Oceanic electric currents induced by fluid convection

In addition to the electric potentials induced by the gyral motions in the oceans, horizontal electric fields and currents result from the exchange of water between the depths and the surface in the presence of the main geomagnetic field. In this note a simple model representing such a circulation is considered, and the spatial distribution of the corresponding induced electric fields is calculated. A surface velocity of 2 knots could induce electric currents up to 10^?4 Amp/m². These steady currents in the ocean could be comparable with the normal oceanic daily variation. Since a proportion of the electric current would return through the earth below the sea floor, this calculation provides an upper limit to this component. An indication is also given of the magnetic field distortion and associated electric currents which occur in a highly conducting (Jovian) ocean.     

Physical models of coupling in the lithosphere-atmosphere-ionosphere system before earthquakes

The most important models of coupling in the lithosphere-atmosphere-ionosphere system are considered. In some of these models, it is assumed that atmospheric acoustic and acoustic gravity waves (AGWs), which propagate through the atmosphere and reach ionospheric altitudes (resulting in the generation of electric field disturbances and modulation of charged particle density), are generated in the near-Earth atmosphere over the earthquake preparation region. In other models it is assumed that ionospheric disturbances originate owing to the modification of electric fields and currents due to electric processes in the lithosphere or near-Earth atmosphere. It seems impossible to stress on only one model and reject the remaining models because the characteristic spatial scales of effects observed in the ionosphere before earthquakes vary from 200–300 km to several thousand kilometers, and the characteristic times vary from several minutes to several days. We can assume that there are several physical mechanisms by which the lithosphere-ionosphere coupling is actually implemented.     

Formation of density cavities with a nonstationary electric field in the zone of auroral field-aligned currents

The formation of small-scale density cavities with a nonstationary electric field, which are registered in the Earth’s auroral magnetosphere, has been analyzed. It has been indicated that cavities are probably initially caused by quasi-static field-aligned electric currents and currents of kinetic Alfvén waves exceeding threshold values. Urgent variants of the linear and nonlinear stages of density disturbance instabilities have been considered. The determined properties of the parameters of small-scale density cavities and nonstationary electric fields are in agreement with the known experimental data.     

The dynamics of solar activity and anomalous weather of summer 2010: 2. Relationship with the active longitude zone; effects in the west and east

We confirm the close synoptic relationship of the sectoral structure of the Sun’s magnetic field of the with the near-Earth tropospheric pressure with a case study of three European points (Troitsk, Rome, Jungfrau) in the period of the anomalously hot summer of June–August 2010. We substantiate the position that such a relationship was fostered by the anomalously low solar activity as a result of superposition of the minima of the 22- and 180-year cycles. Sectoral analysis of the solar-tropospheric relationships has shown that the appearance of a blocking anticyclone in the Moscow suburbs, its expansion to Rome and Jungfrau, and subsequent retreat at first from these points, and then from the Moscow suburbs was closely related to solar activity phenomena producing, according to contemporary notions, cyclonic activity, shown by simulation of the Earth’s electric field.     

Calculation of atmospheric electric fields penetrating from the ionosphere

The spatial distributions of electric fields and currents in the Earth’s atmosphere are calculated. Electric potential distributions typical of substorms and quiet geomagnetic conditions are specified in the ionosphere. The Earth is treated as a perfect conductor. The atmosphere is considered as a spherical layer with a given height dependence of electrical conductivity. With the chosen conductivity model and an ionospheric potential of 300 kV with respect to the Earth, the electric field near the ground is vertical and reaches 110 Vm⁻¹. With the 60-kV potential difference in the polar cap of the ionosphere, the electric field disturbances with a vertical component of up to 13 V m⁻¹ can occur in the atmosphere. These disturbances are maximal near the ground. If the horizontal scales of field nonuniformity are over 100 km, the vertical component of the electric field near the ground can be calculated with the one-dimensional model. The field and current distributions in the upper atmosphere can be obtained only from the three-dimensional model. The numerical method for solving electrical conductivity problems makes it possible to take into account conductivity inhomogeneities and the ground relief.     

Harmonic sources of the main geomagnetic field in the Earth’s core

The large-scale harmonic magnetic-convective sources of the main geomagnetic field in the Earth’s core have been determined for the first time. The determination is based on a complete system of eigenfunctions of the magnetic diffusion equation in a homogeneously conducting sphere, which is surrounded by an insulator. The sources of the main geomagnetic field observed, which is responsible for the distribution of the electric currents generating this field in the core, are expressed in terms of large-scale eigenfunctions. In this case, the dipole sources are directly related to the observed geomagnetic dipole, whereas the quadrupole sources are related to the quadrupole, etc. The time variations in the obtained sources are responsible for individual spatiotemporal features in the generation or suppression of each Gaussian component of the observed geomagnetic field. When the commonly accepted observational international geomagnetic reference field (IGRF) models were used to partially reveal these time variations, it became possible to specify the estimate of the Earth’s core conductivity and determine the minimum period that can separate us from the commencement of further inversion or excursion.     

Morning polar substorms and variations in the atmospheric electric field

The effects of morning magnetospheric substorms in the variations in near-Earth atmospheric electricity according to the observations of the electric field vertical component (E _z ), at Hornsund polar observatory (Spitsbergen). The E _z, data, obtained under the conditions of fair weather (i.e., in the absence of a strong wind, precipitation, and fog), are analyzed. An analysis of the observations indicated that the development of a magnetospheric substorm in the Earth’s morning sector is as a rule accompanied by positive deviations in E _z, independently of the Hornsund location: in the polar cap or at its boundary. In all considered events, Hornsund was located near the center of the morning convection vortex. In the evening sector, when Hornsund fell in the region of evening convection vortex, the development of a geomagnetic substorm was accompanied by negative deviations in E _z., It has been concluded that the variations in the atmospheric electric field E _z), at polar latitudes, observed during the development of magnetospheric substorms, result from the penetration of electric fields of polar ionospheric convection (which are intensified during a substorm) to the Earth’s surface.     

On the occurrence and characteristics of intense low-altitude electric fields observed by Freja

High-resolution measurements by the double probe electric field instrument on the Freja satellite are presented. The observations show that extremely intense (up to 1 V m⁻¹) and fine-structured (<1 km) electric fields exist at auroral latitudes within the altitude regime explored by Freja (up to 1700 km). The intense field events typically occur within the early morning sector of the auroral oval (01-07 MLT) during times of geomagnetic activity. In contrast to the observations within the auroral acceleration region characterized by intense converging electric fields associated with electron precipitation, upward ion beams and upward field-aligned currents, the intense electric fields observed by Freja are often found to be diverging and located within regions of downward field-aligned currents outside the electron aurora. Moreover, the intense fields are observed in conjunction with precipitating and transversely energized ions of energies 0.5-1 keV and may play an important role in the ion heating. The observations suggest that the intense electric field events are associated with small-scale low-conductivity ionospheric regions void of auroral emissions such as east-west aligned dark filaments or vortex streets of black auroral curls located between or adjacent to auroral arcs within the morningside diffuse auroral region. We suggest that these intense fields also exist at ionospheric altitudes although no such observations have yet been made. This is possible since the height-integrated conductivity associated with the dark filaments may be as low as 0.1 S or less. In addition, Freja electric field data collected outside the auroral region are discussed with particular emphasis on subauroral electric fields which are observed within the 19–01 MLT sector between the equatorward edge of the auroral oval and the inner edge of the ring current.     

Steric Sea-level Variations Inferred from Combined Topex/Poseidon Altimetry and GRACE Gravimetry

In this study, we propose to estimate the steric sea-level variations over a < 2-year period (April 2002 through December 2003) by combining global mean sea level (GMSL) based on Topex/Poseidon (T/P) altimetry with time-variable geoid averaged over the oceans, as observed by the GRACE (Gravity Recovery and Climate Experiment) satellite. In effect, altimetry-derived GMSL changes results from two contributions: Steric (thermal plus salinity) effects due to sea water density change and ocean mass change due to water exchange with atmosphere and continents. On the other hand, GRACE data over the oceans provide the ocean mass change component only. The paper first discusses the corrections to apply to the GRACE data. Then the steric contribution to the GMSL is estimated using GRACE and T/P data. Comparison with available thermal expansion based on in situ hydrographic data is performed. G. García: On leave from Space Geodesy Laboratory, Applied Mathematics Department, EPS, University of Alicante, Alicante, Spain.     

Electrical conductivity anomalies in the earth

Anomalies of short-period geomagnetic variations have been found in various regions over the world. It is known that such anomalies arise from electromagnetic induction within an electrical conductivity anomaly or from local perturbation of induced electric currents by a conductivity anomaly. In order to investigate a regional electric state in the Earth, conductivity anomaly (CA) studies based on anomalous behaviors of geomagnetic variations have been extensively undertaken, as well as studies based on magnetotelluries in which induced currents are directly used.Some of the geomagnetic variation anomalies, however, turned out to be caused by surface conductors, such as sea water and sediments. Anomalies of this sort have been intensively studied and classified into coast, island, peninsula, and strait effects in the case of sea effects. Three-dimensional conduction or channelling of induced electric currents is sometimes observed in the cases of sediments and some crustal conductivity anomalies. However, anomalies of such surface origins often provide some information of the underground conductivity structure.Electrical conductivity anomalies can be classified into two types: anomalies originating in the crust and in the upper mantle. Many of crustal anomalies are well correlated with metamorphic belts, fracture zones, and hydrated layers, and magnetic and gravity anomalies are also often found over the conductivity anomalies. Most of mantle anomalies have been interpreted mainly in terms of high temperature and partial melting, since conductivity anomalies coincide well with anomalies in heat flow and seismic wave velocities.     

A model for the coast-effect

A model for the coast-effect of geomagnetism is presented, in which the horizontal magnetic field induces currents in a circuit including a thin finite ocean. The currents flow horizontally across the ocean, vertically down into the earth, back through the deep interior of the earth, and vertically up to the ocean to complete the current loop. The upper layers of the earth are given non-zero conductivity, allowing the possibility of such current loops.A two-dimensional model involving such currents has been worked out analytically, and it is found that a significant induced magnetic field at the seafloor can be obtained with a reasonable conductivity in the earth's upper layers. A three-dimensional model has also been worked out numerically. It is found that the induced vertical component of magnetic field is of comparable magnitude to the horizontal component induced normal to the coast, whereas the horizontal component parallel to the coast is small. These relations are required to explain the observation of Parkinson arrows.     

Analysis and interpretation of anomalous conductivity and magnetic permeability effects in time domain electromagnetic data: Part I: Numerical modeling

Time domain electromagnetic (TDEM) response is usually associated with eddy currents in conductive bodies, since this is the dominant effect. However, other effects, such as displacement currents from dielectric processes and magnetic fields associated with rock magnetization, can contribute to TDEM response. In this paper we analyze the effect of magnetization on TDEM data. We use a 3-D code based on finite-difference method, developed by Wang and Hohmann [Geophysics 58 (1993) 797], to study transient electromagnetic field propagation through a medium containing bodies with both anomalous conductivity and anomalous magnetic permeability. The remarkable result is that the combination of anomalous conductivity and permeability within the same body could increase significantly the anomalous TDEM response in comparison with purely conductive or purely magnetic anomalies. This effect has to be taken into account in interpretation of TDEM data over electrical inhomogeneous structures with potentially anomalous magnetic permeability.     

Influence of a convective generator on the diurnal behavior of the electric field strength in the near-Earth atmosphere in Kamchatka

Diurnal variations in the electric field strength, electrical conductivity, and temperature in the near-Earth atmosphere under “fair-weather” conditions at the Paratunka observatory (Kamchatka) are considered. It is shown that the morning maximum in the electric field diurnal behavior is caused by air convection in the near-surface layer. The difference in the atmospheric temperatures near the Earth’s surface and at an altitude of 25 m is chosen as a measure of the convective air flow. A high correlation of the values of the temperature difference for these altitudes with the diurnal behavior of the electric field strength is obtained.     

Investigation of hot Flow Anomaly structure observed near the Earth’s bow shock

The work is dedicated to investigation of Hot Flow Anomaly (HFA), formed at the front of Earth’s bows hock. Using Interball-Tail data we estimated orientation of the current sheet that was a cause of the anomaly. From the ion energy-time spectrogram we divided the anomaly into several regions. The motional electric fields near the HFA were estimated with 3D model of Earth’s bow shock. In accordance with previous investigations of HFA’s formation conditions these fields were directed towards the current sheet on both sides of it. We also provided the picture of HFA’s motion along the bow shock and calculated its speed. Analyzing ions’ bulk velocities within the HFA we found that the anomaly is expanding. This conclusion was supported by estimation of thermal and magnetic pressure balance. Ion energy-time spectrogram shows that anomaly is a complicated structure consisting of two parts—leading and trailing. Comparison of ion velocity distributions, magnetic field data and ion energy-time spectrogram provides better understanding of the phenomena and indicated the region that is the source of thermal and convective energy inside HFA.     

On the Generation of ULF Magnetic Variations by Conductivity Fluctuations in a Fault Zone

v--vPrior to the October 18, 1989 Loma Prieta M_s 7.1 earthquake, Fraser-Smith et al. (1990) recorded a 10-100 fold increase in ultra-low frequency (ULF) magnetic fields near the earthquake epicenter. Several mechanisms for generation of these ULF fields by fluid flow in the earth have been advanced, but all appear to require unrealistic fluid velocities or hydraulic permeabilities to match the observations. As an alternative explanation, Merzer and Klemperer (1998) proposed that the increase in ULF magnetic fields could result from induced electric currents flowing in a fault-zone made temporarily much more electrically conductive by stress-induced reorganization of pore geometry. Using a numerical model we show that while this mechanism could produce a significant increase in ULF variations, mutual induction between the fault zone and the surrounding crust would probably limit the amplitude increase to levels well below those observed at Loma Prieta. We consider a variant on this quasi-static conductive fault zone model in which low frequency telluric currents are modulated by small higher frequency variations of bulk fault zone conductivity. We show that because the spectrum of natural EM variations is red, substantially larger relative increases in ULF magnetic fields could be produced by this mechanism with even small conductivity fluctuations at these frequencies. These variations would be easy to detect with a well-designed experiment, if they occurred. In principle this mechanism could explain the Loma Prieta ULF observations, however the magnitude of conductivity fluctuations that would be required to match the very large reported amplification factors still appears to be too large to be physically plausible.     

Verification of magnetic field models based on measurements of solar cosmic ray protons in the magnetosphere

Measurements of solar cosmic ray (SCR) protons in the magnetosphere can be used to verify models of the Earth’s magnetic field. The latitudinal profiles of precipitating SCRs with energies of 1–90 MeV were measured on the CORONAS-F low-orbiting satellite during a strong magnetic storm on October 29–30, 2003. A flux of precipitating protons can remain equal to the interplanetary flux only due to a strong pitch angle diffusion that originates when the radius of the field line curvature is close to that of the particle rotation Larmor radius. The observed boundaries of the strong diffusion region can be compared with the boundaries anticipated according to the models of the magnetic field of the Earth’s magnetosphere. The adiabaticity parameter values, calculated for several instants of the CORONAS-F satellite pass based on the TS05 and parabolic models, do not always correspond to measurements. How possible changes in the model configurations of the magnetic field can allow us to eliminate discrepancies with the experiment and to explain why solar protons with energies of several megaelectronvolts penetrate deep in the Earth’s inner magnetosphere is considered here.     

Global surface currents: a high-resolution product for investigating ocean dynamics

A global 1/4° resolution product of surface currents has been developed by the Centre de Topographie des Océans et de l’Hydrosphère. The surface current is calculated from a combination of Ekman currents derived from wind estimates from QuikSCAT satellite, geostrophic current anomalies derived from altimetry, and a mean geostrophic current derived from climatology. In the equatorial band, the currents are adjusted following the methodology proposed by Lagerloef et al. (J Geophys Res, 104(C10):22313–22326, 1999). These satellite-derived currents have been compared to different types of in situ current observations. A global validation is performed using Lagrangian surface drifting buoys and acoustic Doppler current profiler current observations along ship tracks. The comparison shows a very good agreement in the subtropical and mid-latitude bands. The correlation between the satellite-derived currents and the drifter currents in zonal mean bands is around 0.7 for most of the world oceans, both for the zonal and the meridional components. This correlation rises up to 0.8 in the regions of strong boundary currents. In the equatorial band, the correlation with the surface drifting buoys is reduced. A direct comparison with the TOGA/TAO moored current meter data at the equator shows that the low frequency currents are captured by the satellite current product, but there is a substantial high-frequency signal (<20 days), which is not reproduced. This is especially the case for the meridional component and is mainly related to the tropical instability waves. We also show that using daily QuikSCAT wind forcing improves the satellite current product, particularly in the high-latitude westerly wind belt and in the tropical Indian Ocean.     

Transport of thermal plasma above the auroral ionosphere in the presence of electrostatic ion-cyclotron turbulence

The electron component of intensive electric currents flowing along the geomagnetic field lines excites turbulence in the thermal magnetospheric plasma. The protons are then scattered by the excited electromagnetic waves, and as a result the plasma is stable. As the electron and ion temperatures of the background plasma are approximately equal each other, here electrostatic ion-cyclotron (EIC) turbulence is considered. In the nonisothermal plasma the ion-acoustic turbulence may occur additionally. The anomalous resistivity of the plasma causes large-scale differences of the electrostatic potential along the magnetic field lines. The presence of these differences provides heating and acceleration of the thermal and energetic auroral plasma. The investigation of the energy and momentum balance of the plasma and waves in the turbulent region is performed numerically, taking the magnetospheric convection and thermal conductivity of the plasma into account. As shown for the quasi-steady state, EIC turbulence may provide differences of the electric potential of δ V ≈ 1–10 kV at altitudes of 500 < h < 10 000 km above the Earth’s surface. In the turbulent region, the temperatures of the electrons and protons increase only a few times in comparison with the background values.     

Equatorial dayside minimum of the neutral gas density and temperature: Formation mechanism

The causes of the formation of neutral gas temperature and density equatorial minimums on the dayside, recently detected from satellite measurements, have been studied based on a global numerical model of the Earth’s upper atmosphere (UAM). The performed numerical experiments made it possible to conclude that these minimums are not related to the magnetospheric sources of energy and momentum and electric fields of the dynamo origin. It has been indicated that the absorbed solar ionizing radiation and rotation of the Earth are responsible for the formation of the neutral gas temperature and density equatorial minimums on the dayside.