Generation Of Field-aligned Currents Research Articles

Generation of field-aligned currents during substorm expansion: Updated

Generation of field-aligned currents during substorm expansion: Updated

Unified Theory of the Arc Auroras: Formation Mechanism of the Arc Auroras Conforming General Principles of Convection and FAC Generation

AbstractThe arc aurora can be considered as the visualization of the field‐aligned current (FAC). In various cases, the numerically reproduced FAC morphologically matches well with that of the observed arc aurora. Such example can be seen in the sun‐aligned arc, the fan arc, and the theta aurora under the northward interplanetary magnetic field (IMF), and the substorm, the quiet arc, and the initial brightening under the southward IMF. The FAC reproduced by the global simulation is based on the general principle of the magnetohydrodynamic (MHD). In such general principle, the FAC is transmission of net charge, net charge is divE, and E is equivalent to motion. After all, the FAC is transmission of convection motion. In order to transmit the FAC to the dissipative ionosphere based on the MHD, formations of the dynamo and shear that leads from the magnetosphere to the ionosphere are essential. The dynamo and shear are reproduced so as to properly match with each convection structure. The comparison under various magnetospheric topologies between the global simulation and the arc aurora observation indicates that the arc aurora and the FAC occurs inevitably controlled by convection structure. Such a principle of the FAC and convection holds even for the onset FAC.

A confirmation of vertical acoustic resonance and field-aligned current generation just after the 2022 Hunga Tonga Hunga Ha’apai volcanic eruption

A strong volcanic eruption caused a clear vertical acoustic resonance between the sea surface and the thermosphere. Its effects are observed as geomagnetic and GPS-TEC oscillations near the volcano and its geomagnetic conjugate area. The geomagnetic oscillations are observed at Apia and Honolulu geomagnetic observatories with amplitude of about 2 nT and 0.2 nT, respectively. The volcanic eruption started around 04:14 UT on January 15, 2022. The oscillations appeared at 04:21UT at Apia, Samoa, only about 7 min after the start of eruption. Because the distance between the volcano and Apia is about 841 km, it takes about 40 min for a sound wave to propagate from the volcano to Apia. Therefore, it is more plausible to assume that the magnetic oscillation observed at Apia about 7 min after the eruption is caused by the sound waves propagated vertically upward to the ionosphere and generated an electric current. The coherent appearance of geomagnetic oscillation at Honolulu located near the geomagnetic conjugate point of the volcano strongly support the idea that the ionospheric current generated over the volcano diverted as a field-aligned current which flew to the opposite hemisphere and caused the geomagnetic oscillation at Honolulu. The earliest start of GPS-TEC oscillation was around 04:15UT near the volcanic eruption, and it was around 04:20 UT at KOKV station in Hawaii. The time-lag of the TEC variations between Samoa and Hawaii obtained by a cross-correlation analysis is 4.5 min or 8.5 min. These time differences are much smaller than the travel time of the seismic waves from the volcano to Hawaii islands. Therefore, it is suggested that the electric field transmitted along geomagnetic field caused the TEC variation observed over Hawaii Islands. A sawtooth waveform of geomagnetic oscillation observed at Apia and Honolulu is analyzed and a possible generation mechanism is discussed.Graphical

Importance of the Northward IMF for the Quasistatic Mesoscale Field‐Aligned Currents Embedded in the Diminished Region 1/2 Current System in the Dusk Sector

AbstractTo understand the features of the quasistatic field‐aligned currents (FACs) consisting of multiple mesoscale (∼10–100 km) upward and downward FAC regions in the duskside auroral oval, we examined the magnetic field data obtained through the constellation measurements of Swarm satellites for a duration of more than three years. We introduced a method of automated event identification and took the events in 577 satellite orbits that pass through the high‐latitude part of the 14–18 magnetic local time auroral oval. The results of the analysis reveal that the occurrence ratio of the FAC events increases with the IMF BZ, and that the event can be detected at a ratio exceeding 20% for BZ greater than 1 nT. This indicates that the quasistatic mesoscale FACs in the duskside sector are pertinent to a northward IMF condition and that they are not a simple remnant of the diminished Region 1. The results also show that the FAC density tends to increase with the solar wind proton density, implying that the latter is directly associated with FAC generation. The FAC events can occur in a wide range of solar zenith angles, that is, 65–135°, indicating that whether the ionosphere is sunlit or dark is not crucial for the appearance of the event. These results strongly suggest that multiple mesoscale FACs are produced in the low‐latitude boundary layer through a mechanism related to solar wind plasma entry processes that can be more easily attained as the northward component of the IMF increases.

How do field-aligned currents influence plasma flows? Laboratory and space plasma experiments

It is known that the electrodynamic coupling of plasma flows with a magnetic field and background plasma leads to the generation of field-aligned currents that cause many effects in the auroral regions of the Earth during magnetic storms and sub-storms. Much less is known about the influence of field-aligned currents on the dynamics and evolution of plasma flows themselves. The study of this effect in space is complicated by the scale of the phenomenon and requires simultaneous measurements in various regions of the magnetosphere and ionosphere using a large number of satellites. In addition, taking into account the non-stationary nature of these processes, an extremely complex and expensive experiment must be carried out over a long time period. At the same time, such studies can be quite effective in controlled laboratory and ionospheric plasma experiments. This review is devoted to exploring how the generation and evolution of field-aligned currents influences the state and dynamics of plasma flows moving in the magnetic field.

Sliding Contacts in Planetary Magnetospheres

In the planetary magnetospheres there are specific places connected with velocity breakdown, reconnection, and dynamo processes. Here we pay attention to sliding layers. Sliding layers are formed in the ionosphere, on separatrix surfaces, at the magnetopauses and boundaries of stellar astrospheres, and at the Alfvén radius in the equatorial magnetosphere of rapidly rotating strongly magnetized giant planets. Although sliding contacts usually occur in thin local layers, their influence on the global structure of the surrounding space is very great. Therefore, they are associated with non-local processes that play a key role on a large scale. There can be an exchange between different forms of energy, a generation of strong field-aligned currents and emissions, and an amplification of magnetic fields. Depending on the conditions in the magnetosphere of the planet/exoplanet and in the flow of magnetized plasma passing it, different numbers of sliding layers with different configurations appear. Some are associated with regions of auroras and possible radio emissions. The search for planetary radio emissions is a current task in the detection of exoplanets.

A new scenario applying traffic flow analogy to poleward expansion of auroras

Abstract. Transient westward electric fields from the magnetosphere generate equatorward plasma drifts of the order of kilometers per second in the auroral ionosphere. This flow channel extends in north–south directions and is produced in the initial pulse of Pi2 pulsations associated with the field line dipolarization. Drifts in the ionosphere of the order of kilometers per second that accumulated plasmas at the low-latitude end of the flow channel are of such large degree that possible vertical transport effects (including precipitation) along the field lines may be ignored. In this condition, we suggest that plasma compression in the ionosphere initiated the dynamic ionosphere. The dynamic ionosphere includes a nonlinear evolution of the compressed ionospheric plasmas, generation of field-aligned currents to satisfy the quasi-neutrality of the ionosphere, and parallel potentials associated with the excitation of an ion acoustic wave. We will study how the dynamic ionosphere created auroral expansion.

Structure of magnetospheric current systems and mapping of high latitude magnetospheric regions to the ionosphere

The structures of the main magnetospheric current systems are interesting to researchers because of their key role in magnetospheric physics, including the generation of geomagnetic storms and substorms. Ring current and tail current are usually considered as main sources of geomagnetic activity. We assume that the ring current is divided into the inner part located from ∼ 3 to ∼<7 RE and the outer part located from 7 RE to the magnetopause near noon and to ∼ 10–13 RE near midnight. We named the outer part of the ring current as the cut ring current (CRC). Due to compression of the geomagnetic field by the solar wind, the CRC is not concentrated near the equatorial plane at the noon sector, it spreads along field lines. We discuss the main properties of CRC and its role in the generation of field-aligned currents. We propose a method - based on the conservation of plasma pressure along a current line - that enables us to determine the position of the boundary between the ring and tail currents, and to determine the configuration of current lines which cross the magnetopause. At the same time, conservation of plasma pressure along a field line for plasmas in magnetostatic equilibrium enables us to analyze the auroral oval mapping into the equatorial plane regardless of any magnetic field model. It is well known that the auroral oval has a thick ring-like shape. We discuss that it is topologically impossible to map such a structure to a sheet band like structure in the tail. Our analysis showed that most of the auroral oval is mapped to the CRC region. We argue that including CRC as a part of the ring current allows to re-establish the traditional point of view, in which the ring current has a dominant role in Dst/SYM-H variations.

An explanation of auroral intensification during the substorm expansion phase

AbstractA multiple auroral onset substorm on 28 March 2010 provides an opportunity to understand the physical mechanism in generating auroral intensifications during a substorm expansion phase. Conjugate observations of magnetic fields and plasma from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, of field‐aligned currents (FACs) from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellites, and from ground‐based magnetometers and aurora are all available. The comprehensive measurements allow us to further our understanding of the complicated causalities among dipolarization, FAC generation, particle acceleration, and auroral intensification. During the substorm expansion phase, the plasma sheet expanded and was perturbed leading to the generation of a slow mode wave, which modulated electron flux in the outer plasma sheet. During this current sheet expansion, field‐aligned currents formed, and geomagnetic perturbations were simultaneously detected by ground‐based instruments. However, a magnetic dipolarization did not occur until about 3 min later in the outer plasma sheet observed by THEMIS‐A spacecraft (THA). We believe that this dipolarization led to an efficient Fermi acceleration to electrons and consequently the cause of a significant auroral intensification during the expansion phase as observed by the All‐Sky Imagers (ASIs). This Fermi acceleration mechanism operating efficiently in the outer plasma sheet during the expansion phase could be a common explanation of the poleward auroral development after substorm onset. These results also show a good agreement between the upward FAC derived from AMPERE measurements and the auroral brightening observed by the ASIs.

Conjugate observations of flow diversion in the magnetotail and auroral arc extension in the ionosphere

[1] We present a case study of a plasma flow diversion event in the inner magnetosphere to illustrate the effects of near-Earth pressure evolution, field-aligned current (FAC) generation, and associated auroral evolution during conjugate observations from THEMIS satellites and All-Sky Imagers (ASIs) on 29 January 2008. At ~0831 UT, an earthward flow at THC, at X ~ 18 Re, was observed which was likely to be associated with an auroral streamer observed at the ASI station FSMI. Near-Earth satellites THD and THE, separated in the Y-direction by ~1 Re, and magnetically mapping onto the preexisting aurora arc, also observed the earthward flow but with a small Alfven transit time of ~20 s between the magnetotail and the ionosphere. As the earthward flow arrived at THD and THE, the aurora arc near the footprints of THD and THE brightened. A duskward pressure gradient developed between THD and THE in the equatorial magnetotail, corresponding to an upward FAC and consistent with the auroral brightening. At ~0834 UT, an eastward diversion of the flow was observed first at THE and later at THD, further to the East. At the head of that flow, pressure variations, coherent between THE and THD, were observed to propagate Eastward at ~400 km/s, consistent with an eastward auroral expansion near the footprints of THE and THD. Concurrent perturbations of the By component of the magnetic field signify eastward motion of a FAC pair at the two spacecraft locations. We interpret the pressure variations as the generator of the observed FAC that feeds the arc and its eastward expansion.

Barometric and magnetic observations of vertical acoustic resonance and resultant generation of field-aligned current associated with earthquakes

Three rare occasions are introduced, where the excitation of vertical acoustic resonance between the ground and the ionosphere, and the resultant generation of a field-aligned current, just after earthquakes are observationally confirmed. In the case of two inland earthquakes, barometric observations very close to the epicenters (i.e., only 30 km apart) were available, and they showed a sharp spectral peak which appeared within one hour after the origin time and lasted a few hours. The observed periods of the spectral peaks around 260 seconds are close to the period of the theoretically-expected fundamental mode of the resonance. On the other hand, magnetic observations on the ground showed a dominant period at 220–230 seconds which corresponds to the first overtone among theoretically-expected major resonance peaks. In the third case, i.e., during the 2010 Chile earthquake, a long-period magnetic oscillation in the east-west direction, which has two major resonance periods at 265 and 190–195 seconds, was observed on the night-side magnetic dip equator in Peru, where the distance is more than 2600 km from the epicenter, under a very quiet geomagnetic condition. The oscillation was interpreted as the effect of field-aligned current generated through a dynamo process in the ionosphere over the epicenter caused by the resonance.

Axial symmetry breaking of Saturn’s thermosphere

The source of the various planetary-period signals in Saturn’s magnetosphere is at present unknown. We investigate the possibility that the source of these signals is an axially asymmetric wind system in the thermosphere. We describe a feedback mechanism that has the potential to drive such axially asymmetric wind systems. The proposed mechanism relates thermospheric winds to heating from particle precipitation, via the generation of horizontal and field-aligned currents. The relevant physical processes are investigated using a highly simplified general circulation model of Saturn’s thermosphere and ionosphere. Our principal result is that the feedback mechanism is effective in permanently breaking the axial symmetry of the thermosphere, generating a drifting vortex-like structure at high latitudes. However, the precipitating electron energies required to power this structure are of the order of 5 MeV, 2–3 orders of magnitude greater than the observed auroral electron energies, and the highly axially asymmetric distribution of precipitation required across the polar regions of the planet is also inconsistent with observations. Despite these flaws, the model qualitatively explains several features of the observed variation in the pulsing of SKR emissions; in particular, the seasonal variation and the faster rotation rate in the winter hemisphere. We cannot reproduce the apparent 7 month lag in the response of the Saturn Kilometric Radiation (SKR) rotation rate to seasonal variation, but instead suggest the possibility that this effect may have its origin in long chemical time-scales in the upper atmosphere. We also predict the possible existence of secondary periodic features in the SKR emissions with periods of ∼15 planetary rotations, driven by complex wave behaviour in the thermosphere.

Laboratory experiment on region-1 field-aligned current and its origin in the low-latitude boundary layer

In previous experiments by the authors on a magnetic dipole interacting with a laser-produced plasma the generation of an intense field-aligned current (FAC) system on terrella poles was observed. In this paper the question of the origin of these currents in a low-latitude boundary layer of magnetosphere is investigated. Experimental evidence of such a link was obtained by measurements of the magnetic field generated by tangential drag and sheared stress. This specific azimuthal field was found to have quadruple symmetry and local maxima inside the magnetosphere adjacent to the boundary layer. Cases of metallic and dielectric dipole covers modeling good conductive and non-conductive ionosphere revealed that the presence or absence of FACs results in different amplitudes and spatial structures of the sheared field. The current associated with the azimuthal field flows upward at the dawnside, and toward the equator plane at the duskside. It was found to coincide by direction and to correspond by amplitude to a total cross-polar current measured independently. The results suggest that compressional and Alfvén waves are responsible for FAC generation. The study is most relevant to FAC generation in the magnetospheres of Earth and Mercury following pressure jumps in solar wind.

The Relief of Plasma Pressure and Generation of Field-Aligned Currents in the Magnetosphere

A combined action of plasma convection and pitch-angle diffusion of electrons and protons leads to the formation of plasma pressure distribution in the magnetosphere on the night side, and, as it is known, steady electric bulk currents are connected to distribution of gas pressure. The divergence of these bulk currents brings about a spatial distribution of field-aligned currents, i.e. magnetospheric sources of ionospheric current. The projection (mapping) of the plasma pressure relief onto the ionosphere corresponds to the form and position of the auroral oval. This projection, like the real oval, executes a motion with a change of the convection electric field, and expands with an enhancement of the field. Knowing the distribution (3D) of the plasma pressure we can determine the places of MHD-compressor and MHD-generators location in the magnetosphere. Unfortunately, direct observations of plasma distribution in the magnetosphere are faced with large difficulties, because pressure must be known everywhere in the plasma sheet at high resolution, which in situ satellites have been unable to provide. Modeling of distribution of plasma pressure (on ~ 3-12 Re) is very important, because the data from multisatellite magnetospheric missions for these purposes would be a very expensive project.

Region-1 field aligned currents in experiments on laser-produced plasma interacting with magnetic dipole

AbstractIn previous experiments by the authors a generation of intense field aligned current (FAC) system on Terrella poles was observed. In the present report a question of these currents origin in a low latitude boundary layer of magnetosphere is investigated. Experimental evidence of such a link was obtained by measurements of magnetic field generated by tangential sheared drag. Results suggest that compressional and Alfven waves are responsible for FAC generation. The study is most relevant to FAC generation in the Earth and Hermean magnetospheres following pressure jumps in Solar Wind.

Topology of currents in the high-latitude magnetosphere and magnetospheric response to variations in solar wind parameters

We consider a number of new approaches that arise when the topology of currents in the high-latitude magnetosphere is investigated. We note that the high correlation between magnetospheric processes and solar wind parameters is a well-known feature of the magnetospheric dynamics. The proposed explanations of the observed dependences run into difficulties related to the high level of observed turbulence in the magnetosheath and inside the magnetosphere. The topology of the high-latitude magnetosphere in the transition region from dipole magnetic field lines to those extending into the tail is also poorly known. We consider the topology of transverse magnetospheric currents using satellite measurements of the plasma pressure distribution. The currents of the nearest plasma sheet are shown to be closed inside the magnetosphere. The generation of field-aligned currents in Iijima and Potemra region currents 1 and large-scale magnetospheric convection are discussed.

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.

A numerical model of the dayside aurora

A time‐independent model of the dayside auroras is presented within the framework of the pressure‐gradient‐driven mechanism for the generation of the region 1 field‐aligned current (FAC) in the low‐latitude boundary layer (LLBL). Either of two types of the dayside aurora, the bright spots (BSs) or the radially aligned arcs (RAs), arises from spatial modulation of the inner boundary of an LLBL particle population, but the respective source populations have different contents. BSs are produced when the boundary of a majority of the LLBL particle population is modulated. RAs are produced by large‐amplitude modulation occurring on a sharp boundary of a small fraction of the LLBL particle population. The required flux tube content, δN, of the source population of RAs is roughly 10% of that of the ambient population, if the scale length for ∇δN at the inner boundary is comparable to the magnetopause thickness. Using the 1989 Tsyganenko model, the FAC densities from the perturbed inner boundary of the particle population are evaluated and the energy fluxes of electrons precipitating to the ionosphere are then calculated on the basis of the Knight current‐voltage relation. This model can demonstrate the characteristic patterns of the RAs as well as the BSs. It also predicts that (1) both types of aurora with high luminosity as well as inverted‐V structures appear more frequently in the afternoon than in the morning, (2) this morning‐afternoon asymmetry is more pronounced for the case of BSs than RAs, and (3) during periods of northward interplanetary magnetic field, RAs can be distributed over a large area of the polar cap. Now the MLT‐dependent features of the dayside auroras, the appearances of “hot spot” and “warm spot,” and the large‐scale region 1 FAC generation can be understood systematically in view of the distortion of the (average) inner boundary of an LLBL particle population (i.e., nonalignment of the boundary with the average magnetic drift direction).

Numerical simulation of the combined system of nighttime region 0, region 1, and region 2 field‐aligned currents

A numerical simulation is performed to identify the physical mechanisms for the generation of the nighttime region 0 and region 1 field‐aligned currents (FACs) in conditions of a steady solar wind with a southward or weak interplanetary magnetic field. The generation of region 0/region 1 FACs observed near the poleward edge of the auroral oval is closely associated with the dynamics of ion beams produced by nonadiabatic acceleration in the tail current sheet. After ejection from the current sheet, ion beams advance toward the Earth along the nearly straight field lines. During such a time interval, the ions have insignificant magnetic drift velocities. Subsequently, they are reflected at low‐altitude mirror points, and under the influence of inward convection they pass the equator on magnetic shells that are further in. At that moment they attain large magnetic (curvature) drift velocities. Accordingly, in a steady state, the magnetic drift speed averaged over all the ions in a flux tube increases with decreasing invariant latitude; it starts to increase significantly at a certain magnetic shell. In the present paper this magnetic shell is identified with the interface demarcating the nonadiabatic and adiabatic regions in the tail current sheet. An essential point for the region 1 FAC generation is distortion of this interface, namely inclination of the interface to the direction of the average magnetic drift velocity. The generation of region 0 FACs is due to anomalous cross‐field diffusion of energized ions near the poleward edge of the auroral oval. Latitudinal profiles of magnetic field perturbations arising from region 0/region 1 FACs in the simulation are remarkably similar to observed magnetic field signatures of the region 0/region 1 thin‐current‐sheet systems.

Mercury’s Birkeland current system

A consideration is given to the generation of field-aligned currents under different solar wind conditions. The preliminary results from a set of resistive MHD calculations indicate that the field-aligned current system could be significantly changed by the orientation of the interplanetary magnetic field. For most of the cases studied, the total current is less than or on the order of 10 5 A. Even though this current is at least a factor of 10 smaller than its counter part at Earth, it might still produce some important dynamical effects with interesting consequence on the sporadic behavior of Mercury’s atomic sodium emission.