Narrowband dm-spikes in the 2 GHz frequency range and MHD cascading waves in reconnection outflows

Two examples of clouds of narrowband dm-spikes, observed by the Ondejov radiospectrograph in the 1–2 GHz frequency range, are analyzed. After transformation of the frequency scales to distances in the solar atmosphere, the power spectra analysis of size scales reveals a spectral index of –5/3, resembling that of Kolmogorov spectra of turbulent cascades. The narrowband dm-spikes are interpreted as radio emission from electrons accelerated in MHD cascading waves, probably generated in plasma outflows from magnetic field reconnection.     

The Radio-Silent Start of an Intense Solar Gamma-Ray Flare

Radio-silent -ray flares are solar flares that lack any significant emission in the (non-thermal) radio wave band during their impulsive hard X-ray and -ray emission phases. Flares with extremely suppressed long-wavelength spectra have previously been reported by White et al. (1992) and have been discussed in different context by Hudson and Ryan (1995). A striking example of a radio-silent flare was observed by SMM during the onset of the 6 March 1989 energetic -ray flare. We argue that the absence of radio emission at wavelengths longer than microwave wavelengths is an indication of the compactness of the flare rather than that the flare did not exhibit non-thermal properties. Probably the flare site was restricted to altitudes above the photosphere in a newly emerging loop configuration lower than the equivalent altitude corresponding to an emission frequency of 1.4 GHz. This implies the presence of a dense and highly magnetized closed field configuration confining the electron component which causes the impulsive -ray continuum. Reconnection in such a configuration did not lead to open magnetic fields and streamer formation. Acceleration of particles in the and hard X-ray bursts was restricted to closed field lines. Thermal expansion of the loop system may subsequently lead to the generation of radially propagating blast waves in the solar corona which are accompanied by type II solar radio bursts and decimetre emissions. The emission during the onset of the flare was dominated by a continuum originating from electron bremsstrahlung at X-ray and -ray energies with only little evidence for the presence of energetic ions. It is, therefore, concluded that energetic electrons have been primary and not secondary products of the particle acceleration process.     

The Growth rate of upper-hybrid waves and dynamics of microwave zebra structures

The growth rate of the upper-hybrid waves with different velocities of superthermal electrons is computed considering a finite temperature of the background plasma and relativistic corrections. Based on these computations two examples of high-frequency zebra structures are interpreted. The sequence of the continuum, zebra structure, and continuum observed in the 29 October 2000, event is explained as an increase and following decrease of the velocity of superthermal electrons in the range of v=0.1–0.3 c. On the other hand, the zebra structure observed during the 18 March 2003 event represents an example with fast electron acceleration.     

A granite–gabbro complex from Madagascar: constraints on melting of the lower crust

The Ranomandry Complex is a Neoproterozoic, nested intrusion from central Madagascar composed of a gabbroic core within a coeval peraluminous granite ring intruding pelitic metasediments. Although xenocryst entrainment and magma mixing have both contributed to marginal phases of the granite, the primary melt is characterised by steep LREE/HREE ratios and negligible, or slightly positive, Eu anomalies. Both isotopic and trace element systematics preclude an origin from either partial melting of the metapelitic country rock or from fractional crystallisation of the gabbroic magma. However, trace-element modelling suggests an origin from the dehydration melting of a plagioclase-poor, garnet-bearing metagreywacke at temperatures of 850–900 °C and at lower crustal pressures (>10 kbar). Melting of an enriched subcontinental mantle generated gabbroic magmas that caused advective heating and anatexis at the base of thickened continental crust during their ascent. Transport of the resulting granite magma was facilitated by the pre-existing plumbing system that overcame thermal and mechanical problems associated with both diapirism and self-propagating dykes as mechanisms for long-distance transport of granite magmas. The nested geometry of the intrusions is an indication of a structurally homogeneous lower crust that contains no pre-existing shear zones or fault systems.     

Bosumtwi impact structure,Ghana: Evidence for fluidized emplacement of the ejecta

The about 10.5 km diameter Bosumtwi impact crater is one of the youngest large impact structures on Earth. The crater rim is readily noticed on topographic maps or in satellite imagery. It defines a circular basin filled by water (Lake Bosumtwi) and lacustrine sediments. The morphology of this impact structure is also characterized by a circular plateau extending beyond the rim and up to 9–10 km from the center of the crater (about 2 crater radii). This feature comprises a shallow ring depression, also described as an annular moat, and a subdued circular ridge at its outer edge. The origin of this outermost feature could so far not be elucidated based on remote sensing data only. Our approach combines detailed topographic analysis, including roughness mapping, with airborne radiometric surveys (mapping near‐surface K, Th, U concentrations) and field observations. This provides evidence that the moat and outer ring are features inherited from the impact event and represent the partially eroded ejecta layer of the Bosumtwi impact structure. The characteristics of the outer ridge indicate that ejecta emplacement was not purely ballistic but requires ejecta fluidization and surface flow. The setting of Bosumtwi ejecta can therefore be considered as a terrestrial analog for rampart craters, which are common on Mars and Venus, and also found on icy bodies of the outer solar system (e.g., Ganymede, Europa, Dione, Tethys, and Charon). Future studies at Bosumtwi may therefore help to elucidate the mechanism of formation of rampart craters.     

Radiographic study of the distribution of boron in recent mollusc shells

Studies have been made of the boron content and distribution in recent Mytilus edulis shells by mapping the alpha particle distribution produced by the ¹⁰B (n,α)⁷Li reaction using cellulose nitrate track detectors. Boron is concentrated in the inorganic parts of the shell matrix and is enriched in aragonite relative to calcite by a factor of 1.5–2. The B contents in different parts of individual shells and in different specimens from a given locality are well defined, particularly for the aragonite layer. A correlation was found between the B in the aragonite layer and the salinity of the water in which the shells grew. The method shows promise as a means of using the boron content of fossil shells to determine paleosalinity.     

Gradient Estimation from Irregularly Spaced Data Sets

Topographic data sets are often composed of terrain samples arranged in irregular meshes. Many gradient estimation methods require the data to be arranged in a regular mesh; irregular meshes present a difficulty for them. This paper presents a novel method of estimating surface gradients from irregularly spaced inputs. The method is derived using directional derivatives and is shown to be a generalization of traditional finite difference methods. Analytical tests were used to evaluate the method, which was shown to be accurate and robust.