timescales of mingling in shallow magmatic reservoirs arrival of magma from depth into shallow reservoirs has been documented as one of the possible processes leading to eruption magma intruding and rising to the surface interacts with the already emplaced degassed magmas residing at shallower depths leaving chemical signatures in the erupted products we performed two dimensional numerical simulations of the arrival of gas rich magmas into shallow reservoirs we solve the fluid dynamics for the two interacting magmas evaluating the space time evolution of the physical properties of the mixture convection and mingling develop quickly into the chamber and feeding conduit dyke leading on longer timescales to a density stratification with the lighter gas richer magma mixed with different proportions of the resident magma rising to the top of the chamber due to buoyancy over timescales of hours the magmas in the reservoir appear to have mingled throughout and convective patterns become harder to identify our simulations have been performed changing the geometry of the shallow reservoir and the gas content of the initial end member magmas horizontally elongated magma chambers as well as higher density contrasts between the two magmas cause faster ascent velocities and also increase the mixing efficiency

granite ascent and emplacement during contractional deformation in convergent orogens based on a case study in the central maine belt of west central maine u s a it is proposed that crustal scale shear zone systems provide an eaective focussing mechanism for transfer of granite melt through the crust in convergent orogens during contractional deformation flow of melt in crustal materials at depths below the brittle plastic transition is coupled with plastic deformation of these materials the flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses within the oblique reverse central maine belt shear zone system stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain which suggests percolative flow of melt to form the migmatite leuco somes and viscous flow of melt channelized in sheet like bodies possibly along fractures cyclic fluctuations of melt pressure may cause instantaneous changes in the eaective permeability of the flow network if self propa gating melt filled tensile and or dilatant shear fractures are produced due to melt enhanced embrittlement in homogeneous migmatite and schlieric granite occur in zones of lower strain which suggests migration of partially molten material through these zones en masse by granular flow and channelized flow of melt carrying entrained residue founded on the central maine belt case study we develop a model of melt extraction and ascent using the driving forces stress conditions and crustal rheologies in convergent especially transpressive orogens ascent of melt becomes inhibited with decreasing depth as the solidus is approached for intermedi ate a h2o muscovite dehydration melting the water saturated solidus occurs between 400 and 200 mpa near the brittle plastic transition during high t low p metamorphism where the balance of forces favors sub horizontal fracture propagation emplacement of melt may be accommodated by ductile flow and or stoping of wall rock and inflation may be accommodated by lifting of the roof at shallower crustal levels and or sink ing of the pluton floor the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduits although these plutons may have locally discordant relations with country rock structures when viewed at the crustal scale granite ascent and emplacement in convergent orogens are syn tec tonic processes 1998 elsevier science ltd all rights reserved

logarithmic growth of dikes from a depressurizing magma chamber dike propagation is an intrinsically multiphase problem where deformation and fluid flow are intricately coupled in a fracture process here we perform the first fully coupled simulations of dike propagation in two dimensions accounting for depressurization of a circular magma chamber dynamic fluid flow fracture formation and elastic deformation despite the complexity of the governing equations we observe that the lengthening is well explained by a simple model a t c 1 log 1 t c 2 where a is the dike length t is time and c 1 and c 2 are constants we compare the model to seismic data from 8 dikes in iceland and ethiopia and in spite of the assumption of plane strain we find good agreement between the data and the model in addition we derive an approximate model for the depressurization of the chamber with the dike length these models may help forecast the growth of lateral dikes and magma chamber depressurization

pervasive granitoid magma transfer through the lower middle crust during non coaxial compressional deformation granitic magmas migrated through early proterozoic middle lower crust at mt hay central australia via a diverse network of narrow structurally controlled channelways during a period of progressive w sw directed thrusting d1a d1d they utilized existing folds boudins and shear zones or created new channels by magmatic fracture either parallel to layering or rarely in irregular arrays the magmas rose obliquely parallel to the plunging 50 60 regional elongation direction which was defined by coaxial folds boudin necks and a strong mineral elongation lineation megacrystic charnockitic magmas migrated through metre scale conduits during d1a d1b but leucosomes were generally restricted to smaller centimetre scale structures that existed throughout the entire deformation history thus d1a d1b leucosomes were potential feeders of in situ partial melts to the adjacent larger conduits of charnockite magma thereby providing a pervasive interconnected network that allowed efficient migration of all magma types during the early stages of thrusting the upper middle crust of the anmatjira reynolds range area contains abundant megacrystic granitoid sheets that are of similar age and geochemistry to those at mt hay they are considered to have formed as syntectonic intrusions emplaced during w sw directed thrusting as at mt hay suggesting that granitic magmas formed near the base of the continental crust passed through the mid lower crustal level 25 30 km exposed at mt hay and accumulated in batholithic proportions at shallower crustal levels 12 20 km such as the anmatjira reynolds range area the observations imply that granitoid magmas in the deep crust are capable of pervasive migration through the crust during major compressive noncoaxial shear deformation localization of magmas by sequentially developed narrow compressive structures suggests that dilatancy followed successive foliation forming events a situation that can occur during steady state deformation if the effective confining pressures are low which would be a result of high and possibly variable rates of magma influx the inferred rapid melt segregation and migration during deformation suggest that large chambers do not form until magma reaches neutral buoyancy in the middle to upper continental crust

numerical modelling of melting processes and induced diapirism in the lower crust summary the processes of partial melting and magmatic diapirism within the lower crust are evaluated using a numerical underplating model fully molten basalt t 1200 c is emplaced at the moho beneath a solid granite t 750oc in order that a melt front grows into the granite if diapirism does not occur this melt front in the granite reaches a minimal depth in the crust before like in the molten basalt crystallization takes place the density contrast between the partially molten granite layer and the overlying solid granite can lead to a rayleigh taylor instability rti which results in diapiric rise of the partially molten granite assuming a binary eutectic system for both the granite and the underplating basalt and a temperature and stress dependent rheology for the granite we numerically solve the governing equations and find a that diapirism occurs only within a certain but possibly realistic range of parameters and b that if diapirs occur they do not rise to levels shallower than 15 or perhaps 12km the growth rate depends on the degree of melting and the thickness of the partially molten layer as well as the viscosity of the solid and the partially molten granite from a comparison of the growth rate with the velocity of a stefan front it is possible to predict whether a melt front will become unstable and result in diapiric ascent or whether a partially molten layer is created which remains at depth we carry out such a comparison using our thermodynamically and thermomechanically consistent model of melting and diapirism

the pangong injection complex indian karakoram a case of pervasive granite flowthrough hot viscous crust in the karakoram range of nw india the tangtse gorge cuts across deep crustal rock sexhumed by transpression along two strands of the dextral karakoram fault crustal leucogranitic magmas pervasively intruded amphibolites and migmatites in the form of sheets that locally coalesced and expanded to form kilometre scale plutons sheets resulted from slow seeping of granite into foliation planes magma wedging rather than from dyking driven by magma buoyancy and possibly syn intrusive tectonic deformation high temperature of the country rocks at the time of intrusion is inferred from similar crystallization ages of the intrusive leucogranite and in situ partial melt in the migmatites and reflected in the ductile structures developed whereas low country rock viscosity inhibited dyking high temperature freed the magma from the constraints of freezing and permitted pervasive intrusion the injection complex exposed along the tangtse gorge rather than representing the final emplacement structures represent transient structures of the granite pathways on their upward journey to build the karakoram batholith which crops out a few kilometres north at structurally shallower levels we suggest that pervasive magma flow is a transitional step between magma segregation at the source and later rise through cold crust and it may be one of several factors controlling whether dyking or diapirism becomes dominant during late ascent

empirical models of storm time equatorial zonal electric fields ionospheric plasma drifts often show highly complex and variable signatures during geomagnetically active periods due to the effects of different disturbance processes we describe initially a methodology for the study of storm time dependent ionospheric electric fields we present empirical models of equatorial disturbance zonal electric fields obtained using extensive f region vertical plasma drift measurements from the jicamarca observatory and auroral electrojet indices these models determine the plasma drift perturbations due to the combined effects of short lived prompt penetration and longer lasting disturbance dynamo electric fields we show that the prompt penetration drifts obtained from a high time resolution empirical model are in excellent agreement with results from the rice convection model for comparable changes in the polar cap potential drop we also present several case studies comparing observations with results obtained by adding model disturbance drifts and season and solar cycle dependent average quiet time drift patterns when the disturbance drifts are largely due to changes in magnetospheric convection and to disturbance dynamo effects the measured and modeled drift velocities are generally in good agreement however our results indicate that the equatorial disturbance electric field pattern can be strongly affected by variations in the shielding efficiency and in the high latitude potential and energy deposition patterns which are not accounted for in the model these case studies and earlier results also suggest the possible importance of additional sources of plasmaspheric disturbance electric fields

theory of melt segregation a review abstract the processes by which melt is extracted from partially molten source regions at depth are collectively known as melt segregation during the past three years a dynamical theory has been developed which provides a unified description of the mechanical thermodynamical and chemical aspects of melt segregation in this theory a partially molten region is regarded as a deformable porous medium or matrix saturated with melt migration of melt relative to the matrix can be driven either by the buoyancy of the melt or by nonhydrostatic pressure gradients associated with the deformation of the matrix this paper reviews the basic governing equations of the theory and the solutions which have been obtained during the past three years

two dimensional observations of overshielding during a magnetic storm by the super dual auroral radar network superdarn hokkaido radar 1 two dimensional observations of ionospheric plasma flows possibly caused by overshielding are reported for the first time the observations were made by the midlatitude super dual auroral radar network hokkaido radar in japan during a major magnetic storm on 15 december 2006 the magnetosphere was exposed continuously to a southward interplanetary magnetic field imf for several hours during the main phase of the storm immediately following the subsequent northward turning of the imf an antisunward plasma flow was observed for about 14 min in the predusk sector at magnetic latitudes of 50 60 reaching a maximum line of sight speed of 70 80 m s these features are consistent with a simulation of coupling between the ring current and the ionosphere associated with an overshielding condition within 1 h of the first observation a similar antisunward flow was observed during a period of southward oriented imf however the simulation cannot account for the antisunward flow in this case it is suggested that the shielding overshielding condition is not simply caused by the northward turning of imf this second overshielding like condition is attributable to a sudden contraction of the polar cap associated with the substorm or to a sudden strengthening of the inertial current converted from the abrupt injection of magnetospheric ions however neither fully accounts for the observations

disturbance dynamo effects over low latitude f region a study by network of vhf spaced receivers generation of equatorial spread f esf irregularities resulting from magnetic disturbance is known for past few decades however better prediction models for this phenomenon are still lacking magnetic storms also affects the f region plasma drifts in this work we examined variability in i occurrence of such freshly generated esf and ii low latitude f region zonal plasma drifts over indian longitude for this purpose simultaneous observations of amplitude scintillations on 251 mhz signal recorded by a network of spaced receivers located at low latitude stations are utilized observational stations are situated such that it longitudinally latitudinally covers an area of 5 6 13 here effect of disturbance dynamo dd electric field at low latitude f region and its variability are studied for three magnetic storms occurring in 2011 these magnetic storms are having nearly similar type characteristics except their start time we find that as time difference i e t between local sunset and start of magnetic activity decreases the dd effects seen at low latitude f region zonal irregularity drift around midnight becomes stronger for a given magnetic storm the dd effect on f region zonal irregularity drifts is found to be only marginally stronger at dip equator in comparison to off equatorial stations although effect of dd on f region zonal irregularity drifts are felt simultaneously generation of fresh esf is variable within a smaller longitudinal belt of 5 6 it is attributed to the presence of lsws at the bottomside of f region as initiation of esf is highly likely unlikely in the vicinity of crest trough of such lsws

evidence that coupling to magma chambers controls the volume history and velocity of laterally propagating intrusions 1 in this paper i present a simple analytical solution for the unsolved problem of a propagating dike coupled to a magma chamber as recognized by segall et al 2001 the flow of the magma from a chamber to a dike is described by an ordinary differential equation for the unknown pressures of chamber and dike the most intuitive assumption on magma exchange is that the volume gained by the intrusion equals the volume lost by the chamber this constraint however implies a chamber so large that its pressure does not drop during the feeding process and an incompressible magma that does not expand or vesiculate as it intrudes models assuming constant driving pressure fail to explain observations here mass conservation constrains pressure as magma flows from the chamber to the dike and the compressibilities of magma and sources control volume changes these assumptions allow me to decouple the equations and solve the system analytically the model predicts that chamber and dike volume change exponentially with time as v t v 1 exp t the asymptotic volume v and the timescale are found to be functions of rock magma chamber and dike parameters and of the initial pressure conditions assuming that the intrusion is shaped as an elongating cuboid its velocity is found to change as v v0exp t where v0 is the initial dike velocity the model is successfully tested against the best observations available for lateral dike propagation events the volume history of chamber and dike during the 2000 miyakejima intrusion japan and dike velocity during the 1978 krafla event iceland and during some intrusions following the 2005 event in afar ethiopia this paper confirms and extends the results of a previous study by rivalta and segall 2008 who found the final ratio between dike volume and the volume withdrawn from the chamber to be rv 1 4 m 3 1 where is the host rock rigidity and m is the magma compressibility here i demonstrate that the formula for rv holds at any time during the intrusion not just at the end this model confirms that some magma chambers behave as stiff magma tanks able to inflate large dikes as balloons and demonstrates that high volume ratios are unlikely if the chamber has a pure sill shape fitting observations with the model allows us to estimate several dike chamber magma and rock parameters

long duration penetration of the interplanetary electric field to the low latitude ionosphere during the main phase of magnetic storms 1 it is well known that the interplanetary electric field can penetrate to the low latitude ionosphere it is generally believed that the penetration of electric fields can last only for 30 min because of the shielding effect in the ring current in this paper we present the observations of the dayside ionospheric electric field enhancements at middle and low latitudes in association with reorientations of the interplanetary magnetic field imf in six cases the eastward electric field in the dayside equatorial ionosphere measured by the jicamarca incoherent scatter radar was enhanced for 2 3 hours after the imf turned southward and remained continuously southward in one case the eastward electric field in the dayside midlatitude ionosphere measured by the millstone hill incoherent scatter radar was continuously enhanced for 10 hours during southward imf since millstone hill is close to the equatorward boundary of the auroral zone during magnetic storms the penetration electric field there may be different from that at the equatorial ionosphere the most striking feature of the measurements is that the enhancements of the ionospheric electric field can last for many hours without significant decay the electric field enhancements in the middle and low latitude ionosphere are closely related to magnetic activity and occur during the main phase of magnetic storms the observations show that the interplanetary electric field can continuously penetrate to the low latitude ionosphere without shielding for many hours as long as the strengthening of the magnetic activity is going on under storm conditions

the role of transfer structures on volcanic activity at campi flegrei southern italy abstract the tyrrhenian margin of central italy underwent extension during pliocene and quaternary extension occurred mainly through nw se normal faults bordering a sequence of plio quaternary basins these basins are offset by coeval ne sw faults which show strike slip and normal motions and have been interpreted as transfer faults plio quaternary volcanic activity along the margin occurred along a nw se belt systematically in correspondence with ne sw transverse systems the campi flegrei volcanic district cfvd on the southern tyrrhenian margin consists of an active ne sw volcanic ridge developed along ne sw fractures we performed a structural field analysis with analogue and mechanical models to investigate the role of transverse structures upon volcanism at campi flegrei field analysis at campi flegrei recognized ne sw and to a lesser extent coeval nw se active fractures analogue experiments have simulated the development of transfer fault systems in brittle extensional domains the experiments show that subvertical transfer faults connect offset adjacent normal faults dipping 60 the mechanical model is based on the stress equations in uniaxial lithostatic conditions and absence of regional stresses it shows how pre existing subvertical fractures require the smallest magmatic pressures to be penetrated for a given magmatic pressure subvertical fractures might be penetrated more deeply tapping more easily primitive magmas these results suggest that the cfvd is located along a ne sw transfer zone connecting nw se regional normal faults volcanic activity along such ne sw trend would be induced by the subvertical dip of the transfer faults the subvertical dip of transfer faults also suggests an explanation for the emission of the more primitive products along ne sw systems at campi flegrei these considerations find a wider application on the remaining volcanic districts of the margin located in the same overall structural setting

chemical variations in the 1998 2011 and 2015 lava flows from axial seamount juan de fuca ridge cooling during ascent lateral transport and flow

three different types of electric field disturbances affecting equatorial ionosphere during a long duration prompt penetration event coordinated digisonde and oi 630 0 nm airglow observations from thumba tvm an indian dip equatorial station in conjunction with magnetic and geosynchronous particle flux measurements reveal three different types of electric field disturbances in the equatorial ionosphere thermosphere system its occurring in succession over a period of 6 h on a single night 22 23 january 2012 ap 24 these include 1 westward electric field perturbations owing to a pseudo breakup and a substorm event each lasting for about 30 min 2 eastward electric field perturbations continuing for about an hour owing to the southward excursion of z component of interplanetary magnetic field bz and 3 dp2 type fluctuating period 40 min electric field perturbation sustaining for about 4 h the pseudo breakup and the fully grown substorm events are found to be longitudinally localized and different in terms of response in the westward auroral electrojet index al as well as geosynchronous electron proton injections the polarity of the prompt penetration of interplanetary electric field that affects the equatorial ionosphere is observed to be eastward during 2100 2200 ist indian standard time which is observationally sparse but consistent with modeling studies interestingly on the same night dp2 type electric field fluctuations with 40 min periodicity and occasional eastward polarity akin to daytime are also found to affect the equatorial its for about 4 h 2200 0200 ist the case study thus brings out different processes that constitute a long duration prompt penetration event which otherwise would have been categorized as a single event

the form distribution and anisotropy of magnetic susceptibility of jurassic dykes in h u sverdrupfjella dronning maud land antarctica implications for dyke swarm emplacement abstract the mesozoic dyke swarms of western dronning maud land antarctica form a minor intrusive component of the karoo large igneous province five hundred and sixty one dykes were recorded intruding neoproterozoic gneisses and middle jurassic syenite plutons 40 ar 39 ar geochronology data reveal two temporally distinct components the 178 175 ma alkaline straumsvola dyke swarm that predominantly intrudes a nepheline syenite pluton and the 206 204 ma tholeiitic jutulrora dyke swarm found throughout the study area the straumsvola swarm exhibits highly variably dyke trends that display a restricted opening direction interpreted to be the result of high magma pressure equal to the maximum principal stress the jutulrora swarm displays a fan of dyke trends with dyke thickness and spacing increasing away from the inferred point of fan convergence anisotropy of magnetic susceptibility measurements reveal vertical magma transport within both dyke swarms in the straumsvola area with the southern outer exposures of the jutulrora swarm exhibiting lateral magma transport although associated with a long lived local igneous centre comparison of palaeostress estimates for the straumsvola dyke swarm and contemporaneous dykes in ahlmannryggen and vestfjella indicates the presence of a regional scale radial stress system in western dronning maud land between 178 175 ma supporting a mantle plume origin for the karoo large igneous province

effect of solidification on a propagating dike 1 magma migration through the brittle crust from depth occurs by the propagation of hydraulic fractures or dikes volcanic eruptions occur at the last stage if and when a propagating dike reaches the surface dike propagation involves complex physics because of several processes that are simultaneously occurring such as viscous flow of magma rock fracture elastic deformation of the host rock and potentially large changes of the physical properties of the magma crystallization degassing solidification etc currently the most practical way in the field or in terms of field measurements to follow the migration of magma before it reaches the surface is the analysis of the seismicity generated nevertheless a physical model that quantitatively relates the flux of magma in the dike to the seismicity is lacking we present here laboratory experiments involving propagation of a fluid filled crack such that the fluid solidifies upon contact with the cold elastic host we show that this can lead to a way of estimating the flux of the injection as a function of the surface creation rate the latter is shown to be a more reliable gauge of magma flux than the upward propagating velocity in the geologic application of a propagating dike the rate of creation of surface area may be related to the rate of release of seismic energy although this latter relation needs further work to be quantitatively reliable our new model nevertheless indicates how a new general framework can be constructed

effects of interplanetary electric field on the development of an equatorial spread f event 1 on a geomagnetically disturbed night 7 january 2005 an equatorial spread f esf event was captured during premidnight hours by the indian mst radar operated in ionospheric mode at gadanki 13 5 n 79 2 e dip angle 12 5 n the base height of the ionospheric f region over dip equator and a low latitude station showed similar variation during most of the esf interval except mainly during 2145 2200 ist indian standard time ist universal time ut 5 5 hours the zonal electric field variation over dip equator responded to a prompt penetration event at 2000 ist by gradually changing its polarity from westward to eastward after 2015 ist leading to the initiation of esf at 2035 ist the linear growth rate analysis supports the generation of irregularities only when the eastward electric field owing to the prompt penetration effect is taken into account the zonal electric field became westward again after 2100 ist however it ephemerally turned eastward shortly after 2145 ist 1615 ut that is believed to be associated with an overshielding condition a plasma plume got resurrected after 30 min indicating the association of the development of this plume structure with the overshielding electric field the oi 630 0 nm airglow intensity variations observed by a collocated airglow photometer simultaneously operated in a bidirectional zenith and east mode corroborated well with the esf structure and dynamics during the disturbed period these observations evince the active role of interplanetary electric field ief in the development of esf in the premidnight hours

injection and arrest of dykes implications for volcanic hazards dykes are the principal channels through which magma reaches the surface in volcanic eruptions for this reason dykes observed in the field are commonly assumed to be feeders to lava flows the actual proportion of dykes reaching the surface is however poorly known in order to develop models for the purpose of estimating volcanic hazard this proportion must be known this follows because such models should not only consider the probability of dykes being injected from magma chambers during periods of unrest in the associated volcanoes but also the probability of the injected dykes being arrested this paper presents field data on several thousand dykes from iceland and tenerife canary islands indicating that many and probably most dykes become arrested at various crustal levels and never reach the surface to feed eruptions using the results of analytical and numerical models it is shown that for common loading conditions the stress field in the vicinity of a magma chamber may favour the injection and propagation of dykes while the stress field at a certain distance from the chamber favours dyke arrest this means that many dykes that are injected from the chamber propagate only for a very limited distance from the chamber to the point where they become arrested the implication is that during periods of unrest in volcanoes the probability of volcanic eruption is only a small fraction of the probability of dyke injection from the source magma chamber

migmatites granites and orogeny flow modes of partially molten rocks and magmas associated with melt solid segregation in orogenic belts abstract this paper presents a model for the genesis of migmatites and granites during orogenic evolution based on the analysis of several phanerozoic crustal segments and a review of the physical properties of partially molten rocks and magmas this model inventories the modes of bulk flow of partially molten rocks and magmas and the mechanisms of melt solid segregation for each mode partial melting of rocks is associated with a strength decrease of two to three orders of magnitude leading to strain partitioning expressed by channel flow driven either by forces related to plate tectonics vertical channel flow or by the gravity force associated with lateral variations of crustal thicknesses horizontal channel flow although the structural characteristics of migmatites indicate that deformation plays a role in melt migration the emplacement of laccoliths of leucogranites above migmatites attests to the efficiency of the buoyancy force another decrease in apparent strength of about ten orders of magnitude is associated with the loss of continuity of the solid framework marking the transition from partially molten rocks to magmas the breakdown of the solid framework also allows for settling of the solid in suspension increasing the buoyancy of the remaining magma accordingly domes cored by diatexites former heterogeneous magmas and mantled by metatexites former partially molten rocks are interpreted as gravitational instabilities driven by the relative buoyancy of the magma and permitted by the weakness of the partially molten rocks this model provides a first order framework to elucidate the development of a crustal scale horizontal layering during the evolution of orogenic belts that are affected by partial melting in this case the middle crust is dominated by migmatites with domes cored by diatexites and mantled by metatexites that correspond to a partially molten and magmatic zone respectively the granitic dikes and sills and the associated laccoliths of leucogranites correspond to an intrusive zone overlying the partially molten zone the refractory lower crust is potentially in part formed by accumulation of solids segregated from the overlying heterogeneous magmas

melt segregation structures in granitic plutons when melt fraction increases beyond a critical value migmatites behave as magmas conversely when melt fraction decreases crystallizing magmas form solid framework mushes and behave as solids the richness in preserved mesoscopic melt segregation structures in migmatites is not matched by melt segregation structures in granitic bodies this paper identifies melt segregation structures in granites and compares them to structures in migmatites the rarity of segregation structures in granites is a consequence of the solid framework mush allowing for cryptic melt extraction from its pores the relatively short duration of solidification of the mush and the fragile nature of the solid framework identifying segregation features in granitoids at outcrop scale provides the basis for understanding the physical processes that lead to chemical and mineralogical differentiation in granitic magmas

on the combined effects of vertical shear and zonal electric field patterns on nonlinear equatorial spread f evolution in order to understand observations of modulated irregularities in a localized region on the bottomside of the equatorial ionosphere without the presence of vertically rising large scale plume events an investigation is carried out by means of a nonlinear numerical simulation model using different background ionospheric conditions the investigation revealed that the development of long wavelength 200 km perturbations can only be confined to the bottomside of the ionosphere in a localized region with the combined effects of shear in the zonal plasma drift and a nighttime westward electric field the variabilities in the occurrence characteristic are shown to depend on the variabilities in the prereversal enhancement in the zonal electric field further the resurgence of a plume event at a later time as seen by the jicamarca radar is explained on the basis of confined preseeded structures subject to a fresh instability source first evidence is presented that a source of fresh instability may actually be a wind and or a temperature disturbance launched hours earlier by a small substorm

sheeted and bulbous pluton intrusion mechanisms of a small granitoid from southeastern australia implications for dyke to pluton transformation during emplacement abstract the small late syn tectonic carboniferous davys creek granite dcg of southeastern australia consists of microgranitic intrusive bodies of diverse geometry and structure these bodies include 1 subvertical concordant sheets 2 bulbous peneconcordant plutons with apophyses and discordant lobes and 3 subvertical dykes and stocks the sequence of changing intrusive style is broadly 1 2 3 transition from 1 to 2 was probably a response to rising magma pressures or declining tectonic stresses the parameter of emerman and marrett 1990 which discriminates between stable sheet like and potential stock pluton batholith emplacement modes adequately predicts the transitions between sheet and pluton emplacements for the dcg ductile dyking along actively forming foliations appears to have been an important early intrusive mechanism a transition from sheet to bulbous pluton intrusion style is suggested to have been in response to magma pressure increases

getting granite dikes out of the source region whether a dike can propagate far from a magma reservoir depends upon the competition between the rate at which propagation widens the dike and the rate at which freezing constricts the aperture available for magma flow various formulations are developed for a viscous fluid at temperature t sub m intruding a growing crack in an elastic solid the initial solid temperature equals t sub m at the source and decreases linearly with distance from the source if t sub m is the unique freezing temperature of the fluid dike growth is initially self similar and an essentially exact solution is obtained if t sub m is above the solidus temperature the solution is approximate but is designed to overestimate the distance the dike may propagate the ability of a dike to survive thermally depends primarily upon a single parameter that is a measure of the ratio of the dike frozen margin thickness to elastic thickness perhaps more intuitively one may define a minimum distance from the essentially solid reservoir wall to the point at which the host rock temperature drops below the solidus necessary for dikes to propagate far into subsolidus rock it is concluded that for reasonable material properties and source conditions most basalt dikes will have little difficulty leaving the source region but most rhyolite dikes will be halted by freezing soon after the magma encounters rock at temperatures below the magma solidus while these results can explain why granitic dikes are common near granitic plutons but rare elsewhere the potentially large variation in magmatic systems makes it premature to rule out the possibility that most granites are transported through the crust in dikes nonetheless these results highlight difficulties with such proposals and suggest that it may also be premature to rule out the possibility that most granite plutons ascend as more equidimensional bodies

toward a synthesis of equatorial spread f onset and suppression during geomagnetic storms 1 we present initial steps toward unifying our understanding of storm time equatorial spread f esf by searching for the common elements in past case studies and statistical occurrence patterns we show that the development or inhibition of equatorial irregularities during magnetically active periods can be understood using the ae parameterized fejer scherliess model for disturbance vertical drifts versus storm time and local time this model takes into account the different sources of perturbation electric fields magnetospheric and ionospheric dynamos that ultimately drive the equatorial vertical drifts showing prompt and delayed effects in the premidnight sector where both generation and suppression can occur as well as in the postmidnight period where generation dominates the postsunset period exhibits the greatest variability for storm time esf versus longitude and thus we demonstrate the fejer scherliess model s applicability in a test case 6 april 2000 that had an ae pattern compatible with their parameterization scheme the model successfully accounts for the pronounced longitude confinement in the observed postsunset esf patterns finally we move beyond the empirically derived relationships between geomagnetic indices and the occurrence of esf aarons 1991 into a framework of true solar terrestrial parameters that drive such effects additional case studies taken from the published literature are then used to show a consistent linkage between postsunset esf onset and the interplanetary electric field ief esw while ae dst kp and dst dt indices were used in earlier studies to determine the dusk longitude sector of disturbance electric fields here we attribute to the ief the main role in the determination of this longitude sector

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

sequential nesting of magmas in marble southwestern grenville province quebec from fracture propagation to diapirism abstract in the southwestern grenville province quebec 1090 1070 my old k rich alkaline equidimensional plutons such as the kensington pluton occur in marble rich country rock but coeval dykes bearing lower crustal xenoliths are found only in a quartzofeldspathic gneiss complex the sequential nesting of magmas the various episodes of skarn formation and the progressively more mafic character of the intruding magmas of the kensington pluton are interpreted in terms of magma ascent through a series of propagating fractures cessation of upward fracturing and pooling of magma in marble rich ductile crust diapirism radial shortening of wall rock and in situ pluton expansion this suite provides evidence that at deep crustal levels the country rock rheology influences the mode of magma ascent and emplacement and that diapirism is a possible mechanism of magma transport through rocks behaving rheologically by a power law model

response of the equatorial ionosphere in the south atlantic region to the great magnetic storm of july 15 2000 the effects of the great magnetic storm of july 15 2000 on the equatorial ionosphere have been studied by ground based and satellite in situ measurements a large westward plasma drift in the evening equatorial ionosphere was observed as a result of the ionospheric disturbance dynamo in that environment the imf bz turned southward and presumably caused penetration of e fields to low latitudes this e field initiated the onset of 250 mhz and l band scintillations at ascension island 15 w and precipitous tec decrease at fortaleza brazil 38 w bounding the narrow longitude region in the south atlantic these impulsive ionospheric effects were extremely well correlated with abrupt decreases of sym h 1 min resolution dst the dmsp in situ measurements showed the presence of severe ion density bite outs extending over 30 latitude in the south atlantic magnetic anomaly region the rocsat 1 satellite measured upward and large southward ion drifts in the same sector

isolated waves and eddies in a shallow water model abstract a shallow water beta channel model was used to carry out numerical experiments with cyclonic and anticyclonic disturbances of various strengths the model is inviscid so fluid elements conserve potential vorticity q when unforced regions of closed q contours correspond to lagrangian material eddies all fluid within a lagrangian eddy travels with the eddy in contrast to regions of closed height contours motion is wavelike for very weak disturbances maximum particle speed u long planetary wave speed the height field disperses like a group of linear rossby waves and tracers have small oscillatory mainly north south displacements with very little scatter when u the planetary q field is sufficiently distorted for small lagrangian eddies to appear very small eddies are simply bodily advected by the linear wave field small eddies are to some extent self propelling they move westward and north cyclone or south anticyclone moving fluid elements towards their rest latit

a model for radial dike emplacement in composite cones based on observations from summer coon volcano colorado usa we mapped the geometry of 13 silicic dikes at summer coon an eroded oligocene stratovolcano in southern colorado to investigate various characteristics of radial dike emplacement in composite volcanoes exposed dikes are up to about 7 km in length and have numerous offset segments along their upper peripheries surprisingly most dikes at summer coon increase in thickness with distance from the center of the volcano magma pressure in a dike is expected to lessen away from the pressurized source region which would encourage a blade like dike to decrease in thickness with distance from the center of the volcano we attribute the observed thickness pattern as evidence of a driving pressure gradient which is caused by decreasing host rock shear modulus and horizontal stress both due to decreasing emplacement depths beneath the sloping flanks of the volcano based on data from summer coon we propose that radial dikes originate at depth below the summit of a host volcano and follow steeply inclined paths towards the surface near the interface between volcanic cone and basement which may represent a neutral buoyancy surface or stress barrier magma is transported subhorizontally and radially away from the center of the volcano in blade like dikes the dikes thicken with increasing radial distance and offset segments and fingers form along the upper peripheries of the intrusions eruptions may occur anywhere along the length of the dikes but the erupted volume will generally be greater for dike fed eruptions far from the center of the host volcano owing to the increase in driving pressure with distance from the source observed eruptive volumes vent locations and vent area intrusions from inferred post glacial dike fed eruptions at mount adams washington usa support the proposed model hazards associated with radial dike emplacement are therefore greater for longer dikes that propagate to the outer flanks of a volcano

granitoid pluton formation by spreading of continental crust the wiley glacier complex northwest palmer land antarctica abstract the emplacement mechanism geometry and isotope geochemistry of plutons of the wiley glacier complex suggest that new continental crust grew by multiple injection of tonalitic dykes during dextral transtension in the antarctic peninsula magmatic arc in early cretaceous times the suggested mechanism is analogous to basalt dyke injection during sea floor spreading during normal dextral shear the burns bluff pluton a sheeted moderately east dipping syn magmatically sheared tonalite granodiorite intruded syn magmatically sheared quartz diorite of the creswick gap pluton and 140 5 ma hornblende gabbro u pb dating of zircon and ar ar dating of hornblende and biotite suggest that both granite s l plutons were emplaced between 145 and 140 ma but that extensional shearing was active from the time of emplacement until ca 127 ma the burns bluff pluton is chilled at its margin and grades through mylonitised porphyritic tonalite granodiorite sheets and tonalite granodiorite sheets with minor chilling to a kilometre scale body of coarse grained hypidiomorphic tonalite granodiorite co magmatic microdiorite forms dykes and abundant synplutonic mafic enclaves these dykes opened as echelon veins during episodic dextral shear and were deformed to trains of enclaves during continued normal dextral shear pluton marginal porphyritic and hypidiomorphic tonalite granodiorite forms large fault hosted sheets emplaced progressively under extension with minor dextral shear kinematic indicators from pluton marginal granite s l dykes suggest that early in pluton accretion intrusive sheets cooled rapidly with simple shear prior to full crystallisation changing to ductile simple shear during cooling kinematic indicators towards the pluton core suggest that as the pluton grew and cooled more slowly emplacement switched from sheeting to in situ inflation with simple shear distributed across a broad zone prior to full crystallisation of magma cross cutting relationships with the coeval syn extensional creswick gap pluton suggest that the burns bluff pluton was emplaced in a steeper second generation shear structure like those in normal fault systems this suggests that the wiley glacier complex was emplaced above the base of the brittle ductile transition zone 15 18 km depth the burns bluff pluton has nd and sr isotope values that range from mantle dominated nd141 3 8 87 sr 86 sr 141 0 70468 to more crustally influenced nd141 1 7 87 sr 86 sr 141 0 70652 this range probably represents different degrees of mixing between mantle derived magma and lower crustal partial melts generated in the garnet stability zone 40 km depth addition of new crustal material by mafic underplating at the base of the crust and by redistribution of granitic s l and mafic modified underplated magma to mid crustal levels along extensional shear zones as the arc spread were the primary mechanisms of crustal growth

volatile assisted intrusion and autometasomatism of leucogranites in the khumbu himalaya nepal abstract leucogranite crops out in the upper imja khola khumbu himalaya nepal where it makes up approximately 50 of the exposed bedrock leucogranites and their associated aplite pegmatites typically intruded as concordant sills millimeters to kilometers wide the exceptional nuptse granite is an ellipsoidal pluton of nearly 1 km radius which probably resulted from the ballooning of a sill and is wrapped in concordant country rock foliation small scale structures and mineral paragenesis indicate syntectonic intrusion into high temperature low pressure sillimanite muscovite cordierite bearing country rocks penetrative deformation of the country rock and sills is characterized by foliation striking n 60 e 40 nw and by north northwest plunging mineral lineation subparallel to fold axes of decimeter to meter scale isoclinal folds and centimeter to decimeter scale kink folds and crenulation a volatile rich phase played a fundamental role in the development of the imja khola leucogranites it

evolution of equatorial ionospheric bubbles during a large auroral electrojet index increase in the recovery phase of a magnetic storm 1 we present a model and observations of the evolution of equatorial ionospheric bubbles during a large auroral electrojet ae index increase in the recovery phase of a geomagnetic storm using a three dimensional time dependent numerical simulation model we find for the 19 21 october 1998 storm that the equatorial bubble evolution is different during storm time as compared to quiet time conditions we have found that the storm time vertical drift in conjunction with reduced off equatorial e region shorting is the primary mechanism that distinguishes the large ae increase recovery phase storm time evolution from the quiet time case comparison of the simulation model with ground based storm time radar observations is made

role of imf b y in the prompt electric field disturbances over equatorial ionosphere during a space weather event on 7 january 2005 a p 40 prompt penetration electric field perturbations of opposite polarities were observed over thumba and jicamarca on a few occasions during 13 45 16 30 ut however the electric field was found to be eastward during 14 45 15 30 ut over both thumba and jicamarca contrary to the general expectation wherein opposite polarities are expected at nearly antipodal points on closer scrutiny three important observational features are noticed during 14 10 15 15 ut first during 14 10 14 45 ut despite increasing southward interplanetary magnetic field imf b z condition the already westward electric field over thumba weakened less westward while the eastward electric field over jicamarca intensified more eastward second the electric field not only became anomalously eastward over thumba but also got intensified further during 14 45 15 00 ut similar to jicamarca third during 15 00 15 15 ut despite imf b z remaining steadily southward the eastward electric field continued to intensify over thumba but weakened over jicamarca it is suggested that the changes in imf b y component under southward imf b z condition are responsible for skewing the ionospheric equipotential patterns over the dip equator in such a way that thumba came into the same dp2 cell as that of jicamarca leading to anomalous electric field variations magnetic field measurements along the indian and jicamarca longitude sectors and changes in high latitude ionospheric convection patterns provide credence to this proposition thus the present investigation shows that the variations in imf b y are fundamentally important to understand the prompt penetration effects over low latitudes

variation in the form and distribution of dykes in the mull swarm scotland the dykes of the mull swarm part of the british tertiary igneous province have been examined in a series of well exposed coastal sections traverses through the dyke swarm show systematic changes in the thickness and spacing of dykes both of which are typically log normally distributed both the geometric mean thickness and spacing of dykes increase with distance from the mull centre concordant with a decrease in crustal extension variations in the thickness and spacing result from changes in character of the magma flow it is inferred that the magma flux was fairly evenly distributed near the mull centre but farther away from the source it becomes focused into larger dykes the form orientation and opening direction are also observed to vary with distance away from the volcanic centre on mull the dykes intrude a range of pre existing fractures the range of fractures used decreases with distance from the centre this together with other features of the form of the dykes have been interpreted in terms of the varying magma pressure and regional stress which is expressed as the r ratio analysis of the variable strikes of fracture filled dykes is consistent with decreasing magma pressure with distance from the mull centre assuming that regional stresses were relatively invariant across the same distance

mesoscale pervasive felsic magma migration alternatives to dyking abstract this paper reviews the literature on dyking as a mechanism of felsic magma extraction from a source and transport to shallower crustal levels and review the recent literature suggesting a range of alternative mechanisms of magma migration in hot crustal zones which produce mesoscale pervasive granite sheet intrusions recent papers have strongly favoured dyking as the main mechanism controlling magma migration however the initiation of dykes from a felsic magma source is fraught with difficulties even when magma is immediately available for transportation as in magma chambers within a partially molten source magma may reside in a range of structures with a wide range of shapes sizes and degrees of connectivity whereas the growth of individual dykes within a partially molten zone and the self propagation of large dykes into subsolidus crust have both been studied in some detail little attention has been given to the crucial intermediate step of the growth of a dyke network capable of producing wide crustal scale dykes the rarity of granite dyke swarms suggests that if dyking is the preferred mechanism of magma transport felsic magma sources produce only few major transporting dykes during their lifetime alternatively dyking is not an important mechanism the parameters controlling the volume of the catchment drained by one such dyke as well as other basic geometrical parameters controlling the structure of the dyke network within the source are unknown the ability of dyking to drain a partially molten source depends crucially on these variables and particularly on the horizontal permeability of the source the slow velocity of viscous felsic magmas traveling in rock pores implies that magma drained during dyking is mostly that previously extracted from the pores and resident in irregular magma bodies or dyke networks the observation that large volumes of buoyant magma are commonly present in migmatite zones and that dyking in these zones plays a secondary role suggest that dyking is inefficient and is able to extract only a fraction of the total melt available in the source in support of this conclusion recent detailed studies of exhumed hot crustal zones have revealed a range of alternative migration mechanisms characterized by mesoscale pervasive magma flow outcrop scale as opposed to porous flow pervasive migration gives rise to magma sheets preferentially emplaced parallel to high permeability zones such as foliation or bedding planes apart from local dyking three alternative mechanisms have been proposed to account for pervasive migration of magmas namely tectonic pumping magma wedging into low viscosity rocks and volatile driven intrusion because of their unfocused pervasive character these mechanisms are restricted to hot crust where magmas are not exposed to rapid freezing a model is proposed whereby heat advected with the magma pushes crustal geotherms upwards allowing pervasive magma migration to shallower depths

geochemistry of granitic melts produced during the incongruent melting of muscovite implications for the extraction of himalayan leucogranite magmas granites formed from the incongruent melting of muscovite preserve evidence of chemical disequilibrium both between restite and melt and within individual phases during crystallization miocene leucogranites from the himalayas exemplify melt formation from a pelitic source through muscovite breakdown trace elements rb sr and ba reside almost entirely in micas and feldspars in the protolith and therefore are appropriate for quantitative modeling equilibrium between melt and restite was established for sr under fluid phase absent conditions but some disequilibrium is suggested by rb abundances consistent with known diffusion rates for these elements abundances of zr and ce in the granites which provide essential structural constituents of the accessory phases zircon and monazite indicate that the himalayan granite melts were saturated in these elements if formed at temperatures of 670 700 c but slightly undersaturated if formed at somewhat higher temperatures ion microprobe traverses of y and heavy rare earth element abundances across single garnets from himalayan leucogranites provide evidence for disequilibrium between melt and phenocryst during garnet growth thus constraining the evolution of magmatic compositions during melt formation and crystallization in some cases progressive melting of the protolith and crystallization of garnet from the melt appear to have been concomitant indicating that minimum solidus temperatures were maintained throughout the melting event this suggests that thermal buffering rather than melt extraction provided the rate determining step in magma evolution

dynamics of sill and laccolith emplacement in the brittle crust role of host rock strength and deformation mode igneous intrusions in sedimentary basins exhibit a great diversity of shapes from thin sheets e g sills cone sheets to massive intrusions e g laccoliths plugs presently none of the established models of magma emplacement have the capability to simulate this diversity because they account for either purely elastic or purely plastic or purely viscous host rocks whereas natural rocks are complex elasto plastic materials in this study we investigate the effects of elasto plastic properties of host rock on magma emplacement using laboratory experiments made of dry granular materials of variable cohesion our results show how the deformation mechanism of the host rock controls the emplacement of magma thin sheet sills form in high cohesion materials which dominantly deform by elastic bending whereas massive intrusions such as punched laccoliths form in low cohesion materials which dominantly deform by shear failure our models also suggest that combined elastic shear failure deformation modes likely control the emplacement of cone sheets our experiments are the first to spontaneously produce diverse geologically relevant intrusion shapes our models show that accounting for the elasto plastic behaviour of the host rock is essential to filling the gap between the established elastic and plastic models of magma emplacement and so to reveal the dynamics of magma emplacement in the earth s brittle crust

effects of prolonged southward interplanetary magnetic field on low latitude ionospheric electron density the present work describes the low latitude ionospheric variability during an unusually prolonged 33 h geomagnetically disturbed condition that prevailed during 15 16 july 2012 the low latitude electron density in summer hemisphere investigated using ground and satellite based observations responded to this by generating strong negative ionospheric storm on 16 july the maximum electron density on 16 july over indian low latitudes was reduced by more than 50 compared to that on a geomagnetically quiet day 14 july 2012 in contrast to the extreme reduction in total electron content tec in the northern hemisphere tec from a winter hemispheric station revealed substantial 23 total electron content unit 1 tecu 1016 el m 2 enhancements on the same day this contrasting hemispherical response in tec is suggested to be due to the combined effects of strong interhemispheric and solar driven day night winds further very weak equatorial electrojet eej strength on 16 july indicated that the westward electric field perturbations in the low latitude ionosphere were possibly due to the disturbance dynamo effect associated with meridional circulation from polar to equatorial latitudes interestingly despite reduction in the integrated eej strength on 15 july the low latitude electron density showed substantial enhancement highlighting the significant effect of the positive ionospheric storm on the low latitude ionosphere the roles of electrodynamical neutral dynamical and compositional disturbances are discussed in view of these observations to understand low latitude ionospheric response when geomagnetic disturbance persists for longer duration

on the thermal viability of dikes leaving magma chambers dikes beginning to propagate away from a magma source are thin and grow slowly and thus are susceptible to freezing a self similar solution is obtained for a dike propagating down a temperature gradient when the wallrock and magma temperatures are equal at the chamber wall the solution applies only to the special case of a single component magma intruded at the solidus temperature but provides insight into the case in which the magma temperature is above the solidus the dike propagation velocity relative to that in the absence of freezing is determined by a single dimensionless parameter for sufficiently large magma freezing chokes the dike faster than propagation can elastically widen it and the dike cannot grow if the magma intrudes at a temperature above the solidus there is some value of above which the dike will propagate a certain distance and freeze and below which it will propagate until depletion of the source or three dimensional elastic effects promote freezing for reasonable parameters this process can explain why basaltic dikes are found far from obvious source regions while granitic dikes are scarce except in the immediate vicinity of granitic plutons even if a rhyolite is 5 orders of magnitude more viscous than a basalt however this distinction can be offset if the rhyolite has an excess source pressure only 1 order of magnitude greater than that of the basalt

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

formation of lenticulae on europa by saucer shaped sills abstract europa s surface contains numerous quasi elliptical features called pits domes spots and small chaos we propose that these features collectively referred to as lenticulae are the surface expression of saucer shaped sills of liquid water in europa s ice shell in particular the inclined sheets of water that surround a horizontal inner sill limit the lateral extent of intrusion setting the lateral dimension of lenticulae furthermore the inclined sheets disrupt the ice above the intrusion allowing the inner sill to thicken to produce the observed relief of lenticulae and to fracture the crust to form small chaos scaling relationships between sill depth and lateral extent imply that the hypothesized intrusions are or were 1 5 km below the surface liquid water is predicted to exist presently under pits and for a finite time under chaos and domes

effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread f we use radar observations from the jicamarca observatory from 1968 to 1992 to study the effects of the f region vertical plasma drift velocity on the generation and evolution of equatorial spread f the dependence of these irregularities on season solar cycle and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts in the early night sector the bottomside of the f layer is almost always unstable the evolution of the unstable layer is controlled by the history of the vertical drift velocity when the drift velocities are large enough the necessary seeding mechanisms for the generation of strong spread f always appear to be present the threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux the geomagnetic control on the generation of spread f is season solar cycle and longitude dependent these effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields the occurrence of early night spread f decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset the generation of late night spread f requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour these irregularities occur most often at 0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes the occurrence of late night spread f is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux

eddy induced particle dispersion in the near surface north atlantic abstractthis study investigates the anisotropic properties of the eddy induced material transport in the near surface north atlantic from two independent datasets one simulated from the sea surface height altimetry and one derived from real ocean surface drifters and systematically examines the interactions between the mean and eddy induced material transport in the region the lagrangian particle dispersion which is widely used to characterize the eddy induced tracer fluxes is quantified by constructing the spreading ellipses the analysis consistently demonstrates that this dispersion is spatially inhomogeneous and strongly anisotropic the spreading is larger and more anisotropic in the subtropical than in the subpolar gyre and the largest ellipses occur in the gulf stream vicinity even at times longer than half a year the spreading exhibits significant nondiffusive behavior in some parts of the domain the eddies in this study are defined as deviations from the long term time mean the contr

repressurization following eruption from a magma chamber with a viscoelastic aureole i analyze an approximate solution for spherical magma chambers radius r1 surrounded by maxwell viscoelastic shells radius r2 in elastic half spaces following a sudden decrease in pressure mass flux into the chamber is proportional to the pressure difference between a deep source and chamber p p t solutions depend on r2 r1 c m c where m c are magma and chamber compressibilities the relaxation time tr and the recharge time for the elastic system without recharge the system exhibits either post eruptive deflation or for incompressible magmas partial reinflation more generally immediate post eruptive inflation is favored by large tr and small r2 r1 with short tr and large the time for magma chamber pressure to recover increases significantly post eruptive creep increases the chamber pressure decreasing the pressure gradient driving recharge delaying pressure recovery following eruption the hoop stresses which are initially compressive relax toward the radial stress causing them to go through a local relative tension maximum the magma pressure in excess of the dike normal compression at the chamber wall can recover to pre eruptive values well before the chamber pressure or erupted mass recovers this suggests that dikes could nucleate at the chamber margin well before sufficient pressure has recovered to drive them far from the chamber for some parameters co eruptive deflation has not fully recovered when the magma overpressure relative to hoop stress in the elastic region is restored to pre eruptive values a condition assumed to be sufficient for eruption

domes pits and small chaos on europa produced by water sills pits domes and small chaos on europa s surface are quasi circular features a few to a few tens of kilometers in diameter we examine if injection of water sills into europa s ice shell and their subsequent evolution can induce successive surface deformations similar to the morphologies of these features we study the dynamics of water spreading within the elastic part of the ice shell and show that the mechanical properties of ice exert a strong control on the lateral extent of the sill at shallow depths water makes room for itself by lifting the overlying ice layer and water weight promotes lateral spreading of the sill in contrast a deep sill bends the underlying elastic layer and its weight does not affect its spreading in that case the sill lateral extent is limited by the fracture toughness of ice and the sill can thicken substantially after emplacement cooling of the sill warms the surrounding ice and thins the overlying elastic ice layer as a result preexisting stresses in the elastic part of the ice shell increase locally to the point that they may disrupt the ice above the sill small chaos disruption of the surface also allows for partial isostatic compensation of water weight leading to a topographic depression at the surface pit of the order of 10 2 m complete water solidification finally causes expansion of the initial sill volume and results in an uplifted topography dome of 10 2 m

dyke nucleation and early growth from pressurized magma chambers insights from analogue models abstract nucleation and dyke growth from magma chambers are important yet poorly understood aspects of magma transport in the lithosphere to tackle this issue we conducted qualitative experiments using gelatin and silicone to simulate respectively country rock and magma in our experiments the pressurization of the magma filled reservoir was the main cause of dyke formation rather than the density difference between magma and country rock our results show that an overpressurized magma chamber leads to almost simultaneous nucleation of dykes in different parts of the reservoir especially at places with large angular irregularities where local perturbation of elastic stress occurs early propagation of the leading edge of a dyke results in arc shape tabular intrusions that are controlled by the stresses surrounding the overpressurized magma chamber the overall process of early dyke formation reveals three stages 1 very rapid increase of the fracture length at constant width 2 slow down and probably arrest of the intrusion front and 3 thickening of the intrusion with an almost stationary intrusion front these stages suggest that dyke propagation to the surface occurs through several steps separated by the time needed for the renewal of magma pressure in the reservoir in consequence our experiments suggest that overpressurization of a magma chamber is a necessary condition that must be met to explain why some magmas can reach the surface even when the reservoir is located in a region close to the neutral buoyancy level but that the distribution of such overpressure is a dynamic variable that requires further examination

magnetospheric disturbance induced equatorial plasma bubble development and dynamics a case study in brazilian sector 1 equatorial ionospheric plasma bubble irregularity development and dynamics during the major magnetospheric storm of 26 august 1998 are investigated using the data collected by a multistation and multi instrument diagnostic network operated at equatorial and low latitude sites in brazil and auroral electrojet activity au al imf and dst indices a magnetospheric disturbance onset in the morning of 26 august 1998 was initiated by a solar wind shock and associated imf bz polarity reversals and ssc that were soon followed by a succession of substorm like auroral electrojet ae intensifications and dst development an imf bz southward turning and associated ae intensifications in the brazilian dusk sector produced intense prompt penetration eastward electric field that caused large f region vertical drift and consequently the developments of intense postsunset equatorial anomaly and a series of intense plasma bubbles the latter event lasting the entire night as observed by digital ionosondes at sao luis 2 33 s 315 8 e dip angle 5 and fortaleza 3 9 s 321 55 w dip angle 9 and an all sky imager two scanning photometers and a digisonde at the low latitude site cachoeira paulista 22 6 s 315 e dip angle 28 a notable aspect of the dynamics of the bubbles was their initially very low eastward drift velocity which turned into steadily increasing westward velocity that lasted till early morning hours the results show for the first time a relationship between the zonal drift velocities of optically observed large scale bubbles tens to hundreds of kilometers and that of the smaller scale kilometer sizes structures as observed by a digital ionosonde the results point to the dominant role of a disturbance dynamo associated westward thermospheric wind to maintain the plasma irregularity drift increasingly westward going into postmidnight hours as an important finding the results further show that significant contribution to the westward plasma bubble irregularity drift normally attributed to disturbance dynamo effect could arise from prompt penetration disturbance zonal electric field in the course of a disturbance sequence lasting several hours such effect is attributed to hall electric field arising from the primary disturbance zonal electric field under enhanced nighttime ionospheric conductivities produced possibly by storm associated particle precipitation in the brazilian longitude sector in agreement with recent evidences abdu et al 1998b

effect of geomagnetic activity on equatorial radio vhf scintillations and spread f the paper discusses the occurrence of scintillations of ats 3 137 mhz beacons recorded at huancayo on geomagnetically quiet and disturbed days during the years 1969 1976 and compares the results with the corresponding occurrence of range and frequency spread f at huancayo during the equinoctial months and the december solsticial months the geomagnetic activity reduces the equatorial scintillations during premidnight hours but increases their occurrence during the postmidnight hours these features are very similar to the effect of geomagnetic activity on the occurrence of the range type of equatorial spread f rather than on the occurrence of frequency spread which decreases for any hour of the night during geomagnetic active periods during the june solsticial months the occurrence of both scintillations and spread f is very much reduced however both the phenomena are more frequent on disturbed than on quiet days for any of the hours of the night these effects are consistently the same for any of the years within the solar cycle it is suggested that the equatorial radio scintillations at 137 mhz during the nighttime are produced primarily by the occurrence of the range type of spread f the geomagnetic effects are due to the modifications of the equatorial electric field by the geomagnetic disturbance and thereby affect the development of f region irregularities causing scintillations