Alfvén waves in the solar atmosphere

The nonlinear propagation of Alfvén waves on open solar magnetic flux tubes is considered. The flux tubes are taken to be vertical and axisymmetric, and they are initially untwisted. The Alfvén waves are time-dependent axisymmetric twists. Their propagation into the chromosphere and corona is investigated by solving numerically a set of nonlinear time-dependent equations, which couple the Alfvén waves into motions parallel to the initial magnetic field (motion in the third coordinate direction is artificially suppressed). The principal conclusions are: (1) Alfvén waves can steepen into fast shocks in the chromosphere. These shocks can pass through the transition region into the corona, and heat the corona. (2) As the fast shocks pass through the transition region, they produce large-velocity pulses in the direction transverse to B _o. The pulses typically have amplitudes of 60 km s^–1 or so and durations of a few tens of seconds. Such features may have been observed, suggesting that the corona is in fact heated by fast shocks. (3) Alfvén waves exhibit a strong tendency to drive upward flows, with many of the properties of spicules. Spicules, and the observed corrugated nature of the transition region, may therefore be by-products of magnetic heating of the corona. (4) It is qualitatively suggested that Alfvén waves may heat the upper chromosphere indirectly by exerting time-dependent forces on the plasma, rather than by directly depositing heat into the plasma.     

Basement faulting and the focal depths of the larger earthquakes in the Zagros mountains (Iran)

Summary. The present day seismicity of the Zagros seems to occur on high angle reverse faults distributed across the whole width of the belt. It does not indicate activity on a single inclined thrust surface and there do not seem to have been any well located events at intermediate depths. Modelling of the long period teleseismic body waves of seven large earthquakes presented here shows their focal depths to be in the range 8–15 km. This is thought to indicate faulting in the uppermost basement beneath the sedimentary cover, though the absence of published seismic refraction results renders the sediment thickness uncertain. Faulting of this type and depth may occur on inherited normal faults which have subsequently been reactivated as thrusts. Such reactivation allows considerable shortening to take place in the early stages of continental collision without the subduction or excessive thickening of continental crust.     

The Kirbyville Eucrite

Abstract— The Kirbyville meteorite is an equilibrated monomict eucrite with a mode (calculated in wt.%): plagioclase 37, inverted pigeonite 58, silica 3, ilmenite 1.4 and troilite 0.3. The pyroxene compositional range is Wo 2–42, Fs 27–62, En 30–40 with an average of Wo 7. The plagioclase average mean is An 89. Kirbyville is one of the “main group” eucrites.     

The equation of state of Al,H-bearing SiO2 stishovite to 58 GPa

We have determined the P-V equation of state of Al-rich H-bearing SiO₂ stishovite by X-ray powder diffraction at pressures up to 58 GPa using synchrotron radiation. The sample contained 1.8 wt% Al₂O₃ and up to 500 ppm H₂O, and had a composition that would coexist with Mg-silicate perovskite in a subducted slab. By fitting a third-order Birch-Murnaghan equation of state to our compression data, we obtained a bulk modulus K _T0=298(7) GPa with K′=4.3(5). With K′ fixed to a value of 4, the bulk modulus K _T0=304(3) GPa. Our results indicate that Al³⁺ and H⁺ have a small effect on the elastic properties of stishovite. Compared with data obtained up to 43.8 GPa, peak intensities changed and we observed a decreased quality of fit to a tetragonal unit cell at pressures of 49 GPa and higher. These changes may be an indication that the rutile↔CaCl₂ transition occurs between these pressures. After laser annealing of the sample at 58.3(10) GPa and subsequent decompression to room conditions, the cell volume is the same as before compression, giving strong evidence that the composition of the recovered sample is also unchanged. This suggests that Al and H are retained in the sample under extreme P-T conditions and that stishovite can be an agent for transporting water to the deepest lower mantle.     

Spatial and temporal dynamics of epiphytic microalgae on the cordgrassSpartina alterniflora in North Inlet Estuary, South Carolina

Epiphytic microbial biomass (as chlorophylla) was measured monthly in North Inlet Estuary, South Carolina, for 16 months on spatially distinct stem sections (bottom and middle) of dead and livingSpartina alterniflora growth forms (tall, medium, and short) exposed at low tide. The highest biomass was located on the bottom section of tall plants, presumably due to their relatively longer contact with creek water and associated phytoplankton, and their closer proximity to marsh sediments with associated benthic microalgae, both recruitment sources for epiphytes. Dead plants left standing from the previous year’s growth cycle had higher epiphytic biomass than living plants, which occurred mostly in late spring through fall. Epiphytic biomass was highest in the winter (mean of 1.77 mg chla (m² marsh)⁻¹) and lowest in the summer (mean of 0.34 mg chla (m² marsh)⁻¹). Because phytoplankton andSpartina production are lowest in the winter, the results emphasize the relative importance of epiphytes to growth of herbivores in this season.     

Geography’s changing lexicon: measuring disciplinary change in Anglophone human geography through journal content analysis

Much writing on changing trends in geographical practices is based on subjective interpretations of the discipline’s scholarly output. This paper introduces a data source which provides quantitative information on the discipline’s lexicon through full-text searching of the contents of five of the discipline’s major Anglophone journals. Analysis of the large data set identifies two main trends in geography’s language and practices during the period 1950-1998: one group of terms associated with quantitative work became prominent in the 1970s but declined in relative importance thereafter - though by no means disappearing from the lexicon; the other, comprising a range of terms generally associated with cultural and social geography, increased in prominence from the 1980s on.     

Comment on ‘Testing earthquake prediction methods: “The West Pacific short-term forecast of earthquakes with magnitude MwHRV ≥ 5.8”’ by V.G. Kossobokov

In his paper Kossobokov investigates the efficiency of our short-term forecast for two western Pacific regions. Although we agree with the basic results of his evaluation that the forecast statistics is much better than a random guess, we have reservations about his definition of earthquake prediction, some of his tests, and his interpretation of the test results. We distinguish between deterministic earthquake predictions and statistical forecasts. We argue that some techniques used by Kossobokov may not be appropriate for testing our forecasts and discuss other testing methods, based on the likelihood function. We demonstrate that Kossobokov's null hypothesis may be biased, and this bias can influence some of his conclusions. We show that contrary to Kossobokov's statement, our algorithm predicts mainshocks when they are preceded by foreshocks.     

Strike-slip faulting, rotation, and along-strike elongation in the Kopeh Dagh mountains, NE Iran

The Kopeh Dagh is a linear mountain range separating the shortening in Iran from the stable, flat Turkmenistan platform. In its central part is an array of active right-lateral strike-slip faults that obliquely cut the range and produce offsets of several kilometres in the geomorphology and geological structure. They are responsible for major destructive earthquakes in the 19th and 20th centuries and represent an important seismic hazard for this now-populous region of NE Iran. These strike-slip faults all end in thrusts, revealed by the uplift and incision of Late Quaternary river terraces, and do not continue beyond the Atrak river valley, which forms the southern margin of the Kopeh Dagh. The cumulative offset on these strike-slip faults, and their associated rotation about vertical axes, can account for ∼60 km of N–S shortening. This value is similar to estimates of the Late Quaternary N–S right-lateral shear between central Iran and Afghanistan, which must be accommodated in NE Iran. The strike-slip faults also require ∼30 km of along-strike extension of the Kopeh Dagh, which is taken up by the westward component of motion between the South Caspian Basin and both Eurasia and Central Iran. It is probable that these motions occurred over the last ∼10 Ma.