longitudinal variability of low latitude total electron content tidal influences 1 recently nighttime ultraviolet uv observations obtained by image fuv and timed guvi instruments have revealed a longitudinal wave number four pattern in the nighttime airglow intensity and in the position of the equatorial anomalies during equinox and high solar flux conditions in the present study we have extended this work and determined the longitudinal variability of the low latitude total electron content tec climatology during different geophysical conditions with a special emphasis on the longitudinal wave number four structure in the low latitude ionosphere we have used more than 5 million low latitude topex tec observations covering the entire 13 years of topex tec data from august 1992 until october 2005 this data set was used to determine the local time seasonal solar cycle and geomagnetic activity dependence of the longitudinal variability of tec at equatorial and low latitudes and in particular to address the existence and evolution of the wave number four longitudinal pattern under these conditions our study shows that the wave number four pattern is created during the daytime hours at equinox and june solstice but is absent or washed out by other processes during december solstice during equinox the wave number four pattern is created around noon with well defined longitudinal enhancements in the low latitude tec these enhancements which are symmetric about the geomagnetic equator during this season last for many hours and can be clearly seen past midnight the longitudinal patterns are found to be nearly identical between the vernal march april and autumnal september october equinoxes and largely independent of the solar cycle conditions the wave number four pattern is also observed during geomagnetically active conditions indicating that the processes that create this pattern are also present during active times the variations between the well defined longitudinal maxima and minima are of the order of 20 during june solstice the wave number four pattern is also observed in the afternoon hours but in contrast to the equinox cases it exhibits a strong hemispheric asymmetry and is not observed during the night the low latitude tec exhibits clear longitudinal variations during december solstice with large daytime enhancements over the east asian and pacific regions and a third enhancement emerging in the afternoon over the atlantic ocean but a clear wave number four pattern is not observed during this season although the equatorial and low latitude tec values exhibit clear longitudinal patterns during all seasons a significant amount of scatter remains in the tec data that is not accounted for by changes in the solar cycle the season or the local time or by the longitudinal variability this remaining scatter is largest near the poleward edges of the anomalies and is of the order of 40

on secular geocenter motion the impact of climate changes abstract we investigate the impact of recent climate changes on the long term displacement of the center of figure cf of the earth also defined as the geocenter in our convention with respect to the center of mass of the whole earth cm the two realizations of the international terrestrial reference system itrs entitled itrf2000 and itrf2005 present a difference of 1 8 mm yr between the velocities of their respective frame origins suggesting an acceleration of the geocenter towards the north pole we investigate if such a displacement could be explained by geophysical phenomena such as the present ice melting and the sea level rise using published observations on ice caps and glaciers we calculated the range of geocenter motion that may occur today we found that the global ice melting induces long term displacements of the geocenter mainly along the z axis toward the north pole the geocenter velocity is today between 0 3 and 0 7 0 8 mm yr and has doubled during last decade with the recent acceleration of ice melting combining with greff lefftz 2000 results on post glacial rebound we conclude that a present secular geocenter velocity of 1 mm yr is possible however the recent increase of the geocenter velocity cannot explain the difference observed between the two last realizations of the itrs our results comfort the previous conclusions about itrf2000 and itrf2005 and the pre analysis of itrf2008 data suggesting that the large z translation rate between the itrs realizations is probably due to an imprecise itrf2000 origin finally we show that determining precisely the geocenter velocity would give a new type of information that may be useful to more precisely constrain mass exchanges associated with climate changes

conjugate hemisphere ionospheric response to the st patrick s day storms of 2013 and 2015 in the 100 e longitude sector the effects of the st patrick s day geomagnetic storms of 2013 and 2015 in the equatorial and low latitude regions of both hemispheres in the 100 e longitude sector is investigated and compared with the response in the indian sector at 77 e the data from a chain of ionosondes and gps global navigation satellite systems receivers at magnetic conjugate locations in the 100 e sector have been used the perturbation in the equatorial zonal electric field due to the prompt penetration of the magnetospheric convective under shielded electric field and the over shielding electric field gives rise to rapid fluctuations in the f2 layer parameters the direction of imf bz and disturbance electric field perturbations in the sunset sunrise period is found to play a crucial role in deciding the extent of prereversal enhancement which in turn affect the irregularity formation equatorial spread f in the equatorial region the northward southward imf bz in the sunset period inhibited supported the irregularity formation in 2015 2013 in the 100 e sector large height increase hmf2 during sunrise produced short duration irregularities during both the storms the westward disturbance electric field on 18 march inhibited the equatorial ionization anomaly causing negative positive storm effect in low latitude equatorial region the negative effect was amplified in low midlatitude by disturbed thermospheric composition which produced severe density total electron content depletion the longitudinal and hemispheric asymmetry of storm response is observed and attributed to electrodynamic and thermospheric differences

realizing a terrestrial reference frame using the global positioning system we describe a terrestrial reference frame trf realization based on global positioning system gps data alone our approach rests on a highly dynamic long arc 9 day estimation strategy and on gps satellite antenna calibrations derived from gravity recovery and climate experiment and topex poseidon low earth orbit receiver gps data based on nearly 17 years of data 1997 2013 our solution for scale rate agrees with international terrestrial reference frame itrf 2008 to 0 03 ppb yr 1 and our solution for 3 d origin rate agrees with itrf2008 to 0 4 mm yr 1 absolute scale differs by 1 1 ppb 7 mm at the earth s surface and 3 d origin by 8 mm these differences lie within estimated error levels for the contemporary trf

global geodesy using gps without fiducial sites baseline lengths and geocentric radii have been determined from gps data without the use of fiducial sites data from the first gps experiment for the iers and geodynamics gig 91 have been analyzed with a no fiducial strategy a baseline length daily repeatability of 2 mm 4 parts per billion was obtained for baselines in the northern hemisphere comparison of baseline lengths from gps and the global vlbi solution glb659 caprette et al 1990 show rms agreement of 2 1 parts per billion the geocentric radius mean daily repeatability for all sites was 15 cm comparison of geocentric radii from gps and sv5 murray et al 1990 show rms agreement of 3 8 cm given n globally distributed stations the n n 1 2 baseline lengths and n geocentric radii uniquely define a rigid closed polyhedron with a well defined center of mass geodetic information can be obtained by examining the structure of the polyhedron and its change with time

duskside enhancement of equatorial zonal electric field response to convection electric fields during the st patrick s day storm on 17 march 2015 the equatorial zonal electric field responses to prompt penetration of eastward convection electric fields ppef were compared at closely spaced longitudinal intervals at dusk to premidnight sectors during the intense geomagnetic storm of 17 march 2015 at dusk sector indian longitudes a rapid uplift of equatorial f layer to 550 km and development of intense equatorial plasma bubbles epbs were observed these epbs were found to extend up to 27 13 n and 25 98 s magnetic dip latitudes indicating their altitude development to 1670 km at apex in contrast at few degrees east in the premidnight sector thailand indonesian longitudes no significant height rise and or epb activity has been observed the eastward electric field perturbations due to ppef are greatly dominated at dusk sector despite the existence of background westward ionospheric disturbance dynamo idd fields whereas they were mostly counter balanced by the idd fields in the premidnight sector in situ observations from swarm a and swarm c and communication navigation outage forecasting system satellites detected a large plasma density depletion near indian equatorial region due to large electrodynamic uplift of f layer to higher than satellite altitudes further this large uplift is found to confine to a narrow longitudinal sector centered on sunset terminator this study brings out the significantly enhanced equatorial zonal electric field in response to ppef that is uniquely confined to dusk sector the responsible mechanisms are discussed in terms of unique electrodynamic conditions prevailing at dusk sector in the presence of convection electric fields associated with the onset of a substorm under southward interplanetary magnetic field bz

longitudinal variation of the e region electric fields caused by atmospheric tides 1 polarization electric fields created by the e and f region dynamos cause the uplift of f region plasma the subsequent redistribution of that plasma along the magnetic field lines creates the equatorial ionospheric anomaly eia observations of the post sunset eia made by the image and timed satellites are compared here with champ orsted and sac c observations of the noontime equatorial electrojet eej during magnetically quiet periods around equinox the eia and eej show a remarkably similar four peaked wave like longitudinal variation its structure is consistent with the longitudinal variation in the strength of diurnal tides that drive the e region dynamo this indicates a strong vertical coupling between the ionosphere and troposphere because the four peaked tidal structure is driven by tropospheric weather furthermore the dayside ionospheric conditions are found to perform the global scale longitudinal structure of the post sunset ionosphere at low latitudes

australia s unique influence on global sea level in 2010 2011 1 in 2011 a significant drop in global sea level occurred that was unprecedented in the altimeter era and concurrent with an exceptionally strong la nina this analysis examines multiple data sets in exploring the physical basis for the drop s exceptional intensity and persistence australia s hydrologic surface mass anomaly is shown to have been a dominant contributor to the 2011 global total and associated precipitation anomalies were among the highest on record the persistence of australia s mass anomaly is attributed to the continent s unique surface hydrology which includes expansive arheic and endorheic basins that impede runoff to ocean based on australia s key role attribution of sea level variability is addressed the modulating influences of the indian ocean dipole and southern annular mode on la nina teleconnections are found to be key drivers of anomalous precipitation in the continent s interior and the associated surface mass and sea level responses

equatorial and low latitude ionospheric response to the 17 18 march 2015 great storm over southeast asia longitude sector this study mainly investigates equatorial and low latitude ionospheric response to a great geomagnetic storm occurred on the 17 march 2015 we found that there were some interesting ionospheric phenomena e g short term ionospheric positive effect daytime spread f and morning equatorial ionization anomaly eia in the topside ionosphere emerged at equatorial and low latitude region along the longitude of about 100oe ground based ionosondes and in situ satellite swarm were utilized to study the possible mechanisms for these ionospheric phenomena we found that vertical downward transport of plasma or neutral induced by traveling ionospheric disturbances tids or traveling atmospheric disturbances tads might make a contribution to the short term ionospheric positive effect at the main stage of this great storm additionally results suggested that the occurrence of daytime spread f at low latitudes might be due to the diffusion of equatorial ionospheric irregularities in the topside ionosphere along geomagnetic field lines moreover observational evidence shows that tids also might be the main driver for morning eia like feature recorded by swarm b satellite in the topside ionosphere these ionospheric phenomena mentioned above could make us to gain a better understanding of ionospheric storm effects at equatorial and low latitude region

origin of the international terrestrial reference frame 1 with recent improvements in space geodesy the earth s center of mass cm and center of figure cf are no longer indistinguishable the current origin of the international terrestrial reference frame itrf is defined as the cm which shows measured seasonal variations of several millimeters to 1 cm with respect to true cm as scientists study earth s dynamic deformations on seasonal and shorter timescales and begin to compare observed geocenter motion with predictions from geophysical models the reference frame origin presents significant error due to missing the geocenter motion this paper discusses the nature of the origin of the itrf and explores the sensitivity of gps measurements to geocenter motion we find that since the values of nonlinear geocenter motion are not included in the positions of the itrf sites the behavior of the current itrf origin reflects cm on secular timescale but reflects cf on seasonal and short timescales the nature of the itrf origin depends on both the adopted kinematic model and unmodeled network motion the realized itrf origin should be defined by a new nomenclature to reflect its nature accurately by the new nomenclature the origin will maintain its current long term stability while improving its stability on seasonal timescales to the submillimeter level with the degree 1 deformation approach gps measurements are able to provide potentially valuable information on geocenter variations on seasonal and short timescales

non linearity of geocentre motion and its impact on the origin of the terrestrial reference frame the terrestrial reference frame is a cornerstone for modern geodesy and its applications for a wide range of earth sciences the underlying assumption for establishing a terrestrial reference frame is that the motion of the solid earth s figure centre relative to the mass centre of the earth system on a multidecadal timescale is linear however past international terrestrial reference frames itrfs showed unexpected accelerated motion in their translation parameters based on this underlying assumption the inconsistency of relative origin motions of the itrfs has been attributed to data reduction imperfection we investigated the impact of surface mass loading from atmosphere ocean snow soil moisture ice sheet glacier and sea level from 1983 to 2008 on the geocentre variations the resultant geocentre time series display notable trend acceleration from 1998 onward in particular in the z component this effect is primarily driven by the hydrological mass redistribution in the continents soil moisture snow ice sheet and glacier the acceleration is statistically significant at the 99 per cent confidence level as determined using the mann kendall test and it is highly correlated with the satellite laser ranging determined translation series our study based on independent geophysical and hydrological models demonstrates that in addition to systematic errors from analysis procedures the observed non linearity of the earth system behaviour at interannual timescales is physically driven and is able to explain 42 per cent of the disparity between the origins of itrf2000 and itrf2005 as well as the high level of consistency between the itrf2005 and itrf2008 origins

a new global mode of earth deformation seasonal cycle detected we have detected a global mode of earth deformation that is predicted by theory precise positioning of global positioning system sites distributed worldwide reveals that during february to march the northern hemisphere compresses and the southern hemisphere expands such that sites near the north pole move downward by 3 0 millimeters and sites near the equator are pulled northward by 1 5 millimeters the opposite pattern of deformation occurs during august to september we identify this pattern as the degree one spherical harmonic response of an elastic earth to increased winter loading of soil moisture snow cover and atmosphere data inversion shows the load moment s trajectory as a great circle traversing the continents peaking at 6 9 1022 kilogram meters near the north pole in winter indicating interhemispheric mass exchange of 1 0 1016 0 2 1016 kilograms

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

ionospheric effects of st patrick s storm over asian russia 17 19 march 2015 we have carried out a comprehensive analysis of data from the high frequency coherent radar located near yekaterinburg ground based ionospheric riometric and magnetic stations situated within t

variation of the equatorial height anomaly during the main phase of 2015 st patrick s day geomagnetic storm using annim and tiegcm

a reassessment of global and regional mean sea level trends from topex and jason 1 altimetry based on revised reference frame and orbits 1 mean sea level trends from topex and jason 1 altimeter data are recomputed using unified geophysical modeling and the new itrf2005 terrestrial reference frame for the entire altimetric time series with consistent orbits based on satellite laser ranging slr and doris tracking data we obtain a global rate of 3 36 0 41 mm yr over the 14 year period from 1993 to 2007 the regional sea level trends computed with the new reference frame show significant north south hemispherical offsets of 1 5 mm yr relative to trends based on the previous 1995 era frame regional sea level trend comparisons for the time periods of 1993 1999 and 1999 2005 reveal strong basin scale polarities and pronounced inter decadal variability with a relative increase in the global mean sea level trend of 1 5 0 7 mm yr in the latter seven years

a statistical study of the response of the dayside equatorial f2 layer to the main phase of intense geomagnetic storms as an indicator of penetration electric field 1 the response of the dayside equatorial f2 layer to the main phases of the 22 intense geomagnetic storms dst 150 nt in 1998 2008 is investigated using the digital ionosonde data from the equatorial stations in brazilian indian and australian longitudes together with equatorial electrojet strength and imf bz the storms include 15 superstorms dst 200 nt the observations show that there is a period during all mps when the f2 layer peak rises and falls rapidly with large peak electron density nmax reduction the rise velocity strongly correlates with the intensity dst of the storms and the duration of the nmax reduction corresponds to that of strong eastward electrojet when imf bz remains highly negative the observations indicate the occurrence of strong eastward prompt penetration electric fields ppef during the rapid f2 layer response the ppef drives the f2 layer peak rapidly upward which reduces nmax due to vertical expansion and diffusion the results therefore suggest that the rapid f2 layer response rapid rise and fall of peak height hmax with large nmax reduction observed by ionosondes can be used to detect the occurrence of the daytime eastward ppef during intense geomagnetic storms irrespective of season and level of solar activity the data also show two rare events of strong daytime westward electric fields due to disturbance dynamo and or prompt penetration the results are important when radars are not available to monitor the occurrence of the ppef

troposphere mapping functions for gps and very long baseline interferometry from european centre for medium range weather forecasts operational analysis data 1 in the analyses of geodetic very long baseline interferometry vlbi and gps data the analytic form used for mapping of the atmosphere delay from zenith to the line of site is most often a three parameter continued fraction in 1 sin elevation using the 40 years reanalysis era 40 data of the european centre for medium range weather forecasts for the year 2001 the b and c coefficients of the continued fraction form for the hydrostatic mapping functions have been redetermined unlike previous mapping functions based on data from numerical weather models isobaric mapping functions niell 2000 and vienna mapping functions vmf boehm and schuh 2004 the new c coefficients are dependent on the day of the year and unlike the niell mapping functions niell 1996 they are no longer symmetric with respect to the equator apart from the opposite phase for the two hemispheres compared to vmf this causes an effect on the vlbi or gps station heights that is constant and as large as 2 mm at the equator and that varies seasonally between 4 mm and 0 mm at the poles the updated vmf based on these new coefficients and called vmf1 hereinafter yields slightly better baseline length repeatabilities for vlbi data the hydrostatic and wet mapping functions are applied in various combinations with different kinds of a priori zenith delays in the analyses of all vlbi international vlbi service for geodesy and astrometry ivs r1 and ivs r4 24 hour sessions of 2002 and 2003 the investigations concentrate on baseline length repeatabilities as well as on absolute changes of station heights

a theoretical study of thermospheric composition perturbations during an impulsive geomagnetic storm the compositional response of the neutral thermosphere to an impulsive geomagnetic storm has been investigated using a numerical simulation made with the national center for atmospheric research thermospheric general circulation model ncar tgcm calculated time dependent changes in neutral thermospheric composition have been studied together with detailed neutral parcel trajectories and other diagnostic information from the model to gain a greater understanding of the physical mechanisms responsible for composition variability during geomagnetic storms and in particular to investigate the causes of the positive and negative ionospheric storm effects the following principal results were obtained from this study 1 calculated perturbations in thermospheric composition following the onset of an impulsive geomagnetic storm were found to be in good qualitative agreement with the previous experimental statistical study of storm time thermospheric morphology by prolss 1981 2 during the initial onset phase of the simulated storm upward vertical winds occurred in the auroral zone and downward winds occurred in the central magnetic polar cap 3 the largest perturbations in mass mixing ratio of nitrogen n2 at f region altitudes were found to be associated with parcels of neutral gas that travelled through the cusp region and with parcels that were trapped within the auroral zone for a long time 4 storm time enhancements in n2 were found to occur in the midnight and early morning sectors both within and equatorward of the auroral zone and these were determined to be associated with the advective effects of the large antisunward polar cap neutral winds the physical size of the region of enhanced n2 values was determined to be related to the time taken for individual parcels of air to corotate out of the large day to night thermospheric wind jet and thus to the magnitudes of the antisunward polar cap winds 5 decreases in n2 were found to occur in regions of downward vertical winds and in regions into which relatively nitrogen poor air was advected by the horizontal wind 6 calculated compositional perturbations following the onset of the storm were found to propagate to mid latitudes due to the advective effects of equatorward winds leading to the mid latitude positive ionospheric storm effects this last result is in disagreement with the earlier study of rishbeth et al 1985 who found the compositional disturbance to be largely restricted to the auroral zone on a constant pressure surface however it is consistent with the more recent study undertaken by fuller rowell et al 1991 who demonstrated that composition changes were seen well beyond the auroral oval

estimating geocenter variations from a combination of grace and ocean model output 1 in this study we estimate a time series of geocenter anomalies from a combination of data from the gravityrecoveryandclimateexperiment grace satellitemissionandthe outputfromoceanmodels amatrixequationisderivedrelating totalgeocentervariations to the grace coefficients of degrees two and higher and to the oceanic component of the degree one coefficients we estimate the oceanic component from two state of the art ocean models results are compared to independent estimates of geocenter derived from other satellite data such as satellite laser ranging and gps finally we compute degree one coefficients that are consistent with the processing applied to the grace level 2 gravity field coefficients the estimated degree one coefficients can be used to improve estimates of mass variability from grace which alone cannot provide them directly

was magnetic storm the only driver of the long duration enhancements of daytime total electron content in the asian australian sector between 7 and 12 september 2017

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

effects of atmospheric pressure loading and seven parameter transformations on estimates of geocenter motion and station heights from space geodetic observations 1 variations in fluid loads such as the oceans and the atmosphere deform the surface of the earth the accuracy of station coordinates in particular heights that can be estimated depends on how well one can separate these surface deformations from the associated translational motion between the center of mass of the solid earth and the total earth cm we applied simulated atmospheric pressure loading effects to the coordinates of sites in the cm frame to explore to what level of accuracy both geocenter motion and accurate station coordinates can actually be recovered from geodetic analyses we found that standard seven parameter transformations three rotations three translations scale generally recover about 80 of the geocenter motion however the inclusion of a scale factor permits the aliasing of surface loading deformation introducing scale errors of up to 0 3 ppb and daily height errors as large as 4 mm this limits the geophysical studies that can be performed accurately using the results of geodetic analyses where the magnitudes of the signals are small e g tectonic movement of tide gauges uplift rates for interpreting glacial isostatic adjustment the quality of the geodetic results is extremely sensitive to the number and distribution of sites used to estimate the transformations and becomes worse when regional rather than global sets of sites are used if the scale factor parameter is omitted then the amount of aliasing of surface loading effects is reduced considerably and more accurate site velocities and geocenter motion estimates are achieved

longitudinal structure of the equatorial anomaly in the nighttime ionosphere observed by image fuv 1 the far ultraviolet imager fuv on board the image satellite provides an instantaneous global view of the oi 135 6 nm nightglow with 2 min time resolution because the oi 135 6 nm emission from the nighttime ionosphere is determined by the line of sight integrated plasma density the nightglow images are useful for studying the nighttime low latitude ionosphere globally with the image fuv 135 6 nm observations from march to june 2002 we have examined the global characteristics of the nighttime equatorial anomaly ea by constructing a constant local time map lt map in which pixels within an assigned local time range are extracted from the image fuv nightglow images obtained over an observation period of 3 days or more and are put together to compose a global distribution map of emission intensities at that local time these lt maps show that the development of the ea has a significant longitudinal structure in which peaks and dips of the crest emission intensity and the crest latitude have about 90 longitudinal separation in the longitude range from 0 to 250 although there is not enough data over the american sector this result suggests that the ea longitudinal structure has a prominent zonal component of the wave number 4 the observed longitudinal structure of the nighttime ea could not be fully explained by factors such as the empirical electric field and neutral wind models the geomagnetic declination angle or the displacement of the geomagnetic equator from the geographic equator to explain the observed longitudinal structure of the ea in particular the wave number 4 feature we may need to consider other forcing for example nonmigrating tide originated from the lower atmosphere

low latitude ionospheric disturbance electric field effects during the recovery phase of the 19 21 october 1998 magnetic storm 1 low latitude ionospheric electric fields and currents are often strongly disturbed during periods of enhanced geomagnetic activity these perturbations can last for several hours after geomagnetic quieting we use incoherent scatter radar measurements from jicamarca and arecibo during 19 21 october 1998 to study for the first time the low latitude disturbance electric fields during the recovery phase of a large magnetic storm on 19 october the jicamarca data showed initially large and short lived time scale of about 10 20 min upward and westward drift perturbations in the early afternoon sector due to the penetration of strong magnetospheric electric fields probably driven by an increase in the solar wind dynamic pressure following the decrease of auroral activity very strong afternoon and nighttime zonal and vertical disturbance dynamo drifts were observed over jicamarca but surprisingly only small and sporadic perturbation drifts were present over arecibo the latitudinal variation of the daytime zonal disturbance dynamo drifts during this day is in good agreement with that previously observed for thermospheric disturbance winds the disturbance dynamo drifts also indicate significantly different local storm time and possibly latitudinal dependences for their vertical perpendicular and zonal drift components suggesting large variations in both the amplitude and direction of the disturbance dynamo electric field vector in the next day during moderately disturbed conditions the daytime perturbation drifts were small but following transient increases in geomagnetic activity large upward perpendicular drifts were observed near dusk over jicamarca and arecibo where simultaneous westward perturbation drifts were also seen these perturbations are consistent with the occurrence of strong prompt penetration electric fields reaching the magnetic equator the measured prompt penetration drift patterns are generally in good agreement with predictions from global convection models later at night under moderately disturbed conditions relatively large zonal and meridional disturbance dynamo electric fields were observed by the two radars our results illustrate the large variability of the low latitude perturbation electric fields relative to their climatological values after large storms probably due to the importance of additional disturbance processes they also indicate that a much deeper understanding of solar wind magnetospheric ionospheric processes is required for accurate predictions of these electric fields

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

geospace system responses to the st patrick s day storms in 2013 and 2015 this special collection includes 31 research papers investigating geospace system responses to the geomagnetic storms during the st patrick s days of 17 march 2013 and 2015 it covers observation data assimilation and modeling aspects of the storm time phenomena and their associated physical processes the ionosphere and thermosphere as well as their coupling to the magnetosphere are clearly the main subject areas addressed this collection provides a comprehensive picture of the geospace response to these two major storms we provide some highlights of these studies in six specific areas 1 global and magnetosphere plasmasphere perspectives 2 high latitude responses 3 subauroral and mid latitude processes 4 effects of prompt penetration electric fields and disturbance dynamo electric fields 5 effects of neutral dynamics and perturbation and 6 storm effects on plasma bubbles and irregularities we also discuss areas of future challenges and the ways to move forward in advancing our understanding of the geospace storm time behavior and space weather effects

seismic focusing by a single planar fracture 1 a single plane fracture with an axially symmetric stress distribution behaves as a seismic lens that focuses seismic energy to a beam waist at a focal plane both phase and amplitude effects on a seismic wave propagating across the fracture contribute to the lensing behavior radial gradients in the fracture specific stiffness cause wave refraction through a radially varying group time delay while the fracture transmission amplitude approximates a fresnel zone plate this work demonstrates that a two dimensional planar fracture contrasted with three dimensional geologic structures such as basins and domes can focus seismic waves focusing of seismic waves by fractures should be considered in the interpretation of seismic data from fractured strata with heterogeneous stress distributions

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

the transition to overshielding after sharp and gradual interplanetary magnetic field northward turning overshielding is referred to a shielding status during which the dawnward shielding electric field dominates over the duskward penetration electric field in the inner magnetosphere typically appearing when the interplanetary magnetic field imf suddenly turns northward after a prolonged southward orientation it is expected that the transition to overshielding after imf northward turning can be affected by the shape of northward turning sharp or gradual moreover the initial shielding status undershielding or goodshielding prior to the transition may also have influence on the transition here we analyze two groups of cases in which the transitions appear after sharp duration less than 5 min and gradual duration more than 30 min northward turning each group includes two cases in which the transition initiated from undershielding and goodshielding these cases show that 1 the beginning of the transition to overshielding coincides with sharp imf northward turning but appears in the midst of gradual imf northward turning 2 the transition from goodshielding to overshielding is always associated with convection electric field drop and or polar cap shrinkage regardless of the shape of imf northward turning and 3 the typical solar wind condition in which the imf suddenly turns northward after a prolonged southward orientation is neither a necessary condition nor a sufficient condition for overshielding furthermore we will discuss the effect of substorm processes on overshielding