Field-aligned Currents Research Articles

Inter-hemispheric asymmetries in high-latitude electrodynamic forcing and the thermosphere during the October 8–9, 2012, geomagnetic storm: An integrated data–Model investigation

Inter-hemispheric asymmetry (IHA) in Earth’s ionosphere–thermosphere (IT) system can be associated with high-latitude forcing that intensifies during storm time, e.g., ion convection, auroral electron precipitation, and energy deposition, but a comprehensive understanding of the pathways that generate IHA in the IT is lacking. Numerical simulations can help address this issue, but accurate specification of high-latitude forcing is needed. In this study, we utilize the Active Magnetosphere and Planetary Electrodynamics Response Experiment-revised fieldaligned currents (FACs) to specify the high-latitude electric potential in the Global Ionosphere and Thermosphere Model (GITM) during the October 8–9, 2012, storm. Our result illustrates the advantages of the FAC-driven technique in capturing high-latitude ion drift, ion convection equatorial boundary, and the storm-time neutral density response observed by satellite. First, it is found that the cross-polar-cap potential, hemispheric power, and ion convection distribution can be highly asymmetric between two hemispheres with a clear By dependence in the convection equatorial boundary. Comparison with simulation based on mirror precipitation suggests that the convection distribution is more sensitive to FAC, while its intensity also depends on the ionospheric conductance-related precipitation. Second, the IHA in the neutral density response closely follows the IHA in the total Joule heating dissipation with a time delay. Stronger Joule heating deposited associated with greater high-latitude electric potential in the southern hemisphere during the focus period generates more neutral density as well, which provides some evidences that the high-latitude forcing could become the dominant factor to IHAs in the thermosphere when near the equinox. Our study improves the understanding of storm-time IHA in high-latitude forcing and the IT system.

Vanadium pentoxide thin films deposited by the thermionic vacuum arc plasma

The Vanadium pentoxide (V2O5) thin films were deposited on glass substrates by a Thermionic vacuum arc (TVA) plasma system. The structural, elemental, surface morphological, and optical properties of the deposited thin films were investigated in detail using X-ray diffraction (XRD), Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR), Ultraviolet-visible (UV–VIS) spectroscopy, field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), and atomic force microscopy (AFM). In this study, the V2O5 thin films deposited on the glass substrates were produced at three different arc powers at fixed cathode filament current and deposition time, and all characteristics of the deposited thin films were analyzed separately for these three arc powers. The thicknesses of the V2O5 thin films deposited at arc powers of 80, 280, and 640 W were measured as 92, 98, and 204 nm, respectively. The main objective of the research is to show that V2O5 thin films can be deposited by the TVA with the features specified by the technology, which cannot be arranged easily in other methods. While all the deposited thin films have an amorphous structure and roughness as small as 0.4 – 1.8 nm, the arc power did not cause any different bonding in the atomic structure. The transmittance of each film, in the spectral range of 300 to 900 nm, was measured and the optical energy band gap (Eg) was calculated for three films deposited at various arc powers and found to be 3.84, 3.42, and 3.20 eV for the direct allowed transitions respectively.

Universal Time variations in the magnetosphere

We study the dependencies of Earth’s magnetosphere on Universal Time, UT. These are introduced because Earth’s magnetic axis is not aligned with the rotational axis and complicated because it is eccentric, which makes the offset of the magnetic and rotation poles considerably greater in the Southern hemisphere and the longitudinal separation of the magnetic poles less than 180°: hence consequent UT variations in the two hemispheres are not in equal in amplitude nor in exact antiphase and do not cancel, as they would for a geocentric dipole. We use long series of a variety of geomagnetic data to demonstrate the inductive effect of motions of the polar caps in a “geocentric-solar” frame, which is phase-locked to the Russell-McPherron (R-M) effect on solar-wind magnetosphere coupling. This makes the response of the magnetosphere-ionosphere system different for the two polarities of the Y-component of the Interplanetary Magnetic Field in the GSEQ reference frame, explaining the difference in response to the March and September equinox peaks in solar wind forcing. The sunward/antisunward pole-motion effect is detected directly in satellite transpolar voltage data and is shown to have a greater effect on the geomagnetic data than the full dipole tilt effect which generates the equinoctial pattern, the potential origins of which are discussed in terms of the dipole tilt effect on ionospheric conductivities and the stability of the near-Earth tail. Persistent UT variations in Region-1 and Region-2 field-aligned currents and in partial ring current indices are presented: their explanation is an important challenge for numerical modelling of the magnetosphere-ionosphere-thermosphere system which we need to quantify the relative contributions of the various mechanisms and to give understanding of the effect of arrival time on the response of the system to large, geoeffective disturbances in interplanetary space.Plain language summary: The effect on terrestrial space weather of Earth’s magnetic axis not being aligned with the rotational axis is investigated. It is complex because not only do these two axes not align in direction (the “dipole tilt”), the magnetic axis does not pass through the centre of the Earth, which sets a requirement for an “eccentric” model of the field and not the commonly-used “geocentric” one. For many years, it has been known that the dipole tilt gives a peak in geomagnetic activity at the equinoxes (the semi-annual variation) through the “Russell-McPherron” (R-M) effect. However, although the variation with Universal Time is consistent with the R-M effect for the September equinox, it is not for the March equinox. We here solve this long-standing puzzle by investigating the effects of the motions of the two poles in a frame fixed with respect to both the Earth and the Sun for an eccentric dipole model. But solving one puzzle generates many others. We present observations of the Universal Time variations that these mechanisms combine to generate, which set an important challenge to the numerical modelling of the near-Earth space environment.

Multi-Periodicity of High-Frequency Type III Bursts as a Signature of the Fragmented Magnetic Reconnection

Using the radio spectra of the 2 April 2022 eruptive flare, we analyze a group of highfrequency type III bursts by our new wavelet method. In this analysis, we found a multi-periodicity of these bursts that is interpreted by the electron beams accelerated in the fragmented magnetic reconnection in the rising magnetic rope. We propose that each period in these type III bursts is a result of the periodic interaction of sub-ropes formed in the rising magnetic rope. In each interaction, the period depends on the diameter of interacting sub-ropes and local Alfvén velocity. This interpretation is supported by detection of the specific EUV structure which was, according to our knowledge, observed for the first time. All proposed processes occur in the rising magnetic rope. Thus, this flare deviates from the standard flare model, where the main magnetic reconnection is located below the rising magnetic rope.

A case study in support of closure of bow shock current through the ionosphere utilizing multi-point observations and simulation

On the bow shock in front of Earth’s magnetosphere flows a current due to the curl of the interplanetary magnetic field across the shock. The closure of this current remains uncertain; it is unknown whether the bow shock current closes with the Chapman-Ferraro current system on the magnetopause, along magnetic field lines into the ionosphere, through the magnetosheath, or some combination thereof. We present simultaneous observations from Magnetosphere Multiscale (MMS), AMPERE, and Defense Meteorological Satellite Program (DMSP) during a period of strong By, weakly negative Bz, and very small Bx. This IMF orientation should lead to a bow shock current flowing mostly south to north on the shock. AMPERE shows a current poleward of the Region 1 and Region 2 Birkeland currents flowing into the northern polar cap and out of the south, the correct polarity for bow shock current to be closing along open field lines. A southern Defense Meteorological Satellite Program F18 flyover confirms that this current is poleward of the convection reversal boundary. Additionally, we investigate the bow shock current closure for the above-mentioned solar wind conditions using an MHD simulation of the event. We compare the magnitude of the modeled bow shock current due to the IMF By component to the magnitude of the modeled high-latitude current that corresponds to the real current observed in AMPERE and by Defense Meteorological Satellite Program. In the simulation, the current poleward of the Region 1 currents is about 37% as large as the bow shock Iz in the northern ionosphere and 60% in the south. We conclude that the evidence points to at least a partial closure of the bow shock current through the ionosphere.

Generation of Field‐Aligned Currents During Substorm Expansion: An Update

AbstractWe investigated generation processes of field‐aligned currents (FACs) that are abruptly intensified at the beginning of the substorm expansion phase by tracing a packet of the Alfvén wave backward in time from the onset position in the ionosphere in the global magnetohydrodynamics (MHD) simulation. The generation region is found in the near‐Earth plasma sheet, in which (a) azimuthally moving plasma pulls the magnetic field line, and performs negative work against the magnetic tension force to excite the Alfvén waves, (b) FACs are generated from the requirement of Ampère and Faraday laws, and (c) field‐perpendicular current is converted to FACs. We call this near‐Earth FAC dynamo. The plasma involved originates in the tail lobe region. When near‐Earth reconnection occurs in the plasma sheet, the plasma is accelerated earthward by the Lorentz force, and decelerated by the plasma pressure gradient force, followed by the Lorentz force. The flow is deflected to the west and east directions by the plasma pressure gradient force and the Lorentz force, resulting in the excitation of Alfvén waves and FACs. The Alfvén waves propagate along the magnetic field in the rest frame of the moving plasma. When it arrives at the ionosphere, the auroral electrojet starts developing and the substorm expansion phase begins. The near‐Earth FAC dynamo can be distinguished from the near‐Earth dynamo (J · E < 0, where J is the current density and E is the electric field). We suggest that the evolution of the substorm can be understood in terms of the development of FACs.

Mean Energy Flux, Associated Derived Height‐Integrated Conductances, and Field‐Aligned Current Magnitudes Evolve Differently During a Substorm

AbstractWe examine the average evolution of precipitation‐induced height‐integrated conductances, along with field‐aligned currents (FACs), in the nightside sector of the polar cap over the course of a substorm. Conductances are estimated from the average energy flux and mean energies derived from auroral emission data. Data are binned using a superposed epoch analysis on a normalized time grid based on the time between onset and recovery phase (δt) of each contributing substorm. We also examine conductances using a fixed time binning of width 0.25 hr. We split the data set by magnetic latitude of onset. We find that the highest conductances are observed for substorms with onsets that occur between 63 and 65° magnetic latitude, peaking at around 11 mho (Hall) and 4.8 mho (Pedersen). Substorms with onsets at higher magnetic latitudes show lower conductances and less variability. Changes in conductance over the course of a substorm appear primarily driven by changes (about 40% at onset) in the average energy flux, rather than the average energy of the precipitation. Average energies increase after onset slower than energy flux, later these energies decrease slowly for the lowest latitude onsets. No clear expansion of the main region 1 and region 2 FACs is observed. However, we do see an ordering of the current magnitudes with magnetic latitude of onset, particularly for region 1 downwards FAC in the morning sector. Peak current magnitudes occur slightly after or before the start of the recovery phase for the normalized and fixed‐time grids.

Multipoint Observations of Dispersive Scale Alfvénic Field‐Line Resonances Associated With Substorm Auroral Beads

AbstractWe present a case study of the field‐aligned current (FAC) systems that transpire within the high‐altitude auroral acceleration region of an “auroral bead” initiated double oval substorm observed on 23 February 2001 by the Cluster fleet. Conjunctive Cluster measurements and auroral images from IMAGE reveal that auroral bead current system formation and evolution is a multi‐scale, injection‐mediated process. The FACs at large scales vary on substorm evolution time scales (∼minutes) in response to the injection and evolution of hotter denser magnetospheric plasma. Embedded within the large‐scale FACs are intense short‐scale (≲ few 10s of km) currents comprising dispersive scale Alfvén wave (DAW) fluctuations. The DAWs are a complex mixture of ingoing and reflected components that regularly interfere to form a broad spectrum of kinetic (dispersive) scale Alfvénic field‐line resonances (KFLRs). The Alfvénic currents appear as a nested series of upward and downward FAC densities with amplitudes reaching a few 100 nA/m2. Energized field‐aligned or counterstreaming electrons near keV energies and below are observed with parallel skews that vary in concert with variations in the DAW current sense. Positive correlations between DAW electric field energy densities and the energies of energized H+, He+, and O+ outflow are observed, indicative of ion energization within the DAW fields. Due to their L‐shell location (L ∼5.8–7.0) and associations with injections, the KFLRs are interpreted as the high‐altitude auroral zone analog of KFLRs observed in the equatorial inner magnetosphere.

Impacts of Ionospheric Conductance on Magnetosphere‐Ionosphere Coupling

AbstractUnderstanding the consequence of the complex interplay between solar flare and geomagnetic storm on the magnetosphere‐ionosphere (M‐I) coupling is a critical aspect to space weather research. This is the first attempt to simulate the concurrent solar flare and geomagnetic storm effects on M‐I coupling using the state‐of‐art geospace model which integrates Lyon‐Fedder‐Mobarry ‐Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model —the Rice Convection ring current Model (LTR) in a self‐consistent way. Our results indicate that dayside E‐region electron density enhancements peak nearly at the same time as the flare and these electron density enhancements at middle latitudes are not very sensitive to storm forcing. F‐region electron densities also have an immediate response to solar flares but take longer to recover compared to E‐region ionosphere, and dayside F‐region electron density enhancements are more prominent for more stormy conditions. These dayside middle‐latitude F‐region enhancements intensify tongue of ionization in the polar cap region. Both E‐ and F‐region electron density increases contribute to polar ionosphere conductance enhancements that have both local and global consequences. Flare‐induced conductance enhancements tend to reduce the amount of Joule dissipation into upper atmosphere and cross polar cap potential around solar flare peak (12:00 UT), and increase dayside field aligned currents and Joule dissipation afterward (12:20–13:30 UT). These effects are more appreciable for stronger solar wind driving conditions. These simulation results provide important references for interpreting observed solar activity/cycle dependence of magnetosphere‐ionosphere coupling phenomena.

Formation of a Plasma Layer During the Passage of the Moon through the Magnetic Ropes of the Solar Wind

Formation of a Plasma Layer During the Passage of the Moon through the Magnetic Ropes of the Solar Wind

Kinetic Scale Magnetic Reconnection with a Turbulent Forcing: Particle-in-cell Simulations

Turbulent magnetic reconnection has been observed by spacecraft to occur commonly in terrestrial magnetosphere and the solar wind, providing a new scenario of kinetic scale magnetic reconnection. Here by imposing a turbulent forcing on ions in particle-in-cell simulations, we simulate kinetic scale turbulent magnetic reconnection. We find formation of fluctuated electric and magnetic fields and filamentary currents in the diffusion region. Reconnection rate does not change much compared to that in laminar Hall reconnection. At the X-line, the electric and magnetic fields both exhibit a double power-law spectrum with a spectral break near local lower-hybrid frequency. The energy conversion rate is high in turbulent reconnection, leading to significant electron acceleration at the X-line. The accelerated electrons form a power-law spectrum in the high energy range, with a power-law index of about 3.7, much harder than one can obtain in laminar reconnection.

Generation of intense magnetic wakes by relativistic laser pulses in plasma

The emergence of petawatt lasers focused to relativistic intensities enables all-optical laboratory generation of intense magnetic fields in plasmas, which are of great interest due to their ubiquity in astrophysical phenomena. In this work, we study generation of spatially extended and long-lived intense magnetic fields. We show that such magnetic fields, scaling up to the gigagauss range, can be generated by interaction of petawatt laser pulses with relativistically underdense plasma. With three-dimensional particle-in-cell simulations we investigate generation of magnetic fields with strengths up to 10^{10} G and perform a large multi-parametric study of magnetic field in dependence on dimensionless laser amplitude a_{0} and normalized plasma density n_{e}/n_{c}. The numerical results yield scaling laws that closely follow derived analytical result B propto sqrt{a_{0}n_{e}/n_{c}}, and further show a close match with previous experimental works. Furthermore, we show in three-dimensional geometry that the decay of the magnetic wake is governed by current filament bending instability, which develops similarly to von Kármán vortex street in its nonlinear stage.

Avalanche photodiode detection system enables X-ray communication up to 10 Mbps

As the core part of the X-ray communication (XCOM) system, the X-ray detector directly affects the data rate and bit error ratio (BER) performance. An avalanche photodiode (APD) with fast response time and high intern gain is chosen as the detector sensor to improve the data rate of XCOM. A three-stage fast preamplifier using radio frequency amplifier chips is designed to amplify the weak signal from the APD sensor. The bit synchronization signal acquisition and signal judgment modules are established to demodulate the detected pulse X-ray signals. The single photon pulse response and signal-to-noise ratio characteristic of the APD detector are tested. The pulse response width is only 25 ns. The BER characteristic with 211,709-bit pseudo-random sequence and the eye diagram of the XCOM system based on the developed APD detector are evaluated. Results show that the BER is zero at the data rate lower than 6 Mbps and reaches 1.98 × 10−4 at 10 Mbps under 55 kV anode voltage, 1.58 A filament current, and a 10 cm air channel. The designed APD detector increases the data rate of XCOM from the level of 1 Mbps to 10 Mbps, indicating its potential application for a higher data rate.

Influence of magnetic reconnection on the eruptive catastrophes of coronal magnetic flux ropes

Introduction: Large-scale solar eruptive activities have a close relationship with coronal magnetic flux ropes. Previous numerical studies have found that the equilibrium of a coronal flux rope system could be disrupted if the axial magnetic flux of the rope exceeds a critical value, so that the catastrophe occurs, initiating the flux rope to erupt. Further studies discovered that the catastrophe does not necessarily exist: The flux rope system with certain photospheric flux distributions could be non-catastrophic. It is noteworthy that most previous numerical studies are under the ideal magnetohydrodynamic (MHD) condition, so that it is still elusive whether there is the catastrophe associated with the critical axial flux if magnetic reconnection is included in the flux rope system.Methods: In this paper, we carried out numerical simulations to investigate the evolutions of coronal magnetic rope systems under the ideal MHD and the resistive condition.Results and discussions: Under the ideal MHD condition, our simulation results demonstrate that the flux rope systems with either too compact or too weak photospheric magnetic source regions are non-catastrophic versus varying axial flux of the rope, and thus no eruption could be initiated; if there is magnetic reconnection in the rope system, however, those flux rope systems could change to be capable of erupting via the catastrophe associated with increasing axial flux. Therefore, magnetic reconnection could significantly influence the catastrophic behaviors of flux rope system. It should be both the magnetic topology and the local physical parameters related to magnetic reconnection that determine whether the increasing axial flux is able to cause flux rope eruptions.

Electron Polarization in Ultrarelativistic Plasma Current Filamentation Instabilities

Plasma current filamentation of an ultrarelativistic electron beam impinging on an overdense plasma is investigated, with emphasis on radiation-induced electron polarization. Particle-in-cell simulations provide the classification and in-depth analysis of three different regimes of the current filaments, namely, the normal filament, abnormal filament, and quenching regimes. We show that electron radiative polarization emerges during the instability along the azimuthal direction in the momentum space, which significantly varies across the regimes. We put forward an intuitive Hamiltonian model to trace the origin of the electron polarization dynamics. In particular, we discern the role of nonlinear transverse motion of plasma filaments, which induces asymmetry in radiative spin flips, yielding an accumulation of electron polarization. Our results break the conventional perception that quasisymmetric fields are inefficient for generating radiative spin-polarized beams, suggesting the potential of electron polarization as a source of new information on laboratory and astrophysical plasma instabilities.

Empirical models of the Earth’s ionosphere field-aligned currents: a review

In this paper, we present a review of empirical models of the Earth’s ionosphere field-aligned currents (FAC). In total, seven FAC models were considered. Iijima and Potemra’s model was one of the earliest ones. It described a general structure of FAC distribution and could be used as a FAC common pattern. Papitashvili & co-auth., Lukianova & co-auth., and Weimer improved descriptions of FAC distributions using the larger amount of data from various satellite missions. Their models were published as a set of printed maps of the FAC distributions for various geophysical situations. Recent models (MFACE, AMPS, and EW-FAC) are provided as MATLAB, Python, and IDL software, respectively. These models are the high-resolution (finer spatial grid) description of FAC distributions with improved dependence on the Interplanetary Magnetic Field and Solar wind impacts.

SUBSTORMS CAUSED BY LARGE-AMPLITUDE SOLAR WIND DYNAMIC PRESSURE PULSES: CASE STUDY ON 3 NOVEMBER 2021

We studied the geomagnetic effects of abrupt and large-amplitude changes in the solar wind dynamic pressure (Psw) on 3 November 2021. when there were observed three large-amplitude Psw pulses (up to 20 nPa) under the strong (up to −18 nT) southward IMF Bz and significantly varying IMF By (from +20 to −15 nT). Basing on IMAGE magnetometer data, we found three substorms associated with these Psw impulses. These substorms followed one after another with a short interval and each subsequent substorm began developing during the unfinished recovery phase of the previous one under disturbed space weather conditions. Under strong negative IMF Bz there was significant input of energy into the magnetosphere that indicated by the increasing PC-index values. It was shown that the spatial-temporal features of the substorm subsequence development was complicated, differed from a typical isolated “normal” substorm and changed from one substorm to another. According to the AMPERE 66 ionospheric satellite data, the global distribution of the ionospheric and field-aligned currents (FAC) was established during the considered substorms. We found that during all these substorms, there were strong FACs and corresponding ionospheric electrojets in the morning sector indicating an enhanced magnetospheric convection which formed the DP2 current system. In addition, in the night sector, the DP1 current system was observed, the clearest in the second event.

Influences of various space current systems on the geomagnetic field in near-Earth space

Ground and space-based observations of the geomagnetic field are usually a superposition of different sources from the Earth’s core, lithosphere, ocean, ionosphere, and magnetosphere, and also from field-aligned currents coupling the ionosphere and magnetosphere—the meridional currents that connect the two hemispheres and the induced currents due to the variations of fields over time. The fluctuation of magnetic fields generated by these highly dynamic space currents greatly limits the accuracy of the geomagnetic models. In order to better accomplish the scientific objectives of Macau Science Satellite-1 (MSS-1), and to improve existing geomagnetic field models, we present here for the first time a self-consistent coupling of solar wind, magnetosphere, and ionosphere, which represents the most developed numerical simulation method for space physics research so far, making it possible to quantify the contribution of different current systems to the total observed magnetic field (<italic><bold>B</bold></italic>). The results show that numerical simulation can capture main magnetic disturbance characteristics with significant precision. Partial ring current is a major contributor to the latitudinal magnetic perturbation near the equator. Magnetopause and magnetotail currents affect the radial magnetic perturbation around the mid-latitudes. Field-aligned and Pedersen currents produce significant longitudinal and latitudinal magnetic perturbations at high latitudes.

Formation of a Plasma Layer During the Passage of the Moon through the Magnetic Ropes of the Solar Wind

In the absence of a dense atmosphere and a general magnetic field around the Moon, solar wind particles reach the lunar surface and are almost completely absorbed. When the Moon passes through the plasma medium of the solar wind magnetic ropes, the electric currents of the rope can strongly change the electric potential of the lunar surface on the day and night surfaces, and in the case when the current density vectors of the rope and the direction to the Sun are close to colinear, there is the possibility of sufficiently strong ring currents, the magnetic field of which tends to displace the magnetic field of the rope and lead to the formation of a plasma layer with a height of the order of the electron Larmor radius.

Improved FCDI algorithm for tokamak plasma equilibrium reconstruction

The paper presents the improved FCDI plasma equilibrium reconstruction algorithm implemented at the Globus-M2 tokamak. The algorithm has been improved to be able to operate in real time. The FCDI-FF (Fixed Filaments) operation mode was added, in which the plasma is modeled by current filaments at fixed positions. New user interface for the algorithm displays reconstructed plasma separatrix and contours of the internal magnetic surfaces, the poloidal flux distribution, and graphs of the reconstructed plasma parameters, including profiles of plasma pressure, poloidal current and safety factor q, plasma energy, poloidal beta and internal inductance, which can be exported in *.mat, *.json and G-eqdsk formats. Keywords: tokamak, equilibrium reconstruction, plasma shape.