Geomagnetic Dipole Axis Research Articles

Can machine learning reveal precursors of reversals of the geomagnetic axial dipole field?

SUMMARY It is well known that the axial dipole part of Earth’s magnetic field reverses polarity, so that the magnetic North Pole becomes the South Pole and vice versa. The timing of reversals is well documented for the past 160 Myr, but the conditions that lead to a reversal are still not well understood. It is not known if there are reliable ‘precursors’ of reversals (events that indicate that a reversal is upcoming) or what they might be. We investigate if machine learning (ML) techniques can reliably identify precursors of reversals based on time-series of the axial magnetic dipole field. The basic idea is to train a classifier using segments of time-series of the axial magnetic dipole. This training step requires modification of standard ML techniques to account for the fact that we are interested in rare events—a reversal is unusual, while a non-reversing field is the norm. Without our tweak, the ML classifiers lead to useless predictions. Perhaps even more importantly, the usable observational record is limited to 0–2 Ma and contains only five reversals, necessitating that we determine if the data are even sufficient to reliably train and validate an ML algorithm. To answer these questions we use several ML classifiers (linear/non-linear support vector machines and long short-term memory networks), invoke a hierarchy of numerical models (from simplified models to 3-D geodynamo simulations), and two palaeomagnetic reconstructions (PADM2M and Sint-2000). The performance of the ML classifiers varies across the models and the observational record and we provide evidence that this is not an artefact of the numerics, but rather reflects how ‘predictable’ a model or observational record is. Studying models of Earth’s magnetic field via ML classifiers thus can help with identifying shortcomings or advantages of the various models. For Earth’s magnetic field, we conclude that the ability of ML to identify precursors of reversals is limited, largely due to the small amount and low frequency resolution of data, which makes training and subsequent validation nearly impossible. Put simply: the ML techniques we tried are not currently capable of reliably identifying an axial dipole moment (ADM) precursor for geomagnetic reversals. This does not necessarily imply that such a precursor does not exist, and improvements in temporal resolution and length of ADM records may well offer better prospects in the future.

Can machine learning reveal precursors of reversals of the geomagnetic axial dipole field?

Can machine learning reveal precursors of reversals of the geomagnetic axial dipole field?

A new power spectrum and stochastic representation for the geomagnetic axial dipole

SUMMARY Earth’s internal magnetic field is dominated by the contribution of the axial dipole whose temporal variations are wide ranging and reflect characteristic timescales associated with geomagnetic reversals and large scale palaeosecular variation, ranging down to decadal and subannual field changes inferred from direct observations. We present a new empirical power spectrum for the axial dipole moment based on composite magnetic records of temporal variations in the axial dipole field that span the frequency range 0.1 to 5 × 105 Myr–1 (periods from 10 million to 2 yr). The new spectrum is used to build a stochastic representation for these time variations, based on an order 3 autoregressive (AR) process and placed in the context of earlier stochastic modelling studies. The AR parameter estimates depend on the frequency of transitions in the spectral regime and may be influenced by Ohmic diffusion, advection and torsional oscillations in Earth’s core. In several frequency ranges across the interval 200–5000 Myr–1(5000 to 200 yr periods) the empirical power spectrum lies above the AR3 model and may be influenced by Magneto–Coriolis (MC) waves in Earth’s core. The spectral shape and parameter estimates provide a potentially useful guide for developing assessments of whether numerical dynamo simulations meet criteria for being considered Earth like.

Statistical Study on the North‐South Asymmetric Distribution of the Mid‐Low‐Latitude Nightside Disturbed Magnetic Fields

AbstractThe nightside magnetic field at the middle and low latitude observatory is usually disturbed, decreasing in its north component (noted as Bx) during the periods of enhanced space weather activity. As indicated by the SYM‐H index, the decreasing field mainly originated from the magnetospheric westward ring current and the cross‐tail currents. In the meridian direction, the maximum of the disturbed field has been observed not always located at the geomagnetic dipole equator. One possible explanation is that the latitudinal position of the magnetotail current systems is affected by the orientation of solar wind to the Earth's geomagnetic dipole axis (noted as tilt angle). For a non‐zero tilt angle, the current deviates from the equatorial position (e.g., Malin & Mete Işikara, 1976; 10.1111/j.1365-246x.1976.tb07096.x). In this study, the mid‐low latitudinal distribution of the nightside disturbed field is systematically investigated every ∼1.5 hr, based on the magnetic surveys of the Swarm satellite constellation at polar orbits of ∼450 km altitude. The meridian distribution of Bx regularly appears like an upward‐opening parabolic shape with its vertex representing the magnetic minimum. In summer, the minimum is found generally located in the southern hemisphere, while in winter the pattern is reversed. The overall distribution demonstrates north‐south asymmetry. When the disturbed level is low, the deviation of the magnetic minimum from the dipole equator usually gets larger. The north‐south asymmetric distribution during storm times is further analyzed. In the initial phase, the magnetic minimum was shifted prominently (about 10°–30°) from the equator, then moved back (with the offset less than 10°) in the main phase and recovery phase in most cases. Moreover, the latitudinal deviation undergoes a diurnal variation, which is conspicuous in spring and autumn. It seems the asymmetric distribution is anti‐correlated to the contribution of the ring current, indicated by the SYM‐H index. The warping structure of the cross‐tail current sheet probably plays a predominant role in generating the north‐south asymmetry of the magnetic field disturbances for non‐zero dipole tilt angles. Monitoring the external magnetic configuration via the low‐Earth‐orbit satellites will provide fundamental evidence to identify the relative position of the ring current and the cross‐tail current, as well as the dynamic processes in the near magnetotail.

Analyzing the geomagnetic axial dipole field moment over the historical period from new archeointensity results at Bukhara (Uzbekistan, Central Asia)

Since the mid-19th century, direct measurements of both intensity and direction of the Earth's magnetic field have been available, allowing an accurate determination of its spatio-temporal variations. Prior to this time, between ~1600 and 1840, only direct directional measurements are available. Therefore, the construction of global field models over this period requires either a specific treatment of the axial dipole field component or the use of archeomagnetic intensity data. In this study, we use a regional approach based on the construction of an archeointensity variation curve in Central Asia. We analyze baked clay brick fragments sampled in Bukhara (Uzbekistan), dated between the end of the 16th century and the beginning of the 19th century. This city is of particular interest for archeomagnetism due to the well-preserved old buildings accurately dated by documentary archives. A series of archeointensity results is obtained using the Triaxe experimental protocol, which shows a decreasing trend in intensity from ~1600 to ~1750, with intensities during the 18th century lower than expected from global geomagnetic field models. These new data appear consistent with other Triaxe data previously obtained in western Europe and western Russia, when transferred to Bukhara using the field geometry of the gufm1 model. Together, these data are used to recalibrate the axial dipole moment evolution provided by this model. The resulting evolution appears non-linear, with a clear relative minimum in the magnitude of the axial dipole during the late 18th century. We illustrate the fact that at present this evolution can neither be satisfactorily confirmed nor refuted by other datasets available in western Eurasia (as well as at a wider spatial scale), mainly due to the significant dispersion of the data. Our interpretation relies on the accuracy of the field geometry of the gufm1 model, which appears less reliable prior to ~1750. Nevertheless, the minimum proposed in the 18th century seems to be a true feature of axial dipole behavior.

Quantitative estimates of average geomagnetic axial dipole dominance in deep geological time

A defining characteristic of the recent geomagnetic field is its dominant axial dipole which provides its navigational utility and dictates the shape of the magnetosphere. Going back through time, much less is known about the degree of axial dipole dominance. Here we use a substantial and diverse set of 3D numerical dynamo simulations and recent observation-based field models to derive a power law relationship between the angular dispersion of virtual geomagnetic poles at the equator and the median axial dipole dominance measured at Earth’s surface. Applying this relation to published estimates of equatorial angular dispersion implies that geomagnetic axial dipole dominance averaged over 107–109 years has remained moderately high and stable through large parts of geological time. This provides an observational constraint to future studies of the geodynamo and palaeomagnetosphere. It also provides some reassurance as to the reliability of palaeogeographical reconstructions provided by palaeomagnetism.

One Hundred Thousand Years of Geomagnetic Field Evolution

AbstractPaleomagnetic records from sediments, archeological artifacts, and lava flows provide the foundation for studying geomagnetic field changes over 0–100 ka. Late Quaternary time‐varying spherical harmonic models for 0–100 ka produce a global view used to evaluate new data records, study the paleomagnetic secular variation on centennial to multimillennial timescales, and investigate extreme regional or global events such as the Laschamp geomagnetic excursion. Recent modeling results (GGF100k and LSMOD.2) are compared to previous studies based on regional or global stacks and averages of relative geomagnetic paleointensity variations. Time‐averaged field structure is similar on Holocene, 100 ky, and million‐year timescales. Paleosecular variation activity varies greatly over 0–100 ka, with large changes in field strength and significant morphological changes that are especially evident when field strength is low. GGF100k exhibits a factor of 4 variation in geomagnetic axial dipole moment, and higher‐resolution models suggest that much larger changes are likely during global excursions. There is some suggestion of recurrent field states resembling the present‐day South Atlantic Anomaly, but these are not linked to initiation or evolution of excursions. Several properties used to characterize numerical dynamo simulations as “Earth‐like” are evaluated and, in future, improved models may yet reveal systematic changes linked to the onset of geomagnetic excursions. Modeling results are useful in applications ranging from ground truth and data assimilation in geodynamo simulations to providing geochronological constraints and modeling the influence of geomagnetic variations on cosmogenic isotope production rates.

Seasonal and Solar Wind Control of the Reconnection Line Location on the Earth's Dayside Magnetopause

AbstractWe investigate the average location of magnetic reconnection on the Earth's dayside magnetopause, based on spatial distributions of northward and southward reconnection jets observed by the THEMIS spacecraft at the near‐noon (10–14 magnetic local time) magnetopause. A total of 711 reconnection jets were identified by applying the Walén relation, the tangential stress balance relation to be satisfied for a reconnected (rotational discontinuity) magnetopause, to magnetopause crossings identified from 10 years of THEMIS observations. The directions and positions of jets indicate that during southward interplanetary magnetic field (IMF) conditions, the dayside X‐line location shifts from the subsolar point toward the winter hemisphere by about 6 Earth radii under the largest tilt of the geomagnetic dipole axis. The X‐line location also shifts northward (southward) by at most 2.5 Earth radii when the IMF is predominantly radial and its x component is positive (negative). The dipole tilt effect on the shift of the X‐line location becomes smaller for higher solar wind Alfvén Mach numbers. The dipole tilt effect being larger than the IMF Bx effect suggests that the X‐line location has a seasonal dependence. Since models and theory show that the reconnection rate away from the subsolar magnetopause is lower than that at the subsolar magnetopause, the dipole tilt dependence of the X‐line location suggests that the efficiency of solar wind energy transfer into the magnetosphere may decrease under larger dipole tilt; this may partially account for seasonal variations of geomagnetic activity, which is known to decrease under larger dipole tilts.

Estimating the Kinetic Energy Budget of the Polar Wind Outflow

AbstractIonospheric outflow from the polar cap through the polar wind plays an important role in the evolution of the atmosphere and magnetospheric dynamics. Both solar illumination and solar wind energy input are known to be energy sources of the polar wind. However, observational studies of the energy transfer from these two energy sources to the polar wind are difficult. Because of their low energy, polar wind ions are invisible to regular ion detectors onboard a positively charged spacecraft. Using a new technique that indirectly measures these low‐energy ions, we are able to estimate the energy budget of the polar wind. Our results show that solar illumination provides about 107 W of the kinetic energy of the polar wind, in addition to the energy transferred from the solar wind with a maximum rate of about 108 W. The energy transfer efficiency of solar illumination to the kinetic energy of the polar wind is about 6 to 7 orders of magnitude lower than that of the solar wind. Moreover, daily and seasonal changes in the orientation of the geomagnetic dipole axis control solar illumination over the polar cap, modulating both energies of the polar wind and energy transfer efficiencies from the two energy sources.

A detailed paleomagnetic and rock-magnetic investigation around Cretaceous-Paleogene boundary: the Autlan (Western Mexico) volcanic sequence revisited

We present a detailed rock-magnetic and paleomagnetic survey from Autlan volcanic succession in western Mexico. The principal aim of this study is to extend paleomagnetic data from Autlan lavas in order to confirm vertical-axis rotation observed in reconnaissance study and to evaluate long-term variation of the geomagnetic field strength based on existing and global data. The mean inclination (44.7°) is in agreement with the expected inclination for 60 and 70 Ma, as derived from available reference poles for the North American craton. The declination (333.6°), however, is significantly different from those expected, which suggests a statistically significant counterclockwise tectonic rotation ranging between 10° ± 6° and 14° ± 7°. As a measure of paleosecular variation (PSV), we obtained a geomagnetic field dispersion of 9.6° (upper and lower limits: 7.2°–11.9°) in perfect agreement with the previously published PSV compilation of selected Cretaceous data from lavas. The mean virtual dipole moments available for Autlan lavas are about 65% of the present geomagnetic axial dipole but are in reasonably good agreement with other comparable quality determinations between 5 and 90 Ma. This reinforces the hypothesis that low geomagnetic field strengths persisted for the entire Jurassic extending into the Upper Cretaceous.

Geophysicochemical model of an ionospheric auroral gyroscope

AbstractThis study presents a geophysicochemical model of an ionospheric auroral gyroscope. The gyroscopic effect occurs due to the electromagnetic interaction in Earth's polar regions between two types of vertical cavity auroras: the herpolhodic cone (proton cavity aurora), operating in the cusp polar region, and two polhodic cones (an electronic cone and a protonic cone), operating in the aurora region. The ratio between the angular speeds of the herpolhodic and polhodic cones is established by the angle between Earth's rotational axis and the geomagnetic dipole axis. We have developed a theory of the ionospheric auroral gyroscope as a kinematic part of the terrestrial magnetosphere and ionosphere that enables a unified explanation of important macroscopic phenomena that occur at this level. Accordingly, we have explained the oval shape of the polar auroras, Schumann resonances, geomagnetic micropulsation excitation, and the structuring of Earth's areas of radiation. The terrestrial gravitomagnetic field and dark matter are implicated in the initiation and behavior of the auroral ionospheric gyroscope, both of which provide stability and accuracy. Viewed in a wider context, the ionospheric auroral gyroscope theory could offer a way to experimentally investigate dark matter on Earth. Furthermore, it may have a potential value as a predictive tool, providing information about the large earthquakes and Earth's phenomena.

Geomagnetic forecasts driven by thermal wind dynamics in the Earth's core

There exists a fundamental as well as practical interest in being able to accurately forecast the future evolution of Earth's magnetic field at decadal to secular ranges. This work enables such forecasts by combining geomagnetic data with an Earth-like numerical model of a convection-driven fluid dynamo. The underlying data assimilation framework builds on recent progress in inverse geodynamo modelling, a method which estimates an internal dynamic structure for Earth's core from a snapshot of the magnetic field and its instantaneous rate of change at the surface, and takes advantage of linear relationships and long-range correlations between observed and hidden state variables. Here the method is further evolved into a single-epoch ensemble Kalman filter, in order to initialise at a given epoch an ensemble of states compatible with the observations and representative of the uncertainties in the estimation of hidden quantities. The ensemble dynamics, obtained by subsequent numerical integration of the prognostic model equations, are found to be governed by a thermal wind balance or equilibrium between buoyancy forces, the Coriolis force and the pressure gradient. The resulting core fluid flow pattern is a quasi-steady eccentric gyre organised in a column parallel to Earth's rotation axis, in equilibrium with a longitudinal hemispheric convective density anomaly pattern. The flow provides induction for the magnetic field, which also undergoes a realistic amount of diffusion. Predictions of the present magnetic field from data taken within the past century show that the ensemble has an average retaining good consistency with the true geomagnetic evolution and an acceptable spread well representative of prediction errors, up to at least a secular range. The predictability of the geodynamo thus appears to significantly exceed previous theoretical expectations based on the chaotic divergence of ensemble members. The assimilation generally outperforms the linear mathematical extrapolations from a 30-yr prediction range onwards, with a 40 per cent improvement in Earth-surface error at a secular range. The geomagnetic axial dipole decay observed over the past two centuries is predicted to continue at a similar pace in the next century, with a further loss of 1.1 ± 0.3 µT by year 2115. The focal (or minimum intensity) point of the South Atlantic geomagnetic anomaly is predicted to enter the South Pacific region in the next century, with the anomaly itself further deepening and widening. By year 2065, the minimum intensity is predicted to decrease by 1.46 ± 0.4 µT at the Earth surface and the focal point to move 12.8 ± 1.4 deg westwards with a slight northward component. This corresponds to a drift rate of 0.26 deg yr −1 , similar to the typical geomagnetic westward drift observed over the past four centuries. The same drift rate is also predicted until 2115 with a further (but more uncertain) intensity decrease

Counterclockwise block rotation linked to southward propagation and overlap of sub-aerial Red Sea Rift segments, Afar Depression: Insight from paleomagnetism

Thirty-four cooling units from the Pleistocene extrusive volcanic rocks exposed in regions bordered by the active Red Sea magmatic segments, in the northwestern central Afar Depression, were sampled for paleomagnetic study. Six to 12 samples were collected from each paleomagnetic site. Samples were demagnetized using Alternating Field (AF) and Thermal (TH) demagnetization techniques. The samples were then measured using the JR–6A spinner magnetometer available at the department of Earth Sciences of Addis Ababa University. The Natural Remanent Magnetization (NRM) direction reveals mostly one or two simple and straightforward components of magnetizations; the first, low-stability component is isolated by heating to 100°C–300°C or by AF of 10–30mT. The magnetization directions after these steps generally define straight lines that are directed towards the origin, and interpreted as primary NRM or ChRM (Characteristic Remanent Magnetization) and the direction of magnetization was determined. Results of the magnetization decay curve plots and rock magnetic analyses using a Variable Field Translation Balance (VFTB) indicate that magnetic mineralogy is dominated by Ti-poor titano-magnetite with magnetic grain sizes in the pseudo-single domain (PSD) range, with minor presence of goethite and maghemite in a few cases. An overall mean direction calculated for the 26 sites located within the overlap zone is obtained (D=354.4°, I=13.2°, N=26, K=43.5, α95=4.3°), and compared to the expected Geomagnetic Axial Dipole (GAD) field, based on the Apparent Polar Wander Path (APWP) Curve of the African plate (Besse and Courtillot, 1991, 2003), a difference in declination ΔD=−6.5°±4.0° is obtained. This declination difference is interpreted as counterclockwise rotation about a vertical axis, in agreement with rift propagation and right stepping overlap geometry of the Alayta–Dabbahu magmatic segments. The difference in inclination ΔI=7.8°±3.9° is believed to be related to the long-standing non-dipole field in Afar (Courtillot et al., 1984; Kidane et al., 2003).

The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind–magnetosphere‐ionosphere coupling

The angle μ between the geomagnetic dipole axis and the geocentric solar magnetospheric (GSM) zaxis, sometimes called the “dipole tilt,” varies as a function of UT and season. Observations have shown that the cross‐polar cap potential tends to maximize near the equinoxes, when on averageμ= 0, with smaller values observed near the solstices. This is similar to the well‐known semiannual variation in geomagnetic activity. We use numerical model simulations to investigate the role of two possible mechanisms that may be responsible for the influence ofμon the magnetosphere‐ionosphere system: variations in the coupling efficiency between the solar wind and the magnetosphere and variations in the ionospheric conductance over the polar caps. Under southward interplanetary magnetic field (IMF) conditions, variations in ionospheric conductance at high magnetic latitudes are responsible for 10–30% of the variations in the cross‐polar cap potential associated withμ, but variations in solar wind–magnetosphere coupling are more important and responsible for 70–90%. Variations in viscous processes contribute slightly to this, but variations in the reconnection rate with μare the dominant cause. The variation in the reconnection rate is primarily the result of a variation in the length of the section of the separator line along which relatively strong reconnection occurs. Changes in solar wind–magnetosphere coupling also affect the field‐aligned currents, but these are influenced as well by variations in the conductance associated with variations inμ, more so than the cross‐polar cap potential. This may be the case for geomagnetic activity too.

Incompatible Ediacaran paleomagnetic directions suggest an equatorial geomagnetic dipole hypothesis

Paleomagnetic results obtained from rocks of Ediacaran age in several localities in Laurentia and Baltica persistently display co-existence of two magnetization components, one shallowly and the other steeply inclined. Both components pass criteria for a primary magnetization while geological considerations and radiometric age dating indicate that these magnetizations are surprisingly close in age. The conventional interpretation of these results, translating the inclination into paleolatitudes using the geocentric axial dipole hypothesis, would imply that rocks acquired magnetizations in positions switching back and forth between equatorial and near-polar latitudes. In a geographic reference frame, such large-scale and fast (>45 cm/yr) migrations of a continent have been rejected as dynamically implausible; neither plate tectonics nor True Polar Wander are thought to be able to attain the required velocities. A highly irregular behavior of the geomagnetic field during the Ediacaran, possibly an alternation of the geomagnetic dipole axis between a co-axial and an equatorial alignment, remains the only viable explanation for the paleomagnetic data. Such a behavior entails specific outer core conditions, which in turn impose strong constraints on the possible models for the thermal evolution of the Earth's interior.

Paleomagnetic and rock-magnetic survey of Brunhes lava flows from Tancitaro volcano, Mexico

Este estudio presenta los resultados paleomagnéticos del Volcán Tancitaro, (perteneciente al Campo Volcánico Michoacán Guanajuato), en el oeste de México, como una contribución a la base de datos de promediados en el tiempo del campo global. Se realizó el estudio paleomagnético y de magnetismo de roca en once flujos de lava. Se colectaron 120 núcleos orientados en el volcán Tancitaro y sus áreas aledañas. Todos los sitios fueron previamente fechados por 40Ar -39Ar (Ownby et al., 2007,) y van desde 793 ka hasta el presente. Se realizaron experimentos de magnetismo de roca, curvas de susceptibilidad e histéresis magnética y en la mayoría de los casos la remanencia magnética fue por la presencia de titanomagnetita pobre en Titanio con estructura magnética de pseudodominio simple. Ocho de los once flujos de lava dieron polaridad normal, mientras que los tres restantes produjeron paleodirecciones inconsistentes y no se tomaron en cuenta para el análisis y se debió probablemente por efectos de rayos. La paleodirección principal obtenida de los ocho flujos es Inc=39.5º, Dec= 356.4º, k=29, α95=9.1º lo cual corresponde a la posición del polo con Plat=84.4º, Plong=219.9º, K=33 y α95=8.5º. Esta dirección es prácticamente consistente con la paleodirección esperada del Plio Cuaternario, derivada del polo de referencia para el Craton Norteamericano. La variación paleosecular coincide con otros estudios de la misma latitud y con modelos estadísticos recientes. La inclinación media cae dentro de la incertidumbre del Eje Dipolar Geomagnético más 5% de contribuciones cuadrupolares.

Historical variation of the geomagnetic axial dipole

The geomagnetic axial dipole (hereinafter denoted g 1 0 ) is the largest component of our planet’s magnetic field. Its magnitude determines the morphology of solar-terrestrial electrical current systems and it is the most fundamental diagnostic property of the core-generated geodynamo. Elucidating past and future variations of g 1 0 ( t ) is consequently of central importance in geomagnetism. Previous historical geomagnetic field models, such as gufm1 of Jackson et al. [Jackson, A., Jonkers, A.R.T., Walker, M.R., 2000. Four centuries of geomagnetic secular variation from historical records. Philos. Trans. R. Soc. Lond. A 358, 957–990], used direct observations to constrain g 1 0 ( t ) only after 1840 A.D.; before this time a crude linear extrapolation of the post-1840 A.D. rate of change (15 nT/year) was employed. In this contribution I construct historical field models with g 1 0 ( t ) instead constrained from 1590 A.D. to 1840 A.D. by an archaeointensity dataset compiled by Korte et al. [Korte, M., Genevey, A., Constable, C.G., Frank, U., Schnepp, E., 2005. Continuous geomagnetic field models for the past 7 millennia. 1. A new global data compilation. Geochem. Geophys. Geosyst. 6, doi:10.1029/2004GC000800]. A range of possible linear models of the form g 1 0 ( t ) = g 1 0 ( 1840 ) + β ( t − 1840 ) are first explored; β = 2.74 ± 42.32 nT/year is found to explain the archaeointensity dataset with maximum likelihood, consistent with the recent findings of Gubbins et al. [Gubbins, D., Jones, A.L., Finlay, C.C., 2006. Fall in Earth’s magnetic field is erratic. Science 312, 900–902]. Relaxing the linear constraint in an effort to find more physically plausible models, I find it is necessary to artificially increase the weight given to the archaeointensity data in order to obtain acceptable models. Despite satisfactorily explaining both the historical and archaeointensity data, and possessing reasonable spatial and temporal complexity, such free evolution models perform worse than the simpler linearly constrained models when tested against the independent dataset of Gallet et al. [Gallet, Y., Genevey, A., Fluteau, F., 2005. Does Earth’s magnetic field secular variation control centennial climate change? Earth Plan. Sci. Lett. 236, 159–173]. Bayesian model comparison techniques indicate that a model ( gufm1-g10c) involving no change in g 1 0 ( t ) between 1590 A.D. and 1840 A.D. is most probable given the presently available data and current modelling techniques. I propose that this new, empirically derived, constraint on the evolution of the geomagnetic axial dipole be incorporated into the next generation of historical field models.

Variations in the geomagnetic dipole moment during the Holocene and the past 50 kyr

All absolute paleointensity data published in peer-reviewed journals were recently compiled in the GEOMACIA50 database. Based on the information in GEOMAGIA50, we reconstruct variations in the geomagnetic dipole moment over the past 50 kyr, with a focus on the Holocene period. A running-window approach is used to determine the axial dipole moment that provides the optimal least-squares fit to the paleointensity data, whereas associated error estimates are constrained using a bootstrap procedure. We Subsequently compare the reconstruction from this study with previous reconstructions of the geomagnetic dipole moment, including those based on cosmogenic radionuclides (Be-10 and C-14). This comparison generally lends Support to the axial dipole moments obtained in this study. Our reconstruction shows that the evolution of the dipole moment was highly dynamic, and the recently observed rates of change (5% per century) do not appear unique. We observe no apparent link between the occurrence of archeomagnetic jerks and changes in the geomagnetic dipole moment, Suggesting that archeomagnetic jerks most likely represent drastic changes in the orientation of the geomagnetic dipole axis or periods characterized by large secular variation of the non-dipole field. This Study also shows that the Holocene geomagnetic dipole moment was high compared to that of the preceding similar to 40 kyr, and that similar to 4.10(22) Am-2 appears to represent a critical threshold below which geomagnetic excursions and reversals occur. (C) 2008 Elsevier B.V. All rights reserved. (Less)

Regional cosmic ray induced ionization and geomagnetic field changes

Abstract. Cosmic ray induced ionization (CRII) is an important factor of outer space influences on atmospheric properties. Variations of CRII are caused by two different processes – solar activity variations, which modulate the cosmic ray flux in interplanetary space, and changes of the geomagnetic field, which affects the cosmic ray access to Earth. Migration of the geomagnetic dipole axis may greatly alter CRII in some regions on a time scale of centuries and longer. Here we present a study of CRII regional effects of the geomagnetic field changes during the last millennium for two regions: Europe and the Far East. We show that regional effects of the migration of the geomagnetic dipole axis may overcome global changes due to solar activity variations.

Ion Drag with Electrodynamic Drift; a Computer Experiment for the F2 Layer

Summary The model F2 layer considered is one in which the ions move under the influence of electrodynamic drift and simultaneously diffuse along the geomagnetic field lines. Both these motions are assumed to set the neutral air moving horizontally. The problem is made soluble by assuming that the ratio b of the horizontal velocity of the neutral air to the horizontal velocity of the ions is constant. Thus b = 0 corresponds to a stationary neutral atmosphere, while b = 1 corresponds to the case of fully matched horizontal velocities. Allowance is made for curvature and convergence of the (dipole) magnetic field lines, and for variations of electron density with latitude and height. The model layer is isothermal, and is made symmetrical about the equator by assuming that the geomagnetic dipole axis coincides with the Earth's axis of rotation. Equilibrium is assumed; thus the calculations hold only for some fixed time in the early or middle afternoon. The results show that the value chosen for b chiefly affects the overall magnitude, rather than the distribution, of electron density. Electrodynamic drift is much more important than ion-drag in determining the distribution. The calculations therefore support others made recently, which confirm Martyn's theory that electrodynamic lifting is a major feature of the low latitude ionospheric F2 layer. However, the motion of the neutral air in this particular model does have some effect. The results are discussed in relation to other theoretical and experimental work.