Showing papers on "Earth's magnetic field published in 2012"

Geomagnetic Polarity Time Scale

Abstract: The patterns of marine magnetic anomalies for the Late Cretaceous through Neogene (C-sequence) and Middle Jurassic through Early Cretaceous (M-sequence with deep-tow extension) have been calibrated through magnetostratigraphic studies to biostratigraphy, cyclostratigraphy and selected radio-isotope-dated levels. The majority of the geomagnetic polarity time scale for the past 160 myr is constructed by fitting spreading-rate models to these constraints. The status of the geomagnetic polarity time scale for each geological period is summarized in the appropriate period chapters.

Thermal and electrical conductivity of iron at Earth/'s core conditions

Abstract: First principles calculations of the thermal and electrical conductivities of liquid iron mixtures under Earth's core conditions suggest a relatively high adiabatic heat flux of 15 to16 terawatts at the core–mantle boundary, indicating that the top of the core must be thermally stratified. The thermal and electrical properties of iron are important for understanding the thermal evolution of the deep Earth and the power available to drive the dynamo that generates Earth's magnetic field. These parameters have previously been estimated by extrapolating results from conditions with lower pressure or temperature, but Monica Pozzo and colleagues now present a calculation from first principles of these parameters at the pressure and temperature of Earth's outer core. Both conductivities are found to be two to three times higher than earlier estimates, prompting a re-evaluation of power estimates for the dynamo. The results greatly restrict models for powering the geodynamo, and indicate that the top of the core must be thermally stratified. The Earth acts as a gigantic heat engine driven by the decay of radiogenic isotopes and slow cooling, which gives rise to plate tectonics, volcanoes and mountain building. Another key product is the geomagnetic field, generated in the liquid iron core by a dynamo running on heat released by cooling and freezing (as the solid inner core grows), and on chemical convection (due to light elements expelled from the liquid on freezing). The power supplied to the geodynamo, measured by the heat flux across the core–mantle boundary (CMB), places constraints on Earth’s evolution1. Estimates of CMB heat flux2,3,4,5 depend on properties of iron mixtures under the extreme pressure and temperature conditions in the core, most critically on the thermal and electrical conductivities. These quantities remain poorly known because of inherent experimental and theoretical difficulties. Here we use density functional theory to compute these conductivities in liquid iron mixtures at core conditions from first principles—unlike previous estimates, which relied on extrapolations. The mixtures of iron, oxygen, sulphur and silicon are taken from earlier work6 and fit the seismologically determined core density and inner-core boundary density jump7,8. We find both conductivities to be two to three times higher than estimates in current use. The changes are so large that core thermal histories and power requirements need to be reassessed. New estimates indicate that the adiabatic heat flux is 15 to 16 terawatts at the CMB, higher than present estimates of CMB heat flux based on mantle convection1; the top of the core must be thermally stratified and any convection in the upper core must be driven by chemical convection against the adverse thermal buoyancy or lateral variations in CMB heat flow. Power for the geodynamo is greatly restricted, and future models of mantle evolution will need to incorporate a high CMB heat flux and explain the recent formation of the inner core.

Magnetically sensitive light-induced reactions in cryptochrome are consistent with its proposed role as a magnetoreceptor

Abstract: Among the biological phenomena that fall within the emerging field of “quantum biology” is the suggestion that magnetically sensitive chemical reactions are responsible for the magnetic compass of migratory birds. It has been proposed that transient radical pairs are formed by photo-induced electron transfer reactions in cryptochrome proteins and that their coherent spin dynamics are influenced by the geomagnetic field leading to changes in the quantum yield of the signaling state of the protein. Despite a variety of supporting evidence, it is still not clear whether cryptochromes have the properties required to respond to magnetic interactions orders of magnitude weaker than the thermal energy, kBT. Here we demonstrate that the kinetics and quantum yields of photo-induced flavin—tryptophan radical pairs in cryptochrome are indeed magnetically sensitive. The mechanistic origin of the magnetic field effect is clarified, its dependence on the strength of the magnetic field measured, and the rates of relevant spin-dependent, spin-independent, and spin-decoherence processes determined. We argue that cryptochrome is fit for purpose as a chemical magnetoreceptor.

THEMIS observations of electromagnetic ion cyclotron wave occurrence: Dependence on AE, SYMH, and solar wind dynamic pressure

Abstract: [1] Electromagnetic ion cyclotron (EMIC) waves are transverse plasma waves generated by anisotropic proton distributions with Tperp > Tpara. They are believed to play an important role in the dynamics of the ring current and potentially, of the radiation belts. Therefore it is important to know their localization in the magnetosphere and the magnetospheric and solar wind conditions which lead to their generation. Our earlier observations from three Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes demonstrated that strong magnetospheric compressions associated with high solar wind dynamic pressure (Pdyn) may drive EMIC waves in the inner dayside magnetosphere, just inside the plasmapause. Previously, magnetospheric compressions were found to generate EMIC waves mainly close to the magnetopause. In this work we use an automated detection algorithm of EMIC Pc1 waves observed by THEMIS between May 2007 to December 2011 and present the occurrence rate of those waves as a function of L-shell, magnetic local time (MLT), Pdyn, AE, and SYMH. Consistent with earlier studies we find that the dayside (sunward of the terminator) outer magnetosphere is a preferential location for EMIC activity, with the occurrence rate in this region being strongly controlled by solar wind dynamic pressure. High EMIC occurrence, preferentially at 12–15 MLT, is also associated with high AE. Our analysis of 26 magnetic storms with Dst < −50 nT showed that the storm-time EMIC occurrence rate in the inner magnetosphere remains low (<10%). This brings into question the importance of EMIC waves in influencing energetic particle dynamics in the inner magnetosphere during disturbed geomagnetic conditions.

Paleomagnetism: Continents and Oceans

Abstract: Geomagnetism and Paleomagnetism. Geomagnetism. Historical. Main Features of the Geomagnetic Field. Origin of the Main Field. Variations of the Dipole Field with Time. Paleomagnetism. Early Work in Paleomagnetism. Magnetism in Rocks. Geocentric Axial Dipole Hypothesis. Archeomagnetism. Paleointensity Over Geological Times. Paleosecular Variation. Rock Magnetism. Basic Principles of Magnetism. Magnetic Fields, Remanent and Induced Magnetism. Diamagnetism and Paramagnetism. Ferro-, Antiferro- and Ferrimagnetism. Hysteresis. Magnetic Minerals in Rocks. Mineralogy. Titanomagnetites. Titanohematites. Iron Sulfides and Oxyhydroxides. Physical Theory of Rock Magnetism. Magnetic Domains. Theory for Single Domain Grains. Magnetic Viscosity. Critical Size for Single Domain Grains. Thermoremanent Magnetization. Crystallization (or Chemical)Remanent Magnetization. Detrital and Post-Depositional Remanent Magnetization. Viscous and Thermoviscous Remanent Magnetization. Stress Effects and Anisotropy. Methods and Techniques. Sampling and Measurement. Sample Collection in the Field. Sample Measurement. Statistical Methods. Some Statistical Concepts. The Fisher Distribution. Statistical Tests. Calculating Paleomagnetic Poles and Their Errors. Other Statistical Distributions. Field Tests for Stability. Constraining the Age of Magnetization. The Fold Test. Conglomerate Test. Baked Contact Test. Unconformity Test. Consistency and Reversals Tests. Laboratory Methods and Applications. Progressive Stepwise Demagnetization. Presentation of Demagnetization Data. Principal Component Analysis. Analysis of Remagnetization Circles. Identification of Magnetic Minerals and Grain Sizes. Curie Temperatures. Isothermal Remanent Magnetization. The Lowrie-Fuller Test. Hysteresis and Magnetic Grain Sizes. Low-Temperature Measurements. Magnetic Field Reversals. Evidence for Field Reversal. Background and Definition. Self-Reversal in Rocks. Evidence for Field Reversal. The Geomagnetic Polarity Time Scale. Polarity Dating of Lava Flows 0 6 Ma. Geochronometry of Ocean Sediment Cores. Extending the GPTS to 160 Ma. Magnetostratigraphy. Terminology in Magnetostratigraphy. Methods in Magnetostratigraphy. Quality Criteria for Magnetostratigraphy. Late Creatceous-Eocene: The Gubbio Section. Late Triassic GPTS. Superchrons. Polarity Transitions. Recording Polarity Transitions. Directional Changes. Intensity Changes. Polarity Transition Duration. Geomagnetic Excursions. Analysis of Reversal Sequences. Probability Distributions. Filtering of the Record. Non-Stationarity in Reversal Rate. Polarity Symmetry and Superchrons. Oceanic Paleomagnetism. Marine Magnetic Anomalies. Sea-floor Spreading and Plate Tectonics. Vine-Matthews Crustal Model. Measurement of Marine Magnetic Anomalies. Nature of the Magnetic Anomaly Source. Modeling Marine Magnetic Anomalies. Factors Affecting the Shape of Anomalies. Calculating Magnetic Anomalies. Analysing Older Magnetic Anomalies. The Global Magnetic Anomaly Pattern. Magnetic Anomaly Nomenclature. The Cretaceous and Jurassic Quiet Zones. Paleomagnetic Poles for Oceanic Plates. Skewness of Magnetic Anomalies. Magnetization of Seamounts. Calculating Mean Pole Positions from Oceanic Data. Evolution of Oceanic Plates. The Hotspot Reference Frame. Evolution of the Pacific Plate. Continental Paleomagnetism. Analysing Continental Data. Data Selection and Reliability Criteria. Selecting Data for Paleomagnetic Analysis. Reliability Criteria. The Global Paleomagnetic Database. Testing the Geocentric Axial Dipole Model. The Past 5 Million Years. The Past 3000 Million Years. Global Paleointensity Variations. Paleoclimates and Paleoaltitudes. Apparent Polar Wander. The Concept of Apparent Polar Wander. Determining Apparent Polar Wander Paths. Magnetic Blocking Temperature and Isotopic Ages. Phanerozoic APWPs for the Major Blocks. Selection and Grouping of Data. North America and Europe. Asia. The Gondwana Continents. Paleomagnetism and Plate Tectonics. Plate Motions and Paleomagnetic Poles. Combining Euler and Paleomagnetic Poles. Making Reconstructions From Paleomagnetism. Phanerozoic Supercontinents. Laurussia. Paleo-Asia. Gondwana. Pangea. Paleogeography--300 Ma to Present. Displaced Terranes. Western North America. The East and West Avalon Terranes. Armorica. The Western Mediterranean. South and East Asia. Rodinia and the Precambrian. Rodinia. Paleomagnetism and Rodinia. Earth History--1000 Ma to the Present. Precambrian Cratons. Non Plate Tectonic Hypotheses. True Polar Wander. An Expanding Earth? References. Index.

Generation of 100-year geomagnetically induced current scenarios

Abstract: A series of 100-year extreme geoelectric field and geomagnetically induced current (GIC) scenarios are explored by taking into account the key geophysical factors associated with the geomagnetic induction process. More specifically, we derive explicit geoelectric field temporal profiles as a function of ground conductivity structures and geomagnetic latitudes. We also demonstrate how the extreme geoelectric field scenarios can be mapped into GIC. Generated statistics indicate 20 V/km and 5 V/km 100-year maximum 10-s geoelectric field amplitudes at high-latitude locations with poorly conducting and well-conducting ground structures, respectively. We show that there is an indication that geoelectric field magnitudes may experience a dramatic drop across a boundary at about 40°–60° of geomagnetic latitude. We identify this as a threshold at about 50° of geomagnetic latitude. The sub-threshold geoelectric field magnitudes are about an order of magnitude smaller than those at super-threshold geomagnetic latitudes. Further analyses are required to confirm the existence and location of the possible latitude threshold. The computed extreme GIC scenarios can be used in further engineering analyses that are needed to quantify the geomagnetic storm impact on conductor systems such as high-voltage power transmission systems. To facilitate further work on the topic, the digital data for generated geoelectric field scenarios are made publicly available.

Possible links between long-term geomagnetic variations and whole-mantle convection processes

Abstract: The Earth's internal magnetic field varies on timescales of months to billions of years. The field is generated by convection in the liquid outer core, which in turn is influenced by the heat flowing from the core into the base of the overlying mantle. Much of the magnetic field's variation is thought to be stochastic, but over very long timescales, this variability may be related to changes in heat flow associated with mantle convection processes. Over the past 500 Myr, correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late Mesozoic era, beginning about 180 Myr ago. Simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability — such as occurred between the Middle Jurassic and Middle Cretaceous periods — may have been triggered by a decrease in core-mantle boundary heat flow either globally or in equatorial regions. This decrease in heat flow could have been linked to reduced mantle-plume-head production at the core-mantle boundary, an episode of true polar wander, or a combination of the two.

Geomagnetically induced currents in the New Zealand power network

Abstract: [1] Adverse space weather conditions have been shown to be directly responsible for faults within power networks at high latitudes. A number of studies have also shown space weather to impact power networks at lower latitudes, although most of these studies show increases in GIC activity within networks not directly related to hardware faults. This study examines a GIC event that occurred in New Zealand's South Island power network on 6th November 2001. A transformer failure that occurred during this day is shown to be associated with a change in the solar wind dynamic pressure of nearly 20 nPa. Measurements of GICs recorded on the neutral lines of transformers across the Transpower network during this event show good correlation with a GIC-index, a proxy for the geoelectric field that drives GIC. Comparison of this event with GIC activity observed in the Transpower network during large space weather storms such as the “2003 Halloween storm,” suggests that solar wind shocks and associated geomagnetic sudden impulse (SI) events may be as hazardous to middle latitude power networks as GIC activity occurring during the main phase of large storms. Further, this study suggests that the latitudinal dependence of the impacts of SI events on power systems differs from that observed during large main phase storms. This study also highlights the importance of operating procedures for large space weather events, even at middle latitude locations.

Dynamical similarity of geomagnetic field reversals

Abstract: Volcanic records of the reversals of the geomagnetic field can be well matched under the assumption of a common reversal duration, and imply that the reversal process comprises three phases—a precursor, a fast polarity switch and a rebound—the properties of which have remained unchanged for about 180 million years. The geomagnetic variations that prevail during magnetic-field reversals convey important information on the nature of the geodynamo that drives Earth’s magnetic field. In this paper, Jean-Pierre Valet and co-authors show that the ten most detailed volcanic records of geomagnetic reversals have a common duration, as well as common dynamical characteristics. They propose that the reversing process has remained the same for at least the past 180 million years, characterized by three phases: a precursor event, a 180o polarity switch and then a rebound. No consensus has been reached so far on the properties of the geomagnetic field during reversals or on the main features that might reveal its dynamics. A main characteristic of the reversing field is a large decrease in the axial dipole and the dominant role of non-dipole components1,2,3. Other features strongly depend on whether they are derived from sedimentary or volcanic records. Only thermal remanent magnetization of lava flows can capture faithful records of a rapidly varying non-dipole field, but, because of episodic volcanic activity, sequences of overlying flows yield incomplete records. Here we show that the ten most detailed volcanic records of reversals can be matched in a very satisfactory way, under the assumption of a common duration, revealing common dynamical characteristics. We infer that the reversal process has remained unchanged, with the same time constants and durations, at least since 180 million years ago. We propose that the reversing field is characterized by three successive phases: a precursory event, a 180° polarity switch and a rebound. The first and third phases reflect the emergence of the non-dipole field with large-amplitude secular variation. They are rarely both recorded at the same site owing to the rapidly changing field geometry and last for less than 2,500 years. The actual transit between the two polarities does not last longer than 1,000 years and might therefore result from mechanisms other than those governing normal secular variation. Such changes are too brief to be accurately recorded by most sediments.

Field-aligned distribution of the plasmaspheric electron density: An empirical model derived from the IMAGE RPI measurements

Abstract: [1] We present a newly developed empirical model of the plasma density in the plasmasphere. It is based on more than 700 density profiles along field lines derived from active sounding measurements made by the radio plasma imager on IMAGE between June 2000 and July 2005. The measurements cover all magnetic local times and vary from L = 1.6 to L = 4 spatially, with every case manually confirmed to be within the plasmasphere by studying the corresponding dynamic spectrogram. The resulting model depends not only on L-shell but also on magnetic latitude and can be applied to specify the electron densities in the plasmasphere between 2000 km altitude and the plasmapause (the plasmapause location itself is not included in this model). It consists of two parts: the equatorial density, which falls off exponentially as a function of L-shell; and the field-aligned dependence on magnetic latitude and L-shell (in the form of invariant magnetic latitude). The fluctuations of density appear to be greater than what could be explained by a possible dependence on magnetic local time or season, and the dependence on geomagnetic activity is weak and cannot be discerned. The solar cycle effect is not included because the database covers only a fraction of a solar cycle. The performance of the model is evaluated by comparison to four previously developed plasmaspheric models and is further tested against the in situ passive IMAGE RPI measurements of the upper hybrid resonance frequency. While the equatorial densities of different models are mostly within the statistical uncertainties (especially at distances greater than L = 3), the clear latitudinal dependence of the RPI model presents an improvement over previous models. The model shows that the field-aligned density distribution can be treated neither as constant nor as a simple diffusive equilibrium distribution profile. This electron density model combined with an assumed model of the ion composition can be used to estimate the time for an Alfven wave to propagate from one hemisphere to the other, to determine the plasma frequencies along a field line, and to calculate the raypaths for high frequency waves propagating in the plasmasphere.

The interaction between the Moon and the solar wind

Abstract: We study the interaction between the Moon and the solar wind using a three-dimensional hybrid plasma solver. The proton fluxes and electromagnetical fields are presented for typical solar wind conditions with different magnetic field directions. We find two different wake structures for an interplanetary magnetic field that is perpendicular to the solar wind flow, and for one that is parallell to the flow. The wake for intermediate magnetic field directions will be a mix of these two extreme conditions. Several features are consistent with a fluid interaction, e.g., the presence of a rarefaction cone, and an increased magnetic field in the wake. There are however several kinetic features of the interaction. We find kinks in the magnetic field at the wake boundary. There are also density and magnetic field variations in the far wake, maybe from an ion beam instability related to the wake refill. The results are compared to observations by the WIND spacecraft during a wake crossing. The model magnetic field and ion velocities are in agreement with the measurements. The density and the electron temperature in the central wake are not as well captured by the model, probably from the lack of electron physics in the hybrid model.

Stratospheric warmings and the geomagnetic lunar tide: 1958–2007

Abstract: [1] A quantitative comparison of the geomagnetic lunar tide and lower stratospheric parameters (zonal mean air temperature T and zonal mean zonal wind U) is carried out for the period 1958–2007. The correlation between the amplitude of the geomagnetic lunar tide at an equatorial station, Addis Ababa, and the lower stratospheric parameters from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP-NCAR) reanalysis is examined. It is found that the lunar tidal amplitude tends to be positively and negatively correlated with the stratospheric T and U, respectively, in high latitudes of the Northern Hemisphere during December and January. High correlations are observed in approximately 70% of stratospheric sudden warming (SSW) events. The results suggest that variability of the geomagnetic lunar tide during the northern winter is closely linked with dynamical changes in the lower stratospheric parameters associated with SSWs.

Geomagnetic field variability during the Cretaceous Normal Superchron

Abstract: During the Cretaceous Normal Superchron 121–83 million years ago, the polarity of the Earth’s geomagnetic field remained stable for an unusually long time. Deep-tow magnetic data suggest that despite the stability of the polarity, the field varied greatly throughout the interval.

Geomagnetically induced currents in a power grid of northeastern Spain

Abstract: Using the geomagnetic records of Ebro geomagnetic observatory and taking the plane wave assumption for the external current source and a homogeneous Earth conductivity, a prediction of the effects of the geomagnetic activity on the Catalonian (northeastern Spain) power transmission system has been developed. Although the area is located at midlatitudes, determination of the geoelectric field on the occasion of the largest geomagnetic storms during the last solar cycles indicates amplitudes that are higher than those recorded in southern Africa, where some transformer failures on large transmission systems have been reported. A DC network model of the grid has been constructed, and the geomagnetically induced current (GIC) flows in the power network have been calculated for such extreme events using the electric field at Ebro as a regional proxy. In addition, GICs have been measured at one transformer neutral earthing of the power grid, so that there the accuracy of the model has been assessed. Although the agreement is quite satisfactory, results indicate that better knowledge of the ground conductivity structure is needed. This represents the first attempt to study and measure GICs in southern European power grids, a region considered to have low GIC-risk up to the present.

Dynamics of the region 1 Birkeland current oval derived from the active magnetosphere and planetary electrodynamics response experiment (AMPERE)

Abstract: [1] The region 1 (R1) and region 2 current systems typically form concentric rings of field-aligned currents in the polar ionospheres; we term the inner ring the R1 oval We apply an automated fitting scheme to field-aligned current densities provided by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) and identify the latitude of maximum R1 current at all magnetic local times to yield the size of the R1 oval We investigate the dynamics of the R1 oval size in response to geomagnetic activity for two cases corresponding to: repeated substorm activations with a minimally enhanced ring current; a significant ring current enhancement with multiple substorms During the first event the dynamics of the R1 oval size reflected an expanding-contracting polar cap: during substorm growth phase dayside reconnection added open magnetic flux to the polar cap, expanding the R1 oval equatorward Tail reconnection during the substorm expansion phase converted open into closed magnetic flux and the polar cap contracts as reflected by the poleward retreat of the R1 oval During the period of enhanced ring current intensity the R1 oval grew to larger sizes during each substorm growth phase than it did during the other event, consistent with the suggestion that a stronger ring current stabilizes the magnetospheric tail to the onset of magnetic reconnection The presented methodology allows AMPERE data to be condensed into a single parameter, the R1 oval size, which reflects magnetospheric dynamics and provides a convenient measure of the instantaneous magnetospheric system state in both hemispheres

Indoor positioning system using geomagnetic anomalies for smartphones

Abstract: In this paper, we propose an indoor positioning system using smartphones. To localize the position of target that is pedestrian in usual indoors, we make use of perturbations of the geomagnetic field caused by structural steel elements in a building. The system based on magnetic field does not need any physical infrastructure, so it is possible to be realized with low cost. To estimate the target's position using the geomagnetic anomaly, the proposed system measures the magnetic field on its own position using a magnetometer embedded in smartphones and compares the sensor measurement with the magnetic map that has been created for the building in advance. The estimated position is calculated by a stochastic system based on the particle filter. To calculate control inputs for the particle filter, such as moving distance and direction, we exploit the inertial measurement unit (IMU) that is composed of 3-axis accelerometer and gyroscope built in the smartphone. The experimental results show that accuracy is within 3 meter. These results imply the potential to track people in the buildings that have geomagnetic map in advance at the meter scale using only smartphone with embedded sensors.

CEDAR Electrodynamics Thermosphere Ionosphere (ETI) Challenge for systematic assessment of ionosphere/thermosphere models: Electron density, neutral density, NmF2, and hmF2 using space based observations

Abstract: [1] In an effort to quantitatively assess the current capabilities of Ionosphere/Thermosphere (IT) models, an IT model validation study using metrics was performed. This study is a main part of the CEDAR Electrodynamics Thermosphere Ionosphere (ETI) Challenge, which was initiated at the CEDAR workshop in 2009 to better comprehend strengths and weaknesses of models in predicting the IT system, and to trace improvements in ionospheric/thermospheric specification and forecast. For the challenge, two strong geomagnetic storms, four moderate storms, and three quiet time intervals were selected. For the selected events, we obtained four scores (i.e., RMS error, prediction efficiency, ratio of the maximum change in amplitudes, and ratio of the maximum amplitudes) to compare the performance of models in reproducing the selected physical parameters such as vertical drifts, electron and neutral densities, NmF2, and hmF2. In this paper, we present the results from comparing modeled values against space-based measurements including NmF2 and hmF2 from the CHAMP and COSMIC satellites, and electron and neutral densities at the CHAMP satellite locations. It is found that the accuracy of models varies with the metrics used, latitude and geomagnetic activity level.

Magnetic declination and zonal wind effects on longitudinal differences of ionospheric electron density at midlatitudes

Abstract: [1] A prominent ionospheric longitudinal variation at midlatitudes, in particular, over the continental US, was found recently. This variation is characterized as a higher east-side electron density in the evening and a higher west-side electron density in the morning, and with clear seasonal and solar activity dependencies. A combined effect of geomagnetic declination and changing zonal winds was proposed to explain it. This paper represents a comprehensive investigation of this effect by examining climatology for both electron density longitudinal differences and the nighttime zonal winds in the eastern US. Electron density is from incoherent scatter radar extra-wide coverage experiments during 1978–2011 over Millstone Hill for which the spatial separation of the data can be up to 50° in longitude. The thermospheric zonal wind is from the on-site Fabry-Perot interferometer measurements during 1989–2001. The observed zonal wind climatology is found to be perfectly consistent with the expectation based on the east-west electron density differences in terms of local time, seasonal, and solar cycle dependencies. The correlation between the zonal wind and the east-west differential ratio is extremely high with an overall correlation coefficient of 0.93. The observed time delay of ∼3 hours in the response of electron density differences to zonal winds is a marked feature. Thus these results confirm positively the declination-zonal wind mechanism and provide new insight into longitudinal variations at midlatitudes for other geographic sectors.

The first in situ observation of Kelvin‐Helmholtz waves at high‐latitude magnetopause during strongly dawnward interplanetary magnetic field conditions

Abstract: We report the first in situ observation of high-latitude magnetopause (near the northern duskward cusp) Kelvin-Helmholtz waves (KHW) by Cluster on January 12, 2003, under strongly dawnward interplanetary magnetic field (IMF) conditions. The fluctuations unstable to Kelvin-Helmholtz instability (KHI) are found to propagate mostly tailward, i.e., along the direction almost 90 deg. to both the magnetosheath and geomagnetic fields, which lowers the threshold of the KHI. The magnetic configuration across the boundary layer near the northern duskward cusp region during dawnward IMF is similar to that in the low-latitude boundary layer under northward IMF, in that (1) both magnetosheath and magnetospheric fields across the local boundary layer constitute the lowest magnetic shear and (2) the tailward propagation of the KHW is perpendicular to both fields. Approximately 3-hour-long periods of the KHW during dawnward IMF are followed by the rapid expansion of the dayside magnetosphere associated with the passage of an IMF discontinuity that characterizes an abrupt change in IMF cone angle, Phi = acos (B(sub x) / absolute value of Beta), from approx. 90 to approx. 10. Cluster, which was on its outbound trajectory, continued observing the boundary waves at the northern evening-side magnetopause during sunward IMF conditions following the passage of the IMF discontinuity. By comparing the signatures of boundary fluctuations before and after the IMF discontinuity, we report that the frequencies of the most unstable KH modes increased after the discontinuity passed. This result demonstrates that differences in IMF orientations (especially in f) are associated with the properties of KHW at the high-latitude magnetopause due to variations in thickness of the boundary layer, and/or width of the KH-unstable band on the surface of the dayside magnetopause.

Cosmic ray modulation by different types of solar wind disturbances

Abstract: Context. Solar wind disturbances such as interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) cause short-term cosmic ray depressions, generally denoted as Forbush decreases. Aims. We conduct a systematic statistical study of various aspects of Forbush decreases. The analysis provides empirical background for physical interpretations of short-term cosmic ray modulations. Methods. Firstly, we analyzed the effects of different types of solar wind disturbances, and secondly, we focused on the phenomenon of over-recovery (the return of the cosmic ray count to a value higher than the pre-decrease level). The analysis is based on groundbased neutron monitor data and the solar wind data recorded by the Advanced Composition Explorer. The correlations between various cosmic ray depressions and solar wind parameters as well as their statistical significance are analyzed in detail. In addition, we performed a normalized superposed epoch analysis for depressions and magnetic field enhancements. Results. The analysis revealed differences in the relationship between different solar wind disturbances and cosmic ray depression parameters. The amplitude of the depression for ICMEs was found to correlate well with the amplitudes of magnetic field strength and fluctuations, whereas for CIRs we found only the correlation between the amplitude of the depression and the solar wind disturbance dimension proxy vtB. Similar behavior was found for shock and no-shock events, respectively. The CIR/ICME composites show a specific behavior that is a mixture of both ICMEs and CIRs. For all analyzed categories we found that the duration of the depression correlates with the duration of the solar wind disturbance. The analysis of the over-recovery showed that there is no straightforward relationship to either “branching-effect” or geomagnetic effects, therefore we propose a scenario where the “branching-effect” is caused by several factors and is only indirectly related to the over-recovery.

Sources of low-latitude ionospheric E × B drifts and their variability

Abstract: [1] The complete mechanism of how upward propagating tropospheric tides connect to the upper atmosphere is not yet fully understood. One proposed mechanism is via ionospheric wind dynamo. However, other sources can potentially alter the vertical E × B drift: gravity and plasma pressure gradient driven current, the geomagnetic main field, and longitudinal variation in the conductivities. In this study we examine the contribution to the vertical drift from these sources, and compare them. We use March equinox results from the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model. We found that the gravity and plasma pressure gradient driven current and the longitudinal variation of the conductivities excluding the variation due to the geomagnetic main field do not change the longitudinal variation of the vertical drift significantly. Modifying the geomagnetic main field will change the vertical drift at 5–6 LT, 18–19 LT and 23–24 LT at almost all longitudes. In general the influence of the geomagnetic main field on the vertical drift is larger, with respect to the maximum difference, at 18–19 LT and 23–24 LT, equal at 5–6 LT, and smaller at 14–15 LT than the influence due to nonmigrating tidal components in the neutral winds. Examination of the contribution from E- and F-region neutral winds to the vertical drift shows that their importance depends on the local time and the solar activity. This implies that the vertical drift has to be analyzed at specific local times to examine the relation between the wave number in the vertical drift and in the neutral winds.

Diversion of plasma due to high pressure in the inner magnetosphere during steady magnetospheric convection

Abstract: [1] Steady magnetospheric convection (SMC) events in the Earth's magnetosphere are thought to result from balancing the rate of opening flux through solar wind-magnetosphere reconnection at the dayside magnetopause to the rate of closing flux through reconnection in the magnetotail. For this to occur, reconnected flux in the tail must return to the dayside to balance the dayside reconnection rate. Using Geotail and THEMIS data over a span of 14 years, we examine the average plasma conditions and fast Earthward flows during SMC intervals and compare them to other types of geomagnetic activity, such as quiet intervals, isolated substorm phases, and the two hours before an SMC (Pre-SMC intervals). We show that the average total pressure in the inner magnetosphere is higher during SMC events than for other types of activity. This higher pressure region extends to larger radial distances, and causes fast Earthward flows to divert toward the dawn or dusk flanks and continue to the dayside. This pattern is contrasted to substorms, during which flows are directed toward the inner magnetosphere and flux remains there in the “pile-up region.” We suggest that the SMC pattern of flow deflection carries enough flux from the tail to the dayside to allow for balanced reconnection. Finally, the Pre-SMC intervals have plasma conditions that are similar to, but slightly weaker than, SMC events. Since most SMCs begin with a substorm, this indicates that preconditioning of the magnetosphere by prior geomagnetic activity is important in setting up the magnetotail for an SMC state.

Wavelet Analysis on Solar Wind Parameters and Geomagnetic Indices

Abstract: The geomagnetic activity is the result of the solar wind–magnetosphere interaction. It varies following the basic 11-year solar cycle; yet shorter time-scale variations appear intermittently. We study the quasi-periodic behavior of the characteristics of solar wind (speed, temperature, pressure, density) and the interplanetary magnetic field (B x , B y , B z , β, Alfven Mach number) and the variations of the geomagnetic activity indices (D ST, AE, A p and K p). In the analysis of the corresponding 14 time series, which span four solar cycles (1966 – 2010), we use both a wavelet expansion and the Lomb/Scargle periodograms. Our results verify intermittent periodicities in our time-series data, which correspond to already known solar activity variations on timescales shorter than the sunspot cycle; some of these are shared between the solar wind parameters and geomagnetic indices.

Global 3‐D imaging of mantle conductivity based on inversion of observatory C‐responses—II. Data analysis and results

Abstract: SUMMARY The global 3-D electrical conductivity distribution in the mantle (in the depth range between 400 and 1600 km) is imaged by inverting C-responses estimated on a global net of geomagnetic observatories. Very long time-series (up to 51 years; 1957–2007) of hourly means of three components of the geomagnetic field from 281 geomagnetic observatories are collected and analysed. Special attention is given to data processing in order to obtain unbiased C-responses with trustworthy estimates of experimental errors in the period range from 2.9 to 104.2 d. After careful inspection of the obtained C-responses the data from 119 observatories are chosen for the further analysis. Squared coherency is used as a main quality indicator to detect (and then to exclude from consideration) observatories with a large noise-to-signal ratio. During this analysis we found that—along with the C-responses from high-latitude observatories (geomagnetic latitudes higher than 58°)—the C-responses from all low-latitude observatories (geomagnetic latitudes below 11°) also have very low squared coherencies, and thus cannot be used for global induction studies. We found that the C-responses from the selected 119 mid-latitude observatories show a huge variability both in real and imaginary parts, and we investigated to what extent the ocean effect can explain such a scatter. By performing the systematic model calculations we conclude that: (1) the variability due to the ocean effect is substantial, especially at shorter periods, and it is seen for periods up to 40 d or so; (2) the imaginary part of the C-responses is to a larger extent influenced by the oceans; (3) two types of anomalous C-response behaviour associated with the ocean effect can be distinguished; (4) to accurately reproduce the ocean effect a lateral resolution of 1°× 1° of the conductance distribution is needed, and (5) the ocean effect alone does not explain the whole variability of the observed C-responses. We also detected that part of the variability in the real part of the C-responses is due to the auroral effect. In addition we discovered that the auroral effect in the C-responses reveals strong longitudinal variability, at least in the Northern Hemisphere. Europe appears to be the region with smallest degree of distortion compared with North America and northern Asia. We found that the imaginary part of the C-responses is weakly affected by the auroral source, thus confirming the fact that in the considered period range the electromagnetic (EM) induction from the auroral electrojet is small. Assuming weak dependence of the auroral signals on the Earth’s conductivity at considered periods, and longitudinal variability of the auroral effect, we developed a scheme to correct the experimental C-responses for this effect. With these developments and findings in mind we performed a number of regularized 3-D inversions of our experimental data in order to detect robust features in the recovered 3-D conductivity images. Although differing in details, all our 3-D inversions reveal a substantial level of lateral heterogeneity in the mantle at the depths between 410 and 1600 km. Conductivity values vary laterally by more than one order of magnitude between resistive and conductive regions. The maximum lateral variations of the conductivity have been detected in the layer at depths between 670 and 900 km. By comparing our global 3-D results with the results of independent global and semi-global 3-D conductivity studies, we conclude that 3-D conductivity mantle models produced so far are preliminary as different groups obtain disparate results, thus complicating quantitative comparison with seismic tomography or/and geodynamic models. In spite of this, our 3-D EM study and most other 3-D EM studies reveal at least two robust features: reduced conductivity beneath southern Europe and northern Africa, and enhanced conductivity in northeastern China.

Where on earth can animals use a geomagnetic bi-coordinate map for navigation?

Abstract: Many animal taxa have been shown to possess the ability of true navigation. In this study we investigated the possibilities for geomagnetic bi-coordinate map navigation in different regions of the earth by analysing angular differences between isolines of geomagnetic total intensity and inclination. In no-grid zones where isolines were running almost parallel, efficient geomagnetic bi-coordinate navigation would probably not be feasible. These zones formed four distinct areas with a north-south extension in the northern hemisphere, whereas the pattern in the southern hemisphere was more diffuse. On each side of these zones there was often a mirror effect where identical combinations of the geomagnetic parameters appeared. This may potentially cause problems for species migrating long distances east-west across longitudes, since they may pass areas with identical geomagnetic coordinates. Migration routes assumed for four populations of migratory passerine birds were used to illustrate the possibilities of geomagnetic bi-coordinate map navigation along different routes. We conclude that it is unlikely that animal navigation is universally based on a geomagnetic bi-coordinate map mechanism only, and we predict that the relative importance of geomagnetic coordinate information differs between animals, areas and routes, depending on the different conditions for bi-coordinate geomagnetic navigation in different regions of the earth. (Less)

Morphological evolution of a three-dimensional coronal mass ejection cloud reconstructed from three viewpoints

Abstract: The propagation properties of coronal mass ejections (CMEs) are crucial to predict its geomagnetic effect. A newly developed three-dimensional (3D) mask fitting reconstruction method using coronagraph images from three viewpoints has been described and applied to the CME ejected on 2010 August 7. The CME's 3D localization, real shape, and morphological evolution are presented. Due to its interaction with the ambient solar wind, the morphology of this CME changed significantly in the early phase of evolution. Two hours after its initiation, it was expanding almost self-similarly. The CME's 3D localization is quite helpful to link remote sensing observations to in situ measurements. The investigated CME was propagating to Venus with its flank just touching STEREO B. Its corresponding interplanetary CME in the interplanetary space shows a possible signature of a magnetic cloud with a preceding shock in Venus Express (VEX) observations, while from STEREO B only a shock is observed. We have calculated three principal axes for the reconstructed 3D CME cloud. The orientation of the major axis is, in general, consistent with the orientation of a filament (polarity inversion line) observed by SDO/AIA and SDO/HMI. The flux rope axis derived by the Minimal Variance Analysis from VEX indicates a radial-directed axis orientation. It might be that locally only the leg of the flux rope passed through VEX. The height and speed profiles from the Sun to Venus are obtained. We find that the CME speed possibly had been adjusted to the speed of the ambient solar wind flow after leaving the COR2 field of view and before arriving at Venus. A southward deflection of the CME from the source region is found from the trajectory of the CME geometric center. We attribute it to the influence of the coronal hole where the fast solar wind emanated from.

Statistical analysis of phase space density buildups and dropouts

Abstract: [1] The dynamics of the radiation belt Phase Space Density (PSD) is analyzed using measurements from four spacecraft taken during two hundred days in 1990 and 1991. In situ measurements from CRRES, Akebono, GPS, and GEO and a realistic model of the magnetic field are used to infer values of PSD. The inferred values of PSD are assimilated into a radial diffusion model by means of Kalman filtering to produce a reanalysis of the relativistic electron PSD during this time interval. The statistical analysis shows that the plasmapause location is well correlated with the location of the peak of PSD. Positive innovation outside of the plasmasphere shows that local acceleration is present in the trough region. The peak of PSD and the local source, as inferred from the innovation, are displaced inward during times of increased geomagnetic activity. Analysis of non-adiabatic dropouts in PSD shows that the dropouts often coincide with sudden increases in the solar wind dynamic pressure. Approximately 73% of the dropouts can be associated with the simultaneous sudden jumps (>7 nPa over several hours) in the solar wind dynamic pressure, approximately 15% could be associated with small jumps or gradual increases in solar wind dynamic pressure, and the remaining 12%, which consists of only 3 events, occurred during relatively steady solar wind dynamic pressure.