Field-aligned Electric Currents Research Articles

Uncovering the heterogeneity of a solar flare mechanism with mixture models

The physics of solar flares occurring on the Sun is highly complex and far from fully understood. However, observations show that solar eruptions are associated with the intense kilogauss fields of active regions, where free energies are stored with field-aligned electric currents. With the advent of high-quality data sources such as the Geostationary Operational Environmental Satellites (GOES) and Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI), recent works on solar flare forecasting have been focusing on data-driven methods. In particular, black box machine learning and deep learning models are increasingly being adopted in which underlying data structures are not modeled explicitly. If the active regions indeed follow the same laws of physics, similar patterns should be shared among them, reflected by the observations. Yet, these black box models currently used in the literature do not explicitly characterize the heterogeneous nature of the solar flare data within and between active regions. In this paper, we propose two finite mixture models designed to capture the heterogeneous patterns of active regions and their associated solar flare events. With extensive numerical studies, we demonstrate the usefulness of our proposed method for both resolving the sample imbalance issue and modeling the heterogeneity for rare energetic solar flare events.

Kinetic Equilibrium of Two-dimensional Force-free Current Sheets

Force-free current sheets are local plasma structures with field-aligned electric currents and approximately uniform plasma pressures. Such structures, widely found throughout the heliosphere, are sites for plasma instabilities and magnetic reconnection, the growth rate of which is controlled by the structure’s current-sheet configuration. Despite the fact that many kinetic equilibrium models have been developed for one-dimensional force-free current sheets, their two-dimensional (2D) counterparts, which have a magnetic field component normal to the current sheets, have not received sufficient attention to date. Here, using particle-in-cell simulations, we search for such 2D force-free current sheets through relaxation from an initial, magnetohydrodynamic equilibrium. Kinetic equilibria are established toward the end of our simulations, thus demonstrating the existence of kinetic force-free current sheets. Although the system currents in the late equilibrium state remain field aligned as in the initial configuration, the velocity distribution functions of both ions and electrons systematically evolve from their initial drifting Maxwellians to their final time-stationary Vlasov state. The existence of 2D force-free current sheets at kinetic equilibrium necessitates future work in discovering additional integrals of motion of the system, constructing the kinetic distribution functions, and eventually investigating their stability properties.

Effects of geomagnetic disturbances in daytime variations of the atmospheric electric field in polar regions

We have studied the influence of daytime polar substorms (geomagnetic bays under the IMF Bz > 0) on variations of the vertical gradient of the atmospheric electric field potential (Ez) observed at the Polish Hornsund Station (Svalbard, Norway). Only the observations of Ez under “fair weather” conditions were used, i.e. in the absence of strong wind, precipitations, low cloud cover, etc. We studied more than 20 events of daytime polar substorms registered by the Scandinavian chain of IMAGE magnetometers in 2010–2014 during the “fair weather” periods at the Hornsund Station. Analysis of the observations showed that Ez significantly deviates from the its background variations during daytime, as a rule, when the Hornsund Station is in the region of projection of the daytime auroral oval, the position of which was determined from OVATION data. It was shown that the development of a daytime polar substorm leads to fluctuating enhance of Ez values. It was found that Ez surges are accompanied by intensification of field-aligned electric currents outflowing from the ionosphere, which were calculated from the data of low-orbit communication satellites of the AMPERE project.

Dynamics of debye-scale nonstationary plasma structures in the region of auroral field-aligned currents

We consider the formation of small-scale nonstationary plasma structures in the region of relatively strong field-aligned electric currents. The formation of these structures has been shown to be associated with the density instability developed when the current velocity exceeds a critical value. The conditions for the development of this instability can be most favorable in the region of low-density plasma. Numerical calculations have been performed for the initial nonlinear stage of the structure development. The main parameters of the structure, i.e., the times of its formation and destruction, spatial scales, and electric field, have been estimated. The features of the structures are consistent with the existing data from space experiments in the region of auroral field-aligned currents of the Earth.

Planetary distribution of geomagnetic pulsations during a geomagnetic storm at solar minimum

We investigate the features of the planetary distribution of wave phenomena (geomagnetic pulsations) in the Earth’s magnetic shell (the magnetosphere) during a strong geomagnetic storm on December 14–15, 2006, which is untypical of the minimum phase of solar activity. The storm was caused by the approach of the interplanetary magnetic cloud towards the Earth’s magnetosphere. The study is based on the analysis of 1-min data of global digital geomagnetic observations at a few latitudinal profiles of the global network of ground-based magnetic stations. The analysis is focused on the Pc5 geomagnetic pulsations, whose frequencies fall in the band of 1.5–7 mHz (T ∼ 2–10 min), on the fluctuations in the interplanetary magnetic field (IMF) and in the solar wind density in this frequency band. It is shown that during the initial phase of the storm with positive IMF Bz, most intense geomagnetic pulsations were recorded in the dayside polar regions. It was supposed that these pulsations could probably be caused by the injection of the fluctuating streams of solar wind into the Earth’s ionosphere in the dayside polar cusp region. The fluctuations arising in the ionospheric electric currents due to this process are recorded as the geomagnetic pulsations by the ground-based magnetometers. Under negative IMF Bz, substorms develop in the nightside magnetosphere, and the enhancement of geomagnetic pulsations was observed in this latitudinal region on the Earth’s surface. The generation of these pulsations is probably caused by the fluctuations in the field-aligned magnetospheric electric currents flowing along the geomagnetic field lines from the substorm source region. These geomagnetic pulsations are not related to the fluctuations in the interplanetary medium. During the main phase of the magnetic storm, when fluctuations in the interplanetary medium are almost absent, the most intense geomagnetic pulsations were observed in the dawn sector in the region corresponding to the closed magnetosphere. The generation of these pulsations is likely to be associated with the resonance of the geomagnetic field lines. Thus, it is shown that the Pc5 pulsations observed on the ground during the magnetic storm have a different origin and a different planetary distribution.

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 Alfven 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.

Accelerated ions observed in the plasma sheet boundary layer: Beams or streams?

We present an analysis of two crossings of the plasma sheet boundary layer (PSBL) in Earth’s magnetotail by a quartet of Cluster satellites, accomplished both in the absolutely quiet geomagnetic interval and in the restoration period between two substorms. It is found that in the quiet period, field-aligned electric currents were not observed in the PSBL, in spite of presence in this region of high-velocity ions moving toward Earth along the magnetic field lines at velocities of ∼1400 km/s. This means that in the given event, ions and electrons moved together; i.e., high-velocity ions were a part of the accelerated plasma flow. In the disturbed period, the Cluster satellites detected a system of two oppositely directed field-aligned currents, which can be associated with the presence of the X line rather close to the Earth. Owing to multipoint Cluster observations, we managed to estimate the spatial size of a current structure (along the normal to the PSBL surface), which was equal, for a current flowing toward Earth along the high-latitude PSBL boundary, to about 1600 km, which is comparable with the value of the inertial radius of ions. This agrees with the scenario of spatial separation of charges and formation of the Hall system of currents in the magnetic reconnection region. The duration of observation of the given current structure was ∼12 min., which points to the quasi-steady-state character of reconnection.

MESSENGER: Exploring Mercury’s Magnetosphere

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury offers our first opportunity to explore this planet’s miniature magnetosphere since the brief flybys of Mariner 10. Mercury’s magnetosphere is unique in many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands off the solar wind only ~1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic particles and, hence, no radiation belts. Magnetic reconnection at the day side magnetopause may erode the subsolar magnetosphere, allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury’s interior may act to modify the solar wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects may be an important source of information on the state of Mercury’s interior. In addition, Mercury’s magnetosphere is the only one with its defining magnetic flux tubes rooted beneath the solid surface as opposed to an atmosphere with a conductive ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived, ~l-2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury’s magnetic tail. Because of Mercury’s proximity to the sun, 0.3-0.5 AU, this magnetosphere experiences the most extreme driving forces in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and recycling of neutrals and ions among the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury’s magnetosphere are expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection, and pick-up of planetary ions all playing roles in the generation of field-aligned electric currents. However, these field-aligned currents do not close in an ionosphere, but in some other manner. In addition to the insights into magnetospheric physics offered by study of the solar wind-Mercury system, quantitative specification of the “external” magnetic field generated by magnetospheric currents is necessary for accurate determination of the strength and multipolar decomposition of Mercury’s intrinsic magnetic field. MESSENGER’S highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin of Mercury’s magnetic field and the acceleration of charged particles in small magnetospheres. In this article, we review what is known about Mercury’s magnetosphere and describe the MESSENGER science team’s strategy for obtaining answers to the outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere.

Auroral Plasma Acceleration Above Martian Magnetic Anomalies

Aurora is caused by the precipitation of energetic particles into a planetary atmosphere, the light intensity being roughly proportional to the precipitating particle energy flux. From auroral research in the terrestrial magnetosphere it is known that bright auroral displays, discrete aurora, result from an enhanced energy deposition caused by downward accelerated electrons. The process is commonly referred to as the auroral acceleration process. Discrete aurora is the visual manifestation of the structuring inherent in a highly magnetized plasma. A strong magnetic field limits the transverse (to the magnetic field) mobility of charged particles, effectively guiding the particle energy flux along magnetic field lines.The typical, slanted arc structure of the Earth’s discrete aurora not only visualizes the inclination of the Earth’s magnetic field, but also illustrates the confinement of the auroral acceleration process. The terrestrial magnetic field guides and confines the acceleration processes such that the preferred acceleration of particles is frequently along the magnetic field lines. Field-aligned plasma acceleration is therefore also the signature of strongly magnetized plasma.This paper discusses plasma acceleration characteristics in the night-side cavity of Mars. The acceleration is typical for strongly magnetized plasmas – field-aligned acceleration of ions and electrons. The observations map to regions at Mars of what appears to be sufficient magnetization to support magnetic field-aligned plasma acceleration – the localized crustal magnetizations at Mars (Acuña et al., 1999). Our findings are based on data from the ASPERA-3 experiment on ESA’s Mars Express, covering 57 orbits traversing the night-side/eclipse of Mars. There are indeed strong similarities between Mars and the Earth regarding the accelerated electron and ion distributions. Specifically acceleration above Mars near local midnight and acceleration above discrete aurora at the Earth – characterized by nearly monoenergetic downgoing electrons in conjunction with nearly monoenergetic upgoing ions. We describe a number of characteristic features in the accelerated plasma: The “inverted V” energy-time distribution, beam vs temperature distribution, altitude distribution, local time distribution and connection with magnetic anomalies. We also compute the electron energy flux and find that the energy flux is sufficient to cause weak to medium strong (up to several tens of kR 557.7 nm emissions) aurora at Mars.Monoenergetic counterstreaming accelerated ions and electrons is the signature of field-aligned electric currents and electric field acceleration. The topic is reasonably well understood in terrestrial magnetospheric physics, although some controversy still remains on details and the cause-effect relationships. We present a potential cause-effect relationship leading to auroral plasma acceleration in the nightside cavity of Mars – the downward acceleration of electrons supposedly manifesting itself as discrete aurora above Mars.

Formation mechanism of the lower-ionospheric disturbances by the atmosphere electric current over a seismic region

Anomalous sporadic- E layers were observed before the earthquakes. At the same time seismic activity is accompanied by the formation of an external electric current in the lower atmosphere resulting from the emanation of soil gases and transferring charged aerosols. This current is a source of conducting current perturbation in the atmosphere – ionosphere circuit. It is shown that the electric current flowing into the ionosphere from the atmosphere leads to an increase in the E-layer plasma density. The cause of plasma-density enhancement is that the atmospheric electric currents carry positive charged ions upward and the magnetospheric field-aligned electric currents carry electrons downward to the ionosphere. Furthermore, the horizontal electric field of conductive current forms a thin layer of the electron number density by drift of the long-lived metallic ions in the ionosphere. This paper presents the method for calculating the spatial distribution of electron number density perturbations in the bottom ionosphere due to the appearance of external electric current in the near-earth atmosphere. The results can be applied to the interpretation of observation data of the lower-ionospheric perturbations over seismic regions, typhoons and other catastrophes.

On a mechanism of intensification of field-aligned currents at the solar chromosphere-quiescent prominence boundaries

Field-aligned electric currents (FAC) are assumed to support various prominence configurations and it is usually supposed that the quiescent prominence (QP) destabilisation can be initiated by some plasma instability. In this work we study another possibility of QP destabilisation connected with FAC changes. Such a destabilisation may occur when FAC flowing along a loop-shaped QP structure are intensified by bouncing surface MHD modes, presumably generated by motions in the photosphere under, or adjacent to, the QP. The MHD disturbances at the prominence "feet" propagate upwards as Alfven waves and surface modes. We study the possibility that the generated MHD modes, which carry FAC, bounce in the loop-shaped QP structure, provided that they undergo multiple reflection at the prominence feet. This is an interesting physical situation leading to intensification of FAC and promising to be another source of QP destabilisation.

Helical Magnetic Structure of White Light Polar Plumes

We describe the fine structure of white light polar plumes observed using SOHO/LASCO C2 coronagraph. The evolving helical structures of different scales are clearly seen on processed images (the processing is made to reveal the faint contrast objects). The observed structures trace the magnetic field lines, so the electric currents flow along the axes of plumes. An MHD model of a plume which takes into account field-aligned electric currents is developed. The model permits to understand the existence of high-density plasma inside the plume due to the balance between the magnetic forces and the transverse pressure gradient.

Numerical studies on the dynamics of the ionospheric current system

We study the dynamics of the ionospheric current system by means of computational simulations. The initial configuration for the simulations represents numerical solutions of a stationary two-dimensional fluid model that focuses on the closure of one pair of field-aligned electric currents in the lower ionosphere. In this model the momentum transfer between the plasma and the neutral gas is included as well as the Hall term. The dynamical evolution of the current system caused by enhanced magnetospheric sheared plasma flows is investigated by means of numerical simulations carried out with a plasma-neutral gas fluid code. It is shown that in a self-consistent fluid model the intensification of electric field dominated auroral electrojets in the moring and evening sectors of the auroral oval can be understood as the ionospheric response to enhanced magnetospheric plasma convection.

Discussion 4: On restrictions

This discussion focused on specific areas where the concept of the magnetic field being frozen to the plasma breaks down. Each of the panelists, R. L. Lysak, D. Le Queau, and L. P. Block was asked to submit a write-up of their comments, and they writeups are attached below. In addition, Y. Song made an extended comment which is found in a separate paper in this issue. Lysak's comment emphasizes the formation of field-aligned electric fields and currents and the necessity of considering three dimensions to fully understand field-aligned current generation. Le Queau's comment emphasizes the inability of the MHD equations to describe “meso-scale” plasma motions, and Block's comment focuses on the breaking down of the frozen-in approximation in the presence of field-aligned electric fields and in the auroral ionosphere. Song's comment addresses the breaking down of the frozen-in approximation and the magnetic field reconfiguration associated with driven reconnection.

Studies of polar current systems using the IMS Scandinavian magnetometer array

As a contribution to the International Magnetospheric Study (IMS, 1976–1979) a two-dimensional array of 42 temporary magnetometer stations was run in Scandinavia, supplementary to the permanent observatories and concentrated in the northern part of the region. This effort aimed at the time-dependent (periods above about 100 s) determination of the two-dimensional structure of substorm-related magnetic fields at the Earth's surface with highest reasonable spatial resolution (about 100 km, corresponding to the height of the ionosphere) near the footpoints of field-aligned electric currents that couple the disturbed magnetosphere to the ionosphere at auroral latitudes. It has been of particular advantage for cooperative studies that not only simultaneous data were available from all-sky cameras, riometers, balloons, rockets, and satellites, but also from the STARE radar facility yielding colocated two-dimensional ionospheric electric field distributions. In many cases it therefore was possible to infer the three-dimensional regional structure of substorm-related ionospheric current systems. The first part of this review outlines the basic relationships and methods that have been used or have been developed for such studies. The second short part presents typical equivalent current patterns observed by the magnetometer array in the course of substorms. Finally we review main results of studies that have been based on the magnetometer array observations and on additional data, omitting studies on geomagnetic pulsations. These studies contributed to a clarification of the nature of auroral electrojets including the Harang discontinuity and of ionospheric current systems related to auroral features such as the break-up at midnight, the westward traveling surge, eastward drifting omega bands, and spirals.

Joule heating as an explanation for the differential emission measure structure and systematic redshifts in the sun's lower transition region

view Abstract Citations (22) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Joule Heating as an Explanation for the Differential Emission Measure Structure and Systematic Redshifts in the Sun's Lower Transition Region Roumeliotis, George Abstract A new model is described for the sun's lower transition region which extends the ideas of Antiochos and Noci (1986) and Rabin and Moore (1984) but avoids their use of ad hoc heating functions and extremely small spatial scales, respectively. Specifically, it is proposed that most of the lower transition region plasma is contained in sheared magnetic boundary layers separating adjacent flux loops above the network lanes, and that this plasma is heated by the Joule dissipation of intense, field-aligned electric currents. A simple model for a sheared magnetic boundary layer is developed, and the field-aligned electric currents flowing inside it are calculated. A 1D hydrodynamic model is used to determine the atmospheric structure along a Joule-heated field line embedded in a sheared magnetic boundary layer. It is found that when the electric current density is less than a certain critical value, the atmosphere along the field line evolves toward a 'cool' equilibrium state, with T(max) less than 80,000 K. Publication: The Astrophysical Journal Pub Date: September 1991 DOI: 10.1086/170514 Bibcode: 1991ApJ...379..392R Keywords: Chromosphere; Plasma Heating; Red Shift; Resistance Heating; Solar Corona; Current Density; Electric Current; Field Aligned Currents; Magnetohydrodynamics; Ohmic Dissipation; Plasma Temperature; Solar Physics; HYDROMAGNETICS; PLASMAS; SUN: TRANSITION REGION full text sources ADS |

A model of the magnetospheric electric convection field

The observed magnetospheric convection field at ionospheric heights is simulated by a simple analytic quasi‐static electric potential field. In particular, the transition region within the auroral zone where the meridional electric field component reverses is brought into close agreement with Heppner's (1977) observations. The location of zero meridional electric field strength in the model is compared with the observed Harang discontinuity (Maynard, 1974), where the zonal plasma flow reverses. The field‐aligned electric currents associated with the model electric field occur within two regions. In region 1, within the auroral zone, maximum outflow out of the ionosphere occurs at 1600 LT. In region 2, equatorward of the auroral zone, maximum outflow is near 0700 LT, consistent with measurements of Iijima and Potemra (1976).

Magnetic Field-Aligned Electric Currents and Fields in the Magnetosphere and Precipitation of Particles

Magnetic Field-Aligned Electric Currents and Fields in the Magnetosphere and Precipitation of Particles

Field-aligned electric currents and their measurement by the incoherent backscatter technique

Field-aligned electric currents flow in the magnetosphere in many situations of fundamental geophysical interest. It is shown here that the incoherent backscatter technique can be used to measure these currents when the plasma line can be observed. To our knowledge this has not been proposed before. The technique provides a ground-based means of measuring these currents which complements the rocket and satellite ones.

A semiempirical model of large-scale magnetospheric electric fields

Semiempirical analytic formulas for large-scale electric magnetospheric fields are developed that are valid within the inner magnetosphere as well as within the ionosphere. These electric fields drive ionospheric electric currents. Moreover, field-aligned electric currents must exist in order to maintain the magnetospheric electric field configuration. The model calculations are compared and are shown to be consistent with the shape of the plasmapause, the equivalent electric current systems of DP 1 and DP 2, the available electric field observations at ionospheric altitudes, and the observations of field-aligned electric currents.