tracking slabs beneath northwestern pacific subduction zones article i nfo article history this study uses the amplitudes of bottom side reflected shear waves to constrain the morphology and dynam ics of subducted oceanic lithosphere beneath northwestern pacific subduction zones across honshu arc the 410 and 660 km seismic discontinuities are detected at the respective depths of 395 5 and 685 5 km within the wadati benioff zone their topographies are negatively correlated along slab dip showing the dominant effect of temperature on the olivine phase changes within the upper mantle transition zone the base of the upper mantle shows broad depressions as well as localized zones of shallow average depths be neath korea and northeast china the 15 km peak to peak topography west of the wadati benioff zones suggests that the stagnant part of the subducted pacific plate is not as flat as previously suggested eastward slab pile up is also possible at the base of the upper mantle across southern kuril arc the shear wave reflec tion coefficients of major olivine phase boundaries fall below 5 within the wadati benioff zone the appar ent reflection gaps and the spatial connection between a strong reflector at 900 km depth may imply 1 possible compositional variations at the top and bottom of the transition zone and 2 substantial mass heat flux across the 660 km seismic discontinuity we also identify strong reflectors within the subducted oceanic lithosphere at mid transition zone depths the depths and strengths of these reflectors are highly variable be tween honshu and southern kuril islands

evidence from earthquake data for a partially molten crustal layer in southern tibet earthquake data collected by the indepth ii passive source experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern tibet north of the zangbo suture beneath the southern lhasa block a midcrustal low velocity zone is revealed by inversion of receiver functions rayleigh wave phase velocities and modeling of the radial component of teleseismic p waveforms conversely to the south beneath the tethyan himalaya no low velocity zone was observed the presence of the midcrustal low velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern yadong gulu rift and possibly much of southern tibet

response of large scale ionospheric convection to substorm expansion onsets a case study 1 we have studied the response of large scale ionospheric convection to substorm expansion onsets on the basis of two weak substorms of 1 may 2001 during which a large part of the dawn cell of the two cell ionospheric convection pattern was monitored by the superdarn radars ionospheric convection began to enhance first in a localized region of the equatorward part of the dawn cell 2 minutes before the expansion onsets of both substorms and then enhanced in the entire dawn cell successively the enhanced convection persisted throughout their expansion phase possibly even near the footprint of a plasma sheet region without fast flows observed by geotail these observations suggest that ionospheric convection begins to enhance just before substorm expansion onset and then enhances in the entire cell possibly regardless of the presence of fast earthward flows in the corresponding plasma sheet region of the magnetotail the global enhancement of ionospheric convection is consistent with that of magnetotail convection which also begins just before onset

sharp mantle transition from cratons to cordillera in southwestern canada the western canada sedimentary basin marks the transition from the old north american continental lithosphere to young accreted terranes earlier studies in this region have suggested a large number of intricate basement domains as well as major seismic velocity gradients in the mantle to investigate the effect of the accretion and subduction on the mantle structure beneath the western margin of the north american craton we analyze p to s converted waves from upper mantle discontinuities from the canadian rockies and alberta network a regional broadband seismic array based in alberta canada the depths of the 410 and 660 km seismic discontinuities are correlated and on average are 9 km and 7 km greater than their respective global estimates the largest depression is observed beneath the rocky mountain foreland belt in southern alberta which highlights a steep southward westward structural gradient from the cratons to cordillera at lithospheric mantle depths the severity of the depressions especially in the southernmost alberta may be triggered by diffuse partial melt or increased water content above the 410 km discontinuity this result is corroborated by locally increased impedance contrast across the 410 km discontinuity and a strongly depressed 660 km discontinuity a relic mesozoic slab fragment may be partially responsible for the deep olivine phase boundary at the base of the upper mantle

the equatorial electrojet during geomagnetic storms and substorms the climatology of the equatorial electrojet during periods of enhanced geomagnetic activity is examined using long term records of ground based magnetometers in the indian and peruvian regions equatorial electrojet perturbations due to geomagnetic storms and substorms are evaluated using the disturbance storm time dst index and auroral electrojet ae index respectively the response of the equatorial electrojet to rapid changes in the ae index indicates effects of both prompt penetration electric field and disturbance dynamo electric field consistent with previous studies based on f region equatorial vertical plasma drift measurements at jicamarca the average response of the equatorial electrojet to geomagnetic storms dst 50 nt reveals persistent disturbances during the recovery phase which can last for approximately 24 h after the dst index reaches its minimum value this after storm effect is found to depend on the magnitude of the storm solar euv activity season and longitude

a low velocity zone atop the transition zone in northwestern canada 1 seismic studies over the past decade have identified an s wave low velocity zone lvz above the transition zone at various locations around the globe this layer is hypothesized to be a lens of dense hydrous silicate melt ponding atop the 410 km discontinuity beneath the silicate melt density crossover predicted to exist within the upper mantle we have assembled a p and s receiver function data set to quantify the physical properties and geographical extent of the layer in northwestern canada geographic profiles formed from 1 d migration of receiver functions computed for the canadian northwest experiment canoe and portable observatories for lithospheric analysis and research investigating seismicity polaris slave arrays reveal an lvz beneath many stations at a nominal depth of 340 km to constrain layer thickness and poisson s ratio we performed a grid search over a suite of 1 d velocity profiles to model the relative delay times of direct conversions and reverberations from the top of the lvz and 410 km discontinuity as recorded at the yellowknife array in addition we performed linearized inversion of transmission coefficient amplitudes to estimate s velocity contrasts at the bounding interfaces the lvz is characterized by a thickness of 36 km with an s velocity contrast of 7 8 and poisson s ratio of 0 42 taken at face value the two latter results require an increase in p velocity into the lvz the poisson s ratio lies well above the iasp91 average of 0 29 0 3 for this depth range and favors the presence of high melt or fluid fractions

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

formation timescales of large martian valley networks abstract assessing timescales for formation of the ancient martian valley networks is key to interpreting the early climate on mars we determined the likely formation times for seven of the largest ancient valley networks on mars using the darcy weisbach equation for average flow velocity three different sediment transport models and a range of input parameters to encompass possible formation conditions with runoff rates similar to intense storms in arid regions on earth the minimum formation timescales of these martian valley networks range from 10 5 to 10 7 yr depending on the specific valley network shorter formation timescales require hurricane scale flows that if minimized with assumptions of continuous formation unlike even terrestrial rates could complete large valley network formation in as little as 200 to 5000 yr though this is not the preferred interpretation while these results do not support impact induced climate change as the sole mechanism for creating precipitation sufficient to incise the large valley networks neither do they extend the amount of time required to form the networks beyond their range in age consistent with hypotheses that valley incision was constrained to a relatively short period of martian history near the noachian hesperian boundary approximately 3 6 to 3 8 ga

a detailed map of the 660 kilometer discontinuity beneath the izu bonin subduction zone dynamical processes in the earth s mantle such as cold downwelling at subduction zones cause deformations of the solid state phase change that produces a seismic discontinuity near a depth of 660 kilometers observations of short period shear to compressional wave conversions produced at the discontinuity yield a detailed map of deformation beneath the izu bonin subduction zone the discontinuity is depressed by about 60 kilometers beneath the coldest part of the subducted slab with a deformation profile consistent with the expected thermal signature of the slab the experimentally determined clapeyron slope of the phase transition and the regional tectonic history

crustal imprints of precambrian orogenesis in western laurentia crustal low velocity zones lvzs have been reported in active orogens such as the himalayas and the andes but rarely in stable cratonic regions in this study we provide compelling evidence for a significant midcrustal lvz beneath eastern central alberta an integral part of the precambrian canadian shield covered by thick phanerozoic sedimentary deposits this 200 km wide over 10 km thick midcrustal lvz is well resolved by shear velocity inversions using p to s receiver functions from more than 4600 earthquakes it is generally overlain by a high velocity upper crust in the depth range of 8 15 km especially in western central alberta which coincides with the previously documented winagami reflection sequence we interpret the lvz to be of granitic composition potentially in connection with the crystallization of partially molten crust during the paleoproterozoic eon in addition to the precambrian tectonic history of western laurentia which featured plate convergence conducive to crustal melting our crustal model is further supported by 1 a moderate spatial correlation between the lvz and heat flow and 2 shear velocities consistent with that of granite the well preserved winagami reflection sequence and the lvz are potential evidence of distinct episodes of magmatism and crust modification in the precambrian basement of the western canada sedimentary basin the existence of a broad crustal lvz suggests extensive subduction orogenesis and crustal melting during the precambrian assembly of the north american craton

determination and analysis of long wavelength transition zone structure using ss precursors summary global mapping of 410 and 660 km discontinuity topography and transition zone thickness has proven to be a powerful tool for constraining mantle chemistry dynamics and mineralogy numerous seismic and mineral physics studies suggest that the 410 km discontinuity results from the phase change of olivine to wadsleyite and the 660 km discontinuity results from the phase change of ringwoodite to perovskite and magnesiowustite underside reflections of the 410 and 660 km discontinuities arrive as precursors to ss with the recent development of a semi automated method of determining ss arrivals we have more than tripled the flanagan and shearer 1998a data set of handpicked ss waveforms we are able to increase resolution by stacking waveforms in 5 rather than 10 radius bins as well as increasing data coverage significantly in the southern hemisphere the resulting ss s410s and ss s660s times are heavily influenced by upper mantle velocity structure we perform a joint inversion for discontinuity topography and velocity heterogeneity as well as performing a simple velocity correction to the precursor differential times and find little difference between the two methods the 660 km discontinuity topography and transition zone thickness are correlated with velocities in the transition zone whereas the 410 km discontinuity topography is not in addition the 410 km discontinuity topography is not correlated with the 660 km discontinuity topography rather anticorrelated as expected due to the opposite signs of the clapeyron slopes of their respective phase changes these results suggest that whereas the topography of 660 km discontinuity could be dominated by thermal effects the topography of the 410 km discontinuity is likely dominated by compositional effects in addition unlike previous studies which find less topography on the 410 km discontinuity than on the 660 km discontinuity our 410 and 660 km topography have similar amplitudes

a detailed receiver function image of the upper mantle discontinuities in the japan subduction zone abstract we have imaged the upper mantle discontinuities in a 30 20 large region at the active continental margin of the japan subduction zone and neighboring areas using p to s converted phases from teleseismic records of permanent broadband stations the 410 km discontinuity is detected within 10 km of its global average position an interesting exception in its observation is a gap near 135 e very close to the slab penetration of the 410 km discontinuity and in an area where we have rather high data density the 660 km discontinuity reaches 700 km depth at two places where it is hit directly by the slab these data generally show good agreement with tomographic results both the 660 km discontinuity depression and the p velocity anomalies suggest about 400 500 k temperature deficit within the slab however no downward bending of the 660 km discontinuity is observed in eastern china where the flat lying slab is imaged by tomography this suggests that the slab does not cool the 660 km discontinuity in this region therefore the positive buoyancy required to keep the slab lying flat cannot have been provided by the negative clapeyron slope of the spinel perovskite phase transition another mechanism is needed which could possibly be metastable olivine in the cold core of the slab we have also imaged the shallower portion of the slab down to about 150 km underneath some seismic stations likely because metastable gabbro is still existing to this depth providing a sufficient velocity contrast a strong negative converted phase is observed at 150 200 km depth underneath a volcanic region in japan if real observed at one station only the zone which produces this conversion and which may begin exactly above the slab would require fluid fed melting

early mars volcanic sulfur storage in the upper cryosphere and formation of transient so2 rich atmospheres during the hesperian in a previous paper chassefiere et al 2013 we have shown that most volcanic sulfur released to the early mars atmosphere could have been trapped in the upper cryosphere under the form of co2 so2 clathrates huge amounts of sulfur up to the equivalent of an 1 bar atmosphere of so2 would have been stored in the noachian upper cryosphere then massively released to the atmosphere during the hesperian due to rapidly decreasing co2 pressure it could have resulted in the formation of the large sulfate deposits observed mainly in hesperian terrains whereas no or little sulfates are found at the noachian in the present paper we first clarify some aspects of our previous work we discuss the possibility of a smaller cooling effect of sulfur particles or even of a net warming effect we point out the fact that co2 so2 clathrates formed through a progressive enrichment of a pre existing reservoir of co2 clathrates and discuss processes potentially involved in the slow formation of a so2 rich upper cryosphere we show that episodes of sudden destabilization at the hesperian may generate 1000 ppmv of so2 in the atmosphere and contribute to maintaining the surface temperature above the water freezing point

statistical analysis of dayside equatorial ionospheric electric fields and electrojet currents produced by magnetospheric substorms during sawtooth events 1 substorms cause significant disturbances in the ionosphere however it has not been well understood how the electric field and electrojet in the dayside equatorial ionosphere respond to substorm onset previous studies found that the equatorial electric field after substorm onset could be eastward or westward because the onset of isolated substorms is often related to a northward turning of the interplanetary magnetic field imf the measured total electric field is determined by contributions from both imf northward turning and substorm onset and is not necessarily the signature of the onset in order to exclude the effect of imf northward turning we analyze the variations of ionospheric electric field and electrojet during storm time substorms when the imf remains stable thus the ionospheric variations can be identified to be caused solely by substorms the electric field data are measured by the jicamarca radar and the electrojet is derived from magnetometers at jicamarca and piura it is found that substorm onset induces an eastward electric field and electrojet in the dayside equatorial ionosphere when the imf remains continuously southward across the onset the equatorial electrojet starts to increase at the onset reaches a maximum value 30 min after the onset and then decreases to the pre onset value 60 min after the onset westward electric field and counter electrojet occur only if the substorm onset is associated with a northward turning of the imf it is concluded that the effect of substorm onset on the dayside equatorial ionosphere without involvement of imf reorientations is an enhanced eastward electric field

early mars serpentinization derived ch4 reservoirs h2 induced warming and paleopressure evolution ch4 has been observed on mars both by remote sensing and in situ during the past 15 yr it could have been produced by early mars serpentinization processes that could also explain the observed martian remanent magnetic field assuming a cold early mars a cryosphere could trap such ch4 as clathrates in stable form at depth the maximum storage capacity of such a clathrate cryosphere has been recently estimated to be 2 x 10 19 to 2 x 10 20 moles of methane we estimate how large amounts of serpentinization derived ch4 stored in the cryosphere have been released into the atmosphere during the noachian and the early hesperian due to rapid clathrate dissociation and photochemical conversion of ch4 to h 2 these episodes of massive ch4 release may have resulted in transient h 2 rich atmospheres at typical levels of 10 20 in a background 1 2 bar co2 atmosphere the collision induced heating effect of h 2 present in such an atmosphere has been shown to raise the surface temperature above the water freezing point we show how local and rapid destabilization of the cryosphere can be induced by large events such as the hellas basin or tharsis bulge formation and lead to such releases our results show that the early mars cryosphere had a sufficient ch4 storage capacity to have maintained h 2 rich transient atmospheres during a total time period up to several million years or tens of million years having potentially contributed to the formation of valley networks during the noachian early hesperian

storm time dependence of equatorial disturbance dynamo zonal electric fields we use jicamarca radar observations of f region vertical plasma drifts and auroral electrojet indices during 1968 1988 to study the characteristics and temporal evolution of equatorial disturbance dynamo zonal electric fields these electric fields result from the dynamo action of storm time winds and or thermospheric composition changes driven by enhanced energy deposition into the high latitude ionosphere during geomagnetically active conditions the equatorial vertical drift perturbations last for periods of up to 30 hours after large increases in the high latitude currents on the average this process can be described by two basic components with time delays of about 1 12 hours and 22 28 hours between the high latitude current enhancements and the equatorial velocity perturbations our data indicate strong coupling between dynamo processes with different timescales the short term disturbance dynamo drives upward equatorial drifts eastward electric fields at night with largest amplitudes near sunrise and small downward drifts during the day these perturbation drifts are in good agreement with results from the blanc richmond disturbance dynamo theory the dynamo process with time delays of about a day drives upward drift velocities at night with largest values near midnight and downward drifts in the sunrise noon sector in this case the amplitudes of the disturbance drifts maximize during geomagnetically quiet times preceded by strongly disturbed conditions we also present results of a new equatorial storm time dependent empirical model which illustrate the characteristics of the vertical disturbance dynamo drifts

a comparison of the receiver structure beneath stations of the canadian national seismograph network three component broadband data from the recently deployed canadian national seismograph network provide a new opportunity to examine the structure of the crust and upper mantle beneath the canadian landmass receiver function analysis is an ideal method to use with this data set as it can provide constraints on the s velocity structure beneath each station of this seismograph network this analysis method is particularly useful in that it provides site specific information i e within 5 15 km of the station low velocity layers can be identified and it is possible to examine structure to upper mantle depths in this study receiver functions were computed for each of the 19 stations that made up the seismograph network in june 1994 five stations sampling a variety of tectonic environments including the appalachian orogen the canadian shield the western canadian sedimentary basin and the cascadia subduction zone were chosen for detailed modelling the results presented here are the first estimates of the s velocity structure beneath these five stations for those stations where comparisons can be made with seismic reflection and refraction results there is excellent agreement in eastern canada simple receiver functions and clear moho ps conversions at most stations indicate a relatively transparent crust and a moho depth of 40 45 km in northwestern canada moho ps phases indicate a crustal thickness of 33 38 km beneath inuvik northwest territories the moho is interpreted as two velocity steps separated in depth by 5 km and an upper mantle low velocity zone is near 47 km depth in western canada the data indicate a mid crustal low velocity zone beneath edmonton the moho of the subducting juan de fuca plate is interpreted at 52 krn depth beneath southern vancouver island several stations exhibiting complex receiver functions warrant further study they include stations at scheffewille quebec in the canadian shield deer lake newfoundland on the boundary of the grenville province and the appalachian orogen and yellowknife northwest territories at the intersection of the churchill and slave provinces and the western plains

global de correlation of the topography of transition zone discontinuities analysis of long period shear wave underside reflections from upper mantle discontinuities shows the shape of the 410 km discontinuity to be neither globally nor locally correlated with that of the 660 km discontinuity the long wavelength pattern of 410 topography is dominated by degree 1 spherical harmonics and has a reasonable correlation with the seismic velocity perturbations in the mantle above it the global topography of 660 dominated by degree 2 spherical harmonics is strongly correlated with shear wave velocity variations in the transition zone the poor correlation between the observed topography of the 410 and 660 km discontinuities is inconsistent with a simple model of large scale whole mantle flow thus implying a more complex pattern which includes at least temporary accumulation of subducted material in the transition zone ringwood and irifune nature 331 1988 131 136 tackley et al nature 361 1993 699 704 the modest seismic discontinuity at 520 km is well observed when the reflections are under mid age oceans but is not observed when they are under continental shields interpretation of this observation in terms of olivine depletion in the deep continental mantle indicates the possible existence of continent ocean chemical heterogeneity at depths exceeding 500 km

seasonal and longitudinal dependence of equatorial disturbance vertical plasma drifts 1 we used equatorial measurements from the rocsat 1 satellite to determine the seasonal and longitudinal dependent equatorial f region disturbance vertical plasma drifts following sudden increases in geomagnetic activity the prompt penetration vertical drifts are upward during the day and downward at night and have strong local time dependence at all seasons the largest prompt penetration drifts near dusk and dawn occur during june solstice the daytime disturbance dynamo drifts are small at all seasons they are downward near dusk with largest smallest values during equinox june solstice the nighttime drifts are upward with the largest magnitudes in the postmidnight sector during december solstice during equinox the downward disturbance dynamo drifts near sunset are largest in the eastern hemisphere while the late night upward drifts are largest in the western hemisphere the longitudinal dependence of the disturbance dynamo drifts is in good agreement with results from simulation studies

low velocity zone atop the 410 km seismic discontinuity in the northwestern united states the seismic discontinuity at 410 km depth in the earth s mantle is generally attributed to the phase transition of mg fe 2sio 4 from the olivine to wadsleyite structure variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations for example a cold anomaly would elevate the 410 km discontinuity because of its positive clapeyron slope whereas a warm anomaly would depress the discontinuity but trade offs between seismic wave speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities and structure often appears very complicated here we simultaneously model seismic refracted waves and scattered waves from the 410 km discontinuity in the western united states to constrain structure in the region we find a low velocity zone with a shear wave velocity drop of 5 on top of the 410 km discontinuity beneath the northwestern united states extending from southwestern oregon to the northern basin and range province this low velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic receiver function observations observed above the region we interpret the low velocity zone as a compositional anomaly possibly due to a dense partial melt layer which may be linked to prior subduction of the farallon plate and back arc extension the existence of such a layer could be indicative of high water content in the earth s transition zone

new bulk sulfur measurements of martian meteorites and modeling the fate of sulfur during melting and crystallization implications for sulfur transfer from martian mantle to crust atmosphere system abstract sulfur storage and transport between different reservoirs such as core mantle crust and atmosphere of mars are tied to igneous processes martian meteorites carry a record of mantle melting and subsequent differentiation history of martian magmas investigation of s geochemistry of martian meteorites can thus provide an understanding of how s is transferred from the martian interior to the exosphere in this study we measured bulk s concentration of 7 martian meteorites and modeled the behavior of s during both isobaric crystallization of primary martian magmas and isentropic partial melting of martian mantle comparisons between measured data and modeled results suggest that 1 sulfides may become exhausted at the source during decompression melting of the mantle and mantle derived basalts may only become sulfide saturated after cooling and crystallization at shallow depths and 2 in addition to degassing induced s loss mixing between these differentiated sulfide saturated basaltic melts and cumulus minerals with without cumulate sulfides could also be responsible for the bulk sulfur contents in some martian meteorites in this case a significant quantity of s could remain in martian crust as cumulate sulfides or in trapped interstitial liquid varying from 2 to 95 percent by weight our modeling also suggests that generation of sulfide undersaturated parental magmas requires that the mantle source of martian meteorites contain

westward ionospheric electric field perturbations on the dayside associated with substorm processes 1 a controversy has risen in the direction of ionospheric electric field perturbation on the dayside caused by substorm expansion phase in recent years i e eastward or westward to exclude the effect of interplanetary magnetic field northward turning the substorms without interplanetary field imf trigger are required to investigate this issue previous works such as that by huang et al 2004 showed that the eastward electric field perturbations caused by substorms can be observed however our case suggests that some substorms can produce strong westward electric field perturbations and drive westward equatorial electrojets on the dayside ionosphere this westward electric field is created by an overshielding like imbalance state of field aligned currents facs region 2 r2 fac greater than region 1 r1 fac which is built up through r2 fac enhancement rather than r1 fac reduction due to imf northward turning the substorm processes should be responsible for the westward electric field especially through polar cap shrinkage and magnetic field dipolarization

carbon sequestration on mars on earth carbon sequestration in geologic units plays an important role in the carbon cycle scrubbing co 2 from the atmosphere for long term storage while carbonate is identified in low abundances within the dust and soils of mars at 1 wt in select meteorites and in limited outcrops no massive carbonate rock reservoir on mars has been identified to date here we investigate the largest exposed carbonate bearing rock unit the nili fossae plains combining spectral thermophysical and morphological analyses to evaluate the timing and carbon sequestration potential of rocks on mars we find that the olivine enriched 20 25 basalts have been altered by low temperature in situ carbonation processes to at most 20 fe mg carbonate thus limiting carbon sequestration in the nili fossae region to 0 25 12 mbar of co 2 during the late noachian early hesperian before or concurrent with valley network formation while this is large compared to modern day co 2 reservoirs the lack of additional comparably sized post late noachian carbonate bearing deposits on mars indicates ineffective carbon sequestration in rock units over the past 3 7 b y this implies a thin atmosphere 500 mbar during valley network formation extensive post noachian atmospheric loss to space or diffuse deep sequestration by a yet to be understood process in stark contrast to earth s biologically mediated crust atmosphere carbon reservoir ratio of 10 4 10 5 mars ratio is a mere 10 10 3 even if buried pre noachian crust holds multiple bars

a superposed epoch study of superdarn convection observations during substorms 1 case studies of flow patterns during substorms have revealed characteristic patterns in the period leading to expansion onset velocities in the premidnight region have been observed to become strongly sheared and the premidnight convection reversal has been observed to extend across the midnight meridian at expansion onset the shear decreases and flow at midnight becomes meridional out of the polar cap the study presented here superposes flow patterns from 10 substorm intervals to determine the average flow patterns the study confirms the findings of the case studies and extends them to additional local time sectors in addition changes to the flow pattern after expansion onset are examined

waveform inversion of ss precursors an investigation of the northwestern pacific subduction zones and intraplate volcanoes in china abstract the arrival time and amplitude of underside reflections from mantle seismic discontinuities ss precursors have made major contributions to the understanding of mantle composition and dynamics in this study we introduce a nonlinear waveform inversion technique to simultaneously constrain shear velocities and discontinuity depths beneath the northwestern pacific subduction system based exclusively on a large ss precursor waveform dataset we are able to clearly delineate the morphology of the descending pacific plate which flattens at the base of the upper mantle and extends westward by 1500 km toward northern central china our grid search yields the maximum correlation between shear velocity and transition zone thickness at an angle of 30 consistent with the reported average slab dip beneath the study region the strongly positive correlation suggests predominantly thermal rather than compositional variations along the descending pacific plate the joint depth velocity solution also shows a 5 10 km depression of the 410 km discontinuity and an average decrease of 1 2 in upper mantle shear velocity beneath the intraplate volcanic fields in northeastern china this anomaly which reaches the middle of the upper mantle transition zone beneath the changbai hotspot initiates at a significantly shallower 320 km depth beneath the wudalianchi region high amplitude reflections at depths greater than 410 km suggest a water poor melt layer in possible association with 1 decompression melting from passive upwelling and or 2 active upwelling through a slab window

polar equatorial ionospheric currents driven by the region 2 field aligned currents at the onset of substorms 1 the dawn to dusk convection electric field increases during the growth phase of substorms driving dp2 currents composed of two cell current vortices in high latitude and leading to an increase in the eastward electrojet at the dayside dip equator eej during the expansion phase of substorms electric field and currents are often reversed in direction to the normal dp2 currents at subauroral to equatorial latitudes when the convection electric field reduces abruptly the reversed current at the dayside dip equator appears as a counterelectrojet cej and causes an equatorial enhancement of the negative bay in the afternoon sector conversely the convection electric field increases during substorms as deduced from sawtooth events causing an increase in the eej in this study by analyzing isolated substorms with magnetometer array data and superdarn super dual auroral radar network convection maps we deduce that the reversed electric field and currents develop at the subauroral to equatorial latitudes at the onset of substorms while the convection electric field increases at auroral latitudes these observations imply that both the region 1 and region 2 field aligned currents r1 and r2 facs develop on the dayside concurrently with the current wedge responsible for the midlatitude positive bay at midnight the substorm associated r2 facs are strong enough to cause the reversed current at the subauroral latitude and the cej at the equator we also deduce that the dayside equatorial cej begins simultaneously with or even earlier than the midlatitude positive bay at midnight while the midlatitude positive bay onset is delayed by several minutes as the station departs from the midnight meridian these observations suggest that the substorm begins with the intensification of the r2 facs responsible for the equatorial cej

on the latitudinal variations of the ionospheric electric field during magnetospheric disturbances a joint alert campaign was organized during the month of october 1980 by the incoherent scatter radars in the american sector namely jicamarca arecibo millstone hill and chatanika the campaign which met with success was designed to study the behavior of the ionospheric electric field as a function of latitude during magnetically active conditions the arecibo data in this campaign support present and previous observations at jicamarca that suggest that when the convection e field suddenly decreases the alfven layer shielding field becomes unbalanced and penetrates the plasmasphere while this type of observation is reasonably convincing others are more difficult to categorize we suggest that beside the high latitude electric fields time varying auroral conductivity models will have to be considered in order to understand the morphology of the low latitude e field disturbances we present the first correlation analysis and determination of the amplitude ratio of the disturbed zonal electric field at 30 geometric latitude arecibo to the field at 0 jicamarca other highlights of the paper are a discussion of dp2 which may help clarify the controversy surrounding it and a discussion of the sensitivity of low and mid latitude radars to disturbances of magnetospheric origin we show that this sensitivity maximizes at the magnetic equator

global variability of transition zone thickness 1 we analyze recordings from more than 3000 shallow earthquakes and measure s410s s660s differential travel times on a global scale these differential times are obtained using a novel cross correlation technique that eliminates effects associated with a priori assumptions of crustal thickness and velocity variations above the upper mantle transition zone our measurements show absolute perturbations up to 8 s from the global average 242 km which imply transition zone thickness variations of 20 km on a length scale of 2000 km the transition zone under major subduction zones e g the western pacific and south america is significantly thicker than the global average this could be associated with large scale depressions of the 660 km discontinuity due to the accumulation of cold oceanic lithosphere at the base of the upper mantle a narrow transition zone is observed under the central pacific and northern atlantic oceans we also observe a modest anticorrelation between s410s s660s times and the delay times of shear waves predicted using s12 wm13 su et al 1994 in the transition zone which suggests considerable thermal influence on the large scale topography of the 410 and 660 km discontinuities the lack of a stronger anticorrelation suggests that 1 compositional heterogeneities may be present and 2 a significant fraction of thermal anomalies near the transition zone do not extend through the transition zone and influence both the 410 and the 660 km discontinuities a regional analysis shows that the transition zone is on average 6 8 km thicker under continents than under oceans this implies a temperature difference of 50 70 c however we do not observe a clear correlation between thickness and the age of the crust

signatures for magnetospheric substorms in the geomagnetic field of dayside equatorial region origin of the ionospheric component 1 an event based study is made of the conditions that lead to the ionospheric current component in the geomagnetic field signature for magnetospheric substorms in the dayside dip equatorial region the substorm activity studied here is characterized by two onsets separated by 40 min the first one occurred during steady southward interplanetary magnetic field imf and steadily increasing cross polar cap potential and enhanced energy input into the magnetosphere while the second one occurred in close association with a northward transition of imf and a rapid reduction of the polar cap potential and energy input into the magnetosphere earlier studies using multispacecraft data as well as ground based optical radar and magnetic recordings ascertained the first onset to be associated with a pseudo breakup and the second onset with a true breakup magnetometer recordings from a meridional network of stations in the indian and pacific sectors revealed the response of daytime equatorial h field to the pseudo breakup to be weak and not readily identifiable in contrast a distinct negative bay like disturbance with an unambiguous dip equator enhancement prevailed in the h field starting with the onset of the expansive phase of the fully developed substorm this behavior indicative of a significant contribution of ionospheric currents to the negative h field bay is seen both in the indian and pacific sectors covering the 1200 1600 lt region the present case study thus suggests that a sudden and prominent reduction in the cross polar cap potential at the substorm expansive phase is responsible for the prevalence of the ionospheric current component in the geomagnetic disturbance in the dayside equatorial region it is also found for the first time that the amplitude of the negative h field disturbance in the afternoon sector exhibits a marked hemisphere asymmetry at midlatitudes it is higher by a factor of about 3 in the summer hemisphere than in the winter hemisphere the equatorial enhancement of the negative h component disturbance is interpreted as the signature of the over shielding effect namely direct penetration of the perturbation component of the large scale electric field associated with the rapid reduction in convection at the onset of the substorm expansion phase the marked summer winter asymmetry of the h field disturbance at midlatitudes is suggested as the outcome of the competing magnetic effects of field aligned currents and polar origin dp2 currents

photochemical and climate consequences of sulfur outgassing on early mars abstract ancient mars might have been warm and wet compared to today but climate models have trouble reproducing this warmth partly because of the faintness of the young sun and partly because of inherent limitations to co2 h2o greenhouse warming in particular rayleigh scattering of incoming sunlight by a dense co2 rich atmosphere leads to a high planetary albedo thereby reducing the amount of sunlight absorbed by the planet it has been recently suggested that the presence of 1 100 ppmv so2 in mars early atmosphere might have provided enough additional greenhouse warming to maintain a warm wet early mars we show however that this warming should have been more than offset by cooling from sulfate and sulfur aerosols in early martian atmosphere hence the paradox of mars early climate remains unresolved

low palaeopressure of the martian atmosphere estimated from the size distribution of ancient craters the decay of the martian atmosphere which is dominated by carbon dioxide is a component of the long term environmental change on mars from a climate that once allowed rivers to flow to the cold and dry conditions of today the minimum size of craters serves as a proxy for palaeopressure of planetary atmospheres because thinner atmospheres permit smaller objects to reach the surface at high velocities and form craters the aeolis dorsa region near gale crater on mars contains a high density of preserved ancient craters interbedded with river deposits and thus can provide constraints on atmospheric density at the time of fluvial activity here we use high resolution images and digital terrain models from the mars reconnaissance orbiter to identify ancient craters in deposits in aeolis dorsa that date to about 3 6 gyr ago and compare their size distribution with models of atmospheric filtering of impactors we obtain an upper limit of 0 9 0 1 bar for the martian atmospheric palaeopressure rising to 1 9 0 2 bar if rimmed circular mesas interpreted to be erosionally resistant fills or floors of impact craters are excluded we assume target properties appropriate for desert alluvium if sediment had rock mass strength similar to bedrock at the time of impact the paleopressure upper limit increases by a factor of up to two if mars did not have a stable multibar atmosphere at the time that the rivers were flowing as suggested by our results then a warm and wet co 2 h 2o greenhouse is ruled out and long term average temperatures were most likely below freezing

contribution of storm time substorms to the prompt electric field disturbances in the equatorial ionosphere this study tries to bring out the fact that storm time substorms can compete and at times significantly contribute to the geomagnetically disturbed time prompt penetration electric field effects on low and equatorial latitudes observations of unusual equatorial plasma drift data from jicamarca unattended long term investigations of the ionosphere and atmosphere during two space weather events show that substorms can induce both eastward and westward penetration electric fields under steady southward interplanetary magnetic field imf bz conditions during the first event on 2 january 2005 the enhancement of the daytime eastward electric field over jicamarca due to substorm is found to be comparable with the sq and interplanetary electric field iefy generated electric fields combined during the second event on 19 august 2006 the substorm is seen to weaken the daytime eastward field thereby inducing a westward field in spite of the absence of northward turning of imf bz overshielding the westward electric field perturbation in the absence of any overshielding events is observationally sparse and contrary to the earlier results further the substorm induced field is found to be strong enough to compete or almost nullify the effects of storm time iefy fields this study also shows quantitatively that at times substorm contribution to the disturbed time prompt electric fields can be significant and thus should be taken into consideration in evaluating penetration events over low latitudes

eastward electric field enhancement and geomagnetic positive bay in the dayside low latitude ionosphere caused by magnetospheric substorms during sawtooth events 1 substorms cause significant disturbances in the magnetosphere and ionosphere although many substorm induced phenomena have been relatively well understood a serious controversy arises as to how the magnetospheric and ionospheric convection responds to the expansion onset in this paper we present the observations of the low latitude ionospheric electric field and geomagnetic field at substorm onsets during sawtooth events when the interplanetary magnetic field imf remains steady southward so the ionospheric disturbances are attributed solely to the substorm effects in the substorm events we have analyzed the dayside equatorial ionospheric electric field shows an eastward enhancement at the onset and the geomagnetic field northward component at middle and low latitudes shows an increase positive bay on both the dayside and nightside we suggest that the magnetospheric convection is enhanced at substorm onset and that the enhanced magnetospheric convection electric field penetrates to the low latitude ionosphere the penetration electric field driven by substorm onset causes the eastward ionospheric electric field enhancement and geomagnetic positive bay on the dayside and the substorm current wedge causes the geomagnetic positive bay on the nightside our findings are significantly different from the results of most previous studies and provide new insight into the substorm dynamic process