Average Reconnection Rate Research Articles

An Encounter With the Ion and Electron Diffusion Regions at a Flapping and Twisted Tail Current Sheet

AbstractWe analyze data returned by the Magnetospheric Multiscale mission (MMS) constellation during a rapid (∼1.5 s) traversal of a flapping and reconnecting current sheet (CS) in the near‐Earth magnetotail (X ∼−20 RE). The CS was highly tilted, with its normal pointing strongly duskward. Its extreme thinness was confirmed by a curvature analysis of the magnetic field lines. The event was associated with a guide field of 8% of the reconnecting components. From the pitch angle distributions of low‐energy electrons we infer a crossing earthward of the X‐line. Traveling practically normal to the CS, MMS encountered an ion diffusion region (IDR) in which was embedded an electron diffusion region (EDR). IDR signatures included breaking of the ion frozen‐in condition in the presence of Hall B and E fields. EDR signatures included a strong out‐of‐plane current associated with a superAlfvénic electron jet, positive energy transfer, and a temperature anisotropy (Te∥ > Te⊥) which disappeared at the field reversal. Derived scale sizes normal to the CS are: ∼6.9 de (EDR) and ∼0.4 di (IDR; 40 and 100 km). We estimate the average dimensionless reconnection rate as 0.077 ± 0.050. The observations and inferences are supported by particle‐in‐cell (PIC) numerical simulations. We find very good agreement in the reconnection rates. We also discuss the effects of asymmetries in the density, temperature and magnetic field strength on the Hall fields and length of the outflow jets. The event is associated with a substorm onset which began 7 min after the MMS observations.

Scaling of Magnetic Reconnection With a Limited X‐Line Extent

AbstractContrary to all the 2‐D models, where the reconnection x‐line extent is infinitely long, we study magnetic reconnection in the opposite limit. An internal x‐line asymmetry along the current direction develops because of the transport of reconnected magnetic flux by electrons beneath the ion kinetic scale, resulting in a suppression region identified in Liu et al. (2019, https://doi.org/10.1029/2019JA026539). In this letter, we incorporate the length scale of this suppression region ≃10di to quantitatively model the reduction of the reconnection rate and the maximum outflow speed observed in the short x‐line limit. The average reconnection rate drops because of the limited active region (where the current sheet thins down to the electron inertial scale) within an x‐line. The outflow speed reduction correlates with the decrease of the J × B force, that can be modeled by the phase shift between the J and B profiles, also as a consequence of the flux transport.

Substorm‐Related Near‐Earth Reconnection Surge: Combining Telescopic and Microscopic Views

AbstractA strong ~11‐min‐long surge of the lobe reconnection was observed during a substorm on the tailward side of the near‐Earth neutral line. In the southern lobe near the reconnection separatrix the MMS spacecraft observed short‐duration earthward electron beams providing the local Hall current, tailward propagating Alfven wave (AW) bursts with Poynting flux up to 10−4 W/m2, and large‐amplitude E field spikes (e‐holes) and low hybrid waves. The reconnection surge was accompanied by substorm current wedge formation and fast poleward expansion of auroral bulge‐related westward electrojet in the conjugate ionosphere. During its meridional crossing above the expanding bulge the Metop‐2 spacecraft observed an intense energetic precipitation spike near the expected X line foot point and confirmed the dipolarized character of magnetic field lines inside of the bulge. Globally the observed average reconnection rate (<Ey > ~3.3 mV/m) was sufficient to produce the magnetic flux increase in the bulge, associated with observed fast poleward expansion (about 6° latitude in 5 min).

Properties of Magnetic Reconnection and FTEs on the Dayside Magnetopause With and Without Positive IMF Bx Component During Southward IMF

AbstractThis paper describes properties and behavior of magnetic reconnection and flux transfer events (FTEs) on the dayside magnetopause using the global hybrid‐Vlasov code Vlasiator. We investigate two simulation runs with and without a sunward (positive) Bx component of the interplanetary magnetic field (IMF) when the IMF is southward. The runs are two‐dimensional in real space in the noon‐midnight meridional (polar) plane and three‐dimensional in velocity space. Solar wind input parameters are identical in the two simulations with the exception that the IMF is purely southward in one but tilted 45° toward the Sun in the other. In the purely southward case (i.e., without Bx) the magnitude of the magnetosheath magnetic field component tangential to the magnetopause is larger than in the run with a sunward tilt. This is because the shock normal is perpendicular to the IMF at the equatorial plane, whereas in the other run the shock configuration is oblique and a smaller fraction of the total IMF strength is compressed at the shock crossing. Hence, the measured average and maximum reconnection rate are larger in the purely southward run. The run with tilted IMF also exhibits a north‐south asymmetry in the tangential magnetic field caused by the different angle between the IMF and the bow shock normal north and south of the equator. Greater north‐south asymmetries are seen in the FTE occurrence rate, size, and velocity as well; FTEs moving toward the Southern Hemisphere are larger in size and observed less frequently than FTEs in the Northern Hemisphere.

On the role of system size in Hall MHD magnetic reconnection

We study the effects of the Hall electric field on magnetic island coalescence in the large island limit and find evidence for both an elongated electron current sheet layer with a Sweet-Parker-like reconnection rate and a collapsed, Petschek-like electron sheet with a peak reconnection rate approaching the 0.1vAB0 Hall MHD rate. The state observed in our simulations appears to depend on the grid scale. Furthermore, even at the largest system sizes, we find that flux-pileup effects cause the islands to “bounce” despite the presence of a collapsed current sheet allowing fast instantaneous reconnection. The average reconnection rate in the large island limit is slow though the peak reconnection rate is fast.

Using radiative energy losses to constrain the magnetization and magnetic reconnection rate at the base of black hole jets

We calculate the severe radiative energy losses which occur at the base of black hole jets using a relativistic fluid jet model, including in-situ acceleration of non-thermal leptons by magnetic reconnection. Our results demonstrate that including a self-consistent treatment of radiative energy losses is necessary to perform accurate MHD simulations of powerful jets and that jet spectra calculated via post-processing are liable to vastly overestimate the amount of non-thermal emission. If no more than 95% of the initial total jet power is radiated away by the plasma travels as it travels along the length of the jet, we can place a lower bound on the magnetisation of the jet plasma at the base of the jet. For typical powerful jets, we find that the plasma at the jet base is required to be highly magnetised, with at least 10,000 times more energy contained in magnetic fields than in non-thermal leptons. Using a simple power-law model of magnetic reconnection, motivated by simulations of collisionless reconnection, we determine the allowed range of the large-scale average reconnection rate along the jet, by restricting the total radiative energy losses incurred and the distance at which the jet first comes into equipartition. We calculate analytic expressions for the cumulative radiative energy losses due to synchrotron and inverse-Compton emission along jets, and derive analytic formulae for the constraint on the initial magnetisation.

Magnetic flux pileup and plasma depletion in Mercury's subsolar magnetosheath

AbstractMeasurements from the Fast Imaging Plasma Spectrometer (FIPS) and Magnetometer (MAG) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft during 40 orbits about Mercury are used to characterize the plasma depletion layer just exterior to the planet's dayside magnetopause. A plasma depletion layer forms at Mercury as a result of piled‐up magnetic flux that is draped around the magnetosphere. The low average upstream Alfvénic Mach number (MA ~3–5) in the solar wind at Mercury often results in large‐scale plasma depletion in the magnetosheath between the subsolar magnetopause and the bow shock. Flux pileup is observed to occur downstream under both quasi‐perpendicular and quasi‐parallel shock geometries for all orientations of the interplanetary magnetic field (IMF). Furthermore, little to no plasma depletion is seen during some periods with stable northward IMF. The consistently low value of plasma β, the ratio of plasma pressure to magnetic pressure, at the magnetopause associated with the low average upstream MA is believed to be the cause for the high average reconnection rate at Mercury, reported to be nearly 3 times that observed at Earth. Finally, a characteristic depletion length outward from the subsolar magnetopause of ~300 km is found for Mercury. This value scales among planetary bodies as the average standoff distance of the magnetopause.

Plasmoid instability in high-Lundquist-number magnetic reconnection

Our understanding of magnetic reconnection in resistive magnetohydrodynamics has gone through a fundamental change in recent years. The conventional wisdom is that magnetic reconnection mediated by resistivity is slow in laminar high Lundquist (S) plasmas, constrained by the scaling of the reconnection rate predicted by Sweet-Parker theory. However, recent studies have shown that when S exceeds a critical value ∼104, the Sweet-Parker current sheet is unstable to a super-Alfvénic plasmoid instability, with a linear growth rate that scales as S1/4. In the fully developed statistical steady state of two-dimensional resistive magnetohydrodynamic simulations, the normalized average reconnection rate is approximately 0.01, nearly independent of S, and the distribution function f(ψ) of plasmoid magnetic flux ψ follows a power law f(ψ)∼ψ−1. When Hall effects are included, the plasmoid instability may trigger onset of Hall reconnection even when the conventional criterion for onset is not satisfied. The rich variety of possible reconnection dynamics is organized in the framework of a phase diagram.

Multispacecraft observation of magnetic cloud erosion by magnetic reconnection during propagation

During propagation, Magnetic Clouds (MC) interact with their environment and, in particular, may reconnect with the solar wind around it, eroding away part of its initial magnetic flux. Here we quantitatively analyze such an interaction using combined, multipoint observations of the same MC flux rope by STEREO A, B, ACE, WIND and THEMIS on November 19–20, 2007. Observation of azimuthal magnetic flux imbalance inside a MC flux rope has been argued to stem from erosion due to magnetic reconnection at its front boundary. The present study adds to such analysis a large set of signatures expected from this erosion process. (1) Comparison of azimuthal flux imbalance for the same MC at widely separated points precludes the crossing of the MC leg as a source of bias in flux imbalance estimates. (2) The use of different methods, associated errors and parametric analyses show that only an unexpectedly large error in MC axis orientation could explain the azimuthal flux imbalance. (3) Reconnection signatures are observed at the MC front at all spacecraft, consistent with an ongoing erosion process. (4) Signatures in suprathermal electrons suggest that the trailing part of the MC has a different large‐scale magnetic topology, as expected. The azimuthal magnetic flux erosion estimated at ACE and STEREO A corresponds respectively to 44% and 49% of the inferred initial azimuthal magnetic flux before MC erosion upon propagation. The corresponding average reconnection rate during transit is estimated to be in the range 0.12–0.22 mV/m, suggesting most of the erosion occurs in the inner parts of the heliosphere. Future studies ought to quantify the influence of such an erosion process on geo‐effectiveness.

Effects of plasma β on the plasmoid instability

The effects of the initial upstream plasma β on the plasmoid instability are studied via two-dimensional resistive magnetohydrodynamic simulations. For cases with nonuniform β dependent initial plasma mass density and uniform temperature, our numerical results indicate that the critical Lundquist number for onset of the plasmoid instability depends on the initial plasma β. The critical Lundquist number is approximately 2000−3000 for β=50 and is 8000−10 000 for β=0.2. The higher the β, the smaller the critical Lundquist number is. Similar to previous studies of high-β systems, the average reconnection rate in low β systems is found to become weakly dependent on the Lundquist number in the plasmoid-unstable regime. However, the average reconnection rate, normalized to the asymptotic value of upstream BVA, is lower in a low β system than that in a high β system. The magnetic energy spectral index, which characterizes fragmentation of the reconnection layer, is approximately two and is insensitive to β in the high-Lunquist number regime. It is also found that the magnetic reconnection rate becomes similar for different β cases, if the initial force-balance is provided by temperature gradient instead of density gradient. Therefore, it is concluded that the β-dependence mentioned above may be largely attributed to the density variation.

Magnetic reconnection during collisionless, stressed, X-point collapse using particle-in-cell simulation

Magnetic reconnection during collisionless, stressed, X-point collapse was studied using a kinetic, 2.5D, fully electromagnetic, relativistic particle-in-cell numerical code. Two cases of weakly and strongly stressed X-point collapse were considered. Here the descriptors “weakly” and “strongly” refer to 20% and 124% unidirectional spatial compression of the X-point, respectively. In the weakly stressed case, the reconnection rate, defined as the out-of-plane electric field in the X-point (the magnetic null) normalized by the product of external magnetic field and Alfvén speeds, peaks at 0.11, with its average over 1.25 Alfvén times being 0.04. During the peak of the reconnection, electron inflow into the current sheet is mostly concentrated along the separatrices until they deflect from the current sheet on the scale of electron skin depth, with the electron outflow speeds being of the order of the external Alfvén speed. Ion inflow starts to deflect from the current sheet on the ion skin depth scale with the outflow speeds about four times smaller than that of electrons. Electron energy distribution in the current sheet, at the high-energy end of the spectrum, shows a power-law distribution with the index varying in time, attaining a maximal value of −4.1 at the final simulation time step (1.25 Alfvén times). In the strongly stressed case, the magnetic reconnection peak occurs 3.8 times faster and is more efficient. The peak reconnection rate now attains the value 2.5, with the average reconnection rate over 1.25 Alfvén times being 0.5. Plasma inflow into the current sheet is perpendicular to it, with the electron outflow seeds reaching 1.4 Alfvén external Mach number and ions again being about four times slower than electrons. The power-law energy spectrum for the electrons in the current sheet attains now a steeper index of −5.5, a value close to those observed near the X-type region in the Earth’s magnetotail. Within about one Alfvén time, 2% and 20% of the initial magnetic energy is converted into heat and accelerated particle energy in the case of weak and strong stress, respectively. In both cases, during the peak of the reconnection, the quadruple out-of-plane magnetic field is generated, hinting possibly at the Hall regime of the reconnection. These results strongly suggest the importance of the collisionless, stressed X-point collapse as an efficient mechanism of converting magnetic energy into heat and superthermal particle energy.

Low‐latitude dayside magnetopause and boundary layer for high magnetic shear: 2. Occurrence of magnetic reconnection

Quantitative comparisons of the flow velocity change across the magnetopause (MP) with the prediction from local tangential stress balance in a one‐dimensional time‐stationary rotational discontinuity, that is, with the Walén relation, have been performed on 69 Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) low‐latitude (<30°) dayside (0800–1600 LT) high‐shear (>45°) MP crossings. It is found that in 61% of the crossings the observed flow changes agree with the prediction to better than 50%. No dependence of the occurrence of reconnection flows on local magnetic shear, local time/latitude, local tangential magnetosheath flow speed, and local magnetosheath Alfvén Mach number is found. We confirm an earlier result that the agreement with the Walén relation becomes worse with increasing magnetosheath plasma β (β is the ratio of plasma pressure to magnetic pressure) but find that the velocity change itself, predicted by the Walén relation, decreases with increasing β. Moreover, the motion and thickness of the boundary also depend on β: the higher the β value, the faster the speed and the smaller the thickness. These effects combine to make velocity changes in high β events more difficult to measure accurately, which may contribute to the poor agreement with the Walén relation in these events. The 42 events which exhibit plasma flows in reasonable agreement with the Walén relation include 21 cases where the flow direction is inconsistent with a single X line hinged at the subsolar point. The discrepancies between the former result arid dayside X line locations reported earlier may be due to a bias in selection of reconnection events in earlier studies. An average (dimensionless) reconnection rate that is substantially lower than 0.1 is inferred for the 42 events.

Reconnection of the geomagnetic tail deduced from solar-particle observations

It is shown that particle intensity structures bounded by magnetic discontinuities and convected past the earth with the solar wind favor an open-tail interpretation. By using polar-cap measurements of these discontinuities expressed in a new coordinate system described by Morfill (1972) the average reconnection rate between the interplanetary and magnetospheric field lines is calculated. Furthermore, the cross-tail electric field, the mean free path in the tail, and the transverse component of power in the fluctuations of the tail magnetic field at the particle gyrofrequency are derived.