Zonal Magnetic Field Research Articles

Large-scale dynamo model for accretion disks

Context. Magnetic fields in accretion disks play an important role in the rich dynamics of these systems. A dynamo theory describing the generation of these magnetic field is in general very complex and requires many assumptions in order to be of practical use. In this respect, a theory with as few assumptions as possible is desirable. Aims. To investigate the generation of magnetic fields in accretion disks around magnetized central objects, a large-scale dynamo model is employed that includes feedback effects on the mass motion due to the Lorentz force. The dynamo model was developed from the fundamental magnetohydrodynamics equations with a minimum of hypothesis, and was tested in the case of the Sun and other stars. It is applied to accretion disks for the first time. Methods. The magnetic field in the disk, generated by the mentioned dynamo theory, was matched to that of the central object, considered dipolar, and to that of a magnetosphere described with the Grad–Shafranov equation. The relation between axial current and magnetic flux required in the Grad–Shafranov equation was not imposed, but was self-consistently determined along with the full solution. Results. The model is able to reproduce the patterns of magnetic field lines obtained in several works, such as closed magnetic lines near the central object and open lines for larger radii. The maximum value of the field is located near the internal radius of the accretion disk, where the currents in the disk force the concentration of field lines of the central object in the magnetosphere around this region. By varying the values of stellar mass, stellar magnetic field, mass accretion rate, and internal radius of the disk, it is found that the stellar magnetic field is the most important parameter in the determination of the disk magnetic field. The stellar mass is of secondary importance. It affects the azimuthal component of the disk magnetic field. The internal radius of the disk affects the disk zonal magnetic field and is likewise less important.

The Response of Geomagnetic Daily Variation and Ionospheric Currents to the Annular Solar Eclipse on 21 June 2020

AbstractIn this study, we examined the response of geomagnetic daily variation and ionospheric currents to the solar annular eclipse that passed through China at 10:00–17:30 LT on 21 June 2020. We utilized geomagnetic field data obtained from the Geomagnetic Network Center of China, Institute of Geophysics, China Earthquake Administration and International Real‐time Magnetic Observatory Network, which extend from middle latitudes to equatorial regions in China and surrounding areas. It is found that the eclipse obscuration reduces the intensity of geomagnetic daily variation at most stations as compared with the 5‐quiet‐day average. The response of geomagnetic daily variation to the solar eclipse depends on local time and latitude. In most shadowed areas of the eclipse, there is a reduction of northward (southward) component of the meridional magnetic daily field (ΔX) below (above) ∼30° magnetic latitudes. The solar eclipse can also induce a decrease of the zonal magnetic field variation (ΔY). Meanwhile, we observed that the solar eclipse reduces the intensity of vertical geomagnetic component (ΔZ) at the stations around the early afternoon aside the eclipse totality. In general, these geomagnetic responses imply that the solar eclipse can reduce the ionospheric solar quiet (Sq) current system. These ionospheric currents and ground geomagnetic field disturbances during the solar eclipse are also verified by a global physical model. The simulation demonstrates that the ionospheric currents in the southern hemisphere, where the solar eclipse does not cover, also have a weak response. Overall, the ionospheric current disturbance induced by the solar eclipse shows a counter‐Sq pattern.

On developing a new ionospheric plasma index for Brazilian equatorial F region irregularities

Abstract. F region vertical drifts (Vz) are the result of the interaction between ionospheric plasma with the zonal electric field and the Earth's magnetic field. Abrupt variations in Vz are strongly associated with the occurrence of plasma irregularities (spread F) during the nighttime periods. These irregularities are manifestations of space weather in the ionosphere's environment without necessarily requiring a solar burst. In this context, the Brazilian Space Weather Study and Monitoring Program (Embrace) of the National Institute for Space Research (INPE) has been developing different indexes to analyze these ionospheric irregularities in the Brazilian sector. Therefore, the main purpose of this work is to produce a new ionospheric scale based on the analysis of the ionospheric plasma drift velocity, named AV. It is based on the maximum value of Vz (Vzp), which in turn is calculated through its relationship with the virtual height parameter, h′F, measured by the Digisonde Portable Sounder (DPS-4D) installed in São Luís (2∘ S, 44∘ W; dip: −2.3∘). This index quantifies the time relationship between the Vz peak and the irregularity observed in the ionograms. Thus, in this study, we analyzed 7 years of data, between 2009 and 2015, divided by season in order to construct a standardized scale. The results show there is a delay of at least 15 min between the Vzp observation and the irregularity occurrence. Finally, we believe that this proposed index allows for evaluating the impacts of ionospheric phenomena in the space weather environment.

Dynamo action of the zonal winds in Jupiter

The new data delivered by NASA’s Juno spacecraft significantly increase our understanding of the internal dynamics of Jupiter. The gravity data constrain the depth of the zonal flows observed at cloud level and suggest that they slow down considerably at a depth of about 0.96 rJ, where rJ is the mean radius at the one bar level. The magnetometer onboard Juno reveals the internal magnetic field of the planet. We combine the new zonal flow and magnetic field models with an updated electrical conductivity profile to assess the zonal-wind-induced dynamo action, concentrating on the outer part of the molecular hydrogen region of Jupiter where the conductivity increases very rapidly with depth. Dynamo action remains quasi-stationary and can therefore reasonably be estimated where the magnetic Reynolds number remains smaller than one, which is roughly the region above 0.96 rJ. We calculate that the locally induced radial magnetic field reaches rms values of about 10−6 T in this region and may just be detectable by the Juno mission. Very localized dynamo action and a distinct pattern that reflects the zonal wind system increases the chance to disentangle this locally induced field from the background field. The estimates of the locally induced currents also allow calculation of the zonal-flow-related Ohmic heating and associated entropy production. The respective quantities remain below new revised predictions for the total dissipative heating and total entropy production in Jupiter for any of the explored model combinations. Thus, neither Ohmic heating nor entropy production offer additional constraints on the depth of the zonal winds.

Global gyrokinetic simulation of turbulence driven by kinetic ballooning mode

Nonlinear evolution of the kinetic ballooning mode (KBM) is investigated by extending the global toroidal gyrokinetic simulation code (GKNET) to an electromagnetic regime. It is found that the saturation process of KBM, which is unstable at high normalized pressure β, is significantly different from the ion temperature gradient (ITG) mode, which is unstable at low β. The KBMs get saturated by producing zonal flows and zonal magnetic fields. The production of zonal flow is weak in the initial saturation phase of KBM, which is in contrast to the ITG mode which produces strong zonal flows in the initial saturation phase. However, strong zonal flows are produced in the subsequent evolution of KBM, and a quasisteady state of KBM turbulence is established. In addition to the zonal flows, some low toroidal number modes, which are linearly stable against the KBM, dominate the KBM turbulence. The strong zonal magnetic field is also produced by the KBM. These zonal modes regulate the KBM turbulence.

Large-scale Model of the Axisymmetric Dynamo with Feedback Effects

A dynamo model is presented, based on a previously introduced kinematic model, in which the effect of the magnetic field on the mass flow through the Lorentz force is included. Given the base mass flow corresponding to the case with no magnetic field, and assuming that the modification of this flow through the Lorentz force can be treated as a perturbation, a complete model of the large-scale magnetic field dynamics can be obtained. The input needed consists of the large-scale meridional and zonal flows, the small-scale magnetic diffusivity, and a constant parameter entering the expression of the $\alpha $ -effect. When applied to a Sun-like star, the model shows realistic dynamics of the magnetic field, including cycle duration, consistent field amplitudes with the correct parity, progression of the zonal magnetic field toward the equator, and motion toward the poles of the radial field at high latitudes. In addition, the radial and zonal components show a correct phase relation, and at the surface level, the magnetic helicity is predominantly negative in the northern hemisphere and positive in the southern hemisphere.

Suppression of explosive bursts triggered by neo-classical tearing mode in reversed magnetic shear tokamak plasmas via ECCD

The suppression of explosive bursts, triggered by the neo-classical tearing mode, in the reversed magnetic shear configuration tokamak plasmas by electron cyclotron current drive (ECCD) are numerically studied by making use of a reduced magnetohydrodynamic model including both bootstrap current and self-consistently evolving EC driven current. It is found that the ECCD with appropriate input power and switch-on time can effectively stabilize the neo-classical islands. In comparison with the classical one, it is far more difficult to control the neo-classical island due to the strong zonal magnetic field induced during the nonlinear evolution. This strong zonal field can lead to intense fluctuations of the magnetic surfaces once the ECCD is turned on, which can damage the steady deposition of the driven current. To avoid the strong zonal field, the switch-on time should be put forward, which is proved to be effective. By adopting this scheme, the explosive burst triggered by neo-classical current can also be effectively suppressed. Moreover, the steady-state distributions of driven current are displayed. The influence of the radial misalignment of ECCD is discussed in detail. Based on the numerical results in different fractions of bootstrap current, some suggestions are proposed for the tokamak experiments.

The Direct Effect of Toroidal Magnetic Fields on Stellar Oscillations: An Analytical Expression for the General Matrix Element

Abstract Where is the solar dynamo located and what is its modus operandi? These are still open questions in solar physics. Helio- and asteroseismology can help answer them by enabling us to study solar and stellar internal structures through global oscillations. The properties of solar and stellar acoustic modes are changing with the level of magnetic activity. However, until now, the inference on subsurface magnetic fields with seismic measures has been very limited. The aim of this paper is to develop a formalism to calculate the effect of large-scale toroidal magnetic fields on solar and stellar global oscillation eigenfunctions and eigenfrequencies. If the Lorentz force is added to the equilibrium equation of motion, stellar eigenmodes can couple. In quasi-degenerate perturbation theory, this coupling, also known as the direct effect, can be quantified by the general matrix element. We present the analytical expression of the matrix element for a superposition of subsurface zonal toroidal magnetic field configurations. The matrix element is important for forward calculations of perturbed solar and stellar eigenfunctions and frequency perturbations. The results presented here will help to ascertain solar and stellar large-scale subsurface magnetic fields, and their geometric configuration, strength, and change over the course of activity cycles.

Zonal flow magnetic field interaction in the semi-conducting region of giant planets

All four giant planets in the Solar System feature zonal flows on the order of 100 m/s in the cloud deck, and large-scale intrinsic magnetic fields on the order of 1 Gauss near the surface. The vertical structure of the zonal flows remains obscure. The end-member scenarios are shallow flows confined in the radiative atmosphere and deep flows throughout the entire planet. The electrical conductivity increases rapidly yet smoothly as a function of depth inside Jupiter and Saturn. Deep zonal flows will inevitably interact with the magnetic field, at depth with even modest electrical conductivity. Here we investigate the interaction between zonal flows and magnetic fields in the semi-conducting region of giant planets. Employing mean-field electrodynamics, we show that the interaction will generate detectable poloidal magnetic field perturbations spatially correlated with the deep zonal flows. Assuming the peak amplitude of the dynamo α-effect to be 0.1 mm/s, deep zonal flows on the order of 0.1–1 m/s in the semi-conducting region of Jupiter and Saturn would generate poloidal magnetic perturbations on the order of 0.01%–1% of the background dipole field. These poloidal perturbations should be detectable with the in-situ magnetic field measurements from the Juno mission and the Cassini Grand Finale. This implies that magnetic field measurements can be employed to constrain the properties of deep zonal flows in the semi-conducting region of giant planets.

Generation of Zonal Flow and Magnetic Field by Planetary Waves in the Earth’s Ionosphere

Possibility of generation of large-scale sheared zonal flow and magnetic field by coupled under the typical ionospheric conditions short-scale planetary low-frequency waves is shown. Propagation of coupled internal-gravity-Alfven, Rossby-Khantadze, Rossby-Alfven-Khantadze and collision-less electron skin depth order drift-Alfven waves is revealed and investigated in detail. To describe the nonlinear interaction of such coupled waves with sheared zonal flow the corresponding nonlinear equations are deduced. The instability mechanism is based on the nonlinear parametric triple interaction of the finite amplitude short-scale planetary waves leading to the inverse energy cascade toward the longer wavelengths. It is shown that under such interaction intense sheared magnetic fields can be generated. Appropriate growth rates are discussed in detail.

Generation of zonal flow and magnetic field in the ionospheric E-layer

We review the generation of zonal flow and magnetic field by coupled electromagnetic ultra-low-frequency waves in the Earth’s ionospheric E-layer. It is shown that, under typical ionospheric E-layer conditions, different planetary low-frequency waves can couple with each other. Propagation of coupled internal-gravity–Alfvén, coupled Rossby–Khantadze and coupled Rossby–Alfvén–Khantadze waves is revealed and studied. A set of appropriate equations describing the nonlinear interaction of such waves with sheared zonal flow is derived. The conclusion on the instability of short-wavelength turbulence of such coupled waves with respect to the excitation of low-frequency and large-scale perturbation of the sheared zonal flow and sheared magnetic field is deduced. The nonlinear mechanism of the instability is based on the parametric triple interaction of finite-amplitude coupled waves leading to the inverse energy cascade towards longer wavelength. The possibility of generation of an intense mean magnetic field is shown. Obtained growth rates are discussed for each case of the considered coupled waves.

LARGE-SCALE QUASI-GEOSTROPHIC MAGNETOHYDRODYNAMICS

We consider the ideal magnetohydrodynamics (MHD) of a shallow fluid layer on a rapidly rotating planet or star. The presence of a background toroidal magnetic field is assumed, and the "shallow water" beta-plane approximation is used. We derive a single equation for the slow large length scale dynamics. The range of validity of this equation fits the MHD of the lighter fluid at the top of Earth's outer core. The form of this equation is similar to the quasi-geostrophic (Q-G) equation (for usual ocean or atmosphere), but the parameters are essentially different. Our equation also implies the inverse cascade; but contrary to the usual Q-G situation, the energy cascades to smaller length scales, while the enstrophy cascades to the larger scales. We find the Kolmogorov-type spectrum for the inverse cascade. The spectrum indicates the energy accumulation in larger scales. In addition to the energy and enstrophy, the obtained equation possesses an extra (adiabatic-type) invariant. Its presence implies energy accumulation in the 30° sector around zonal direction. With some special energy input, the extra invariant can lead to the accumulation of energy in zonal magnetic field; this happens if the input of the extra invariant is small, while the energy input is considerable.

Zonal flows and magnetic fields driven by large‐amplitude Rossby‐Alfvén‐Khantadze waves in the E‐layer ionosphere

AbstractPlanetary‐scale waves and vortices in the weakly ionized ionospheric E layer are dominated by the Hall conductivity that couples the Rossby and Alfvén dynamics giving rise to what are called Rossby‐Alfvén‐Khantadze electromagnetic structures. At finite amplitudes we show that the nonlinearities in the dynamics generate sheared zonal‐flow velocities and zonal magnetic field fluctuations. The zonal‐flow mechanism is based on the parametric excitation of the zonal variations through three‐wave mode coupling in the planetary‐scale waves. The coupled dynamics of the nonlinear 3‐D incompressible flows and the magnetic field fluctuations are derived and used to derive the structure and growth rates for the zonal flows and zonal magnetic fields. Large‐amplitude planetary waves are shown to drive up magnetic fluctuations up to 100 nT.

Generation of zonal flow and magnetic field by coupled Rossby–Alfvén–Khantadze waves in the Earth's ionospheric E-layer

It is shown that in the Earth's weakly ionized ionospheric E-layer with the dominant Hall conductivity, a new type of coupled Rossby–Alfvén–Khantadze (CRAK) electromagnetic (EM) planetary waves, attributable by the latitudinal inhomogeneity of both the Earth's Coriolis parameter and the geomagnetic field, can exist. Under such coupling, a new type of dispersive Alfvén waves is revealed. The generation of a sheared zonal flow and a magnetic field by CRAK EM planetary waves is investigated. The nonlinear mechanism of the instability is based on the parametric excitation of a zonal flow by interacting four waves, leading to the inverse energy cascade in the direction of a longer wavelength. A three-dimensional (3D) set of coupled equations describing the nonlinear interaction of pumping CRAK waves and zonal flow is derived. The growth rate of the corresponding instability and the conditions for driving them are determined. It is found that the growth rate is mainly stipulated by Rossby waves but the generation of the intense mean magnetic field is caused by Alfvén waves.

Formation of coronal mass ejections at different phases of solar activity

This paper investigates the dependence of the observed coronal mass ejections and their parameters on evolutionary changes in the global solar magnetic field at different phases of solar cycles 23–24. Four periods in the evolution of the solar cycle are identified, depending on the dominance ratio of the sectoral and zonal magnetic field structures. The parameters of coronal mass ejections observed during these periods are analyzed. The evolving structure in the global magnetic field is identified, and its influence on coronal mass ejections is examined.

Generation of zonal flow and magnetic field by coupled internal-gravity and alfvén waves in the ionospheric E-layer

Nonlinear dynamics of coupled internal-gravity (IG) and alfvén electromagnetic planetary waves in the weakly ionized ionospheric E-layer is investigated. Under such coupling new type of alfvén waves is revealed. It is shown that such short wavelength turbulence of IG and alfvén waves is unstable with respect to the excitation of low-frequency and large-scale perturbations of the zonal flow and magnetic field. A set of coupled equations describing the nonlinear interaction of coupled IG and alfvén waves with zonal flows is derived. The nonlinear mechanism of the instability is driven by the advection of vorticity and is based on the parametric excitation of convective cells by finite-amplitude coupled IG and alfvén waves leading to the inverse energy cascade toward the longer wavelength. The growth rates of the corresponding instability and the conditions for driving them are determined. The possibility of generation of the intense mean magnetic field is shown.

Excitation of zonal flow and magnetic field by Rossby–Khantadze electromagnetic planetary waves in the ionospheric E-layer

Nonlinear dynamics of Rossby–Khantadze electromagnetic planetary waves in the weakly ionized ionospheric E-layer is investigated. Along with the prevalent effect of Hall conductivity for these waves, the latitudinal inhomogeneity of both the Earth’s angular velocity and the geomagnetic field becomes essential. It is shown that such short wavelength turbulence of Rossby–Khantadze waves is unstable with respect to the excitation of low-frequency and large-scale perturbations of the zonal flow and magnetic field. The nonlinear mechanism of the instability is driven by the advection of vorticity, leading to the inverse energy cascade toward the longer wavelength. The growth rate of the corresponding instability is found. It is shown that the generation of the intense mean magnetic field is caused by the latitudinal gradient of the geomagnetic field.

Nonlinear generation of sheared flows and zonal magnetic fields by electron whistlers in plasmas

The nonlinear generation of shear field and flow in whistler waves is considered. It is shown that a coherent parametric process leads to modulational instability of four waves whistler interaction. Growth rates for the flow/field are compared with published simulation results.

Characteristics of F‐region dynamo currents deduced from CHAMP magnetic field measurements

Using magnetic field observations of the CHAMP satellite we provide the first comprehensive study of F‐region dynamo currents as a function of season, local time, geographic longitude, and solar activity. From bipolar variations of the zonal magnetic field component the density of vertical current driven by the F‐region dynamo is deduced. The current strength is smallest around June solstice, which is attributed to a reduced F‐region Pedersen conductance caused by a lower electron density and neutral density at that season. During the hours around noon highest current densities are observed. They are flowing downward over the dip equator. A secondary peak of upward currents appears at dusk. The polarity switch occurs between15 and 16 (local time) independent of season. The noontime F‐region dynamo current peaks at longitudes connected to the South Atlantic Anomaly, which can be explained by the enhanced conductivity in the region of reduced B field. The F‐region current at dusk exhibits no peak in the longitude sector of the South Atlantic Anomaly. At noon, the F‐region dynamo currents exhibit a wave 4 longitudinal structure during equinoxes and June solstice. The wave 4 signature becomes weak during December solstice. At dusk the wave 4 signature of F‐region dynamo currents is much reduced in all seasons. This behavior can be explained by the DE3 tidal signature in the zonal wind at CHAMP altitude. F‐region dynamo currents increase linearly with the solar flux index, F10.7, during both noon and dusk time sectors. The increase in current strength with increasing F10.7 is slightly higher at dusk than at noon.

Generation of zonal magnetic fields by drift waves in a current carrying nonuniform magnetoplasma

It is shown that zonal magnetic fields (ZMFs) can be nonlinearly excited by incoherent drift waves (DWs) in a current carrying nonuniform magnetoplasma. The dynamics of incoherent DWs in the presence of ZMFs is governed by a wave-kinetic equation. The governing equation for ZMFs in the presence of nonlinear advection force of the DWs is obtained from the parallel component of the electron momentum equation and the Faraday law. Standard techniques are used to derive a nonlinear dispersion relation, which depicts instability via which ZMFs are excited in plasmas. ZMFs may inhibit the turbulent cross-field particle and energy transport in a nonuniform magnetoplasma.