multipoint observations of a small substorm in this paper the authors present multipoint observations of a small substorm which occurred just after 0110 ut on april 25 1985 the observations were made by spacecraft ampte cce ampte irm dmsp f6 and dmsp f7 ground auroral stations eiscat magnetometer cross syowa narssarssuaq great whale river and fort churchill and mid latitude stations furstenfeldbruck toledo and argentine island these data provide them with a broad range of observations including the latitudinal extent of the polar cap visual identification of substorm aurorae and the magnetic perturbations produced directly beneath them in situ magnetic field and energetic particle observations of the disruption of the cross tail current sheet and observations concerning the spatial expansion of the current disruption region from two radially aligned spacecraft the dmsp data indicate that the event took place during a period when the polar cap was relatively contracted yet the disruption of the current sheet was observed by cce at 8 56 r sub e they have been able to infer a considerable amount of detail concerning the structure and westward expansion of the auroral features associated with the event and they show that those auroral surges were located more than 10 degree equatorward of the boundary between open andmore closed field lines moreover they present evidence that the current sheet disruption observed by cce in the neutral sheet was located on field lines which mapped to the westward traveling surge observed directly overhead of the ground station at syowa furthermore the observations strongly imply that disruption of the cross tail current began in the near earth region and that it had a component of expansion which was radially antisunward less

parameterized mesoscale forcing mechanisms for initiating numerically simulated isolated multicellular convection abstract two methods designed to parameterize mesoscale ascent in a three dimensional numerical cloud model via near surface momentum and heat fluxes are presented and compared to the commonly used technique of an initial perturbation placed within the model initial condition the flux techniques use a continuously reinforced thermal or convergent low level wind field to produce upward vertical motion on the order of 10 cm s 1 by which deep moist convection can be initiated the sensitivity of the convective response to the type strength and size of the forcing is evaluated using numerical simulations of a conditionally unstable environment with weak unidirectional shear precipitation free cloud processes are used to further simplify the model response to the forcing the three methods tested produce an initial convective response but only the momentum and heat flux methods are able to produce sustained deep convection that approximately resembles isolated multicellular convection cell regeneration

external versus internal triggering of substorms an information theoretical approach the role of external triggering of substorms through northward turning of the interplanetary magnetic field has been examined in a number of recent studies while hsu and mcpherron 2002 2004 argue that the strong association between external triggers defined by lyons et al 1997 and substorm onsets could be responsible for most substorms morley and freeman 2007 argue that the association between northward interplanetary magnetic field imf turnings and substorm onsets are coincidental rather than causal because the same external triggers are also closely associated with an artificial list of substorm onsets generated with the minimal substorm model which has no requirement of northward imf turning we examine an expanded list of substorms using conditional redundancy an entropy based measure of conditional dependency to examine whether northward imf turning as an external trigger provides any additional information about substorm onset beyond knowing that there has been a period of sustained loading of energy flux southward imf our analysis reveals that only a few percent additional information is provided by the northward turning criterion which is consistent with the statistics of surrogate data sets of external triggers constructed to coincide with 2 of substorms we therefore conclude that northward turning of the imf is in general coincidentally rather than causally associated with substorm onsets

tailward leap of multiple expansions of the plasma sheet during a moderately intense substorm themis observations a moderately intense substorm on 1 march 2008 from 0830 to 1000 ut observed by themis probes and the ground based observatory gbo is examined to investigate the global evolution of substorm phenomena during this interval all five themis probes are closely aligned along the tail axis near midnight covering a radial range from similar to 9 re to similar to 18 re after the substorm onset plasma sheet expansions take place successively at multiple locations in the magnetotail as measured by different probes the positions of the plasma sheet expansions have a tailward leap progression with an average velocity of similar to 36 km s there are two types of dipolarization detected in this substorm the first type is the dipolarization front which is associated with the bursty bulk flow bbf while the second type which we call global dipolarization is associated with plasma sheet expansions in the substorm studied there are four intensifications as shown in the themis ae index we can detect the effects of localized and short lived magnetic energy release processes occurring in the magnetotail corresponding to each of the four ae intensifications furthermore the inner four probes can detect the global dipolarization signatures similar to 4 15 min earlier than plasma sheet expansions while the outermost probe p1 cannot detect this before the plasma sheet expansion these two phenomena are caused by the same process magnetic energy release process but the effects detected by probes locally appear delayed the observations in this case are not sufficient to distinguish between the two competing substorm models

identification of substorm onset location and preonset sequence using reimei themis gbo pfisr and geotail 1 we present state of the art multiple instrument observations of an isolated substorm on october 12 2007 the auroral breakup was observed simultaneously by reimei themis asi and pfisr the footprint of geotail was also near the breakup these observations allow for detailed study of the breakup location in terms of large and small scale auroral morphology particle precipitation and ionospheric convection which has not previously been achieved it also allows for detailed identification of the sequence leading to the breakup we report the first spaceborne high spatial and temporal resolution images of part of a breakup arc and a wave like auroral enhancement captured by reimei observations suggest a sudden plasma sheet thinning initiated 10 min before the onset wave like auroral enhancements were observed twice at the most equatorward arc 3 min and 1 min before the breakup these enhancements are likely due to some near earth instability such as ballooning instability unlike the usual substorm sequence this most equatorward arc did not develop into the breakup arc but remained almost stable until being engulfed by the auroral equatorward expansion from higher latitude after onset the wave like auroral enhancement was associated with three fine inverted v arcs and embedded within energetic ion precipitation following this enhancement an arc likely a poleward boundary intensification formed at higher latitude just adjacent to the plasma sheet boundary layer psbl this arc then extended southwestward and led to the breakup arc which was located poleward of the wavy structures assuming longitudinal homogeneity of ion precipitation over 1 this breakup arc was located in a region without ion precipitation just poleward of the energetic ion precipitation these observations suggest the possible existence of a low entropy flow channel associated with the arc adjacent to the psbl which might be associated with instability in the near earth plasma sheet responsible for the auroral breakup

airborne doppler radar and sounding analysis of an oceanic cold front abstract an analysis of a cold front over the eastern atlantic ocean based on airborne doppler wind syntheses and dropsonde data is presented the focus and unique aspect of this study is a segment of the front that was near the center of the cyclone the dual doppler wind synthesis of the frontal zone combined with an average dropsonde spacing of 30 km covers a total distance of 450 km in the cross frontal direction the finescale resolution and areal coverage of the dataset are believed to be unprecedented the cold front was characterized by a distinct wind shift and a strong horizontal temperature gradient the latter was most intense aloft and not at the surface in contrast to the classical paradigm of surface cold fronts the shear of the alongfront component of the wind was relatively uniform as a function of height within the frontal zone this observation is contrary to studies suggesting that frontal zones decrease in intensity above the surface the surface convergence within the frontal zon

two component auroral electrojet importance for substorm studies this paper attempts to synthesize the diverse number of observations of electric fields and currents in the high latitude ionosphere during substorms by demonstrating that there are often spatial shifts among regions of high ionospheric conductivity large electric fields and intense currents in the auroral electrojet it is shown that substorm time variations of the current patterns over the entire polar region consist of two basic components the first is related to the two cell convection pattern and the second to the westward electrojet in the dark sector which is in turn related to the three dimensional wedge current system these two components result from the relative strength of electric fields and conductivities in the intensification of the auroral electrojet and are identified as the signatures for directly driven and the unloading components in solar wind magnetosphere interactions we contend that disturbed intervals do not necessitate the presence of substorm expansion phase activity and that the vast number of earlier complex results concerning the auroral electrojet can be ascertained from the high degree of variability of the two components depending on substorm events substorm phases and their own spatial temporal scale sizes it is demonstrated that several major issues that have remained controversial are now accounted for reasonably well in terms of this two component electrojet model we also predict specific features of the substorm auroral electrojet that have not yet been observed

dynamics of the substorm expansive phase meridian scanning photometer msp and magnetometer data from the canadian auroral network for the open program unified study canopus ground based array have been used to study the dynamics of the substorm expansive phase the relative latitudinal motions of the msp 630 0 557 7 and 486 1 nm emissions for eight isolated events have been studied the data show the expansive phase to comprise three stages an explosive tens of seconds onset rapid poleward motion of 557 7 nm emissions of the order of a few minutes and a period of slower tens of minutes poleward moving 630 0 nm emissions we interpret the rapid poleward motion of the 557 7 nm data in terms of a region of instability expanding rapidly down the magnetotail possibly accelerating plasma sheet electrons as it proceeds all events show that lobe flux reconnection occurs after near earth onset with typical time delays between 1 and 5 min the extremely short time interval between near earth onset and the beginning of lobe flux reconnection presents a severe observational constraint on any substorm model which attempts to explain the substorm expansive phase magnetometer data from the canopus churchill line have been used to study the dynamics of the substorm electrojets during the growth and early expansive phases of the substorm the borders of these electrojets closely follow the motion of the msp data during the growth and expansive phases at the instant of expansive phase onset the growth phase electrojets as observed along the churchill meridian line disappear being replaced by the substorm westward electrojet associated with the current wedge the equatorward border of the substorm westward electrojet is embedded within the proton aurora emissions and appears at expansive phase onset 2 equatorward of the growth phase electrojets further showing the near earth proximity of expansive phase onset no indication of reconnection on closed field lines prior to expansive phase onset was observed in either the magnetometer or the photometer data

on relative timing in substorm onset signatures we investigate the timing of popular substorm onset signatures to understand their temporal relationship proxies for substorm onsets include auroral breakups high latitude magnetic bays low latitude pi2 bursts dispersionless injections at geostationary orbits and auroral kilometric radiation we use the auroral breakup identified with polar uvi images as a common reference time frame to calibrate the others results illustrated by two well defined auroral substorms unambiguously indicate that none of the four frequently used substorm onset proxies can provide a consistent timing of substorm onset this inconsistency in substorm onset timing is attributed as a consequence of temporal and spatial limitations on each observational technique a delay between the proxy identifiers and the auroral breakup is found to be typical it is therefore strongly suggested from this study result that a common reference time frame for substorm onset is necessary and we propose it should be auroral breakups we argue that there is a need for an intercalibration of magnetospheric substorm phenomenology by using a unified definition of the substorm onset

themis observations of substorms on 26 february 2008 initiated by magnetotail reconnection 1 on 26 february 2008 the themis satellites observed two substorms that occurred at about 0405 and 0455 ut angelopoulos et al 2008 made a comprehensive study of the second event in this paper we display detailed features of the two substorms with emphasis on the first in both substorms a distinct auroral intensification occurred during the earliest stage of onset about 1 to 2 min after midtail reconnection began this initial intensification was weak and localized and thus had the signatures of a pseudobreakup in both substorms a second major intensification occurred next in the substorm onset sequence followed by rapid and extensive poleward expansion this second intensification had the features of the major expansion onset and was nearly coincident with observations of earthward flows and magnetic dipolarization in the near earth tail during the growth phase of the two substorms open magnetic flux accumulated in the polar cap in the expansion recovery phase the polar cap open flux was quickly reduced these observations are in agreement with the assertion that tail reconnection initiates the initial pseudobreakup and the ensuing major expansion and releases and transports energy to eventually cause near earth dipolarization and the expansion phase onset of these two substorms

identification of substorm expansive phase onsets 1 over the years many different particle and electromagnetic signatures have been invoked to identify the onset of what is called a substorm in this paper i shall first clarify what is generally meant by the term substorm onset and i shall show that some disturbances identified as substorm onsets should be treated with care i shall review the substorm in terms of its two recognized characteristic components directly driven and storage release i shall then discuss the importance of pseudobreakups in deciding what constitutes a legitimate substorm onset and in doing so will illustrate the subjective nature of present substorm onset identification techniques i shall stress that substorm like disturbances can take place both near the equatorward edge of the midnight sector oval mapping to the near earth magnetotail inside 12 re and near the poleward edge of the evening sector oval mapping to regions much farther downtail i shall present case studies that illustrate that some poleward edge disturbances have magnetic and auroral signatures that may be incorrectly interpreted as legitimate expansive phase onsets on the basis of this study i suggest that what may appear to be a lengthy series of closely spaced expansive phases may in fact be a single expansive phase followed by an arbitrarily long period of poleward border activity

substorms fundamental observational features distinction from other disturbances and external triggering fundamental observational signatures of substorms and other auroral zone disturbances are summarized in this review it is shown that at least three distinct types of major geomagnetic disturbance have been identified in the auroral zones and it is suggested that care should be taken in distinguishing substorms from the other types of disturbance when reaching observational conclusions concerning substorms substorms can be uniquely identified by the formation of the current wedge and by the associated westward electrojet and region of active aurora that move poleward within the ionosphere during the expansion phase periods of enhanced convection have been observed to continue without substorms for as long as the interplanetary magnetic field imf remains stably southward they can have al signatures that look very much like intense substorms but these signatures result from strong dp 2 ionospheric currents driven by enhanced convection rather than from a nightside westward electrojet a third type of disturbance is characterized by short intervals of enhanced equatorward flow within the ionosphere and enhanced auroral emissions these enhancements move equatorward from near the polar cap boundary and they may be associated with several minute long enhancements of earthward flow within the plasma sheet studies that have examined the triggering of substorm expansions by imf changes are also reviewed when considered together and substorms are distinguished from the other disturbances these studies imply that most and perhaps all expansions are triggered by imf changes these changes include both northward turnings and reductions in the magnitude of the y component if most substorms are indeed externally triggered as suggested here then substorms must generally not be the result of an internal magnetospheric instability

near earth initiation of a terrestrial substorm 1 despite the characterization of the auroral substorm more than 40 years ago controversy still surrounds the processes triggering substorm onset initiation that stretching of the earth s magnetotail following the addition of new nightside magnetic flux from dayside reconnection powers the substorm is well understood the trigger for explosive energy release at substorm expansion phase onset is not using ground based data sets with unprecedented combined spatial and temporal coverage we report the discovery of new localized and contemporaneous magnetic wave and small azimuthal scale auroral signature of substorm onset these local auroral arc undulations and magnetic field signatures rapidly evolve on second time scales for several minutes in advance of the release of the auroral surge we also present evidence from a conjugate geosynchronous satellite of the concurrent magnetic onset in space as the onset of magnetic pulsations in the ionosphere to within technique error throughout this time period the more poleward arcs that correspond to the auroral oval which maps to the central plasma sheet remain undisturbed there is good evidence that flows from the midtail crossing the plasma sheet can generate north south auroral structures yet no such auroral forms are seen in this event our observations present a severe challenge to the standard hypothesis that magnetic reconnection in stretched magnetotail fields triggers onset indicating substorm expansion phase initiation occurs on field lines that are close to the earth as bounded by observations at geosynchronous orbit and in the conjugate ionosphere

the role of forcing in cell morphology and evolution within midlatitude squall lines abstract this study assesses the role of mesoscale forcing on cell morphology and early evolution of midlatitude squall lines the forcing chosen was a cold front simulated to frontal collapse to produce a specific set of thermodynamic profiles at the leading edge of the front use of a realistic balanced and persistent forced state allowed a unique evaluation of its importance in thunderstorm evolution compared with a traditional homogeneous environment without forcing three dimensional squall lines were modeled with and without the front present in low and high bulk richardson number environments the forced convection evolved in significantly different ways than their isolated unforced counterparts in low shear conditions the line of isolated convective cells split with the adjacent split cells interfering destructively with neighboring cells in the line with forcing present differences in anticyclonic cell intensity and propagation prevented this interaction from occurring leading to longe

impact of the alluvial style on the geoarcheology of stream valleys abstract the extensive archeological record within river valleys provides an environment long favored for study by the archeologic community this record of human occupation varies with the alluvial style of the river resulting in each river valley having a unique archeological record controlled by three factors location of occupation sites preservation of those sites and recognition of the sites if problems of site preservation and recognition in a river valley can be recognized and addressed a better insight into settlement patterns and complex human environment interactions can result this study represents a synthesis of extensive published and secondary data sets pertaining to archaeology and alluvial geomorphology of the lower missouri red and mississippi rivers to examine the relationship of prehistoric site distribution with fluvial geomorphology the archeologic data are from published literature and government agencies the geomorphic data are from a variety of published and unpublished investigations including maps of alluvial geomorphology digital elevation models aerial photographs and topographic maps the data were input into spreadsheets and a gis for qualitative and quantitative analysis prehistoric human occupation of the narrow missouri valley floor is limited with only 0 061 recorded sites km2 additional sites are likely buried by thick vertical accretion sediment on the limited prehistoric portions of the floodplain and within alluvial colluvial fans along the valley margin non random preservation patterns such as the absence of sites from the youngest cultural period are attributed to shifts in settlement patterns in comparison to the lower missouri river valley prehistoric occupation of the lower red and mississippi rivers was much more extensive and the density of occupation was 0 274 sites km2 and 0 277 sites km2 respectively significant differences do exist in the age of sites and their relationship to different floodplain depositional environments many sites along the active meander belt of the red river were likely lost due to high rates of channel migration nevertheless intense occupation of paleo indian through caddoan groups assures that many significant sites have been preserved the wide valley and high rates of sedimentation suggest that many sites are likely buried particularly along natural levees flanking an older meander belt recorded sites are also present along streams that incise the terrace along the terrace margin and along small channels that cross the backswamp the spatial and temporal site distribution along the lower mississippi valley is distinct in comparison with the red river because the mississippi valley is wider the terraces are younger and less dissected and the floodplain has more relief and is not broadly inundated by floodwater with relatively low sediment concentrations most of the significant archaic through mississippian period floodplain sites are located along the surfaces of abandoned mississippi and distributary meander belts which aggraded above the adjacent backswamp and have rarely been influenced by flood sedimentation since abandonment abundant paleo indian through mississippian period sites are also widely distributed across undissected terraces only a few significant mississippian sites are present along the active meander belt and in the backswamp where flooding was common in comparison with the red river the archaeological record of the floodplain is less likely to be influenced by burial or erosion due to the mississippi river having lower rates of overbank sedimentation and channel migration respectively the variable alluvial styles of the lower missouri river lower red river and the lower mississippi river valleys presented varying opportunities and constraints to occupation and sustained settlement and resulted in widely varying archaeological patterns that are partly dependent upon the alluvial geomorphology

westward traveling surges sliding along boundary arcs and distinction from onset arc brightening 1 after onset auroral brightening rapidly spreads azimuthally along the onset arc typically but not always extending substantially farther westward than eastward expansion phase auroras also expand poleward and can contact an arc along the auroral poleward boundary for such events we find that poleward expansion of the region of active aurora accelerates after contact forming a bulge region that protrudes into the preexisting polar cap and expands westward as the westward traveling surge this westward motion is slower than and initiates later than the azimuthal spreading of the onset arc brightening the poleward boundary arc remains connected to the surge as the surge slides along the equatorward moving edge of the boundary arc and this connection appears to set off the westward traveling poleward protruding activity of the surge via localized polar cap flow adjacent to the boundary arc ground magnetic signatures show a relatively weak h component depression moving westward with the onset arc brightening and a delayed substantially larger depression moving with the westward propagating surge that must be associated with a substantially larger westward ionospheric current the clear distinction between onset arc brightening and the westward traveling surge seen in auroral and magnetometer current signatures implies that their underlying processes are very different evidence suggests that flow channels adjacent to the boundary arc possibly feed surge development these results are consistent with current wedge formation being more associated with post onset activity than with onset and with flow channels from the polar cap being important for surge bulge development

comment on the importance of auroral features in the search for substorm onset process by syun ichi akasofu a t y lui and c i meng

automated determination of auroral breakup during the substorm expansion phase using all sky imager data this paper describes a novel method for quantitatively and routinely identifying auroral breakup following substorm onset using the time history of events and macroscale interactions during substorms all sky imagers substorm onset is characterized by a brightening of the aurora that is followed by auroral poleward expansion and auroral breakup this breakup can be identified by a sharp increase in the auroral intensity i t and the time derivative of auroral intensity i t utilizing both i t and i t we have developed an algorithm for identifying the time interval and spatial location of auroral breakup during the substorm expansion phase based solely on quantifiable characteristics of the optical auroral emissions we compare the time interval determined by the algorithm to independently identified auroral onset times from three previously published studies in each case the time interval determined by the algorithm is within error of the onset independently identified by the prior studies we further show the utility of the algorithm by comparing the breakup intervals determined using the automated algorithm to an independent list of substorm onset times we demonstrate that 50 of the breakup intervals characterized by the algorithm are within the uncertainty of the times identified in the list the quantitative description and routine identification of an interval of auroral brightening during the substorm expansion phase provides a foundation for unbiased statistical analysis of the aurora and to probe the physics of the auroral substorm and identify the processes leading to auroral substorm onset

auroral fragmentation into patches auroral patches in diffuse auroras are very common features in the postmidnight local time however the processes that produce auroral patches are not yet well understood in this paper we present two examples of auroral fragmentation which is the process by which uniform aurora is broken into several fragments to form auroral patches these examples were observed at athabasca canada geomagnetic latitude 61 7 n and tromso norway 67 1 n captured in sequences of images the auroral fragmentation occurs as finger like structures developing latitudinally with horizontal scale sizes of 40 100 km at ionospheric altitudes the structures tend to develop in a north south direction with speeds of 150 420 m s without any shearing motion suggesting that pressure driven instability in the balance between the earthward magnetic tension force and the tailward pressure gradient force in the magnetosphere is the main driving force of the auroral fragmentation therefore these observations indicate that auroral fragmentation associated with pressure driven instability is a process that creates auroral patches the observed slow eastward drift of aurora during the auroral fragmentation suggests that fragmentation occurs in low energy ambient plasma

simultaneous themis in situ and auroral observations of a small substorm 1 we present ground based and in situ observations from march 13 2007 the themis satellites were in the evening sector conjugate to themis ground based imagers at 0507 ut there was an optical onset on inner cps field lines this involved near simultaneous brightening of 1 mlt hour longitudinal segment of the onset arc the part of the arc that brightened was that closest to the equatorward boundary of the diffuse proton aurora within one minute a dipolarization front moved across four themis satellites based on their locations the order in which they detected the dipolarization front and the auroral evolution we assert that the expansion phase began earthward of the four satellites and evolved radially outwards we conclude that this onset occurred in an azimuthally localized region of highly stretched field lines

the response of the dayside equatorial electrojet to step like changes of imf bz 1 the equatorial electrojet eej is driven by zonal electric fields which are known to be well correlated with the interplanetary electric field and therefore with the interplanetary magnetic field imf bz component in the present study we investigate how the equatorial h magnetic component and therefore the eej responds to step like changes of imf bz the reduction of southward imf bz northward turning and that of northward imf bz southward turning are examined separately the result shows that for the northward turnings the eej immediately starts to weaken with the accuracy of the estimates of the travel times of the imf changes the time constant of the response is much longer and the equatorial h component decreases continuously by 40 nt for 30 min after the northward turnings in contrast the response of the eej to the southward turnings is far less clear in both magnitude and timing and it does not depend on whether or not imf bz actually becomes southward the difference in the eej response to the northward and southward turnings reflects at least partially the fact that the magnetosphere ionosphere system is more sensitive to imf bz when imf is southward than northward it is suggested that the electric field penetrates from the polar region to the dip equator through a global current system that connects the auroral electrojets and the eej and the ionospheric conductance in the polar region may play an important role in the formation of such a current system

substorm triggering by new plasma intrusion themis all sky imager observations 1 a critical long standing problem in substorm research is identification of the sequence of events leading to substorm auroral onset based on event and statistical analysis of themis all sky imager data we show that there is a distinct and repeatable sequence of events leading to onset the sequence having similarities to and important differences from previous ideas the sequence is initiated by a poleward boundary intensification pbi and followed by a north south n s arc moving equatorward toward the onset latitude because of the linkage of fast magnetotail flows to pbis and to n s auroras the results indicate that onset is preceded by enhanced earthward plasma flows associated with enhanced reconnection near the pre existing open closed field line boundary the flows carry new plasma from the open field line region to the plasma sheet the auroral observations indicate that earthward transport of the new plasma leads to a near earth instability and auroral breakup 5 5 min after pbi formation our observations also indicate the importance of region 2 magnetosphere ionosphere electrodynamic coupling which may play an important role in the motion of pre onset auroral forms and determining the local times of onsets furthermore we find motion of the pre onset auroral forms around the harang reversal and along the growth phase arc reflecting a well developed region 2 current system within the duskside convection cell and also a high probability of diffuse appearing aurora occurrence near the onset latitude indicating high plasma pressure along these inner plasma sheet field lines which would drive large region 2 currents

moist convection at a surface cold front abstract the motion of a surface cold front in an environment that is unstable to moist convection is studied with the aid ofboth hydrostatic and nonhydrostatic two dimensional models simulations with the hydrostatic model essentially extend the work reported by ross and orlanski it is shown that when deep convection occurs coriolis turning of the flow into the convective line creates a poleward low level jet ahead of the front it is also shown that after the generation and decay of the first convective element another line develops on the order of a day later it is found that the intensity of this line increases significantly ira north south gradient of moisture is specified the periodicity in convective activity at the front is explained in terms of an inertial gravity oscillation in the low level convergence the first convective system which decays when the subeloud layer is dried out by the convection forces a geostrophic imbalance in the surface front and the surrounding environment in ret

substorm onset location and the equatorward boundary of the proton auroral oval 1 an examination of the substorm onset location inferred from polar uvi and the location of the equatorward boundary of the proton auroral oval inferred from ground based superdarn radars is presented a study of 96 individual substorm events reveals that the substorms can be initiated either near the equatorward boundary of the proton auroral oval or far poleward of the equatorward boundary of the proton auroral oval depending on the preceding interplanetary conditions when the interplanetary magnetic field imf is predominantly southward prior to the substorm onset the onset location is near the equatorward boundary of the proton auoral oval when the imf is predominantly northward prior to the onset the onset location is far poleward of the equatorward boundary of the proton auroral oval the latitudinal separation between the onset location and the equatorward boundary of the proton auroral oval shows a linear dependence on the imf

relation of substorm pre onset arc to large scale field aligned current distribution 1 substorm auroral onset often occurs along a pre existing arc that is associated with an enhanced upward field aligned current fac to determine the location of the upward fac of the pre onset arc relative to the large scale region 1 r1 and 2 r2 facs we present coordinated observations of the themis all sky imagers and champ satellite where champ passed over relatively stable pre onset arcs within the 30 min period before substorm auroral onset we found that the upward fac corresponding to the pre onset arc is latitudinally separated from the upward r1 fac within the onset mlt range in contrast the arc is located near the r1 r2 boundary to the west and east of the onset mlt range this mlt dependence suggests that an east west oriented upward fac sheet forms within the downward r2 fac as a precursor fac signature for substorm auroral onset

importance of auroral features in the search for substorm onset processes 1 auroral features at about the onset time of substorms are revisited to emphasize their importance in considering substorm onset processes this study is based on all sky camera photographs and meridian scanning photometer records from a single station first in considering substorm onset processes it is crucial to pay attention to the implication of the fact that an auroral arc located just poleward side of the initially brightening arc becomes active only after not before onset second prior to substorm onset there occurs a rapid equatorward shift or more like narrowing the width of the equatorward half of the oval not the whole oval without any distinct and immediate changes of the interplanetary magnetic field imf bz component except that the shift tends to occur after the southward turning of the imf these phenomena suggest that some processes take place spontaneously at a distance of less than 10 re perhaps near the boundary between the central plasma sheet and the boundary plasma sheet after the magnetosphere is primed by the imf southward turning thus it is concluded that substorm models in the near earth initiation category satisfy the observational constraints provided by the present paper because of our limited data set in terms of observations at single stations new ground based observations are suggested to confirm our results

in situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge the substorm current wedge scw is a fundamental component of geomagnetic substorms models tend to describe the scw as a simple line current flowing into the ionosphere toward dawn and out of the ionosphere toward dusk linked by a westward electrojet we use multispacecraft observations from perigee passes of the cluster 1 and 4 spacecraft during a substorm on 15 january 2010 in conjunction with ground based observations to examine the spatial structuring and temporal variability of the scw at this time the spacecraft traveled east west azimuthally above the auroral region we show that the scw has significant azimuthal substructure on scales of 100 km at altitudes of 4000 7000 km we identify 26 individual current sheets in the cluster 4 data and 34 individual current sheets in the cluster 1 data with cluster 1 passing through the scw 120 240 s after cluster 4 at 1300 2000 km higher altitude both spacecraft observed large scale regions of net upward and downward field aligned current consistent with the large scale characteristics of the scw although sheets of oppositely directed currents were observed within both regions we show that the majority of these current sheets were closely aligned to a north south direction in contrast to the expected east west orientation of the preonset aurora comparing our results with observations of the field aligned current associated with bursty bulk flows bbfs we conclude that significant questions remain for the explanation of scw structuring by bbf driven wedgelets our results therefore represent constraints on future modeling and theoretical frameworks on the generation of the scw

mid 1980s distribution of tritium 3he 14c and 39ar in the greenland norwegian seas and the nansen basin of the arctic ocean abstract the distributions of tritium 3 he 14c and 39ar observed in the period between 1985 and 1987 in the greenland norwegian seas and the nansen basin of the arctic ocean are presented the data are used to outline aspects of the large scale circulation and the exchange of deep water between the greenland norwegian seas and the nansen basin additionally semi quantitative estimates of mean ages of the main water masses found in these regions are obtained apparent tritium 3 he ages of the upper waters depth 1 500m depth of the greenland norwegian seas show apparent tritium 3 he ages between about 17 years in the greenland sea and 30 years in the norwegian sea 39ar based estimates of the nansen basin intermediate deep and bottom water ages are 91 26 23 161 50 44 and 277 33 31 years for arctic intermediate water aiw eurasian basin deep water ebdw and eurasian basin bottom water ebbw respectively within the errors age estimates of ebdw and ebbw based on 14 c tritium correlations are consistent with those derived from 39ar 163 to 287 years for ebdw and 244 to 368 years for ebbw a quantitative evaluation of the data in terms of deep water formation and exchange rates based on box model calculations is presented in an accompanying paper

substorm associated changes in large scale convection during the november 24 1996 geospace environment modeling event we have used data from the november 24 1996 geospace environment modeling program substorm event to investigate the critical question of substorm expansion phase triggering by interplanetary magnetic field imf changes that lead to a reduction in large scale convection coverage of ionospheric convection with the super dual auroral radar network superdarn radars was excellent during this period this has allowed us to investigate the changes in large scale convection associated with two expansion phase onsets that followed a 80 min period of strongly southward and quite steady imf we have found that reductions in global convection occurred in association with two observed northward turnings of the imf and to within observation uncertainty these reductions occurred at the same times that the two onsets were first observed in the magnetosphere the first smaller northward turning was found to give a small convection reduction and the onset of a small substorm while the second much larger northward turning was found to give a large reduction in global convection and the onset of a large substorm taken together with other more localized studies which also show convection reductions associated with expansion onsets our results imply that imf driven reductions in global convection may be generally associated with expansion onset our results also suggest that substorm expansions may represent an adjustment to conditions of reduced convection following a growth phase of enhanced convection and magnetotail energy storage under this suggestion for a given amount of energy stored in the tail a small reduction in convection would lead to a small substorm and a large reduction in convection would lead to a large substorm

on the place of the pseudo breakup in a magnetospheric substorm it is common to view the expansive phase of a magnetospheric substorm as a large amplitude disturbance as a result any magnetic disturbance that has the normal characteristics of an expansive phase onset but has an amplitude below some subjective limit is not classed as an expansive phase onset but is labelled instead as a pseudo breakup in this paper it will be shown that a pseudo breakup is in fact a disturbance which represents the same physical process as the onset of an expansive phase or any of the subsequent intensifications examination of simultaneous interplanetary magnetic field data suggests the possibility that a pseudo breakup is distinguished from the onset or intensifications of an expansive phase by its occurrence during a period of increasing energy input into the magnetosphere thus the pseudo breakup is not accompanied by a large scale poleward motion of the high latitude edge of the auroral oval in the substorm disturbed region characteristic of an expansive phase

earthward plasma sheet flows during substorm phases 1 magnetic reconnection in the earth s magnetotail converts open magnetic flux to closed producing fast flows in the plasma sheet earthward fast flows can thus be used as a proxy for energy and momentum transfer into the closed magnetosphere during the course of a substorm characteristics of flows observed before substorm onset can provide information on the role of the flows in substorm onset triggering as well as the sequence of substorm related processes we use 15 years of data obtained by geotail cluster and themis to study statistically the distribution of earthward plasma sheet flows during substorms substorm phases are determined from time series of the al index and the z component of the interplanetary magnetic field we find that the occurrence frequency of medium speed 100 km s 500 km s flows on the other hand does not clearly increase during the substorm growth phase the increase at substorm onset is more gradual than for the medium speed flows the occurrence frequency of high speed flows peaks at the beginning of substorm recovery these results indicate that any effect of the flows on substorm onset triggering would be caused by medium rather than very high speed flows

wavelet based ulf wave diagnosis of substorm expansion phase onset 1 using a discrete wavelet transform with a meyer wavelet basis we present a new quantitative algorithm for determining the onset time of pi1 and pi2 ulf waves in the nightside ionosphere with 20 to 40 s resolution at substorm expansion phase onset we validate the algorithm by comparing both the ulf wave onset time and location to the optical onset determined by the imager for magnetopause to aurora global exploration image far ultraviolet imager fuv instrument in each of the six events analyzed five substorm onsets and one pseudobreakup the ulf onset is observed prior to the global optical onset observed by image at a station closely conjugate to the optical onset the observed ulf onset times expand both latitudinally and longitudinally away from an epicenter of ulf wave power in the ionosphere we further discuss the utility of the algorithm for diagnosing pseudobreakups and the relationship of the ulf onset epicenter to the meridians of elements of the substorm current wedge the importance of the technique for establishing the causal sequence of events at substorm onset especially in support of the multisatellite time history of events and macroscale interactions during substorms themis mission is also described

dynamics of the substorm growth phase as observed using canopus and superdarn instruments the substorm growth phase in four substorm events was studied using ground based observations from the canadian open unified study canopus array of meridian scanning photometers and magnetometers the gillam all sky camera and two canadian super dual auroral radar network superdarn radars the following common features were revealed a proton aurora that has a latitudinal width of 2 moves equatorward remaining near the equatorward boundary of the intense electron precipitation zone the proton aurora intensifies during the growth phase toward the onset of the substorm intensification the latitudinal width of the electron precipitation region narrows during the growth phase reaching 2 5 of latitudinal width just before the substorm intensification which indicates significant stretching of magnetic field lines and thinning of the plasma sheet at the beginning of the growth phase convection is intensified near the proton aurora band forming a flow with a large amplitude westward component this enhanced westward component of the convection is in agreement with magnetometer observations of the increased eastward electrojet in this region the maximum of the flow moves equatorward following the proton aurora band prior to substorm intensification the convective flow develops significant latitudinal gradients all sky images available for one event indicate that just before a substorm intensification a large auroral vortex develops from an auroral arc imbedded in the proton aurora region and then intensifies these observational constraints are discussed with respect to a possible mechanism which can lead to a large scale auroral vortex at substorm intensification

in situ observations of the preexisting auroral arc by themis all sky imagers and the fast spacecraft 1 auroral substorms were first described more than 40 years ago and their atmospheric and magnetospheric signatures have been investigated extensively however because magnetic mapping from the ionosphere to the equator is uncertain especially during active times the magnetospheric source regions of the substorm associated features in the upper atmosphere remain poorly understood in optical images auroral substorms always involve brightening followed by poleward expansion of a discrete auroral arc the arc that brightens is usually the most equatorward of several auroral arcs that remain quiescent for 30 min or more before the break up commences in order to identify the magnetospheric region that is magnetically conjugate to this preexisting arc we combine auroral images from ground based imagers magnetic field and particle data from low altitude spacecraft and maps of field aligned currents based on ground magnetometer arrays we surveyed data from the themis all sky imager asi array and the fast spacecraft from 2007 to april 2009 and obtained 5 events in which the low altitude fast spacecraft crossed magnetic flux tubes linked to a preexisting auroral arc imaged by themis asi prior to substorm onset the observations show that in each of the five cases 1 the precipitating electrons associated with the preexisting arc are accelerated by a field aligned potential drop with characteristic energy ranging from a few hundred ev to a few kev 2 the preexisting arc is located 1 2 poleward of the equatorward edge of the 1 kev electron plasma sheet in the ionosphere and it maps to equatorial locations within the electron plasma sheet and tailward of its inner edge 3 the preexisting arc is located at or very near the boundary between the region 1 and region 2 field aligned currents the localization relative to the region 1 region 2 current is confirmed by comparison with maps of field aligned currents inferred from ground magnetometer data

predictions of substorms following northward turnings of the interplanetary magnetic field substorms are often observed to occur when the interplanetary magnetic field imf turns northward at the end of intervals of southward imf despite the significant correlation between northward turnings and substorms a direct causal relationship between northward turnings and substorms has not been demonstrated if such a causal relationship exists then we should be able to predict that substorms will occur within a particular interval following the observation of a northward turning in the imf we have searched wind imf data and identified 16 strong northward turnings that following intervals of steady southward imf to enhance the chances that the selected northward turnings impacted the magnetosphere we required that each of them be also observed at either geotail or imp 8 while the separation of the second spacecraft from wind was more that 10 re in the y direction additionally we required that either the second satellite was on the opposite side of the y 0 plane from wind or that one of the satellites was near y 0 these two spacecraft observations also allowed us to more accurately predict the arrival times of the northward turnings at the earth than we could by using data only from one spacecraft of the predicted substorms 10 predictions were clearly successful to within 8 min five predictions failed but these failures reveal important new criteria that must be satisfied for a northward turning to cause a substorm three of the failures were caused by an increase in bygsm that occurred simultaneously with the northward turning this suggests that the triggering is associated with sharp reductions in the magnetospheric electric field since increases in bygsm negate the convection reduction associated with northward turnings two of the failures occurred for cases with the weakest southward imf preceding the northward turning which improves our criteria for the strength of the southward imf that must precede a substorm the final northward turning resulted in unusual auroral activity these results establish the predictability of substorms following sharp northward turnings of the imf and demonstrate that a direct causal relationship between northward turnings and substorm onset exists

substorm associated changes in the high latitude ionospheric convection 1 a study of polar cap dayside convection response to substorm intervals using canadian advanced digital ionosondes cadis situated well within the polar cap and superdarn radars during steady and prolonged southward imf bz conditions showed three distinct features 1 gradual prolonged decrease of dayside polar cap convection speed until the substorm onset 2 sudden decrease of convection following the onset of the substorm and 3 increase of convection during the recovery phase of the substorm we hypothesize that the observed features of the convection are due to the modulation of the region 1 current system associated with the substorm the ground magnetic response of magnetometers situated inside the polar cap is at least consistent with our postulate

airborne observations of a front near a col during fastex abstract an analysis of an oceanic front situated near a col defined by the surface pressure field is presented there have been few observational examples of this type of front presented in the literature the primary source of information for this study was data recorded by an aircraft equipped with a doppler radar the front was approximately two dimensional and the cross frontal scale at low levels was 30 40 km a prefrontal low level jet was identified in the high resolution analyses and was shown to be supergeostrophic surface pressure measurements and the horizontal temperature gradients were used to calculate the geostrophic wind and the thermal wind imbalance twi in the alongfront direction large negative values of twi the vertical shear is less than predicted for the given horizontal temperature gradient were located near a region of frontogenesis the strong ageostrophic component of the wind parallel to the front suggests that the alongfrontal component of the wind may not have been in g

enhanced stage and stage variability on the lower missouri river benchmarked by lewis and clark data from the 1803 1806 lewis and clark expedition and nineteenth century stage records are used to quantitatively benchmark natural premanagement hydrology of the lower missouri river and assess the magnitude and timing of hydrologic change over nearly two centuries data show doubling in daily stage variability from the nineteenth century to 2005 annual maximum stages have at some sites become more extreme and their seasonality is less regular observed changes adversely affect riverine habitat and flood levels their timing beginning as early as 1900 suggests that channelization is the major driver

intensification of preexisting auroral arc at substorm expansion phase onset wave like disruption during the first tens of seconds 1 with the deployment of the all sky imager array of the themis mission we were able to construct a preliminary database of auroral substorm expansion phase onsets from which we have established a number of common features characterizing the first tens of seconds of the substorm auroral intensification we find that the intensification occurs within 10 sec over an arc segment extending approximately 1 h mlt and featuring wave like formations distributed in longitude the longitudinal wave number ranges between 100 and 300 such that the wavelength is comparable to the ion gyroradius in the central plasma sheet the scale the intensification is about 10 30 sec this study casts important observational constraints on substorm onset theories

flow and form in rehabilitation of large river ecosystems an example from the lower missouri river abstract on large intensively engineered rivers like the lower missouri the template of the physical habitat is determined by the nearly independent interaction of channel form and flow regime we evaluated the interaction between flow and form by modeling four combinations of modern and historical channel form and modern and historical flow regimes the analysis used shallow slow water shallow water habitat swh defined as depths between 0 and 1 5 m and current velocities between 0 and 0 75 m s as an indicator of habitat that has been lost on many intensively engineered rivers and one that is thought to be especially important in rearing of young fishes two dimensional hydrodynamic models for modern and historical channels of the lower missouri river at hermann missouri indicate substantial differences between the two channels in total availability and spatial characteristics of swh in the modern channel swh is maximized at extremely low flows and in overbank flows whereas the historical channel had substantially more swh at all discharges and swh increased with increasing discharge the historical channel form produced 3 7 times the swh area of the modern channel regardless of flow regime the effect of flow regime is evident in increased within year swh variability with the natural flow regime including significant seasonal peaks of swh associated with spring flooding comparison with other reaches along the lower missouri river indicates that a channel form is the dominant control of the availability of habitat even in reaches where the hydrograph is more intensively altered and b rehabilitation projects that move toward the historical condition can be successful in increasing topographic diversity and thereby decreasing sensitivity of the availability of habitat to flow regime the relative efficacy of managing flow and form in creating swh is useful information toward achieving socially acceptable rehabilitation of the ecosystem in large river systems

distinction between auroral substorm onset and traditional ground magnetic onset signatures 1 to evaluate whether ground magnetic responses traditionally viewed as signatures of substorm onset may be related to postonset auroral streamers rather than to substorm onset seen in aurora we have analyzed ground magnetic and all sky image responses for 14 substorm onsets selected solely on imager coverage we find evidence supporting this possibility the response being approximately coincident for midlatitude positive bay and pi2 pulsation enhancements and for abrupt auroral zone h decreases at stations near a streamer these are all signatures of current wedge formation indicating that substorm current wedge formation may be more of a response to plasma sheet flow channels associated with postonset streamers than to the process leading to auroral onset and that the current wedge may develop via a series of distinctly narrower wedge like structures we also find evidence that periods of streamers can occur more than once during a substorm expansion phase and can give magnetic signatures of multiple onsets even if there are no additional auroral onsets furthermore the peak auroral zone h decreases are seen in association with streamers and at times varying from just a few minutes to well over a half hour after substorm auroral onset if there is a prolonged period of streamers this indicates that peak substorm ae and al values are likely related to features of postonset streamers and their locations relative to observing stations and are not a measure of the strength of the substorm auroral onset processes we discuss possible additional implications on substorm expansion phase development

multipoint study of a substorm on february 9 1995 an extended interval of strong northward interplanetary magnetic field imf was observed by the wind spacecraft located at an upstream distance of 193 re from february 8 10 1995 with a brief break of southward imf from 0200 to 0400 ut on february 9 this brief interval of southward imf led to an isolated substorm of moderate intensity 500 nt with expansion phase starting at 0431 ut this substorm may be triggered by the northward turning of the imf since its onset time matched well with the time expected for the arrival of the northward turning of the imf at earth the substorm activities were monitored by 11 spacecraft in space wind imp 8 geotail six geosynchronous satellites one dmsp satellite and freja and two networks of ground stations canopus and superdarn covering both the northern and southern hemispheres the extensive coverage of this event provides us with results 1 showing some unusual characteristics possibly related to the isolated nature of the substorm and 2 revealing some surprising features difficult to reconcile with the traditional substorm model in the first category is unusually long duration of the growth phase and the long time delay between substorm expansion onset and particle injection onset at the geosynchronous orbit in the second category is new evidence for multiple particle acceleration sites during substorm expansion and for sunward flow during the late expansion phase of a substorm being unrelated to a single acceleration site x line moving from the near earth tail to the more distant tail we also present observations which show the possible optical signature on the ground of bursty bulk flows in the magnetotail

determination of substorm onset timing and location using the themis ground based observatories 1 the nasa themis mission is studying substorms by timing the substorm signatures at multiple satellite locations in the magnetosphere and in the aurora using 20 ground based observatories gbo s the time resolution requirement is 10 sec the gbo s provide a near contiguous array over north america each contains an all sky imager 3 s exposure cadence and a magnetometer with 2 hz sampling rate in one example substorm the onset brightening of the equatorward arc was a gradual process 27 seconds with minimal morphology changes until the arc break up the break up was timed to the nearest frame 3 sec and occurred at 58 n latitude and 256 3 e longitude geographic 67 n magnetic latitude 22 1 hours mlt the brightening of the arc was accompanied by a slow increase of the westward electrojet but this was too gradual for accurate timing of the event high pass filtered magnetic data showed some wave activity but with significant delay 40 sec after the arc break up similar break up occurred in alaska 10 minutes later highlighting the need for an array to distinguish prime onset

global and local equatorward expansion of the ion auroral oval before substorm onsets 1 the temporal variation of the equatorward boundary of the proton aurora high energy ion precipitation is a manifestation of diurnal and seasonal i e dipole tilt effects as well as magnetic activity in particular during the substorm growth phase this boundary moves equatorward an effect due primarily to thinning and earthward motion of the cross tail current in the inner magnetosphere as the field evolves toward a more stretched topology recent advances in monitoring this boundary using ground based instruments have opened up the possibility of following its temporal evolution across several hours in local time this in turn allows one to explore whether this magnetotail stretching is a global or local phenomenon we have examined this boundary evolution during the growth phases of 68 substorms over the canadian sector we use the equatorward boundary of superdarn e region echoes as a proxy for the proton auroral boundary as described by jayachandran et al 2002b we find that in 21 of the 68 substorms the equatorward motion of the auroral boundary is restricted to several hours of local time in the evening sector in the remaining 47 substorms the equatorward motion was global so that the boundary retained its shape throughout the growth phase our results indicate dramatically different growth phase phenomenology in these two classes of substorms in one the growth phase involves stretching in the inner magnetosphere that is most pronounced around the onset meridian in the other the stretching extends many hours in local time away from the onset meridian

large flow shears around auroral beads at substorm onset 1 we present a fine scale electrodynamic structure behind the auroral beads observed immediately before substorm onset as inferred from high spatial and temporal resolution measurements of substorm with an all sky tv camera atv and a coherent high frequency radar of super dual auroral radar network in iceland on the night of 24 september 2006 the atv observed eastward propagating auroral beads in the initial brightening arc of a substorm just prior to the poleward expansion during the sequential passage of the beads across the radar beams the radar detected large velocity flow shears whose magnitude was in excess of 0 27 s 1 the observations suggest that flow shears were located very close to the center of the beads thus they corresponded to electric fields converging toward the beads which is consistent with the existence of upward field aligned currents facs flowing out of the beads the temporal and spatial resolutions of the current radar measurement were still insufficient for fully resolving the detailed electrodynamic structure behind the fast moving auroral beads at least however we can suggest the existence of highly localized filamentary fac structures behind the beads which would be an important consequence of possible plasma instabilities operating in the m i coupling region or near the equatorial plane of the magnetosphere

coincidence of the ion precipitation boundary with the hf e region backscatter boundary in the dusk midnight sector of the auroral oval 1 we have examined the location of the superdarn e region ionospheric backscatter boundary in the dusk midnight sector and compared it with the equatorward boundary of the hard ion precipitation b2i determined from the dmsp satellite particle spectrograms in the dusk midnight sector of the auroral oval the locations of these two boundaries were found to be nearly coincident this study evinces the capability of the superdarn radars to monitor the equatorward ion precipitation boundary diffuse auroral boundary and its dynamics irrespective of magnetic activity

revised timing and onset location of two isolated substorms observed by time history of events and macroscale interactions during substorms themis 1 we report timing analysis on two previously published substorm events captured by the time history of events and macroscale interactions during substorms themis spacecraft during the 29 january 2008 0713 ut substorm the solar wind velocity had a strong southward component corresponding to a 6 5 southward tilt of the magnetotail viewed in a rotated system the magnetic field at the most distant probe p1 xgsm 29 5 re shows a bipolar magnetic signature interpreted herein as a tailward moving plasmoid p2 xgsm 18 5 re also observed magnetic signatures indicating tailward motion at onset p3 xgsm 10 8 re and p4 xgsm 10 6 re captured dipolarization fronts and earthward flows at the same time after allowing for the more general case of different magnetosonic speeds on the two sides of the reconnection site timing of the first signatures in space and ground reveals that tail reconnection initiated at 18 re down tail 2 min prior to auroral intensification allowing for different magnetosonic speeds on either side of the reconnection site is warranted by the large separation between the inner 10 12 re and outer 25 30 re probe locations and differing ion temperatures and equatorial magnetic fields expected at those locations the same technique was applied for the 2 february 2008 0740 ut substorm event during which midtail data from p2 were unavailable a previous study obtained a reconnection site location of xgsm 11 17 re assuming the same propagation speed on both sides of the reconnection site relaxation of the constant speed condition results in a reconnection location of 22 re and an inferred reconnection time of 3 4 min before the auroral intensification our results are consistent with other themis event studies that are unaffected by large solar wind deflections or incomplete probe coverage suggesting that reconnection triggering of substorm onset is a common occurrence

near earth plasma sheet azimuthal pressure gradient and associated auroral development soon before substorm onset 1 the azimuthal plasma pressure gradient in the near earth plasma sheet makes crucial contributions to field aligned current fac formation numerical simulations and statistical observations have shown that a plasma pressure peak tends to build up in the premidnight region of the near earth plasma sheet during the substorm growth phase owing to enhanced magnetic drift this leads to azimuthal pressure gradients in this region the temporal variation of the azimuthal pressure gradient may provide an indication for the fac variations associated with the substorm growth phase and may set up a plasma sheet precondition for the substorm onset being triggered near this region we take advantage of two of the time history of events and macroscale interactions during substorms themis spacecraft separated azimuthally near the orbit apogee and investigate the azimuthal plasma pressure gradient before substorm onset in the r 10 12 re region equatorial plasma pressure is estimated by removing the curvature force effect five events with the spacecraft footprints mapped very close to the aurora onset region were selected these events show substantial duskward pressure gradient enhancement 1 5 min before onset the onset arc which results from enhanced energetic electron precipitation was found to intensify simultaneously with the pressure gradient enhancement before onset breakup occurs since the energy and energy flux of precipitating electrons reflect the upward fac strength these results indicate that the duskward azimuthal pressure gradient enhancement is associated with enhanced upward fac during the late growth phase and leads to the intensification of the onset auroral arc soon before it breaks up it is possible that this pressure gradient enhancement may lead to ballooning mode instability and thus substorm onset along the intensifying arc

detailed analysis of a substorm event on 6 and 7 june 1989 1 growth phase evolution of nightside auroral activities and ionospheric convection toward expansion phase onset 1 we have analyzed in detail the evolution of nightside auroral activities and ionospheric convection during the growth phase of an isolated substorm the following three characteristic auroral activities were identified by meridian scanning photometers msp at ground stations 1 proton main oval 2 fast equatorward moving fem arc and 3 near plasma sheet boundary layer npsbl aurora the proton main oval corresponds to the main part of the proton auroral region and was located at the equatorward most region of the electron auroral emissions and gradually moved equatorward during the growth phase the fem arc was a discrete auroral arc having a clear longitudinally elongated form it appeared about 20 min before the onset on the high latitude side of the auroral region and moved equatorward toward the proton main oval its equatorward motion gradually slowed as it approached the proton main oval as the fem arc moved equatorward the location of the peak velocity in the nightside return flow of the two cell convection also moved equatorward the auroral breakup occurred in a localized area around the demarcation region between the two return flows close to the latitude of the fem arc this was observed by the uv imager aboard the akebono satellite the npsbl aurora appeared a few minutes before the onset around the poleward most region of the aurora and continued even after the onset our detailed analysis of these phenomena can give us important clues as to how the growth phase proceeds toward the expansion phase onset

relation of substorm breakup arc to other growth phase auroral arcs 1 high resolution canopus meridian scanning photometer and all sky imager observations of preonset and expansion phase auroral arcs are analyzed for expansion phase onsets that evolve into full substorms and into pseudobreakups one or more arcs are seen across the sky throughout the growth phase prior to onset the onsets we have examined indicate that auroral breakup at onset does not generally occur along one of these arcs but instead often occurs along a thin breakup arc that forms equatorward of all growth phase arcs a few minutes prior to onset the intensity of this breakup arc increases monotonically for the few minutes prior to the time normally identified as substorm onset and then typically increases dramatically these results imply that the processes responsible for auroral breakup initiate a few minutes prior to the time normally identified as substorm expansion phase onset we also find that arcs poleward of the arc that breaks up appear to be unaffected by substorm onset until expansion phase auroral activity moves poleward to the location of such arcs arcs poleward of the poleward most extent of pseudobreakup auroral activity show no apparent effects of a pseudobreakup these results imply that the process that initiates the onset of substorms does not require the occurrence of plasma sheet changes significant enough to affect magnetosphere ionosphere electrodynamics along field lines that cross the equator tailward of the substorm onset region

on the simultaneity of substorm onset between two hemispheres simultaneous observations of auroral kilometric radiation from the northern and southern hemispheres showed some cases in which the buildup of field aligned acceleration occurred only in one hemisphere at the substorm onset this indicates that a substorm does not always complete the current system by connecting the cross tail current with both northern and southern ionospheric currents conjugate auroral observations showed that in one case the auroral breakup in the northern and southern hemispheres was not simultaneous rather they occurred a few minutes apart this time difference in the breakup between two hemispheres suggests that the local auroral ionosphere controls auroral breakup in each hemisphere independently the evidence in this study may indicate that the buildup of the field aligned acceleration region at the auroral breakup does not result only from the magnetospheric process and that the auroral ionosphere finally controls and or ignites the substorm onset that is the auroral breakup

radial expansion of the tail current disruption during substorms a new approach to the substorm onset region the substorm onset region and the radial development of the tail current disruption are examined from a new viewpoint the reconfiguration of the magnetotail field at substorm onset can be understood in terms of a sudden decrease disruption in tail current intensity the north south component bz is very sensitive to whether the spacecraft position is earthward or tailward of the disruption region while the change in the sun earth component bx is most sensitive to the change in tail current intensity near the spacecraft if the current disruption starts in a localized range of radial distance and expands radially a distinctive phase relationship between the changes in bx and bz is expected to be observed this phase relationship depends on whether the current disruption starts on the earthward side or the tailward side of the spacecraft thus it is possible to infer the direction of the radial expansion of the current disruption from magnetic field data of a single spacecraft this method is applied to isee observations of a tail reconfiguration event that occurred on march 6 1979 the phase relationship indicates that the disruption region expanded tailward from the earthward side of the spacecraft during the event this model prediction is consistent with the time lag of magnetic signatures observed by the two isee spacecraft the expansion velocity is estimated at 2 re min 200 km s for this event furthermore it is found that the observed magnetic signatures can be reproduced to a good approximation by a simple geometrical model of the current disruption the method is used statistically for 13 events selected from the isee magnetometer data it is found that the current disruption usually starts in the near earth magnetotail x 20re and often within 15 re of the earth the phase relationship between bx and bz is discussed in terms of the near earth neutral line model and the result is compared with previous studies on the substorm onset region

triggering of expansive phase intensifications of magnetospheric substorms by northward turnings of the interplanetary magnetic field over the past decade evidence has arisen suggesting that northward turnings of the interplanetary magnetic field after an interval of sustained southward imf can trigger the onset of major substorm expansive phase effects however there has been some question in the past as to just how many substorms are triggered and whether indeed the correlation of substorm onsets and northward turnings was just a matter of chance one reason for doubt was that the examples presented in the literature appeared during disturbed periods where the ims was fluctuating significantly on a time scale of less than one hour and many substorm expansive phase intensifications were observable in the time frame where the correlation between the imf and ground data was reported the purpose of this paper is to present three clear cut examples of isolated substorms where the reader may be able to convince himself of the validity of the reported triggering effect for these events it will be clear that the substorm current wedge triggered by the northward turning of the imf carries substantially larger currents than the driven system associated with the overall substorm event the role of the imf in externally triggering episodes of joule heating and particle precipitation within the substorm process will be discussed

detailed analysis of a substorm event on 6 and 7 june 1989 2 stepwise auroral bulge evolution during expansion phase 1 we have analyzed in detail the auroral bulge evolution during the expansion phase of an isolated substorm which was observed by the uv imager aboard the akebono satellite it was found that there were three distinct stages in the evolution stage 1 was characterized by rapid poleward and azimuthal predominantly westward expansions in a short time about 2 min stage 2 was characterized by a very slow poleward and slower and continuous azimuthal expansions there was a certain period for transition between stage 1 and stage 2 and it was characterized by a very slow poleward and rapid eastward expansions stage 3 started about 11 min after the onset and was characterized by a sudden reactivation of the rapid poleward and azimuthal expansions the reactivation started around the initial onset meridian and then spread both eastward and westward at the azimuthal front the expansion first occurred at the lowest latitudes spread poleward to around the highest latitudes of stage 1 and then spread further poleward after a brief interval hence the local expansion also had three distinct stages similar to the global one the ground based observations showed that the highest latitude of the local first stage was very close to the latitude of auroral activity that appeared near the ionospheric plasma sheet boundary layer psbl region a few minutes before the onset the further poleward expansion during the local third stage started with a significant intensification of the poleward most auroral activity during the local third stage the bright electron auroral region was bifurcated into a poleward expanding part and an equatorward moving part the proton auroral emission coexisted in the bulge during the local first and second stages and almost disappeared soon after the bifurcation during the local third stage based on these observations we discuss the evolution in the magnetosphere during the expansion phase

multipoint observations of substorm pre onset flows and time sequence in the ionosphere and magnetosphere 1 in this paper we use a rare and close longitudinal conjunction of the themis spacecraft with the sondrestrom radar to present multipoint observations of important features of the time sequence of substorm pre onset plasma flows in the ionosphere and magnetotail on march 5 2008 we found that the onset was preceded sequentially by enhanced polar cap flows heading equatorward near the polar cap boundary and then by tail fast flows from the mid tail to the near earth region we also observed in situ fluctuations in short period pi2 band 30 50 s in both the magnetic field and plasma pressure during the initial couple of min of the fast flows occurring nearly simultaneously with the dipolarization and the fast flows and propagating earthward our results suggest that these fluctuations may be triggered by the fast flows and may play an important role in the substorm onset process our event suggests that localized tail reconnection may be triggered by the enhanced polar cap flows though the reconnection location cannot be unambiguously determined earthward fast flows are generated as a result of the tail reconnection and reach the inner magnetosphere to initiate the substorm onset the presented case is consistent with that predicted by the nishimura et al 2010a scenario

relationship between substorm onset locations and nightside convection pattern features 1 this paper presents a study using the super dual auroral radar network superdarn convection database in conjunction with a list of substorm onsets determined from the image satellite to examine the relationship between substorm onset locations and features of the convection pattern particular attention is devoted to the latitude of onset locations compared to the latitude of the near midnight convection reversal boundary crb and the latitude at which the highest flow velocity the flow peak was observed equatorward of the crb because of this focus the set of observations examined is limited to cases where superdarn provided sufficient observations in the midnight sector to determine the latitude of the crb distributions of the latitude difference between the onset location and the crb and flow peak were examined for different times leading up to substorm onset and for a few different conditions it was found that the onsets occurred equatorward of the crb in more than 95 of the cases and that the distribution was approximately centered on the flow velocity peak

no evidence for externally triggered substorms based on superposed epoch analysis of imf bz superposed epoch analyses have shown that on average the interplanetary magnetic field imf turns northward close to substorm onset this has been commonly accepted as evidence for the substorm onset being triggered by a rapid northward turning of the imf here we show that the tendency arises in any superposed epoch analysis of the imf in which event onset is biased to occur for southward imf irrespective of a coincident rapid northward turning of the imf the overall imf variation found in the largest superposed epoch analysis of this kind is also well reproduced using a minimal substorm model in which substorm onsets are determined without the requirement of a northward imf turning trigger we discuss the explanation underlying these results and conclude that there is no conclusive evidence in favour of the hypothesis that substorm onsets are triggered by a rapid northward turning of the imf citation freeman m p and s k morley 2009 no evidence for externally triggered substorms based on superposed epoch analysis of imf b z geophys res lett 36 l21101 doi 10 1029 2009gl040621

flood enhancement through flood control flood stages for constant discharge have increased 2 4 m over the past century at numerous locations in the mississippi river basin however no increases are observed on rivers such as the meramec and the upper missouri which have been spared extensive river engineering projects flood stage increases on the middle mississippi river and lower missouri river are mostly attributable to channelization

substorm simulation quiet and n s arcs preceding auroral breakup auroral breakup at the onset of substorm expansion is sometimes preceded by auroral forms known as quiet arcs and n s arcs observations have shown that both the auroral forms tend to move equatorward and the initial brightening takes place in or near one of the quiet arcs the auroral forms attract great attention but generation of auroral forms and their association with the initial brightening are poorly understood recent global magnetohydrodynamic simulations are capable of producing upward field aligned currents facs that resemble the auroral forms in both shape and temporal evolution based on the simulation results we propose the following scenarios 1 when the convection electric field is weak northward interplanetary magnetic field imf the high pressure region is elongated from the plasma sheet toward higher latitudes and is structured by a coupling between the magnetosphere and the ionosphere interchange like instabilities 2 when the convection electric field is strong southward imf the structured high pressure region moves equatorward toward the plasma sheet upward currents are generated around it which can be observed as arcs in the ionosphere the upward current can be tentatively intensified in the course of the equatorward movement before the formation of a near earth neutral line nenl 3 the nenl releases magnetic tension and results in the enhancement of plasma pressure at off equator in the near earth region sudden formation of the off equatorial high pressure region generates the onset current system that manifests initial brightening our scenario can explain the observational fact that poleward arcs remained undisturbed at the onset

high time resolution dayside convection monitoring by incoherent scatter radar and a sample application 1 convection and its changes driven by the solar wind are related to geomagnetic disturbances such as substorms dynamic pressure disturbances and convection bays furthermore convection is theoretically related to the evolution of the harang electric field reversal or discontinuity which has been related observationally and theoretically to these disturbances interplanetary measurements generally used for studying and making associations of convection with disturbances are acceptable for statistical studies but spatial structure and propagation timing errors due to the highly variable orientation of interplanetary structures create significant ambiguity for case studies and have prevented definitive understanding moreover small but geoeffective structure in the solar wind may be missed by even multispacecraft observations direct observation of dayside convection with high time resolution offers a potential solution to this problem in this paper we demonstrate that sondrestrom incoherent scatter radar isr observations have the ability to accurately monitor the dayside convection with 2 5 min temporal resolution the observations clearly show the dayside convection responses to imf bz by and solar wind dynamic pressure variations and thus shed new light on how the magnetosphere and ionosphere respond to different external driving then we illustrate an application of this technique by showing how it can be applied to study substorm triggering we plan to use this technique in the future to study the relationships between convection other geomagnetic disturbances and the harang reversal as well as to evaluate the preceding convection strength and duration and the convection changes that lead to substorm onset

conjugate observations of flow diversion in the magnetotail and auroral arc extension in the ionosphere 1 we present a case study of a plasma flow diversion event in the inner magnetosphere to illustrate the effects of near earth pressure evolution field aligned current fac generation and associated auroral evolution during conjugate observations from themis satellites and all sky imagers asis on 29 january 2008 at 0831 ut an earthward flow at thc at x 18 re was observed which was likely to be associated with an auroral streamer observed at the asi station fsmi near earth satellites thd and the separated in the y direction by 1 re and magnetically mapping onto the preexisting aurora arc also observed the earthward flow but with a small alfven transit time of 20 s between the magnetotail and the ionosphere as the earthward flow arrived at thd and the the aurora arc near the footprints of thd and the brightened a duskward pressure gradient developed between thd and the in the equatorial magnetotail corresponding to an upward fac and consistent with the auroral brightening at 0834 ut an eastward diversion of the flow was observed first at the and later at thd further to the east at the head of that flow pressure variations coherent between the and thd were observed to propagate eastward at 400 km s consistent with an eastward auroral expansion near the footprints of the and thd concurrent perturbations of the by component of the magnetic field signify eastward motion of a fac pair at the two spacecraft locations we interpret the pressure variations as the generator of the observed fac that feeds the arc and its eastward expansion

coherent regeneration and the role of water vapor and shear in a long lived convective episode abstract in a recent radar based climatological study of warm season precipitation over the continental united states carbone et al found a high frequency of long lived coherent rainfall episodes many of the events were of longer duration than normally associated with mesoscale convective complexes and exhibited phase speeds 10 m s 1 in excess of the phase speed associated with synoptic systems the observations led to the speculation that cold pool dynamics and wavelike propagation mechanisms were responsible for the longevity of the systems one of the long lived episodes included in the statistics of the carbone et al study is described here occurring on 14 15 july 1998 the system lasted 50 h and traveled over 2800 km at its peak intensity the system was a bow echo producing damaging wind large hail and local flash flooding an interesting aspect of the event was an abrupt 90 turn in the storm s orientation and propagation vector midway through its life the environmental factors that led t

statistical description of the bulge type auroral substorm in the far ultraviolet 1 using global auroral images at ultraviolet wavelengths during 116 substorms we have obtained quantitative measures of key features of the bulge aurora and oval aurora their temporal variations their locations rates and characteristics of gross expansion and decay and the variability of these parameters the expansion period identified solely from images varied primarily from 10 to 40 minutes with an average of 30 9 minutes to avoid mixing expansion data with recovery data we normalized the time of each substorm to one unit from onset to maximum expansion the average onset location was 22 6 magnetic local time mlt and 66 8 invariant latitude ilat in good agreement with previous analyses we found that the bulge aurora rapidly expanded out of the onset location approximately equally to the west surge and to the east so that the average center of the bulge remained close to the onset mlt this is also the case for average location of the maximum expansion in latitude of the bulge thus the bulge is offset about 1 1 2 hours west of midnight by half the expansion period the bulge has usually expanded poleward sufficiently to reveal a brightened portion of the original auroral oval this brightening expands less than 1 hour mlt to the west but rapidly to the east farther than the east end of the bulge thus the two auroras are offset in mlt the bulge expansion is fastest initially but slows for the second half of the expansion period the ends of the bulge continue a small expansion poleward during early recovery when the center of the bulge slowly retreats the large spreads of substorm expansion times the onset locations and in the locations in mlt and ilat of the key features of these auroras argue strongly for the need to normalize the time of expansion and location of key features of the substorm for any kind of superposed epoch analysis to be meaningful

substorm convection patterns observed by the super dual auroral radar network a study of convection patterns during substorms observed by the super dual auroral radar network superdarn radar network is presented specific attention is given to the growth phase and the time immediately following expansion onset it is found that the growth phase is characterized by the enhancement of the velocity shear near midnight and its extension to low latitudes and to local times across the midnight meridian the velocity shear was observed to diminish at expansion onset in addition the ionospheric flow velocity magnitude was observed to be enhanced for some period of time prior to expansion onset and to decrease at expansion onset possible explanations for the establishment of the midnight velocity shear during growth phase and its subsequent disappearance at expansion onset are presented also arguments are presented regarding the decrease of ionospheric flow velocity at expansion onset

solar wind control of magnetospheric energy content substorm quenching and multiple onsets in this paper we report coordinated multispacecraft and ground based observations of a double substorm onset close to scandinavia on november 17 1996 the wind and the geotail spacecraft which were located in the solar wind and the subsolar magnetosheath respectively recorded two periods of southward directed interplanetary magnetic field imf these periods were separated by a short northward imf excursion associated with a solar wind pressure pulse which compressed the magnetosphere to such a degree that geotail for a short period was located outside the bow shock the first period of southward imf initiated a substorm growth phase which was clearly detected by an array of ground based instrumentation and by interball in the northern tail lobe a first substorm onset occurred in close relation to the solar wind pressure pulse impinging on the magnetopause and almost simultaneously with the northward turning of the imf however this substorm did not fully develop in clear association with the expansion of the magnetosphere at the end of the pressure pulse the auroral expansion was stopped and the northern sky cleared we will present evidence that the change in the solar wind dynamic pressure actively quenched the energy available for any further substorm expansion directly after this period the magnetometer network detected signatures of a renewed substorm growth phase which was initiated by the second southward turning of the imf and which finally lead to a second and this time complete substorm intensification we have used our multipoint observations in order to understand the solar wind control of the substorm onset and substorm quenching the relative timings between the observations on the various satellites and on the ground were used to infer a possible causal relationship between the solar wind pressure variations and consequent substorm development furthermore using a relatively simple algorithm to model the tail lobe field and the total tail flux we show that there indeed exists a close relationship between the relaxation of a solar wind pressure pulse the reduction of the tail lobe field and the quenching of the initial substorm

determination of the substorm initiation region from a major conjunction interval of themis satellites 1 we investigate in detail the time history of substorm disturbances in the magnetotail observed during a major tail conjunction of time history of events and macroscale interactions during substorms themis satellites on 29 january 2008 0700 0900 ut during this interval all themis satellites were closely aligned along the tail axis near midnight and were bracketed in local time by goes 11 and 12 the radial distance covered ranges from the geosynchronous altitude to 30 r e in the tail this interval consists of three activations detected by the themis satellites with good ground all sky camera observations of auroral activity the first activation is a small substorm with spatially limited disturbance in the tail the onset arc was equatorward of an undisturbed arc the second activation is a moderate size substorm with the onset arc also being equatorward of an undisturbed arc the third activation is an intensification of the substorm with its onset indicated by the second activation the active auroral arc for this intensification was near the poleward boundary of the auroral oval analysis of these observations indicates that the first activation is a small substorm initiated in the near earth plasma sheet and does not involve magnetic reconnection of open magnetic field lines magnetic reconnection on closed field lines can be ruled out for this substorm because it cannot generate the observed high speed plasma flow the second and third activations are part of a moderate size substorm initiated also in the near earth plasma sheet with a subsequent substorm intensification involving activity initiated tailward of 30 r e overall the time history of substorm activity for these two substorms is consistent with the near earth initiation model

ionospheric flow structures associated with auroral beading at substorm auroral onset auroral observations have shown that brightening at substorm auroral onset often consists of azimuthally propagating beads forming along a preexisting arc however the ionospheric flow structure related to this wavy auroral structure has been poorly understood we investigated ionospheric flow patterns associated with auroral onset beads using line of sight flow observations from the super dual auroral radar network superdarn and auroral images from the time history of events and macroscale interactions during substorms themis ground based all sky imager asi array we selected events when superdarn radars operated in a high temporal resolution themis mode 6 s along northward looking beams a time resolution comparable to that of the imagers providing a unique tool to detect properties of flows associated with the substorm onset instability we have found very fast oscillating flows 1000 m s that are correlated with the onset beads propagating across the themis mode beam meridian two dimensional radar measurements also show a wavy pattern in the azimuthal direction with a wavelength of 78 km which is close to the azimuthal separation of individual beads we also used an imager and superdarn in iceland and identified weak but significant azimuthal flow modulations associated with beads these strong correlations in time and space between auroral beading and the fast ionospheric flows suggest that substorm onset occurs via an instability in the inner plasma sheet and is associated with intense flow shears the flow shear is clockwise around auroral beads consistent with converging electric fields associated with upward field aligned currents in the shear center

auroral substorm timescales imf and seasonal variations 1 the timescales and phases of auroral substorm activity are quantified in this study using the hemispheric power computed from polar ultraviolet imager uvi observations with this approach we are able to determine substorm phase self consistently from the uvi optical measurements without needing any ancillary metrics for auroral activity this technique also allows us to combine a quantitative measurement of substorm timescales with the qualitative description of auroral morphology during each event we show how the characteristics of substorms vary with imf bz orientation and season for several hundred substorm events substorm intensity as measured by the rate of energy deposition by electron precipitation exhibits greater variation with imf bz orientation than with season the timescale of substorm expansion shows comparable variation with both imf bz orientation and season on the other hand substorm recovery times vary more strongly with season than with imf bz direction the recovery time for substorm activity is well ordered by whether or not the nightside auroral region is sunlit substorms occurring in the winter and equinox periods have similar recovery timescales which are both roughly a factor of two longer than that for summer when the auroral oval is sunlit however imf bz orientation is shown to have a strong modulating effect on both substorm expansion and recovery timescales during the summer the winter summer asymmetries in the aurora described by previous studies are more completely explained by the seasonal variations of substorm recovery timescales characterized in this work our results support the hypothesis that the ionosphere plays an active role in governing the dynamics of the aurora

counterstreaming auroral structures during substorm expansion 1 using of high quality keograms constructed by filtered and averaged images from the lovozero all sky imager asi station and themis all sky imager array allowed us to investigate in detail optical signatures of poleward forms propagating equatorward and getting involved in the auroral pattern in the course of substorm expansion we found that weak nearly east west oriented auroral arcs often penetrate into poleward expanding regions of active auroras from preceding breakup activations in the south in total 40 such events have been analyzed the major point that we aimed to clarify is whether these weak forms fade disperse or keep moving equatorward in such occasions we were able to trace their fate and demonstrate that they keep their equatorward motion against the background of bright poleward expanding auroras forming a counterstreaming auroral pattern at substorm expansion

numerical study of precipitation core gap structure along cold fronts abstract the mechanism responsible for the core gap structure of precipitation along narrow cold frontal rainbands ncfrs is investigated through analyses of idealized cloud resolving simulations of cold fronts the control simulation in which the prefrontal thermal stratification is characterized by a weak convective instability at low levels with convective available potential energy cape of 60 j kg 1 reproduces the typical alongfront variability of observed ncfrs the simulated ncfr is broken up into regularly spaced ellipsoidal cores oriented at a clockwise angle to the cold front while horizontal shear instability hsi has frequently been proposed as a mechanism leading to the alongfront variability of ncfrs no characteristic features of hsi are recognized in the simulated vertical vorticity field at the leading edge of the cold front the alongfront variability in precipitation is attributed to the formation of a wavelike disturbance just above the leading edge of the cold front the wave

response of dayside net downward field aligned current to changes in the interplanetary magnetic field and to substorm perturbations we have studied the response of the auroral eastward and westward electrojets and of the downward field aligned currents across the noon sector which feed them to southward turnings of the interplanetary magnetic field imf and to substorm onsets in the nightside magnetosphere we have found that while southward turnings of the imf lead to electrojet intensifications and low level substorm activity northward turnings of the imf appear capable of triggering large substorm disturbances both northward and southward turnings of the imf cause the downward field aligned current across the noon sector as well as the auroral electrojets to strengthen within a few minutes of the impact of the changed imf at the magnetopause since large substorms involve the sudden release of energy stored in the magnetotail we conclude that the tail current system links directly to the overall magnetosphere ionosphere current system involving the auroral electrojets in this context a substorm associated with a northward turning of the imf reflects a reconfiguration of the magnetotail in response to a change in the level of the solar terrestrial interaction involving a decrease in the rate at which solar wind energy penetrates into the magnetosphere

reply to comment by y i feldstein v g vorobjev and v l zverev on the importance of auroral features in the search for substorm onset process