Evolving Photospheric Flux Concentrations and Filament Dynamic Changes

We analyze the role of weak photospheric flux concentrations that evolve in a filament channel, in the triggering of dynamic changes in the shape of a filament. The high polarimetric sensitivity of THEMIS allowed us to detect weak flux concentrations (few Gauss) associated with the filament development. The synoptic instruments (MDI, SOLIS) even if their sensitivity is much less than THEMIS were useful to follow any subsequent strengthening of these flux concentrations after their identification in the THEMIS magnetograms. We found that (1) the northern part of the filament develops an Hα barb at the same time that weak minority polarity elements develop near a plage; (2) a section in the southern part of the Hα filament gradually disappears and later reforms at the same time that several mixed-polarity magnetic elements appear, then subsequently cancel or spread away from each other. These changes correspond to increases in EUV emission, as observed by TRACE, EIT, and CDS. This suggests that the plasma is temporarily heated along the filament spine. An idealized sequence of force-free models of this filament channel, based on plasma-supporting magnetic dips occurring in the windings of a very weakly twisted flux tube, naturally explains the evolution of its southern part as being due to changes in the topology of the coronal magnetic field as the photospheric flux concentrations evolve.     

The heliographic latitude dependence and sector structure of the interplanetary magnetic field 1969–1974: Results from the HEOS satellites

Data from the two HEOS satellites obtained during the period December 1968 to August 1974 are used to investigate the large-scale properties of the interplanetary magnetic field.The sector structure has been deduced from the observed times of sector boundary crossings which are tabulated. A two-sector pattern existed throughout most of the period with occasional intervals of 2–3 months duration in which four sectors appeared. The variation of the dominant sector polarity with heliographic latitude showed a reversal in sense during 1971 at the time of the reported reversal in the Sun's polar field. A statistical analysis of the change in polarity distribution with latitude suggests that at Earth's orbit the sector boundaries are inclined to the solar equator on average at an angle of 12 deg.No evidence was found in the HEOS measurements of the north-south field component to confirm the systematic latitude-dependent deviation of the plasma flow away from the solar equatorial plane suggested by several analyses of data from previous spacecraft. The mean field magnitude and the average amplitude of the directional fluctuations appeared to be independent of heliographic latitude within the ±7.3° range explored.     

AN ANALYTICAL SCHEME FOR CHONDRITIC METEORITES

A chemical scheme is reported for the analysis of chondritic materials and results are presented for the Cocklebiddy meteorite. Analytical problems associated with chondrite inhomogeneity including phase leaching selectivity and the completeness of individual phase attack are considered. Data obtained by this method can be used to classify chondrites according to the accepted weight and molar ratio methods (Van Schmus and Wood, 1967: Wasson, 1974) The analytical scheme uses selective leaching techniques to determine, in each sample, the chemical composition of metallic, sulphide and silicate phases. Elemental concentrations are obtained by atomic absorption spectrometry, flame photometry, spectrophotometry or titration techniques, and the total sample average compositions are determined on fully oxidised specimens by XRF     

Equivalent width of interstellar molecular lines

We have made an analysis of the observed equivalent widths of the lines of Lyman and Werner bands by Smith. The H₂ column densities of 3×10¹⁹ to 10²⁰ and the excitation temperature of 80 to 150 K satisfy the observations. This temperature refers to the kinetic temperature. We have also discussed the importance of getting the excitation temperature from lines of H₂ and other heteronuclear diatomic molecules for the same star and from different regions of space.     

Fine structure of a very bright active region at a wavelength of 2.8 cm

The radio structure of the active region McMath 11976 has been investigated with an east-west resolution of 16″ at a wavelength of 2.8 cm. Six bright components were found with angular sizes of the order of 20″. From the position measure of these components in two following days the heights above the solar photosphere are determined. Their positions are compared with the Hα and magnetic structure of the active region. The derived brightness temperatures reach unusual values, up to 6 × 10⁶K for the brightest component.     

Solar wind induction in Mercury: Constraints on the formation of a magnetosphere

The discovery of Mercury's magnetosphere by Mariner 10 was surprising since the conventional view of regenerative planetary dynamos had been that the spin requirement would likely have been in excess of the observed spin rate of Mercury. Also internal fluid motions were not expected to be sufficiently large. This paper explores the alternative model of the formation of Mercury's magnetosphere via electromagnetic induction forced by the solar wind. It is shown, however, that the constraints are so severe as to limit severely the applicability of such a model. Although induction is easily observed on the Moon, the modification of the magnetic boundary condition associated with a plasma magnetosphere on Mercury rules out its formation via induction except for interplanetary driving fields which are decreasing in amplitude. That model is explored but retains the difficulty that induced magnetospheres tend to be of small radial and temporal extent compared to that inferred by Ness et al. for Mercury.     

Radiation pressure — a stabilizing agent of dust clouds in comets?

The internal dynamics of an illuminated dust cloud of finite optical thickness is investigated. The dependence of the radiation pressure on the optical depth makes the individual particles oscillate, in one dimension, around the accelerated centre of gravity of the cloud. The cloud moves as an entity, irrespectively of the velocity dispersion of the particles and their efficiency for radiation pressure. If the optical depth does not change, i.e. if the cloud does not expand laterally, its lifetime is unlimited. A contraction caused by energy dissipation in mechanical collissions between the dust particles is expected. The range of particle sizes which can be transported by such a “coherent cloud” is estimated, as well as the acceleration of the whole cloud. The structure of the cloud in real space and in velocity space is investigated. A comparison with the “striae” observed in the dust tails of great comets shows that the parent clouds of these striae may have been of the kind considered.