Lower midlatitude ionospheric disturbances and the perkins instability

This paper presents a preliminary study of ionospheric disturbances at dip latitudes less than 40° as seen by the ion drift meter and the retarding potential analyzer on board Atmosphere Explorer E during a period of low solar activity. The altitude of observation was relatively low, mostly below 300 km. The emphasis is on the midlatitude region, where some features resemble equatorial bubbles; no clear demarcation in latitude could be recognized between the bubbles and other midlatitude disturbances. Excellent evidence was found that the Perkins instability is responsible for very structured disturbances which were frequently observed. In most cases where regions of inward and outward E × B drift were encountered, diffusive motion up or down the field line partially cancelled the vertical component of the cross-field drift. In some cases cancellation was almost perfect, but in others it appeared not to occur at all (probably cases involving rapid changes). Within the larger structures caused by the Perkins instability there were places where the secondary gradient drift instability was also active.     

Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude

We have observed a number of strong echoes with the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5° and 78.6° with the geomagnetic field and at about 100-km altitude north in the auroral zone. The echoes are shortlived and occur in single 2- or 10-s data dumps. They are offset by 125–130 kHz with respect to the transmitted frequency. In most cases the offset compares well with the frequency of gyro lines in the incoherent-scatter spectrum, as given by the standard linear dispersion relation. But sometimes the measured offsets deviate significantly from the model calculations, and the interpretation in terms of gyro lines becomes questionable. The discrepancy could possibly be explained by local deviations in the magnetic field from the model (IGRF 1987), which are generated by incoming particle beams. A more serious problem with the gyro-line theory is how the line can be excited at altitudes where the collisional damping is substantial. The high intensity and short lifetime of the signal point to a fast-growing plasma instability as the likely excitation mechanism, if the gyro-line interpretation is correct. The cause of the instability could be the same particle beams as those causing the disturbances in the magnetic field. Alternatively, the observations may be interpreted as meteor head echoes. The large Doppler shifts, the short lifetimes and the altitudes of the signals support this explanation. The main difficulty is that the distribution of measured offsets appears to be different in magnetically active conditions and in less active conditions. Also, the occurrence of echoes does not seem to follow the expected changes in meteor density. More observations in different conditions are needed to decide between the two interpretations. As it is, we are inclined to believe in the meteor head echo theory, the objections to the gyro-line theory being more fundamental.     

Operational Processing of Hydroacoustics at the Prototype International Data Center

— The Prototype International Data Center (PIDC) has designed and implemented a system to process data from the International Monitoring System's hydroacoustic network. The automatic system detects and measures various signal characteristics that are then used to classify the signal into one of three categories. The detected signals are combined with the seismic and infrasonic detections to automatically form event hypotheses. The automatic results are reviewed by human analysts to form the Reviewed Event Bulletin (REB). Continuous processing of hydroacoustic data has been in place since May 1997 and during that time a large database of hydroacoustic signals has been accumulated. For a two-year period, the REB contains 13,582 T phases that are associated to 8,437 events. This is roughly 25% of REB events after taking station downtime into account. Predicted travel times used in locations are based on the arrival time of the peak a mplitude mode calculated from a normal mode propagation model. Global sound velocity and bathymetry databases are used to obtain reliable 2-D, seasonally dependent, travel-time tables for each hydroacoustic station in the PIDC. A limited number of ground-truth observations indicate that the predicted travel times are good to within 5 seconds for paths extending to over 7,000?km – corresponding to a relative error of less than 0.1%. The ground truth indicates that the random errors in measuring arrival times for impulsive signals are between 1 and 6 seconds. This paper describes and evaluates the automatic hydroacoustic processing compared to the analyst reviewed results. In addition, special studies help characterize the overall performance of the hydroacoustic network.     

Pb and Nd isotope constraints on the origin of high Mg and tholeiitic amphibolites,Kolar Schist Belt,South India

The late Archean, north-south trending Kolar Schist Belt in south India, 4 km wide by 80 km long, is thought to be a suture between two gneiss terranes (Krogstad et al. 1989). Within this volcanics-dominated belt are recognized both tholeiitic and high Mg (komatiitic and picritic) amphibolites, which make up some 70% and 5% respectively of the exposed outcrops. A massive tholeiitic amphibolite separates the belt into western and eastern parts. A Pb-Pb whole-rock age of 2732±155 Ma on samples from a single outcrop of massive tholeiite is a minimum age for this rock. Samples of this rock have _Nd values at 2700 Ma that range between +3.8 and +6.8, ₁ (initial ²³⁸U/²⁰⁴Pb) of 7.5 and K ₁ (initial ²³²Th/²³⁸U) of about 4. Two different types of high-Mg amphibolites are recognized from the western part of the belt: a picritic or P-type, and a komatiitic or K-type. The P-type have highly variable Ce/Al ratios all greater than chondritic, Nd/Yb ratios greater than chondritic, _Nd at 2700 Ma of +1.5 to +8, and Pb isotope compositions variable in ²⁰⁷Pb/²⁰⁴Pb with ₁ of about 8.0 and k ₁ of about 4. The trace-element data suggest that the light-REE enrichment is a character of the mantle source and is not due to residual garnet. The K-type amphibolites have near chondritic Ce/Al and Nd/Yb ratios, _Nd at 2700 Ma of +1.5 to +8, and ₁ of about 8 and k ₁ of about 4. Although the P-type is light-REE enriched compared to the K-type, both types have similar Ce/Nd ratios as well as initial Pb and Nd isotopes. If the 2696±136 Ma age for the Sm-Nd isochron which includes both types of high-Mg amphibolite has any significance it dates the time of light-REE enrichment of the mantle source for the P-type komatiitic amphibolites. The high-Mg amphibolites in the eastern part of the belt are light-REE enriched, have Pb isotopic compositions that are variable in ²⁰⁷Pb/²⁰⁴Pb with a ₁ about 8.5 and _Nd at 2700 Ma of +1.8 to +4.5. Hydrothermal fluids associated with metamorphism and shearing prior to about 2400 Ma ago were responsible for the introduction of gold-quartz-carbonate veins into the Kolar Schist Belt. The Pb isotope composition of galena in these veins suggests that these fluids may have also introduced extraneous Pb from adjacent older granitoid gneisses into the amphibolites, which could be responsible for the variability in the ²⁰⁷Pb/²⁰⁴Pb ratios of the samples. This extraneous Pb probably is not responsible for the distinct Pb isotope character of each type of amphibolite.     

Consistent fractionation of 13C in nature and in the laboratory: growth-rate effects in some haptophyte algae

The carbon isotopic fractionation accompanying formation of biomass by alkenone-producing algae in natural marine environments varies systematically with the concentration of dissolved phosphate. Specifically, if the fractionation is expressed by epsilon p approximately delta e - delta p, where delta e and delta p are the delta 13C values for dissolved CO2 and for algal biomass (determined by isotopic analysis of C37 alkadienones), respectively, and if Ce is the concentration of dissolved CO2, micromole kg-1, then b = 38 + 160*[PO4], where [PO4] is the concentration of dissolved phosphate, microM, and b = (25 - epsilon p)Ce. The correlation found between b and [PO4] is due to effects linking nutrient levels to growth rates and cellular carbon budgets for alkenone-containing algae, most likely by trace-metal limitations on algal growth. The relationship reported here is characteristic of 39 samples (r2 = 0.95) from the Santa Monica Basin (six different times during the annual cycle), the equatorial Pacific (boreal spring and fall cruises as well as during an iron-enrichment experiment), and the Peru upwelling zone. Points representative of samples from the Sargasso Sea ([PO4] < or = 0.1 microM) fall above the b = f[PO4] line. Analysis of correlations expected between mu (growth rate), epsilon p, and Ce shows that, for our entire data set, most variations in epsilon p result from variations in mu rather than Ce. Accordingly, before concentrations of dissolved CO2 can be estimated from isotopic fractionations, some means of accounting for variations in growth rate must be found, perhaps by drawing on relationships between [PO4] and Cd/Ca ratios in shells of planktonic foraminifera.     

A new formula for the total longshore sediment transport rate

A new predictive formula for the total longshore sediment transport (LST) rate was developed from principles of sediment transport physics assuming that breaking waves mobilize the sediment, which is subsequently moved by a mean current. Six high-quality data sets on hydrodynamics and sediment transport collected during both field and laboratory conditions were employed to evaluate the predictive capability of the new formula. The main parameter of the formula (a transport coefficient), which represents the efficiency of the waves in keeping sand grains in suspension, was expressed through a Dean number based on dimensional analysis. The new formula yields predictions that lie within a factor of 0.5 to 2 of the measured values for 62% of the data points, which is higher than other commonly employed formulas for the LST rate such as the CERC equation or the formulas developed by Inman–Bagnold and Kamphuis, respectively. The new formula is well suited for practical applications in coastal areas, as well as for numerical modeling of sediment transport and shoreline change in the nearshore.     

Ion temperature troughs in the equatorial topside ionosphere

The Retarding Potential Analyzer aboard OGO-6 sometimes recorded marked depressions of ion temperature as the satellite crossed the equatorial region. These “T_i troughs” occur at heights between about 700 km and the satellite apogee at 1100 km. At the centre of a trough, close to the dip equator, T_i is frequently 500–1000 K below its value at the northern and southern edges, which are usually 15°–20° in latitude from the centre of the trough. At a given season and local time, the occurrence, symmetry, depth and position of the troughs often vary markedly with longitude. The troughs have no particular association with equatorial troughs of ion concentration N_i.As suggested by Hanson, Nagy and Moffett, the T_i troughs appear to be caused by transequatorial winds that drive F region plasma along geomagnetic field lines. The plasma is adiabatically cooled as it is driven upwards on the “upwind” side of the dip equator, and heated as it descends on the “downwind” side. The available data on the occurrence of troughs at different longitudes, local times and seasons are reasonably consistent with wind directions deduced from Jacchia's model and the OGO-6 thermospheric model of Hedin et al., and with the north-south asymmetries of the tropical 630 nm airglow observed by OGO-4 and OGO-6. Factors determining the latitudinal extent of the troughs are discussed and some questions for further study are listed.     

Geochemistry and origin of massif-type anorthosites

Samples of Proterozoic anorthosite complexes from the Adirondack Mountains of New York, Burwash Area of Ontario, and the Nain Complex of Labrador, ranging in composition from anorthosite to anorthositic gabbro, have been analyzed for major elements, Rb, Sr, Ba and nine rare-earth elements (REE), in order to set limits on the compositions and origins of their parent magmas. Similar rock types from the different areas have similar major and trace element compositions. The anorthosites have high Sr/Ba ratios, low REE abundances (Ce about 10, Yb about 0.5–1.5 times chondrites) and large positive Eu anomalies. The associated anorthositic gabbros have lower Sr/Ba ratios, REE abundances nearly an order of magnitude higher than the anorthosites, and small to negligible positive Eu anomalies.Model calculations using the adcumulate rocks with the lowest REE abundances and published distribution coefficients yield parent liquids having REE abundances and patterns similar to those of the associated anorthositic gabbros with the highest REE abundances. Rocks with intermediate REE abundances are the result of incorporation of a liquid component by a plagioclase-rich cumulate similar to the adcumulate samples. The analytical data and model calculations both suggest parent liquids having compositions of 50–54% SiO₂, greater than 20% Al₂O₃, about 1% K₂O, atomic Mg/(Mg+Fe²⁺) ratios (Mg No.'s) of less than 0.4, 15–30 ppm Rb, 400–600 ppm Sr and 400–600 ppm Ba, 40–50 times chondrites for Ce and 8–10 times chondrites for Yb.The low atomic Mg/(Mg+Fe²⁺) values for these rocks combined with geophysical evidence suggesting there are not large quantities of ferromagnesian material at depth, indicate that the anorthositic masses are not products of fractional crystallization of mafic melt derived from melting of the mantle. Rather, it is suggested that they are a result of partial melting of tholeiitic compositions at depths shallower than the basalt-eclogite transformation, leaving a pyroxene-dominated residue.     

The application of trace elements to the petrogenesis of igneous rocks of granitic composition

Although trace element modeling has been used to great advantage for petrogenetic interpretations of basaltic systems, similar studies on igneous rocks of granitic composition have been fewer. In general the mineral/melt distribution coefficients for rare earth elements (REE) in granitic melts are equal to or greater than those for similar minerals in the basaltic system. Thus the effects of these minerals on the REE patterns of granitic melts during partial melting or differentiation are exaggerated as compared to basaltic systems, making detection of residual phases easier. For the K/Rb ratio, if neither a K-feldspar component nor biotitephlogopite is present in the residue, it is difficult to reduce the K/Rb ratio of the melt relative to the parent by a factor of two by either differentiation or partial melting.The petrogenesis of four distinctly different rocks are received: (1) an Archean tonalite presumably derived by partial melting of an Archean tholeiite at mantle depths, leaving a garnet plus clinopyroxene residue; (2) an Archean quartz monzonite presumably derived by partial melting of a short-lived graywacke-argillite sequence at crustal depths; (3) a dacite from Saipan presumably derived by differentiation from a basaltic parent; and (4) a trachyte from Ross Island, Antarctica, presumably derived by differentiation from a basanitoid parent and contaminated by continental crustal components.