eof analysis and modeling of gps tec climatology over north america an empirical ionospheric model of the total electron content tec over north america 20 60 n 40 140 w is constructed using the gps tec data collected by massachusetts institute of technology mit haystack observatory during the years 2001 2012 this model is based on an analysis of quiet time monthly averages using the empirical orthogonal function eof decomposition technique allowing for separation of spatial and temporal variations the importance of different types of spatial temporal variations to the overall tec variability can be well represented by the characteristics of eof basis functions and associated principal components coefficients with various modes the mode one eof decomposition constitutes 97 5 of the total variance and therefore represents the essential feature of north america spatial and diurnal variation of the tec the mode two eof as reported in an earlier study reveals a large and significant symmetric longitudinal variation of the ionosphere organized with respect to magnetic declination the mode three eof decomposition shows midlatitude latitudinal structure that varies with season in a manner very similar to the so called winter anomaly because of the quick convergence of eof decomposition modes the first four eof modes are utilized for constructing a tec empirical model for each of the eof modes the temporal variations are expressed analytically in terms of local time season and solar activity and the spatial variations by cubic spline functions an analysis of accuracy and quality indicates that this regional empirical tec model can reflect the majority of the quiet time monthly means and represent characteristic temporal spatial variations in the north america

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

ionospheric annual anomaly new insights to the physical mechanisms the ionospheric annual anomaly or nonseasonal anomaly of the ionosphere is characterized by globally increased ionization in december solstice than in june solstice though this phenomenon was reported several decades ago the causal mechanisms have not been fully understood till today in this paper the f2 layer peak electron density nmf2 data from formosa satellite 3 constellation observing system for meteorology ionosphere and climate radio occultation observations during the low solar activity year 2009 were systematically analyzed to investigate the physical mechanisms responsible for annual anomaly and its local time latitudinal and longitudinal variability it is found that the annual anomaly is primarily dominant at southern hemisphere at all local times with significant enhancements at equatorial ionization anomaly crest latitudes during noon to afternoon hours and at high latitudes during nighttimes the annual anomaly in northern hemisphere occurs with relatively smaller magnitudes and confined only to morning to early afternoon hours 08 14 lt this study brings out the important roles of effective neutral winds due to the geomagnetic field configuration and the offset between geomagnetic equator and subsolar point for the enhanced plasma density in the southern hemisphere during december that majorly contributes to the ionospheric annual anomaly these results provide new insights to the responsible mechanisms behind the ionospheric annual anomaly and its local time latitudinal and longitudinal variation

sequence stratigraphy and carbon isotope stratigraphy of an aptian mixed carbonate siliciclastic platform to basin transition galve sub basin ne spain

seasonal variations of the ionospheric total electron content in asian equatorial anomaly regions the ionospheric total electron contents tec in both northern and southern equatorial anomaly regions are examined by using the global positioning system gps in asian area the tec contour charts obtained at ymsm 25 2 n 121 6 e 14 0 n geomagnetic and dgar 7 3 s 72 4 e 16 2 s geomagnetic stations in 1997 solar minimum are investigated it is found that the ionospheric crests manifest remarkable seasonal variations the tec values on both northern and southern equatorial anomaly crests yield their maximum values during the vernal and autumnal months but the winter anomaly does not appear in the southern region results show that both crests are fully developed around midday in winter postnoon in equinoxes and late afternoon in summer and the two crests move significantly equatorward in winter but slightly poleward in summer and autumn these phenomena can be fully explained by a combined theory of the transequatorial netural wind the subsolar point and the auroral equatorward wind

on the relationship between the postmidnight thermospheric equatorial mass anomaly and equatorial ionization anomaly under geomagnetic quiet conditions the equatorial mass anomaly ema in the thermosphere and equatorial ionization anomaly eia in the ionosphere are two interesting phenomena in low equatorial latitude regions previous studies have shown that the ema appears between 1000 and 2000 local time lt and its location of trough is aligned with dip equator indicating the plausible role of the eia structure in the development of ema in this report we conducted a statistical study of the occurrence of postmidnight ema and eia on the basis of the champ in situ measurements during 2002 2008 our results revealed that clear ema and eia structures are sometimes visible in the postmidnight sector 0100 0600 lt during geomagnetic quiet periods kp 3 the postmidnight ema is not necessarily accompanied by the eia signature in both case study and statistics sense being distinct from the daytime situation in addition the occurrence rates of postmidnight ema and eia display contrasting behavior with respect to their local time longitudinal and solar activity dependences the highest occurrence rate for ema is 8 at around 0300 lt while the occurrence rate of the eia decreases gradually from about 30 at around 2300 lt to similar to 5 at 0600 lt longitudinal occurrence of postmidnight eia presents a wave like pattern however no salient feature appears for the longitudinal occurrence of ema postmidnight ema is more likely to occur at lower solar activity whereas an opposite trend presents in the eia on the basis of above results our findings imply that a simple eia ema cause effect relationship does not hold in the postmidnight sector

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

tidal variability in the ionospheric dynamo region 1 the seasonal and interannual variability of migrating sun synchronous and nonmigrating solar atmospheric tides at altitudes between 100 and 116 km are investigated using temperature measurements made with the saber instrument on the timed spacecraft during 2002 2006 quasi biennial variations of order 10 15 in migrating diurnal and semidiurnal tidal amplitudes are found presumably due to modulation by the quasi biennial oscillation qbo as the tides propagate from their troposphere and stratospheric sources to the lower thermosphere a number of nonmigrating tidal components are found that have the potential to produce significant longitudinal variability of the total tidal fields the most prominent of these i e those that appear at amplitudes of order 5 10 k in a 5 year mean climatology include the zonally symmetric s 0 diurnal tide d0 the eastward propagating diurnal and semidiurnal tides with zonal wave numbers s 2 de2 and se2 and s 3 de3 and se3 and the following westward propagating waves diurnal s 2 dw2 semidiurnal s 1 sw1 s 3 sw3 and s 4 sw4 and terdiurnal s 5 tw5 these waves can be plausibly accounted for by nonlinear interaction between migrating tidal components and stationary planetary waves with s 1 or s 2 or by longitudinal variations of tropospheric thermal forcing additional waves that occur during some years or undergo phase cancellation within construction of a 5 year climatology include dw5 se1 se4 sw6 te1 tw1 and tw7 it is anticipated that the winds that accompany all of these waves in the 100 170 km region will impose longitudinal variability in the electric fields produced through the ionospheric dynamo mechanism thereby modulating vertical motion of the equatorial ionosphere and the concomitant plasma densities in addition to the wave 4 modulation of the equatorial ionosphere that has recently been discovered and replicated in modeling studies the waves revealed here will generate wave 1 sw1 sw3 d0 dw2 wave 2 sw4 tw1 wave 3 de2 se1 wave 4 de3 se2 dw5 sw6 te1 tw7 wave 5 se3 and wave 6 se4 components of this ionospheric variability depending on year and time of year however the absolute and relative efficiencies with which these waves produce electric fields remains to be determined

theoretical study of the ionospheric weddell sea anomaly using sami2 1 the ionospheric weddell sea anomaly wsa was first reported more than five decades ago based on ionosonde data near the antarctica peninsula the wsa is an ionospheric structure characterized by a larger nighttime electron density than daytime density recent satellite observations indicate that the wsa can extend from south america and antarctica to the central pacific the major physical mechanisms that have been suggested for the wsa formation are an equatorward neutral wind an electric field the photoionization and the downward diffusion from the plasmasphere on the basis of the theoretical modeling performed in this study using the sami2 model an equatorward neutral wind is identified as the major cause of the wsa while the downward flux from the plasmasphere provides an additional plasma source to enhance or maintain the density of the anomalous structure

connections between deep tropical clouds and the earth s ionosphere 1 we report on a series of simulations with the national center for atmospheric research ncar thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm which were designed to replicate and facilitate the interpretation of the longitudinal structure discovered in image satellite airglow observations of the equatorial ionization anomaly eia at the far ultraviolet fuv 135 6 nm wavelength during march april 2002 equinox our time gcm results indicate that the four peaked longitudinal variation in the eia observed by image fuv near 20 00 local solar time can be explained by the effects of an eastward propagating zonal wavenumber 3 diurnal tide de3 that is excited by latent heat release associated with raindrop formation in the tropical troposphere

behavior of the f2 peak ionosphere over the south pacific at dusk during quiet summer conditions from cosmic data 1 the six satellite constellation observing system for meteorology ionosphere and climate cosmic mission makes routine ionospheric measurements over the entire globe using occultation techniques these observations have been used in this study to develop global scale climate maps of nmf2 and hmf2 during the southern northern summer winter enhanced electron densities that appear to be associated with the southern equatorial appleton anomaly are displaced far southward at dusk and within about an hour form the weddell sea anomaly coincidentally the height of the f2 peak increases on the northern boundary of this anomaly this height increase is also displaced southward as the enhanced electron densities are displaced southward suggesting that the electron density increases are associated with the f2 peak rising as well as being an interesting phenomenon in its own right this behavior may shed new light on the formation of the weddell sea anomaly no unambiguous explanation for this behavior can be determined from the data presently available but an examination of some possibilities suggests that an evening downward flux of plasma from the plasmasphere may be at least partly responsible for the phenomenon

midlatitude summer nighttime anomaly of the ionospheric electron density observed by formosat 3 cosmic 1 this paper presents monthly variations of the midlatitude summer nighttime anomaly msna of the ionosphere for the first time by global observations of the formosat 3 cosmic f3 c mission the anomaly is characterized by the greater nighttime 1800 lt 0200 lt ionospheric electron density than during daytime 0800 1800 lt at middle latitudes during months around june and december solstices the anomaly shown during december solstice was known as the weddell sea anomaly wsa occurring around the antarctica and nearby the pacific ocean this paper demonstrates that the wsa like feature also exists in the northern hemisphere and is most prominent near the northeast asia europe africa and central pacific longitudes around june solstice in both hemispheres the anomalies with similar electron density characteristics and variations caused by the similar mechanism prompts us to name this phenomenon the msna the monthly f3 c observations indicate that the anomaly appears as the most prominent structure of the global ionosphere around midnight hours

seismic structure of the crust and uppermost mantle in the incipient stage of back arc rifting northernmost okinawa trough 1 in 1999 a marine seismic structure survey was conducted in the northernmost okinawa trough outcrops of the acoustic basement through the sedimentary layers are seen in the obtained structures velocities of lower layer in upper crust are varying laterally from 5 7 to 6 2 km s and thickness of 6 0 km s layer ranges from 3 to 9 km the lower crust has uniform thickness of about 15 km the pn velocity is 7 7 7 8 km s and the total crustal thickness is about 25 km including sedimentary layers the structure is in a first approximation similar to that of island arcs however the large heterogeneity of upper crust can be interpreted as a result of crustal thinning the low pn velocity and the outcrops of the acoustic basement relate to igneous activity in this area these results suggest that the northernmost part of the okinawa trough is in the incipient stage of a back arc rifting

structure and origins of the weddell sea anomaly from tidal and planetary wave signatures in formosat 3 cosmic observations and gaia gcm simulations the weddell sea anomaly wsa is a recurrent feature of the austral summer midlatitude ionosphere where electron densities are observed to maximize during the local nighttime in this study tidal decomposition is applied to formosat 3 formosa satellite constellation observing system for meteorology ionosphere and climate cosmic total electron content tec and electron density observations between 2007 and 2012 to quantify the components dominating local time and spatial variation in the wsa region our results present some of the first three dimensional spaceborne analyses of the wsa from a tidal perspective over multiple years we find that the features of the wsa can be reconstructed as the result of superposition between the dominant diurnal standing d0 eastward wave number 1 de1 westward wave number 2 dw2 and stationary planetary wave 1 spw1 components in tecs producing the characteristic midnight wsa peak the d0 de1 dw2 and spw1 components are found to be an interannually recurring feature of the southern midlatitude to high latitude ionosphere during the summer manifesting as enhancements in electron density around 300 km altitude of the summer middle to high magnetic latitudes the phases of the aforementioned nonmigrating diurnal signatures in electron density in this region are near evanescent suggesting in situ generation rather than upward propagation from below however the spw1 signature shows some signs of an eastward tilt with altitude suggesting possible downward propagation the relation of these components to possible generation via in situ photoionization or plasma transport along magnetic field lines is also discussed using results from the ground to topside model of atmosphere and ionosphere for aeronomy gaia general circulation model gcm connecting the tidal interpretation of the wsa to previously examined generation mechanisms

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

first observations of the midlatitude evening anomaly using super dual auroral radar network superdarn radars 1 under geomagnetically quiet conditions the daytime midlatitude ionosphere is mainly influenced by solar radiation typically electron densities in the ionosphere peak around solar noon previous observations from the millstone hill incoherent scatter radar isr have evidenced the presence of evening electron densities higher than daytime densities during the summer the recent development of midlatitude super dual auroral radar network superdarn radars over north america and japan has revealed an evening enhancement in ground backscatter during the summer superdarn observations are compared to data from the millstone hill isr confirming a direct relation between the observed evening enhancements in electron densities and ground backscatter statistics over a year of data from the blackstone radar show that the enhancement occurs during sunset for a few hours from april to september the evening enhancement observed by both superdarn and the millstone hill isr is shown to be related to recent satellite observations reporting an enhancement in electron densities over a wide range of longitudes in the northern hemisphere midlatitude sector during summer time finally global results from the international reference ionosphere iri and the horizontal wind model hwm07 are presented in relation with previously published experimental results and proposed mechanisms of the evening enhancement namely thermospheric horizontal winds and geomagnetic field configuration it is shown that the iri captures the features of the evening enhancement as observed by superdarn radars and satellites

unusual nighttime impulsive fof2 enhancement below the southern anomaly crest under geomagnetically quiet conditions 1 an unusual nighttime impulsive electron density enhancement was observed on 6 march 2010 over a wide region of south america below the southern crest of the equatorial anomaly under low solar activity and quiet geomagnetic conditions the phenomenon was observed almost simultaneously by the f2 layer critical frequency fof2 recorded at three ionospheric stations which are widely distributed in space namely cachoeira paulista 22 4 s 44 6 w magnetic latitude 13 4 s sao jose dos campos 23 2 s 45 9 w magnetic latitude 14 1 s brazil and tucuman 26 9 s 65 4 w magnetic latitude 16 8 s argentina although in a more restricted region over tucuman the phenomenon was also observed by the total electron content tec maps computed by using measurements from 12 gps receivers the investigated phenomenon is very particular because besides being of brief duration it is characterized by a pronounced compression of the ionosphere this compression was clearly visible both by the virtual height of the base of the f region h f recorded at the aforementioned ionospheric stations and by both the vertical electron density profiles and the slab thickness computed over tucuman consequently neither an enhanced fountain effect nor plasma diffusion from the plasmasphere can be considered as the single cause of this unusual event a thorough analysis of isoheight and isofrequency ionosonde plots suggest that traveling ionospheric disturbances tids caused by gravity wave gw propagation could have likely played a significant role in causing the phenomenon

topside ionospheric electron temperature and density along the weddell sea latitude it has been well known that the ionospheric electron density ne is greater in the summer nighttime than daytime around the weddell sea region which is named weddell sea anomaly wsa this paper for the first time reports unusual increases decreases of the daytime nighttime electron temperature te at about 830 km altitude over the wsa latitudes probed by tatiana 2 during december 2009 to january 2010 concurrent measurements at 660 830 km altitude observed by tatiana 2 detection of electro magnetic emissions transmitted from earthquake regions demeter and formosa satellite 3 constellation observing system for meteorology ionosphere and climate f3 c reveal the anticorrelation between te and ne along the wsa latitudes in the daytime and nighttime based on f3 c ne along the wsa latitudes observed at various local times the associated te values are computed the tatiana 2 and demeter observations as well as the computed results show that te yield the maximum values over the wsa region during daytime and over the indian and atlantic ocean area during nighttime the maxima or minima in f3 c ne and the computed te reveal eastward phase shifts

three dimensional ionospheric electron density structure of the weddell sea anomaly 1 this paper provides the first three dimensional description of the ionospheric density structure of the weddell sea anomaly wsa the wsa is characterized by a nighttime ionospheric density greater than that in daytime in the weddell sea region around the antarctic peninsula during the southern hemisphere summer it was first observed by ground based ionosondes located in the antarctica back in the 1950s and was further investigated by two dimensional maps over the oceans using tec measurements collected by the topex poseidon recently although these tec maps have provided two dimensional views for tracking the time evolution and spatial coverage of the wsa the vertical distribution of this peculiar feature is still unavailable with the vertical ionospheric density profiles observed by the formosat 3 cosmic three dimensional density structure of the wsa is presented here for the first time meanwhile a similar wsa signature is observed in the northern and eastern hemisphere during june solstice by both the gps tec and the formosat 3 cosmic electron density observations from the observed altitudinal structure of the wsa during 1800 2400 lt and the similar feature occurred in the opposite hemisphere suggest that the southward offset of the magnetic equator with respect to the geographic equator plays a major role for the wsa formation

longitudinal structure of the equatorial ionosphere time evolution of the four peaked eia structure 1 longitudinal structure of the equatorial ionosphere during the 24 h local time period is observed by the formosat 3 cosmic f3 c satellite constellation by binning the f3 c radio occultation observations during september and october 2006 global ionospheric total electron content tec maps at a constant local time map local time tec map referred as lt map can be obtained to monitor the development and subsidence of the four peaked longitudinal structure of the equatorial ionosphere from lt maps the four peaked structure starts to develop at 0800 1000 lt and becomes most prominent at 1200 1600 lt the longitudinal structure starts to subside after 2200 2400 lt and becomes indiscernible after 0400 0600 lt in addition to tec ionospheric peak altitude also shows a four peaked longitudinal structure with variation very similar to tec during daytime the four peaked structure of the ionospheric peak altitude is indiscernible at night with global local time maps of ionospheric tec and peak altitude we compare temporal variations of the longitudinal structure with variations of e b drift from the empirical model our results indicate that the observations are consistent with the hypothesis that the four peaked longitudinal structure is caused by the equatorial plasma fountain modulated by the e3 nonmigrating tide additionally the four maximum regions show a tendency of moving eastward with propagation velocity of several 10 s m s

motions of the equatorial ionization anomaly crests imaged by formosat 3 cosmic 1 the equatorial ionization anomaly eia structures and evolutions are imaged using radio occultation observation of the newly launched formosat 3 cosmic f3 c satellite constellation three dimensional ionospheric images provide unprecedented detail of the eia structure globally this paper presents images of the eia structures during july august 2006 and discusses the development and subsidence of the eia clear seasonal asymmetries in both ionospheric electron density and layer height are observed two dimensional cross section maps at a meridian provide dynamic variations and motions of the northern and southern eia crests results suggest that in addition to the asymmetric neutral composition effect interactions between the summer to winter transequatorial neutral winds and strength of the equatorial plasma fountain effect play important roles in producing asymmetric development of the eia crests as imaged by the f3 c

the influence of nonmigrating tides on the longitudinal variation of the equatorial electrojet 1 the climatological model of the equatorial electrojet eejm 1 derived from orsted champ and sac c satellite measurements provides the opportunity to investigate the longitudinal variation of the current strength in detail special emphasis is put in this study on the effect of nonmigrating tides we have found that the influence of the diurnal eastward propagating mode with wavenumber 3 de3 is particularly strong in polar orbiting satellite observations the de3 tidal signal appears as a four peaked longitudinal structure we have put special emphasis in our analysis to isolate the de3 contribution from other sources contributing to the wavenumber 4 structure in satellite data the amplitude of the de3 signature in the eej not only peaks during equinox seasons but is also strong around the june solstice when looking at the modulation of the eej intensity the de3 accounts for about 25 during the months of april through september it is thus the dominant cause for longitudinal variations during december solstice months the influence of de3 is negligible a secondary three peaked longitudinal pattern emerges during solstice seasons when the de3 influence is removed from the data available it is however not clear whether this pattern is related to any tidal drivers