searching for ulf signatures of the cusp observations from search coil magnetometers and auroral imagers in svalbard 1 spacecraft traveling through the cusp at a variety of altitudes have consistently found the cusp to be filled with intense often irregular power in the upper ulf frequency range some ground based studies have observed narrowband waves in this same frequency range in the vicinity of the cusp foot point but it has not been possible with magnetometers alone to either confirm or deny a cusp source for these waves we report here on the first simultaneous collocated observations of a set of induction magnetometers installed at three near cusp sites on svalbard and an all sky auroral imager located at longyearbyen data during northern winters of 2006 2007 and 2007 2008 when the cusp foot point was in darkness showed occasional broadband noise when energetic particle precipitation occurred overhead but on most days no broadband ulf power was observed above the noise level near noon when only soft cusp precipitation or poleward moving auroral forms occurred overhead however on 3 days including 15 january 2007 several bursts of band limited pc 1 2 waves were observed in association with regions of intense soft precipitation that peaked near the poleward edge of the cusp their properties are consistent with an origin in the plasma mantle as observed in a recent satellite ground study by engebretson et al 2005 these observations confirm that even intense cusp precipitation is not effective in generating ion cyclotron waves that penetrate to the ground if it is embedded within the central regions of the cusp whereas regions of enhanced precipitation at the poleward edge of the cusp are associated with observed waves

crustal evolution in the central congo kasai craton luebo d r congo insights from zircon u pb ages hf isotope and trace element data abstract analysis of u pb ages hf isotopes and trace element compositions in zircon collected from modern drainages is a robust methodology for tracking crustal and magmatic evolution the application of the technique to zircons from the luebo area in the central part of the congo kasai craton indicates the presence of a 3 6 ga crust that underwent late archean 2 9 2 5 ga reworking very low initial hf isotope ratios in zircons with u pb ages between 1 9 and 2 6 ga suggest the presence of crust even older than 3 6 ga at depth the archean crust underwent further reworking during paleoproterozoic mesoproterozoic and neoproterozoic thermal events post archean generation of juvenile crust appears to have occurred only sporadically during meoproterozoic neoproterozoic time the magmatic activity and tectonic events observed in the neoproterozoic katangan belt had a regional extent as they also impacted the congo kasai craton where alkaline magmatism has occurred from late archean to early cambrian time the ages of zircons with low hree and u contents suggest three separate episodes of kimberlite magmatism in the central part of the congo kasai craton in late archean 2 8 2 6 ga neoproteorozoic 850 582 ma and cretaceous 116 70 ma time

instantaneous dynamics of the cratonic congo basin understanding the formation mechanisms of cratonic basins provides an examination of the rheological compositional and thermal properties of continental cratons however these mechanisms are poorly understood because there are few currently active cratonic basins one cratonic basin thought to be active is the congo basin located in equatorial africa the congo basin is coincident with a large negative free air gravity anomaly an anomalous topographic depression and a large positive upper mantle shear wave velocity anomaly localized admittance models show that the gravity anomaly cannot be explained by a flexural support of the topographic depression at the congo we analyze these data and show that they can be explained by the depression of the congo basin by the action of a downward dynamic force on the lithosphere resulting from a high density object within the lithosphere we formulate instantaneous dynamic models describing the action of this force on the lithosphere these models show that the gravity and topography of the congo basin are explained by viscous support of an anomalously dense region located at 100 km depth within the lithosphere the density anomaly has a magnitude within the range of 27 60 kg m 3 and is most likely compositional in origin our models do not provide a constraint on the lithospheric viscosity of the congo craton because the shallow location of the anomaly ensures strong coupling of the anomaly to the surface regardless of viscosity structure

ring complexes in the peninsular ranges batholith mexico and the usa magma plumbing systems in the middle and upper crust subvolcanic ring complexes are unusual in that they preserve a rapidly frozen record of intrusive events this sequential history is generally lost or complicated in plutons owing to mixing and mingling in a dynamic state thus subvolcanic ring complexes are more like erupted rocks in their preservation of instantaneous events but the self contained nature of the complexes allows detailed structural and chemical work to be conducted in environments where the relative timing between individual magmatic events is commonly well preserved we suggest that development of subvolcanic ring complexes in the western peninsular ranges batholith prb involved the following three stage generalized sequence 1 fracturing of the roof above a buoyant or overpressured magma chamber which resulted in moderately inward dipping conical fractures that locally hosted cone sheets 2 subsequent loss of magma from the chamber combined with degassing of the melt which facilitated collapse of the roof along near vertical ring faults that locally hosted ring dikes and 3 resurgence of the chamber and or intrusion of a broadly cogenetic nested pluton which locally destroyed evidence for the earlier history of the system this sequence has been repeated twice in one of the ring complexes that we have identified which resulted in nested intrusive centers calderas subvolcanic ring complexes and plutons may represent progressively deeper sections through linked magma plumbing systems and the systematic occurrences of these features in the western prb are consistent with progressively deeper along strike exposures of the batholith from south to north over a distance greater than 250 km in addition to subvolcanic complexes in the western prb deeper crustal levels exposed in the transition zone between eastern and western parts of the batholith preserve ring complexes emplaced at depths of up to 18 km occurrence of these deeper level complexes suggests either that caldera subsidence can extend to mid crustal levels or that other processes can produce ring complexes d 2002 elsevier science b v all rights reserved

reply to comment by roberta m hotinski lee r kump and karen l bice on could the late permian deep ocean have been anoxic

ionospheric observations of magnetospheric low latitude boundary layer waves on august 4 1991 we report on a coordinated set of observations from greenland magnetometers sondrestrom incoherent scatter radar dmsp f9 and f10 satellite data and imp 8 measurements of the interplanetary magnetic field imf and solar wind plasma these data were acquired during an interval august 4 1991 1100 1400 ut which is characterized by magnetic ulf pulsations observed on the ground which occur in a limited band between 73 and 78 magnetic latitude the spatial characteristics of the ground ulf magnetic pulsation disturbances are consistent with a continuous train of ionospheric convection vortices moving westward across local magnetic noon sector with a phase velocity of 1 3 km s the vortices occur at the ionospheric convection reversal boundary defense meterological satellite program dmsp drift meter data are consistent with the incoherent scatter convection measurements and dmsp particle data indicate that the ionospheric reversal boundary maps to the low latitude boundary using magnetometer stations across greenland from the east coast central greenland and the west coast we can determine that individual vortices are long lived lasting over 15 min cross correlations of the pulsation wave forms at stations separated longitudinally by about 500 km produce coefficients between 0 65 and 0 75 cross correlations between stations separated longitudinally by 1000 km produce coefficients of only 0 39 this indicates that while individual vortices are long lived they are evolving significantly in shape latitudinal location and intensity as they move distances of 1000 km across the ionosphere using a field aligned current filament model to fit the data we find the strength 50 to 100 ka of these currents to be larger than typical currents previously reported we find solar wind pressure variations an unlikely source for the generation of these pulsations and suggest that they are the ionospheric signature of either a kelvin helmholtz mechanism or perhaps magnetic merging during an interval of imf oriented predominantly in the by direction

detailed structural analysis of the rim of a large complex impact crater bosumtwi crater ghana the 1 ma bosumtwi crater in ghana is an 11 km diameter presumably complex well preserved impact structure that is associated with the ivory coast tektite strewnfield detailed structural geologic studies along a complete traverse through the northwestern rim section indicated four zones characterized by distinct deformation styles from just outside of the crater rim to near the crater floor zone 1 is dominated by thick deposits of lithic impact breccia intercalated in places with products of local mass wasting zone 2 contains inward dipping thrust planes conjugate radial fractures isoclinal folding and overturned stratigraphic sequences zone 3 represents a megabreccia zone in which block size decreases upward and outward toward the rim crest the innermost zone 4 is dominated by intense thrust faulting of multiple orientations resulting in complex duplex and lens shaped bodies these deformation styles generally correspond to those previously reported from the rims of simple bowl shaped meteorite impact craters and appear to be characteristic of impact structures in general

gps tec variations in the polar cap ionosphere solar wind and imf dependence this statistical study examines the solar wind dependence of total electron content tec variations arising from mesoscale tens to hundreds of kilometers structuring of the polar cap ionosphere six years of tec measurements were collected from five high data rate global positioning system gps receivers of the canadian high arctic ionospheric network chain from which high resolution magnetic local time latitude maps of tec variation occurrence rate and amplitude were created ionosonde radars were used to identify tec variations arising from ionization of the e and f region ionospheres statistical tec maps were examined as a function of solar wind and interplanetary magnetic field imf measurements statistical results showed that occurrence rate of tec variations was highest in localized dayside regions with exact local time and latitude of peak occurrence depending primarily on the dayside coupling rate of the solar wind and magnetosphere as well as imf orientation and magnitude in the y z plane occurrence of tec variations throughout the polar cap increased with solar wind magnetosphere coupling rate and imf magnitude the solar wind dependence of occurrence rate largely reflected the location and rate of dayside magnetic reconnection and subsequent particle precipitation and polar cap convection amplitudes of tec variations were largest around noon and increased throughout the polar cap with increased solar wind magnetosphere coupling rate these statistical results improve upon the existing observational picture of the polar ionosphere and will potentially facilitate development of models and techniques for mitigating impacts of the polar ionosphere on navigation signals and communication links

permian climates evaluating model predictions using global paleobotanical data the most recent global icehouse hothouse climate transition in earth history occurred in the permian warmer polar conditions relative to today existed from the middle permian through the mesozoic and into the cenozoic we focus here on one particularly well correlated middle permian stage that postdated the deglaciation the wordian 267 264 ma integrating floral and lithological data to determine wordian climates globally paleobotanical data provide the best means of interpreting terrestrial paleoclimates often revealing important information in the continuum between dry and wet end member lithological indicators such as evaporites and coals new statistical analyses of wordian floras worldwide have enabled a greater understanding of original vegetation patterns and prevailing climate conditions the derived climate interpretations are compared with new wordian atmospheric general circulation model simulations the model matches the data well in the tropics and northern high latitudes but predicts colder conditions in southern high latitudes we discuss possible reasons for this discrepancy

characteristics of gps tec variations in the polar cap ionosphere this paper presents statistical characteristics occurrence rate amplitude and frequency of low frequency 100 mhz variations in total electron content tec observed in the polar cap ionosphere tec variations were primarily associated with mesoscale tens to hundreds of kilometers ionization structures and were observed by five global positioning system gps receivers over a 6 year period 2009 2014 the altitude of ionization structures was estimated by using colocated ionosonde radars high data rate receivers combined with broad spatial coverage of multisatellite tec measurements provided high resolution magnetic local time latitude maps of tec variation characteristics which were examined as a function of solar cycle and season these high resolution maps improve upon the current observational picture of mesoscale structuring in the polar cap and provide accurate links to particular magnetospheric source regions occurrence of tec variations was consistently highest in dayside regions mapping to low latitude and plasma mantle boundary layers while largest amplitude tec variations were observed in dayside regions close to the polar cusp and lower latitudes around midnight occurrence and amplitude of tec variations increased significantly during the ascending phase of the solar cycle independent of solar wind conditions while seasonal statistics showed highest dayside occurrence and amplitude in winter months lowest in summer and highest nightside occurrence and amplitude around equinox a surprising result in the frequency distributions of tec variations was discrete frequencies of about 2 and 4 mhz which appeared to originate from regions corresponding to the plasma mantle immediately poleward of the polar cusp

a proterozoic rift origin for the structure and the evolution of the cratonic congo basin abstract we interpret the cratonic congo basin a large circular cuvette centrale filled with up to 9 km of proterozoic to neogene sediments as the consequence of a neo proterozoic rift firstly the magnitude and the long term subsidence are consistent with the thermal time constant of a 200 250 km thick lithosphere inferred from several tomographic studies secondly the surface accumulation of sediments is compensated at depth by crustal thinning whose magnitude can be estimated from the analysis of the surface gravity after backstripping the effect of the sediments a residual nw se positive and narrow gravity anomaly is observed across the cuvette centrale and is interpreted as the remaining crustal thinning associated with this rift assuming that isostasy is governed by a necking level and a flexural response to sediment loads we have estimated the combination of the depth of necking and the equivalent elastic thickness of the lithosphere that provide the best fit with the residual gravity i e 10 km and 100 km respectively the corresponding uplift of the upper mantle is in the continuity of the mbuji mayi supergroup to the se and the liki bembian group to the nw these two groups represent older stages of rifting in the congo craton which shows that rifting has periodically affected and weakened the cuvette centrale during a long period of time

location of highhaltitude cusp during steady solar wind conditions we have investigated the highhaltitude cusp location and its dependence on the interplanetary magnetic field limfr bz solar wind ey and solar wind dynamic pressure during steady solar wind conditions we have adopted strict event selection criteria where we have studied only events during stationary imf bz when the cusp latitude is supposed to be stationary prior to this study the statistical studies have shown a great deal of scatter between the cusp latitude and the imf bz however with the strict event selection criteria we have been able to reduce the scatter the strongest source of scatter comes from large and largely fluctuating imf by when the events are selected during small imf by the scattering between the cusp latitude and the imf bz is reduced when imf is southward also the fluctuation of imf by has strong influence on the cusp latitude whereas for northward imf the imf by fluctuation has less influence on the cusp latitude for larger southward imf the cusp appears further equatorward than with smaller southward imf when the scatter is reduced by introducing the imf by control the larger northward imf brings the cusp to lower latitudes than the smaller northward imf i e the maximum cusp latitude is obtained for small positive imf bz for increasing solar wind ez the cusp moves equatorward furthermore with increasing solar wind dynamic pressure the cusp appears at lower latitudes this behavior depends on the orientation of the imf bz with southward imf the cusp moves faster equatorward than with northward imf

four large scale field aligned current systems in the dayside high latitude region a system of four current sheets of large scale field aligned currents facs was discovered in the data set of simultaneous viking and dmsp f7 crossings of the dayside high latitude region this paper reports four examples of this system that were observed in the prenoon sector the flow polarities of facs are upward downward upward and downward from equatorward to poleward the lowest latitude upward current is flowing mostly in the cps precipitation region often overlapping with the bps at its poleward edge and is interpreted as a region 2 current the pair of downward and upward facs in the middle of the structure are collocated with structured electron precipitation the precipitation of high energy 1 kev electrons is more intense in the lower latitude downward current sheet the highest latitude downward flowing current sheet is located in a weak low energy particle precipitation region suggesting that this current is flowing on open field lines simultaneous observations in the postnoon local time sector reveal the standard three sheet structure of facs sometimes described as region 2 region 1 and mantle referred to the midday region 0 currents a high correlation was found between the occurrence of the four fac sheet structure and negative interplanetary magnetic field imf by we discuss the fac structure in terms of three types of convection cells the merging viscous and lobe cells during strongly negative imf by two convection reversals exist in the prenoon sector one is inside the viscous cell and the other is between the viscous cell and the lobe cell this structure of convection flow is supported by the viking electric field and auroral uv image data based on the convection pattern the four fac sheet structure is interpreted as the latitudinal overlap of midday and morning fac systems we suggest that the four current sheet structure is common in a certain prenoon local time sector during strongly negative imf by

global warming and the end permian extinction event proxy and modeling perspectives abstract the mass extinction event that occurred at the close of the permian period 252 million years ago represents the most severe biodiversity loss in the ocean of the phanerozoic the links between the global carbon cycle climate change and mass extinction are complex and involve a whole range of often inter related geochemical biological ecologic and climatic factors it has become widely accepted that the end permian mass extinction was associated with a global warming event because the age of the siberian trap eruption a potentially massive source of carbon dioxide coincides within error with the extinction event however geologic data that are in support of this global warming event are relatively sparse the chain of events and the causal relationship between the eruption of siberian traps and mass extinction is not well established global warming in particular has only been reported from limited proxy data and climate models for which the pco2 in the atmosphere just before and during the end permian extinction event is poorly known this study critically assesses both the proxy climate data and the existing paleoclimate simulations with the goals of assessing our current understanding of the link between mass extinction and climate change and providing some guidance for future studies proxies indicate that prior to the end permian extinction event tropical sea surface temperatures ranged from 22 to 25 c and possible pco2 values ranged from 500 to 4000 ppm before the extinction event during the peak extinction tropical temperatures rose up to 30 c while pco2 perhaps increased up to 8000 ppm climate models that use different pre event pco2 values show similar amount of co2 doubling to replicate the observed carbon isotope excursions we find that the temperature anomaly during the end permian extinction is consistent with 1 5 doublings of atmospheric pco2 and that the implied climate sensitivity is 5 6 c within the upper range of values produced by climate models

characteristics of the postnoon auroras inferred from eiscat radar measurements svalbard european incoherent scatter eiscat radar and polar satellite ultraviolet imager data have been used to investigate structure and dynamics of the postnoon auroral oval in the so called 14 mlt region southward turnings of the interplanetary magnetic field were associated with bright afternoon and evening arcs which drifted equatorward increase in the equatorward arc velocity occurred about 27 7 min after the imf bz turned southward at the subsolar point the observed equatorward motion may occur independently of the convection in one of the cases the velocity of the drifting arc was observed to be about 100 200 m s southward whereas the ionospheric plasma drifted northward at a velocity of 160 m s the arcs were located in the region of convection shear that was shown by the ground magnetometers and the eiscat data on ion velocity and temperature in the cases when electric field data were available the meridional component of the electric field varied from about 60 mv m southward equatorward of the arc to 30 mv m northward poleward of the arc ionospheric plasma parameters were measured during the passes of the arcs through the radar beam downward field aligned ion flow at the 300 550 km altitude was observed poleward of the arc whereas upward flows were observed equatorward of the arc this variation is explained by the difference in the vertical gradient of plasma density convection flows associated with arc and differences in electron precipitation and sunlight conditions create the vertical gradients of plasma density that determine the ambipolar diffusion and observed ion motion

premonitory acoustic emissions and stick slip in natural and smooth faulted westerly granite 1 a stick slip event was induced in a cylindrical sample of westerly granite containing a preexisting natural fault by loading at constant confining pressure of 150 mpa continuously recorded acoustic emission ae data and computer tomography ct generated images of the fault plane were combined to provide a detailed examination of microscale processes operating on the fault the dynamic stick slip event considered to be a laboratory analog of an earthquake generated an ultrasonic signal that was recorded as a large amplitude ae event first arrivals of this event were inverted to determine the nucleation site of slip which is associated with a geometric asperity on the fault surface ct images and ae locations suggest that a variety of asperities existed in the sample because of the intersection of branch or splay faults with the main fault this experiment is compared with a stick slip experiment on a sample prepared with a smooth artificial saw cut fault surface nearly a thousand times more ae were observed for the natural fault which has a higher friction coefficient 0 78 compared to 0 53 and larger shear stress drop 140 compared to 68 mpa however at the measured resolution the ultrasonic signal emitted during slip initiation does not vary significantly between the two experiments suggesting a similar dynamic rupture process we propose that the natural faulted sample under triaxial compression provides a good laboratory analogue for a field scale fault system in terms of the presence of asperities fault surface heterogeneity and interaction of branching faults

resolving the richat enigma doming and hydrothermal karstification above an alkaline complex the richat structure sahara mauritania appears as a large dome at least 40 km in diameter within a late proterozoic to ordovician sequence erosion has created circular cuestas represented by three nested rings dipping outward from the structure the center of the structure consists of a limestone dolomite shelf that encloses a kilometer scale siliceous breccia and is intruded by basaltic ring dikes kimberlitic intrusions and alkaline volcanic rocks several hypotheses have been presented to explain the spectacular richat structure and breccia but their origin remains enigmatic the breccia body is lenticular in shape and irregularly thins at its extremities to only a few meters the breccia was created during karst dissolution and collapse internal sediments fill the centimeter to meter scale cavities alkaline enrichment and the presence of cretaceous automorphous neoformed k feldspar demonstrate the hydrothermal origin of these internal sediments and their contemporaneity with magmatism a model is proposed in which doming and the production of hydrothermal fluids were instrumental in creating a favorable setting for dissolution the circular richat structure and its breccia core thus represent the superficial expression of a cretaceous alkaline complex with an exceptionally well preserved hydrothermal karst infilling at its summit

signatures of the ionospheric cusp in digital ionosonde measurements of plasma drift above casey antarctica signatures of the ionospheric cusp in hf digital ionosonde measurements of plasma drift made at the polar cap station casey antarctica 80 8 geomagnetic latitude are investigated measurements recorded during the campaign interval february 13 17 1996 are considered in this case study because the summer dipole tilt effect and an interplanetary magnetic field imf northward condition on february 16 were favorable for the detection of the cusp at a higher than usual latitude on february 14 and 15 the magnitude of the imf was about 4 6 nt and the station probably passed just poleward of the cusp the most general signatures of the cusp were enhanced electric field and electric field turbulence shown by increased drift velocity and velocity scatter in the convection throat respectively broadband unstructured fluctuations in the geomagnetic field measured by magnetometers near to noon are well known to be a signature of cusp currents and were associated with the intervals of enhanced convection turbulence a major cusp event occurred above casey on february 16 when the magnitude of the imf increased from about 5 to 10 nt cusp signatures during this event included the drift velocity surging to large values just before and after an interval during which the f region echoes were lost because of an absence of f region ionization and the formation of electron density patches the loss of echoes was only partly explained by increased absorption and scatter of the transmitted radio waves although the spectral width of doppler peaks increased this by itself was not a unique signature of the cusp because the obliquity of echoes also controlled the spectral width in our near vertical interferometry however signatures of the cusp were easily recognized in digisonde data and the cusp s location and dynamics can be monitored using digisondes

the early triassic ammonoid recovery paleoclimatic significance of diversity gradients ammonoids recovered much faster than other marine shelly invertebrates after the end permian mass extinction based on a refined global data set at the basin level we investigate the paleobiogeographical global latitudinal and longitudinal diversity patterns in terms of climatic changes during the early triassic such analysis differs from already published qualitative or quantitative studies in that it estimates faunal patterns and endemicity at an ecologically meaningful level of spatial resolution i e at the basin level during the early triassic the global first order trend in increasing ammonoid diversity was accompanied by a progressive change from cosmopolitan to latitudinally restricted distributions this led to the emergence of a clear latitudinal diversity gradient during most of the smithian and spathian stages which entails increased steepness of the sea surface temperature gradient during the late early triassic however two brief episodes of ammonoid cosmopolitanism combined with low global diversity interrupted the first order increasing trend at the very beginning of the smithian and at its very end the longitudinal analysis of smithian distributions indicates a westward decrease of diversity within the tethys which faded away during the spathian analysis of endemicity indicates a rapid biogeographical maturing and structuring of faunas concomitant with the edification of the latitudinal diversity gradient

the low latitude boundary layer and the boundary plasma sheet at low altitude prenoon precipitation regions and convection reversal boundaries the dayside zone of soft precipitation can be divided into four distinct types of plasma regimes each corresponding to the respective magnetospheric source region the cusp the mantle the low latitude boundary layer llbl and the dayside extension of the bps based on a detailed spectral study including comparisons with nonsimultaneous isee 1 satellite llbl data we identify regions of llbl type plasma in the dmsp data set and compare these plasma boundaries with convection reversal boundaries crbs as determined by either sondrestrom or the drift meter instrument on board the dmsp f9 spacecraft the nine cases considered are all in the prenoon local time sector we find that in eight of the nine cases the crb occurs within the llbl as expected generally near to but not coincident with the equatorward edge of the llbl type plasma in our sample set chosen for cases with latitudinally wide easily identifiable llbl signatures the average latitudinal width was 1 85 magnetic latitude the crb defined as the onset of steady antisunward convection occurred about 30 of this width beyond the equatorward onset of llbl type particles the most equatorward portion of the region with llbl type plasma usually had near zero or erratic convection and may correspond to the stagnation region reported from isee observations the potential drop observed across the low altitude llbl is roughly estimated to be typically 5 kev a summary is given on how the various high altitude sources can be identified when plasma regions are observed at low altitude in the dayside auroral oval

late palaeozoic deformation in central africa a result of distant collision seismic reflection profiles from the cuvette centrale central basin of zaire show that a major contractional deformation affected a part of central africa during the late permian and early triassic periods at the same time other regions of central africa experienced extensional and strike slip deformation and associated rift formation the coexistence of such varied styles of tectonic activity over a large continental area resembles the cenozoic tectonics of central asia and northwest europe the intracontinental deformation seen in these latter areas is attributed to collisional processes at a distant continental margin1 3 by analogy we argue here that deformation of central africa in the late palaeozoic period was a direct result of collisional processes some distance away at the southern margin of gondwana

temporal and spatial aspects of the cusp inferred from local and global ground and space based observations in a case study 1 global and local observations from both ground and space facilities are brought to bear to understand the origin of a stepped cusp ion dispersion observed 3 hours prenoon under an interplanetary magnetic field characterized by strongly positive by conditions under which the cusp is believed to be shifted postnoon our main aim is to distinguish the temporal from the spatial cusp features the local observations from svalbard are from approximately the 0900 1200 mlt sector while the in situ observations are from polar at 0900 mlt the global auroral observations were acquired by the earth camera on polar the aurora from the ground perspective appeared as a periodic sequence of brightening events located on either side of a midday gap aurora centered at about 1030 mlt at noon the aurora appeared as a semiperiodic sequence of brightening events these were concurrent with enhancements of the overhead ionospheric hall current polar observed two steps of a staircase ion energy latitude dispersion these two steps were in turn concurrent with two consecutive auroral brightening events we argue from this complement of observations that the stepped cusp was temporal in nature and resulted from a time varying reconnection process global uv imagery was employed to complement the ground observations for the purpose of separating the temporal from the spatial aspects the imager captured namely two bands of strong uv emission in the prenoon and postnoon sectors between which there appeared a persistent weak uv aurora we interpret this to be an underlying spatial substratum patchy and bursty uv enhancements coincided with the transients seen in the local ground and space measurements corroborating further our distinction between space and time in the cusp comparing these inferences with recent work on the response of the dayside aurora to reconnection at the magnetopause we find a close correspondence in addition we document the field aligned currents associated with the stepped cusp which in turn compare favorably with recent theoretical ideas this work is a contribution to the current debate on the nature of the stepped cusp and suggests that multiple data sets are essential to study this problem

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

investigation of shuttle radar topography mission data of the possible impact structure at serra da cangalha brazil the serra da cangalha crater structure in northeast brazil 13 km in diameter has long been widely considered to be a confirmed impact structure based on reports of shatter cone findings only very limited field work has been carried out at this crater structure landsat thematic mapper tm and shuttle radar topography mission srtm data sets for the region around this crater structure are compared here with regard to their suitability to determine first order structural detail of impact crater structures the srtm data provide very detailed information regarding drainage patterns and topography a pronounced central ring of up to 300 m elevation above the surrounding area two comparatively subdued intermediate rings of 6 and 10 5 km diameter respectively and the broad complex crater rim of up to 100 m elevation can be distinguished in the serra da cangalha data the maximum cratering related regional deformation radial and concentric features seems to be limited to a radial distance of 16 18 km from the center of the structure a first comparison of macrostructural information from several impact structures with that from serra da cangalha does not yield firm trends but the database is still very small at this stage the varied nature of the target geology strongly influences the development of structural features in any impact event

simulated circulation of an idealized ocean for pangaean time an eight level dynamical ocean model driven by prescribed surface forcing wind stress temperature precipitation minus evaporation from a previously published pangaean time climate simulation is used to study possible features of ocean circulation temperature and salinity for the early mesozoic the simulations are sensitivity experiments rather than estimates of the actual circulation because many simplifying assumptions are made concerning the prescribed surface forcing and the ocean basin s shape and depth simulated features include relatively warm water in high latitudes warm deep water a westward flowing equatorial current across panthalassa and into the tethys sea poleward and eastward flowing currents along the tethys coasts and relatively vigorous meridional and longitudinal vertical overturning cells cool relatively fresh polar water combines with warm saline tethys water to produce the relatively warm deep water there are few geological estimates of early mesozoic pangaean time ocean circulation available for comparison with the simulated circulation

structure and evolution of the intracratonic congo basin 1 surface wave tomography heat flow and crustal thickness measurements have demonstrated that the thickness of the continental lithosphere varies by at least a factor of 2 since the thermal time constant of the lithosphere depends upon the square of its thickness subsidence records of extensional sedimentary basins offer a potential way of extending these observations into the past here we examine the congo basin a large and iconic intracratonic sedimentary basin in central africa this roughly circular basin covers an area in excess of 1 4 106 km2 with more than 5 km thickness of sedimentary rocks the oldest parts of which are late precambrian in age first we assess the thickness of the lithosphere we have estimated its thickness across africa using maps of shear wave velocity obtained by inversion of fundamental and higher mode surface waveforms the congo basin sits on 220 30 km thick lithosphere and appears to be part of a southern core to the continent encompassing both archean cratons and proterozoic mobile belts this thickness is consistent with published estimates from kimberlites reappraisal of legacy seismic reflection images demonstrates that the sedimentary section is underlain by a late precambrian rift zone and that the basin is still subsiding today subsidence modeling of two deep wells is consistent with uniform extension and cooling of the lithosphere by a factor of 1 2 during latest precambrian and cambrian time we argue that the exceptional 0 55 ga history of the basin is a direct consequence of the lithospheric thermal time constant being a factor of 4 longer than normal today the basin coincides with a long wavelength 30 to 40 mgal gravity anomaly we interpret this gravity anomaly as the surficial manifestation of 400 600 m of recent mantle convective drawdown in response to the onset of upwelling plumes around the flanks of the southern african continent the alternative explanation that it is the static manifestation of locally thick lithosphere is inconsistent with global trends of mantle density depletion our interpretation is consistent with fast seismic velocities observed throughout the sublithospheric upper mantle underneath the basin and recent geodynamic modeling

hf radar signatures of the cusp and low latitude boundary layer continuous ground based observations of ionospheric and magnetospheric regions are critical to the geospace environment modeling gem program it is therefore important to establish clear intercalibrations between different ground based instruments and satellites in order to clearly place the ground based observations in context with the corresponding in situ satellite measurements hf radars operating at high latitudes are capable of observing very large spatial regions of the ionosphere on a nearly continuous basis in this paper we report on an intercalibration study made using the polar anglo american conjugate radar experiment radars located at goose bay labrador and halley station antarctica and the defense meteorological satellite program dmsp satellites the dmsp satellite data are used to provide clear identifications of the ionospheric cusp and the low latitude boundary layer llbl the radar data for eight cusp events and eight llbl events have been examined in order to determine a radar signature of these ionospheric regions this intercalibration indicates that the cusp is always characterized by wide complex doppler power spectra whereas the llbl is usually found to have spectra dominated by a single component the distribution of spectral widths in the cusp is of a generally gaussian form with a peak at about 220 m s the distribution of spectral widths in the llbl is more like an exponential distribution with the peak of the distribution occurring at about 50 m s there are a few cases in the llbl where the doppler power spectra are strikingly similar to those observed in the cusp

mapping the dayside ionosphere to the magnetosphere according to particle precipitation characteristics a long standing and vexing problem in magnetospheric physics is mapping the ionosphere to the magnetosphere to date almost all information on the topic has come from magnetic field mappings herein a complementary approach is tried regions are identified based on the plasma characteristics as observed by low altitude satellites using an automated identification scheme applied to approximately 60 000 individual satellite passes through the dayside oval probability maps are computed for observing various types of plasma precipitating into the ionosphere the synthesis of the various individual maps allows the construction of an ionospheric map into the magnetosphere based on plasma characteristics the results suggest a cusp which is about 2 5 hours in total local time extent an llbl which typically extends to about 0900 mlt and a mantle which reaches to about 0800 1600 consistent with earlier results the extension of the nightside region of discrete aurora into the dayside is far more substantial at dusk than at dawn

global 30 240 kev proton precipitation in the 17 18 april 2002 geomagnetic storms 1 patterns 1 global 30 240 kev proton precipitation patterns during the 17 18 april 2002 magnetic storm events were generated using newly developed 3 hour data products of the medium energy proton and electron detector meped on board the noaa 15 and 16 satellites the observational data set was sorted in 1 degree corrected magnetic latitude cml and 8 min magnetic local time mlt bins to achieve global coverage the missing mlt data were filled by interpolation techniques in each 1 degree cml interval the resulting global 30 240 kev proton precipitation maps available on a 3 hour cadence provide new information on the development and variability in the structure of incident high energy protons on a timescale commensurate with the growth and decay of the plasma sheet and ring current the change of energetic proton precipitation patterns in the april 2002 storms is quantified in terms of three aspects hemispheric integrated total particle energy input midnight proton oval equatorward boundary and position of proton precipitation peak in a general sense as magnetospheric activity intensified the midnight equatorward boundary tended to move to the lower latitude while the precipitation peak moved equatorward and westward away from midnight in agreement with ring current motion it is well illustrated that energetic proton precipitation patterns can serve as a valuable diagnostic tool for investigating the inner magnetospheric activity

could the late permian deep ocean have been anoxic models of ocean circulation and biogeochemical cycles are used to speculate about possible mechanisms of deep sea anoxia across the permo triassic boundary two modes of likely late permian ocean thermohaline circulation are identified a vigorous thermal mode driven by cooling in southern polar latitudes and a weaker haline mode driven by evaporation from the subtropics we find that the thermal mode typical of climates such as our own is unlikely to surpport deep sea anoxia the haline mode which might exist in warm climates with enhanced hydrological cycles can lead to significant but periodic depletion of oxygen in the panthalassic deep ocean however owing to the inherent instability of the haline mode a deep sea anoxia persisting for millions of years is not clearly supported by this study though mechanisms emerge which might be consistent with the observed record the enhanced hydrological cycle or reduced oceanic diapycnal mixing required to obtain the haline mode remain unconstrained for the late permian

response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field the response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field imf is examined with multi instrument data from the viking and dmsp f7 satellites as well as ground magnetometer data the event selected which occurred on october 5 1986 was previously identified as a high latitude dayside form by murphree and elphinstone 1988 imf by was positive during most of the viking auroral uv observation whereas imf bz made a transient southward excursion followed by a sharp increase to 8 nt the results are summarized as follows 1 there were two auroral belts extending westward from the early afternoon sector the equatorward belt was persistent throughout the event and was embedded in the cps bps precipitation region whereas the poleward belt was in the open field line region and was associated with northward imf bz 2 a new auroral spot emerged in the afternoon sector just poleward of the previously active region delayed 10 min from the sudden increase in imf bz this time lag is ascribed to the travel time of the new imf orientation from the subsolar point to a solar wind magnetosphere interaction site tailward of the dayside cusp as well as to the response time of the dayside auroral acceleration process 3 the new auroral activity expanded both eastward and westward during the first few minutes and then expanded primarily westward across the noon meridian forming the poleward belt the speed of the westward extension in the second phase was comparable to the speed of the ionospheric convection 4 the dayside auroral process has a finite decay constant which is inferred to be at least 10 min 5 the response time of the midday ionospheric convection to imf variations is estimated to be a few to several minutes

ocean stagnation and end permian anoxia ocean stagnation has been invoked to explain the widespread occurrence of organic carbon rich laminated sediments interpreted to have been deposited under anoxic bottom waters at the time of the end permian mass extinction however to a first approximation stagnation would severely reduce the upwelling supply of nutrients to the photic zone reducing productivity moreover it is not obvious that ocean stagnation can be achieved numerical experiments performed with a three dimensional global ocean model linked to a biogeochemical model of phosphate and oxygen cycling indicate that a low equator to pole temperature gradient could have produced weak oceanic circulation and widespread anoxia in the late permian ocean we find that polar warming and tropical cooling of sea surface temperatures cause anoxia throughout the deep ocean as a result of both lower dissolved oxygen in bottom source waters and increased nutrient utilization buildup of quantities of h2s and co2 in the late permian ocean sufficient to directly cause a mass extinction however would have required large increases in the oceanic nutrient inventory

electrostatic potential drop across the ionospheric signature of the low latitude boundary layer 1 a study of the behavior of the ionospheric signature of the low latitude boundary layer is conducted from signatures of the precipitating electrons and the ionospheric convection velocity observations are made under southward interplanetary magnetic field imf conditions in a highly dynamic environment where temporal changes produce large excursions in the location of both the convection reversal boundary and the poleward edge of the particle precipitation associated with the outer edge of the low latitude boundary layer we describe the latitudinal width of the boundary layer as the displacement between these boundaries and the bulk plasma flow as the potential difference across the region on either side of local noon the boundary layer has a latitudinal width of about one degree widening to about two degrees near dawn and dusk the potential drop across the boundary layer increases toward local noon and accounts for no more than 30 of the total potential across the polar cap while the boundary layer is physically wider on the duskside than on the dawnside the average potential across the dawnside boundary layer is larger than that across the duskside layer the resulting electric field across the dawnside layer is approximately twice that found in the duskside layer we also find a higher level of variability in the boundary layer width and potential on the dawnside than on the duskside that may be related to higher levels of turbulence and wave activity associated with the orientation of the imf with respect to the bow shock

use of the hough transform in automated lineament most lineament mapping is done visually using enhanced images to automate this procedure partially algorithms have been developed to imitate some of the visual rules employed by geologists when mapping lineaments the hough transform is described and its capabilities for automated lineament analysis are demonstrated using part of a landsat tm image of the canadian shield near sudbury on canada the results of the automated analyses are compared with the major faults shown on a geologic map of the area and with a visual assessment of an image of the same area undertaken by two remote sensing geology specialists the results show that automated interpretation identifies more of the faults than visual interpretation

the auroral oval position structure and intensity of precipitation from 1984 onward an automated on line data base an on line data base consisting of the auroral oval boundaries structure and particle fluxes from the dmsp f7 and f9 satellites from december 1983 through the present with about an 7 month lag is announced the data are divided into distinct regions e g diffuse aurora discrete aurora cusp proper polar rain etc the magnetospheric sources of the various particle precipitation regions are identified using a sophisticated pattern recognition technique a neural network for each region the boundaries are specified in both geographic and geomagnetic coordinates and the average and peak values of the particle fluxes are given along with the average energies the dmsp satellites are continuously monitored so that data gaps are comparatively rare it is anticipated that the location of the auroral oval and precipitation intensities will be of particular interest to ground based observers although anyone interested in the state of the auroral oval may find it of value span data requests are processed on a completely automated basis with up to 48 hours of auroral oval parameters provided in response to a single request

dependencies of high latitude plasma convection consideration of interplanetary magnetic field seasonal and universal time factors in statistical patterns 1 the database of the nine radars of the super dual auroral radar network superdarn in the northern hemisphere has been analyzed for information on factors that influence the convection of plasma in the high latitude ionosphere the velocity measurements were collected over the period 1998 2002 the data were first used to derive a new statistical model of convection that improves upon the earlier one radar model of ruohoniemi and greenwald 1996 in its specification of the dependence of the convection pattern on the magnitude and direction of the imf in the gsm y z plane we then derived average patterns for secondary sortings by season year and radar such dependencies as emerged were most clearly seen by contrasting the results for by and by the seasonal effect in the convection pattern is found to have similarities to that of the sign of by in particular the combination of by summer by winter reinforces the tendency of the by sign factor to sculpt the dusk and dawn cells into more round crescent crescent round shapes and to shift the crescent cell across the midnight mlt meridian however these combinations are associated with lower estimates of the total cross polar cap potential drop pc while the nonreinforcing combinations produce elevated pc especially by summer there is an overall tendency for pc to increase from winter to summer although the pure seasonal effect on the potential drop is weaker than that of the by sign season factor we did not find pronounced differences among the patterns derived for the 5 individual years which spanned the most recent interval of solar cycle maximum sorting by radar we found few differences among the patterns for by but for by variations emerged that are consistent with a possible dependence on universal time ut the impacts of season and ut on convection in the high latitude ionosphere thus depends on the imf especially the sign of by we speculate that variability in the ionospheric conductivity has a greater effect on magnetosphere ionosphere coupling under by conditions

the response of the hf radar spectral width boundary to a switch in the imf by direction ionospheric consequences of transient dayside reconnection in the high latitude dayside ionosphere the movement of the hf radar spectral width boundary swb provides a good proxy for the movement of the open closed field line boundary around magnetic local noon by studying the dynamics of the spectral width boundary we can investigate features of the dayside ionospheric response to changes in the interplanetary magnetic field imf the high temporal and spatial resolution of the super dual auroral radar network superdarn hf radars make them good tools to study these features in this paper we use the halley hf radar in antarctica to study the equatorward motion of the swb which appears to occur in response to a large change in the direction of imf by the spectral width boundary initially moves equatorward in the form of a u shaped bulge close to magnetic local noon this bulge then expands longitudinally to earlier and later magnetic local times merged velocity vectors from two antarctic hf radars describe the flow velocity variation in the boundary region the flow equatorward of the boundary follows the contours of the boundary as it expands the flow poleward of the boundary is directed at more oblique angles to the boundary this study represents the first clear two dimensional observation of the formation of an equatorward bulge on the polar cap boundary which may be associated with changes in dayside reconnection and presents a unique observation of the variation of the ionospheric flow in the locality of the boundary we discuss the possible interpretations of this event and its consequences to our present understanding of the ionospheric response to changes in the imf

finding multiscale connectivity in our geospace observational system network analysis of total electron content we present the first complex network theory based analysis of high latitude total electron content tec data including dependencies on interplanetary magnetic field imf clock angle and hemisphere we examine several network measures to quantify the spatio temporal correlation patterns in the tec data for winter and summer months in 2016 we find that significant structure exists in the correlation patterns distinguishing the dayside and nightside ionosphere and specific features in the high latitudes such as the polar cap and auroral oval including the cusp and ionospheric footpoints of magnetospheric boundary layers these features vary with the imf exhibiting a strong dependence on the north south direction and generally larger variations during the winter months in both hemispheres our exploratory results suggest that network analysis of tec data can be used to study characteristic ionospheric spatial scales at high latitudes thereby extending the utility of these data we explore meso and large scales 10s km and 100s km respectively as a function of winter summer season hemisphere and imf direction and conclude that the relative importance of different ionospheric scales is not a constant relationship together with an identification of important areas of future work our findings provide a foundation for the application of network analysis techniques to ionospheric tec our results suggest that network analysis can reveal new physical connections in the ionospheric system

relationship between birkeland current regions particle precipitation and electric fields data from eight dmsp f7 satellite passes coincident with sondrestrom radar observations have been examined to determine how the large scale dayside birkeland currents are related to the particle precipitation regions and to the convection pattern the classification schemes recently developed from dmsp particle data were adopted the observations were limited to the prenoon local time hours and led to the following conclusions 1 the local time of the mantle currents which were traditionally called cusp currents is not limited to the longitude of the cusp proper but covers a larger local time extent 2 the mantle currents flow entirely on open field lines where open field lines is defined as a region where the ion precipitation and electron precipitation have the characteristics of plasma mantle cusp or polar rain this confirms and extends to all local times similar results obtained from other observations 3 about half of region 1 currents flow on open field lines this is consistent with the assumption that the region 1 currents are generated by the solar wind dynamo and flow within the surface that separates open and closed field lines 4 more than 80 of the birkeland current boundaries do not correspond to particle precipitation boundaries region 2 currents extend beyond the plasma sheet poleward boundary region 1 currents flow in part on open field lines mantle currents and mantle particles are not coincident 5 on most passes when a triple current sheet is observed region 2 region 1 and mantle currents the convection reversal is located on closed field lines when only two current sheets are observed either region 2 region 1 or region 1 mantle currents the convection reversal is on open field lines 6 the data appear to be more consistent with a topology such that mantle currents are an extension of region 1 currents rather than a separate system located poleward of the region 1 current system

omeonga a possible large impact structure on the eastern kasai province d r congo abstract the omeonga ring structure d r congo shows a remarkable drainage pattern encircling an area up to 45 km wide and encompassing a central smoothed relief 20 km wide this inner circular ridge is elevated about 70 m above the ring depression corresponding to the bed of the unia river which flows between the inner ridge and an outer irregular ridge landsat 7 etm and aster dem show that the structural characteristics resemble those of several wide impact structures known on earth other geological modes of origin that could produce ring structures such as magmatic activity salt diapirism and karst dissolution have been considered however after evaluating the regional stratigraphy the distribution of volcanism and morphometry these processes seem to be rather unlikely if of impact origin the age of the omeonga structure can be constrained to the late cretaceous cenozoic according to the youngest units in which the ring structure was formed

optical signatures and sources of prenoon auroral precipitation optical observations from ny alesund svalbard of prenoon auroral structures are compared with simultaneous measurements of energetic particle precipitation and plasma drifts taken during a nearby overpass of the dmsp f9 satellite near the 0830 mlt meridian close to the time of this pass a stable band of red line emissions was detected from south of ny alesund very weak transient auroral activity was observed from the sky above and north of the station an automatic classification algorithm applied to the dmsp f9 particle precipitation data identified source regions in the central plasma sheet the boundary plasma sheet the plasma mantle and the polar rain the authors interpreted the poleward and equatorward parts of the red auroral band as being collocated with regions of mantle and boundary plasma sheet precipitation on tailward and sunward convecting field lines respectively however the mantle part of the red auroral band as interpreted by the automated algorithm is difficult to reconcile with simultaneously measured high latitude convection distributions another interpretation of the data which include magnetic mapping considerations and constraints imposed by the global electric field pattern suggests that the entire band of red line emissions from south of ny alesund is excited by electrons originated in the nightside boundarymore plasma sheet although neither interpretation is definitive resolving dilemmas that they pose is critical for understanding this common and simple form of dayside aurora 18 refs 7 figs less

pangaean climates megamonsoons of the megacontinent the climate for a highly idealized geographic representation of the megacontinent of pangaea corresponding to the period 250 200 m y ago is explored using a low resolution atmospheric general circulation model that is coupled to a static mixed layer ocean of constant depth with a provision for sea ice formation and decay soil moisture is included as a predicted variable on the continent because there are large uncertainties about key factors that might influence the pangaean climate sensitivity experiments are used to examine the climate for two different mean elevations 0 and 1000 m two different snowcover prescriptions and two different levels of implied greenhouse heating and solar luminosity modern co2 concentration and modern solar luminosity fivefold increase in modern co2 concentration and a 1 decrease in modern solar luminosity in all of the experiments the large bipolar land masses of laurasia and gondwanaland joined near the equator in the west and separated by the tethys sea in the east exhibit extreme continentality with hot summers cold winters and large scale summer and winter monsoon circulations year round or seasonal aridity is simulated for all but the eastern coastal regions the tropical western coast and the regions poleward of 40 latitude the simulated climate of the idealized pangaean continent has many features that correspond qualitatively with geologic evidence for tropical warmth and aridity in the late permian and triassic however the model results serve mainly to indicate the importance of various climate forming factors geography topography heating detailed comparisons will require new experiments with better models and more accurate geologic boundary conditions

interhemispheric observations of dayside convection under northward imf 1 the reverse convection potential and electric field under northward interplanetary magnetic field imf has been shown to saturate at a larger value in the summer than in the winter previous studies of reverse convection under northward imf have suggested that in the winter hemisphere much of the reconnection involves internal reconnection with over draped field lines from the summer hemisphere we present a case study in which reverse convection was simultaneously visible in the northern and southern hemispheres near the northern winter solstice we use superdarn radar velocity measurements to demonstrate that the sunward reverse convection along the noon meridian is significantly faster in the summer hemisphere we also use dmsp energetic particle data to demonstrate that in the winter hemisphere some of the convection circulates on closed field lines which is a signature of internal reconnection these results suggest that internal reconnection is much less effective in transmitting the interplanetary electric field ief into the ionosphere

statistical analysis of the sources of the cross polar potential for southward imf based on particle precipitation characteristics there are several proposed physical processes which may contribute to the cross polar potential and thus drive ionospheric convection around the polar caps it is generally believed that magnetic r

ground based signatures for the nightside polar cap boundary this work describes an application of incoherent scatter is radar and all sky imaging techniques to the identification of signatures for the nightside auroral zone polar cap boundary this identification is made by estimating the characteristic energy of precipitating electrons across all latitudes measured by the is radar the primary method used to discern the boundary is based on an algorithm wherein the detailed shape of e region ionization profiles is mapped to the required number flux from a library of monoenergetic ionization profiles this procedure results in a reconstructed energy distribution for the precipitating electrons from which a characteristic energy is derived the latitudinal gradient of the characteristic energy is subsequently used to identify the boundary monochromatic all sky imagers are used to establish the time history of the boundary and estimate characteristic energy at magnetic zenith additional plasma signatures such as auroral ionospheric cavities and localized f region te enhancements are shown to provide contextual clues for the location of the boundary three in depth case study periods are used to qualify the various boundary identification methods a detailed study of 30 is radar scans in the geomagnetic meridian is used to conclude that a latitudinal gradient in characteristic energy of 5 to 11 kev per degree when measured adjacent to an extended region of low energy precipitation can locate the boundary with a latitudinal precision of 0 2

transient magnetic field signatures at high latitudes we survey goes 2 5 6 geosynchronous and huancayo peru ground magnetometer observations at the times of 70 transient 2 10 min events recorded at south pole station antarctica the simultaneous observations indicate that most south pole events correspond to sudden sharp variations in the equatorial magnetospheric and low latitude ground magnetic field the exceptions occur when the south pole events have weak amplitudes and or huancayo and goes 2 5 6 are far from local noon the corresponding features observed at goes 5 and goes 6 are generally similar with a lag indicating antisunward motion a similar antisunward motion may be inferred from the ground observations themselves on a case by case and statistical basis the characteristics of the events observed in south pole ground magnetograms resemble those previously interpreted as sudden impulse and sudden storm commencement signatures at other high latitude stations these observations suggest that the transient events at south pole form part of the magnetospheric and ionospheric response to a sudden change in the fraction of the solar wind dynamic pressure applied to the magnetosphere 57 refs 14 figs

interplanetary magnetic field control of mantle precipitation and associated field aligned currents dayside reconnection which is particularly effective for a southward interplanetary magnetic field imf allows magnetosheath particles to enter the magnetosphere where they form the plasma mantle the motions of the reconnected flux tubes produce convective flows in the ionosphere it is known that the convection patterns in the polar cap are skewed to the dawnside for a positive imf by or duskside for a negative imf by in the northern polar cap correspondingly one would expect to find asymmetric distributions of mantle particle precipitation but previous results have been unclear in this paper the correlation between by and the distribution of mantle particle precipitation is studied for steady imf conditions with southward imf ion and electron data from the dmsp f6 and f7 satellites are used to identify the mantle region and imp 8 is used as a solar wind monitor to characterize the imf we study the local time extension of mantle precipitation in the prenoon and postnoon regions we find that in accordance with theoretical expectations for a positive negative imf by mantle particle precipitation mainly appears in the prenoon region of the northern southern hemisphere the mantle particle precipitation can extend to as early as 0600 magnetic local time mlt in the prenoon region but extends over a smaller local time region in the postnoon sector we did not find mantle plasma beyond 1600 mlt in our data set although coverage is scant in this area magnetometer data from f7 are used to determine whether part of the region 1 current flows on open field lines we find that at times part of the region 1 sense current extends into the region of mantle particle precipitation and is therefore on open field lines in other cases region 1 currents are absent on open field lines most of the observed features can be readily interpreted in terms of the open magnetosphere model

separating and quantifying ionospheric responses to proton and electron precipitation over svalbard on 16 december 2001 a variable and structured aurora associated with a period of high solar wind velocity and low solar wind density was recorded through optical radar and particle measurements

observation of a correspondence between poleward moving auroral forms and stepped cusp ion precipitation in a case study we document for the first time a one to one association between three auroral forms which are separated in latitude and moving poleward and three intervals of almost constant but different low energy cutoffs in the ion precipitation in the cusp region stepped cusp precipitation the ground observations are from ny alesund at 75 magnetic latitude mlat and the precipitation pattern is observed by the dmsp f13 satellite as it crosses the noon meridian into the morning sector the energy cutoffs increase with decreasing latitude the auroral forms brighten up at different times but at practically the same mlat 73 and start fading 5 min later at 76 mlat interplanetary conditions monitored by wind show a stable slow and cold solar wind with an interplanetary magnetic field which points south and west by 0 staircase ion precipitation signatures such as observed in this example have been successfully interpreted in terms of a sequence of reconnection bursts at the low latitude dayside magnetopause the time history of the auroral activity observed from the ground confirms this interpretation we exclude the possibility that the auroral forms are triggered by changes in the interplanetary medium because the latter is very steady in particular the low dynamic pressure varies little about a mean value of 1 6 npa

permo triassic boundary superanoxia and stratified superocean records from lost deep sea pelagic cherts of japan and british columbia canada recorded a long term and worldwide deep sea anoxic oxygen depleted event across the permo triassic or paleozoic and mesozoic boundary 251 2 million years ago the symmetry in lithostratigraphy and redox condition of the boundary sections suggest that the superocean panthalassa became totally stratified for nearly 20 million years across the boundary the timing of onset climax and termination of the oceanic stratification correspond to global biotic events including the end guadalupian decline the end permian extinction and mid triassic recovery

tectonic evolution of the cuvette centrale zaire the cuvette centrale central basin of zaire is an infra cambrian to recent basin with up to 9 km of sedimentary section basin subsidence can be largely explained by a postulated late proterozoic failed rift and subsequent thermal relaxation seismic reflection profiles and well data reveal a palaeozoic development of the basin punctuated by two regional deformational episodes one in the early palaeozoic late cambrian and a second in the late palaeozoic late permian triassic both episodes result from ne sw contraction with the late palaeozoic episode being largely a reactivation of the early palaeozoic structures the timing of the two deformations correlates with collisional events at continental margins some distance from the congo basin in the early palaeozoic the deformation appears to be related to the late stages of the west congolian fold and thrust belt and coincides regionally with the pan african event sensu stricto the late palaeozoic deformation coincides with collisional tectonics along the southern margin of gondwana and the generation of the cape and sierra ventana foldbelts it is postulated that in both the early and late palaeozoic africa and gondwana experienced widespread intracontinental deformation generated by distant collisional processes much as central asia is experiencing today

nearshore euxinia in the photic zone of an ancient sea abstract evidence for photic zone euxinia pze in ancient oceans is widespread and commonly associated with global climate change and biotic events however evidence of pze in marginal settings is commonly extrapolated to infer basin scale characteristics to explore the relationship between euxinia in marginal settings and the wider basin in one palaeozoic context we have conducted a detailed spatial and temporal examination the palaeowater column redox state from the polish sector of the southern permian basin spb during deposition of the zechstein lopingian second carbonate main dolomite cycle sediments from toe of slope slope and hypersaline lagoonal settings of the northeastern margin of the spb contain abundant chlorobactane isorenieratane and their likely degradation products c 15 to c 31 2 3 6 aryl isoprenoids these indicate that part of the photic zone was euxinic during significant intervals of the main dolomite deposition further evidence for strongly anoxic conditions includes the occurrence of c 28 bisnorhopane high concentrations of pentakishomohopanes i e high homohopane indices and the occurrence of framboidal and native sulphur crystals in slope facies however a lack of these signatures in basinal settings indicates that strongly reducing conditions were restricted to the slope and toe of slope locations even in these settings however highly variable homohopane indices and concentrations of isorenieratene and chlorobactene derivatives as well as the presence of benthic fauna and bioturbation indicate that anoxia fluctuated throughout deposition of the main dolomite likely on a variety of timescales sterane distributions are also variable suggesting a dynamic ecosystem likely exhibiting a strong response to environmental forcing overall it appears that high but episodic primary bioproductivity of organic matter was concentrated on the northeastern slope and restricted lagoons leading to the formation of source rocks for petroleum however the temporal and geographical restriction of anoxia appears to have prevented the accumulation of large or more widespread quantities of organic matter and in fact toc contents exhibit poor correlation with ecological and anoxia indicators this model indicates that strong evidence for pze in shelf and slope facies need not be associated with widespread basin scale anoxia with implications for organic matter burial carbon cycling and biotic crises

events of enhanced convection and related dayside auroral activity in this paper we study the high latitude plasma flow variations associated with a periodic 8 min sequence of auroral forms moving along the polar cap boundary which appear to be the most regularly occuring dayside auroral phenomenon under conditions of southward directed interplanetary magnetic field satellite data on auroral particle precipitation and ionospheric plasma drifts from dmsp f10 and f11 are combined with ground based optical and ion flow measurements for january 7 1992 ionospheric flow measurements of 10 s resolution over the range of invariant latitudes from 71 to 76 were obtained by operating both the european incoherent scatter eiscat uhf and vhf radars simultaneously the optical site ny alesund svalbard and the eiscat radar field of view were located in the postnoon sector during the actual observations the west greenland magnetometers provided information about temporal variations of high latitude convection in the prenoon sector satellite observations of polar cap convection in the northern and southern hemispheres show a standard two cell pattern consistent with a prevailing negative by component of the interplanetary magnetic field the 630 0 nm auroral forms located poleward of the persistent cleft aurora and the flow reversal boundary in the 1440 1540 mlt sector were observed to coincide with magnetosheath like particle precipitation and a secondary population of higher energy ions and they propagated eastward tailward at speeds comparable with the convection velocity it is shown that these optical events were accompanied by bursts of sunward return flow at lower latitudes in both the morning and the afternoon sectors consistent with a modulation of dungey cell convection the background level of convection was low in this case kp 2 the variability of the high latitude convection may be explained as resulting from time varying reconnection at the magnetopause in that case this study indicates that time variations of the reconnection rate effectively modulates ionospheric convection