On the NOx production by laboratory electrical discharges and lightning

Different approaches are used in estimating the global production of NO_x by lightning flashes, including field measurements carried out during thunderstorm conditions, theoretical studies combining the physics and chemistry of the electrical discharges, and measurements of NO_x yield in laboratory sparks with subsequent extrapolation to lightning. In the latter procedure, laboratory data are extrapolated to lightning using the energy as the scaling quantity. Further, in these studies only the return strokes are considered assuming that contributions from other processes such as leaders, continuing currents, M components, and K processes are negligible. In this paper, we argue that the use of energy as the scaling quantity and omission of all lightning processes other than return strokes are not justified. First, a theory which can be used to evaluate the NO_x production by electrical discharges, if the current flowing in the discharge is known, is presented. The results obtained from theory are compared with the available experimental data and a reasonable agreement is found. Numerical experiments suggest that the NO_x production efficiency of electrical discharges depends not only on the energy dissipated in the discharge, but also on the shape of current waveform. Thus, the current signature, can influence extrapolation of laboratory data to lightning flashes. Second, an estimation of the NO_x yield per lightning flash is made by treating the lightning flash as a composite event consisting of several discharge processes. We show that the NO_x production takes place mainly in slow discharge processes such as leaders, M components, and continuing currents, with return strokes contributing only a small fraction of the total NO_x. The results also show that cloud flashes are as efficient as ground flashes in NO_x generation. In estimating the global NO_x production by lightning flashes the most influencing parameter is the length of the lightning discharge channel inside the cloud. For the total length of channels inside the cloud of a typical ground flash of about 45 km, we estimate that the global annual production of NO_x is about 4 Tg(N).     

Retrograde Schists of the Amphibolite Facies at broken hill,New South Wales

Aluminous, mafic, felsic, calcareous, and sulphide‐rich rocks have been involved in localized deformation and retrograde metamorphism at Broken Hill, western New South Wales, where retrograde schist‐zones intersect high‐grade, regional metamorphic rocks of the lower granulite facies (or the amphibolite‐granulite facies transition). Although technically retrograde, the schists contain mineral assemblages indicative of the lower amphibolite facies. The schist‐zones were formed by local folding, apparently as part of the third stage of deformation in the Broken Hill area.     

High Frequency PKKP<Subscript>BC</Subscript> Around 2.5 Hz Recorded Globally

Anomalous high frequency PKKP_BC signals (displaying a large amount of energy around 2.5 Hz), recorded globally for deep and intermediate depth earthquakes, are compared to PKKP_AB signals. The attenuation difference t_\textAB^* - t_\textBC^* t_{\text{AB}}^{*} - t_{\text{BC}}^{*} is evaluated from spectral amplitudes in the range 96–111°, being approximately twice the results provided by full-wave theory and PREM (with no low Q_μ zone in the lowermost mantle and a nearly infinite Q_K in the outer core). Most ray paths for such recordings are piercing the D″ region in the proximity of regions where ultra-low velocity zones (ULVZ) have been previously reported beneath the North Atlantic Ocean, the Southwest Pacific and the southwestern part of South America. If BC amplitudes around 2.5 Hz and at low frequencies (0.5–1.5 Hz) are comparable, the observed attenuation difference (in the frequency range 0.2–2.5 Hz) is small (around 0.25 s) and close to the PREM value. The particle motion of the high-frequency PKKP_BC at 2.5 Hz is quite similar to that of the raw recording, suggesting a deep source. An explanation for this might be scattering of the BC branch in some very restricted areas of the lowermost mantle. Alternately, the presence of a thin layer with high attenuation in the D″ region would most likely be associated with either the ultra-low velocity zone (ULVZ) or light sediments on the underside of the core-mantle boundary (CMB). Correlated to other methods to investigate the lowermost mantle, the high-frequency PKKP_BC can be used to map lateral variations of attenuation above the CMB, possibly associated with the boundary of the superplumes, especially when PKKP_AB is observed.     

Surface layer variability in the Ross Sea,Antarctica as assessed by in situ fluorescence measurements

Phytoplankton fluorescence, temperature and salinity were measured from December through February using in situ instruments deployed at two locations in the southern Ross Sea, Antarctica during the austral summers of three consecutive years (2003–2004, 2004–2005, and 2005–2006) to assess the short-term, seasonal and interannual variations in phytoplankton biomass and oceanographic conditions. The seasonal climatologies of physical forcing variables were also determined from satellite measurements, and the data from the two sites compared to the 2000–2009 mean. In situ fluorometers were deployed at three depths at 77°S, 172.7°E and 77.5°S, 180°. Significant differences between the two sites were consistently observed, confirming the anticipated high level of spatial and temporal heterogeneity. Chlorophyll fluorescence was maximal in late December, and generally decreased rapidly to modest levels in January and February. However, during 1 year (2003–2004) a secondary bloom was found, with summer maxima being similar to those observed during spring. Fluorescence displayed a strong diel cycle, with strong quenching during periods of maximum irradiance. The magnitude of this reduction was large (the minimum average fluorescence was 25% of the daily mean) and decreased with depth. Fluorescence varied interannually, with the absolute levels and temporal patterns being different among years. The two sites had different temperature/salinity properties as measured at 24 m, and both variables changed with time. During 2004–2005 we were able to continuously measure the photosynthetic quantum efficiency of PSII (F_v/F_m) at 11 m, which revealed a minimum in December, and an increase in January, whereas the absolute fluorescence (F_o) decreased simultaneously. We suggest that this reflected a mixing event, whereby available irradiance increased, allowing a short period of growth in a more favorable optical environment. While substantial variations from the mean physical forcing were observed, the linkage of these physical variations with fluorescence was not always clear. Short-term (over 24-h) changes in fluorescence occurred, and were likely related to advective events. Wind events altered fluorescence in the surface layer, and these redistributed phytoplankton in the surface. The variability in chlorophyll fluorescence and physical forcing over a variety of scales in the Ross Sea provides insights into temporal–spatial coupling of phytoplankton.     

The ecology of worm populations in the Erins Vale formation (late permian) southern Sydney Basin

The distribution, mode of occurrence and population densities of extensive colonies of worms, and the morphology of their thick‐walled tubes are described. Interaction between two species occurring in certain strata enables the feeding habits of the tube‐builders to be reconstructed, and they are thus identified as terebellid worms. The worms when alive were less than 5 mm in diameter and had tentacles 27 mm long. Their environmental situation was near the southern edge of an extensive area of marine deposition covering much of the Sydney Basin. The depth was below wave base. The worm populations were buried alive during deposition of sheets of sediment, probably resulting from intermittent sudden flooding events in coastal rivers.     

Efficiency of Lg Propagation from SmS Dynamic Ray Tracing in Three-dimensionally Varying Crustal Waveguides

The effects of variations in crustal and basin thicknesses on the efficiency of Lg propagation in Central Asia are investigated by comparing the predictions of ray theoretical modeling of Lg as multiple, critically reflected, SmS rays with observations at the KNET, CNET, and ILPA arrays. Ray modeling predicts that strong spatial gradients in crustal thickness within 10 to 50 km of either the source or receiver can affect Lg efficiency over paths exceeding 500 km. Variations in the efficiency of Lg propagation to KNET, including instances of strong variations across the aperture of the array, are consistent with predicted blockage of SmS by high slopes of Moho topography associated with the Hindu Kush and Pamir mountains. At CNET and ILPA the blocking effects of Moho slopes associated with the Caucausus and Zagros mountain belts and Black and Caspian Seas are accentuated by the effects of thick basins. Thick basins (greater than 2000 m) are found to have large effects on the efficiency of Lg propagation from shallow (1 km) crustal sources, blocking either a portion or all of the Lg phase along paths crossing these basins. The basin effects are strongly sensitive to source depth, nearly vanishing for sources 15 km and deeper.     

Reducing the bias of multitaper spectrum estimates

The power spectral density of geophysical signals provides information about the processes that generated them. We present a new approach to determine power spectra based on Thomson's multitaper analysis method. Our method reduces the bias due to the curvature of the spectrum close to the frequency of interest. Even while maintaining the same resolution bandwidth, bias is reduced in areas where the power spectrum is significantly quadratic. No additional sidelobe leakage is introduced. In addition, our methodology reliably estimates the derivatives (slope and curvature) of the spectrum. The extra information gleaned from the signal is useful for parameter estimation or to compare different signals.