Hall Current System Research Articles

Sub‐Structures of the Separatrix Region During Magnetic Reconnection

AbstractUtilizing data from the Magnetospheric Multiscale mission, the sub‐structures of the separatrix region during reconnection are investigated. A small current system in the separatrix region is embedded in the large Hall current system, which appears as a quadrupole current. This small current system has an opposite direction to the Hall current and cancels part of the Hall magnetic field. The formation of the quadrupole current is possibly related to the patchy parallel electric field. The electric field in the separatrix region can be divided into three sub‐layers according to the electron frozen‐in condition. En enhances in two outside sub‐layers, where the strong En is supported by the Hall term and electron pressure gradient term, and the nearly zero En, separating the enhanced electric fields, is caused by a small flow and electron density gradient. Our observations imply complex dynamics in the separatrix region during magnetic reconnection.

Magnetic Reconnection Within the Boundary Layer of a Magnetic Cloud in the Solar Wind

AbstractThe twisted local magnetic field at the front or rear regions of the magnetic clouds (MCs) associated with interplanetary coronal mass ejections (ICMEs) is often nearly opposite to the direction of the ambient interplanetary magnetic field. There is also observational evidence for magnetic reconnection (MR) outflows occurring within the boundary layers of MCs. In this study, a MR event located at the western flank of the MC occurring on October 3, 2000 is studied in detail. Both the large‐scale geometry of the helical MC and the MR outflow structure are scrutinized in a detailed multipoint study. The ICME sheath is of hybrid propagation‐expansion type. Here, the freshly reconnected open field lines are expected to slip slowly over the MC resulting in plasma mixing at the same time. As for MR, the current sheet geometry and the vertical motion of the outflow channel between ACE‐Geotail‐WIND spacecraft were carefully studied and tested. The main findings on MR include (a) first‐time observation of non‐Petschek‐type slow‐shock‐like discontinuities in the inflow regions; (b) observation of turbulent Hall magnetic field associated with a Lorentz‐force‐deflected electron jet; (c) acceleration of protons by reconnection electric field and their back‐scatter from the slow‐shock‐like discontinuity; (d) observation of relativistic electron near the MC inflow boundary/separatrix; these electron populations can presumably appear as a result of nonadiabatic acceleration, gradient B drift, and via acceleration in the electrostatic potential well associated with the Hall current system; and (e) observation of Doppler‐shifted ion‐acoustic and Langmuir waves in the MC inflow region.

Signatures of Magnetic Separatrices at the Borders of a Crater Flux Transfer Event Connected to an Active X‐Line

AbstractIn this paper, we present Magnetospheric Multiscale (MMS) observations of a flux transfer event (FTE) characterized by a clear signature in the magnetic field magnitude, which shows maximum at the center flanked by two depressions, detected during a period of stable southward interplanetary magnetic field. This class of FTEs are called “crater‐FTEs” and have been suggested to be connected with active reconnection X line. The MMS burst mode data allow the identification of intense fluctuations in the components of the electric field and electron velocity parallel to the magnetic field at the borders of the FTE, which are interpreted as signatures of the magnetic separatrices. In particular, the strong and persistent fluctuations of the parallel electron velocity at the borders of this crater‐FTE reported for the first time in this paper, sustain the field‐aligned current part of the Hall current system along the separatrix layer, and confirm that this FTE is connected with an active reconnection X line. Our observations suggest a stratification of particles inside the reconnection layer, where electrons are flowing toward the X line along the separatrix, are flowing away from the X line along the reconnected field lines adjacent to the separatrices, and more internally ions and electrons are flowing away from the X line with comparable velocities, forming the reconnection jets. This stratification of the reconnection layer forming the FTE, together with the reconnection jet at the trailing edge of the FTE, suggests clearly that this FTE is formed by the single X line generation mechanism.

Magnetospheric Multiscale Observations of an Ion Diffusion Region With Large Guide Field at the Magnetopause: Current System, Electron Heating, and Plasma Waves

AbstractWe report Magnetospheric Multiscale (MMS) observations of a reconnecting current sheet in the presence of a weak density asymmetry with large guide field at the dayside magnetopause. An ion diffusion region (IDR) was detected associated with this current sheet. Parallel current dominated over the perpendicular current in the IDR, as found in previous studies of component reconnection. Electrons were preferentially heated parallel to the magnetic field within the IDR. The heating was manifested as a flattop distribution below 400 eV. Two types of electromagnetic electron whistler waves were observed within the regions where electrons were heated. One type of whistler wave was associated with nonlinear structures in E|| with amplitudes up to 20 mV/m. The other type was not associated with any structures in E||. Poynting fluxes of these two types of whistler waves were directed away from the X‐line. We suggest that the nonlinear evolution of the oblique whistler waves gave rise to the solitary structures in E||. There was a perpendicular super‐Alfvénic outflow jet that was carried by magnetized electrons. Intense electrostatic lower hybrid drift waves were localized in the current sheet center and were probably driven by the super‐Alfvénic electron jet, the velocity of which was approximately equal to the diamagnetic drift of demagnetized ions. Our observations suggest that the guide field significantly modified the structures (Hall electromagnetic fields and current system) and wave properties in the IDR.

A shear BY field in the Earth's magnetotail and its variations in the current sheet

We use Cluster and THEMIS simultaneous observations to study the spatial distributions of a shear BY field in the Plasma Sheet (PS) of the Earth's magnetotail at −31 RE < X < −9 RE. The best correlation between the BY field in the PS (BY_PS) and the Y-component of the Interplanetary Magnetic Field (IMF) (BY_IMF) was observed during the quiet PS periods when high speed plasma flows were not detected. During active PS periods the correlation between the BY_PS and BY_ IMF was poor. The analysis of spatial distribution of the BY field along the direction perpendicular to the Current Sheet (CS) plane showed the presence of one of the following configurations, which can be self-consistently generated in the CS: 1) the “quadrupole” distribution of the BY field usually associated with the Hall current system in the vicinity of X-line and 2) the symmetrical “bell-shaped” distribution formed due to the BY amplification near the neutral plane of the CS. Multipoint observations revealed the transient appearance of the “quadrupole” BY distribution during the periods of X-line formation in the mid-tail. This distribution was observed during a few minutes within, at least, 12 RE from the estimated X-line position. On the contrary, the symmetrical “bell-shaped” distribution is more localized in the radial direction and, generally, has a larger observation time (up to ∼10 min). Thus, the internal CS perturbations caused either by the Hall currents related to reconnection or by the peculiarities of the local quasi-adiabatic ion dynamics sufficiently affect the shear BY field existing in the magnetotail due to the partial IMF penetration.

Quantitative analysis of a Hall system in the exhaust of asymmetric magnetic reconnection

AbstractTaking advantage of high‐resolution measurements from the MMS mission, we find evidence for a complete Hall system in the exhaust of asymmetric magnetic reconnection 40 Di downstream of the X line. The investigation of the fine structure of the Hall system reveals that it displays features in the exhaust similar to those reported previously in the ion diffusion region by simulations and observations. This finding confirms the importance of particle‐scale processes in the reconnection exhaust as well. On the magnetospheric side of the exhaust, electrons are strongly accelerated by parallel electric fields. This process significantly contributes to feed the Hall current system, resulting in a nonnegligible Hall magnetic field signature on this side despite an otherwise lower density. Calculation of the induced out‐of‐plane magnetic field by in‐plane currents (based on Biot‐Savart law) provides direct quantitative evidence for the process of Hall magnetic field generation by the Hall current system. A strong normal Hall electric field is present only on the magnetospheric side of the exhaust region, consistent with previous works. Multipoint data analysis shows that the ion pressure gradient in the ion momentum equation produces this Hall electric field. This global pattern of the Hall system can be explained by kinetic Alfvén wave theory.

Birkeland current effects on high‐latitude ground magnetic field perturbations

AbstractMagnetic perturbations on ground at high latitudes are directly associated only with the divergence‐free component of the height‐integrated horizontal ionospheric current, J⊥,df. Here we show how J⊥,df can be expressed as the total horizontal current J⊥ minus its curl‐free component, the latter being completely determined by the global Birkeland current pattern. Thus, in regions where J⊥=0, the global Birkeland current distribution alone determines the local magnetic perturbation. We show with observations from ground and space that in the polar cap, the ground magnetic field perturbations tend to align with the Birkeland current contribution in darkness but not in sunlight. We also show that in sunlight, the magnetic perturbations are typically such that the equivalent overhead current is antiparallel to the convection, indicating that the Hall current system dominates. Thus, the ground magnetic field in the polar cap relates to different current systems in sunlight and in darkness.

Slow magnetosonic waves detected in reconnection diffusion region in the Earth's magnetotail

We studied the ULF waves with a period of 30 s in the ion diffusion region of reconnection observed by Cluster satellites on 21 August 2002. The fast convective tailward ion flow accompanied with a large southward magnetic field component, a strong Hall magnetic field, and a Hall electric field demonstrates that the Cluster satellites are located in the ion diffusion region of reconnection. The analyses of wave properties using minimum variance analysis and timing method indicate that this wave is an obliquely propagating slow magnetosonic wave which is characterized by the anti phase relation between the magnitude of the magnetic field and the ion number density and by a propagation angle of around 130° with respect to the ambient magnetic field. This wave is generated by periodic magnetic reconnection. The periodic oscillations in the By and Bx components are driven by the periodic Hall current system, while the oscillation in the Bz component is driven by quasiperiodic convective tailward ion flows. The slow magnetosonic wave significantly influences the frequency of whistler waves and low hybrid waves excited by magnetic reconnection by changing the magnitude of the local magnetic field and subsequently the electron cyclotron frequency.

Favorable conditions for energetic electron acceleration during magnetic reconnection in the Earth's magnetotail

[1] We have studied favorable conditions for energetic electron acceleration during magnetic reconnection in the Earth's magnetosphere using the Geotail data. We have found the strong energetic electron acceleration in some reconnection events. On the other hand, the other reconnection events show weak electron acceleration. To discuss what reconnection characteristics determine energetic electron acceleration efficiency, we have studied the reconnection characteristics for 10 events in which the Geotail satellite observed the vicinity of the diffusion region. We have classified the relationship between the reconnection characteristics and the electron acceleration efficiency into three types: (1) good correlation (absolute value of correlation coefficient ∣r∣ > 0.6); (2) ambiguous correlation (0.6 > ∣r∣ > 0.3); and (3) no correlation (0.3 > ∣r∣). We found that ion heating, electron heating, current sheet thickness, reconnection electric field, and converging normal electric field could be categorized into good correlation. Ion/electron temperature ratio, total amount of reconnected magnetic energy, and reconnection rate were classified in ambiguous correlation. We could not find any correlation between energetic electron acceleration efficiency and absolute value of outflow velocity, current density parallel to magnetic field (Hall current system), and satellite location in the Earth's magnetosphere. From our analysis we claimed that the electrons are efficiently accelerated in a thin current sheet during fast reconnection events.

Cluster observations of bidirectional beams caused by electron trapping during antiparallel reconnection

So‐called bidirectional electron beams have been observed by a number of spacecraft missions mainly in the inflow of reconnection regions. Here we show that these beam‐like features in the electron distribution function are explained by electron trapping. The trapping is mainly controlled by a positive acceleration potential, Φ∥, which is related to the structure of the parallel electric fields in the vicinity of the reconnection region. Guided by the results of a kinetic simulation, we extend a recent analytical model for the electron distribution function applicable to the inflow region in antiparallel reconnection. The model is successfully compared to data observed by the four Cluster spacecraft inside an active reconnection region. The anisotropy recorded in the electron distributions is consistent with mainly electric trapping of electrons by Φ∥. In the analysis we determine the profiles of Φ∥ along the paths of the Cluster spacecraft during their encounter with a reconnection region. Typical values of eΦ∥ are in excess of 1 keV (much higher than the electron temperature in the ambient lobe plasma) and Φ∥ traps all thermal electrons. This is important for the internal structure of the Hall current system associated with the ion diffusion region because extended trapping significantly alters the electrical and kinematic properties of the electron fluid. Finally, at the boundary between the inflow and exhaust regions the values of eΦ∥ in the inflow region smoothly approach the shoulder energies (up to 15 keV) of the so‐called flat‐top distribution observed in the reconnection exhaust. This suggests that Φ∥ may be important also to the formation of the flat‐top distributions.

Reconnection Hall current system observed in the magnetotail and in the ionosphere

On 07 Sep 2001 around 21:40 UT, Cluster was located 19 RE downtail, and observed earthward fast flows and strong perturbations in the dawn‐dusk component of the magnetic field (BY). The perturbations corresponded to a sheet of tailward field aligned current close to the neutral sheet and another sheet of earthward field aligned current closer to the lobe. At Cluster footpoint, the ionospheric equivalent current density was monitored by the IMAGE magnetometer network. The observed equivalent current pattern is found to be consistent with a longitudinally extended current out of the ionosphere at 67° magnetic latitude and another sheet into the ionosphere 10 km further north. These observations provide evidence that the Hall current system generated in the ion diffusion region of a tail reconnection X‐line, can close in the ionosphere.

The Hall current system revealed as a statistical significant pattern during fast flows

Abstract. We have examined the dawn-dusk component of the magnetic field, BY, in the night side current sheet during fast flows in the neutral sheet. 237 h of Cluster data from the plasma sheet between 2 August 2002 and 2 October 2002 have been analysed. The spatial pattern of BY as a function of the distance from the centre of the current sheet has been estimated by using a Harris current sheet model. We have used the average slopes of these patterns to estimate earthward and tailward currents. For earthward fast flows there is a tailward current in the inner central plasma sheet and an earthward current in the outer central plasma sheet on average. For tailward fast flows the currents are oppositely directed. These observations are interpreted as signatures of Hall currents in the reconnection region or as field aligned currents which are connected with these currents. Although fast flows often are associated with a dawn-dusk current wedge, we believe that we have managed to filter out such currents from our statistical patterns.

Wind observations of low energy particles within a solar wind reconnection region

Abstract. We report characteristics of thermal particle observations during the encounter of the Wind satellite with the separatrix and the outflow domains of a reconnection event on 22 July 1999 in the solar wind. During the studied event the electrostatic analyzers on Wind were transmitting three-dimensional electron and proton distributions in a burst mode every 3 s, the spin period of the spacecraft. The event was associated with a magnetic shear angle of 114° and a large guide magnetic field. The observations suggest that Wind crossed the separatrix and outflow regions about a thousand of ion skin depths from the X-line. At the leading separator boundary, a strong proton beam was identified that originated from the direction of the X-line. In the separatrix and the outflow regions, the phase space distributions of thermal electrons displayed field aligned bidirectional anisotropy. During the crossings of the current sheets bounding the outflow region, we identified two adjacent layers in which the dominant thermal electron flows were towards the X-line at the inner edges of the current sheets and away from the X-line at the outer edges. Interestingly, simulation studies and observations in the Earth's magnetosphere have revealed that the electron flows are reversed, consistent with the Hall current system.

Cluster observation of plasma flow reversal in the magnetotail during a substorm

Abstract. We investigate in detail a reversal of plasma flow from tailward to earthward detected by Cluster at the downstream distance of ~19 RE in the midnight sector of the magnetotail on 22 August 2001. This flow reversal was accompanied by a sign reversal of the Bz component and occurred during the late substorm expansion phase as revealed by simultaneous global view of auroral activity from IMAGE. We examine the associated Hall current system signature, current density, electric field, Lorentz force, and current dissipation/dynamo term, the last two parameters being new features that have not been studied previously for plasma flow reversals. It is found that (1) there was no clear quadrupole Hall current system signature organized by the flow reversal time, (2) the x-component of the Lorentz force did not change sign while the other two did, (3) the timing sequence of flow reversal from the Cluster configuration did not match tailward motion of a single plasma flow source, (4) the electric field was occasionally dawnward, producing a dynamo effect, and (5) the electric field was occasionally larger at the high-latitude plasma sheet than near the neutral sheet. These observations are consistent with the current disruption model for substorms in which these disturbances are due to shifting dominance of multiple current disruption sites and turbulence at the observing location.

Detailed analysis of low-energy electron streaming in the near-Earth neutral line region during a substorm

A synaptic view of Cluster FGM, CIS, EDI, and PEACE observations of the near-Earth neutral line region during 1800–1830 UT on 13 September 2002 is discussed. Associated with the flow reversal and a thin cross-tail current sheet, a clear signature of a quadrupolar magnetic field is observed, which indicates the formation of the Hall current system around the X line in the large-scale picture. Associated with this magnetic field structure, low energy field-aligned electron beams mainly directed toward the X line are observed. High resolution data of FGM and EDI further show the fine structure of the low energy electron beams near the plasma sheet–lobe boundary. The beam becomes much more intense in the narrower region associated with the small magnetic field disturbances. In some cases, complicated bi-directional or oppositely directed beams are observed.

Cluster electron observations of the separatrix layer during traveling compression regions

We present Cluster 4‐point observations of electrons during traveling compression regions (TCRs) on 19 September 2001. The electron and ∣B∣ signatures vary with distance from the plasma sheet, confirming that transient plasma sheet bulges propagate past Cluster. TCRs with ∣B∣ increases have either no electron signature, or unidirectional ∼1 keV electrons at the plasma sheet edge. However, spacecraft initially near the plasma sheet edge are engulfed within the bulge and observe a diamagnetic reduction in ∣B∣. In cases where the underlying plasma sheet bulge moves earthward, electrons at the plasma sheet edge stream tailward. We suggest this represents either a remote observation of electrons closing the Hall current system in an ion diffusion region located farther tailward, or the outflow jets along the separatrix formed by a second neutral line located farther earthward of the spacecraft. The latter case implies the simultaneous action of multiple X‐lines in the near‐Earth tail.

Current sheet structure around the near‐Earth neutral line observed by Geotail

The current sheet structure around the near‐Earth neutral line during substorms is investigated using ion and electron velocity moments directly obtained by the Geotail satellite. The intense cross‐tail current sheet is found to be thinner closer to the X line and becomes as thin as 500 km. This is less than the ion inertial length (720 km), where ions are decoupled from electrons. The current sheet is found to be bifurcated from the ion diffusion region to the off‐neutral sheet and forms a double‐peaked current sheet away from the X line. Fast dawnward moving electrons with the strong ambipolar electric field toward the neutral sheet (Ens) carry the intense electron‐dominated current, and such Ens is larger than Ey in the bursty bulk flows. So‐called Hall current system into/out of the X line caused by the large separation between ion and electron motions is fully identified. The Hall current system is observed around the neutral sheet in the vicinity of the X line and bifurcates to the north/south boundary regions as the distance from the X line increases. There, the inward and outward currents form antiparallel current layers, whose boundaries are identified as 0.8 ∼ 0.85 of lobe magnetic field intensity.

Ionospheric currents estimated simultaneously from CHAMP satelliteand IMAGE ground-based magnetic field measurements: a statisticalstudy at auroral latitudes

Abstract. One important contribution to the magnetic field measured at satellite altitude and at ground level comes from the external currents. We used the total field data sampled by the Overhauser Magnetometer on CHAMP and the horizontal magnetic field measurements of the IMAGE ground-based magnetometer network to study the ionospheric Hall current system in the auroral regions. For the CHAMP data a current model consisting of a series of lines and placed at a height of 110km is fitted to the magnetic field signature sampled on the passage across the polar region. The derived current distributions depend, among others, on season and on the local time of the satellite track. At dawn/dusk the auroral electrojets can be detected most clearly in the auroral regions. Their intensity and location are evidently correlated with the A E activity index. For a period of almost two years the results obtained from space and the currents determined from ground-based observations are studied. For the full IMAGE station array a newly-developed method of spherical elementary current systems (SECS) is employed to compute the 2-D equivalent current distribution, which gives a detailed picture of an area covering latitudes 60° – 80° N and 10° – 30° E in the auroral region. Generally, the current estimates from satellite and ground are in good agreement. The results of this survey clearly show the average dependence of the auroral electrojet on season and local time. This is particularly true during periods of increased auroral activity. The correlation coefficient of the results is close to one in the region of sizeable ionospheric current densities. Also the ratio of the current densities, as determined from above and below the ionosphere, is close to unity. It is the first time that the method of Hall current estimate from a satellite has been validated quantitatively by ground-based observations. Among others, this result is of interest for magnetic main field modelling, since it demonstrates that ground-based observations can be used to predict electrojet signatures in satellite magnetic field scalar data. Key words. Ionosphere (auroral Ionosphere; electric fields and currents; ionosphere-magnetosphere interactions)

The role of upper hybrid waves in magnetic reconnection

Wind observations of upper hybrid (UH) waves adjacent to a magnetic X‐line are compared with simultaneous Wind electron observations to clarify wave‐electron interactions. Electron beams and “football” (T∥ &gt; T⟂) distributions accompany the UH waves. The energy density of the inward‐directed beams, at 10−14 J/m3, is sufficient to supply the free energy for the waves. The electron beams may be part of the larger Hall current system, and the UH wave/electron instability represent a Hall current dissipation process (i.e., an effective resistance) which removes energy as the currents travels along the separatrix.

Structure of the Hall current system in the vicinity of the magnetic reconnection site

The spacecraft Geotail has observed the Hall current system in the vicinity of the magnetic reconnection site of the near‐Earth magnetotail for substorm onsets. In the outermost region near the plasma sheet/tail lobe boundary, field‐aligned currents flow out of the magnetic reconnection site. In the adjacent region, just inside the outflowing current layer, field‐aligned currents flow into the magnetic reconnection site. Hence, the Hall current circuit forms a thin double‐sheet structure near the separatrix layer.