Unusual coronal activity following the flare of 6 November 1980

For almost 30 hr after the major (gamma-ray) two-ribbon flare on 6 November 1980, 03:30 UT, the Hard X-Ray Imaging Spectrometer (HXIS) aboard the SMM satellite imaged in > 3.5 keV X-rays a gigantic arch extending above the active region over the limb. Like a similar configuration on 22 May 1980, this arch formed the lowest part of a stationary post-flare radio noise storm recorded at metric wavelengths at Nançay and Culgoora. 6.5 hr after the flare a coronal region below the arch started quasi-periodic pulsations in X-ray brightness, observed by several SMM instruments. These brightness variations had no response in the chromosphere (H), very little in the transition layer (O v), but they clearly correlated with similar variations in brightness at 169 MHz. There were 13 pulses of this kind, with apparent periodicity of about 20 min, until another flare occurred in the active region at 15:00 UT. All the brightenings appeared within a localized area of about 30000 km² in the northern part of the active region, but they definitely did not occur all at the same place.The top of the X-ray arch, at an altitude of 155 000 km, was continuously and smoothly decaying, taking no part in the striking variations below it. Therefore, the area variable in brightness does not seem to be the footpoint of the arch, as we supposed for similar variations on 22 May. More likely, it is a separate region connected directly with the source of the radio storm; particles accelerated in the storm may be dumped into the low corona and cause the X-ray enhancements. The X-ray arch was enhanced by two orders of magnitude in 3.5–5.5 keV X-ray counts and the temperature increased from 7.3 × 10⁶ to 9 × 10⁶ K when the new two-ribbon flare occurred at 15:00 UT. Thus, it is possible that energy is brought into the arch via the upper parts of the reconnecting flare loops - a process that can continue for hours.     

A three-dimensional numerical sensitivity study of mesoscale circulations induced by circular lakes

Summary A three-dimensional mesoscale planetary boundary layer model with theE- turbulence closure is used to simulate airflow over a lake of circular shape. A series of model sensitivity studies are performed to examine the effects of lake-land temperature difference, ambient wind magnitude and direction, lake size, surface roughness, the Coriolis force and baroclinic ambient wind conditions on mesoscale lake circulations.The lake-land temperature difference is essentially the basic energy source driving the mesoscale circulations over the lake on synoptically undisturbed days. A lake-breeze convergence zone is predicted by the model due to the differential heating between the land and the water. It is found that spatial and temporal variations of this convergence zone and associated convection are strongly controlled by the direction and the magnitude of the ambient wind. Under southeasterly and southwesterly ambient winds, the lake-breeze convergence zone and the associated convection occur primarily along up wind and lateral sides of the lake with reference to the general direction of the ambient flow. In contrast to the southeasterly and southwesterly ambient winds, the lake-breeze convergence zone and the convection are predicted all around the coastline of the lake under calm wind.The model also predicts a cloudless region over the lake in all the case studies due to divergent nature of the lake-breeze circulation. The lake size is found to have a significant effect in intensifying convection. Surface roughness over the land surface is found to be important in determining the intensity of the convection. The combined effect of the Coriolis force and the differential surface roughness between land and water appear to be the responsible mechanism for producing the asymmetric shape of the lake-breeze convergence zone around the symmetric circular lake. Finally, it was found that an initial baroclinic flow has different mesoscale lake-breeze circulation patterns as compared to an initial barotropic flow.With 16 Figures     

The character of the fine fractions of the bottom sediments of the Bohemian part of the Labe (Elbe) river

The amounts of fractions with size 63 m in the bottom sediments of the Bohemian part of the Labe (Elbe) river is variable. The grain-size distribution is poorly to very poorly sorted with a mean particle size from 5.1 to 12.2 m.The crystalline phase of recent sediments of the Labe river draining various types of parent rocks has a simple mineral composition, with predominant mica/illite, quartz, kaolinite and chlorite (±vermiculite ±smectite). There are also very small amounts of interstratification of 10 Å and 14 Å sheet silicates (11–13.6 Å). Small amounts of Na-rich plagioclase, K-feldspar and calcite have also been found; accessory amounts of amphibole, authigenic gypsum and pyrite are also present. Opal in diatomic tests is an omnipresent constituent. In the amorphic inorganic phase, hydrated oxides predominate, where Si >Al >Fe >Mn in a ratio of 64:28:22:1. The greatest amount of amorphous inorganic and organic phases were found in the 4 m fraction.A considerable part of the detrital material originates from soils formed on various igneous, metamorphic and sedimentary rocks, transported to the Labe by other rivers or directly by erosion in the immediate vicinity of the Labe river.     

Experimental investigation of the elastic modulus of a fractal system—A model of fractured rocks

The elastic properties of a physical model representing a damaged rock matrix were studied using a square lattice deformed under tensile stress. The elastic modulusM of such a system varies in agreement with percolation theory as|x–x _c | ^f , wherex is the damage parameter andx _c the threshold value of the damage parameter,f3.6. Atxx _c the scale dependence ofM can be expressed asML ^–f/v , whereL is the size of the sample andv the correlation exponent in percolation theory.The experimental results are of interest in assessing elastic properties in earthquake focal zones and fault zones in general.     

A chlorine isotope study of DSDP/ODP serpentinized ultramafic rocks: Insights into the serpentinization process

Eight DSDP/ODP cores were analyzed for major ion concentrations and δ³⁷Cl values of water-soluble chloride (δ³⁷Cl_WSC) and structurally bound chloride (δ³⁷Cl_SBC) in serpentinized ultramafic rocks. This diverse set of cores spans a wide range in age, temperature of serpentinization, tectonic setting, and geographic location of drilled serpentinized oceanic crust. Three of the cores were sampled at closely spaced intervals to investigate downhole variation in Cl concentration and chlorine isotope composition.The average total Cl content of all 86 samples is 0.26 ± 0.16 wt.% (0.19 ± 0.10 wt.% as water-soluble Cl (X_WSC) and 0.09 ± 0.09 wt.% as structurally bound Cl (X_SBC)). Structurally bound Cl concentration nearly doubles with depth in all cores; there is no consistent trend in water-soluble Cl content among the cores. Chlorine isotope fractionation between the structurally bound Cl⁻ site and the water-soluble Cl⁻ site varies from − 1.08‰ to + 1.16‰, averaging to + 0.21‰. Samples with negative fractionations may be related to reequilibration of the water-soluble chloride with seawater post-serpentinite formation. Six of the cores have positive bulk δ³⁷Cl values (+ 0.05‰ to + 0.36‰); the other two cores (173-1068A (Leg-Hole) and 84-570) have negative bulk δ³⁷Cl values (− 1.26‰ and − 0.54‰). The cores with negative δ³⁷Cl values also have variable Cl⁻ / SO₄² ratios, in contrast to all other cores. The isotopically positive cores (153-920D and 147-895E) show no isotopic variation with depth; the isotopically negative core (173-1068A) decreases by ∼1‰ with depth for both the water-soluble and structurally bound Cl fractions.Non-zero bulk δ³⁷Cl values indicate Cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling. Cores with positive δ³⁷Cl values are most likely explained by open system fractionation during hydrothermal alteration, with preferential incorporation of ³⁷Cl from seawater into the serpentinite and loss of residual light Cl back to the ocean. Fluid / rock ratios were probably low as evidenced by the presence of water-soluble salts. The two isotopically negative cores are characterized by a thick overlying sedimentary package that was in place prior to serpentinization. We believe the low δ³⁷Cl values of these cores are a result of hydration of ultramafic rock by infiltrating aqueous pore fluids from the overlying sediments. The resulting serpentinites inherit the characteristic negative δ³⁷Cl values of the pore waters. Chlorine stable isotopes can be used to identify the source of the serpentinizing fluid and ultimately discern chemical and tectonic processes involved in serpentinization.