radial variations of melt viscosity around growing bubbles and gas overpressure in vesiculating magmas abstract the viscosity of silicic melts depends strongly on their water content as bubbles grow in a supersaturated melt water evaporates from the bubble melt interface a diffusive profile develops and leads to steep viscosity gradients across the melt shell here we investigate the effects of radial viscosity profiles on the dynamics of bubble growth we find that the effective melt viscosity resisting gas overpressure in the bubbles is close to the viscosity at the dehydrated rind and may be higher than that of the surrounding melt by more than an order of magnitude as a result bubbles may retain pressures that are higher than ambient pressure for longer times magma degassing is delayed to shallower depth and fragmentation of magma due to gas overpressure may occur over a wider range of conditions measured water content in eruption products yields information on the average melt viscosity however additional information about the concentration profiles is needed for estimating the effective viscosity that controlled the evolution of bubble growth in the ejecta

longitudinal development of low latitude ionospheric irregularities during the geomagnetic storms of july 2004 1 during the period 22 28 july 2004 three geomagnetic storms occurred due to a sequence of coronal mass ejections in this paper we present and discuss the ionospheric observations from a set of in situ satellites and ground based gps total electron content and scintillation receivers a vhf radar and two chains of ionosondes 300 e and 120 e respectively that provide the evolutionary characteristics of equatorial and low latitude ionospheric irregularities versus longitude during these storm periods it is found that the irregularities occurred over a wide longitudinal range extending from around 300 e to 120 e on storm days 25 and 27 july 2004 on 25 july plasma bubbles pbs began premidnight in america and postmidnight in southeast asia on 27 july the occurrence of irregularities followed the sunset terminator and was observed sequentially after sunset from american to southeast asian longitudes past studies have reported that storm time low latitude ionospheric irregularities are mostly confined to a narrower longitude range 90 after sunset hours and are associated with the prompt penetration of eastward electric fields ppefs into low latitudes in june solstice months the occurrence of range type spread f or pbs is very low in southeast asian and south american sectors in contrast the present results indicate that geomagnetic storms triggered the wide longitudinal development of pbs in the american sector this was probably due to the effects of ppefs on both storm days however in the southeast asian sector the pbs on the 2 days probably arose from disturbance dynamo electric field ddef ppef and gravity wave seeding effects this study further shows that under complex storm conditions besides the long duration or multiple penetrations the combined effects of ppefs and ddefs could result in a wide longitude extent of ionospheric irregularities at times

a scaling growth model for bubbles in basaltic lava flows abstract pahoehoe an and massive lavas from mount etna show common statistical properties from one sample to another which are independent of scale size over certain ranges the gas vesicle distribution shows two scale invariant regimes with number density n v v b 1 where v is the volume and empirically b 0 for small bubbles and b 1 for medium to large bubbles we introduce a bubble growth model which explains the b 1 range by a strongly non linear cascading growth regime dominated by a quasi steady state coalescence process the small bubble region is dominated by diffusion its role is to supply small bubbles to the coalescence regime the presence of measured dissolved gas in the matrix glass is consistent with the notion that bubbles generally grow in quasi steady state conditions the basic model assumptions are quite robust with respect to the action of a wide variety of processes since we only require that the dynamics are scaled over the relevant range of scales and that during the coalescence process bubble volumes are approximately conserved the model also predicts a decaying coalescence regime with b 1 associated with a depletion of the gas source or alternatively a loss of large vesicles through the surface of the flow our model thus explains the empirical evidence pointing to the coexistence of two different growth mechanisms in subsurface lava flows but acting over distinct ranges of scale with non linear coalescence as the primary growth process the total vesicularity of each sample can then be well estimated from the partial vesicularity of each growth regime without any outlier problems

coupled textural and compositional characterization of basaltic scoria insights into the transition from strombolian to fire fountain activity at mount etna italy strombolian and fire fountain activities represent a common expression of explosive basaltic eruptions however the transition between these two eruptive styles and their source mechanisms are still debated we use textural and compositional studies to characterize pyroclastic material from both the strombolian and hawaiian style fire fountain phases of the january june 2000 etna activity we find that basaltic scoria presents distinctive textural and compositional features that reflect different modes of magma vesiculation and crystallization in the two eruptive regimes overall magma that forms strombolian scoria is far more crystallized less vesicular and more evolved indicating strong volatile depletion and longer residence time before being erupted fire fountain scoria indicates a fast rising magma with evidence of moderate syneruptive volatile exsolution the new textural and compositional data set is integrated with previous volcanological and geophysical investigations to provide further insights into the dynamics of fire fountains and to frame the transition from strombolian explosions to fire fountain activity into a model that may apply to future eruptions at mount etna as well as other active basaltic volcanoes

textural heterogeneities in pumices from the climactic eruption of mount pinatubo 15 june 1991 and implications for magma ascent dynamics the climactic event of mount pinatubo represents one of the most thoroughly studied eruptions of the century and has provided important insights into the dynamics of explosive volcanism we have performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions white pumice has higher vesicularity deformed and highly coalesced vesicles with thin walls euhedral phenocrysts and microlite free groundmass gray pumice shows lower vesicularity wider ranges in vesicle number density limited coalescence vesicles with thick walls that are less deformed phenocrysts and microphenocrysts with abundant solution pitting and groundmass containing ubiquitous microlites and crystal fragments the presence of white and gray pumice varieties and the broad range in vesicularity and vesicle number density that characterizes both of them appear to record the complexities of conduit processes such as magma vesiculation and fragmentation and the development of conduit regions marked by different rheological behaviors in particular the results of this study suggest the likely importance of intense shear and viscous dissipation at the conduit walls a mechanism that may be responsible for the creation and discharge of the gray pumice of this eruption along with the dominant white variety

the multi instrumented studies of equatorial thermosphere aeronomy scintillation system climatology of zonal drifts a spaced antenna scintillation system was installed at ancon peru in may 1994 to measure scintillation of 250 mhz signals from a geostationary satellite by three antennas spaced in the magnetic east west direction these measurements were used to establish the climatology of the zonal drift of the irregularities which cause equatorial scintillations the major objective of this study is to compare this drift climatology to the climatology of zonal neutral wind which is the driver of the equatorial electrodynamics a comparison of these two climatologies in conjunction with scintillation statistics may provide some clues regarding factors which help or hinder the formation of equatorial spread f esf with these objectives in mind the first year s drift and scintillation statistics have been presented as a function of local time season and magnetic activity and compared with the statistics of ion drift published earlier from incoherent scatter radar observations the scintillation drift is in good agreement with the jicamarca radar observations except for the fact that the local time dependence of our drift observations exhibit a broader maximum the broad maximum may be attributed to lower ion drag experienced in the presence of esf due to sustained uplifting of the ionosphere during magnetically active periods the scintillation drift often exhibits east to west reversals presumably because of the disturbance dynamo effects the westward drifts during such reversals may be as large as 100 m s we have also modeled the zonal drifts as a seasonal basis by using hedin s neutral wind model and anderson s fully analytical ionospheric model the modeled zonal drifts present good quantitative agreement with the drifts obtained with the scintillation technique

vesiculation in magmas from stromboli and implications for normal strombolian activity and paroxysmal explosions in basaltic systems 1 we performed a series of x ray tomographic experiments and lattice boltzmann permeability simulations on pyroclastic products from explosive activity at stromboli between december 2004 and may 2006 we reconstructed the 3 d textures of vesicles to investigate the relationship between the nature of vesiculation in the erupted products and the dynamics of gas transport in the shallow conduit in order to derive implications for the eruptive behavior of basaltic volcanoes scoriae from normal strombolian explosions display remarkably consistent vesicle volume distributions fit by power laws with an exponent of 1 0 2 we ascribe the origin of such distributions to the combined effect of coalescence and continuous nucleation events in the steady state shallow magma system that supplies normal strombolian activity volume distributions and textures of vesicles in pumice clasts from the 5 april 2003 and 15 march 2007 paroxysmal activity are markedly different from those in the scoriae besides a power law function with a higher exponent portions of these distributions can be also fit by an exponential function suggesting the attempt of the system to reach near equilibrium conditions the investigated pumice clasts also lack the large connecting vesicles responsible for the development of degassing pathways in the stromboli magma that erupts the scoriae this testifies to a decreased degassing efficiency of the magma associated with paroxysmal explosions and potential overpressure buildup at depth by comparison with degassing experiments on basaltic melts we derive a time constraint on the order of minutes to hours for the incubation of paroxysms at stromboli

evolution of melt vapor surface tension in silicic volcanic systems experiments with hydrous melts 1 we evaluate the melt vapor surface tension of natural water saturated dacite melt at 200 mpa 950 1055 c and 4 8 5 7 wt h2o we experimentally determine the critical supersaturation pressure for bubble nucleation as a function of dissolved water and then solve for at those conditions using classical nucleation theory the solutions obtained give dacite melt vapor surface tensions that vary inversely with dissolved water from 0 042 0 003 j m 2 at 5 7 wt h2o to 0 060 0 007 j m 2 at 5 2 wt h2o to 0 073 0 003 j m 2 at 4 8 wt h2o combining our dacite results with data from published hydrous haplogranite and high silica rhyolite experiments reveals that melt vapor surface tension also varies inversely with the concentration of mafic melt components e g cao feototal mgo we develop a thermodynamic context for these observations in which melt vapor surface tension is represented by a balance of work terms controlled by melt structure overall our results suggest that cooling crystallization and vapor exsolution cause systematic changes in that should be considered in dynamic modeling of magmatic processes

texture and composition of pumices and scoriae from the campi flegrei caldera italy implications on the dynamics of explosive eruptions 1 the campi flegrei cf caldera is one of the most dangerous quiescent volcanic systems in the world its activity mostly resulted in low magnitude explosive eruptions such as that of the monte nuovo tuff cone that represents the last eruptive event within the caldera a d 1538 however there have been more energetic plinian events e g the agnano monte spina eruption 4 1 ka and very highly explosive caldera forming eruptions e g the campanian ignimbrite eruption 39 ka here we integrate new and literature data on the groundmass texture and composition of pyroclastic products from the three above eruptions with the aim of unraveling how volatiles content degassing mechanisms and crystallization processes influence magma explosivity and eruption dynamics at cf previous studies indicate that the investigated rocks share similar major element bulk and phenocryst chemistry also similar is the water content of their trapped melt inclusions these observations suggest that the magmas feeding these eruptions had comparable physicochemical properties during storage in the shallow crust however our investigations indicate that the studied rocks differ in texture and composition of the groundmass and viscosity of the related magmas we ascribe such differences to the variable style of volatile exsolution and outgassing from the melt primarily in response to changes of the rate of magma ascent to the surface we conclude that the magma ascent rate was the key parameter in driving explosive eruptions at cf and we suggest that this parameter may be influenced by magma water interaction and or magma chamber geometry and replenishment

new evidence for degassing processes during explosive eruptions measurements of pore size distribution in five samples of pumice formed during explosive eruptions of plinian type show three distinct sizes of vesicle the characteristics of the three pore size populations are similar in the samples although they were collected from plinian deposits of five different volcanoes we interpret the pore size populations as representing the degassing of silicic magma on three very different time scales the coarse population 60 m represents slow vesiculation within the magma chamber over periods of weeks months or years at small levels of gas supersaturation the medium sized population 50 5 m represents rapid vesiculation of magma on ascent to the surface during the plinian eruption over periods of minutes to hours the fine population 5 0 5 m represents bubbles formed during explosive disruption of magma and rapid ejection into the atmosphere involving short time scales a few seconds but large decompression rates

the evolution of bubble size distributions in volcanic eruptions we review observations of bubble size distributions bsds generated during explosive volcanic eruptions and laboratory explosions as inferred from vesicle size distributions found in the end products unimodal polymodal exponential and power law bsds are common even in the absence of coalescence and both power law and exponential distributions have been generated in the same eruption to date theoretical models have proposed incompatible mechanisms for producing the various distributions we here present a unifying mechanism data from our laboratory analogue experiments suggest that power law distributions are associated with highly non equilibrium degassing a numerical model is developed in which bubbles nucleate repeatedly and grow in the spaces between those of previous generations where in a non equilibrium degassing scenario the volatile concentration remains high this process causes the bsd to evolve from unimodal through exponential into a power law the exponent of the power law is a measure of the number of nucleation events or the duration of the nucleation period compared with the timescale of bubble growth the mathematical inevitability of the evolution from unimodal poissonian to power law is discussed the findings may resolve the apparent contradiction between the equilibrium degassing conduit flow models and the non equilibrium degassing conditions derived from bubble growth models of explosive volcanic eruptions the process of ongoing nucleation is the mechanism whereby the volcanic system maintains near equilibrium in the case of rapid depressurisation and slow volatile diffusion

inferring volcanic degassing processes from vesicle size distributions both power law and exponential vesicle size distributions vsds have been observed in many different types of volcanic rocks we present results of computer simulations and laboratory analogue experiments which reproduce these findings and show that the distributions can be interpreted as the product of continuous bubble nucleation resulting from non equilibrium degassing this ongoing nucleation causes the bubbles to evolve through an exponential size distribution into a power law size distribution as nucleation and growth progress these findings may explain the apparent contradiction between present models of bubble growth in magmas which predict that degassing in explosive eruptions is a non equilibrium process and models of conduit flow which assume perfect equilibrium degassing the process of continuous nucleation is the mechanism whereby the volcanic system maintains near equilibrium in the case of rapid depressurization and slow volatile diffusion

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

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

development of intermediate scale structure at different altitudes within an equatorial plasma bubble implications for l band scintillations an important aspect of the development of intermediate scale length 100 m few km irregularities in an equatorial plasma bubble epb that has not been considered in the schemes to predict the occurrence pattern of l band scintillations in low latitude regions is how these structures develop at different heights within an epb as it rises in the post sunset equatorial ionosphere due to the growth of the rayleigh taylor r t instability irregularities at different heights over the dip equator map to different latitudes and their spectrum as well as the background electron density determine the strength of l band scintillations at different latitudes in this paper vhf and l band scintillations recorded at different latitudes together with theoretical modelling of the scintillations are used to study the implications of this structuring of epbs on the occurrence and strength of l band scintillations at different latitudes theoretical modelling shows that while s4 index for scintillations on a vhf signal recorded at an equatorial station may be 1 s4 index for scintillations on a vhf signal recorded near the crest of the equatorial ionization anomaly eia generally does not exceed the value of 1 because the intermediate scale irregularity spectrum at f layer peak near the eia crest is shallower than that found in the equatorial f layer peak this also explains the latitudinal distribution of l band scintillations thus it is concluded that there is greater structuring of an epb on the top side of the equatorial f region than near the equatorial f layer peak

h2o diffusion models in rhyolitic melt with new high pressure data abstract water diffusion in silicate melts is important for understanding bubble growth in magma magma degassing and eruption dynamics of volcanos previous studies have made significant progress on water diffusion in silicate melts especially rhyolitic melt however the pressure dependence of h2o diffusion is not constrained satisfactorily we investigated h2o diffusion in rhyolitic melt at 0 95 1 9 gpa and 407 1629 c and 0 2 5 2 wt total water h2ot content with the diffusion couple method in a piston cylinder apparatus compared to previous data at 0 1 500 mpa h2o diffusivity is smaller at higher pressures indicating a negative pressure effect this pressure effect is more pronounced at low temperatures assuming h2o diffusion in rhyolitic melt is controlled by the mobility of molecular h2o h2om the diffusivity of h2om dh2om at h2ot 7 7 wt 403 1629 c and 1 9 gpa is given by d h 2 o m d 0 e x p a x with d 0 e x p 13 375 1 8875 p 12939 3625 6 p t and a 37 256 75884 t where d0 is in m2 s x is mole fraction of h2ot on a single oxygen basis t is temperature in k and p is pressure in gpa h2ot diffusivities dh2ot in m2 s can be calculated from h2om diffusivity or directly from the following expression l n d h 2 o t x 13 47 49 996 x 7 0827 x 1 8875 p 9532 3 91933 x 13403 x 3625 6 p t at low h2ot content up to 2 wt if an error of a factor of 2 is allowed h2ot diffusivity is approximately proportional to h2ot content d h 2 o t c c 0 e x p 9 5279 1 8875 p 9698 5 3625 6 p t where c is h2ot content in wt and c0 is 1 wt the new expressions for h2o diffusion not only reproduce our own data but also match data in literature from different laboratories and using different methods indicating good inter laboratory and multi method consistency the new expressions cover a wide range of geological conditions and can be applied to h2o diffusion in rhyolitic melts in various volcanic and magmatic processes

delayed disequilibrium degassing in rhyolite magma decompression experiments and implications for explosive volcanism recent numerical models and analog shock tube experiments show that disequilibrium degassing during magma ascent may lead to violent vesiculation very near the surface in this study a series of decompression experiments using crystal free rhyolite melt were conducted to examine the development of large supersaturations due to delayed homogenous spontaneous bubble nucleation melts were saturated at 900 c and 200 mpa with either 5 2 wt dissolved h2o or with 4 2 wt h2o and 640 ppm co2 and isothermally decompressed at linear rates of either 0 003 0 025 or 8 5 mpa s to final pressures between 25 and 175 mpa additional isobaric saturation experiments 900 c 200 25 mpa using pure h2o or mixed h2o co2 fluids establish reference equilibrium solubility curves values homogenous nucleation is triggered in both h2o only and h2o co2 experiments once the supersaturation pressure pss reaches 120 150 mpa and the melt contains two times its equilibrium water contents bubble number density and nucleation rate depend on the supersaturation pressure with values on the order of 102 cm3 and 1 cm3 s for pss 120 mpa 106 cm3 and 103 105 cm3 s for pss 130 150 mpa and 107 cm3 and 106 cm3 s for pss 160 175 mpa nucleation rates are consistent with classical nucleation theory and infer an activation energy for nucleation of 1 5 10 18 j nucleus a critical bubble radius of 2 10 9 m and an effective surface tension for rhyolite at 5 2 wt h2o and 900 c of 0 10 0 11 n m the long nucleation delay limits the time available for subsequent diffusion such that disequilibrium dissolved h2o and co2 contents persist to the end of our runs the disequilibrium degassing paths inferred from our experiments contrast markedly with the equilibrium or quasi equilibrium paths found in other studies where bubble nucleation occurs heterogenously on crystals or other discontinuities in the melt at low pss homogenous and heterogenous nucleation rates are comparable however as are bubble number densities so that at a given decompression rate it appears that nucleation mechanism rather than nucleation rate determines degassing efficiency by fixing the pressure depth at which vesiculation commences and hence the time available for equilibration prior to eruption although real systems are probably never truly crystal free our results show that rhyolitic magmas containing up to 104 crystals cm3 and perhaps as high as 106 crystals cm3 are controlled by homogenous rather than heterogenous nucleation during ascent

global thermospheric neutral density and wind response to the severe 2003 geomagnetic storms from champ accelerometer data 1 measurements of atmospheric density near 410 km from the star accelerometer on the champ satellite are used to illustrate the spatial temporal dependence of the thermospheric response to the severe solar storms occurring during 29 october to 1 november 2003 this interval includes periods of elevated magnetic activity with kp values of 5 9 as well as undisturbed intervals that serve to define quiet time baseline densities measurements are available from 87 to 87 latitude during both day and night at local times near 1300 and 0100 hours respectively during times of maximum geomagnetic activity for this study density measurements exhibit enhancements of 200 300 northern hemisphere daytime responses are much larger than in the southern hemisphere the origins of this effect are unknown nighttime density disturbances more readily propagate to equatorial latitudes possibly facilitated by the predominant equatorward flow in both hemispheres due to the diurnal tides driven by in situ euv heating the champ density measurements are compared with density predictions from the nrl msise00 empirical density model and demonstrate some model shortcomings measurements of cross track accelerations provide the opportunity to estimate zonal winds from the equator to about 60 latitude transitioning to a measure of purely meridional winds at the turning point of the orbit near 87 latitude a periodic variation in cross track winds with an apparent period of 24 hours appears at high latitudes and exhibits similar amplitudes and temporal latitudinal structures to the empirical hwm 93 wind model when projected into the cross track direction this periodicity is due to the displacement of geomagnetic and geographic coordinates at low latitudes champ and hwm 93 both yield westward winds of order 100 ms 1 during midday under quiet magnetic conditions however during severely disturbed periods the hwm 93 winds generally show a greater westward intensification to 250 ms 1 than the champ measurements at night champ winds are near zero under quiet conditions whereas hwm 93 indicates eastward winds of order 50 100 ms 1 under disturbed conditions the champ winds shift to westward values of order 200 to 250 ms 1 while hmw 93 values do not exceed about 50 ms 1 in the westward direction the physical origins of the observed effects are difficult to isolate and unequivocal interpretation will require sophisticated numerical modeling taking into account self consistent interactions between the neutral winds drifts and ionization densities

bnd bubble number density decompression rate meter for explosive volcanic eruptions abstract this decade has been witness to substantial progress in the understanding of bubble dynamics and its role in volcanic eruptions a comparison between the results of experiments and a model for bubble nucleation shows that bubble number density bnd can be calculated as a function of decompression rate for given physical properties such as the diffusivity of water in a silicate melt and interfacial tension within an acceptable margin of error conversely in this paper we propose a method hereafter referred to as bnd decompression rate meter to estimate the decompression rate by using bnd data of natural pumice samples from volcanic eruptions as a result of the application of the bnd decompression rate meter to pumice and scoria from explosive eruptions it is found that the decompression rates increase in the range from 10 6 to 10 8 pa s with the eruption column heights these rates are proportional to the 4th power of the eruption column heights and are linear to the discharge rates the absolute values of the estimated decompression rates are very high and cannot be explained by the existing models of conduit flows in order to explain such high rates of decompression and the correlation with discharge rate we propose a possible model according to which the bubble nucleation propagates downward as a rarefaction shock wave in the conduit

deformation mechanisms of antigorite serpentinite at subduction zone conditions determined from experimentally and naturally deformed rocks abstract we performed deformation dia experiments on antigorite serpentinite at pressures of 1 3 5 gpa and temperatures of between 400 and 650 c bracketing the stability of antigorite under subduction zone conditions for each set of pressure temperature p t conditions we conducted two runs at strain rates of 5 10 5 and 1 10 4 s 1 we complemented our study with a sample deformed in a griggs type apparatus at 1 gpa and 400 c chernak and hirth 2010 and with natural samples from cuba and the alps deformed under blueschist eclogitic conditions optical and transmission electron microscopies were used for microstructural characterization and determination of deformation mechanisms our observations on experimentally deformed antigorite prior to breakdown show that deformation is dominated by cataclastic flow with observable but minor contribution of plastic deformation microkinking and 001 gliding mainly expressed by stacking disorder mainly in contrast in naturally deformed samples plastic deformation structures are dominant stacking disorder kinking pressure solution with minor but also perceptible contribution of brittle deformation when dehydration occurs in experiments plasticity increases and is coupled to local embrittlement that we attribute to antigorite dehydration in dehydrating samples collected in the alps embrittlement is also observed suggesting that dehydration may contribute to intermediate depth seismicity our results thus show that semibrittle deformation operates within and above the stability field of antigorite however the plastic deformation recorded by naturally deformed samples was likely acquired at low strain rates we also document that the corrugated structure of antigorite controls the strain accommodation mechanisms under subduction conditions with preferred inter and intra grain cracking along 001 and gliding along both a and b we also show that antigorite rheology in subduction zones is partly controlled by the presence of fluids which can percolate within the exhumation channel via deformation induced interconnected porosity

homogeneous bubble nucleation in rhyolitic magmas an experimental study of the effect of h2o and co2 1 rapid decompression experiments were performed to study homogeneous bubble nucleation in a crystal free rhyolitic liquid at 800 c bubble nucleation was produced by lowering the pressure at 1 10 mpa s 1 from an initial value between 200 and 295 mpa to a final value below the volatile saturation pressure psat six volatile compositions with 4 1 7 7 wt h2o and 10 1200 ppm co2 were investigated for each composition we determined the critical pressure phon below which homogeneous nucleation can proceed the samples quenched below phon showed a nucleated core with a large number of uniformly spaced bubbles with decreasing pressure bubble number densities increased from 1015 m 3 the degree of supersaturation required for homogeneous nucleation phon psat phon increased with decreasing h2o content or increasing co2 content huge values of phon 135 to 310 mpa were measured in the h2o poor compositions 4 1 4 6 wt h2o 50 1100 ppm co2 much lower values from 60 to 160 mpa were obtained in the h2o rich compositions 7 0 7 7 wt h2o 10 630 ppm co2 the high phon in liquids with 4 5 wt h2o should result in the buildup of large degrees of supersaturation during magma ascent a very late nucleation event and a rapid explosive vesiculation by contrast rhyolitic liquids with much larger water contents have higher saturation pressures and much lower phon bubble nucleation may therefore occur at depth in the volcanic conduit favoring a subsequent near equilibrium degassing

the plinian phase of the campanian ignimbrite eruption phlegrean fields italy evidence from density measurements and textural characterization of pumice textural characterization of pumice clasts from explosive volcanic eruptions provides constraints on magmatic processes through the quantification of crystal and vesicle content size shape vesicle wall thickness and the degree of interconnectivity the plinian fallout deposit directly underlying the campanian ignimbrite ci eruption represents a suitable case to investigate pumice products with different textural characteristics and to link the findings to processes accompanying conduit magma ascent to the crater the deposit consists of a lower lfu and upper ufu pumice lapilli bed generated by the sub steady eruption of trachytic magma with 0 90 with vesicle number density ranging from 107 108 cm 3 the degree of vesicle coalescence is high for all pumice types with interconnected vesicles generally representing more than 90 of the bulk vesicle population the results show a high degree of heterogeneous textures among pumice clasts from both phases of the eruption and within each eruption phase the different pumice types and also within each single pumice type fragment the origin of pumice clasts with different textural characteristics is ascribed to the development of conduit regions marked by different rheological behavior the conclusions of this study are that vesicle deformation degree of coalescence and intense shear at the conduit walls play a major role on the degassing process hence affecting the entire conduit dynamics

fragmentation of magma during plinian volcanic eruptions the ratio of the volume of vesicles gas to that of glass liquid in pumice clasts v g v l reflects the degassing and dynamic history experienced by a magma during an explosive eruption v g v l in pumices from a large number of plinian eruption deposits is shown here to vary by two orders of magnitude even between pumices at a given level in a deposit these variations in v g v l do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities despite large ranges in these variables gas volume ratios of pumices do however vary systematically with magma viscosity estimated at the point of fragmentation and we infer that pumices do not quench at the level of fragmentation but undergo some post fragmentary evolution on the timescale of plinian eruptions pumices with viscosities 109 pa s can expand after fragmentation as long as their bubbles retain gas at a rate inversely proportional to their viscosity once the bubbles connect to form a permeable network and lose their gas expansion halts and pumices with viscosities 105 pa s can collapse under the action of surface tension textural evidence from bubble sizes and shapes in pumices indicates that both expansion and collapse have taken place the magnitudes of expansion and collapse therefore depend critically on the timing of bubble connectivity relative to the final moment of quenching we propose that bubbles in different pumices become connected at different times throughout the time span between fragmentation and quenching after accounting for these effects we derive new information on the fragmentation process from two characteristics of pumices the most important is a relatively constant minimum value of v g v l of 1 78 64 vol vesicularity in all samples with viscosities 105 pa s this value is independent of magma composition and thus reflects a property of the eruptive mechanism the other characteristic is that highly expanded pumices 85 vol vesicularities are common which argues against overpressure in bubbles as a mechanism for fragmenting magma we suggest that magma fragments when it reaches a vesicularity of 64 vol but only if sheared sufficiently strongly the intensity of shear varies as a function of velocity in the conduit which is related to overpressure in the chamber so that changes in overpressure with time are important in controlling the common progression from explosive to effusive activity at volcanoes

vesiculation of basaltic magma during eruption vesicle size distributions in vent lavas from the pu9u9o9o kupaianaha eruption of kilauea volcano are used to estimate nucleation and growth rates of h 2 o rich gas bubbles in basaltic magma nearing the earth9s surface 120 m depth by using well constrained estimates for the depth of volatile exsolution and magma ascent rate nucleation rates of 35 9 events cm 3 s 1 and growth rates of 3 2 x 10 4 cm s are determined directly from size distribution data the results are consistent with diffusion controlled growth as predicted by a parabolic growth law this empirical approach is not subject to the limitations inherent in classical nucleation and growth theory and provides the first direct measurement of vesiculation kinetics in natural settings in addition perturbations in the measured size distributions are used to examine bubble escape accumulation and coalescence prior to the eruption of magma

a super bubble detected by dense gps network at east asian longitudes a post sunset bubble manifested by total electron content depletion was observed at midlatitudes similar to 30 degrees 34 degrees n similar to 130 degrees 134 degrees e during the main phase of a storm on 12 february 2000 with loss of lock and the rate of the total electron content index maps the bubble was seen to bifurcate at its early growth phase the upward drift speed was observed similar to 300 m s at similar to 2150 km and decreasing with increasing altitude and time the bubble had unusually large latitudinal extension reaching midlatitude of 36 5 degrees n 31 5 degrees n magnetic latitude indicating an apex height of similar to 2500 km in process of the evolution the bubble drifted eastward at a speed of similar to 50 m s the f region peak height and density obtained by a meridional ionosonde chain suggested a prompt penetrating magnetospheric electric field helped to trigger the super bubble

local time dependent response of postsunset esf during geomagnetic storms 1 development or inhibition of esf during magnetically active periods has been an important space weather topic of interest during the recent past in view of its applications in the satellite based navigational systems particularly the postsunset period exhibits significant variability for storm time development of esf versus longitude in this paper we report the results of a multi instrumental ground based and space borne and multistation study on the development inhibition of postsunset esf during five moderate to intense geomagnetic storms occurred during the low and descending phase of the solar activity period 2004 2006 it has been observed that the prompt penetration of eastward electric fields into low latitudes and subsequent development of esf occurred in all longitudinal sectors where the local time corresponds to postsunset hours during the entire main phase of the storm in this paper we show the development of plasma bubble irregularities over a wide longitudinal extent of 92 owing to the dusk time penetration of eastward electric fields into low latitudes either the sudden increase in ae index and or a marked decrease in sym h index may be used as proxies to determine the occurrence as well as the time of penetration of electric fields into equatorial and low latitudes however in such cases where the ae index does not represent any sudden increase the dsymh dt seems to be the better index to determine the time of penetration in this paper is also presented an interesting case where the prompt penetration eastward electric fields dominated the existing strong westward electric fields and subsequently caused the onset of spread f and scintillations at both vhf 244 mhz as well as l band 1 5 ghz frequencies

homogeneous bubble nucleation in rhyolitic melt experiments and nonclassical theory 1 the transfer of volatiles from the earth s interior to the atmosphere occurs through degassing of magma the dynamics of which assert a significant control on volcanic eruptions the first and most critical step in degassing is the nucleation of gas bubbles which requires that a sufficient number of volatile molecules cluster together to overcome the free energy associated with the formation of a new interface between nucleus and surrounding melt this free energy is a function of surface tension typically assumed to equate to the macroscopically measurable value surface tension estimates inferred from bubble nucleation experiments in silicate melts are however lower than direct macroscopic measurements making it difficult to accurately predict magma ascent and decompression rates from measured bubble number densities in pyroclasts we provide a potential resolution to this problem through an integrated study of bubble nucleation experiments and modeling thereof based on nonclassical nucleation theory we find that surface tension between critical bubble nuclei and the surrounding melt depends on the degree of supersaturation and is lower than the macroscopically measured value this is consistent with the view that far from equilibrium the interface between a nucleus and surrounding metastable bulk phase is diffuse instead of sharp as a consequence the increase in nucleation rate with supersaturation is significantly larger at high supersaturations than predicted by classical nucleation theory

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

the structure of basaltic scoria and reticulite and inferences for vesiculation foam formation and fragmentation in lava fountains in this investigation pyroclast structures are used to constrain degassing in basaltic lava fountains vesicle size shape number density interconnectedness and packing character are quantified and related to 1 the kinetics of bubble nucleation and growth 2 the structural evolution of magmatic foams and 3 the influence of vesiculation rate on magma fragmentation measurements made on a diverse suite of pyroclasts from kilauea volcano indicate that basaltic foams evolve through an initially disordered closed celled spherical state to a well ordered open celled polyhedral state as the vesicularity rises from 75 to 98 the structural changes occur rapidly 10 s in the conduit and fountain in response to an intense vesiculation burst vesicle size distribution systematics indicate bubble nucleation rates 2 104 events cm 3s 1 that are approximately three orders of magnitude greater than those found for effusive eruptive activity bubble growth rates 9 10 4 cm s exceed effusive estimates by a factor of 3 the observed runaway rate of bubble production indicates strong supersaturations at the onset of nucleation we speculate that the rise speed of the magma as it reaches the level where significant volatile exsolution begins determines the intensity of the vesiculation burst and hence the vigor of the eruption rapid expansion and acceleration of the magma under these conditions may provide the impetus for fragmentation

magma degassing during the plinian eruption of novarupta alaska 1912 1 we have modeled the nucleation and isothermal growth of bubbles in dacite from the 1912 plinian eruption of novarupta alaska bubble growth calculations account for the exsolution of h2o and co2 beginning with bubble nucleation and ending when bubble sizes reproduced the observed size distribution of vesicles in novarupta pumice clasts assuming classical nucleation theory bubbles nucleated with a diameter of the order of 10 8 m and grew to sizes ranging from 10 6 m to greater than 10 3 m the typical range of vesicle sizes found in novarupta pumice the smallest vesicles in novarupta pumices are also the most abundant and bubbles with radii of 10 6 m to 10 5 m comprise almost 90 of the entire bubble population we find that these bubbles must have nucleated and grown to their final size within a few 100 milliseconds despite these extremely fast growth rates the pressures of exsolved volatiles contained within the bubbles remained high up to about 107pa in excess of ambient pressure assuming a closed system the potential energy of these compressed volatiles was sufficient to cause magma fragmentation even though only a fraction of the pre eruptive volatiles had exsolved unless the matrix glasses of novarupta pyroclasts retains a large fraction of pre eruptive volatiles the majority of magmatic volatiles 80 90 was likely lost by open system degassing between magma fragmentation and quenching

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

measurement of bubble size distributions in vesiculated rocks with implications for quantitative estimation of eruption processes abstract this paper outlines methods for determining a bubble size distribution bsd and the moments of the bsd function in vesiculated clasts produced by volcanic eruptions it reports the results of applications of the methods to 11 natural samples and discusses the implications for quantitative estimates of eruption processes the analysis is based on a quantitative morphological stereological method for 2 dimensional imaging of cross sections of samples one method determines with some assumptions the complete shape of the bsd function from the chord lengths cut by bubbles the other determines the 1st 2nd and 3rd moments of distribution functions by measurement of the number of bubbles per unit area the surface area per unit volume and the volume fraction of bubbles comparison of procedures and results of these two distinct methods shows that the latter yields rather more reliable results than the former though the results coincide in absolute and relative magnitudes results of the analysis for vesiculated rocks from eleven subplinian to plinian eruptions show some interesting systematic correlations both between moments of the bsd and between a moment and the eruption column height or the sio2 content of magma these correlations are successfully interpreted in terms of the nucleation and growth processes of bubbles in ascending magmas this suggests that bubble coalescence does not predominate in sub plinian to plinian explosive eruptions the moment moment correlations put constraints on the style of the nucleation and growth process of bubbles the scaling argument suggests that a single nucleation event and subsequent growth with any kind of bubble interaction under continuous depressurization which leads to an intermediate growth law between the diffusional growth r m t 2 3 at a constant depressurization rate and the ostwald ripening r m t 1 3 under a constant pressure where rm and t are the mean radius of bubble and the effective time of diffusion respectively occurred in the eruptions it is emphasized that the bsd in vesiculated rocks from terrestrial volcanoes can be used to estimate quantitatively eruption processes such as the initial saturation pressure and magma ascent velocity in a volcanic conduit

kinetics of bubble nucleation in a rhyolitic melt an experimental study of the effect of ascent rate in order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyolitic magma we performed three series of experiments decompressed at rates of either 1000 167 or 27 8 kpa s the experiments were carried out in an externally heated pressure vessel at 800 c and in the pressure range 260 59 mpa the starting material was a crystal free and bubble free rhyolitic glass containing 7 0 wt dissolved h2o in all the decompression experiments homogeneous bubble nucleation began at 90 2 mpa that is 150 mpa below the water saturation pressure of the silicate liquid 240 mpa the degree of supersaturation phon required to trigger homogeneous bubble nucleation was almost independent of decompression rate phon is the difference between the saturation pressure and the nucleation pressure nucleation pressure decreased by 3 mpa for a 36 fold increase in decompression rate these results are in good agreement with the classical theory of nucleation assuming a rhyolite h2o surface tension of 0 106 n m 1 our major experimental finding is that after a short nucleation event the nucleation rate dropped and the bubble number density n reached a stationary value that was strongly sensitive to decompression rate 6 8 mm 3 at 27 8 kpa s 470 mm 3 at 167 kpa s and 5800 mm 3 at 1000 kpa s the smaller value of n at low decompression rate was compensated by a larger mean bubble size so that at a given pressure vesicularity was almost independent of decompression rate the experimental values of n can be reproduced within a factor 0 3 1 4 using a relationship derived from published numerical simulations of vesiculation in ascending magmas the nucleation behavior in our experiments is dictated by a competition between bubble nucleation and diffusive bubble growth which depletes in water the surrounding liquid and therefore reduces the degree of volatile supersaturation once a critical value of n is attained diffusive bubble growth can keep pace with decompression and prevent the degree of volatile supersaturation in the liquid to increase with decreasing pressure the strong correlation between bubble number density and decompression rate has fundamental volcanological implications if we extrapolate the experimental data to the typical ascent rates of silicic magmas we obtain bubble number densities 10 3 101 mm 3 for homogeneous nucleation 10 1 103 mm 3 for heterogeneous nucleation that are orders of magnitude smaller than those measured in most natural pumices we therefore propose that the large values of n in silicic pumices may be due to two successive nucleation events 1 a first event which occurs relatively deep in the volcanic conduit and which yields a moderate number of bubbles and 2 a second nucleation event yielding a very large number of small bubbles and presumably related to the dramatic increase of decompression rate that precedes fragmentation the small bubble number densities associated with homogeneous nucleation suggest that a strong departure from equilibrium degassing should be the rule even at slow ascent rates

heterogeneous bubble nucleation in highly viscous silicate melts during instantaneous decompression from high pressure abstract the nucleation of h 2 o bubbles in magmas has been proposed as a trigger for volcanic eruptions to determine how bubbles nucleate heterogeneously in silicate melts experiments were carried out in which high silica rhyolitic melts were hydrated at 800 c and either 50 or 125 mpa and then decompressed by 20 91 mpa at temperatures that ranged between 550 and 700 c and held at the lower pressures for 10 720 s before being quenched bubbles nucleated in number densities n b that vary between 3 10 7 and 2 10 8 cm 3 blocky shaped magnetite or the ends of needle shaped hematite acted as sites for nucleation but only if a minimum super saturation was exceeded which increases with increasing melt viscosity bubbles did not nucleate along the lengths of hematite needles nor on plagioclase both the beginning and ending times of the nucleation event increases with increasing melt viscosity using nucleation theory predictions neither the slower nucleation rates nor the changing activation of nucleation sites can be adequately explained by the differences in temperature water diffusivity or viscosity instead the variations in nucleation kinetics are best explained by changes in surface tension between melt and vapor resulting from the increasing polymerization of the melt at lower temperatures and water contents because only 10 8 bubbles cm 3 nucleate on magnetite in the rhyolite melt used almost regardless of experimental conditions results from this study may not be directly comparable to vesicle numbers in volcanic pumice of different compositions

abrupt transitions during sustained explosive eruptions examples from the 1912 eruption of novarupta alaska plinian ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes we focus here on the processes leading to the termination of the second and third of these three episodes early erupted pumice from both episodes show a very similar range in bulk vesicularity but the modal values markedly decrease and the vesicularity range widens toward the end of episode iii clasts erupted at the end of each episode represent textural extremes at the end of episode ii clasts have very thin glass walls and a predominance of large bubbles whereas at the end of episode iii clasts have thick interstices and more small bubbles quantitatively all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 m vesicle diameter and cumulative number densities ranging from 107 109 cm 3 patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles compared to experimental data for bubble growth in silicic melts the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma the most distinct clast populations occurred toward the end of plinian activity preceding effusive dome growth distributions skewed toward small sizes thick walls and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing the data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes ii and iii had very different causes through episode iii the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open system outgassing as a precursor to effusive dome growth no such trend is visible in the episode ii clast assemblages we suggest that external changes involving failure of the conduit vent walls are more likely to have effected the break in explosive activity at 45 h

turbulent upwelling of the mid latitude ionosphere 2 theoretical framework in a companion paper data from the mu radar have been presented which show that during sunspot minimum conditions in japan the mid latitude ionosphere is sometimes characterized by regions of rapid and turbulent upwelling in this paper we explore possible mechanisms for these events the most likely process seems to be the instability of the equilibrium which occurs when the mid latitude plasma is supported against gravity either by an eastward electric field component or by a southward neutral wind as was proposed by perkins 1973 we show for example that the growth rate determined by perkins is considerably higher in sunspot minimum conditions than at sunspot maximum for comparable altitudes of the ionospheric f layer the growth rate is not very large however and we argue here that the observed structures must evolve from preexisting undulations of the bottomside of the f region which are generated by gravity waves that is the gravity waves create finite amplitude structures which are amplified by the plasma instability an intriguing feature of the gravity wave role in this process is that the echoing patches detected by the mu radar and the height bands detected by the arecibo radar which we believe to be related phenomena all seem to propagate to the west this is the same direction reported for the angle of arrival measurements of classic mid latitude spread f by a number of researchers using ionosonde techniques since the mu radar detects the upwelling regions from nonthermal 3 m irregularities there must be mechanism to create such tracers we propose that secondary structures are created at intermediate scales via the e b instability operating on the dome of the upwelling structure and by the neutral wind driven process on either the west or the east wall of the structure depending on the direction of the zonal wind the 3 m waves are then created in a cascade process which brings energy into a range of k space in which the structures are linearly damped finally we discuss the fine structure of the echoing patches and suggest several plausible mechanisms two of which involve e region coupling and one which deals with the vertical structure of the gravity wave seeding process

highly localized unique electrodynamics and plasma irregularities linked with the 17 march 2015 severe magnetic storm observed using multitechnique common volume observations from gadanki india in this paper we study equatorial electrodynamics and plasma irregularities linked with the 17 march 2015 severe magnetic storm in the indian sector using common volume observations made by the gadanki ionospheric radar interferometer airglow imager digisonde and gps receiver established at gadanki 13 5o n 79 2o e observations show that with the initiation of the storm at 6 ut on 17 march which happened to be midday in the indian sector the low latitude ionosphere responded in tune with the storm induced electric field and by the sunset time the base of the f layer ascended to an altitude of 470 km with a peak upward velocity of 50 m s 1 eventually manifesting equatorial plasma bubble and irregularities causing strong gps scintillation the most important finding found in this study is the confinement of plasma bubble and irregularities in a narrow longitude zone of 69o 98o e results also show reversal of zonal drift of the irregularities from 120 m s 1 eastward drift to 120 m s 1 westward drift in a time span of 30 min both observations are shown to be linked with very special electrodynamical conditions induced by the magnetic storm related electric field in the dusk sector intriguing details of the longitudinally localized electrodynamics and plasma irregularities are discussed in terms of prompt penetration and disturbed dynamo electric field effects

bubble nucleation growth and coalescence during the 1997 vulcanian explosions of soufriere hills volcano montserrat abstract soufriere hills volcano had two periods of repetitive vulcanian activity in 1997 each explosion discharged the contents of the upper 0 5 2 km of the conduit as pyroclastic flows and fallout frothy pumices from a deep gas rich zone lava and breadcrust bombs from a degassed lava plug and dense pumices from a transition zone vesicles constitute 1 66 vol of breadcrust bombs and 24 79 of pumices all those larger than a few tens of m being interconnected small vesicles few hundreds of m in pumices are interpreted as pre dating explosion implying pre explosive conduit porosities up to 55 about a sixth of large vesicles in pumices and all those in breadcrust bombs are angular voids formed by syn explosive fracturing of amphibole phenocrysts an intermediate sized vesicle population formed by coalescence of the small syn explosive bubbles bubble nucleation took place heterogeneously on titanomagnetite number densities of which greatly exceed those of vesicles and growth took place mainly by decompression development of pyroclast vesicle textures was controlled by the time interval between the onset of explosion decompression and surface quench in contact with air lava plug fragments entered the air quickly after fragmentation 10 s so the interiors continued to vesiculate once the rinds had quenched forming breadcrust bombs deeper gas rich magma took longer 50 s to reach the surface and vesiculation of resulting pumice clasts was essentially complete prior to surface quench this accounts for the absence of breadcrusting on pumice clasts and for the textural similarity between pyroclastic flow and fallout pumices despite different thermal histories after leaving the vent it also allowed syn explosive coalescence to proceed further in the pumices than in the breadcrust bombs uniaxial boudinage of amphibole phenocrysts in pumices implies significant syn explosive vesiculation even prior to magma fragmentation probably in a zone of steep pressure gradient beneath the descending fragmentation front syn explosive decompression rates estimated from vesicle number densities 0 3 6 5 mpa s 1 are consistent with those predicted by previously published numerical models

bubble nucleation in rhyolite and dacite melts temperature dependence of surface tension surface tension profoundly influences the ability of gas bubbles to nucleate in silicate melts to determine how temperature impacts experiments were carried out in which high silica rhyolite melts with 5 wt dissolved water were decompressed at temperatures that ranged from 775 to 1 085 c decompressions were also carried out using dacite melts with 4 3 wt dissolved water at 1 150 c water bubbles nucleated in rhyolite only when decompressions exceeded 95 mpa at all temperatures bubbles nucleated in number densities that increased as decompression increased and at hotter temperatures at a given amount of decompression after correcting decompression amounts for temperature differences values for were estimated from nucleation rates and found to vary between 0 081 and 0 093 n m 1 surface tension decreases as temperature increases from 775 to 875 c but then increases as temperature increases to 1 085 c those values overlap previous results but only when melt viscosity is less than 104 pa s for low viscosity rhyolite there is a strong correlation of with temperature in which increases by 6 9 10 5 n m 1 c 1 that variation is robust for 5 9 wt dissolved water as long as melt viscosity is 104 pa s more viscous rhyolite deviates from that correlation probably because nucleation is retarded in stiffer melts bubbles nucleated in dacite when decompressions exceeded 87 mpa and occured in one or more events as decompression increased surface tension is estimated to be 0 083 0 001 n m 1 and when adjusted for temperature agrees well with previous results for colder and wetter dacite melts at a given water content dacite melts have lower surface tensions than rhyolite melts when corrected to a fixed temperature

numerical study of nucleation and growth of bubbles in viscous magmas the nucleation and growth processes of bubbles in viscous magmas with a constant decompression rate have been numerically investigated based on a formation which accounts for effects of viscosity as well as diffusivity interfacial tension and decompression rate the numerical solutions show two regimes in the nucleation and growth process a diffusion controlled regime and a viscosity controlled regime mainly depending on the decompression rate initial saturation pressure and viscosity the basic mechanism common to both regimes is that growth governs nucleation through depletion of degassing components in basaltic eruptions the vesiculation is essentially controlled by diffusion and the viscosity controlled regime is limited to very high decompression rate and very small water content when andesitic magma saturated by water at 10 mpa is decompressed through the propagation of rarefraction wave induced by a landslide as took place in the mount st helens 1980 eruption the vesiculation is controlled by the viscosity up to 100 m depth on the other hand in a rhyolitic magma for the same situation vesiculation is controlled by the viscosity over the whole depth of the magma column in the viscosity controlled regime the vesicularity may be 90 or less as seen in silicic pumice whereas in the diffusion controlledmore regime the vesicularity equals or exceeds 98 such as in reticulite in hawaiian basalt an observed variation of the number density of bubbles by several orders of magnitude in plinian eruptions and the correlation with the sio2 content can be attributed approximately to the dependence of diffusivity of viscosity on sio2 content and temperature assuming the apparent correlation between sio2 content and temperature of magma less

geomagnetically induced currents around the world during the 17 march 2015 storm geomagnetically induced currents gics represent a significant space weather issue for power grid and pipeline infrastructure particularly during severe geomagnetic storms in this study magnetometer data collected from around the world are analyzed to investigate the gics caused by the 2015 st patrick s day storm while significant gic activity in the high latitude regions due to storm time substorm activity is shown for this event enhanced gic activity was also measured at two equatorial stations in the american and south east asian sectors this equatorial gic activity is closely examined and it is shown that it is present both during the arrival of the interplanetary shock at the storm sudden commencement ssc in south east asia and during the main phase of the storm 10 hours later in south america the ssc caused magnetic field variations at the equator in south east asia that were twice the magnitude of those observed only a few degrees to the north strongly indicating that the equatorial electrojet eej played a significant role the large equatorial magnetic field variations measured in south america are also examined and the coincident solar wind data are used to investigate the causes of the sudden changes in the eej 10 hours into the storm from this analysis it is concluded that sudden magnetopause current increases due to increases in the solar wind dynamic pressure and the sudden changes in the resultant magnetospheric and ionospheric current systems are the primary drivers of equatorial gics

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

vesiculation of may 18 1980 mount st helens magma the may 18 1980 eruption of mount st helens washington produced both white and gray pumice similar in composition but varying in phenocryst microlite and vesicle content the white pumice has fewer phenocrysts no microlites and higher vesicularity and is thus less dense than the gray in addition vesicles in the white pumice are larger and more interconnected than those in the gray both white and gray have effective crystal free vesicularities 85 7 and 72 2 respectively close to the traditionally accepted fragmentation vesicularity of 75 77 given that the two pumice types were erupted concurrently for several phases of the eruption and therefore probably had similar eruption paths we suggest that differences in vesicularity and vesicle size distributions result from the presence or absence of microlites the presence of microlites in the gray pumice not only increased the magma viscosity and effective vesicularity but appears to have aided bubble nucleation and hindered bubble expansion and coalescence thus magmas with microlites may fragment at a lower bulk vesicularity than those without microlites fragmented microlite bearing clasts are also likely to expand less after fragmentation and therefore more closely preserve the bubble distribution and structure at the time of magma fragmentation

linking variable explosion style and magma textures during 2002 at stromboli volcano italy microtextural characteristics of fresh ejecta from stromboli volcano were examined from three periods of differing eruption style and intensity in 2002 activity shifted from relatively weak and infrequent ash charged explosions during january through may into two broad cycles of waxing activity in june through late september and late september through december followed by the onset on 28 december of the 2002 2003 effusive eruption analyzed sets of lapilli from may september october and 28 december show contrasts in the physical properties of magma resident in the shallow conduit during this range of activity three distinct textures are observed among the analyzed pyroclasts low density ld with an abundance of subspherical bubbles the presence of large irregularly shaped bubbles and a light to transparent glass matrix transitional texture tt with an intermediate number of subspherical bubbles a high frequency of large irregularly shaped bubbles and a honey colored glass matrix and high density hd with sparse relatively small bubbles conspicuous large irregular bubbles and a dark glass matrix observational and quantitative data density vesicle size indicate that these textures are linked through variable residence time in stromboli s shallow conduit with an ongoing evolution from ld to hd magma calculations suggest that residual ld magma will evolve to hd texture in a period of hours to days contrasting amounts of the ld tt and hd magmas are present in each sample with the most tt in may the most ld in september october and the most hd in december this implies that the shallow magma had a different rheology at each collection period the viscosity of ld and hd magmas are calculated to be in the range of 2 000 to 2 600 and 3 000 to 5 000 pa s respectively which with their changing proportions must have implications for rates of bubble slug ascent and processes of fragmentation this study suggests that an increasing maturity of magma in stromboli s shallow conduit with resultant increase in viscosity feeds back to reduce the intensity of explosions whereas a steady flux of ld magma favors more powerful explosions

scaling vesicle distributions and volcanic eruptions models of coalescence decompressive expansion of the later stages of bubble growth predict that for diverse types of volcanic products the vesicle number densities n v are of the scaling form text n left text v right propto text v text b 3 1 where v is the volume of the vesicles and b3 the 3 dimensional scaling power law exponent we analyze cross sections of 9 pumice samples showing that over the range of bubble sizes from 10 m to 3 cm they are well fit with b3 0 85 we show that to within experimental error this exponent is the same as that reported in the literature for basaltic lavas and other volcanic products the importance of the scaling of vesicle distributions is highlighted by the observation that they are particularly effective at packing bubbles allowing very high vesicularities to be reached before the critical percolation threshold a process which for highly stressed magmas would trigger explosion in this way the scaling of the bubble distributions allows them to be key actors in determining the rheological properties and in eruption dynamics

bubble nucleation in rhyolitic melts experiments at high pressure temperature and water content abstract we report the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release two rhyolitic obsidians one containing less than 1 of microlites of fe ti oxides and the other about 20 of various crystals were hydrated at 150 mpa and 780 850 c after saturation was reached 5 3 5 5 wt water pressure was lowered and the samples were allowed to nucleate and grow bubbles for various amounts of time before the final rapid quenching of the experiments the results demonstrate the importance of heterogeneous nucleation microlites of fe ti oxides are very efficient as sites for bubble nucleation in their presence modest nucleation was observed even after decompression by 6 10 8 bubbles cm 3 in the absence of microlites no nucleation occurred at p mpa at p 10 mpa bubbles also nucleated on crystals of biotite zircon and apatite modest nucleation 10 3 10 5 cm 3 took place even in crystal free samples but it was still heterogeneous when p exceeded 80 mpa nucleation in crystal free samples became extensive 10 5 10 7 cm 3 the lack of correlation of bubble density with either time or decompression suggests that nucleation was still heterogeneous nucleation rates were controlled mainly by the availability of sites rates were faster than 10 6 cm 3 s 1 when microlites were present and faster than 10 5 cm 3 s 1 in the absence of microlites at p 70 mpa narrow size distributions in most samples suggests that nucleation took place immediately after the pressure drop the experimental data we present here indicate that the presence or absence of efficient nucleation sites can lead to two distinct modes of bubble formation when a large number of efficient sites e g fe ti oxide are present bubble nucleation requires very little supersaturation and to a good approximation gas and magma are in equilibrium in magmas that are crystal free or contain crystals that are inefficient at nucleating bubbles very high degrees of supersaturation are required in order to initiate nucleation these two modes of exsolution may lead to contrasting styles of convection pressure build up and eruption

experimental constraints on degassing of magma isothermal bubble growth during continuous decompression from high pressure numerical models predict that rapid ascent of hydrous magma can lead to supersaturation of dissolved volatile constituents possibly leading to explosive eruption we have performed controlled decompression experiments to investigate the ascent rates required to maintain bubble melt equilibrium high silica rhyolitic melts were saturated with water at 200 mpa and 825 c decompressed to lower pressures at constant rates of 0 025 0 25 0 5 and 1 0 mpa s 1 and then rapidly quenched isobarically other samples were saturated with water over the pressure range investigated to determine equilibrium water solubility in order to quantify degassing efficiency during decompression at a decompression rate of 0 025 mpa s 1 melt vapor equilibrium was maintained over the entire pressure range examined 200 to 17 5 mpa a single bubble nucleation event occurred in response to decompression and quenched bubble sizes can be modeled by a equilibrium bubble growth model that takes into account the number density of bubbles at decompression rates of 0 25 0 5 and 1 0 mpa s 1 rhyolitic melts could not degas in equilibrium when pressure decreased from 200 mpa to 140 mpa and water supersaturation p in the melt reached up to 60 mpa with higher values at faster decompression rates further pressure release resulted in near equilibrium degassing and p dropped significantly in each case p decreased when bubbles exceeded 10 vol a single heterogeneous bubble nucleation event occurred in each experiment when p 20 mpa no other bubbles nucleated despite p reaching 60 mpa which is probably too low to trigger homogeneous nucleation compared to estimates for magma decompression rates during lava dome eruptions our results indicate that magmas can degas efficiently throughout their ascent to the surface in explosive eruptions decompression rates may exceed those of this study and hence melts may become supersaturated with water such fast decompressions are expected however only when magma is highly vesicular which would aid approach to equilibrium degassing

insights into eruption dynamics from textural analysis the case of the may 2008 chaiten eruption the may 2008 chaiten southern chile eruption was characterized by several explosive events each associated with plumes which reached up to about 19 km above sea level on may 6 a study of the textural and physical features of the juvenile clasts erupted during the climactic phase of the 2008 eruption of chaiten is presented pumice clasts show unimodal density distribution main mode at 600 kg m3 average vesicularity of about 69 a glassy groundmass with no microcrystals and vesicles with dimension between 1 m and 2 mm they also show a unimodal vesicle size distribution with most frequent vesicle size in the range 0 05 0 08 mm and an estimated vesicle number density of 1 3 0 5 105 mm 3 related to a rapid nucleation event produced during the late phases of magma rise this is confirmed by the absence of microcrystals that could otherwise have delayed vesicle formation and allowed the magma to maintain a low viscosity and a supersaturation in volatiles vesiculation and fragmentation were triggered by a sudden decompression of the melt associated with the opening of the volcanic conduit 10 mpa s 1

kinetics of heterogeneous bubble nucleation in rhyolitic melts implications for the number density of bubbles in volcanic conduits and for pumice textures we performed decompression experiments to simulate the ascent of a phenocryst bearing rhyolitic magma in a volcanic conduit the starting materials were bubble free rhyolites water saturated at 200 mpa 800 c under oxidizing conditions they contained 6 0 wt dissolved h2o and a dense population of hematite crystals 8 7 2 105 mm 3 pressure was decreased from the saturation value to a final value ranging from 99 to 20 mpa at constant temperature 800 c the rate of decompression was either 1 000 or 27 8 kpa s in all experiments we observed a single event of heterogeneous bubble nucleation beginning at a pressure pn equal to 63 3 mpa in the 1 000 kpa s series and to 69 1 mpa in the 27 8 kpa s series below pn the degree of water supersaturation in the liquid rapidly decreased to a few 0 1 wt the nucleation rate dropped and the bubble number density bnd stabilized to a value strongly sensitive to decompression rate 80 mm 3 at 27 8 kpa s vs 5 900 mm 3 at 1 000 kpa s this behaviour is like the behavior formerly described in the case of homogeneous bubble nucleation in the rhyolite h2o system and in numerical simulations of vesiculation in ascending magmas similar degrees of water supersaturation were measured at 27 8 and 1 000 kpa s implying that a faster decompression rate does not result in a larger departure from equilibrium our experimental results imply that bnds in acid to intermediate magmas ascending in volcanic conduits will depend on both the decompression rate left left text d p text d t right right and the number density of phenocrysts especially the number density of magnetite microphenocrysts 1 100 mm 3 which is the only mineral species able to reduce significantly the degree of water supersaturation required for bubble nucleation very low bnds 1 mm 3 are predicted in the case of effusive eruptions left left text d p text d t right right 0 1 kpa s high bnds up to 107 mm 3 and bimodal bubble size distributions are expected in the case of explosive eruptions 1 a relatively small number density of bubbles 1 100 mm 3 will first nucleate in the lower part of the conduit left left text d p text d t right right 10 kpa s either at high pressure on magnetite or at lower pressure on quartz and feldspar or by homogeneous nucleation in the liquid and 2 then extreme decompression rates near the fragmentation level left left text d p text d t right right 103 kpa s will trigger a major nucleation event leading to the multitude of small bubbles typically a few micrometers to a few tens of micrometers in diameter which characterizes most silicic pumices

diverse patterns of ascent degassing and eruption of rhyolite magma during the 1 8 ka taupo eruption new zealand evidence from clast vesicularity k lauea hawaii like vesuvius and etna is a frequently active volcano associated with a national park which is a focus for volcano tourism two contrasting summit eruptions at k lauea in 2008 2009 and 1959 emphasize how even small eruptions on populous volcanoes have significant impact on both hawaii volcanoes national park hvnp and the surrounding communities the effects are typically more indirect and social than direct physical but have significant fiscal repercussions the former include both positive benefits and negative impacts as an illustration direct consequences of the 1959 eruption were the rerouting of the major park access road and destruction of a summer camp but benefits included an unprecedented surge in visitor numbers and construction of nature trails that are now the most visited asset in the park both positive and negative impacts have increased in intensity with time due to growth in the resident population and visitor numbers for example the entire park has been evacuated twice during the ongoing 2008 2009 eruption

structuring of intermediate scale equatorial spread f irregularities during intense geomagnetic storm of solar cycle 24 here we examine the structuring of equatorial plasma bubble epb during intense geomagnetic storm of solar cycle sc 24 that occurred on 17 march 2015 using spaced receiver scintillation observations on a 251 mhz radio signal recorded by a network of stations in indian region as yet this is the strongest geomagnetic storm dstmin 223nt that occurred in present sc present study reveals that the structuring of equatorial spread f esf irregularities was significantly different on 17 march as compared to quiet days of corresponding month esf irregularities of intermediate scale 100 m to few kilometers are observed at unusually higher altitudes 800 km covering wider longitudinal latitudinal belt over indian region a presence of large scale irregularity structures with stronger n at raised f peak with small scale irregularities at even higher altitudes is observed it caused strong focusing effect s4 1 that prevails throughout premidnight hours at dip equatorial station tirunelveli other observational aspect is that zonal irregularity drifts over low latitude station kolhapur exhibited a large deviation of 230 m s from their average quiet time pattern during this geomagnetic storm two southward turnings of significant strength bz 15 nt occurred at 11 4 ist indian standard time and 17 9 ist the later southward turning of interplanetary magnetic field imf bz resulted in a large eastward prompt penetration electric field ppef close to sunset hours in indian longitude estimates of ppef obtained from real time ionospheric model are too low to explain the observed large upliftment of f region in the post sunset hours possible reason for observed enhanced ppef linked effects is discussed

scintillations plasma drifts and neutral winds in the equatorial ionosphere after sunset an equatorial campaign was conducted during september 25 to october 7 1994 to investigate the neutral and plasma dynamics in the equatorial ionosphere after sunset in relation to the day to day variability of the occurrence of equatorial spread f esf the campaign was organized under the auspices of national science foundation s multi instrumented studies of the equatorial thermosphere aeronomy program miseta which included the jicamarca radar spaced antenna satellite scintillation digisonde all sky imager and fabry perot interferometer fpi measurements near the magnetic equator in peru during a part of the period september 27 to october 3 the geophysics directorate of phillips laboratory performed measurements away from the magnetic equator at aguaverde chile magnetic latitude 11 s located 800 km to the east of the jicamarca meridian using geostationary and gps satellite scintillation digisonde and all sky imager systems the incoherent scatter radar results indicate that the postsunset enhancement of upward plasma drift even though of the order of only 20 m s 1 during the solar minimum period is a necessary condition for the generation of esf in view of the extreme difficulty of determining the neutral wind speed during the early evening hours by the fpi due to low airglow intensity it was not possible to unequivocally associate the observed postsunset enhancements with strong eastward neutral winds however considering a few observations contiguous to the campaign period it appears that such a causal relationship may exist the scintillation drift measurements in peru and chile indicated that the zonal irregularity drift was smaller away from the magnetic equator implying a variation of neutral wind with latitude this is reproduced in the altitude variation of zonal drift observed by the jicamarca radar during a magnetic storm scintillation measurements indicated that eastward drifts near the magnetic equator are accompanied by westward drifts near the anomaly peak which is consistent with the effects of a disturbance dynamo the campaign results indicate that in order to resolve the variability of esf a careful probing of neutral dynamics as a function of latitude needs to be undertaken during the postsunset period

mid latitude ionospheric irregularities persisting into late morning during the magnetic storm on 19 march 2001 1 post sunrise intense ionospheric irregularities at low to mid latitudes are reported for the first time a method based on spaced receiver technique is proposed to estimate the apparent drift velocity of the irregularities by using the correlation of the slant total electron content tec from 3 gps receivers the irregularities were detected at 23 45 n by the gps earth observation network geonet of japan from 0455 lt to 1051 lt on 21 march 2001 during the recovery phase of the magnetic storm at the corresponding time spread f was observed with 3 ionosondes located meridionally at okinawa 127 8 e 26 3 n 20 7 n magnetic latitude kokubunji 139 5 e 35 7 n 30 0 n magnetic latitude and wakkanai 141 7 e 45 4 n 40 5 n magnetic latitude while an intense negative ionospheric storm was processing and propagating from mid to lower latitudes the irregularities were seen as plasma bubbles by the defense meteorological satellite program dmsp in situ measurements westward drift with the speed of 100 m s was estimated by spaced receivers from geonet and confirmed by dmsp during the evolution of the irregularities the plasma bubbles at mid latitude sustained 1 5 hour after sunrise and at the lower latitudes they survived more than 4 hours after sunrise the observations suggested that the irregularities were generated by an eastward electric field associated with the disturbance dynamo and the long time existence of the irregularities after sunrise seemed to be related to the daytime intense negative ionospheric storm on 21 march

heterogeneous bubble nucleation on fe ti oxide crystals in high silica rhyolitic melts the nucleation of h2o bubbles in magmas has been proposed as a trigger for volcanic eruptions to determine how bubbles nucleate heterogeneously in silicate melts experiments were carried out in which high silica rhyolitic melts were hydrated at 740 800 c and 50 175 mpa decompressed by 20 70 mpa and held at the lower pressures for 10 s before being quenched the hydration conditions were subliquidus and all samples contain blocky magnetite needle shaped hematite plagioclase magnetite is abundant at 800 c and high pressures whereas hematite becomes more abundant at lower temperatures and pressures bubbles nucleated in a single event in all samples with the number density nt of bubbles varying between 2 107 and 1 109 cm 3 at low degrees of supersaturation one to a few bubbles nucleate on faces of magnetite but at medium to high degrees of supersaturation multiple bubbles nucleate on single magnetite grains on hematite one to a few bubbles nucleated at the ends of the needle shaped crystals at medium supersaturations but formed along their entire lengths at high supersaturations nt increases as water diffusivity decreases indicating that the number of bubbles nucleated is influenced by their growth which depletes the melt with respect to h2o and lowers supersaturation if volcanic eruptions are triggered by bubble formation in magmas stored in shallow level magma chambers then the supersaturations needed for heterogeneous nucleation suggest that only small amounts of crystallization are needed whereas homogeneous nucleation is unlikely to trigger eruptions

bubble growth during decompression of magma experimental and theoretical investigation a model of bubble growth during decompression of supersaturated melt was developed in order to explore the conditions for preservation of gas overpressure in bubbles or for maintaining supersaturation of the melt the model accounts for the interplay of three dynamic processes decompression rate of the magma deformation of the viscous melt around the growing bubble and diffusion of volatiles into the bubble generally these processes are coupled and the evolution of bubble radius and gas pressure is solved numerically for a better understanding of the physics of the processes we developed some analytical solutions under simplifying assumptions for cases where growth is controlled by viscous resistance diffusion or linear decompression rate we show that the solutions are a function of time and two dimensionless numbers which are the ratios of either the diffusive or viscous time scales over the decompression time scale the conditions for each growth regime are provided as a function of the two governing dimensionless parameters analytical calculations for some specific cases compare well with numerical simulations and experimental results on bubble growth during decompression of hydrated silicic melts the model solutions including the division to the growth regimes as function of the two parameters provide a fast tool for estimation of the state of erupting magma in terms of gas overpressure supersaturation and gas volume fraction the model results are in agreement with the conditions of plinian explosive eruption e g mount st helens 18 may 1980 where high gas overpressure is expected the conditions of effusion of lava domes with sudden onset of explosive activity are also in agreement with the model predictions mostly in equilibrium degassing and partly in overpressure conditions we show that in a situation of quasi static diffusion during decompression the diffusive influx depends on the diffusivity away from the bubble insensitive to the diffusivity profile

equatorial disturbance dynamo electric field longitudinal structure and spread f a case study from guar eits campaigns digisondes ionosondes an hf doppler radar and magnetometers were operated in brazil and india during the september october 1994 guara eits campaigns analysis of the data for the two disturbed intervals 2 4 october and 25 27 september provided evidence of a longitudinal structure in the disturbance dynamo dd electric field at low latitudes the dd electric field which is westward in the evening inhibited the developments of the equatorial prereversal electric field and postsunset esf whereas the simultaneous eastward field in the predawn sector did not lead to esf contrary to what is normally expected this nondevelopment of esf is suggested as evidence of the stabilizing effect of transequatorial neutral winds associated with stormtime circulation the campaign observations clearly demonstrate that the dd electric fields could often mask the low latitude ionospheric responses to prompt penetration electric fields in the course of sustained substorm activity

low latitude ionosphere response to super geomagnetic storm of 17 18 march 2015 results from a chain of ground based observations over indian sector in this paper we present unique results of equatorial and low latitude ionosphere response to one of the major geomagnetic storms of the current solar cycle that occurred during 17 18 march 2015 where dst reached its minimum of 228 nt here we utilized data from magnetometers chain of ionosondes located at tirunelveli 8 73 n 77 70 e geometry 0 32 n hyderabad 17 36 n 78 47 e geometry 8 76 n and allahabad 25 45 n 81 85 e geometry 16 5 n along with multistation gps receivers over indian sector the observations showed a remarkable increase of h f to as high as 560 km over tirunelveli magnetic equator with vertical drift of 70 m s at 13 30 ut due to direct penetration of storm time eastward electric fields which exactly coincided with the local time of pre reversal enhancement pre and caused intense equatorial spread f irregularities in ionosondes and scintillations in gps receivers at wide latitudes plasma irregularities are so intense that their signatures are seen in allahabad lucknow storm time thermospheric meridional winds as estimated using two ionosondes suggest the equatorward surge of gravity waves with period of 2 h suppression of anomaly crest on the subsequent day of the storm suggests the complex role of disturbance dynamo electric fields and disturbance wind effects our results also show an interesting feature of traveling ionospheric disturbances possibly associated with disturbance meridional wind surge during recovery phase in addition noteworthy observations are nighttime westward zonal drifts and pre related total electron content enhancements at anomaly crests during main phase and counter electrojet signatures during recovery phase

first observation of interhemispheric asymmetry in the epbs during the st patrick s day geomagnetic storm of 2015 in this work we have studied the characteristics of epbs such as their zonal drift and tilt from the low latitude and dip equatorial region in the indian longitude sector during the main phase of the 17 march 2015 storm all sky airglow imaging observations from tirunelveli 8 7on 77 8oe geographic 1 7on dip latitude and kolhapur 16 7on 74 3oe geographic 11 5on dip latitude are utilized here on 17 march 2015 epbs were observed to drift eastward during 14 30 16 30 ut between 3os and 15on dip latitudes a westward drift presumably under the influence of the disturbance dynamo electric field ddef initially appeared at higher dip latitudes almost 10 hours after the storm onset and subsequently the same was observed at lower dip latitudes the epbs attained a peak westward drift at 17 00 ut followed by a gradual decrease in their speed till 18 30 ut after regaining their westward speed the epbs continued to drift westward till 22 00 ut moreover a latitudinal gradient in the drift motion of the epbs was also observed on this night another interesting observation made from the images obtained from tirunelveli was the presence of a large westward tilt of the epbs the most intriguing finding of this study however was the asymmetry in the tilt of the epbs at conjugate points during the pre midnight hours on 17 march 2015 in this study the possible mechanisms that can explain these observations are discussed in the light of the current understanding of the equatorial electrodynamics and epbs

simulating bubble number density of rhyolitic pumices from plinian eruptions constraints from fast decompression experiments decompression experiments of a crystal free rhyolitic liquid with 6 6 wt h2o were carried out at a pressure range from 250 mpa to 30 75 mpa in order to characterize effects of magma ascent rate and temperature on bubble nucleation kinetics especially on the bubble number density bnd the number of bubbles produced per unit volume of liquid a first series of experiments at 800 c and fast decompression rates 10 90 mpa s produced huge bnds 2 1014 m 3 at 10 mpa s 2 1015 m 3 at 90 mpa s comparable to those in natural silicic pumices from plinian eruptions 1015 1016 m 3 a second series of experiments at 700 c and 1 mpa s produced bnds 9 1012 m 3 close to those observed at 800 c and 1 mpa s 6 1012 m 3 showing that temperature has an insignificant effect on bnds at a given decompression rate our study strengthens the theory that the bnds are good markers of the decompression rate of magmas in volcanic conduits irrespective of temperature huge number densities of small bubbles in natural silicic pumices from plinian eruptions imply that a major nucleation event occurs just below the fragmentation level at which the decompression rate of ascending magmas is a maximum 1 mpa s