Secular Acceleration Research Articles

Rotational motion of Phobos

Context. Phobos is in synchronous spin-orbit resonance around Mars, like our Moon around the Earth. As a consequence, the rotational period of Phobos is equal in average to its orbital period. The variations of its rotational motion are described by oscillations, called physical librations, which yield information of its interior structure. The largest libration of Phobos rotational motion was first detected in 1981 and the determination of this libration has recently been improved using Mars EXpress observations. Aims. The objective of this paper is to present the spectrum of Phobos’ librations by using recent orbital ephemerides and geophysical knowledge of this Martian satellite. The analysis of the librational spectrum highlights the relationship between dynamical and geophysical properties of the body, but is also useful for cartographic and geodetic purposes for future space missions dedicated to Phobos. Methods. We developed a numerical model of Phobos’ rotation that includes the point-mass Mars acting on the dynamical shape of Phobos, expanded to the third degree, and the e ect of Mars’ oblateness. The forced librations spectrum is extracted through a frequency analysis. Results. We find that the libration in longitude presents a quadratic term that coincides with the secular acceleration of Phobos falling onto Mars. The primary libration in longitude has a period equal to the anomalistic mean motion, whereas the primary libration in latitude has a period equal to the draconic mean motion (node to node). Both librations have amplitudes of about one degree leading to a surface displacement of about 200 m. These two components dominate the libration spectrum by a factor one hundred. Phobos’ third degree gravity harmonics and Mars’ oblateness a ect the librations amplitude at 10 4 degree. This is small but detectable from long-term tracking of a lander. The determination of the librational spectrum would bring strong constraints on the principal torques acting on the Martian moon, as well as on the possible presence of lateral variations in density predicted by certain geophysical models of the Stickney crater formation. We also investigate the obliquity variations of Phobos and find that their amplitudes are larger than the mean value of the obliquity. Conclusions. Phobos exhibits a rich and varied set of librational oscillations. The main librations and the librations close to the proper frequencies are the most sensitive to the interior structure. On the other hand, the superimposed e ect of large amplitude oscillations is likely to make the determination of the mean obliquity challenging.

Assessing the importance and expression of the 6 year geomagnetic oscillation

The first time derivative of residual length‐of‐day observations is known to contain a distinctive 6 year periodic oscillation. Here we theorize that through the flow accelerations at the top of the core the same periodicity should arise in the geomagnetic secular acceleration. We use the secular acceleration of the CHAOS‐3 and CM4 geomagnetic field models to recover frequency spectra through both a traditional Fourier analysis and an empirical mode decomposition. We identify the 6 year periodic signal in the geomagnetic secular acceleration and characterize its spatial behavior. This signal seems to be closely related to recent geomagnetic jerks. We also identify a 2.5 year periodic signal in CHAOS‐3 with unknown origin. This signal is strictly axially dipolar and is absent from other magnetic or geodetic time series.

Localized analysis of polar geomagnetic jerks

A new method is introduced to find the relatively sudden temporal changes or jerks of the geomagnetic field over the polar regions. Geomagnetic jerk events during the 20th century have mostly been identified from direct measurements at mid-latitude geomagnetic observatories. Recently, however, global magnetic spherical harmonic models like CM4 and CHAOS have been found effective in identifying the Antarctic events of 1969, 1978, 1985, 1991 and 2003 that were followed by jerks in the Arctic region with time delays of one to three years. The present study extends these events into earlier decades from 1900 to 1985 using the global harmonic model gufm1 with localized spherical coefficients or Slepian functions for the polar regions. Power spectral minima for the localized coefficients correlate with and thus help identify the abrupt changes in geomagnetic secular acceleration models. The time delay between the Antarctic and Arctic geomagnetic jerks was also confirmed.

Timescales of geomagnetic secular acceleration in satellite field models and geodynamo models

Magnetic satellite data from the last decade allow to model geomagnetic secular acceleration, the second time derivative of the field, in a highly precise manner. Robust estimates of the secular acceleration (SA) are obtained by using order six B-Splines as representation of the field variability, which in turn allows us to estimate the characteristic SA timescale, τSA. We confirm a recent finding that τSA is of order 10 years and fairly independent of the spherical harmonic degree n. This contrasts with the characteristic timescale of geomagnetic secular variation τSV, which is a decreasing function of n and is 100yr for n≤ 5. Conceivably the SA timescale might be related to short-term processes in the core, distinct from convective overturn whose timescale is reflected by τSV. Previously it had been shown that dynamo simulations reproduce the shape of the secular variation (SV) spectrum and, provided their magnetic Reynolds number Rm has an Earth-like value of order 1000, also the absolute values of τSV. The question arises if dynamo simulations can capture the observed timescales of geomagnetic SA. We determined τSA(n) for a set of dynamo models, covering a range of values of the relevant control parameters. The selection of models was based on the morphological similarity of their magnetic fields to the geomagnetic field and not on criteria related to the time dependence of the field, or on any aspect of the spectra associated with their field variation. We find that τSA depends only weakly on n up to degree 10, but for larger n it asymptotically approaches the 1/n-dependence that is also found for τSV(n). The acceleration timescale at low n varies with magnetic Reynolds number more strongly than τSV and may also depend on magnetic field strength. For an Earth-like Rm≃ 1000, τSA is of order 10yr for n≃ 2–10, as found in the field models from satellite data. A simple scaling analysis based on the frozen flux assumption for magnetic variations suggests two contributions to the SA, an advective part that scales with velocity U and has a length scale dependence corresponding to n-, and a part that depends on the acceleration of the flow without explicit dependence on the length scale. Their combination can explain the spectral shape of τSA(n) in numerical models, with the latter term dominating at n < 10. The characteristic timescale of acceleration of the near surface flow correlates with τSA in the different numerical models and is of the same order as τSA. This suggests that the observed 10yr timescale of geomagnetic SA reflects the characteristic time of core flow acceleration. To explain the geomagnetic SV and SA timescales, we find that the rms velocity near the core surface must be 18 kmyr- and the rms flow acceleration approximately 2 kmyr-2, although a statistical analysis of the induction equation suggests that most of the latter may occur at flow scales corresponding to harmonic degrees n > 12. The ability of dynamo models to match simultaneously SV and SA timescales suggests that dynamic processes in the core at the decadal timescale are not fundamentally different from those at the centennial timescale.

An extended version of the C3FM geomagnetic field model: application of a continuous frozen-flux constraint

SUMMARY A recently developed method of constructing core field models that satisfies a frozen-flux constraint is used to built a field model covering 1957–2008. Like the previous model C3FM, we invert observatory secular variation data and adopt satellite-based field models to constrain the field morphology in 1980 and 2004. To derive a frozen-flux field model, we start from a field model that has been derived using ‘classical’ techniques, which is spatially and temporally smooth. This is achieved by using order six B-splines as basis functions for the temporal evolution of the Gauss coefficients and requiring that the model minimizes the integral of the third-time derivative of the field taken over the core surface. That guarantees a robust estimate of the secular acceleration. Comparisons between the ‘classical’ and the frozen-flux field models are given, and we describe to what extend the frozen-flux constraint is adhered. Our models allow the interpretation that magnetic diffusion does not contribute to the observed secular variation. Additionally, the resulting frozen-flux field model shows a more or less constant spatial complexity, so that the spatial complexity of the magnetic field imposed by the 2004 satellite field model is maintained backward in time to the beginning of the model period, 1957. Therefore, we understand the frozen-flux constraint as an instrument that aids the backward projection of high spatial resolution in core field models to earlier times.

Core surface magnetic field evolution 2000-2010

SUMMARY We present new dedicated core surface field models spanning the decade from 2000.0 to 2010.0. These models, called gufm-sat, are based on CHAMP, Orsted and SAC-C satellite observations along with annual differences of processed observatory monthly means. A spatial parametrization of spherical harmonics up to degree and order 24 and a temporal parametrization of sixth-order B-splines with 0.25 yr knot spacing is employed. Models were constructed by minimizing an absolute deviation measure of misfit along with measures of spatial and temporal complexity at the core surface. We investigate traditional quadratic or maximum entropy regularization in space, and second or third time derivative regularization in time. Entropy regularization allows the construction of models with approximately constant spectral slope at the core surface, avoiding both the divergence characteristic of the crustal field and the unrealistic rapid decay typical of quadratic regularization at degrees above 12. We describe in detail aspects of the models that are relevant to core dynamics. Secular variation and secular acceleration are found to be of lower amplitude under the Pacific hemisphere where the core field is weaker. Rapid field evolution is observed under the eastern Indian Ocean associated with the growth and drift of an intense low latitude flux patch. We also find that the present axial dipole decay arises from a combination of subtle changes in the southern hemisphere field morphology.

Investigating the 2003 geomagnetic jerk by simultaneous inversion of the secular variation and acceleration for both the core flow and its acceleration

At observatory locations, the Earth’s magnetic field displays an almost constant secular acceleration, except at times of geomagnetic jerks when this quantity suddenly changes its value. The 2003 geomagnetic jerk was the first to have been recorded globally and uniformly over the Earth, and is well captured by time-varying spherical harmonic models of the geomagnetic field, based on satellite data. We jointly derive instantaneous estimates of the core surface flow and acceleration accounting for the field secular variation and acceleration given by such models. We consider epochs just before and after the 2003 event and show that no steady flow, no matter how general, can simultaneously account for the observed secular variation and acceleration at either epoch. Assuming the flow to be tangentially geostrophic, we next show that even purely toroidal zonal flow accelerations cannot account for the observed secular acceleration and conclude that more general tangentially geostrophic flow accelerations are required by the data. These flow accelerations, however, may well be equatorially symmetric, as predicted by theoretical considerations. Investigating the flow acceleration change throughout the 2003 event shows that these also may be equatorially symmetric, but definitely not purely toroidal zonal. This unambiguously confirms earlier suggestions that the 2003 geomagnetic jerk was a more complex phenomena than a simple consequence of torsional oscillations.

From superchrons to secular variation: A broadband dynamo frequency spectrum for the geomagnetic dipole

We build a broadband frequency spectrum for geomagnetic dipole intensity variations originating in the core using results from numerical dynamos. Scaling dipole intensity spectra from numerical dynamos with diverse input parameters (Ekman, Rayleigh, and magnetic Prandtl numbers) and magnetic Reynolds numbers from 170 to 1985 yields a composite spectrum ranging from ultra-low frequencies (∼100Myr period) to very high frequencies (∼1yr period). The amplitude and shape of our composite spectrum compare favorably with estimates of the geomagnetic and paleomagnetic dipole moment spectra. Significant power is found in an ultra-low frequency band f<0.1Myr−1 that corresponds to geomagnetic superchrons and also in a low frequency band f=0.1−5Myr−1 that corresponds to the time scales of geomagnetic polarity chrons. In higher frequency bands our composite spectrum varies with frequency approximately like f−n. In a transitional frequency band from f=5Myr−1 to f=2kyr−1, characteristic of relative paleointensity variations in sediments, we find n≃1.8. In a high frequency band with f>2kyr−1, representing archaeomagnetic and historical geomagnetic secular variation, we find n≃4 on average, with spectral slope increasing at very high frequencies to near n≃6 at f=1yr−1. The secular variation and the secular acceleration spectra for the dipole field reach their peak values in the high frequency band, close to frequencies where spectra for the dynamo fluid velocity decrease sharply.

Mapping geomagnetic secular variation at the core-mantle boundary

SUMMARY Data from recent satellite missions have vastly increased the resolution of models of the geomagnetic field, and itsfirst and second time derivatives – secular variation (SV) and secular acceleration (SA). The spectra of both SV and SA are ‘blue’ at the core–mantle boundary, both well-fit by functions proportional to l(l + 1) where l is the spherical harmonic degree. The ratio of the two spectra defines a timescale for geomagnetic variations of approximately 10 yrs for all resolvable harmonic degrees. The blue spectra should prevent meaningful maps of the SV being generated; nevertheless, the coherence of the maps up to harmonic degree 13 suggests that it is possible to obtain useful insight from their examination. Low SV is confirmed under the Pacific, but also revealed under the North Atlantic and Antarctica. These features are more readily explained in terms of dynamo control through thermal core–mantle coupling than by electromagnetic screening. Comparison with maps from measurements prior to the recent satellites, using the ‘Comprehensive Model’, suggests that models back to at least 1970 are sufficiently good to enable direct comparison of the SV.

The second generation of the GFZ Reference Internal Magnetic Model: GRIMM-2

We present the second generation of the GFZ Reference Internal Magnetic Model (GRIMM-2), that was derived for the preparation of the GFZ candidate for the 11th generation of the IGRF. The model is built by fitting a vector data set made of CHAMP satellite and observatory data, spanning the period 2001.0 to 2009.5. The data selection technique and the model parametrization are similar to that used for the derivation of the GRIMM model (Lesur et al., 2008). The obtained model is robust over the time span of the data. However, the secular variation above spherical harmonic degree 13 becomes less controlled by the data and is constrained by the applied regularization before 2002 and after 2008.5. At best, only the spherical harmonic degrees 3 to 6 are robustly estimated for the secular acceleration. The problem associated with the first two spherical harmonic degrees of the secular acceleration model arise from the difficulty in separating the core field signal from the external fields and their internally induced counterparts. The regularization technique applied smoothes the magnetic field model in time. This affects all spherical harmonic degrees, but starts to be significant at spherical harmonic degree 5.

The 1991 geomagnetic jerk as seen at the Earth's surface and the core-mantle boundary

SUMMARY We determine the occurrence times of geomagnetic impulses (jerks) around the year 1991 in the three geomagnetic secular variation components for the Earth's surface by a simple optimization algorithm. The geomagnetic field models we use are the low-degree parts of the models CM4 and C3FM. We find that the temporal jerk pattern can be detected in fields (n≤ 4), from which the spherical harmonic degrees n = 2 or n = 3 (tangential) and n = 4 (radial) are representative. To calculate the secular variation components at the core–mantle boundary (CMB) we apply the non-harmonic downward continuation (NHDC) method. For the mantle conductivity, three estimates, dependent on the radius, are assumed with conductances between 107 S and 2 × 108 S. The knowledge of the secular variation components at the CMB allows us to track the global distributions of jerk occurrence times in dependence on the mantle conductivity estimate. We find for each component a typically shaped, global topology for the location of the jerk occurrence times at the Earth's surface and the CMB. For the tangential (ϑ, ϕ)-components, these global topologies show always the well-known temporal bimodality on each surface. Another characteristic feature is found for the jerk of the r-component. It displays a double jerk centred around 1991 consisting of a v-shaped and a reversely v-shaped part, which are significantly correlated. Between the CMB and the Earth's surface, we find time delays in the range of 1–2 yr for the tangential and less than 1 yr for the radial jerk components. To understand these time delays, comparisons at fixed locations are carried out to check the influence of the respective conductivity function. For studying the time delay effects, we apply the inversion set-up of the NHDC to calculate simulated temporal oscillations and derive analytical expressions approximating the phase shifts. We find that jerk occurrence time delays and simulated phase shifts of temporal oscillations have a similar behaviour with respect to the influence of the conductivity and for the radial and the tangential components, respectively. In addition, a new concept for determining a jerk amplitude is presented briefly. This so-called dynamical jerk morphology, which forms a portion of the geomagnetic secular acceleration, is defined for each component by a time function on the considered surface. Its temporal motion patterns at the CMB are likely related with jerk originating processes in the fluid outer core.

MILLISECOND PULSAR AGES: IMPLICATIONS OF BINARY EVOLUTION AND A MAXIMUM SPIN LIMIT

In the absence of constraints from the binary companion or supernova remnant, the standard method for estimating pulsar ages is to infer an age from the rate of spin-down. While the generic spin-down age may give realistic estimates for normal pulsars, it can fail for pulsars with very short periods. Details of the spin-up process during the low-mass X-ray binary (LMXB) phase pose additional constraints on the period (P) and spin-down rates that may consequently affect the age estimate. Here, we propose a new recipe to estimate millisecond pulsar (MSP) ages that parametrically incorporates constraints arising from binary evolution and limiting physics. We show that the standard method can be improved by this approach to achieve age estimates closer to the true age while the standard spin-down age may overestimate or underestimate the age of the pulsar by more than a factor of ∼10 in the millisecond regime. We use this approach to analyze the population on a broader scale. For instance, in order to understand the dominant energy loss mechanism after the onset of radio emission, we test for a range of plausible braking indices. We find that a braking index of n = 3 is consistent with the observed MSP population. We demonstrate the existence and quantify the potential contributions of two main sources of age corruption: the previously known "age bias" due to secular acceleration and "age contamination" driven by sub-Eddington progenitor accretion rates. We explicitly show that descendants of LMXBs that have accreted at very low rates will exhibit ages that appear older than the age of the Galaxy. We further elaborate on this technique, the implications and potential solutions it offers regarding MSP evolution, the underlying age distribution, and the post-accretion energy loss mechanism.

Four decades of European geomagnetic secular variation and acceleration

Geomagnetic secular variation, the generally slow, continuous change in the core magnetic field, is characterized by occasional rapid variations known as geomagnetic jerks. Recent studies on magnetic data obtained by satellites with a good global coverage suggest that more rapid and smaller scale features than previously thought occur in the field change. We have taken advantage of the comparatively high density of geomagnetic observatories in Europe and have derived a regional model for the detailed study of secular variation and acceleration over the past four decades from 1960 to 2001 by means of improved and regularized spherical cap harmonic analysis. We show the improvements to our regional model over a global model. All the known jerks are seen in our model, but further times with rapid changes in secular variation exist. Moreover, times of zero acceleration in general do not occur simultaneously in all magnetic field components, although this nearly is the case in 1969.6 and 1982.2. Secular variation and acceleration show very dynamic patterns indicating rapid and complex causal processes in the Earth’s fluid core.

Inelastic Processes in the Shell

It has been known for some thousands of years that the Moon keeps a nearly constant face to the Earth. Only one explanation has been offered of how this state can have come about, namely tidal friction in the interior of the Moon. Darwin made an extensive study of the effects of tidal friction in the Earth and other planets, with special reference to the apparent secular acceleration of the Moon. Mercury and known satellites appear to keep constant faces to their primaries (there seems to be some d,oubt now about Mercury, but the rotation still appears to be slow). Turbulence in tidal currents in shallow seas seems to account for most of the friction for the Earth; but the other bodies have probably never had shallow seas, and we must infer some imperfection of elasticity at strains of the order of It is very hard to detect such imperfection in the laboratory. Other types however are very conspicuous. Fracture in igneous rocks and well compacted metamorphic ones such as quartzites occurs at strains of the order of and there is a complicated non-linear behaviour when the elastic strains are somewhat less than this; this may lead to permanent deformation far exceeding the direct elastic one. It was shown by various geodesists that mountains produce far smaller disturbances of gravity than would be expected if they were simply added loads. An explanation was given by Airy, who postulated a lower layer of denser but weaker matter. Extra load on a thin crust would push the interface down, and the resulting lack of heavy matter would largely compensate for the extra attraction of the mountain. Later Suess made the crucial inference on geological grounds that most surface rocks are essentially derived from granite or similar rocks with densities about 2.7, and overlie a layer with a density of 3-0 or so. Then A. MohorovilSiC in 1909 detected a duplication of earthquake pulses, as if one pair, Pg and Sg, travelled in an upper layer but another, Pn and Sn, in a lower one with higher velocities. Curiously, none of the continental seismologists at the time appear to mention Suess or Airy. The first mention in geophysics of the geological inference was, I think, in the paper by Winch and me in 1922 on the Oppau explosion. Later comparison with experiment confirmed these early results except that the lower layer seemed to need a density more like 3.3 and the upper was a great deal more complicated. But this is still too simple. Hayford’s survey of the United States assumed exact uniformity of mass per unit area and greatly reduced the residuals in comparison with the hypothesis that extra mass on the surface is not accompanied by defect below. But Helmert noticed that successive residuals along a survey line tended to keep the same sign, and on this ground multiplied Hayford’s uncertainties by about 3. Barrel1 inferred hence that the lower layer cannot be completely weak; he supposed a layer about lOOkm down where the strength is much less than that of surface rocks, but still appreciable. He called this the asthenosphere. Again confirmation came from Turner’s discovery of deep-focus earthquakes in 1921. Earthquakes comparable with the strongest shallow ones occur to depths of about a tenth of the Earth‘s radius. They send out strong S pulses. This is definite evidence that a discontinuous change occurs in the stress-difference and the material must have been able to stand that until the earthquake took place. (Incidentallyl it is hard to

THE ORBITS AND MASSES OF THE MARTIAN SATELLITES AND THE LIBRATION OF PHOBOS

This paper reports on an update to the orbits and masses of the Martian satellites Phobos and Deimos. We obtained the orbits by fitting a numerical integration to all available Earth-based astrometry through the opposition of 2003, spacecraft imaging observations through 2007, and the Doppler tracking of the Viking and Phobos 2 spacecraft; the Doppler data provide information on the satellite masses. Our dynamical model included the figure acceleration due to a librating Phobos; we determined the amplitude of the forced libration. We also took into account the secular acceleration of Phobos due to the tide that it raises on Mars and estimated the Martian tidal quality factor Q. We provide an assessment of the accuracy of the orbits and a geometrical description of the orbits in the form of mean elements.

Geomagnetic Core Field Secular Variation Models

We analyse models describing time changes of the Earth’s core magnetic field (secular variation) covering the historical period (several centuries) and the more recent satellite era (previous decade), and we illustrate how both the information contained in the data and the a priori information (regularisation) affect the result of the ill-posed geomagnetic inverse problem. We show how data quality, frequency and selection procedures govern part of the temporal changes in the secular variation norms and spectra, which are sometimes difficult to dissociate from true changes of the core state. We highlight the difficulty of resolving the time variability of the high degree secular variation coefficients (i.e. the secular acceleration), arising for instance from the challenge to properly separate sources of internal and of external origin. In addition, the regularisation process may also result in artificial changes in the model norms and spectra. Model users should keep in mind that such features can be mis-interpreted as the signature of physical mechanisms (e.g. diffusion). Finally, we present perspectives concerning core field modelling: imposing dynamical constraints (e.g. by means of data assimilation) reduces the non-uniqueness of the geomagnetic inverse problem.

Crecimiento y maduración de los pacientes con hipotiroidismo congénito detectados por el programa de cribado neonatal en Cataluña (1986–1997)

Background and objective Many countries have developed screening programs for CH that bring about a normal somatic and psychological development. The aim of this study is to evaluate the evolution of cases detected by the screening program of congenital hypothyroidism in Catalonia. Patients and methods The was a descriptive, longitudinal study of a series of 136 cases of congenital hypothyroidism detected by the screening program in Catalonia from 1986 to 1997 and who had been checked in the Maternal-infantile Vall d′Hebron Hospital in Barcelona. Follow-up was carried out for growth and a psychological exploration was performed (McCarthy test and WISC-R). Results Males (30 cases, 22.1%) and females (106 cases, 77.9%) with congenital hypothyroidism of this study had a height, weight and BMI that were not different from those of the current population of Barcelona. Height was 0.5 SD above the Tanner reference values of 1966 and BMI was one SD above the Roland-Cachera reference values of 1982. This secular acceleration of growth has been observed in all developed countries. In a sample of 37.5% of the cases, psychological development did not differ from that of the reference population. However, a greater intensity of neonatal hypothyroidism and its most delayed normalization correlated negatively with the score in the psychological tests. Conclusions Once normalization of growth and psychological development are achieved, the screening program of congenital hypothyroidism in Catalonia should be optimized by initiating treatment as soon as possible to avoid the possibility that some children do not reach their complete intellectual potential.

The M dwarf planet search programme at the ESO VLT + UVES

We present radial velocity (RV) measurements of our sample of 40 M dwarfs from our planet search programme with VLT+UVES begun in 2000. Although with our RV precision down to 2–2.5 m/s and timebase line of up to 7 years, we are capable of finding planets of a few Earth masses in the close-in habitable zones of M dwarfs, there is no detection of a planetary companion. To demonstrate this we present mass detection limits allowing us to exclude Jupiter-mass planets up to 1 AU for most of our sample stars. We identified 6 M dwarfs that host a brown dwarf or low-mass stellar companion. With the exception of these, all other sample stars show low RV variability with an rmsm/s. Some high proper motion stars exhibit a linear RV trend consistent with their secular acceleration. Furthermore, we examine our data sets for a possible correlation between RVs and stellar activity as seen in variations of the H<i>α<i/> line strength. For Barnard's star we found a significant anticorrelation, but most of the sample stars do not show such a correlation.

Geomagnetic jerks in the polar regions

[1] The occurrence of geomagnetic jerks over the Arctic and Antarctic regions is here investigated. Maps of geomagnetic secular acceleration over the polar regions are produced from the CM4 and CHAOS models and the occurrence of geomagnetic jerks is associated with jumps in secular acceleration. The obtained results confirm that in Antarctica geomagnetic jerks systematically follow geomagnetic jerks in the Arctic region with a time delay from one to three years. Evidence is found of an abrupt change in secular acceleration in both polar regions around 1985, suggesting that the 1985 local jerk could actually be a worldwide event. Combining our results with the results previously obtained on the occurrence of a geomagnetic jerk at low-mid latitudes around 2003, we support the hypothesis of a global extension of an event occurred at the beginning of the 21st century.

Characteristics of the Secular Variation and Secular Acceleration Distributions of the Main Geomagnetic Field from the CHAMP Satellite

AbstractUsing the new geomagnetic model POMME‐4.2S derived from the CHAMP satellite data, we have analyzed the characteristics of the distributions of seven secular variation components and three secular acceleration components , Ÿ , in China and the world. We also compared the differences of the distributions between the secular variation models POMME‐4.2S and CGRF‐SV (Chinese Geomagnetic Reference Field‐Secular Variation). The results suggest that the quadrupole plays the main role in the geomagnetic secular variation, and the octupole occupies the largest portion in the contribution to the secular acceleration. The decrease of global secular variation is much larger than its increase in 2004.0 A.D. in the background of a decreasing trend of the geomagnetic field. The secular acceleration changes more at middle and low latitudes than that at higher latitudes. Moreover, the negative and positive magnetic anomalies of Ÿ and components form several alternating areas along longitudes, and these two kinds of anomalies of component are nearly symmetrical to the equator. An evident relationship is present for the magnetic anomaly distributions of the three secular acceleration components , Ÿ , . That is, the foci of magnetic anomalies exist in the south and north, and Ÿ in the east and west sides of the foci of magnetic anomalies. The amounts of change of the secular variation and acceleration in China are small and their distributions are relatively uniform.