Showing papers on "Earth's magnetic field published in 2009"

Global distribution of whistler‐mode chorus waves observed on the THEMIS spacecraft

Abstract: [1] Whistler mode chorus waves are receiving increased scientific attention due to their important roles in both acceleration and loss processes of radiation belt electrons. A new global survey of whistler-mode chorus waves is performed using magnetic field filter bank data from the THEMIS spacecraft with 5 probes in near-equatorial orbits. Our results confirm earlier analyses of the strong dependence of wave amplitudes on geomagnetic activity, confinement of nightside emissions to low magnetic latitudes, and extension of dayside emissions to high latitudes. An important new finding is the strong occurrence rate of chorus on the dayside at L > 7, where moderate dayside chorus is present >10% of the time and can persist even during periods of low geomagnetic activity. Citation: Li, W., R. M. Thorne, V. Angelopoulos, J. Bortnik, C. M. Cully, B. Ni, O. LeContel, A. Roux, U. Auster, and W. Magnes (2009), Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft, Geophys. Res. Lett., 36, L09104, doi:10.1029/2009GL037595.

Geomagnetic field for 0–3 ka: 2. A new series of time‐varying global models

Abstract: [1] Steadily increasing numbers of archeomagnetic and paleomagnetic data for the Holocene have allowed development of temporally continuous global spherical harmonic models of the geomagnetic field extending present and historical global descriptions of magnetic field evolution. The current work uses various subsets of improved data compilations, details of which are given in a companion paper by Donadini et al. (2009), and minor modifications of standard modeling strategies (using temporally and spatially regularized inversion of the data and cubic spline parameterizations for temporal variations) to produce five models with enhanced spatial and temporal resolution for 0–3 ka. Spurious end effects present in earlier models are eliminated by enforcing large-scale agreement with the gufm1 historical model for 1650–1990 A.D. and by extending the model range to accommodate data older than 3 ka. Age errors are not considered as a contribution to data uncertainties but are included along with data uncertainties in an investigation of statistical uncertainty estimates for the models using parametric bootstrap resampling techniques. We find common features but also significant differences among the various models, indicating intrinsic uncertainties in global models based on the currently available Holocene data. Model CALS3k.3 based on all available archeomagnetic and sediment data, without a priori quality selection, currently constitutes the best global representation of the past field. The new models have slightly higher dipole moments than our previous models. Virtual axial dipole moments (VADMs) calculated directly from the data are in good agreement with all corresponding model predictions of VADMs. These are always higher than the spherical harmonic dipole moment, indicating the limitations of using VADMs as a measure of geomagnetic dipole moments.

Evidence of a long-term trend in total solar irradiance

Abstract: Aims. During the solar minimum of 2008, the value of total solar irradiance at 1 AU (TSI) was more than 0.2 Wm-2 lower than during the last minimum in 1996, indicating for the first time a directly observed long-term change. On the other hand, chromospheric indices and hence solar UV irradiance do not exhibit a similar change. Methods. Comparison of TSI with other activity parameters indicates that only the open solar magnetic field, , observed from satellites at 1 AU show a similar long-term behaviour. The values at the minima correlate well and the linear fit provides a direct physical relationship between TSI and during the minimum times.Results. This correlation allows an unambiguous reconstruction of TSI back in time, provided the open solar magnetic field can be determined from e.g. geomagnetic indices or cosmogenic radionucleides. Since the solar UV irradiance has no long-term trend, the mechanism for the secular change of TSI must differ from the effect of surface magnetism, as manifested by sunspots, faculae, and network which indeed explain well the intra-cycle variability of both total and spectral irradiance. Conclusions. The long-term trend of TSI is most probably caused by a global temperature change of the Sun that does not influence the UV irradiance in the same way as the surface magnetic fields.

Modelling the palaeo-evolution of the geodynamo

Abstract: Although it is known that the geodynamo has been operating for at least 3.2 Ga, it remains difficult to infer the intensity, dipolarity and stability (occurrence of reversals) of the Precam-brian magnetic field of the Earth. In order to assist the interpretation of palaeomagnetic data, we produce models for the long-term evolution of the geodynamo by combining core ther-modynamics with a systematic scaling analysis of numerical dynamo simulations. We update earlier dynamo scaling results by exploring a parameter space, which has been extended in order to account for core aspect ratios and buoyancy source distributions relevant to Earth in the Precambrian. Our analysis highlights the central role of the convective power, which is an output of core thermodynamics and the main input of our updated scalings. As the thermal evolution of the Earth's core is not well known, two end-member models of heat flow evolution at the core–mantle boundary (CMB) are used, respectively, terminating at present heat flows of 11 TW (high-power scenario) and 3 TW (low power scenario). The resulting models predict that until the appearance of the inner core, a thermal dynamo driven only by secular cooling, and without any need for radioactive heating, can produce a dipole moment of strength comparable to that of the present field, thus precluding an interpretation of the oldest palaeomagnetic records as evidence of the inner core presence. The observed lack of strong long-term trends in palaeointensity data throughout the Earth's history can be rationalized by the weakness of palaeointensity variations predicted by our models relatively to the data scatter. Specifically, the most significant internal magnetic field increase which we predict is associated to the sudden power increase resulting from inner core nucleation, but the dynamo becomes deeper-seated in the core, thus largely cancelling the increase at the core and Earth surface, and diminishing the prospect of observing this event in palaeointensity data. Our models additionally suggest that the geodynamo has lied close to the transition to polarity reversals throughout its history. In the Precambrian, we predict a dynamo with similar dipolarity and less frequent reversals than at present times, due to conditions of generally lower convective forcing. Quantifying the typical CMB heat flow variation needed for the geodynamo to cross the transition from a reversing to a non-reversing state, we find that it is unlikely that such a variation may have caused superchrons in the last 0.5 Ga without shutting down dynamo action altogether.

Oblique Whistler-Mode Waves in the Inhomogeneous Magnetospheric Plasma: Resonant Interactions with Energetic Charged Particles

Abstract: A consistent account is given of the theory of resonant interactions between energetic charged particles and a whistler-mode wave propagating obliquely to the non-uniform geomagnetic field in the inhomogeneous magnetospheric plasma. The basic equations for the wave field and charged particle dynamics are presented, with the emphasis being placed on the parameters governing the problem. A Hamiltonian approach is consistently used in the analysis of the particle equations of motion which are discussed in detail and solved analytically in various cases. Two applications of the theory are considered. First, we calculate the growth (or damping) rate for a whistler-mode wave propagating obliquely to geomagnetic field in the magnetosphere. Secondly, we estimate the proton precipitation into the upper atmosphere induced by a VLF transmitter signal.

Evolution of electron fluxes in the outer radiation belt computed with the VERB code

Abstract: [1] Three-dimensional simulations of the dynamics of outer radiation belt electrons with the recently developed Versatile Electron Radiation Belt (VERB) code are presented. Simulations are preformed for an idealized storm with geomagnetic activity–dependent wave amplitudes that are parameterized as a function of the level of geomagnetic activity. Numerical experiments using the VERB code with various scattering processes (pitch angle diffusion, radial diffusion, and energy diffusion) indicate that diffusive processes are strongly coupled with each other and that they all should be included in realistic simulations of the radiation belts. We show that during storms, inward radial diffusion can produce significant accelerations to relativistic energies, while pitch angle scattering and energy diffusion produce a decrease and an increase in fluxes, respectively. We show that in the presence of high-latitude and low-latitude chorus, peaks in the radial profile of phase space density are formed between L of 4 and 6 during the recovery phase of a storm and are later smoothed by radial diffusion. Sensitivity experiments show that geomagnetic control of wave intensities plays a controlling role in the dynamics of radiation belt electrons. Numerical simulations indicate that electrons of 10–100 keV near geosynchronous orbit can reach MeV energies in the heart of the radiation belts by combined radial diffusion and in situ acceleration. We present two scenarios of acceleration of the plasma sheet electrons: (1) in the range of hundreds of keV by means of radial diffusion and (2) in the range of tens of keV by means of radial diffusion combined with local acceleration.

Geomagnetic activity and polar surface air temperature variability

Abstract: Here we use the ERA-40 and ECMWF operational surface level air temperature data sets from 1957 to 2006 to examine polar temperature variations during years with different levels of geomagnetic activity, as defined by the A(p) index. Previous modeling work has suggested that NOx produced at high latitudes by energetic particle precipitation can eventually lead to detectable changes in surface air temperatures (SATs). We find that during winter months, polar SATs in years with high A(p) index are different than in years with low A(p) index; the differences are statistically significant at the 2-sigma level and range up to about +/- 4.5 K, depending on location. The temperature differences are larger when years with wintertime Sudden Stratospheric Warmings (SSWs) are excluded. We take into account solar irradiance variations, unlike previous analyses of geomagnetic effects in ERA-40 and operational data. Although we cannot conclusively show that the polar SAT patterns are physically linked by geomagnetic activity, we conclude that geomagnetic activity likely plays a role in modulating wintertime surface air temperatures. We tested our SAT results against variation in the Quasi Biennial Oscillation, the El Nino Southern Oscillation and the Southern Annular Mode. The results suggested that these were not driving the observed polar SAT variability. However, significant uncertainty is introduced by the Northern Annular Mode, and we cannot robustly exclude a chance linkage between sea surface temperature variability and geomagnetic activity.

Generation of a strong magnetic field using uniform heat flux at the surface of the core

Abstract: Numerical simulations that assume realistic core-fluid viscosities have been unsuccessful in fully reproducing the unique characteristics of the Earth’s geomagnetic field. An evaluation of boundary conditions suggests that the prescription of a uniform heat flux at the core’s surface could generate a more Earth-like magnetic field. The Earth’s main magnetic field is thought to be generated by motions in the planet’s fluid outer core, which lead to an effect similar to that of a dynamo1,2,3. Recent high-resolution numerical simulations produce only a non-dipolar4 or a dipolar but comparatively weak magnetic field5,6 unlike that of the Earth. Older models that did generate a strong, Earth-like field needed to use unrealistically high viscosities for the core fluid7,8,9,10. Common to most of the models is the assumption of a laterally uniform core-surface temperature. Here we use a low-viscosity geodynamo model to evaluate the effect of a different and more realistic boundary condition—a uniform heat flux at the surface of the core—on the simulation of an Earth-like magnetic field. Our results show that when the surface temperature is laterally uniform, only a weak magnetic field is generated because planetary-scale fluid circulations are suppressed. In contrast, a laterally uniform heat flux at the core’s surface leads to large-scale convective flows, and a comparatively strong dipole-type magnetic field. Contrary to previous work11,12, we suggest that thermal conditions at the core surface have a strong effect on low-viscosity geodynamo models.

Simple mechanism for reversals of earth's magnetic field.

Abstract: We show that a model, recently used to describe all the dynamical regimes of the magnetic field generated by the dynamo effect in the von Karman sodium experiment, also provides a simple explanation of the reversals of Earth's magnetic field, despite strong differences between both systems. The validity of the model relies on the smallness of the magnetic Prandtl number.

Meridional transport and deposition of atmospheric 10 Be

Abstract: . 10Be concentrations measured in ice cores exhibit larger temporal variability than expected based on theoretical production calculations. To investigate whether this is due to atmospheric transport a general circulation model study is performed with the 10Be production divided into stratospheric, tropospheric tropical, tropospheric subtropical and tropospheric polar sources. A control run with present day 10Be production rate is compared with a run during a geomagnetic minimum. The present 10Be production rate is 4–5 times higher at high latitudes than in the tropics whereas during a period of no geomagnetic dipole field it is constant at all latitudes. The 10Be deposition fluxes, however, show a very similar latitudinal distribution in both the present day and the geomagnetic minimum run indicating that 10Be is well mixed in the atmosphere before its deposition. This is also confirmed by the fact that the contribution of 10Be produced in the stratosphere is dominant (55%–70%) and relatively constant at all latitudes. The contribution of stratospheric 10Be is approximately 70% in Greenland and 60% in Antarctica reflecting the weaker stratosphere-troposphere air exchange in the Southern Hemisphere.

The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database

Abstract: We firstly present the results of a detailed palaeointensity study performed on 54 samples from 9 volcanic units of late Archaean age (2724-2772 Ma) from the Pilbara Craton, Western Australia. These results were severely affected by magnetomineralogical alteration occurring during the laboratory heating process necessitating the application of a correction procedure. The correction allowed results from three lavas to pass strict selection criteria but we deem that only one of these exhibits sufficient internal consistency to be considered moderately reliable. It yields a virtual dipole moment of 47±6 ZAm2which is 60% of the present-day value. We combine this determination with a filtered dataset from the updated IAGA (International Association of Geomagnetism and Aeronomy) palaeointensity database, PINT08. Directional secular variation has recently been shown to have changed fundamentally since the Archaean, probably as a consequence of inner core growth since that time. However, here we argue that it is still unclear whether this evolution was accompanied by a single long timescale change in average poloidal field intensity. While the distribution of Precambrian palaeointensity determinations as a whole is significantly lower than that for the last 300 Myr, we show that this finding largely reflects data from the Proterozoic aeon. The distribution of more ancient measurements from the late Archaean-earliest Proterozoic is indistinguishable from that of the last 300 Myr which might suggest that a ‘Proterozoic dipole low’ period existed between two periods of higher field intensity. Were this pattern of long-term geomagnetic intensity variation to be supported by the addition of new data in the future, then it could indicate a related three-stage evolution in core dynamics, namely: vigorous thermal convection caused by high core-mantle heat flux early in the Earth’s history, weaker thermal convection later as the heat flux fell, and finally, strong compositional convection since the inner core nucleated.

Modeling ionospheric outflows and their impact on the magnetosphere, initial results

Abstract: [1] Ionospheric outflow has been shown to be a significant contributor to the plasma population of the magnetosphere during active geomagnetic conditions. We present the results of new efforts to model the source and effects of out-flowing plasma in the Space Weather Modeling Framework (SWMF). In particular, we develop and use the Polar Wind Outflow Model (PWOM), a field-aligned, multifluid, multifield line polar wind code to simulate the ionospheric outflow. The PWOM is coupled to the ionosphere electrodynamics and global magnetosphere components of the SWMF, so we can calculate the outflow and its resulting impact on magnetospheric composition and dynamics. By including the outflow as part of a coupled system, we study the consequences of outflow on the larger space environment system. We present our methodology for the magnetosphere-ionosphere coupling, as well as the effect of outflow on the magnetosphere during two geomagnetic storms. Moreover, we explore the use of multispecies MHD to track the resulting plasma composition in the magnetosphere. We find that, by including ionospheric outflow during geomagnetic storms, we can reduce the RMS error in the simulated magnetic field as compared with various GOES satellites by as much as 50%. Additionally, we find that the outflow causes a strong decrease in Dst and in the cross-polar cap potential.

CHAOS-2—a geomagnetic field model derived from one decade of continuous satellite data

Abstract: SUMMARY We have derived a model of the near-Earth's magnetic field using more than 10 yr of high-precision geomagnetic measurements from the three satellites Orsted, CHAMP and SAC-C. This model is an update of the two previous models, CHAOS (Olsen et al. 2006) and xCHAOS (Olsen & Mandea 2008). Data selection and model parameterization follow closely those chosen for deriving these models. The main difference concerns the maximum spherical harmonic degree of the static field (n= 60 compared to n= 50 for CHAOS and xCHAOS), and of the core field time changes, for which spherical harmonic expansion coefficients up to n= 20 are described by order 5 splines (with 6-month knot spacing) spanning the years from 1997.0 to 2009.5. Compared to its predecessors, the temporal regularization of the CHAOS-2 model is also modified. Indeed, second and higher order time derivatives of the core field are damped by minimizing the second time derivative of the squared magnetic field intensity at the core–mantle boundary. The CHAOS-2 model describes rapid time changes, as monitored by the ground magnetic observatories, much better than its predecessors.

Space weather events in July 1982 and October 2003 and the effects of geomagnetically induced currents on Swedish technical systems

Abstract: . In this paper, we analyse in detail two famous space weather events; a railway problem on 13–14 July 1982 and a power blackout on 30 October 2003. Both occurred in Sweden during very intensive space weather storms and each of them a few years after the sunspot maximum. This paper provides a description of the conditions on the Sun and in the solar wind leading to the two GIC events on the ground. By applying modelling techniques introduced and developed in our previous paper, we also calculate the horizontal geoelectric field at the Earth's surface in southern Sweden during the two storms as well as GIC flowing in the southern Swedish 400 kV power grid during the event in October 2003. The results from the calculations agree with all measured data available. In the July-1982 storm, the geomagnetic field variation, ΔBx, reached values up to ~2500 nT/min and the geoelectric field reached values in the order of several volts per kilometer. In the October-2003 storm, the geomagnetic field fluctuations were smaller. However, GIC of some hundreds of amperes flowed in the power grid during the October-2003 event. Technological issues related to the railway signalling in July 1982 and to the power network equipment in October 2003 are also discussed.

Survey of cold ionospheric outflows in the magnetotail

Abstract: . Low-energy ions escape from the ionosphere and constitute a large part of the magnetospheric content, especially in the geomagnetic tail lobes. However, they are normally invisible to spacecraft measurements, since the potential of a sunlit spacecraft in a tenuous plasma in many cases exceeds the energy-per-charge of the ions, and little is therefore known about their outflow properties far from the Earth. Here we present an extensive statistical study of cold ion outflows (0–60 eV) in the geomagnetic tail at geocentric distances from 5 to 19 RE using the Cluster spacecraft during the period from 2001 to 2005. Our results were obtained by a new method, relying on the detection of a wake behind the spacecraft. We show that the cold ions dominate in both flux and density in large regions of the magnetosphere. Most of the cold ions are found to escape from the Earth, which improves previous estimates of the global outflow. The local outflow in the magnetotail corresponds to a global outflow of the order of 1026 ions s−1. The size of the outflow depends on different solar and magnetic activity levels.

Measurements of geomagnetically induced current in a power grid in Hokkaido, Japan

Abstract: [1] There have been numerous reports showing that space weather affects power grids through a geomagnetically induced current (GIC). Generally, power grids consist of power lines connected to transformers, of which neutral points are directly grounded. The GIC flows into those transformers through the neutral points if geomagnetic variations cause a ground level potential. These currents can damage power grids, especially transformers. It has been tacitly assumed, however, that the effect of the GIC is minor in Japan because of the country's location at geomagnetically lower latitudes. To examine the GIC effect in Japan, we conducted approximately 2 years of GIC measurements in Hokkaido, Japan. It is found that GICs associated with substorms can be detected in Japan even at the solar minimum although intense GICs do occur mostly during geomagnetic storms. Temporal variations of GICs show high correlation with geomagnetic field variations, rather than time derivatives of the geomagnetic field.

Extremely low-frequency electromagnetic fields disrupt magnetic alignment of ruminants

Abstract: Resting and grazing cattle and deer tend to align their body axes in the geomagnetic North-South direction. The mechanism(s) that underlie this behavior remain unknown. Here, we show that extremely low-frequency magnetic fields (ELFMFs) generated by high-voltage power lines disrupt alignment of the bodies of these animals with the geomagnetic field. Body orientation of cattle and roe deer was random on pastures under or near power lines. Moreover, cattle exposed to various magnetic fields directly beneath or in the vicinity of power lines trending in various magnetic directions exhibited distinct patterns of alignment. The disturbing effect of the ELFMFs on body alignment diminished with the distance from conductors. These findings constitute evidence for magnetic sensation in large mammals as well as evidence of an overt behavioral reaction to weak ELFMFs in vertebrates. The demonstrated reaction to weak ELFMFs implies effects at the cellular and molecular levels.

Modeling seasonal variations of auroral particle precipitation in a global-scale magnetosphere-ionosphere simulation

Abstract: [1] A variety of observations have shown strong seasonal variations in a vast array of magnetosphere-ionosphere parameters, including field-aligned currents, cross polar cap potential, and precipitating electron energy flux. In this paper we examine how these variations are modeled in the Lyon-Fedder-Mobarry (LFM) global-scale magnetohydrodynamic simulation of the coupled solar wind-magnetosphere-ionosphere system. In order to account for changes in the solar wind conditions caused by the seasonal variation of the Earth's dipole tilt we carefully select the solar wind parameters so that the effective driving conditions are the same across the March, June, and December intervals examined. The seasonal variation of the field-aligned current strengths is in good agreement with observations, with the sunlit hemisphere having more current than the dark hemisphere in the June and December intervals. However, in order to bring the modeled precipitating electron energy flux into better agreement with the observations we need to utilize a modified current-voltage relationship which includes a proxy for illumination effects. We provide a detailed description of the LFM's magnetosphere-ionosphere coupling interface including how illumination effects are incorporated into the model. This methodology for including these effects does not allow for determining if changes in conductance or ionospheric density are responsible for the changes. In addition to improving the agreement with observations the new version of the current-voltage relationship results in enhanced geomagnetic activity in the March interval examined and suppression of activity during the June interval.

Geomagnetic storm effects on GPS based navigation

Abstract: . The energetic events on the sun, solar wind and subsequent effects on the Earth's geomagnetic field and upper atmosphere (ionosphere) comprise space weather. Modern navigation systems that use radio-wave signals, reflecting from or propagating through the ionosphere as a means of determining range or distance, are vulnerable to a variety of effects that can degrade the performance of the navigational systems. In particular, the Global Positioning System (GPS) that uses a constellation of earth orbiting satellites are affected due to the space weather phenomena. Studies made during two successive geomagnetic storms that occurred during the period from 8 to 12 November 2004, have clearly revealed the adverse affects on the GPS range delay as inferred from the Total Electron Content (TEC) measurements made from a chain of seven dual frequency GPS receivers installed in the Indian sector. Significant increases in TEC at the Equatorial Ionization anomaly crest region are observed, resulting in increased range delay during the periods of the storm activity. Further, the storm time rapid changes occurring in TEC resulted in a number of phase slips in the GPS signal compared to those on quiet days. These phase slips often result in the loss of lock of the GPS receivers, similar to those that occur during strong(>10 dB) L-band scintillation events, adversely affecting the GPS based navigation.

Ensemble inversion of time‐dependent core flow models

Abstract: Quasi-geostrophic core flow models are built from two secular variation models spanning the periods 1960–2002 and 1997–2008. We rely on an ensemble method to account for the contributions of the unresolved small-scale magnetic field interacting with core surface flows to the observed magnetic field changes. The different core flow members of the ensemble solution agree up to spherical harmonic degree l ≃ 10, and this resolved component varies only weakly with regularization. Taking into account the finite correlation time of the small-scale concealed magnetic field, we find that the time variations of the magnetic field occurring over short time scales, such as the geomagnetic jerks, can be accounted for by the resolved (large-scale) part of the flow to a large extent. Residuals from our flow models are 30% smaller for recent epochs, after 1995. This result is attributed to an improvement in the quality of geomagnetic data. The magnetic field models show little frozen flux violation for the most recent epochs, within our estimate of the apparent magnetic flux changes at the core-mantle boundary arising from spatial resolution errors. We associate the more important flux changes detected at earlier epochs with uncertainties in the field models at large harmonic degrees. Our core flow models show, at all epochs, an eccentric and planetary-scale anticyclonic gyre circling around the cylindrical surface tangent to the inner core, at approximately 30 and 60 latitude under the Indian and Pacific oceans, respectively. They account well for the changes in core angular momentum for the most recent epochs.

Cosmic Rays in Magnetospheres of the Earth and other Planets

Abstract: First Measurements of Cosmic Ray Geomagnetic Effects and the Problem of CR Nature.- Cosmic Rays in the Dipole Geomagnetic Field.- Cosmic Rays in the Real Geomagnetic Field.- Cosmic Ray Planetary Surveys on Ships, Trains, Tracks, Planes, Balloons, and Satellites.- Main Results of Cosmic Ray Survey to Antarctica on the Ship Italica in 1996/97.- Geomagnetic Variations of Cosmic Rays.- Magnetospheric Models and their Checking by Cosmic Rays.- Galactic Cosmic Rays in Atmospheres and Magnetospheres of Other Planets.

Storm induced large scale TIDs observed in GPS derived TEC

Abstract: . This work is a first statistical analysis of large scale traveling ionospheric disturbances (LSTID) in Europe using total electron content (TEC) data derived from GNSS measurements. The GNSS receiver network in Europe is dense enough to map the ionospheric perturbation TEC with high horizontal resolution. The derived perturbation TEC maps are analysed studying the effect of space weather events on the ionosphere over Europe. Equatorward propagating storm induced wave packets have been identified during several geomagnetic storms. Characteristic parameters such as velocity, wavelength and direction were estimated from the perturbation TEC maps. Showing a mean wavelength of 2000 km, a mean period of 59 min and a phase speed of 684 ms−1 in average, the perturbations are allocated to LSTID. The comparison to LSTID observed over Japan shows an equal wavelength but a considerably faster phase speed. This might be attributed to the differences in the distance to the auroral region or inclination/declination of the geomagnetic field lines. The observed correlation between the LSTID amplitudes and the Auroral Electrojet (AE) indicates that most of the wave like perturbations are exited by Joule heating. Particle precipitation effects could not be separated.

Time evolution of the South Atlantic Magnetic Anomaly

Abstract: The South Atlantic Magnetic Anomaly (SAMA) is one of the most outstanding anomalies of the geomagnetic field. The SAMA secular variation was obtained and compared to the evolution of other anomalies using spherical harmonic field models for the 1590-2005 period. An analysis of data from four South American observatories shows how this large scale anomaly affected their measurements. Since SAMA is a low total field anomaly, the field was separated into its nondipolar, quadrupolar and octupolar parts. The time evolution of the non-dipole/total, quadrupolar/total and octupolar/total field ratios yielded increasingly high values for the South Atlantic since 1750. The SAMA evolution is compared to the evolution of other large scale surface geomagnetic features like the North and the South Pole and the Siberia High, and this comparison shows the intensity equilibrium between these anomalies in both hemispheres. The analysis of non-dipole fields in historical period suggests that SAMA is governed by (i) quadrupolar field for drift, and (ii) quadrupolar and octupolar fields for intensity and area of influence. Furthermore, our study reinforces the possibility that SAMA may be related to reverse fluxes in the outer core under the South Atlantic region.

Storm-dependent radiation belt electron dynamics

Abstract: [1] Using recently published electron phase space densities (PSD) as a function of L* (L* is approximately the radial distance in Earth radii at the equator) and time, energization and loss in the Earth's outer electron radiation belt were studied quantitatively and numerically using a radial diffusion model that included finite electron lifetimes and an internal source parameterized as a function of geomagnetic indices. We used PSD data at fixed values of the first and second adiabatic invariants, corresponding to electrons mirroring near the Earth's equator with an energy of ∼2.7 MeV at L* = 4. Model results for the second half of 2002 reproduced the average variations of the radiation belt electron PSD between L* = 2.5 and L* = 6 but with overprediction and underprediction at different times, implying that the same set of parameters cannot be applied to all storms. A detailed analysis of four individual storms showed that while electrons in three storms could be well simulated by energization from either radial diffusion only or internal heating only, incorporating both yielded the best results. For the other storm, an additional source of electrons was required to account for the enhanced PSD. The model results indicated that each storm is best simulated when a combination of radial diffusion and internal heating is used. Different storms required different magnitudes of radial diffusion and internal heating, and the relative contributions of these two acceleration mechanisms varied from storm to storm. A comparison of the results from different runs for the four storms and an analysis of the radial diffusion coefficients further suggest that internal heating contributes more to the enhancement of 2.7 MeV electrons at L* = 4 than radial diffusion.

No asymmetry in geomagnetic reversals recorded by 1.1-billion-year-old Keweenawan basalts

Abstract: 1.1-billion-year-old volcanic rocks in North America are thought to record asymmetric geomagnetic reversals, indicating non-axial dipolar behaviour of the magnetic field. High-resolution data from Ontario suggest that the reversals were instead symmetric, and that the apparent reversal asymmetry is an aliasing effect of the low resolution of earlier samples combined with the rapid motion of North America.

Natural magnetic disturbance fields, not precursors, preceding the Loma Prieta earthquake

Abstract: [1] Available records of the magnetic indices Dst and ap together with standard observatory recordings of 1-min field levels were examined for the period preceding the earthquake of October 1989, centered near Loma Prieta, California. The magnetic records showed that the Fraser-Smith et al. (1990) report claiming the existence of a 100-s (ultralow frequency) geomagnetic field precursor signal at Corralitos, California, foretelling a nearby earthquake is not valid. My study shows that the Stanford ULF signal was not local but rather widespread throughout the western United States and, therefore, expected to be due to a coincidental geomagnetic solar-terrestrial disturbance field.