exchange through the bab el mandab during the last 10 years there have been a number of important developments in the understanding of the exchange flow in the bab el mandab new and improved estimates of the fluxes of mass heat and salt through the strait have been made directly from oceanographic measurements and indirectly from air sea flux data hydraulic theory has been applied to explain the seasonal variations in the exchange flow and in particular the three layer summer circulation the effects of wind stress friction and seasonal sea level variability on hydraulic control are addressed in this paper analyses of oceanographic data have failed to provide direct evidence of hydraulic control but they have highlighted the importance of tidal scale variability and differences between real continuously stratified fluids and idealised layer flows often used in hydraulic theory other studies have investigated the coupling between the flow in the strait and the circulation within the red sea

direct observations of seasonal exchange through the bab el mandab strait the exchange flow between the red sea and the gulf of aden indian ocean through the bab el mandab strait was measured continuously for 10 months june 1995 march 1996 adcp and temperature salinity chain moorings allow an unprecedented look at the magnitude and seasonal evolution of the inflow layer from the gulf of aden and the high salinity outflow layer from the red sea the timing structure and evolution of the summer season mid depth intrusion of cold low salinity water into the red sea from the gulf of aden is measured for the complete intrusion cycle of 1995 we unexpectedly find the deep outflow still strong in june 1995 with speeds of 0 6 m sec and transport of 0 4 sv 1 sv 106 m sec from july to mid september the deep outflow persists but is attenuated to speeds of 0 2 m sec and transport of 0 05 sv the dominant summer feature the cold low salinity intermediate layer intrusion persists for 3 months occupies 70 of the water column in the strait and carries approximately 1 7 1012 m of cold nutrient rich water into the red sea the winter regime begins in mid september is fully developed by early november and continues to the end of our first observation interval in march 1996 speeds in the lower layer are 0 8 1 0 m sec and 0 4 0 6 m sec in the upper layer at maximum exchange in mid february outflow transport reaches 0 7 sv ubiquitous oscillations in current and salinity at synoptic and intraseasonal periods appear closely related to fluctuations in the along channel wind forcing and perhaps to coastally trapped waves

archipelago strait exchange processes an overview archipelagos consist of a set of islands forming a collection of basins interconnected by straits and are typically characterized by widely varying spatial and temporal scales regarding geometry and forcing conditions focusing on the strait exchange parameterization we describe an archipelago water exchange model in which the archipelago is subdivided into a network of discrete basins and interconnecting straits and where the time integration assumes a series of quasi steady states we propose an algorithm that should be sufficiently flexible to provide reasonable strait exchange estimates under the variety of forcing conditions encountered in the stockholm archipelago we start from the functional formulation of two layer hydraulic theory which allows numerical schemes to be designed that given the forcing conditions at the ends of a given strait distinguish between maximal and sub maximal flow cases and solve the flow accordingly we relax the assumption of two homogeneous layers when necessary using an approximate method based on a self similarity assumption and with the sea level difference over the strait as an explicit part of the problem this method allows exchange flows with two groups of layers to be solved for the same set of geometries that the pure two layer theory can handle including sill contraction combinations and non rectangular cross sections we further show how aspiration of dense water from below the sill crest can be quantified with hydraulic theory and be included in the method for stratified strait exchange rotational control in wide straits and in parallel straits connecting the same two basins is treated with a simple but robust scheme we evaluate the calculations with data from the oxdjupet strait in the stockholm archipelago simulations with a three dimensional non hydrostatic numerical model are performed to compensate for sparsity in data

a possible foreshock of a future large earthquake near odawara center japan the 1990 odawara earthquake m5 1 occurred within an area where a large earthquake of m7 class is expected to occur in the future this area the odawara area has been seismically quiescent since 1933 analysis of near field broadband seismograms and the extent of fore and after shocks suggests that the 1990 event had a high stress drop and that the odawara area remains in a state of high stress which could potentially cause a large earthquake

theoretical effects of geomagnetic activity on low latitude ionospheric electric fields 1 the influence of geomagnetic activity on middle and low latitude thermospheric winds and ionospheric electric fields is investigated using model results from the national center for atmospheric research thermosphere ionosphere electrodynamics general circulation model model runs are made for different levels of geomagnetic activity model results show that the equatorward ionospheric currents produced by disturbance winds develop positive charge accumulation at low latitudes that maximizes in the premidnight sector the local time of maximum electric potential perturbation depends significantly on universal time so that the local time of reversal of the equatorial zonal perturbation electric field varies with longitude by 2 to 3 hours depending on the intensity of geomagnetic activity the westward perturbation electric field in the postsunset period indicates that stronger geomagnetic activity will produce a lower driven height of the evening f region after geomagnetic activity ceases model results show that the zonal disturbance winds can last for many days in the postrecovery period while the meridional disturbance winds decay more rapidly the long lasting zonal winds through the pedersen currents they drive help maintain meridional disturbance potential drops that decay much more slowly than the zonal disturbance potential drops after the activity ceases

solar flare effects on 150 km echoes observed over jicamarca waccm x simulations submitted manuscript accepted for publication in geophysical research letters https doi org 10 1029 2019gl084790 copyright 2019 american geophysical union further reproduction or electronic distribution is not permitted

direct observation of the f region dynamo currents and the spatial structure of the eej by champ 1 the satellite champ in its low altitude polar orbit and with its high resolution magnetic field measurements provides the opportunity to investigate ionospheric currents in great detail in this study we focus on average features of equatorial current systems during day time considering quiet time observations from the period aug 2000 to oct 2004 we determined the mean spatial structure of the eej current the existence of reverse currents at the flanks of the primary eastward current is confirmed more importantly average characteristics of the f region dynamo driven currents are observed for the first time throughout the daytime the meridional current system driven by westward thermospheric winds peaks around noon similar meridional currents but flowing in the opposite sense are observed at the dusk sector at champ altitudes we find over the dip equator in the f region a vertical sheet current of about 6 ma m oriented downward at noon and an upward current of 4 ma m around sunset

euro seistest determination of the geological structure of the volvi basin and validation of the basin response the volvi basin northern greece is a 5 km wide graben of neogene age it has been chosen by european union directorate general xii for science research and development in climatology and natural hazards as a test site euro seistest for studying site effects in one of europe s most seismic active areas the site is located in the epicentral area of the 1978 ms 6 5 earthquake that struck the city of thessaloniki the geometry velocity and stratigraphy of the basin were not well known consequently a number of geophysical and geotechnical studies were performed to determine the 2d geometry and the dynamic characteristics vp vs qs of the valley investigations included 78 refraction tests 20 surface wave inversions and 8 borehole seismic tests as well as geotechnical tests this article presents the methodology used for the geophysical investigation and the results in its center the graben depth reaches 190 m and at least four normal faults have been located along the section the overall graben structure and the fault locations have been confirmed by the records of a shot generated at one edge of the valley and by the measurements of a local earthquake wave propagation appears to be strongly influenced by the thickness of the sediments which in turn is controlled by fault displacements simple 2d modeling of the basin response was performed and the results are in good agreement with weak motion data confirming the validity of the seismic structure inferred from seismic prospecting

three layer view of transports and hydraulics in the strait of gibraltar a three dimensional model study 1 the ability of a three dimensional sigma coordinate free surface model to reproduce the interfacial layer between the atlantic and mediterranean waters in the strait of gibraltar is verified the model uses a coastal following curvilinear orthogonal grid that includes the gulf of cadiz and the alboran sea with very high resolution in the strait less than 500 m it is forced by imposing the two major semidiurnal tidal components m2 and s2 along the atlantic and the mediterranean open boundaries the main experiment covers an entire spring neap cycle fortnight period results show that the model is able to reproduce a thick interfacial layer that carries a substantial fraction of the total transport the interfacial thickness is strongly modulated by tidal forcing that enhances the vertical exchanges between layers in order to take into account the thick interfacial layer a three layer approximation is used to study the hydraulics results show that the exchange regime is intermittently maximal due to cyclically controlled regions over camarinal sill and within tarifa narrows the analysis has been repeated in a two layer framework to evaluate the impact of the introduction of an interfacial layer on the calculation of hydraulics in the two layer approximation there is not any controlled region extending on the whole cross section so the exchange is always submaximal concluding the two layer approximation underestimates hydraulic controls in the strait respect to the three layer approach

seasonal and longitudinal variations of the solar quiet sq current system during solar minimum determined by champ satellite magnetic field observations 1 vector magnetometer observations from the challenging minisatellite payload champ satellite are used to determine the solar quiet sq current system during the recent solar minimum observations from 2006 to 2008 are combined and after removal of a main field model and accounting for field aligned currents the longitudinal and seasonal variation of the sq currents are determined through the method of spherical harmonic analysis comparison with sq currents derived from ground based magnetometers in the african european longitude sector reveals similar amplitudes and seasonal variations indicating that the champ observations can reliably determine the sq current system the seasonal variation is consistent with prior observations during solar minimum conditions and in the northern hemisphere exhibits a primarily annual variation with peak currents during local summer the seasonal variation in the southern hemisphere is characterized by a semiannual variation with the maxima occurring around the equinoxes significant longitudinal variations are also observed and they display a seasonal variability during northern hemisphere summer the predominant feature at local noon is a wave number 1 variation in longitude during the remainder of the year a wave 3 longitudinal structure is observed at this local time the longitudinal variations are considered to be due to a combination of the orientation and strength of the geomagnetic field as well as the tidal winds in the lower thermosphere variations in tidal winds due to nonmigrating tides may influence the dynamo generated electric fields and currents resulting in the observed longitudinal variations of the sq current function

excursions of interhemispheric field aligned currents in africa the interhemispheric field aligned currents ihfacs are the most evident currents connecting the solar quiet sq current systems in the northern and southern hemispheres understanding these coupling currents has practical importance for space weather forecast of the magnetosphere ionosphere interactions and their potential impacts on low orbiting spacecraft in this paper we have examined digital magnetic records from september 2008 to august 2009 at nine magnetic data acquisition system magdas stations in order to uniquely determine the variability of ihfacs in terms of their excursions in the african sector by excursions we mean the direction in which the ihfacs are moving as a function of local time or latitude different from theoretical prediction we found prominent positive ihfacs around dusk in february and march equinox a novel feature of this study is the semidiurnal variation control of the coupling between the northern and southern hemispheres in addition we found terdiurnal variation in september with their associated strong diurnal variations from october through december quite unexpectedly in january a significant latitudinal depletion in the intensities of ihfacs was observed from dawn to dusk compared to any other months under investigation accompanied with a marked reduction of the prominent diurnal variations at the magnetic equator addis ababa aab the fact that the excursions of ihfacs in january experience an unusual retardation with respect to their in

the effects of nonrectangular cross section friction and barotropic fluctuations on the exchange through the strait of gibraltar abstract the inviscid two layer hydraulic model of farmer and armi has two types of solution in the eastern end of the strait of gibraltar for flow that is critical at camarinal sill the solution corresponding to maximal exchange has a fast supercritical flow with a shallow interface whereas the solutions corresponding to submaximal exchange show a much slower subcritical flow with a rather deep interface we extend their model to include physically important effects such as nonrectangular cross section friction and barotropic fluctuations allowing for realistic cross sections reduces the exchange and raises the interface everywhere along the strait realistic interfacial friction has little influence on the exchange but lateral friction at the sloping sides of the cross sections has a significant effect on the flow and brings the two solutions supercritical and subcritical closer together the effect of the earth s rotation is also summarized but dealt with in more detail in a companion paper by bor

the interhemispheric and f region dynamo currents revisited with the swarm constellation based on magnetic field data sampled by the swarm satellite constellation it is possible for the first time to determine uniquely f region currents at low latitudes initial results are presented from the first 200 days of formation flight 17 april to 5 november 2014 detailed results have been obtained for interhemispheric field aligned currents connecting the solar quiet day magnetic variation sq current systems in the two hemispheres we obtain prominent currents from the southern winter hemisphere to the northern around noon weaker currents in opposite direction are observed during morning and evening hours furthermore we could confirm the existence of vertical currents above the dip equator downward around noon and upward around sunset for both current systems we present and discuss longitudinal variations

longitudinal variation of the lunar tide in the equatorial electrojet the atmospheric lunar tide is one known source of ionospheric variability the subject received renewed attention as recent studies found a link between stratospheric sudden warmings and amplified lunar tidal perturbations in the equatorial ionosphere there is increasing evidence from ground observations that the lunar tidal influence on the ionosphere depends on longitude we use magnetic field measurements from the champ satellite during july 2000 september 2010 and from the two swarm satellites during november 2013 february 2017 to determine for the first time the complete seasonal longitudinal climatology of the semidiurnal lunar tidal variation in the equatorial electrojet intensity significant longitudinal variability is found in the amplitude of the lunar tidal variation while the longitudinal variability in the phase is small the amplitude peaks in the peruvian sector 285 e during the northern hemisphere winter and equinoxes and in the brazilian sector 325 e during the northern hemisphere summer there are also local amplitude maxima at 55 e and 120 e the longitudinal variation is partly due to the modulation of ionospheric conductivities by the inhomogeneous geomagnetic field another possible cause of the longitudinal variability is neutral wind forcing by nonmigrating lunar tides a tidal spectrum analysis of the semidiurnal lunar tidal variation in the equatorial electrojet reveals the dominance of the westward propagating mode with zonal wavenumber 2 sw2 with secondary contributions by westward propagating modes with zonal wavenumber 3 sw3 and 4 sw4 eastward propagating waves are largely absent from the tidal spectrum further study will be required for the relative importance of ionopsheric conductivities and nonmigrating lunar tides

the equatorial electrojet an integrated treatment of observations and models of the undisturbed equatorial electrojet is presented previous reviews have concentrated on particular aspects of the electrojet phenomenon such as ground magnetic variations the counterelectrojet or e region electron density irregularities and associated plasma instabilities here accomplishments during the past 15 years involving all aspects of the phenomenon are covered and where possible interrelated observational and theoretical studies during recent years have emphasized the local time longitude vertical and latitudinal structure of electric fields and currents in the electrojet this review similarly places its emphasis along these lines and by necessity involves a detailed examination of electrodynamic coupling with the neutral atmosphere tidal oscillations in temperature composition and winds the review concludes with an assessment of current knowledge concerning the equatorial electrojet and suggestions for the future direction of observational and theoretical research on electrodynamical processes in the equatorial ionosphere

longitudinal variation of the e region electric fields caused by atmospheric tides 1 polarization electric fields created by the e and f region dynamos cause the uplift of f region plasma the subsequent redistribution of that plasma along the magnetic field lines creates the equatorial ionospheric anomaly eia observations of the post sunset eia made by the image and timed satellites are compared here with champ orsted and sac c observations of the noontime equatorial electrojet eej during magnetically quiet periods around equinox the eia and eej show a remarkably similar four peaked wave like longitudinal variation its structure is consistent with the longitudinal variation in the strength of diurnal tides that drive the e region dynamo this indicates a strong vertical coupling between the ionosphere and troposphere because the four peaked tidal structure is driven by tropospheric weather furthermore the dayside ionospheric conditions are found to perform the global scale longitudinal structure of the post sunset ionosphere at low latitudes

modeling strong motion in the ashigara valley for the 1990 odawara japan earthquake finite difference seismograms are calculated for the 1990 odawara earthquake using a basin structure derived from a geotechnical survey and a double couple solution determined from the p first motion data a two dimensional sh finite difference calculation reproduces the observed spatial variation in amplitude and waveform of the transverse records at the ashigara valley blind prediction sites for frequencies lower than 2 hz comparison of the synthetics and the observed seismograms suggests that the observed amplitude variation of the transverse records in the ashigara valley is mainly due to the source radiation pattern rather than the basin structure several different double couples are used in the waveform modeling to show the sensitivity of ground motion to the source mechanism we thus conclude that it is important to use an appropriate focal mechanism to model near field ground motion

equatorial electrojet responses to intense solar flares under geomagnetic disturbance time electric fields large enhancement in the equatorial electrojet eej current can occur due to sudden increase in the e layer density arising from solar flare associated ionizing radiations as also from background electric fields modified by magnetospheric disturbances when present before or during a solar flare we investigate the eej responses at widely separated longitudes during two x class flares that occurred at different activity phases surrounding the magnetic super storm sequences of 28 29 october 2003 during the 28 october flare we observed intense reverse electrojet under strong westward electric field in the sunrise sector over jicamarca sources of westward disturbance electric fields driving large eej current are identified for the first time model calculations on the e layer density with and without flare and comparison of the results between jicamarca and sao luis suggested enhanced westward electric field due to the flare occurring close to sunrise over jicamarca during the flare on 29 october which occurred during a rapid ae recovery a strong overshielding electric field of westward polarity over jicamarca delayed an expected eej eastward growth due to flare induced ionization enhancement in the afternoon this eej response yielded a measure of the overshielding decay time determined by the storm time region 2 field aligned current this paper will present a detailed analysis of the eej responses during the two flares including a quantitative evaluation of the flare induced electron density enhancements and identification of electric field sources that played dominant roles in the large westward eej at the sunrise sector over jicamarca

tidal variability in the ionospheric dynamo region 1 the seasonal and interannual variability of migrating sun synchronous and nonmigrating solar atmospheric tides at altitudes between 100 and 116 km are investigated using temperature measurements made with the saber instrument on the timed spacecraft during 2002 2006 quasi biennial variations of order 10 15 in migrating diurnal and semidiurnal tidal amplitudes are found presumably due to modulation by the quasi biennial oscillation qbo as the tides propagate from their troposphere and stratospheric sources to the lower thermosphere a number of nonmigrating tidal components are found that have the potential to produce significant longitudinal variability of the total tidal fields the most prominent of these i e those that appear at amplitudes of order 5 10 k in a 5 year mean climatology include the zonally symmetric s 0 diurnal tide d0 the eastward propagating diurnal and semidiurnal tides with zonal wave numbers s 2 de2 and se2 and s 3 de3 and se3 and the following westward propagating waves diurnal s 2 dw2 semidiurnal s 1 sw1 s 3 sw3 and s 4 sw4 and terdiurnal s 5 tw5 these waves can be plausibly accounted for by nonlinear interaction between migrating tidal components and stationary planetary waves with s 1 or s 2 or by longitudinal variations of tropospheric thermal forcing additional waves that occur during some years or undergo phase cancellation within construction of a 5 year climatology include dw5 se1 se4 sw6 te1 tw1 and tw7 it is anticipated that the winds that accompany all of these waves in the 100 170 km region will impose longitudinal variability in the electric fields produced through the ionospheric dynamo mechanism thereby modulating vertical motion of the equatorial ionosphere and the concomitant plasma densities in addition to the wave 4 modulation of the equatorial ionosphere that has recently been discovered and replicated in modeling studies the waves revealed here will generate wave 1 sw1 sw3 d0 dw2 wave 2 sw4 tw1 wave 3 de2 se1 wave 4 de3 se2 dw5 sw6 te1 tw7 wave 5 se3 and wave 6 se4 components of this ionospheric variability depending on year and time of year however the absolute and relative efficiencies with which these waves produce electric fields remains to be determined

physical and biological characteristics of the winter summer transition in the central red sea the central red sea crs lies between two distinct hydrographic and atmospheric regimes in the southern red sea seasonal monsoon reversal regulates the exchange of water between the red sea and the indian ocean in the northern red sea intermediate and occasionally deep water are formed during winter to sustain the basin s overturning circulation highly variable mesoscale eddies and the northward flowing eastern boundary current ebc determine the physical and biogeochemical characteristics of the crs ship based and glider observations in the crs between march and june 2013 capture key features of the transition from winter to summer and depict the impact of the eddy activity on the ebc flow less saline and relatively warmer water of indian ocean origin reaches the crs via the ebc initially an anticyclonic eddy with diameter of 140km penetrating to 150m depth with maximum velocities up to 30 35 cm s 1 prevails in the crs this anticyclonic eddy appears to block or at least redirect the northward flow of the ebc dissipation of the eddy permits the near coastal northward flow of the ebc and gives place to a smaller cyclonic eddy with a diameter of about 50km penetrating to 200m depth by the end of may as the northerly winds become stronger and persistent throughout the basin characteristic of the summer southwest monsoon wind regime the ebc and its associated lower salinity water became less evident replaced by the saltier surface water that characterizes the onset of the summer stratification in the crs

zonal currents in the f region deduced from swarm constellation measurements the swarm constellation has been used to estimate zonal currents in the topside f region ionosphere at about 500 km near simultaneous magnetic field measurements from two altitudes but the same meridian are used for the current density calculations we consider the period 15 february to 23 june 2014 for deriving a full 24 h local time coverage of the latitudinal distribution over 50 in magnetic latitude intervals with close orbital phasing at the two heights are considered which repeat every 6 days from such days seven successive orbits are used where the epochs of equator crossings differ by less than 2 min deduced current densities are predominantly eastward about 20 na m2 on the dayside and westward about 10 na m2 on the nightside a number of different drivers contribute to the observed total current we identified the gravity driven eastward current as the most prominent at low latitudes eastward currents in the northern hemisphere are clearly stronger than in the south this is attributed to the proximity of our study period to june solstice when the solar radiation is stronger in the north in addition interhemispheric winds from the northern summer to the southern winter hemisphere contribute they cause eastward currents in the north and westward in the south we find a relatively large variability of the zonal currents both in space and time the standard deviation is at least twice as large as the mean value of current density this large variability is suggested to be related to gravity wave forcing from below

spatio temporal characterization of the equatorial electrojet from champ orsted and sac c satellite magnetic measurements received 4 may 2007 revised 20 june 2007 accepted 12 june 2007 published 13 september 2007 1 the equatorial electrojet eej is an eastward electric current on the day side flowing in a narrow band along the dip equator in the ionospheric e region recent magnetic observations from the champ orsted and sac c satellites comprising more than 95 000 dip equator crossings from 1999 to 2006 have provided an unprecedented longitudinal coverage of the eej magnetic signature we have used these data to construct an empirical model of the eej current climatological mean and day to day variability as a function of longitude local time season and solar flux our model has been successfully verified against vertical drift data from the julia radar at jicamarca we have also used the eej observations to estimate the self correlation of the eej confirming short longitudinal correlation lengths of 15 and finding a temporal correlation length of 2 4 h our model s predictions of the eastward electric field and its standard deviation may provide useful input to various kinds of ionospheric simulations coefficients and software are available online at http models geomag us eej html and http www earthref org

an empirical model of the earth s horizontal wind fields hwm07 1 the new horizontal wind model hwm07 provides a statistical representation of the horizontal wind fields of the earth s atmosphere from the ground to the exosphere 0 500 km it represents over 50 years of satellite rocket and ground based wind measurements via a compact fortran 90 subroutine the computer model is a function of geographic location altitude day of the year solar local time and geomagnetic activity it includes representations of the zonal mean circulation stationary planetary waves migrating tides and the seasonal modulation thereof hwm07 is composed of two components a quiet time component for the background state described in this paper and a geomagnetic storm time component dwm07 described in a companion paper

sensitivity of red sea circulation to monsoonal variability during the holocene an integrated data and modeling study received 20 october 2009 revised 6 march 2010 accepted 30 june 2010 published 4 november 2010 1 we used an oceanic general circulation model to evaluate the sensitivity of the hydrography and circulation of the red sea in response to reduced sea level and modified atmospheric conditions during the holocene with holocene sea level close to the modern level the red sea was sensitive to changes in atmospheric conditions and it only shows a relatively mild response to sea level change changes in the monsoon system influence the exchange flow through the strait of bab el mandab the meridional overturning circulation of the red sea and its hydrography forced by humid conditions the modeled red sea temperature increased by 1 5 c while when arid conditions were imposed the temperature decreased by 2 5 c similar heating and cooling events during the early and late holocene are seen in a sea surface temperature record from the northern red sea derived from the temperature sensitive tex86 molecular biomarker which suggests that humid conditions prevailed during the early holocene and more arid conditions prevailed during the late holocene the gradual decline in red sea temperature between these two time periods suggests a gradual decline in the summer monsoon strength this monsoon trend and the resulting changes in the red sea circulation are supported by the distribution of crenarchaea fossil lipids in red sea sediments from this period monsoon driven changes in the exchange flow through the strait of bab el mandab affected the crenarchaea population structure and therefore their molecular fossil distribution in the sediments of the red sea potentially provides an index for the summer monsoon strength during the holocene

observations on the characteristics of the exchange flow in the dardanelles strait 1 a mooring section at each end of the dardanelles strait was deployed by the naval research laboratory on 28 29 august 2008 and maintained until 11 october 2009 each section was equipped with two bottom mounted barny moorings and a vertical string that together delivered concurrent bottom pressure temperature salinity and almost full water column current velocity observations these observations at the mooring sections indicated that on longer timescales monthly or longer the exchange through the dardanelles strait showed little variability and had a two layer structure with brackish waters moving approximately southward in the upper layer and more saline aegean waters flowing roughly northward underneath on shorter time scales synoptic there were pronounced fluctuations in both layers in the northern section some manifested as almost simultaneous reversals of the flow direction in both layers with lighter upper layer waters backing into the sea of marmara and dense lower layer waters moving toward the aegean sea in the southern section some of the exchange variability consisted of a three layer flow structure with upper brackish and bottom salty layers moving toward the aegean sea and a middle salty layer flowing toward the sea of marmara this three layer exchange persisted from a day to a week synoptic flow variability in the upper layer was partly related to both the along strait wind stress and the bottom pressure anomaly gradient while lower layer current fluctuations were coherent with the bottom pressure anomaly gradient

stratification and mixing in sea straits abstract quantifying the transport through relatively narrow straits connecting basins with differing fluid properties is an important issue in both surface and abyssal flows these flows range from large scale flows in ocean straits connecting the major ocean basins down to small scale regional flows such as in estuaries and lagoons connected to the coastal ocean by relatively narrow straits or channels the exchange rate through these straits is dependent on the topography mixing and stratification and for steady flows the interplay between these three effects can be quantified using the dimensionless parameter gr t a 2 where the grashoff number gr t and the aspect ratio a are evaluated within the strait when gr t a 2 is large the exchange is determined by the well known hydraulic solution as gr t a 2 becomes small the exchange rates progressively decrease below the hydraulic predictions toward values determined by diffusive control and characterized by strong vertical mixing for systems with simple topographic controls we show how the exchange rate in the strait may be linked to the external basin scale forcing characteristics we review recent work on extending this picture to consider the effects of unsteadiness in two forms time dependent forcing in the basin due to seasonal effects and tidal forcing with an associated barotropic forcing in the strait itself

seasonal and local time dependences of the interhemispheric field aligned currents deduced from the orsted satellite and the ground geomagnetic observations 1 the precise magnetic field measurements by the orsted satellite revealed a distinct spatial variation of the eastward component residual b at mid and low latitudes on the dayside this magnetic field variation was attributed to the interhemispheric field aligned currents ihfacs at mid and low latitudes based on its comparison with ground geomagnetic field variations these ihfacs were found to flow from the summer hemisphere to the winter hemisphere in the dawn sector and from the winter hemisphere to the summer hemisphere in the noon and the dusk sectors these ihfacs flip their direction not at equinoxes but in may and november the current intensity of the ihfacs is largest not at solstices but in february and august the ground based geomagnetic field observations at 17 low latitude stations also support the seasonal dependence and local time distribution of the ihfacs

ionospheric electron content within the equatorial f 2 layer anomaly belt this paper describes some new results on the day to day behavior of the location of the equatorial anomaly in the columnar electron content the measurements were made from a unique network of stations covering dip latitudes from 0 n to 25 n during the period in 1975 1976 near sunspot minimum when the ats 6 satellite was visible from india the latitudinal distribution of the total electron content on a particular day was found to depend only on the strength of the electrojet current as determined by the difference of the horizontal magnetic field at stations on the magnetic equator and another outside the equatorial electrojet rather than on the horizontal magnetic field measured only at the equatorial station during magnetically disturbed periods the presence or absence of the anomaly was also found to depend only upon the strength of the electrojet current the varying electrojet current that is the equatorial electric field is very effective in moving the ionization to the anomaly latitudes of 15 20 but the columnar electron content over the magnetic equator remains relatively constant

elastic finite difference modeling of strong motion in ashigara valley for the 1990 odawara japan earthquake abstract finite difference seismograms are calculated for the 1990 odawara earthquake to model transverse and radial components of strong motion data using a basin structure derived from a geotechnical survey and double couple solution determined from p first motion data and sh sv amplitude ratio two dimensional sh and p sv finite difference calculation along with correction for soft surface layers reproduces the observed spatial variation in amplitude and waveform of the transverse and radial records at the ashigara valley blind prediction sites for frequencies up to 2 hz comparison of the synthetic and the observed seismograms suggests that the observed spatial variation in the sh amplitude is attributed to the source radiation and amplification due to the soft surface layers while the sv amplitude variation is controlled by the amplification due to the sedimentary basin as well as the source and surface layer effects

site effects in parkway basin comparison between observations and 3 d modelling summary this paper is a contribution towards filling the large gap that currently exists between theoretical and observational studies of site effects we present results of numerical modelling of ground motion in parkway valley a small alluvial valley in new zealand we use a 3 d finite difference code and compute ground motion for the vertical incidence of plane waves p and s the results are compared with a thorough analysis of site effects in this basin presented previously based on the records obtained using a dense temporary seismic array the results of the simulations confirm the observation that ground motion in parkway basin is dominated by locally generated surface waves diffracted surface waves are generated mainly towards the south this direction does not depend on the polarization of the incident waves we obtain a very good agreement between observed and predicted transfer functions for the two horizontal components of stations located towards the centre of the valley our model clearly performs less well towards the edges of the basin where the details of the real structure are missing in our model our results suggest that the 3 d shape of the basin may be more important than a detailed knowledge of the subsoil mechanical properties in predicting ground response a correct estimate of the absolute amplification level however requires a correct estimate of the impedance contrast but this also applies to any modelling of site response

seasonal overturning circulation in the red sea 1 model validation and summer circulation the overturning circulation in the red sea exhibits a distinct seasonally reversing pattern and is studied using high resolution mit general circulation model simulations in the first part of this study the vertical and horizontal structure of the summer overturning circulation and its dynamical mechanisms are presented from the model results the seasonal water exchange in the strait of bab el mandeb is successfully simulated and the structures of the intruding subsurface gulf of aden intermediate water are in good agreement with summer observations in 2011 the model results suggest that the summer overturning circulation is driven by the combined effect of the shoaling of the thermocline in the gulf of aden resulting from remote winds in the arabian sea and an upward surface slope from the red sea to the gulf of aden set up by local surface winds in the red sea in addition during late summer two processes associated respectively with latitudinally differential heating and increased salinity in the southern red sea act together to cause the reversal of the contrast of the vertical density structure and the cessation of the summer overturning circulation dynamically the subsurface northward pressure gradient force is mainly balanced by vertical viscosity resulting from the vertical shear and boundary friction in the strait of bab el mandeb unlike some previous studies the three layer summer exchange flows in the strait of bab el mandeb do not appear to be hydraulically controlled

response of the equatorial and low latitude ionosphere to an intense x class solar flare x7 2b as observed on 09 august 2011 1 in this paper we present response of equatorial and low latitude ionosphere to an intense solar flare of class x7 2b that peaked at 08 05 ut on 09 august 2011 in the solar cycle 24 global positioning system total electron content tec observations in the sunlit hemisphere show enhancement of 3 tec units while geomagnetic h component observations indicate sudden decrease and increase in their strength at equatorial and low latitude stations respectively at several stations in the sunlit hemisphere in addition equatorial electrojet strength over indian region reveals commencement of counter electrojet simultaneous canadian advanced digital ionosonde observations at tirunelveli an equatorial station in india show the disappearance of ionogram echoes during the flare event indicating absorption of radio signals in the d region strong equatorial blanketing type es layer was observed in the ionogram records at tirunelveli prior to the occurrence of the solar flare that continued for several hours though it became weak absent during the flare event ionogram records on the control day show regular f layer movement without any blanketing type es layer very low frequency vlf observations at allahabad an indian low latitude station show enhanced vlf amplitude signal during the same time revealing the sudden enhancement of d region ionization using the observations presented here an attempt has been made to study the impact of the solar flares on the electrodynamics of the equatorial and low latitude ionosphere

lateral propagation effects observed at parkway new zealand a case history to compare 1d versus 2d site effects abstract the importance of 2d or 3d site effects has been shown by theoretical studies however there is a lack of observational evidence of site effects due to lateral heterogeneities we have evaluated 1d and 2d site effects in the data obtained by a dense seismograph array that operated for more than 2 months in the alluvial basin of parkway new zealand 1d site effects were evaluated using three independent techniques spectral ratios relative to a reference station spectral ratios of horizontal relative to vertical ground motion and a generalized inversion scheme site effects due to lateral heterogeneity were investigated in detail for two events using frequency wavenumber analysis our results show that it is not possible to separate 1d from 2d site effects using frequency domain techniques this explains the scatter obtained from standard 1d data analysis the 1d resonance peaks will be more or less contaminated with laterally propagating waves for different events

hydraulic criticality of the exchange flow through the strait of gibraltar abstract the hydraulic state of the exchange circulation through the strait of gibraltar is defined using a recently developed critical condition that accounts for cross channel variations in layer thickness and velocity applied to the output of a high resolution three dimensional numerical model simulating the tidal exchange the numerical model uses a coastal following curvilinear orthogonal grid which includes in addition to the strait of gibraltar the gulf of cadiz and the alboran sea the model is forced at the open boundaries through the specification of the surface tidal elevation that is characterized by the two principal semidiurnal and two diurnal harmonics m2 s2 o1 and k1 the simulation covers an entire tropical month the hydraulic analysis is carried out approximating the continuous vertical stratification first as a two layer system and then as a three layer system in the latter the transition zone generated by entrainment and mixing between the atlantic and mediterranean flows

two layer rotating exchange flow between two deep basins theory and application to the strait of gibraltar rotating two layer exchange flow over a sill in a strait separating two relatively deep and wide basins is analyzed upstream of the sill in the deep upstream basin the infinitely deep dense lower layer is assumed to be inactive while the relatively thin upper layer flowing away from the sill forms a detached boundary current in the upstream basin this analysis emphasizes the importance of this upstream boundary current incorporating its width as a key parameter in a formalism for deducing the volume exchange rate and discriminating between maximal and submaximal states hence even for narrow straits in which rotation does not dominate the dynamics within the strait the importance of rotation in the wide upstream basin can be exploited it is shown that the maximal allowable exchange transport through straits wider than 11 2 rossby deformation radii increases as rotation increases unlike for smaller rotations where the exchange decreases as rotation increases the theory is applied to the exchange flow through the strait of gibraltar this application illustrates how images of the oceans taken from space showing the width of the upstream flow in this case a space shuttle photograph might be used to determine the exchange transport through a strait maximal exchange conditions in the strait of gibraltar are predicted to apply at the time the space shuttle photograph was taken

exchange through the strait of gibraltar to measure the exchange between the atlantic and mediterranean through the strait of gibraltar an array of current meter moorings was deployed for a year in the strait during 1985 1986 a novel aspect of these measurements is the inclusion of conductivity as well as temperature and pressure sensors on each current meter so that the salinity of the flows could be monitored continuously these salinity measurements determine the water mass characteristics of the flows crossing the sill they allow definition of the 37 psu isohaline as the interface between inflowing fresher atlantic water and outflowing saltier mediterranean water and they enable time series to be developed for the depth of this interface for the upper layer inflow and for the lower layer outflow from these measurements the time averaged outflow of mediterranean water is estimated to be 0 68sv 1sv 1 106m3s 1 and the outflow salinity transport defined to be the outflow times the salinity excess above a basic atlantic water salinity of 36 1 psu is estimated to be 1 50 103m3s 1 1sv 1psu 1 103m3s 1 equivalent to a net evaporation over the mediterranean basin of 52cm y 1 extrapolated measurements of the inflow from current meters generally deployed below 100m depth yield an estimate for the time averaged inflow of 0 93sv which is believed to be unrealistically high in view of the better measured outflow and net evaporation thus a more realistic estimate of the inflow is 0 72sv equal to the sum of the outflow and net evaporation as required by the mass budget for the mediterranean sea such estimates of the exchange are smaller by almost a factor of 2 than previous values for the exchange by lacombe and richez 1982 the exchange across the gibraltar sill is found to be due in nearly equal parts to the mean currents and to the tidal fluctuations the mean currents are smaller than had been expected reaching a peak value of only about 60 cm s 1 in the deep outflow over the sill the tidal exchange is due to a strong correlation over the tidal period between the depth of the interface and the strength of the inflowing currents for the m2 tide at the sill the amplitude of the interface depth is 51 m and the amplitude of the tidal currents is 1 2m s 1 furthermore the inflow and interface depth have similar phases as a consequence the upper layer is deep on the inflowing tide so that a large slug of atlantic water crosses the sill into the mediterranean on the outflowing tide the interface is shallow so that a large slug of mediterranean water crosses the sill into the atlantic similar processes occur for the s2 o1 and k1 tides though the amplitude are smaller in this manner tidal oscillations lead to a time averaged exchange of water masses across the gibraltar sill the inflow and outflow defined to be the instantaneous transports above and below the 37psu isohaline interface exhibit m2 tidal amplitudes of 2 3 sv and 1 3sv respectively thus the tides are large enough to reverse the mean upper layer inflow and lower layer outflow daily averaged inflow and outflow transports exhibit low frequency fluctuations with standard deviations of 0 37sv and 0 22sv respectively such low frequency fluctuations have been shown previously to be associated with barotropic flows through the strait of gibraltar compensating for sea level variations over the mediterranean due to atmospheric pressure fluctuations candela winant and bryden 1989 finally from these measurements there appear to be little fortnightly or annual period fluctuations in the exchange through the strait of gibraltar

simulations of strong ground motion in sw iberia for the 1969 february 28 ms 8 0 and the 1755 november 1 m 8 5 earthquakes ii strong ground motion simulations summary this is the second paper of a series of two concerning strong ground motion in sw iberia due to earthquakes originating from the adjacent atlantic area the aim of this paper is to use the velocity model that was proposed and validated in the companion paper for seismic intensity modelling of the 1969 m s 8 0 and 1755 m 8 5 8 7 earthquakes first we propose a regression to convert simulated values of peak ground velocity pgv into modified mercalli intensity mmi in sw iberia and using this regression we build synthetic isoseismal maps for a large m s 8 0 earthquake that occurred in 1969 based on information on the seismic source provided by various authors we show that the velocity model effectively reproduces macroseismic observations in the whole region we also confirm that seismic intensity distribution is very sensitive to a small number of source parameters rupture directivity fault strike and fault dimensions then we extrapolate the method to the case of the great m 8 5 8 7 1755 earthquake for a series of hypotheses recently proposed by three authors about the location of the epicentral region the model involving a subduction

on the crossover of boundary currents in an idealized model of the red sea the west to east crossover ofboundarycurrents has been seen in mean circulationschemes fromseveral past models of the red sea this study investigates the mechanisms that produce and control the crossover in an idealized eddy resolving numerical model of the red sea the authors also review the observational evidence and derive an analytical estimate for the crossover latitude the surface buoyancy loss increases northward in the idealized model and the resultant mean circulation consists of an anticyclonic gyre in the south and a cyclonicgyreinthenorth in themidbasin thenorthwardsurfaceflow crossesfromthewestern boundarytothe eastern boundary numerical experiments with different parameters indicate that the crossover latitude of the boundary currents changes with f0 b and the meridional gradient of surface buoyancy forcing in the analytical estimate which is based on quasigeostrophic b plane dynamics the crossover is predicted to lie at the latitude where the net potential vorticity advection including an eddy component is zero various terms in the potential vorticity budget can be estimated using a buoyancy budget a thermal wind balance and a parameterization of baroclinic instability

new evidence of a meridional current system in the equatorial ionosphere an interesting variation in the geomagnetic d component has been found by analysis of the magsat data characteristics of this variation are that it appears everyday on the low latitude duskside and is antisymmetric about the dip equator negative in the northern hemisphere that the magnitude from 5 to 25 nt depends upon the geographic longitude and altitude and that no corresponding change can be seen on the ground magnetic data the change suggests a meridional current system in the equatorial ionosphere produced in association with the electrojet current along the dip equator this effect should be taken into account in the study of geomagnetic anomaly of internal origin with the magsat data

longitudinal and seasonal structure of the ionospheric equatorial electric field 1 the daytime eastward equatorial electric field eef in the ionospheric e region plays an important role in equatorial ionospheric dynamics it is responsible for driving the equatorial electrojet eej current system equatorial vertical ion drifts and the equatorial ionization anomaly due to its importance there is much interest in accurately measuring and modeling the eef in this work we propose a method of estimating the eef using champ satellite derived latitudinal current profiles of the daytime eej along with h measurements from ground magnetometer stations magnetometer station pairs in both africa and south america were used for this study to produce time series of electrojet current profiles these current profiles were then inverted for estimates of the eef by solving the governing electrostatic equations we compare our results with the ion velocity meter ivm instrument on board the communication navigation outage forecasting system satellite we find high correlations of about 80 with the ivm data however we also find a constant offset of about 0 3 mv m between the two data sets in africa further investigation is needed to determine its cause we compare the eef structure in africa and south america and find differences which can be attributed to the effect of atmospheric nonmigrating tides this technique can be extended to any pair of ground magnetometer stations which can capture the day to day strength of the eej

finite difference seismograms for sh waves the accuracy of the finite difference method for generating synthetic seismograms of sh wave propagation in cylindrically symmetric media is discussed the finite difference method has the advantage that arbitrary density and velocity fields in the medium may be specified a point source is generated by a simple transformation of a line source the accuracy of the finite difference seismograms in flat and dipping layered media is confirmed by comparison with the generalized ray method a source radiation pattern is inserted by introducing a near field which has permanent displacement near the source strong motion synthetics are constructed with this new method for the 1968 borrego mountain earthquake as recorded at el centro good fits to the data are achieved using the laterally varying model determined by a detailed refraction survey and the source parameters determined by teleseismic waveform modeling shallow faulting is no longer necessary to explain the long period surface wave development

conjugate point equatorial experiment copex campaign in brazil electrodynamics highlights on spread f development conditions and day to day variability 1 a conjugate point equatorial experiment copex campaign was conducted during the october december 2002 period in brazil with the objective to investigate the equatorial spread f plasma bubble irregularity esf development conditions in terms of the electrodynamical state of the ionosphere along the magnetic flux tubes in which they occur a network of instruments including digisondes optical imagers and gps receivers was deployed at magnetic conjugate and dip equatorial locations in a geometry that permitted field line mapping of the conjugate e layers to dip equatorial f layer bottomside we analyze in this paper the extensive digisonde data from the copex stations complemented by limited all sky imager conjugate point observations the sheffield university plasmasphere ionosphere model supim is used to assess the transequatorial winds tew as inferred from the observed difference of hmf2 at the conjugate sites new results and evidence on the esf development conditions and the related ambient electrodynamic processes from this study can be highlighted as follows 1 large scale bottomside wave structures satellite traces at the equator followed by their simultaneous appearance at conjugate sites are shown to be indicative of the esf instability initiation 2 the evening prereversal electric field enhancement pre vertical drift presents systematic control on the time delay in sf onset at off equatorial sites indicative of the vertical bubble growth under weak transequatorial wind 3 the pre presents a large latitude height gradient in the brazilian sector 4 conjugate point symmetry asymmetry of large scale plasma depletions versus smaller scale structures is revealed and 5 while transequatorial winds seem to suppress esf development in a case study the medium term trend in the esf seems to be controlled more by the variation in the pre than in the tew during the copex period competing influences of the evening vertical plasma drift in favoring the esf development and that of the tew in suppressing its growth are discussed presenting a perspective on the esf day to day and medium term variabilities

on the height variation of the equatorial f region vertical plasma drifts we have used improved incoherent scatter radar measurements at the jicamarca radio observatory to study the height variation of the f region vertical plasma drift velocity driven by the zonal electric field during moderately quiet conditions preliminary results indicate a nearly linear change of the vertical drift velocity with altitude between 200 and 700 km but with considerable day to day variations in the value of the slope on the average the velocity gradients are positive in the late night and morning periods and negative during the afternoon and evening hours simultaneous vertical and zonal drift measurements confirm that the measured height variation of the vertical drift is consistent with the existence of a curl free electric field in the low latitude ionosphere the time dependence of the jicamarca vertical drifts extrapolated to higher altitudes closely resembles the diurnal variation of the drift component due to the zonal electric field observed at f region heights over arecibo

noon time equatorial electrojet its spatial features as determined by the champ satellite 1 new observations obtained by the satellite champ reveal a detailed picture of the noon time equatorial electrojet eej the low orbit of champ and its high precision magnetometers reveal the spatial structure of the eej with unprecedented accuracy data from more than two and a half years have been used to investigate average features and also the global characteristics of the eej rather than interpreting the magnetic signatures we determined the horizontal current distribution by using a very general current model series of line currents this makes the results independent of satellite altitude and ambient field geometry the procedure for determining the structure of the electrojet is fully automated giving an objective response some of the spatial features of the noon time eej are as follows the electrojet current peaks right at the dip equator there is no deviation from it either on a seasonal basis or with longitude the width of the eej 4 in latitude at half the peak value of the current density profile is for a given longitude fairly constant independent of the amplitude return currents north and south of the eastward current are a common feature of the eej they peak at latitudes some 5 away from the dip equator the intensity of the eej varies strongly from day to day the average peak current density exhibits a clear dependence on longitude peaks show up over south america and indonesia the average current density follows closely the monthly mean of the solar flux index f10 7 the total eej eastward current is about three times as strong as the return current the total current and the peak current density are related to each other by a power law this suggests that the longitude dependence of the eej intensity can be explained by the varying cross sectional area of the cowling channel

hydraulie control and flow separation in a multi layered fluid with applications to the vema channel abstract observations from a recent field experiment in the vema channel are briefly described these show a remarkable change in the configuration of isopycnal surfaces within the channel and the development of thick nearly homogeneous regions near the bottom which are capped by sharp vertical gradients contrary to previous speculation that these bottom boundary layer result from enhanced vertical mixing a dynamical mechanism is explored this involves the hydraulic adjustment of an inertial semi geostrophic flow to the channel geometry first an active two layer flow in a rectangular geometry is studied to show that internal flow separation can occur when the flow is accelerated sufficiently by a narrowing channel almost always this separation accompanies hydraulic control the slowest upstream moving kelvin wave is stopped and upstream and downstream states are not symmetric with respect to the channel width an active three layer flow with a variable bottom profile is then presented as a more

evolution and dynamics of the flow through herald canyon in the western chukchi sea abstract the flow of summer and winter pacific water masses through herald canyon is investigated using data from a high resolution hydrographic velocity survey conducted in summer 2004 the survey was part of the russian american long term census of the arctic rusalca program and consisted of four cross canyon transects occupied over a 2 day period at the time of the survey dense winter water was entering the western side of the canyon from the chukchi sea flowing alongside a poleward jet of summer water on the canyon s eastern flank as the dense water progressed northward it switched sides of the canyon and underwent a sudden increase in layer thickness this coincided with vertical mixing near the interface of the winter and summer water producing a new water mass mode exiting the canyon all of these features are consistent with the notion of hydraulic activity occurring in the canyon a three layer hydraulic theory is applied to the flow which suggests that it is supercritical and that hydraulic control is likely a lock exchange formulation accurately predicts the northward transport of the winter water the origin of the winter water and the manner in which it drains into the canyon is investigated using satellite ice concentration data atmospheric re analysis fields historical in situ data and a simple circulation model finally the fate of the pacific water exiting the canyon and its connection to the chukchi shelfbreak current is discussed

study of the equatorial and low latitude electrodynamic and ionospheric disturbances during the 22 23 june 2015 geomagnetic storm using ground based and spaceborne techniques we use a set of ground based instruments global positioning system receivers ionosondes magnetometers along with data of multiple satellite missions swarm c nofs dmsp guvi to analyze the equatorial and low latitude electrodynamic and ionospheric disturbances caused by the geomagnetic storm of 22 23 june 2015 which is the second largest storm in the current solar cycle our results show that at the beginning of the storm the equatorial electrojet eej and the equatorial zonal electric fields were largely impacted by the prompt penetration electric fields ppef the ppef were first directed eastward and caused significant ionospheric uplift and positive ionospheric storm on the dayside and downward drift on the nightside furthermore about 45 min after the storm commencement the interplanetary magnetic field imf bz component turned northward leading to the eej changing sign to westward and to overall decrease of the vertical total electron content vtec and electron density on the dayside at the end of the main phase of the storm and with the second long term imf bz southward turn we observed several oscillations of the eej which led us to conclude that at this stage of the storm the disturbance dynamo effect was already in effect competing with the ppef and reducing it our analysis showed no significant upward or downward plasma motion during this period of time however the electron density and the vtec drastically increased on the dayside over the asian region we show that this second positive storm was largely influenced by the disturbed thermospheric conditions

understanding the red sea response to sea level here we outline a new pragmatic methodology to derive relative sea level estimates from central red sea oxygen isotope records based on a previously published model in this paper the methodology is described in detail and it is shown that sea level change is the dominant factor responsible for the recorded variability in red sea 18o pdb for sea level changes greater than 12 m variables such as temperature or net evaporation are shown to have relatively small effects on the oxygen isotope record the modelled 18o pdb to sea level relationship is given in terms of a fifth order polynomial which may be used to describe relative sea level from central red sea oxygen isotope records we show how established sea level records from fossil reef terraces for the last 20 kyr are successfully simulated from central red sea oxygen isotope records we also examine the spatial variability of 18o pdb in the basin over the last 13 kyr

hydraulic interpretation of direct velocity measurements in the bab al mandab abstract acoustic doppler current profiler velocity measurements in the bab al mandab during the period june 1995 march 1996 are used to assess the hydraulic character of the exchange flow the strait is 150 km long and contains two distinct geometrical choke points the hanish sill and perim narrows the authors use a three layer approximation of the monthly mean velocity and density structure at the sill and narrows to calculate the phase speeds of the first and second internal long gravity waves the first second mode is generally characterized by in phase out of phase motions of the two interfaces the calculations take cross strait topographic variations into consideration by using a piecewise linear representation of the actual bottom the resulting phase speeds are used to determine whether the flow is subcritical supercritical or critical with respect to the first and second modes subcritical supercritical means that the two wave pairs corresponding to a given mode propagate in opposite

a three dimensional simulation of seismic waves in the santa clara valley california from a loma prieta aftershock abstract the finite difference method is used to propagate elastic waves through a 3 d model of the santa clara valley an alluvium filled basin that underlies the city of san jose california the model was based on depth to bedrock information from water wells in the area the simulation corresponded to a region 30 east west by 22 north south by 6 depth km and contained about 4 million grid points synthetic seismograms from the simulation are accurate at frequencies up to 1 hz motions from a magnitude 4 4 aftershock of the loma prieta earthquake were modeled snapshots of ground motion and synthetic seismograms from the simulation are presented the simulation illustrates s to surface wave conversion at the edges of the basin and the large amplitude and long duration of ground motion in the basin compared to the surrounding rock love waves produced at the edge of the basin are the largest arrivals in the transverse synthetics because of the slow group velocity of the love waves sites near the center of the basin have longer durations of significant motions than basin sites near the valley edges sites near the center of the basin also show larger peak amplitudes on the transverse component array analysis of the synthetic seismograms indicates that love waves tend to propagate parallel to the eastern and western edges of the valley rayleigh waves are produced along the southern margin of the basin from incident s waves large radial motions occur where a rayleigh wave impinges on the northeast margin of the valley some rayleigh waves travel westward across the basin after being scattered from the eastern edge of the valley synthetic seismograms from the simulation have similar peak amplitudes as seismograms recorded by the sunnyvale dense array for this aftershock although the duration of the tranverse component is not matched by the synthetic seismogram using this basin model the simulation indicates that the love waves observed on the actual seismograms were produced by conversion of incident s waves at the southern margin of the santa clara valley 2 d simulations show how the s to love wave conversion is affected by the angle of incidence of the s wave and the sharpness of the velocity transition between the alluvium and bedrock as more accurate basin models are developed 3 d simulations should become valuable for predicting ground motions in sedimentary basins for future large earthquakes

3d site effects a thorough analysis of a high quality dataset this article is a contribution toward filling the large gap that currently exists between theoretical and observational studies of site effects we present a detailed analysis of 10 events recorded by a dense temporary seismograph network that operated in the small roughly 1 3 by 0 4 km alluvial valley of parkway new zealand we computed f k spectra for the three recorded components of those 10 events as a function of frequency and time window along the seismograms our results show that a major part of the energy in the records propagates as surface waves the small size of the basin explains the observation that small phase velocity waves appear shortly after direct p and s wave arrivals both love and rayleigh waves are present it was difficult to isolate individual wavetrains but directions of propagation are dominated by southerly directions especially at lower frequencies and earlier times along the seismograms the inversion of observed phase velocity dispersion is in very good agreement with available information on the subsoil structure for this valley a comparison with a different interpretation of one event from our dataset shows clearly how misleading it can be to analyze partial observations the large variability of ground motion in the time domain contrasts with the very stable results obtained for this dataset in the frequency domain and indicates that we still have a long way to go before we are able to incorporate locally generated surface waves into the frame of modern building codes

response of mediterranean circulation to miocene shoaling and closure of the indian gateway a model study abstract in this regional ocean model study we explore the effect of the early to middle miocene shoaling and closure of the indian gateway on mediterranean circulation and its exchange with the adjacent oceans for this we use the regional ocean circulation model sbpom and a collection of bathymetries created from an early burdigalian paleogeographical base map in which the depth of the indian gateway is set to 1000 450 200 or 0 m a significant improvement of this work relative to previous regional modeling studies is that we also consider the possibility that the indian and atlantic gateways accommodated net westward flow as reported in many global climate model studies to this end we superimpose net westward flows of different magnitudes to the gateways because the early middle miocene climate is uncertain we start with atmospheric conditions based on the present and subsequently explore alternative atmospheric forcings similarly we first assume a relatively shallow atlantic gateway of 500 m depth and then study the effect of a connection of 900 m the extensive set of experiments performed allows us to gain detailed physics based insight into the effect of the above mentioned factors on the thermohaline circulation and water properties and provides a solid background to interpret observational evidence by comparing our modeled exchange patterns to data we are able to propose the flow configurations consistent with observations some of them not considered until now during the different stages of closure and infer the conditions climate presence absence of net flow atlantic gateway depth under which these patterns arise

study of ionospheric response to the 4b flare on 28 october 2003 using international gps service network data 1 using the gps data from as many as 114 gps stations of the international gps service for geodynamics igs the morphological features of the ionospheric total electron content tec variations on the sunlit hemisphere during the 4b solar flare on 28 october 2003 is studied it is found that the strongest sudden increase of tec sitec happened during the flare and the magnitudes of sitec vary at regions with different local solar zenith angle sza in the northern hemisphere the tec enhancement is approximately symmetrical to the local noon and its value is usually greater than 14 tecu 1 tecu 1016 m2 if the sza is less than 60 on the whole as the sza increases the value of tec enhancement in the northern hemisphere decreases it is worth mentioning that even in the regions of sza between 90 and 100 the sitec was still seen from the temporal tec curves using a photochemical model the electron production rate over the sunlit boundary region is calculated and some obvious features of sitec over this region are analyzed in the polar region the effect of this flare on the ionosphere exceeds the effect of the ionospheric scintillations and it seems that the ionosphere in the northern polar region responses more sensitively to this flare in the end superimposed on the curves of the rate of tec change there are some small disturbances spikes synchronously appearing on all curves and thus indicating an existence of similar structures in the euv band of the flare

maximal two layer exchange over a sill and through the combination of a sill and contraction with barotropic flow the analysis of two layer exchange flow through contractions with a barotropic component treated by armi farmer 1986 is extended to include exchange flows over sills and through a combination of a sill and contraction it is shown that exchange over a sill is fundamentally different from exchange through a contraction control at the sill crest acts primarily through the deeper layer into which the sill projects and only indirectly controls the surface layer this asymmetry in the control results in asymmetrical flows the interface depth above the crest is not one half the total depth as assumed in other studies by analogy with flow through contractions but is somewhat deeper the maximal exchange rate is less than for flow through a contraction of equal depth when both a sill and a contraction are present the contraction influences control at the sill crest only if it lies between the sill and the source of denser water the response to barotropic flow is also asymmetrical the transition to single layer flow occurs at much lower speeds for a barotropic component in one direction than the other results of the analysis are applied to exchange flow through the strait of gibraltar which includes both a sill and a contraction it is shown that maximal exchange conditions apply throughout part of the tidal cycle and observations illustrate several of the analytical predictions for barotropic flows including the formation of fronts single layer flow submaximal exchange and reverse flow

dwm07 global empirical model of upper thermospheric storm induced disturbance winds 1 we present a global empirical disturbance wind model dwm07 that represents average geospace storm induced perturbations of upper thermospheric 200 600 km altitude neutral winds dwm07 depends on the following three parameters magnetic latitude magnetic local time and the 3 h kp geomagnetic activity index the latitude and local time dependences are represented by vector spherical harmonic functions up to degree 10 in latitude and order 3 in local time and the kp dependence is represented by quadratic b splines dwm07 is the storm time thermospheric component of the new horizontal wind model hwm07 which is described in a companion paper dwm07 is based on data from the wind imaging interferometer on board the upper atmosphere research satellite the wind and temperature spectrometer on board dynamics explorer 2 and seven ground based fabry perot interferometers the perturbation winds derived from the three data sets are in good mutual agreement under most conditions and the model captures most of the climatological variations evident in the data

3 d wave propagation in the los angeles basin for the whittier narrows earthquake abstract elastic wave propagation within and around the los angeles basin during the whittier narrows earthquake in 1988 is simulated with a 3 d finite difference method the model consists mainly of a sedimentary layer over the basement but lateral variations inside the sedimentary basin is also included to approximate observed variations in surface shear velocity the minimum shear velocity is 1 km sec on the surface and fairly reliable results are obtained up to 0 75 hz the total simulation lasts 26 sec our simulation can reproduce the general amplification pattern small east of the epicenter and large in the l a basin and the san fernando valley due to thick sediments of low velocity in these areas surface waves are generated by the primary s wave and trapped inside the basins in the southern part of the l a basin a distinct later phase delayed by 5 to 6 sec relative to the primary s wave is produced in our simulation which matches some strong motion records

features of global geomagnetic sq field from 1980 to 1990 1 seasonal and year to year variations of geomagnetic sq field were examined using the global equivalent current system for one solar cycle from 1980 to 1990 it was confirmed that the sq current intensity is about half in solar minimum of that in solar maximum and its year to year variation is relatively smooth for each month at 0200 ut vortices in the winter and summer hemispheres shift to the afternoon and morning side respectively but with phase delay by about 1 or 2 months on the contrary at 1400 ut the northern vortex is located in the morning side of the southern vortex in most months of all years and seasonal variation of the location is not so clear as for the morphology of the current system it was found that there are only two types in the shape of the sq current system at 0200 ut that is it shows northern summer type from april to september and northern winter type from october to march and there is no equinox type and it clearly differs in march from that in september this feature is similar to that of the seasonal variation type of the tidal winds in the upper mesosphere

three dimensional simulation of the near fault ground motion for the 1995 hyogo ken nanbu kobe japan earthquake abstract the 17 january 1995 hyogo ken nanbu earthquake is a typical example showing that the ground motions along basin edge faults can be very destructive in this study we simulate the near fault ground motion from this earthquake based on a kinematic fault model and a simplified 3d velocity structure of the kobe area the kinematic earthquake rupture and the wave propagation are modeled using a 3d finite difference method fdm our simulation identifies the basin edge effect as an important factor that influenced the ground motion amplification pattern in the kobe area we found that the coupling of the source directivity and basin edge effects causes impulsive ground motions with extremely high amplitude at periods greater than 1 sec and in a narrow zone offset less than 1 km from the basin edge the combination of these effects acted to create a fairly continuous band of amplification that extends about 30 km in an elongated zone parallel to the basin edge boundary in some areas localized site effects might have been as important as the abovementioned effects but they cannot explain the continuity of the extended east west zone of damage

hydraulic control of three layer exchange flows application to the bab al mandab two layer hydraulic models have proven to be extremely useful in understanding a number of exchange flows through straits for example the strait of gibraltar there are though some strait flows that cannot be represented by a two layer model a very striking example is the bab al mandab in the exchange between the red sea and the gulf of aden three distinct water masses are observed in the strait during summer the wintertime flow is though very different with only two layers observed in the strait in this paper a three layer model of hydraulically controlled exchange flows is described the model can represent exchange flows that are controlled with respect to either the first or second or both internal modes a number of different examples are discussed the model is able to explain the qualitative features of the seasonal variation of the flow in the bab al mandab in particular the reversal of the direction of flow of the surface layer and the intrusion into the red sea of cold intermediate water from the gulf of aden during the summer months can be explained by upwelling in the gulf of aden forced by the southwest monsoon

multi layer hydraulic exchange flows flows between ocean basins are often controlled by narrow channels and shallow sills a multi layer hydraulic control theory is developed for exchange flow through such constrictions the theory is based on the inviscid shallow water equations and extends the functional approach introduced by gill 1977 and developed by dalziel 1991 the flows considered are those in rectangular cross section channels connecting two large reservoirs with a single constriction sill and or narrows the exchange flow depends on the stratification in the two reservoirs represented as a finite number of immiscible layers of different uniform density for most cases the flow is controlled at the constriction and often at other points along the channel virtual controls too as with one and two layer hydraulics controls are locations at which the flow passes from one solution branch to another and at which at least one internal wave mode is stationary the theory is applied to three layer flows which have two internal wave modes predicting interface heights and layer fluxes from the given reservoir conditions the theoretical results for three layer flows are compared to a comprehensive set of laboratory experiments and found to give good agreement the laboratory experiments also show other features of the flow such as the formation of waves on the interfaces the implications of the results for oceanographic flows and ocean modelling are discussed

modeling path effects in three dimensional basin structures path effects for seismic wave propagation within three dimensional 3 d basin structures are analyzed using a reciprocal source experiment in this experiment a numerical simulation is performed in which a point source is excited at a given location and then the wave field is propagated and recorded throughout a 3 d grid of points using the principle of reciprocity source and receiver locations are reversed this allows the modeling of path effects into a particular observation site for all possible source locations using only one simulation the numerical technique is based on the use of paraxial extrapolators and currently tracks only acoustic waves however the method is capable of handling arbitrary media variations thus effects due to focusing diffraction and the generation of multiple reflections and refractions are modeled quite well the application of this technique to model path effects for local earthquakes recorded at stations in the los angeles area of southern california indicates the strong influence of the 3 d crustal basins of this region on the propagation of seismic energy the modeling results show that the los angeles san fernando and san gabriel basins create strong patterns of focusing and defocusing for paths into these stations from various source locations these simulations correlate well with earthquake data recorded at both stations by comparing these calculations with earthquake data we can begin to evaluate the importance of these basin effects on observed patterns of strong ground motions

third generation of the potsdam magnetic model of the earth pomme the potsdam magnetic model of the earth pomme is a geomagnetic field model providing an estimate of the earth s core crustal magnetospheric and induced magnetic fields the internal field is represented to spherical harmonic sh degree 90 while the secular variation and acceleration are given to sh degree 16 static and time varying magnetospheric fields are parameterized in geocentric solar magnetospheric gsm and solar magnetic sm coordinates and include disturbance storm time dst index and interplanetary magnetic field imf by dependent contributions the model was estimated from five years of champ satellite magnetic data all measurements were corrected for ocean tidal induction and night side ionospheric f region currents the model is validated using an independent model from a combined data set of orsted and sac c satellite measurements for the core field to sh degree 13 the root mean square rms vector difference between the two models at the center of the model period is smaller than 4 nt at the earth s surface the rms uncertainty increases to about 100 nt for the predicted field in 2010 as inferred from the difference between the two models

quiet time equatorial f region vertical plasma drift model derived from rocsat 1 observations 1 we have used five years of measurements on board the rocsat 1 satellite to develop a detailed quiet time global empirical model for equatorial f region vertical plasma drifts this model describes the local time seasonal and longitudinal dependence of the vertical drifts for an altitude of 600 km under moderate and high solar flux conditions the model results are in excellent agreement with measurements from the jicamarca radar and also from other ground based and in situ probes we show that the longitudinal dependence of the daytime and nighttime vertical drifts is much stronger than reported earlier especially during december and june solstice the late night downward drift velocities are larger in the eastern than in the western hemisphere at all seasons the morning and afternoon december solstice drifts have significantly different longitudinal dependence and the daytime upward drifts have strong wave number four signatures during equinox and june solstice the largest evening upward drifts occur during equinox and december solstice near the american sector the longitudinal variations of the evening prereversal velocity peaks during december and june solstice are anti correlated which further indicates the importance of conductivity effects on the electrodynamics of the equatorial ionosphere

the influence of nonmigrating tides on the longitudinal variation of the equatorial electrojet 1 the climatological model of the equatorial electrojet eejm 1 derived from orsted champ and sac c satellite measurements provides the opportunity to investigate the longitudinal variation of the current strength in detail special emphasis is put in this study on the effect of nonmigrating tides we have found that the influence of the diurnal eastward propagating mode with wavenumber 3 de3 is particularly strong in polar orbiting satellite observations the de3 tidal signal appears as a four peaked longitudinal structure we have put special emphasis in our analysis to isolate the de3 contribution from other sources contributing to the wavenumber 4 structure in satellite data the amplitude of the de3 signature in the eej not only peaks during equinox seasons but is also strong around the june solstice when looking at the modulation of the eej intensity the de3 accounts for about 25 during the months of april through september it is thus the dominant cause for longitudinal variations during december solstice months the influence of de3 is negligible a secondary three peaked longitudinal pattern emerges during solstice seasons when the de3 influence is removed from the data available it is however not clear whether this pattern is related to any tidal drivers

international geomagnetic reference field the eleventh generation the eleventh generation of the international geomagnetic reference field igrf was adopted in december 2009 by the international association of geomagnetism and aeronomy working group v mod it updates the previous igrf generation with a definitive main field model for epoch 2005 0 a main field model for epoch 2010 0 and a linear predictive secular variation model for 2010 0 2015 0 in this note the equations defining the igrf model are provided along with the spherical harmonic coefficients for the eleventh generation maps of the magnetic declination inclination and total intensity for epoch 2010 0 and their predicted rates of change for 2010 0 2015 0 are presented the recent evolution of the south atlantic anomaly and magnetic pole positions are also examined