on external and internal parts of the geomagnetic jerk of 1970 this paper is a short contribution to the debate on sources external or internal of the geomagnetic acceleration phenomenon jerk around 1970 using annual means of very quiet days it is shown here that the external part in the jerk signal in ydot in north europe was rather small there was a clear external part in z but the corresponding change in ydot was only 1 nt y 1

topographically controlled fronts in the ocean and their biological influence headlands islands and reefs generate complex three dimensional secondary flows that result in physical and biological fronts mixing and diffusion processes near these reefs and headlands are quite different from these processes in the open sea and classical advection diffusion models that were developed for the open sea are not valid near shore topographically generated fronts affect the distribution of sediments and they aggregate waterborne eggs larvae and plankton this aggregation influences the distribution and density of benthic assemblages and of pelagic secondary and tertiary predators

a note on the secular variation of the geomagnetic field abstract geomagnetic variations with apparent periods from 13 to 30 y have been reported by alldredge 1977 who has argued that the origin of these signals must be found in the core of the earth rather than outside the earth it is shown in this paper that a portion of these geomagnetic variations perhaps most of the variations might well be due to geomagnetic signals of much longer period originating in the core that appear to have periods of 13 30 y because of an artifact of the data processing much of the remainder of these variations could well be of external origin a method for processing these data is suggested that is superior to alldredge s method in minimizing the generation of spurious waves by long period secular variation

characterizing frontal eddies along the east australian current from hf radar observations the east australian current eac dominates the ocean circulation along south eastern australia however little is known about the submesoscale frontal instabilities associated with this western boundary current one year of surface current measurements from hf radars in conjunction with mooring and satellite observations highlight the occurrence and propagation of meanders and frontal eddies along the inshore edge of the eac eddies were systematically identified using the geometry of the high spatial resolution 1 5 km surface currents and tracked every hour cyclonic eddies were observed irregularly on average every 7 days with inshore radius 10 km among various forms of structures frontal eddies associated with eac meanders were characterized by poleward advection speeds of 0 3 0 4 m s migrating as far as 500 km south based on satellite imagery flow field kinematics show that cyclonic eddies have high rossby numbers 0 6 1 9 and enhance particle dispersion patches of intensified surface divergence at the leading edge of the structures are expected to generate vertical uplift this is confirmed by subsurface measurements showing temperature uplift of up to 55 m over 24 h and rough estimates of vertical velocities of 10s of meters per day while frontal eddies propagate through the radar domain independently of local wind stress upfront wind can influence their stalling and growth and can also generate large cold core eddies through intense shear such coherent structures are a major mechanism for the transport and entrainment of nutrient rich coastal or deep waters influencing physical and biological dynamics and connectivity over large distances

magnetic jerk gaining wider acceptance something seems to have happened within the earth s core that jerked the magnetic field in 1969 during its slow drift across the globe

secular variation in north america during historical times summary the secular variation of the declination inclination and total force of the geomagnetic field has been plotted for 74 locations in north america a comparison of the occurrences of maxima and minima in the sv clirves from different stations shows little evidence of drift in north america although a declination maximum exhibits westward drift up to 1915 all other extrema in declination inclination and total intensity occur almost simultaneously over a wide area the major feature of sv in north america appears to be a 4000 nt decrease in the total field since 1850 which may be due to a decrease in the dipole moment coupled with the decay of a large non dipole anomaly situated under the continent short period changes in the rate of decrease are possibly jerks of the magnetic field maximum entropy spectral analysis of all three components of the field indicates periods of 102 and 53 yr

varying geomagnetic anomalies and secular variation the time variations of the geomagnetic field at 10 european observatories were examined in detail fourier analyses were made for three field components at each of the observatories curves for each component were synthesized for three passbands a low passband for periods greater than 30 years a mid passband for periods equal to or less than 30 years but greater than 13 years and a high passband for periods equal to or less than 13 years the low passband results show only very smooth variations from which the linear prediction of secular variation as is now in use might give a good average result for 10 15 years if there were no other changes the high passband results show the solar cycle effect that has been adequately treated before the mid passband has an amplitude of tens of nt which changes differently at each observatory in such a way as to infer core sources these time varying anomalies are important to the understanding of secular variation

geomagnetic secular variation over spain 1970 1988 by means of spherical cap harmonic analysis abstract a spherical cap harmonic analysis was applied to spain and neighbouring areas covering a region with a cap halfangle of 16 the technique due to haines has allowed us to obtain a model of secular variation sv for a time interval of 18 years reduced to the epoch 1987 5 the total number of selected data points was 1581 provided from ten observatories and 60 repeat stations the sv model has a maximum spatial index of four and temporal degree of two the coefficients were tested and the statistically non significant terms removed another model covering a smaller region with greater density of data has been developed and both results compared the average secular variations derived from the definitive geomagnetic reference field dgrf and the international geomagnetic reference field igrf were compared with these models and maps plotting the calculated first field derivatives are presented the integration of the final model will serve to update satellite data in order to produce a regional main field model

geomagnetic secular variation impulses the secular variation of the earth s internal magnetic field changes on time scales of the order of a year such impulses occurred in 1969 and probably in 1913 they provide constraints on deep mantle conductivity which appears to be rather low and on the motions of the top of the fluid core which are probably dominated by westward drift present data favour the existence of upwelling of core material at the core mantle boundary the impulses can be correlated with extrema in the earth s rotation rate and interpreted in terms of electromagnetic core mantle coupling

the midlatitude ionosphere during the total solar eclipse of march 9 1997 the total solar eclipse of march 9 1997 was visible from some regions of china mongolia and east siberia during 0045 0130 ut the eclipse coincided with a relatively long geomagnetically quiet period during the total solar eclipse the observations included oblique incidence ionograms recording also measurement the total electron content along specified directions to the visible global positioning system satellites and the doppler sounding on various hf ray paths over the region under consideration were carried out this paper presents results derived from studying the spatial pattern of midlatitude ionospheric response to this eclipse on the basis of a numerical simulations calculations have been executed using a model for ionosphere plasmasphere coupling comparison of model results with data of all measurements showed a good qualitative and quantitative agreement it is shown that by and large the behavior of the ionosphere during the eclipse manifests itself as a short lasting l hour rearrangement to nighttime conditions with the ion flow directed downward from the plasmasphere as a rise of the f2 layer maximum by about 20 km and as a twofold increase in electron density at the height of the maximum during the eclipse s totality phase the electron temperature decreases by 200 400 k while the ion temperature drops only slightly it is found that changes in the spatial distribution of electron density along the hf ray paths during the eclipse give rise to variations of doppler frequency shift with amplitudes of about 1 hz and a duration of about 120 min the findings reported in this paper do not validate the hypothesis that solar eclipses generate atmospheric gravity waves and associated traveling ionospheric disturbances

new representation of geomagnetic secular variation over restricted regions by means of spherical cap harmonic analysis application to the case of spain abstract a model of the secular variation sv of the geomagnetic field over spain and adjacent regions using a particular application of spherical cap harmonic analysis scha is given scha was directly applied as presented in a previous paper however some drawbacks were detected making evident the limitations of the technique for fields with low spatial harmonic content as in the case for sv over relatively small regions after trying some alternative approaches to the conventional procedure it has been discovered that the solution lies in increasing the cap half angle but the data continue to be restricted within the original cap to obtain more suitable harmonic contributions it is shown that inspection of the spatial power spectra of the sv field on different spherical caps is useful in selecting the cap half angle

upgraded norwegian magnetic observatories in the arctic a description is given of a new digital fluxgate magnetometer system to be run at the following stations in the norwegian arctic region ny alesund bjornoya jan mayen ramfjordmoen and the auroral observatory in tromso summaries of the annual quiet mean values of the h and d components as well as the total field strength f observed at tromso bjornoya and ny alesund for the last decades are also presented the magnetic field strength reached a maximum at all three stations in the period 1975 1977 the maximum was first reached at the northernmost station indicating a phase shift from north to south

in situ observations of phase transition between perovskite and cairo 3 type phase in mgsio 3 and pyrolitic mantle composition abstract in situ observations of the perovskite cairo3 phase transition in mgsio3 and in pyrolitic compositions were carried out using a laser heated diamond anvil cell interfaced with a synchrotron radiation source for pure mgsio3 the phase boundary between the orthorhombic mg perovskite and cairo3 type phases in the temperature range of 1300 3100 k was determined to be p gpa 130 3 0 0070 0 0030 t 2500 k using platinum as a pressure calibrant we confirmed that the cairo3 type phase remained stable up to pressures of at least 156 gpa and temperatures of 2600 k the consistency of our results with previous theoretical calculations leads us to conclude that the 2700 km seismic discontinuity at the bottom of the lower mantle can be attributed to a phase transition to the cairo3 type phase the phase change from an orthorhombic mg perovskite to a cairo3 type bearing assemblage in a pyrolitic mantle composition was also observed at p 125 gpa which corresponds to the same mantle depth as the seismic discontinuity the phase boundary between the orthorhombic mg perovskite and cairo3 type bearing assemblage was determined to be p gpa 124 4 0 008 0 005 t 2500 k using gold as a pressure calibrant this transition boundary indicates that the temperature at a depth of 2700 km is about 2600 k and the adiabatic temperature gradient in the lower mantle is estimated to be 0 31 k km the partition coefficients and the effect of some elements on the phase equilibrium between the orthorhombic mgsio3 perovskite and cairo3 type mgsio3 were estimated from ab initio calculations our experimental and theoretical results indicate that the d layer consists of a cairo3 type bearing assemblage which is likely to have significant effect on the chemical and thermal evolution of the earth s mantle

secular variation from monthly means from apia and amberley magnetic observatories summary the availability of data from magnetic observatories in machinereadable form means that we can now study secular change internal variations of the magnetic field by analysing time series with a rapid sampling rate rather than annual means the major task is to identify and remove fields due to external sources the major benefit is finer time resolution which is particularly important because of the recent interest in sudden phenomena such as geomagnetic impulses and jerks apia and amberley observatories provide good data for experimentation many external fields are satisfactorily excluded by taking averages of hourly values at local midnight for the five international quiet days of the month and removing annual and semi annual lines by least squares quiet days based on the k index are found to be at least as suitable as those based on the ae and dst indices time series of monthly means are developed for all components at both observatories from 1921 apia and 1923 amberley cubics provide good fits to the long term variations but the residuals to these cubics are highly significant an 11 yr variation is noticeable showing that we have not succeeded in removing this external source of noise residual curves for both observatories which are some 30 apart follow each other quite closely the large swings with a period of approximately 22 yr that were noted to be in phase with the double sunspot cycle by beagley bullen have not continued in phase the magnetic field is widely believed to have undergone a change around 1969 70 this change is now known as the geomagnetic jerk the finer time resolution afforded by monthly means is used to investigate the duration of this event the resolution is limited to 6 18 month by noise in the measurements a rapid change in the y component occurs effectively instantaneously at 1970 6 for apia and 1972 4 for amberley we propose our method as a

an estimate of average lower mantle conductivity by wavelet analysis of geomagnetic jerks it has recently been proposed that geomagnetic jerks observed at the earth s surface could be viewed as singularities in the time behavior of the geomagnetic field with a regularity of about 1 5 when wavelet analyzed such a signal should have suffered some distortion when diffusing from the core mantle boundary cmb through the conducting mantle assuming that the upper mantle is an insulator and given the electromagnetic time constant of the mantle we compute the distortion that a pure singularity introduced at the cmb suffers as it traverses the mantle we compute this distortion through its effects on the so called ridge functions extracted from the wavelet transform of the signal this distortion is very similar to the small but significant one that we observe in real data we therefore speculate that jerks must have been pure singularities at the base of the mantle and infer an average estimate for the mantle electromagnetic time constant from the way the signal is distorted by fitting the synthetic ridge functions to the experimental ones assuming for example a thickness of 2000 km for a uniform lower conducting mantle we find an electrical conductivity smaller than 10 s m 1 this value is in reasonable agreement with values derived from high pressure experiments for a silicate mantle

implications of a long lived basal magma ocean in generating earth s ancient magnetic field 1 observations of earth s magnetic field extending back to 3 45 billion years ago indicate that generation by a core dynamo must be sustained over most of earth s history however recent estimates of thermal and electrical conductivity of liquid iron at core conditions from mineral physics experiments indicate that adiabatic heat flux is approximately 15 tw nearly three times larger than previously thought exacerbating difficulties for driving a core dynamo throughout earth history by convective core cooling alone here we explore the geomagnetic consequences of a basal magma ocean layer in the lowermost mantle hypothesized to exist in the early earth and perhaps surviving until well after the archean while the modern solid lower mantle is an electromagnetic insulator electrical conductivities of silicate melts are known to be higher though as yet they are unconstrained for lowermost mantle conditions we consider a range of possible electrical conductivities and find that for the highest electrical conductivities considered a long lived basal magma ocean could be a primary dynamo source region this would suggest the proposed three magnetic eras observed in paleomagnetic data originate from distinct sources for dynamo generation from 4 5 to 2 45 ga within a basal magma ocean from 2 25 to 0 4 ga within a superadiabatically cooled liquid core and from 0 4 ga to present within a quasi adiabatic core that includes a solidifying inner core

historic and paleomagnetic secular variation and the earth s core dynamo process this paper attempts to summarize briefly the recent and ongoing efforts of the geomagnetism and paleomagnetism community to understand both the earth s magnetic field secular variation and its implications for the core dynamo process secular variation is defined in this paper as the general spatio temporal variation of the field secular change will refer to the first time derivative of the spatial field strict geomagnetic definition of secular variation we will emphasize here the long term 10 year or longer behavior of the historic secular variation hsv for this is the variation that is comparable with the paleomagnetic secular variation psv two important related topics the 1969 1970 geomagnetic jerk in secular acceleration and the spatial geomagnetic field components due to crustal or mantle sources are treated more fully in other reviews

lateral variations in d thickness from long period shear wave data we explore global variations in d shear wave structure by examining 12 years of long period global digital seismograph network gdsn data to identify reflected phases from the d layer in the lowermost mantle we restrict our search to epicentral distances between 63 and 74 where a precritical d reflection will lie between the s and scs arrivals with gdsn long period records a d signal at these ranges is generally obscured by the stronger neighboring phases so to isolate the d reflections we develop a technique for stripping away the interfering s and scs waveforms we interpret travel time variations in the observed sds phases in terms of variations in d thickness and generate maps of inferred d thickness for areas of good seismic coverage the regions of observed d reflections are beneath australasia north central asia the arctic alaska and central america we find considerable variation in d structure in these regions a result consistent with previous studies which have found evidence for lateral heterogeneity in d on average the estimated d thickness is 260 km but there is a fairly uniform distribution of thicknesses between 150 km and 350 km a possible correlation is observed between d thickness and regions of predicted strong horizontal mantle flow

lower mantle conductivity anomalies estimated from geomagnetic jerks 1 the occurrence epochs and duration periods of the 1969 1978 and 1991 geomagnetic jerks are estimated for 65 geomagnetic observatories by applying a statistical time series model to monthly means of the geomagnetic eastward component it is found for every jerk that the occurrence epochs around south africa and south pacific ocean are delayed several years from those in other regions moreover it is found for the 1969 and 1978 jerks that the duration times around south africa which are defined as the lengths of the duration periods are longer than those in other regions the distribution of these anomalous regions of the jerks i e south africa and south pacific ocean may imply the existence of higher conductivities in the lower mantle beneath these regions which possibly relates to the fact that the velocity of a seismic s wave in the d layer beneath these regions is slower than that in other regions the mantle conductivities beneath south africa south pacific ocean and other regions are estimated through a numerical simulation the magnetic diffusion equation in the mantle is solved for an abrupt magnetic change generated at the core mantle boundary comparing the results obtained by the simulation with those obtained by the data analysis it is plausible that the mantle conductivities beneath south africa and south pacific ocean are higher than that beneath other regions although there is an ambiguity in the estimated magnitudes of the mantle conductivity

a study of tomographically reconstructed ionospheric images during a solar eclipse the low latitude ionospheric tomography network litn consists of a chain of six naval navigation satellite system nnss receiving stations established along 121 e longitude from a geographic latitude of 14 6 n to 31 n it is specifically designed to observe large scale ionospheric variations over the equatorial anomaly region by using tomographic imaging techniques recently the network litn was applied to observations of the october 24 1995 solar eclipse two dimensional images of ionospheric electron density during the eclipse period were reconstructed these images and the corresponding results from a nearby ionosonde were compared with those for a reference day it is shown that during the eclipse day the ionosphere experienced some large scale changes in particular four episodes of electron density enhancement or depression have been identified 1 the maximum enhancement occurred before the maximum phase of the solar eclipse at approximately 7 10 n geomagnetic latitude at the 275 300 km ionospheric height 2 the second enhancement appeared roughly 3 1 2 hours after the maximum obscuration at 15 22 n geomagnetic latitude and 300 325 km ionospheric height 3 the largest electron density depression occurred roughly 2 hours after the maximum obscuration at approximately 9 15 n geomagnetic latitude and on both the bottom and topside ionosphere 4 the second depression occurred about 4 hours after the maximum obscuration at approximately 5 5 n geomagnetic latitude and mainly on the topside ionosphere more detailed study suggests that the two enhancements have their origins in the ionospheric day to day variations the first depression is related to the combined photochemical and the equatorial fountain effects and the second depression may have its origin in geomagnetic coupling between conjugate ionospheres these observations are interpreted within the framework of ionospheric dynamics in the equatorial anomaly region

spherical cap harmonic analysis of super dual auroral radar network superdarn observations for generating maps of ionospheric convection 1 a spherical cap harmonic analysis scha technique is introduced for mapping the 2 d high latitude ionospheric convection pattern based on super dual auroral radar network superdarn velocity measurements the current method for generating such maps is the fit technique which generates global scale maps over the entire convection region this is accomplished by combining observations with a statistical model to prevent unphysical solutions in areas away from the observation points and by forcing the plasma flow to zero at the low latitude boundary of the convection zone both constraints distort the mapped convection and require a preconception of where the plasma flow lines should close by focusing on mapping the convection over a region well covered by velocity observations the scha technique is freed of these constraints and more accurately reproduces local convection we generate large scale convection maps from superdarn data for various interplanetary magnetic field imf conditions during periods of widespread radar coverage to show the patterns are consistent with expectations for various imf configurations we validate the scha maps by comparing them with the 2 d ion drifts measured by the dmsp satellites and with the 2 d convection vectors obtained by merging superdarn measurements at beam crossings the scha technique is shown to perform comparably to the fit technique over regions of good data coverage for limited data coverage and over regions of highly variable flow particularly near the equatorward edge of the mapping region the scha technique provides a better solution for mapping ionospheric convection based on superdarn radar observations

on the geomagnetic jerk of 1969 french and british scientists have published reports describing a sudden change in the geomagnetic secular acceleration which took place around 1969 they claimed that this change called an impulse or jerk took place in a period of a year or two and that the sources for the jerk were internal a recent paper by alldredge questioned their method of analysis and concluded that abrupt changes in the observed field which occur in a year or two do not originate in the core of the earth this paper questions alldredge s conclusion and provides further evidence in support of the reports of the french and british investigators the average centroid of the jerk is found to be 1969 5 evidence is presented that indicates that the centroid of the jerk differs for the various spherical harmonics so that the centroid of the degree one spherical harmonic is significantly later than the centroids for the next three harmonics new estimates for the spherical harmonic coefficients for the jerk and the prejerk and postjerk secular acceleration are given some possible further investigations are suggested

geomagnetic jerks as chaotic fluctuations of the earth s magnetic field 1 the geomagnetic field is chaotic and can be characterized by a mean exponential time scale after which it is no longer predictable it is also ergodic so time analyses can substitute the more difficult phase space analyses taking advantage of these two properties of the earth s magnetic field a scheme of processing global geomagnetic models in time is presented to estimate fluctuations of the time scale here considering that the capability to predict the geomagnetic field is reduced over periods of geomagnetic jerks we propose a method to detect these events over a long time span this approach considers that epochs characterized by relative minima of fluctuations in time scale i e those periods when a geomagnetic field is less predictable are possible jerk occurrence dates we analyze the last 400 years of the geomagnetic field covered by the gufm1 model to detect minima of fluctuations i e epochs characterized by low values of the time scale most of the well known jerks are confirmed through this method and a few others have been suggested finally we also identify some short periods when the field is less chaotic more predictable than usual naming these periods as steady state geomagnetic regime to underline their opposite behavior with respect to jerks

gnss observations of ionospheric variations during the 21 august 2017 solar eclipse u s national science foundation grant nasa grant nasa lws funding support muri grant onr

coral reef habitat mapping a combination of object based image analysis and ecological modelling despite being one of the most important and well studied coral reefs in the world the full extent of coral habitat of the great barrier reef gbr is not well mapped and there is no current and comprehensive map of the gbr s geomorphic zonation or benthic composition this paper demonstrates an approach that integrates ecological coral habitat mapping with empirical modelling to map the geomorphic zonation and benthic composition of the shallow offshore reefs of the gbr using the capricorn bunker group cbg as a case study the approach combined environmental data sets and geo ecological rule sets to identify geomorphic zones the benthic composition of individual geomorphic zones was mapped for shallow reef flat zones using object based image analysis with context driven rules based on coral reef ecology and reef slope zones using levels of wave exposure to predict the distribution of coral types the environmental data sets used were field based benthic composition data landsat 8 oli satellite image derived bottom reflectance water depth and slope 15 m 15 m pixel size data reef impact data and modelled wave exposure the study showed that the combination of geomorphic ecological rules and models with remote sensing imagery provided robust mapping results over a large 2500 km2 reef system of which 245 km2 was mapped as shallow coral reefs and 88 km2 of that was mapped as areas containing coral most importantly the method produced defined the geomorphic zones and benthic composition of a study area that is significantly larger than the majority of coral reef remote sensing mapping projects previously published with some modifications the methods presented have the potential to be applied to the full extent of the shallow offshore reefs of the gbr or any large reef globally monitoring and management of coral reefs for conservation and other purposes at regional to global scales will benefit from the ability to produce and use this type of essential information on a regular basis

eddy induced cross shelf export of high chl a coastal waters in the se bay of biscay abstract different remote sensing data were combined to characterise a winter anticyclonic eddy in the southeastern bay of biscay and to infer its effects on cross shelf exchanges in a period when typical along shelf slope currents depict a cyclonic pattern while the joint analysis of available satellite data infrared visible and altimetry permitted the characterisation and tracking of the anticyclone properties and path data from a coastal high frequency radar system enabled a quantitative analysis of the surface cross shelf transports associated with this anticyclone the warm core anticyclone had a diameter of around 50 km maximum azimuthal velocities near 50 cm s 1 and a relative vorticity of up to 0 45f the eddy generation occurred after the relaxation of a cyclonic wind driven current regime over the shelf slope then the eddy remained stationary for several weeks until it started to drift northwards along the shelf break the surface signature of this eddy was observed by means of high frequency radar data for 20 consecutive days providing a unique opportunity to characterise and quantify from a lagrangian perspective the associated transport and its effect on the chl a surface distribution we observed the presence of mesoscale structures with similar characteristics in the area during different winters within the period 2011 2014 our results suggest that the eddy induced recurrent cross shelf export is an effective mechanism for the expansion of coastal productive waters into the adjacent oligotrophic ocean basin

on the identification of magsat anomaly charts as crustal part of the internal field satellite magnetic anomaly charts are derived from the measurements by subtracting a global field model regarded as representative of the core field after removal of the external field effects the field model comprises all spherical harmonic terms of the internal field up to a certain degree n say n 13 hence the large scale constituents of the crustal field are also removed from the anomaly charts the calculated magnetic field of a global magnetization model of the earth s crust gives an estimate of the lacking lower order crustal terms it turns out that the omitted crustal field part is of the same order of magnitude as the rest of the field the geographic distribution of the omitted model field part on a continental scale is closely correlated with the depth of the moho by analogy the same conclusion is drawn with respect to the actual crustal field this result accounts for the fact that in the satellite charts the continental margins are not very well reflected considerable corrections are therefore required if the anomaly charts are intended to display the whole crustal field

a geomagnetic jerk for the early 1990 s abstract records from geomagnetic observatories in europe are showing indications of another impulse in the third time derivative of the east component around 1991 these impulses otherwise known as jerks have occurred in the past at about 1925 1969 and 1978 and possibly also 1900 and 1913 they are believed to be internal in origin and have implications for the understanding of the fluid motions in the core which create them or for lower mantle conductivity or both

long period 30 days 1 year electromagnetic sounding and the electrical conductivity of the lower mantle beneath europe the c response connects the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic variations and forms the basis for deriving the conductivity depth profile of the earth time series of daily mean values at 42 observatories typically with 50 years of data are used to estimate c responses for periods between 1 month and 1 yr the z y method is applied which means that the vertical component is taken locally whereas the horizontal components are used globally by expansion in a series of spherical harmonics in combination with results from previous analyses the method yields consistent results for european observatories in the entire period range from a few hours to 1 yr corresponding to penetration depths between 300 and 1800 km 1 d conductivity models derived from these results show an increase in conductivity with depth z to about 2 s m 1 at z 800 km and almost constant conductivity between z 800 and z 2000 km with values of 3 10 s m 1 in good agreement with laboratory measurements of mantle material below 2000 km the conductivity is poorly resolved however the best fitting models indicate a further increase in conductivity to values between 50 and 150 s m 1

a global analysis of the 1991 geomagnetic jerk summary a recent examination of the geomagnetic annual mean values for the european magnetic observatories has shown the existence of a sudden change in the secular acceleration in about 1991 cafarella meloni 1995 macmillan 1996 using first differences of the y east geomagnetic field component mean values from 74 observatories the worldwide character of the 1991 impulse has been determined de michelis et al 1998 using data from 109 observatories widely distributed all over the world the structure of the secular variation for the x north and z vertical magnetic field intensities around 1990 was investigated and evidence of this most recent jerk was found external effects were removed from the annual mean data by comparing the long term variations of the geomagnetic field components at individual observatories with the long term variations of two geomagnetic indices aa and dst and of a solar index the wolf number r a careful analysis has been carried out on the amplitude of the external disturbance on its dependence on latitude and on the weights of the geomagnetic indices in the evaluation of the resulting external field the secular variation has been evaluated from the corrected annual means around 1990 the secular variation can be fitted at many observatories by two straight lines with a sudden and marked change in slope in this manner the jerk occurrence time and the intensity of the step in the second time derivative x y and z were computed maps of x y and z provide information on the worldwide intensity distribution of the examined event maps of the jerk occurrence time distributions are also given the mean jerk occurrence time is 1990 1 0 6 finally a spherical harmonic analysis was used to complete the quantitative description of this phenomenon in order to study the trend of the energy density spectrum as a function of the harmonic degree n

gps tec response to the 22 july 2009 total solar eclipse in east asia 1 the longest total solar eclipse of this century occurred in east and south asia on 22 july 2009 the eclipse was accompanied with a medium magnetic storm whose main phase onset occurred 27 min after the passage of the moon s umbral shadow using tec data from 60 gps stations we construct differential tec maps to investigate the ionosphere response to the solar eclipse in central china in the range of 26 n 36 n 108 e 118 e i e the magnetic latitude 15 n 25 n during the eclipse s totality a shadow in the ionosphere shown as tec depletion area was formed 100 km south of the moon s umbral path with a maximum decrease of 5 tecu the tec depletion area moved eastward following the movement of the totality area with a time lag of 10 min enhancements of tec due to the storm are observed after the main phase onset the relative drop of tec due to the solar eclipse is evidently larger at lower latitudes than that at higher ones and around noontime than that in the morning by modeling work we find that the latitudinal dependence of the tec response may result from latitudinal variation of magnetic inclination which influences the diffusion of ionization among different layers besides the local time dependence of tec response is closely related to the local time variation of background atmosphere density which affects the electron loss efficiency in the ionosphere

the electrical conductivity of the mantle beneath europe derived from c responses from 3 to 720 hr summary the c response which connects the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic fluctuations forms the basis for estimating the conductivity depth profile of the earth this paper describes new estimates of the c response obtained from observatory hourly mean values the z y method is applied which means that the vertical component is used locally whereas the horizontal components are used globally by expansion into terms of spherical harmonics a special evort is made to obtain unbiased estimates of c when applied to 90 months of hourly mean values from about 90 observatories the method yields consistent results for european observatories in the entire period range from 3 to 720 hr and for two diverent source mechanisms s and d st a good description of the source structure for individual time segments is essential this was achieved by a separate spherical harmonic analysis of the data for each month for d st or each day for s and by estimating a large number 120 of expansion coeycients the results are interpreted by means of 1 d conductivity models which show that the upper mantle has remarkably little structure with a monotonic decrease of resistivity from 100 v m near z 200 km to 0 7 v m below z 1000 km

possible refugia for reefs in times of environmental stress this paper investigates the refuge potential of 1 upwelling areas 2 coral areas at medium depth and 3 offshore bank and island reefs in a scenario of increased global warming and thus increased sea surface temperature sst and increased solar uv radiation 1 observations on coral health and water temperature in the subtropical atlantic eleuthera and cat island bahamas and indian ocean sodwana bay south africa suggest a link between cool water delivered by upwelling and coral health after the 1998 bleaching event caused by strong sst anomalies coral health and recovery from the previous year s bleaching was significantly better on the narrow southern cat island shelf 70 of corals healthy where the presence of cold water was observed which was attributed to small scale upwelling than on the wide northern eleuthera shelf 44 of corals healthy where downwelling of hot bank waters was believed to have damaged corals in south africa regular short term upwelling events in five summers reduced sst to well below bleaching level 2 in the northern red sea safaga bay and in south africa sodwana bay wide areas with either coral frameworks or non framework communities exist calculations show that if the top 10 m 20 m of the ocean became inhospitable to corals still 50 4 17 5 of the coral area would remain intact in the red sea and 99 40 in south africa 3 offshore bank and island reefs investigated in the turks caicos and mouchoir banks and grand and little cayman showed high rates of mortality and coral diseases the most remote sites mouchoir bank were not the healthiest refuge areas appear to exist in 1 and 2 but in 3 only if vigorous water circulation is encountered

the hydraulics of density currents over estuarine sills it is demonstrated here that density currents being sensitive to the underlying topographies are subject to topographic control in estuaries indented with mild sills as a density current flows over a ridge that current shoals downstream but deepens upstream rendering the upstream flow more subcritical the earth s rotation weakens the topographic influence upstream and downstream and the vertical mixing of momentum and density also weakens the influence conventionally it was thought that the two layer estuarine circulation characterized by the upward entrainment of a landward undercurrent will be set up following the seaward passage of the density front in the presence of bottom topographies however there should be a final phase of adjustment during this phase there is a downward entrainment of the flow upstream of sills and a consequent modulation of the estuarine circulation

electrical conductivities of pyrolitic mantle and morb materials up to the lowermost mantle conditions abstract the electrical conductivities of natural pyrolitic mantle and morb materials were measured at high pressure and temperature covering the entire lower mantle conditions up to 133 gpa and 2650 k in contrast to the previous laboratory based models our data demonstrate that the conductivity of pyrolite does not increase monotonically but varies dramatically with depth in the lower mantle it drops due to high spin to low spin transition of iron in both perovskite and ferropericlase in the mid lower mantle and increases sharply across the perovskite to post perovskite phase transition at the d layer we also found that the morb exhibits much higher conductivity than pyrolite the depth conductivity profile measured for pyrolite does not match the geomagnetic field data below about 1500 km depth possibly suggesting the existence of large quantities of subducted morb crust in the deep lower mantle the observations of geomagnetic jerks suggest that the electrical conductivity may be laterally heterogeneous in the lowermost mantle with high anomaly underneath africa and the pacific the same regions as large low shear wave velocity provinces such conductivity and shear wave speed anomalies are also possibly caused by the deep subduction and accumulation of dense morb crust above the core mantle boundary

the near earth magnetic field at 1980 determined from magsat data data from the magsat spacecraft for november 1979 through april 1980 and from 91 magnetic observatories for 1978 through 1982 are used to derive a spherical harmonic model of the earth s main magnetic field and its secular variation at epoch 1980 0 the model is called gsfc 12 83 constant coefficients are determined through degree and order 13 and secular variation coefficients through degree and order 10 the first degree external terms and corresponding induced internal terms are given as a function of dst preliminary modeling using separate data sets at dawn and dusk local time showed that the dusk data contain a substantial field contribution from the equatorial electrojet current the final data set was therefore selected first from dawn data and then augmented by dusk data to achieve a good geographic data distribution for each of three time periods 1 november december 1979 2 january february 1980 and 3 march april 1980 a correction for the effects of the equatorial electrojet was applied to the dusk data utilized the solution included calculation of fixed biases or anomalies for the observatory data although similar in many respects gsfc 12 83 differs from international geomagnetic reference field 1980 by 3 6 nt in the g10 term and shows a slightly negative b in the northern polar region as well as other differences in secular variation pattern

jerks abound an analysis of geomagnetic observatory data from 1957 to 2008 we present a two step method for the removal of external field signals and the identification of geomagnetic jerks in magnetic observatory monthly mean data providing quantitative uncertainty estimates on jerk occurrence times and amplitudes with minimal a priori information we apply the method to the complete time series of x y and z components at up to 103 observatory locations in the period of 1957 2008 we find features fitting the definition of jerks in individual components to be frequent and not globally contemporaneous identified regional jerks have no consistent occurrence pattern and the most widespread in any given year is identified at 30 of observatories worldwide whilst we identify jerks throughout the period of study relative peaks in the global number of jerk occurrences are found in 1968 71 1973 74 1977 79 1983 85 1989 93 1995 98 and 2002 03 with the suggestion of further poorly sampled events in the early 1960s and late 2000s the mean uncertainties on individual jerk occurrence times and amplitudes are found to be 0 3 yrs and 2 1 nt yr2 respectively for all field components jerk amplitudes suggest possible periodic trends across europe and north america which may be related to the 6 yr periods detected independently in the secular variation and length of day

long period geomagnetic variations and mantle conductivity an inversion using bailey s method summary we have implemented an algorithm which is based on bailey s solution of the inverse problem of electromagnetic induction in the earth the study was motivated by recent determinations of very long period data and also benefited from recent redeterminations of high frequency data the algorithm has been successfully tested to provide reliable estimates of conductivity down to a depth of 2000 km using synthetic data in the period range from 4 days to 11 years smooth data sets which are required for the inversion were constructed from various sources at a given depth the range of inverted models is less than one order of magnitude due to the lack of high frequency data the conductivity of the upper 600 km of the mantle which is found to be of the order of lo i2 l m may be overestimated the algorithm performs well in the middle mantle where conductivity rises steadily from 1 to 50 i2 l m the lack of very low frequency data and limitations of the algorithm prevent one from obtaining meaningful estimates in the lower mantle however the study of the propagation of the late 1960s secular variation acceleration provides an estimate of the mean conductivity of the whole mantle thus a complete mantle profile can be constructed it is found that deep mantle conductivity probability does not exceed a few hundred i2 l m

tracking geomagnetic impulses at the core mantle boundary abstract the main magnetic field of the earth is generated and maintained by convective motions in the fluid outer core temporal field variations occur on an impressive variety of scales ranging from months to millions of years among these the most intriguing features are geomagnetic impulses or jerks v courtillot j ducruix and j l le mouel sur une acceleration recente de la variation seculaire du champ magnetique terrestre acad sci paris c r d287 1978 1095 1098 j l le mouel j ducruix and c ha duyen geophys res lett 5 1983 369 1 2 here we investigate these very rapid events at the surface of the earth s core using a global model designed for use at this depth we identify two regions of very active variations where wave like structures propagate the geometrical characteristics of these wave like motions provide new constraints on the intricate dynamics of the core

on the recent variations of the apparent westward drift rate quantitative study of the westward drift w d was initiated by bullard et al 1950 and then followed by many others using more or less similar methods e g yukutake 1962 harwood and malin 1976 more recently magsat results have enhanced the interest in the subject see for example fig 1 of peddie 1982 in the present paper we compute the w d rate in the 1950 1980 time interval using different methods and we establish that the 1969 1970 secular acceleration impulse the so called jerk clearly shows up in the results

nighttime ionospheric enhancements induced by the occurrence of an evening solar eclipse 1 the solar eclipse on 15 january 2010 traversed asia and completed its travel on the shandong peninsula in china at sunset two vertical incidence ionosondes at wuhan and beijing and the oblique incidence ionosonde network in north china were implemented to record the ionospheric response to the solar eclipse following the initial electron density decrease caused by the eclipse the ionosphere was characterized by a strong premidnight enhancement and a subsequent ionospheric decay and a 10 h later postmidnight enhancement neither geomagnetic disturbance occurred during the eclipse day nor did obvious nighttime peak appear for the 10 day mean of the f2 layer critical frequency fof2 the electron density profilogram of the beijing ionosonde indicates that the two enhancements were the result of the plasma flux downward from the top ionosphere possibly due to the steep decrease of the ionospheric electron density and plasma temperature during the solar eclipse the two dimensional differential fof2 maps present the regional variations of the nighttime electron density peaks and decay both the pre and postmidnight enhancements initially appeared in a belt almost in parallel with the eclipse track and then drifted southward the different magnitudes of greatest eclipse in the umbra and outside tend to account for the different occurrence times of the plasma flux the ionospheric decay following the premidnight enhancement is also considered as a consequence of the eclipse shade

electromagnetic core mantle coupling iii laterally varying mantle conductance abstract we consider the implications of lateral variations in deep mantle conductance for electromagnetic core mantle coupling and the consequent decadal variations in length of day large scale variations in conductance can alter long term trends in the torque but do not on average enhance its magnitude or alter its decadal variation many previous estimates of the torque predict an unobserved negative trend during this century this trend may result from modelling assuming an incorrect conductance structure by suitable choice of this structure the trend can be removed but there is no correlation between such a structure and seismological observations of lower mantle heterogeneity runcorn argued that a highly conducting layer in the deep mantle beneath the pacific could account for features of the observed secular variation and also preferred palaeomagnetic vgp reversal paths we argue that the conductance customarily advocated for this mechanism is inconsistent with core mantle angular momentum exchange on decadal time scales

was the 1970 geomagnetic jerk of internal or external origin annual mean data from worldwide magnetic observatories show that there was a jerk1 step change in the second time derivative in the geomagnetic field which took place over an interval of 2 yr around 1970 if such a short lived phenomenon originated in the earth s core and were still detectable after passing through the mantle to the surface this would imply a much lower conductivity for the lower mantle than had been believed previously here we show by an objective test that most of the jerk has an internal origin

measurements and uncertainties of the occurrence time of the 1969 1978 1991 and 1999 geomagnetic jerks geomagnetic jerks are rapid time variations of the magnetic field at the earth s surface that are thought to be of primarily internal origin jerks are relevant for studies of the earth interior they likely give information on core dynamics and possibly on mantle electrical conductivity in such studies a precise determination of the jerk occurrence time and its error bar at each observatory is required we analyze the most well known global jerks 1969 1978 and 1991 and a possible local jerk in 1999 considering all three components of the magnetic field x y and z different data sets are investigated annual means 12 month running averages of observatory monthly means in rotated geomagnetic dipole coordinates and data representing the core field contribution synthesized from the cm4 time dependent field model the secular variation in each component of the field around the time of a jerk was modeled by two straight line segments using both least squares and 1 norm methods the 1969 1978 and 1991 jerks were globally detected while the 1999 event was only locally identified using this simple method enables us to calculate error bars in the jerk occurrence times and to quantify their nonsimultaneous behavior we find that our error bars are not in general symmetric about the mean occurrence time and that the mean errors on the x and z components of 1 7 years and 1 5 years are larger than that of 1 1 years on the y component generally the error bars were found to be larger in the southern hemisphere observatories our results are necessary prerequisites for further studies of the inverse problem that attempt to determine mantle electrical conductivity from variations in jerk occurrence times

the use of magsat data to determine secular variation the appendix is available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 004 2 50 payment must accompany order a combined spatial and secular variation model of the geomagnetic field labeled m061581 is derived from a selection of magsat data secular variation sv data computed from linear fits to midnight hourly values from 19 magnetic observatories were also included in the analysis but were seen to have little effect on the model the sv patterns from this new model are compared with those from the 1980 igrf and with those for 1970 computed by the dgrf and with the 1960 patterns computed using the gsfc 12 66 model most of the features of the m061581 are identical in location and level with those of the 1980 igrf together they confirm that the reversals in sign of field change seen over asia and north america between 1965 and 1975 are reverting to the pre 1965 states the m061581 model gives 32 nt yr for the dipole decay rate larger than the 70 increase already reported since 1965 this abnormally high value is interpreted as being a defect of the model because it appears to result from a much larger 100 nt yr drop in field over the polar regions not indicated by the 1980 igrf this north polar decrease is shown to be of external origin as the result of a combination of the seasonal effect of the north polar ionospheric counterclockwise afternoon sqp cell increasing in intensity from the beginning november 1979 to the end june 1980 of the magsat data collection period coupled with an enhancement of its effect as the orbit lowered from the 350 550 km initial altitudes to near 200 km just prior to burnup this experiment indicates that secular variation can be obtained from satellite data for intervals of less than a full year if corrections can be made for seasonal effects and that annual snapshots of the field by a satellite would allow easy and accurate models of secular change without the use of any surface data

the midlatitude f2 layer during solar eclipses observations and modeling national natural science foundation of china 40725014 40674090 40636032 national important basic research project 2006cb806306

spherical cap harmonic analysis of geomagnetic secular variation over canada 1960 1983 the new technique of spherical cap harmonic anaylsis has enabled a model of secular variation to be constructed that applies to an area of over 30 million square kilometers and a time interval of over 20 years this technique is a regional analog of the ordinary method of spherical harmonic analysis and ensures that the curl and divergence of the secular variation vector are zero by making each harmonic coefficient a polynomial in time the model provides both spatial and temporal continuity the secular variation data from 1960 to 1983 comprise 1227 first central differences of annual mean values from 24 observatories in canada united states greenland and iceland and 939 first central differences from 138 canadian and american repeat stations divided by their respective time intervals the secular variation model includes terms up to spatial index 4 and temporal degree 2 although such a model allows for 75 coefficients only 39 were found to be statistically significant the temporal behavior of the model at observatories and repeat stations is compared on time plots to that of the secular variation data itself the scatter about the regression model or standard error of estimate was 11 8 nt yr the model is required both to update aeromagnetic survey data for producing canadian geomagnetic main field charts at 1985 0 and to provide annual change information for the extrapolation of those charts up to 1990 the average secular variations derived from the definitive dgrf and provisional pgrf international geomagnetic reference fields agree well with the model but the average secular variation for 1980 1985 from the predictive international geomagnetic reference field igrf does not

geomagnetism the next millenium abstract the earth has possessed a magnetic field for most of its history it is therefore strange that the present day field is changing so rapidly at the recent rate of decay the magnetic field will be removed altogether in the next 1500 years if this fall persists as seems likely our future magnetic environment would be quite different from that of today a long term program of observation and development of theory for the origin of the magnetic field are needed in order to understand the processes influencing the magnetic field

ionospheric total electron content response to solar eclipses on october 24 1995 and march 9 1997 two solar eclipse events occur it is therefore of interest to investigate how the ionosphere responded to the eclipses five global positioning system gps ground based receivers are specifically designed to observe large scale ionospheric variations over the geomagnetic equatorial equatorial anomaly crest and midlatitude regions two dimensional images of ionospheric total electron content tec during the two eclipse periods are constructed the deviations in the tec images on eclipse days from those on reference days show that during the eclipse days the ionosphere experienced some large scale changes four features of the tec deviations pre ascension pa major depression md sunset ascension sa and secondary depression sd have been observed a detailed study shows that in geomagnetic low latitudes pas are possibly related to the locations of the equatorial anomaly crest the latitudinal location amplitude and occurrence time of mds suggest that the fountain effect is essential sas and sds occurring in geomagnetic equatorial and low latitudes and appearing respectively before around and after local sunset indicate that the prereversal enhancement plays an important role

a geomagnetic jerk for the end of the 20th century abstract the series of magnetic measurements at some european observatories give some hint of a new geomagnetic jerk around 1999 the geomagnetic impulses would present the remarkable and intriguing property to occur with a frequency of one per decade in the last third of the 20th century the geomagnetic jerks have been proposed to be indicators which anticipate the changes in the earth s rotation rate if this statement is of general validity the 1992 jerk should be followed by an acceleration of the earth rotation which would take place now and along the same lines we might predict a new deceleration in less than 10 yr

world magnetic charts for 1985 spherical harmonic models of the geomagnetic field and its secular variation summary the 1980 world magnetic charts published by the uk hydro graphic department and by the us defense mapping agency hydrographic topographic center have been produced jointly they are based on spherical harmonic models of the main geomagnetic field 168 coefficients and of the secular variation 80 coefficients these models are presented here with a discussion of the methods used to derive them and of the data on which they are based positions of the dip poles the geomagnetic poles and the eccentric dipole together with their rates of change are derived from the models

the worldwide character of the 1969 1970 impulse of the secular acceleration rate abstract this paper continues earlier studies to ascertain whether the distribution of the 1969 1970 jump of the secular variation rate has a worldwide character the geometry of the jump presents some similarities with the geometry of the secular variation itself we give a new estimation of the westward drift rate and information about spatial and temporal variations of the secular variation field

worldwide character of the 1991 geomagnetic jerk a recent examination of the geomagnetic element annual mean values for the european magnetic observatories has shown the existence of a sudden change in the secular acceleration at about 1991 in this work the space time character of the 1991 impulse is investigated and its worldwide character is evidenced an evaluation of the amplitude spatial distribution is given for the y component analysing data relative to 74 magnetic observatories the external effects were removed from the annual mean data and successively the y secular variation has been evaluated from the corrected annual means moreover the worldwide character of the intensity of the step in the second time derivative is compared with the character of the previous well known jerk events of 1969 and 1978

ionospheric and thermospheric response over millstone hill to the may 30 1984 annular solar eclipse the national center for atmospheric research ncar thermospheric general circulation model tgcm is used to calculate the time dependent thermospheric response to the may 30 1984 annular solar eclipse the path of maximum obscurity begins at sunrise in the pacific ocean near 2 n and 135 w it moves northeastward passing across central mexico the eastern united states and then the atlantic before ending near 28 n and 4 e in algeria the area of the partial shadow is relatively large and the total solar flux incident on the dayside of the earth is decreased by about 6 during the eclipse the tgcm calculates the time dependent response of the winds temperature and the mass mixing ratios of the major constituents throughout the thermosphere perturbations follow the path of the annular eclipse with maximum deviations occurring near 1700 ut at about 300 km for the temperature and at higher altitudes for the winds and composition the perturbation winds converge from all directions toward the shadow at speeds reaching 75 m s 1 in the upper thermosphere the maximum temperature anomaly 55 k and vertical wind anomaly 7 m s 1 occur near the center of the shadow at a constant altitude of 300 km both the n2 density and the o density decrease by about 10 and 6 respectively the path of maximum obscuration passes within 3 of latitude of the incoherent scatter radar at millstone hill massachusetts 42 6 n 71 5 w the station experiences a maximum solar obscuration of 86 at 1700 ut 1200 lt a time dependent one dimensional numerical model of the ionosphere that uses the tgcm calculated winds temperature and composition at millstone hill is used to calculate the electron and ion densities and temperatures and the densities of the odd nitrogen species no n 4s and n d during the eclipse the calculated electron density decreases by about factors of 2 in the f region 4 in the f1 region and 3 in the e region compared to a similar control run without an eclipse the f1 region emerges during the eclipse with an increase in the no o ratio the calculated electron temperature decreases by 460 k during the eclipse but then increases 200 k following the eclipse because of the intense solar heating in a region of reduced electron densities the calculated ion temperature generally follows the changes in neutral temperature the calculated thermospheric and ionospheric responses agree well with measurements made by the millstone hill incoherent scatter radar

application and validation of the spherical elementary currents systems technique for deriving ionospheric equivalent currents with the north american and greenland ground magnetometer arrays 1 with data from the canadian magnetic observatory system canadian array for real time investigations of magnetic activity geophysical institute magnetometer array greenland time history of events and macroscale interactions during substorms themis and magnetometer array for cusp and cleft studies ground magnetometer arrays we applied the state of art technique on the basis of spherical elementary currents systems secs developed by amm and viljanen 1999 in order to calculate maps of ionospheric equivalent currents over the whole north american auroral region this study is the first to apply the secs technique to a large nonrectangular area with widely separated ground magnetometers 350 km for this study we will first demonstrate the validity of the technique using synthetic data and then examine equivalent ionospheric currents associated with a harang discontinuity for a case study on 10 december 2007 the results show in detail the dynamic evolution of the currents over the entire north american ground magnetometer network equivalent ionospheric current eic maps inferred at the minimum resolution of the database in this case 10 s can thus be analyzed further in conjunction with near simultaneous images of the themis all sky imager mosaics and super dual auroral radar network radar data the eic maps represent a value added product from the raw magnetometer database and can be used for contextual interpretation as well as help with our understanding of magnetosphere ionosphere coupling mechanisms using the ground arrays and the themis spacecraft data