influence of ionospheric electron density fluctuations on satellite radar interferometry evidence is presented that auroral zone ionospheric disturbances can influence satellite radar interferometry sri obtained with the radarsat ers and jers 1 satellites fluctuations in ionospheric electron density can lead to an azimuth shift modulation in synthetic aperture radar sar imagery which can be detected using sri measurements of azimuth shift in sri can help to differentiate ionospheric from tropospheric propagation problems and to understand better the impact of the ionosphere on spaceborne sar further sri azimuth shift modulation may be useful in mapping patterns of polar auroral zone ionospheric disturbances over large distances

pine island glacier antarctica a study of the catchment using interferometric synthetic aperture radar measurements and radar altimetry we have mapped the catchment region of pine island glacier antarctica using ers 1 and ers 2 radar altimetry and synthetic aperture radar sar interferometry the radar altimeter data were converted to a digital elevation model dem using an algorithm we developed that corrects for slopes and grids the surface elevation estimates in a single step we used the dem and precise postprocessed orbital ephemerides to generate terrain corrected and geolocated complex radar imagery interferometric pairs on both ascending and descending orbits corrected by means of the dem yielded components of ice surface motion in the radar look direction where possible we combined velocity components from ascending and descending tracks to produce vector velocities elsewhere the velocity was calculated by assuming the direction of motion is down the surface slope the velocity map reveals a system of tributaries that channel ice from the basin like catchment area into the fast flowing downstream trunk of pine island glacier none of the tributaries shows a clear onset region instead the ice speed gradually increases along with a gradual decrease in driving stress along each tributary we combined the velocity data with sparse information on ice thicknesses and snow accumulation rates to calculate an approximate mass balance we find that the region is in balance within an uncertainty of 30 of the total mass throughput

satellite radar interferometry for monitoring ice sheet motion application to an antarctic ice stream satellite radar interferometry sri provides a sensitive means of monitoring the flow velocities and grounding line positions of ice streams which are indicators of response of the ice sheets to climatic change or internal instability the detection limit is about 1 5 millimeters for vertical motions and about 4 millimeters for horizontal motions in the radar beam direction the grounding line detected by tidal motions where the ice goes afloat can be mapped at a resolution of 0 5 kilometer the sri velocities and grounding line of the rutford ice stream antarctica agree fairly well with earlier ground based data the combined use of sri and other satellite methods is expected to provide data that will enhance the understanding of ice stream mechanics and help make possible the prediction of ice sheet behavior

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

tributaries of west antarctic ice streams revealed by radarsat interferometry interferometric radarsat data are used to map ice motion in the source areas of four west antarctic ice streams the data reveal that tributaries coincident with subglacial valleys provide a spatially extensive transition between slow inland flow and rapid ice stream flow and that adjacent ice streams draw from shared source regions two tributaries flow into the stagnant ice stream c creating an extensive region that is thickening at an average rate of 0 49 meters per year this is one of the largest rates of thickening ever reported in antarctica

flow of glaciar moreno argentina from repeat pass shuttle imaging radar images comparison of the phase correlation method with radar interferometry high resolution radar images of glaciar moreno argentina acquired by the shuttle imaging radar c sir c on 9 and 10 october 1994 at 24 cm wavelength l band are utilized to map the glacier velocity both interferometrically and using the phase correlation method the precision of the interferometric ice velocities is 1 8 cm d 1 6 m a 1 1 the phase correlation method measures ice velocity with a precision of 14 cm d 1 50 m a 1 with image data at a 6 m sample spacing acquired 1 day apart averaged strain rates are measured with a precision of 10 4 d 1 at a 240 m sample spacing with the phase correlation method and 10 5 d 1 with radar interferometry the phase correlation method is less precise than radar interferometry but it performs better in areas of rapid flow is more robust to temporal changes in glacier scattering and measures the glacier velocity in two dimensions with only one image pair using this technique we find that glaciar moreno flows at 400 m a 1 in the terminal valley and 800 m a 1 at the calving front in agreement with velocities recorded a decade ago assuming steady state flow conditions the vertical strain rates measured by sir c are combined with prior data on mass ablation to estimate the glacier thickness and ice discharge the calculated discharge is 0 6 0 2 km3 ice a 1 at 300 m elevation and 1 1 0 2 km3 ice a 1 at the equilibrium line elevation 1150 m which yields a balance accumulation of 6 1m ice a 1

penetration depth of interferometric synthetic aperture radar signals in snow and ice digital elevation models of glaciated terrain produced by the nasa jet propulsion laboratory jpl airborne interferometric synthetic aperture radar insar instrument in greenland and alaska at the c 5 6 cm wave length and l band 24 cm frequencies were compared with surface elevation measured from airborne laser altimetry to estimate the phase center of the interferometric depth or penetration depth p on cold polar firn at greenland summit p 9 2m at c and 14 4m at l band on the exposed ice surface of jakobshavn isbrae west greenland p 1 2 m at c and 3 3 m at l band except on smooth marginal ice where p 15 5 m on colder marginal ice of northeast greenland p reaches 60 to 120 m at l band on the temperate ice of brady glacier alaska p is 4 2 m at c and 12 6 m at l band with little dependence on snow ice conditions the implications of the results on the scientific use of insar data over snow ice terrain is discussed

circulation and melting beneath george vi ice shelf antarctica introduction george vi ice shelf sandwiched between the western coast of palmer land and the eastern coast of alexander island is the largest and most studied of the west antarctic peninsula ice shelves it covers an area of approximately 25 000 km2 and is underlain by circumpolar deep water cdw with temperatures in excess of 1oc giving rise to rapid basal melting bishop and walton 1981 lennon et al 1982 the maximum ice thickness of about 500 m occurs about 70 km from the southern ice front where a ridge of thick ice extends across george vi sound near 70ow see figure 1 effectively dividing the upper water column into northern and southern regions the northern ice front which faces marguerite bay appears to be near the geographical limit of ice shelf viability and has undergone a gradual retreat in recent decades lucchitta and rosanova 1998 a timeframe over which much of the nearby wordie ice shelf disintegrated doake and vaughan 1991 there is extensive surface melting over the northern parts of the ice shelf and much of the ice column near the northern ice front appears to be temperate paren and cooper 1986 conditions in the south where the ice front faces into ronne entrance are colder and the ice front position appears to be steady the vast majority of the flow into the ice shelf comes from palmer land but basal melting is sufficient to remove most of this so that the ice is derived almost exclusively from local accumulation by the time it reaches the ice fronts potter et al 1984 there is some evidence at the margins of the ice shelf to suggest that it may have disappeared completely during the early holocene before reforming sugden and clapperton 1981 bentley et al 2005

ice motion over lake vostok antarctica constraints on inferences regarding the accreted ice ice motion over lake vostok antarctica is measured using repeat pass synthetic aperture radar sar interferometry the coverage of the lake and the components of the vector field are resolved using 10 overlapping data takes from ascending and descending look directions seventy day temporal baselines provide the sensitivity required to observe the range of ice motion 0 6 m a 1 over the lake and the adjacent ice sheet it is remarkable that the scattering field remained coherent over these time separations this is critical for interferometric analysis and can be attributed to the low surface accumulation and low air temperature at this elevation the regional flow of the ice sheet around lake vostok is from west to east perpendicular to the surface elevation contours as the ice flows past the grounding line a southward component of motion develops that is correlated with the north south surface slope along the length of the lake the surface velocity increases slowly from the northern tip of the lake and then more rapidly south of 77 s at vostok station the ice motion is 4 2 m a 1 across the lake and away from boundary effects the down lake flow pattern takes on a parabolic profile with maximum velocity close to the center line of the lake the overall influence of the subglacial lake is the addition of a down lake motion component to the prevailing west east motion of the ice sheet as a result we estimate 10 of the mass flowing onto the lake is diverted south reconstructions based on the vostok ice core indicate that the ice was grounded up glacier from the core site approximately 5000 years ago this suggests a minimum freezing rate of 40 mm a 1 for the subglacial accretion ice 10 times greater than that inferred from thermodynamic modeling of the upper 2 km of the ice core

east antarctic ice stream tributary underlain by major sedimentary basin marine and rift sediments exert a fundamental control on ice stream flow in the west antarctic ice sheet and hence on its mass balance and stability in contrast most ice streams in the much larger east antarctic ice sheet are thought to be relatively stable features resting on till perhaps underlain by crystalline rock any geological controls on east antarctic ice sheet enhanced flow remain largely unknown we present aerogeophysical evidence indicating that a region of enhanced ice flow in the interior of the east antarctic ice sheet is underlain by subglacial sediments 3 km thick and that these are influencing the flow regime of the overlying ice we show that subglacial sediments are important in modulating ice dynamics not just for the west antarctic ice sheet but also for its much larger neighbor and suggest that the sedimentary basin identified here may contain information on the neogene glacial history of this part of the east antarctic ice sheet

antarctic subglacial conditions inferred from a hybrid ice sheet ice stream model abstract subglacial conditions of large polar ice sheets remain poorly understood despite recent advances in satellite observation major uncertainties related to basal conditions such as the temperature field are due to an insufficient knowledge of geothermal heat flow here a hybrid method is presented that combines numerical modeling of the ice sheet thermodynamics with a priori information using a simple assimilation technique additional data are essentially vertical temperature profiles measured in the ice sheet at selected spots as well as the distribution of subglacial lakes in this way geothermal heat flow datasets are improved to yield calculated temperatures in accord with observations in areas where information is available results of the sensitivity experiments show that 55 of the grounded part of the antarctic ice sheet is at pressure melting point calculated basal melt rates are approximately 65 gt year 1 which is 3 of the total surface accumulation although these sensitivity experiments exhibit small variations in basal melt rates the impact on the ice age of basal layers is quite important in areas that are characterized by relatively low melt rates this may lead to standard deviations up to 50 in a model run over a period of 10 6 years subglacial water flow is concentrated underneath the large outlet glaciers of antarctica the larger subglacial lakes are perched at the head of these subglacial water systems an exception however is the lake system of the recovery ice stream catchment that receives a substantial amount of subglacial melt water from the upstream catchment

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

accuracy and resolution of alos interferometry vector deformation maps of the father s day intrusion at kilauea we assess the spatial resolution and phase noise of interferograms made from l band advanced land observing satellite alos synthetic aperture radar sar data and compare these results with corresponding c band measurements from european space agency remote sensing satellite ers based on cross spectral analysis of phase gradients we find that the spatial resolution of alos interferograms is 1 3times better than ers interferograms the phase noise of alos i e line of sight precision in the 100 5000 m wavelength band is 1 6times worse than ers 3 3 mm versus 2 1 mm in both cases the largest source of error is tropospheric phase delay vector deformation maps associated with the june 17 2007 father s day intrusion along the east rift zone of the kilauea volcano were recovered using just four alos sar images from two look directions comparisons with deformation vectors from 19 continuous gps sites show rms line of site precision of 14 mm and rms azimuth precision flight direction of 71 mm this azimuth precision is at least 4times better than the corresponding measurements made at c band phase coherence is high even in heavily vegetated areas in agreement with previous results this improved coherence combined with similar or better accuracy and resolution suggests that l band alos will outperform c band ers in the recovery of slow crustal deformation

hercynian blueschist metamorphism in north portugal tectonothermal implications aegirine jadeite clinopyroxene 60 mol jadeite locally occurs within blueschists of the lower allochthon exposed in the tras os montes region of northern portugal peak conditions attained during blueschist facies metamorphism are estimated to have been c 420 c and 11 kbar porphyroblastic white mica paragonite phengite within the blueschist assemblage records a 36ar 40ar versus 39ar 40ar isotope correlation age of 329 4 1 6 ma in view of the relatively low t nature of the metamorphism the c 330 ma age is interpreted to date closely the high p recrystallization this tectonothermal activity is interpreted to have resulted from structural emplacement of a previously assembled crystalline nappe complex upper allochthon ophiolite nappe onto iberian protoliths of the lower allochthon during terminal stages of the hercynian orogeny

surface erosion control on the evolution of the deep lithosphere russell n pysklywec geology v 34 no 4 pp 225 228 april 2006 1 figure 1 has been corrected and replaced by the author the corrected figure 1 is below figure 2 1 lookup doi 10 1130 g21963 1 2 pending yes

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

large subglacial lakes in east antarctica at the onset of fast flowing ice streams water has been found to gather in lakes underneath the antarctic ice sheet and it can move between lakes and drain catastrophically despite this relatively little is known about how subglacial lakes influence ice sheet dynamics now satellite imagery has identified four subglacial lakes similar in total area to lake vostok right at the onset of the recovery glacier ice stream in east antarctica this suggests that subglacial lakes may initiate and maintain rapid ice flow a mechanism that will need to be considered in future ice sheet mass balance assessments satellite imagery is used to identify four subglacial lakes similar in total area to lake vostok right at the onset of the recovery glacier ice stream in east antarctica this suggests that the subglacial lakes can initiate and maintain rapid ice flow a mechanism that will need to be considered in future ice sheet mass balance assessments water plays a crucial role in ice sheet stability and the onset of ice streams subglacial lake water moves between lakes1 and rapidly drains causing catastrophic floods2 the exact mechanisms by which subglacial lakes influence ice sheet dynamics are unknown however and large subglacial lakes3 4 have not been closely associated with rapidly flowing ice streams here we use satellite imagery and ice surface elevations to identify a region of subglacial lakes similar in total area to lake vostok at the onset region of the recovery glacier ice stream in east antarctica and predicted by ice sheet models5 we define four lakes through extensive flat featureless regions of ice surface bounded by upstream troughs and downstream ridges using ice velocities determined using interferometric synthetic aperture radar insar we find the onset of rapid flow moving at 20 to 30 m yr 1 of the tributaries to the recovery glacier ice stream in a 280 km wide segment at the downslope margins of these four subglacial lakes we conclude that the subglacial lakes initiate and maintain rapid ice flow through either active modification of the basal thermal regime of the ice sheet by lake accretion or through scouring bedrock channels in periodic drainage events we suggest that the role of subglacial lakes needs to be considered in ice sheet mass balance assessments

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

interferometric estimation of three dimensional ice flow using ascending and descending passes satellite radar interferometry sri provides an important new tool for determining ice flow velocity interferometric measurements made from a single track direction are sensitive only to a single component of the three component velocity vector observations from along three different track directions would allow the full velocity vector to be determined a north south looking synthetic aperture radar sar could provide these observations over large portions of the globe but not over large areas of the polar ice sheets the authors develop and demonstrate a technique that allows the three component velocity vector to be estimated from data acquired along two track directions ascending and descending under a surface parallel flow assumption this technique requires that there are accurate estimates of the surface slope which are also determined interferometrically to demonstrate the technique the authors estimate the three component velocity field for the ryder glacier greenland their results are promising although they do not have yet ground truth data with which to determine the accuracy of their estimates

unsteady flow inferred for thwaites glacier and comparison with pine island glacier west antarctica we present a comparison of surface velocities in 1996 derived from interferometric synthetic aperture radar with an estimate of the long term depth averaged velocity based on the assumption of steady state flow for both pine island glacier and its neighbour thwaites glacier west antarctica the results show that the former was close to balance conditions in 1996 within 9 the ice flux and velocity of the latter is significantly different in magnitude and distribution from that required to maintain the basin in a state of mass balance over the long term the balance flux was found to be 32 19 less than the measured outgoing flux we conclude that the mass imbalance and dramatic difference in the pattern of flow is most likely due to a substantial change in the flow regime of thwaites glacier in the recent past

the accuracy of digital elevation models of the antarctic continent abstract the accuracy of two widely used digital elevation models of antarctica was assessed using data from the geoscience laser altimeter system onboard icesat the digital elevation models were derived from satellite radar altimeter and terrestrial data sets the first termed jlb97 was produced predominantly from ers 1 data while the second termed rampv2 included other sources of data in areas of high relief and poor coverage by ers 1 the accuracy of the models was examined as a function of surface slope and original data source large errors in excess of 100 m were ubiquitous in both models in areas where terrestrially derived elevation data had been used but were more extensive in rampv2 elsewhere the systematic error bias was found to be a monotonic function of slope for jlb97 with a more complex less predictable bias for rampv2 the magnitude of the global slope dependent bias ranged from less than a metre to slightly over 10 m but with much larger regional deviations the random error ranged from about 1 m to over 100 m depending on the dem and slope the random error was consistently over a factor two larger for rampv2 compared to jlb97

a subglacial water flow model for west antarctica the current generation of continental scale ice sheet models cannot successfully reproduce the complex ice water sediment interactions of the west antarctic ice sheet wais in a physically meaningful manner the potential of a thin film based subglacial water flow model for incorporation into a continental scale coupled ice water flow model of the wais is evaluated in this paper the subglacial water flow model is applied to the ross sea sector of the wais in both a steady state and time dependent form to derive the equilibrium water depth for the present day configuration the potential for coupling the model to an ice flow model is then demonstrated using a variable sliding parameter that is a function of the subglacial water depth a coupled ice water flow model using the parameterization tested in this paper could have the potential for reproducing the surface elevation velocity and thermal regime of the wais successfully these requirements are crucial in modelling the evolution of the wais and must be addressed before reliable continental scale predictive models can be utilized

the dynamics of ice sheet outlets a comparison of data from aircraft altimetry landsat imagery and radia echo sounding has shown characteristic surface topographies associated with sheet and stream flow the transition between the two is abrupt and occurs at a step in the subglacial topography this marks the onset of basal sliding and high velocities caused by subglacial water it results in crevassed amphitheatre like basins round the head of outlet glaciers it is also the zone of maximum driving stress beyond which values decline rapidly as velocities increase this abrupt transition appears to be topographically controlled since basal temperatures are at the pressure melting point well inland of the change in regime the marie byrd land ice streams exhibit qualitative differences from other ice sheet outlets however the change to lower driving stresses is much more gradual and occurs several hundred kilometres inland such ice streams have particularly low surface slopes and appear in form and flow regime to resemble confined ice shelves rather than grounded ice the repeated association of the transition to rapid sliding with a distinct subglacial feature implies a stabilizing effect on discharge through outlet glaciers acceleration of the ice is pinned to a subglacial step and propagation of high velocities inland of this feature seems improbable rapid ice flow through subglacial trenches may also ensure a relatively permanent trough through accentuation of the feature by erosion this is concentrated towards the heads of outlet glaciers up stream of the region where significant basal decoupling occurs this may be a mechanism for the overdeepening of fjords at their inland ends and the development of very steep fjord headwalls

basal and thermal control mechanisms of the ragnhild glaciers east antarctica the ragnhild glaciers are three enhanced flow features situated between the sor rondane and yamato mountains in eastern dronning maud land antarctica we investigate the glaciological mechanisms controlling their existence and behavior using a three dimensional numerical thermo mechanical ice sheet model including higher order stress gradients this model is further extended with a steady state model of subglacial water flow based on the hydraulic potential gradient both static and dynamic simulations are capable of reproducing the enhanced ice flow features although basal topography is responsible for the existence of the flow pattern thermomechanical effects and basal sliding seem to locally soften and lubricate the ice in the main trunks lateral drag is a contributing factor in balancing the driving stress as shear margins can be traced over a distance of hundreds of kilometers along west ragnhild glacier different basal sliding scenarios show that central ragnhild glacier stagnates as west ragnhild glacier accelerates and progressively drains the whole catchment area by ice and water piracy

widespread complex flow in the interior of the antarctic ice sheet it has been suggested that as much as 90 of the discharge from the antarctic ice sheet is drained through a small number of fast moving ice streams and outlet glaciers fed by relatively stable and inactive catchment areas here evidence obtained from balance velocity estimates suggests that each major drainage basin is fed by complex systems of tributaries that penetrate up to 1000 kilometers from the grounding line into the interior of the ice sheet this finding has important consequences for the modeled or estimated dynamic response time of past and present ice sheets to climate forcing

basal topography and ice flow in the bailey slessor region of east antarctica an airborne radio echo sounding campaign carried out in the upper reaches of bailey ice stream and slessor glacier in coats land east antarctica has revealed that tributaries of enhanced flow lie within well defined basal troughs and are separated from each other by bed highs these new data indicate significant differences in ice thickness compared with those estimated in the bedmap database a numerical modeling study has revealed that driving stresses are high enough to account for flow by ice deformation alone in intertributary areas most flow in the enhanced flow tributaries of slessor glacier may also be explained by ice deformation alone however although ice deformation is also significant in the bailey ice stream tributary a large amount of basal motion is also required to fully explain flow velocities here it is proposed that the trough in which bailey tributary lies is sufficiently deep that marine sediments may have accumulated here in preglacial times along with water produced by geothermal heating frictional heating and through a reduction in the pressure melting point basal motion may therefore be facilitated by the presence of a deformable saturated till layer at the bed