Dynamo Currents Research Articles

Direct observation of the F region dynamo currents and the spatial structure of the EEJ by CHAMP

The satellite CHAMP in its low‐altitude polar orbit, and with its high‐resolution magnetic field measurements, provides the opportunity to investigate ionospheric currents in great detail. In this study we focus on average features of equatorial current systems during day time. Considering quiet time observations from the period Aug. 2000 to Oct. 2004 we determined the mean spatial structure of the EEJ current. The existence of reverse currents at the flanks of the primary eastward current is confirmed. More importantly, average characteristics of the F region dynamo driven currents are observed for the first time throughout the daytime. The meridional current system, driven by westward thermospheric winds, peaks around noon. Similar meridional currents, but flowing in the opposite sense, are observed at the dusk sector. At CHAMP altitudes we find over the dip equator in the F region a vertical sheet current of about 6 mA/m oriented downward at noon and an upward current of 4 mA/m around sunset.

Magnetic signature of the ionospheric disturbance dynamo at equatorial latitudes: “Ddyn”

During magnetic storms, wind disturbances produced by auroral phenomena can affect the whole thermospheric circulation and associated ionospheric dynamo currents for many hours after the end of the storms. In this paper we define criteria to select a new simple type of ionospheric disturbance dynamo events that allow a simple interpretation over all longitude sectors. These events exhibit a weak auroral activity during at least 24 UT hours, on the day after the storm. We analyze the magnetic disturbances “Ddyn” observed at equatorial latitudes in the three longitude sectors of such selected events. It is found for all the cases that the amplitude of the H component of the Earth's magnetic field is reduced, on the day after storm at equatorial latitudes, in agreement with the ionospheric disturbance dynamo model (Blanc and Richmond, 1980). The observation of H component decrease on the day after storm is longitudinally asymmetric. The observed signature of the ionospheric disturbance dynamo process in a specific longitude sector is strongly dependent on the magnitude, the start time, and the duration of the storm.

Profile-turbulence interactions, magnetohydrodynamic relaxations, and transport in tokamaks

The dynamical behavior of the global, two-fluid, electromagnetic model of a tokamak plasma is explored under conditions corresponding to the Rijnhuizen tokamak project [A. J. H. Donné, Plasma Phys. Rep. 20, 192 (1994)] using the CUTIE code [A. Thyagaraja, Plasma Phys. Controlled Fusion 42, B255 (2000)]. Simulations of an off-axis electron-cyclotron-heated (350kW) hydrogen discharge and a purely Ohmic one over several resistive evolution times (τres≃15−20ms) are described. The results illustrate profile-turbulence interactions and the spectral transfer processes implicated in the spontaneous generation and maintenance of mesoscale zonal flows and dynamo currents. Relaxation phenomena, including off- and on-axis sawteeth and periodically repeating edge ballooning instabilities mediated by these mechanisms, are presented. The CUTIE model reproduces many observed features of the experiment qualitatively and suggests that global electromagnetic simulations may play an essential role in understanding tokamak turbulence and transport.

Effects of the induction field on the Sq dynamo currents in the ionosphere

Effects of the induction field on the ionospheric currents generated by the Sq dynamo were studied using current function. It was shown that the induction field significantly affects the ionospheric currents only if the field is artificially enhanced by 100 times. Existence of the spherical inner conductor reduces the effects because currents induced in the sphere partly cancel vertical component of the variation field. On the other hand, it is probable that the induction field should be considered for the simulation of the ionospheric current with the period of less than 10 min. because the induction field affects the ionospheric Sq currents if the field is enhanced by 100 times.

The effect of gravity and pressure in the electrodynamics of the low‐latitude ionosphere

The source of the plasma drift structure in the low‐latitude ionosphere during magnetically quiet times is generally understood. Nearly all the quiet time electric field structure has been attributed to divergences in the neutral wind current dynamo. However, there are other current drivers active in the low latitude ionosphere: a gravity‐driven current (g × B drift of O+) and a gradient‐pressure current (∇P × B drift of e and O+). The gravity‐driven current is important in the development of the Rayleigh‐Taylor instability and equatorial spread F (ESF) but is ignored in global dynamo electric field models. The gradient pressure and gravity terms in the ionosphere momentum equation normally oppose each other. This paper examines the magnitude of the large‐scale electric fields generated by divergences in these lesser currents in a coupled ionosphere‐electrodynamics model. The low‐latitude electric fields generated by the neutral wind dynamo are altered only slightly by these additional terms. The resulting vertical electric fields (zonal plasma drifts) do not change significantly but the vertical plasma drifts are affected by ∼10 to 15 m/s in the hours before sunrise and after sunset. These low‐latitude plasma drift differences are due entirely to the inclusion of the gravity‐driven current. The gravity current term produces a downward drift after sunset, which reduces the evening prereversal enhancement. Additionally, the positive vertical drift before sunrise is sometimes large enough to cause a predawn vertical enhancement. The gradient pressure terms can be ignored for large and medium scale ionosphere features.

Measurement of current profile dynamics in the Madison Symmetric Torus

The current profile and core magnetic field fluctuation amplitudes in a reversed-field pinch are measured by using a high-resolution polarimetry–interferometry system. This paper presents data showing a redistribution of the current during a sawtooth crash. Also, the core magnetic field fluctuation amplitude is observed to increase at a sawtooth crash consistent with the idea of nonlinearly driven dynamo current. In addition, the parallel current density increases in the outer region of the plasma during auxiliary pulsed parallel current drive. This was expected, as the external application of an edge parallel electric field is designed to flatten the current profile providing an equilibrium closer to the minimum energy Taylor state. However, the current density also increases in the core, relative to standard plasmas. This increase can be explained by a reduction of the dynamo (anti)current drive in the core that should accompany the measured reduction of magnetic fluctuations and by a drop in resistivity caused by the increased confinement of fast electrons.

Long‐lasting disturbances in the equatorial ionospheric electric field simulated with a coupled magnetosphere‐ionosphere‐thermosphere model

The Magnetosphere‐Thermosphere‐Ionosphere‐Electrodynamics General Circulation model of Peymirat et al. [1998] is used to investigate ionospheric‐wind‐dynamo influences on low‐latitude ionospheric electric fields during and after a magnetic storm. Simulations are performed with time‐varying polar cap electric potentials and an expanding and contracting polar cap boundary. Three influences on equatorial electric fields can be of comparable importance: (1) global winds driven by solar heating; (2) direct penetration of polar cap electric fields to the equator that are partially shielded by the effects of Region‐2 field‐aligned currents; and (3) disturbance winds driven by high‐latitude heating and ion‐drag acceleration. The first two influences tend to have similar magnetic local time (MLT) variations in a steady state, while the disturbance‐wind influence tends to have the opposite MLT variations. The nighttime disturbance winds at upper midlatitudes that affect the global ionospheric wind dynamo are predominantly westward after the simulated magnetic storm. The nighttime winds drive an equatorward dynamo current that tends to charge the low‐latitude ionosphere positively around midnight, which can lead to reductions or reversals of the normal equatorial night‐side east‐west electric fields. The simulations partly support the theories of the so‐called “disturbance dynamo” [Blanc and Richmond, 1980] and “fossil wind” [Spiro et al., 1988], both of which predict long‐lasting disturbances in the equatorial eastward electric field associated with magnetic storms. However, the simulations do not support the element of fossil wind theory that links the disturbance‐wind influence on equatorial electric fields to polar cap contraction following the storm. The simulations show a stronger wind‐produced enhancement of steady state shielding than predicted by the model of Forbes and Harel [1989], due to the fact that the disturbance winds extend well equatorward of the Region‐2 currents.

The correlation between the variation in ionospheric conductivity and that of the geomagnetic Sq field

Effects of the solar activity on the geomagnetic Sq field were studied by examining the correlation of the Sq amplitude in the Y-component with the sunspot number or height integrated Pedersen and Hall conductivities of the ionosphere at Kakioka, Chambon La Foret and Port Moresby for a period of 21 years. It was found that solar activity dependence of the Sq amplitude is almost explained by the effect of the local ionospheric conductivity if the month is fixed. That is, the solar activity dependence in each month is mainly caused by the local conductivity. However, the amplitude is clearly small in winter for the same conductivity value. This is probably due to the seasonal difference of the neutral winds driving ionospheric dynamo currents or to the magnetic effect of the field-aligned currents connecting both hemispheres driven by the asymmetric dynamo action in the ionosphere.

Mars' core and magnetism

The detection of strongly magnetized ancient crust on Mars is one of the most surprising outcomes of recent Mars exploration, and provides important insight about the history and nature of the martian core. The iron-rich core probably formed during the hot accretion of Mars approximately 4.5 billion years ago and subsequently cooled at a rate dictated by the overlying mantle. A core dynamo operated much like Earth's current dynamo, but was probably limited in duration to several hundred million years. The early demise of the dynamo could have arisen through a change in the cooling rate of the mantle, or even a switch in convective style that led to mantle heating. Presently, Mars probably has a liquid, conductive outer core and might have a solid inner core like Earth.

Outstanding problems in the equatorial ionosphere–thermosphere electrodynamics relevant to spread F

Dynamic coupling of the atmospheric regions involving upward energy transport coupled with the locally active thermal tidal modes establish the wind system of the thermosphere whose interaction with the magnetized conducting ionospheric layers produces the dynamo electric fields and currents that control the quiet time electrodynamic processes of the equatorial ionosphere–thermosphere system. The plasma fountain responsible for the ionization anomaly, the vertical and zonal plasma drifts of the post-sunset hours leading to plasma bubble/spread F irregularity generation, and the Hall electric field that drives the electrojet current system are among the most notable manifestations of the these processes. The electrodynamic processes to be discussed in this paper will concern mainly the equatorial spread F (ESF) irregularity generation and their variabilities. The key factors that control the ESF generation by generalized Rayleigh–Taylor instability process, such as the the prereversal enhancement electric field (vertical drift) that is controlled by post-sunset zonal wind and longitudinal conductivity gradients, meridional/transequatorial winds, flux tube integrated conducitvities, and the seed perturbations are discussed in some detail, focussing attention on the aspects of their seasonal, logitudinal and day-to-day variabilities. The role of the evening F layer vertical drift in the instability process leading to ESF has been the most extensively investigated experimentally so far. Recent theoretical and computational results have advanced greatly our understanding of the importance of the key factors in the development process of spread F. However, observational identification of the relative importance of some of the key control factors in a given spread F event or in its day-to-day variablity is still lacking, the main examples being that of the seed perturbation, meridional/transequatorial winds and integrated conductivities. However, some progress on understanding the seasonal pattern has been achieved. Enhanced conductivity below F region and in E layer in the longitude sector of the South Atlantic Magnetic Anomaly seems to influence the equatorial spread F electrodynamics.

Generic nonlinear processes in self–exciting dynamos and the long–term behaviour of the main geomagnetic field, including polarity superchrons

When the main geomagnetic field undergoes a reversal in polarity it does so in no more than a few thousand years. Many hundreds of such reversals must have occurred over geological time (the age of...

Amplification of fluctuations and current dynamo due to helical and chiral effects in geophysical and plasma-like media

Dynamo effect is considered in more general case than well known one when not only mean motion but also conducting components possess nonzero mean helicity. Dispersion equation for helical motions is studied, including inhomogeneous case. Criteria for development of instability are found. Interaction between large-scale internal wave and small-scale helical turbulence in plane Couette flow of fluid with statically stable uniform density gradient in gravitational field is studied basing on a set of equations for scalar and quasi-scalar wave fields and for helicity of turbulence. This system of equation describes acceleration of wave growth due to its backward action on turbulence, i.e. wave-turbulent instability. Chiral media are further considered and effects topologically close to helical ones are analyzed. It is shown that in such media ranges of scales exist, in which fluctuations of electric field can be anomalously amplified.

A simple model of low‐latitude electric fields

The low‐latitude electric fields are generally modeled by numerically solving a two‐dimensional potential equation developed from assumptions of equipotential field lines and current continuity. Additional assumptions are used to derive simple formulas for the vertical electric field EL and zonal electric field Eϕ in the plane of the magnetic dip equator. Electric fields obtained from the simple model will be compared against the electric fields of the more complete numerical model to demonstrate the validity of the simple model solution and to identify the range of validity. The simple electric field model is used to directly identify the primary causes of the low‐latitude electric field structure. The observed structure has sources that can be divided into components: (1) polarization fields created by divergences in neutral‐wind dynamo currents and (2) curl‐free effects demanded by ∇ × E = 0. It is shown that the evening prereversal enhancement of Eϕ is caused by “curl‐freeness” demands of the rapidly changing EL near sunset. This supports cursory statements of Rishbeth [1971] on the cause of the postsunset enhancement. Finally, the resulting altitude variation of the zonal electric field depends on the type of source. For polarization field sources the altitude variation is approximately proportional to L2. For curl‐free sources the altitude variation is related to the rate of change of EL with respect to longitude.

Ionospheric current systems on days of low equatorial ΔH

Examination of the day to day variability in the equatorial electrojet strength, in the Indian region, shows that certain quiet days (Ap ≤ 6) are characterized by an abnormally low magnitude of the jet strength. Geomagnetic field data (H-component) on these abnormal jet days along with the normal jet days from equatorial and low latitude stations are subjected to the statistical methodology of Principal Component Analysis. Results suggest that the contributions to the diurnal variations on such abnormal days could be accounted for by an additional noon-time field. The contributing factors for the observed spatial distribution of the additional fields that are responsible for the changes in low latitude dynamo currents are discussed.

The Dynamo-Driven Magnetic Helicity Transport**The project supported by Nuclear Industry Science Foundation of China

In this paper, a new set of the evolution equations for the helicity of the mean magnetic field and the mean helicity of the fluctuating magnetic field is derived from the Maxwell equations and the generalized Ohm's law with the dynamo action. It is shown that there exist two kinds of the dynamo-driven magnetic helicity transport. One of them makes the mean magnetic field helicity transfer to the fluctuating magnetic field, yielding an anomalous loop voltage. The other makes the fluctuating magnetic field helicity transfer to the mean magnetic field, which provides a convincing evidence for the existence of the dynamo current. Therefore, the two kinds of the magnetic helicity transport describe the mutual conversion between the regular and irregular motions. The formulas of the loop voltage and the dynamo current are given. In particular, we give out the formula of the dynamo current-generated equilibrium magnetic field which provides a concrete mode of the magnetic field creation and maintenance in both astrophysical and laboratory (e.g., reversed-field pinch) plasmas.

Inductive instability in heterogeneous nonstationary systems

In this study we analyze a new type of electric dynamo caused by the rapid change of the distribution of the electric conductivity in heterogeneous conducting systems. It is demonstrated that there exist two types of electric dynamos, namely, the regular magnetic dynamo and the electric current dynamo. The magnetic dynamo is associated with the growth of the total energy of the magnetic field. The electric current dynamo is defined as the growth of the total electric current through some cross section of a conductor, whereby the choice of the cross section is determined by the symmetry of the excited electromagnetic field. We show that the condition for the excitation of the electric current dynamo is less restrictive than the condition for the excitation of the magnetic dynamo, and it can be satisfied even without a hydrodynamic flow. The existence of the hydrodynamic flow is cardinal for the excitation of the magnetic dynamo. In contrast to the turbulent magnetic dynamo which is associated with the fact that magnetic-field lines are ‘‘frozen in’’ to the fluid and thus can be excited at high magnetic Reynolds numbers, the laminar magnetic dynamo which is considered in the present study can be excited at the relatively low magnetic Reynolds numbers Rem>1 depending upon the symmetry of the electromagnetic field. In this study we determined the dependence of the magnetic Reynolds number providing the excitation of the instability upon the symmetry of the electromagnetic field. @S1063-651X~97!03209-1#

Observed dynamical coupling through tidal wave propagation in the mesosphere and lower thermosphere at midlatitudes

Abstract The incoherent scatter radar at Millstone Hill (42.6 °N, 71.5 °W) and the nearby meteor wind radar at Durham have been used in coordinated experiments to study the characteristics of the semidiurnal tide that dominates the dynamics of the upper mesosphere and lower thermosphere at midlatitudes. Observations of horizontal neutral winds during 10–16 August 1994 at Millstone Hill have provided the first opportunity to derive neutral wind data at altitudes as low as 94 km, overlapping the Durham meteor wind observations with good agreement. Comparisons of the results with those obtained during 20–30 January 1993 suggest that the seasonal phase transition of the semidiurnal tide is smaller in the lower thermosphere than in the upper mesosphere. The Millstone Hill meridional wind measurements were also extended to 250 km altitude and reveal that the semidiurnal tide reaches a maximum amplitude of 70 m s −1 in a relatively narrow layer with a peak altitude of 110 km. The dynamo currents induced by the observed winds and electric fields agree with observed magnetic field fluctuations, indicating overall consistency of the observations.

Variation of Sq Induced by Atmospheric Tides Simulated by a Middle Atmosphere General Circulation Model.

Dynamo currents and resulting geomagnetic variations induced by neutral winds simulated by Middle Atmosphere Circulation Model at Kyushu University (MACMKU) are examined. Assuming a steady (having a diurnal cycle but no day-to-day variation) conductivity in the dynamo region, dynamo currents induced by the neutral winds and an additional prescribed steady diurnal (1, -2) mode to supply the in situ tides, those are not included in MACMKU, are calculated for the 10 days of the perpetual October 1st condition. As a result of neutral winds fluctuation superposed on the prescribed regular (1, -2) mode, Sq shows day-to-day and time-to-time variations. The present result suggests that neutral winds fluctuations propagated from below can induce significant day-to-day and time-to-time Sq variations even though the electromagnetic environment in the lower thermosphere is kept in a steady state.

Model for relaxation in reversed-field pinch plasmas

In this article, a model for anomalous ion heating, a dynamo current-sustained edge toroidal field, and a sawtooth oscillation during the relaxation in the reversed-field pinch (RFP) plasma is presented. The dynamo (α), the turbulent resistivity (β) and viscosity (χ), dependent on the magnetohydrodynamics (MHD) fluctuations, are incorporated into the model. Turbulent viscous dissipation of the fluctuation energy is proposed as the mechanism of the anomalous ion heating. This is a straightforward corollary of the turbulent viscosity heating of ions in that the temperature of the heavier ions is higher than that of the lighter ions and that the ion temperature increases with the MHD fluctuation level. Correspondingly, the turbulent resistivity heats electrons anomalously. It is shown that the dynamo current, generated by the back-transfer of fluctuating magnetic field helicity to mean magnetic field, sustains the RFP magnetic configuration. In the edge the total current density is approximately equal to the dynamo current density, while at the core the dynamo current opposes the applied electric-field-driven current, flattening the current profile. Provided the α dynamo has a periodic behavior in time, the physical quantities of the RFP plasma have a sawtooth time dependence. The local poloidal current density in the edge increases during the sawtooth crash and peaks at the end of the crash, as do the ion and electron temperatures. In contrast, the toroidal current density at the core decreases during the crash and arrives at its minimum at the end of the crash. Qualitatively, the conclusions drawn from the present model are in good agreement with many of the experimental results [Scime et al., Phys. Rev. Lett. 68, 2165 (1992); Ji et al., ibid. 73, 668 (1994)] and the numerical simulations.

Dynamo effect and current drive by low-frequency waves

The dynamo effect due to low-frequency oscillations and its application to the nonresonant current drive are examined. The relevant single particle forces are analyzed and the possibility of the dynamo effect enhancement by thermal effects is investigated. In the collisional two-fluid theory with isotropic pressure the flux-surface averaged parallel dynamo field is found to be solely due to frictional forces. In the collisionless, double-adiabatic approximation the previously reported steady-state collisionless α-effect is shown to be absent. The origin of this discrepancy is discussed.