up, and was seen by both the SOHO and STEREO coronagraphs to drift slowly away from the Sun. This tail remnant was found to be composed of particles far back from the head of the comet. The motion of the tail remnant shows a loss of angular momentum during the passage through the solar corona. Atmospheric drag is estimated to account for a significant fraction of this change in angular momentum, but indications are that other mechanisms may be required to completely account for the total amount of change.     

Oceanic versus island arc origin of ophiolites

The silica content of basaltic rocks is an unreliable variable with which to distinguish ultramafic-mafic complexes developed at ocean ridges from those potentially formed beneath volcanic island arcs. Data from Appalachian ophiolites supports the view that silica metasomatism is responsible for the high silica content of supposed calc-alkaline basaltic rocks found in ophiolites such as Troodos, and that the high-silica (70 wt.%) leucocratic rocks associated with ophiolites are of tholeiitic rather than calc-alkaline parentage. The use of titanium as a discriminant of tectonic environment is also suspect because the titanium content of basalts associated with Appalachian ophiolites as well as those recently recovered from the Atlantic ocean floor ranges from values even lower than those typical of island arc tholeiites to values typical of abyssal tholeiites. However, the internal stratigraphy of ophiolites in both the Appalachian and Tethyan systems can only be explained on the basis of the postulate that ophiolites originate at oceanic spreading centres rather than beneath island arcs.     

The effect of cracks on the thermal expansion of rocks

Thermal expansion during the first heating cycle at atmospheric pressure was measured in several directions in seven igneous rocks between 25° and 400°C at slow heating rates. The coefficient of thermal expansion measured under these conditions increases more rapidly as temperature is increased than the average thermal expansion coefficient of the constituent minerals. The “extra” expansion is attributed to the formation of cracks by differential expansion of mineral grains. The presence of such cracks in the rocks during the cooling part of the cycle and during any subsequent heating and cooling cycles will result in a substantial decrease in the coefficient of thermal expansion as compared to that measured during the first heating cycles. The effect of cracks initially present in a rock was studied by measuring the full tensor of the coefficient of thermal expansion on two rocks with anisotropic crack distributions. In these two rocks the coefficient of thermal expansion is least in the direction perpendicular to the plane of greatest crack concentration. The implication of our data is that thermal expansion depends greatly on the fracture state of the rock. Both the fractures in the rock and the boundary conditions on the rock are significant for the interpretation of thermal expansion measurements and for their application to other problems.     

On the Principle of Least Interaction Action and the Laplacean satellites of Jupiter and Uranus

The Principle of Least Interaction Action, which explains the observed preference in the Solar System for two-satellite resonant configurations, is shown to apply also to the Laplacean satellites of Jupiter and Uranus, in the sense that these triplet resonant structures lie close to configurations for which the time-mean of the action associated with the mutual interaction of the satellites is an overall minimum. Far from the minimum configuration, significant changes take place in the major semi-axes on time-scales ～10⁶-10⁷y. Both systems require times ～10⁸ y to come close to the minimum configuration; to approach resonance to the observed precision of the Laplace relationship (3×10^?4 for the Uranian case, 2×10^?7 for the Jovian case) requires, for both systems, a time closely comparable with the age of the solar system.