Reconnection Events Research Articles

Hybrid magnetic structures around spinning black holes connected to a surrounding accretion disk.

The hot accretion flow around Kerr black holes is strongly magnetized. Magnetic field loops sustained by a surrounding accretion disk can close within the event horizon. We performed particle-in-cell simulations in Kerr metric to capture the dynamics of the electromagnetic field and of the ambient collisionless plasma in this coupled configuration. We find that a hybrid magnetic topology develops with a closed magnetosphere co-existing with open field lines threading the horizon reminiscent of the Blandford-Znajek solution. Further in the disk, highly inclined open magnetic field lines can launch a magnetically-driven wind. While the plasma is essentially force-free, a current sheet forms above the disk where magnetic reconnection produces macroscopic plasmoids and accelerates particles up to relativistic Lorentz factors. A highly dynamic Y-point forms on the furthest closed magnetic field line, with episodic reconnection events responsible for transient synchrotron emission and coronal heating.

Solar Wind Reconnection Exhausts in the Inner Heliosphere Observed by Helios and Detected via Machine Learning

Abstract Reconnecting current sheets in the solar wind play an important role in the dynamics of the heliosphere and offer an opportunity to study magnetic reconnection exhausts under a wide variety of inflow and magnetic shear conditions. However, progress in understanding reconnection can be frustrated by the difficulty of finding events in long time-series data. Here we describe a new method to detect magnetic reconnection events in the solar wind based on machine learning, and apply it to Helios data in the inner heliosphere. The method searches for known solar wind reconnection exhaust features, and parameters in the algorithm are optimized to maximize the Matthews Correlation Coefficient using a training set of events and non-events. Applied to the whole Helios data set, the trained algorithm generated a candidate set of events that were subsequently verified by hand, resulting in a database of 88 events. This approach offers a significant reduction in construction time for event databases compared to purely manual approaches. The database contains events covering a range of heliospheric distances from ∼0.3 to ∼1 au, and a wide variety of magnetic shear angles, but is limited by the relatively coarse time resolution of the Helios data. Analysis of these events suggests that proton heating by reconnection in the inner heliosphere depends on the available magnetic energy in a manner consistent with observations in other regimes such as at the Earth’s magnetopause, suggesting this may be a universal feature of reconnection.

Latitudinal Dependence of the Kelvin‐Helmholtz Instability and Beta Dependence of Vortex‐Induced High‐Guide Field Magnetic Reconnection

AbstractWe investigate both large‐ and small‐scale properties of a Kelvin‐Helmholtz (KH) event at the dusk flank magnetopause using Magnetospheric Multiscale observations on 8 September 2015. We first use two types of 3‐D simulations (global and local) to demonstrate that Magnetospheric Multiscale is close to the most KH unstable region, and so the occurrence of vortex‐induced reconnection may be expected. Because they produce low‐shear current sheets, KH vortices constitute a perfect laboratory to investigate magnetic reconnection with large guide field and low asymmetry. Recent works suggest that magnetic reconnection may be suppressed when a current sheet combines large guide field and pressure gradient (which induces a diamagnetic drift). We thus perform a statistical analysis of high‐resolution data for the 69 KH‐induced low‐shear magnetic reconnection events observed on that day. We find that the suppression mechanism is not at work for most of the observed reconnecting current sheets, as predicted, but we also find that almost all nonreconnecting current sheets should be reconnecting according to this model. This confirms the fact that the model provides a necessary but not sufficient condition for reconnection to occur. Finally, based on the same data set, we study the latitudinal distribution of these magnetic reconnection events combined with global magnetospheric modeling. We find that reconnection associated with KH vortices occurs over a significant range of latitudes at the flank magnetopause. It is not confined to the plane where the growth rate is maximum, in agreement with recent 3‐D simulations.

The phenomenology of reconnection events in the reversed field pinch

The phenomenology of reconnection events, associated to relaxations in high-current (∼1.5 MA) plasmas of the reversed-field pinch device RFX–mod, is shown. Each relaxation event can be described as a series of stages starting in the core and propagating towards the plasma edge. In an initial stage (trigger), the evolution of the q profile brings resonant layers closer together, allowing for an interaction of the current sheets associated to tearing modes at the respective rational values of q. The phase locking of the resonant modes then initiates the reconnection process that, once started, changes the magnetic topology bringing the initially helical state to a more chaotic configuration.

Maximum amplification of enstrophy in three-dimensional Navier–Stokes flows

This investigation concerns a systematic search for potentially singular behavior in 3D Navier-Stokes flows. Enstrophy serves as a convenient indicator of the regularity of solutions to the Navier Stokes system --- as long as this quantity remains finite, the solutions are guaranteed to be smooth and satisfy the equations in the classical (pointwise) sense. However, there are no estimates available with finite a priori bounds on the growth of enstrophy and hence the regularity problem for the 3D Navier-Stokes system remains open. In order to quantify the maximum possible growth of enstrophy, we consider a family of PDE optimization problems in which initial conditions with prescribed enstrophy $\mathcal{E}_0$ are sought such that the enstrophy in the resulting Navier-Stokes flow is maximized at some time $T$. Such problems are solved computationally using a large-scale adjoint-based gradient approach derived in the continuous setting. By solving these problems for a broad range of values of $\mathcal{E}_0$ and $T$, we demonstrate that the maximum growth of enstrophy is in fact finite and scales in proportion to $\mathcal{E}_0^{3/2}$ as $\mathcal{E}_0$ becomes large. Thus, in such worst-case scenario the enstrophy still remains bounded for all times and there is no evidence for formation of singularity in finite time. We also analyze properties of the Navier-Stokes flows leading to the extreme enstrophy values and show that this behavior is realized by a series of vortex reconnection events.

Formation of Macroscale Flux Transfer Events at Mercury

Abstract Flux transfer events (FTEs) are magnetic flux ropes that are produced via magnetic reconnection at the planetary magnetopause where the solar wind directly interacts with the magnetosphere. Previous observations show that FTEs with a duration of several seconds, corresponding to a spatial scale of ∼0.5–1 R M, can occur at Mercury. However, the formation of these macroscale FTEs at a small dimensional magnetopause with a radius of ∼1.5 R M remains unclear. Here, we report the observations of active magnetic reconnection events at Mercury’s magnetopause by the MESSENGER spacecraft. The reconnection process is dominated by the formation of a series of multi-scale FTEs. Ion-scale flux ropes, typically with durations of ∼1 s or less, may be produced by the tearing instability in the thin current sheet near the subsolar position. Moreover, the commonly observed macroscale FTEs consist of three to tens of successive small-scale FTEs. We propose that macroscale FTEs at Mercury are generated by the interaction and merging of multiple ion-scale flux ropes, probably through two or more steps. This is distinct from the formation of typical FTEs, mainly between a pair of X-lines, at Earth’s magnetopause. Thus, the formation and evolution of FTEs may differ among planetary magnetospheres with a vast range of scale sizes. We further conclude that Mercury’s magnetopause is a natural plasma laboratory to study flux rope dynamics and evolution for the upcoming Bepi-Colombo mission.

Electron-only Reconnection in Kinetic-Alfvén Turbulence

Abstract We study numerically small-scale reconnection events in kinetic, low-frequency, quasi-2D turbulence (termed kinetic-Alfvén turbulence). Using 2D particle-in-cell simulations, we demonstrate that such turbulence generates reconnection structures where the electron dynamics do not couple to the ions, similarly to the electron-only reconnection events recently detected in the Earth’s magnetosheath by Phan et al. Electron-only reconnection is thus an inherent property of kinetic-Alfvén turbulence, where the electron current sheets have limited anisotropy and, as a result, their sizes are smaller than the ion inertial scale. The reconnection rate of such electron-only events is found to be close to 0.1.

Multiple X-line Reconnection Observed in Mercury’s Magnetotail Driven by an Interplanetary Coronal Mass Ejection

Abstract How magnetic reconnection drives Mercury’s magnetospheric dynamics under extreme solar wind conditions is not well understood. Here we report MESSENGER observations of an active reconnection event in Mercury’s magnetotail driven by an interplanetary coronal mass ejection on 2011 November 23. The primary Hall magnetic field, sequential passage of X-lines with Hall field perturbations, and flux ropes (FRs) provide unambiguous evidence of multiple X-line reconnection in an unstable ion diffusion region. In addition, large FRs consisting of multiple successive small-scale FRs are ejected tailward at quasi-periodic intervals of ∼1 minute, which is comparable to the Dungey cycle time. We propose that these large FRs are generated by the interaction and coalescence of multiple ion-scale FRs. This is distinct from the commonly accepted Earth-like substorm process where plasmoids are created by widely separated X-lines in the magnetotail. These observations suggest that during extreme solar wind conditions multiple X-line reconnection may dominate the tail reconnection process and control the global dynamics of Mercury’s magnetosphere.

Magnetic reconnection in partially ionized plasmas.

Magnetic reconnection has been intensively studied in fully ionized plasmas. However, plasmas are often partially ionized in astrophysical environments. The interactions between the neutral particles and ionized plasmas might strongly affect the reconnection mechanisms. We review magnetic reconnection in partially ionized plasmas in different environments from theoretical, numerical, observational and experimental points of view. We focus on mechanisms which make magnetic reconnection fast enough to compare with observations, especially on the reconnection events in the low solar atmosphere. The heating mechanisms and the related observational evidence of the reconnection process in the partially ionized low solar atmosphere are also discussed. We describe magnetic reconnection in weakly ionized astrophysical environments, including the interstellar medium and protostellar discs. We present recent achievements about fast reconnection in laboratory experiments for partially ionized plasmas.

How Does Magnetic Reconnection Drive the Early-stage Evolution of Coronal Mass Ejections?

Abstract Theoretically, coronal mass ejection (CME) kinematics are related to magnetic reconnection processes in the solar corona. However, the current quantitative understanding of this relationship is based on the analysis of only a handful of events. Here we report a statistical study of 60 CME-flare events from 2010 August to 2013 December. We investigate kinematic properties of CMEs and magnetic reconnection in the low corona during the early phase of the eruptions, by combining limb observations from STEREO with simultaneous on-disk views from SDO. For a subset of 42 events with reconnection rate evaluated by the magnetic fluxes swept by the flare ribbons on the solar disk observed from SDO, we find a strong correlation between the peak CME acceleration and the peak reconnection rate. Also, the maximum velocities of relatively fast CMEs (≳600 km s−1) are positively correlated with the reconnection flux, but no such correlation is found for slow CMEs. A time-lagged correlation analysis suggests that the distribution of the time lag of CME acceleration relative to reconnection rate exhibits three peaks, approximately 10 minutes apart, and on average, acceleration-led events have smaller reconnection rates. We further compare the CME total mechanical energy with the estimated energy in the current sheet. The comparison suggests that, for small-flare events, reconnection in the current sheet alone is insufficient to fuel CMEs. Results from this study suggest that flare reconnection may dominate the acceleration of fast CMEs, but for events of slow CMEs and weak reconnection, other mechanisms may be more important.

Magnetospheric Multiscale (MMS) Observations of Magnetic Reconnection in Foreshock Transients

AbstractMagnetic reconnection is a fundamental process of energy conversion in plasmas between electromagnetic fields and particles. Magnetic reconnection has been observed directly in a variety of plasmas in the solar wind and Earth's magnetosphere. Most recently, electron magnetic reconnection without ion coupling was observed for the first time in the turbulent magnetosheath and within the transition region of Earth's bow shock. In the ion foreshock upstream of Earth's bow shock, there may also be magnetic reconnection especially around foreshock transients that are very turbulent and dynamic. With observations from the National Aeronautics and Space Administration's Magnetospheric Multiscale mission inside foreshock transients, we report two events of magnetic reconnection with and without a strong guide field, respectively. In both events, a super‐ion‐Alfvénic electron jet was observed within a current sheet with thickness less than or comparable to one ion inertial length. In both events, energy was converted from the magnetic field to electrons, manifested as an increase in electron temperature. Weak or no ion coupling was observed in either event. Results from particle‐in‐cell simulations of magnetic reconnection with and without a strong guide field are qualitatively consistent with observations. Our results imply that magnetic reconnection is another electron acceleration/heating process inside foreshock transients and could play an important role in shock dynamics.

Concurrent Observations Of Magnetic Reconnection From Cluster, IMAGE and SuperDARN: A Comparison Of Reconnection Rates And Energy Conversion

AbstractAuroral emissions reflect energy dissipation in the atmosphere, while the energy mostly originates from the magnetospheric dynamics of the planet. In the Earth's magnetosphere, the interaction between the solar wind and the terrestrial magnetic field produces magnetic reconnection ultimately causing the appearance of auroras in the polar regions. In this study, we revisit two reconnection events previously identified using ESA Cluster observations during conditions of moderate geomagnetic activity, in the magnetotail. We compare these in situ detections with simultaneous global auroral images from the NASA‐IMAGE satellite and ionospheric convection measurements from SuperDARN. We show for the first time a direct correspondence between the electric voltage estimated at ionospheric altitude and the reconnection rate at the reconnection site. We compute the total auroral precipitation power and compare it to the energy released from the reconnection site. We deduce that the precipitated electron power of the aurora and the electron energy deposition rate from the reconnection site are of the same order of magnitude, which means that the thermal energy released by reconnection is primarily transferred to the ionosphere and contributes to the auroral activity.

Favorable Conditions for Magnetic Reconnection at Ganymede's Upstream Magnetopause

AbstractGanymede is the only Solar System moon known to generate a permanent magnetic field. Jovian plasma motions around Ganymede create an upstream magnetopause, where energy flows are thought to be driven by magnetic reconnection. Simulations indicate Ganymedean reconnection events may be transient, but the nature of magnetopause reconnection at Ganymede remains poorly understood, requiring an assessment of reconnection onset theory. We present an analytical model of steady‐state conditions at Ganymede's magnetopause, from which the first Ganymedean reconnection onset assessment is conducted. We find that reconnection may occur wherever Ganymede's closed magnetic field encounters Jupiter's ambient magnetic field, regardless of variations in magnetopause conditions. Unrestricted reconnection onset highlights possibilities for multiple X lines or widespread transient reconnection at Ganymede. The reconnection rate is controlled by the ambient Jovian field orientation and hence driven by Jupiter's rotation. Future progress on this topic is highly relevant for the JUpiter ICy moon Explorer mission.

Magnetotail Reconnection at Jupiter: A Survey of Juno Magnetic Field Observations.

At Jupiter, tail reconnection is thought to be driven by an internal mass loading and release process called the Vasyliunas cycle. Galileo data have shown hundreds of reconnection events occurring in Jupiter's magnetotail. Here we present a survey of reconnection events observed by Juno during its first 16 orbits of Jupiter (July 2016-October 2018). The events are identified using Juno magnetic field data, which facilitates comparison to the Vogt et al. (2010, https://doi.org/10.1029/2009JA015098) survey of reconnection events from Galileo magnetometer data, but we present data from Juno's other particle and fields instruments for context. We searched for field dipolarizations or reversals and found 232 reconnection events in the Juno data, most of which featured an increase in |B θ |, the magnetic field meridional component, by a factor of 3 over background values. We found that most properties of the Juno reconnection events, like their spatial distribution and duration, are comparable to Galileo, including the presence of a ~3-day quasi-periodicity in the recurrence of Juno tail reconnection events and in Juno JEDI, JADE, and Waves data. However, unlike with Galileo we were unable to clearly define a statistical x-line separating planetward and tailward Juno events. A preliminary analysis of plasma velocities during five magnetic field reconnection events showed that the events were accompanied by fast radial flows, confirming our interpretation of these magnetic signatures as reconnection events. We anticipate that a future survey covering other Juno datasets will provide additional insight into the nature of tail reconnection at Jupiter.

The Creation of Twist by Reconnection of Flux Tubes

A fundamental process in a plasma is the magnetic reconnection of one pair of flux tubes (such as solar coronal loops) to produce a new pair. During this process magnetic helicity is conserved, but mutual helicity can be transformed to self-helicity, so that the new tubes acquire twist. However, until recently, when Wright (Astrophys. J.878, 102, 2019) supplied a solution, the partition of self-helicity between the two tubes was an outstanding puzzle. Here we examine Wright’s result in detail and apply it to a variety of cases. The simplest case, which Wright himself used to illustrate the result, is that of thin ribbons or flux sheets. We first explicitly apply his method to the usually expected standard case (when the tubes approach one another without twisting before reconnection) and confirm his result is valid for flux sheaths and tubes as well as sheets.For the reconnection of sheets, it is shown that the orientation of the sheets needs to be chosen carefully. For flux sheaths and tubes, Wright’s results are demonstrated to hold for the standard case. There is both a local and a global aspect to the effect of reconnection. The local effect of reconnection is to produce an equipartition of the added self-helicity (and therefore of twist), but the extra global effect of the location and orientation of the feet of the sheet, shell or tube in general adds different amounts of magnetic helicity to the two structures.It is important, as Wright realized, to account for any twist or writhe already existing in the fluxes prior to reconnection. Here we show explicitly that, if a section of a flux sheet is twisted by a multiple of pi with its ends held fixed and is then reconnected with another sheet, then the effect of the reconnection is to add that multiple of pi to one sheet and subtract it from the other, while conserving the total helicity. If, on the other hand, the central part of a flux sheath is twisted before reconnection by any angle, then the effect of reconnection is to add that amount of twist to one sheath and subtract it from the other, while conserving the total helicity. Thus, for the local part of the process in both sheets and sheaths, there is no longer helicity equipartition.Finally, we apply Wright’s results explicitly to flux tubes, and in particular to determine the twist that is acquired by an erupting flux rope due to reconnection during an eruptive solar flare or coronal mass ejection.

Statistical analysis of stochastic magnetic fields.

Previous work has introduced scale-split energy density ψ_{l,L}(x,t)=1/2B_{l}·B_{L} for vector field B(x,t) coarse grained at scales l and L, in order to quantify the field stochasticity or spatial complexity. In this formalism, the L_{p} norms S_{p}(t)=1/2||1-B[over ̂]_{l}·B[over ̂]_{L}||_{p},pth-order stochasticity level, and E_{p}(t)=1/2||B_{l}B_{L}||_{p},pth order mean cross energy density, are used to analyze the evolution of the stochastic field B(x,t). Application to turbulent magnetic fields leads to the prediction that turbulence in general tends to tangle an initially smooth magnetic field increasing the magnetic stochasticity level, ∂_{t}S_{p}>0. An increasing magnetic stochasticity in turn leads to disalignments of the coarse-grained fields B_{d} at smaller scales, d≪L, thus they average to weaker fields B_{L} at larger scales upon coarse graining, i.e., ∂_{t}E_{p}<0. Magnetic field resists the tangling effect of the turbulence by means of magnetic tension force. This can lead at some point to a sudden slippage between the field and fluid, decreasing the stochasticity ∂_{t}S_{p}<0 and increasing the energy ∂_{t}E_{p}>0 by aligning small-scale fields B_{d}. Thus the maxima (minima) of magnetic stochasticity are expected to approximately coincide with the minima (maxima) of cross energy density, occurrence of which corresponds to slippage of the magnetic field through the fluid. In this formalism, magnetic reconnection and field-fluid slippage both correspond to T_{p}=∂_{t}S_{p}=0and∂_{t}T_{2}<0. Previous work has also linked field-fluid slippage to magnetic reconnection invoking totally different approaches. In this paper, (a) we test these theoretical predictions numerically using a homogeneous, incompressible magnetohydrodynamic (MHD) simulation. Apart from expected small-scale deviations, possibly due to, e.g., intermittency and strong field annihilation, the theoretically predicted global relationship between stochasticity and cross energy is observed in different subvolumes of the simulation box. This indicate ubiquitous local field-fluid slippage and reconnection events in MHD turbulence. In addition, (b) we show that the conditions T_{p}=∂_{t}S_{p}=0and∂_{t}T_{p}<0 lead to sudden increases in kinetic stochasticity level, i.e., τ_{p}=∂_{t}s_{p}(t)>0 with s_{p}(t)=1/2||1-u[over ̂]_{l}.u[over ̂]_{L}||_{p}, which may correspond to fluid jets spontaneously driven by sudden field-fluid slippage-magnetic reconnection. Otherwise, they may correspond only to field-fluid slippage without energy dissipation. This picture, therefore, suggests defining reconnection as field-fluid slippage (changes in S_{p}) accompanied with magnetic energy dissipation (changes in E_{p}). All in all, these provide a statistical approach to the reconnection in terms of the time evolution of magnetic and kinetic stochasticities, S_{p} and s_{p}, their time derivatives, T_{p}=∂_{t}S_{p},τ_{p}=∂_{t}s_{p}, and corresponding cross energies, E_{p},e_{p}(t)=1/2||u_{l}u_{L}||_{p}. Furthermore, (c) we introduce the scale-split magnetic helicity based on which we discuss the energy or stochasticity relaxation of turbulent magnetic fields-a generalized Taylor relaxation. Finally, (d) we construct and numerically test a toy model, which resembles a classical version of quantum mean field Ising model for magnetized fluids, in order to illustrate how turbulent energy can affect magnetic stochasticity in the weak field regime.

The Distribution of Time Delays Between Nanoflares in Magnetohydrodynamic Simulations

Nanoflares are thought to be an important energy source for heating the solar corona. The exact amount of heating contributed by nanoflares depends, in part, on the frequency of the heating and the time interval between nanoflares. Several numerical models have attempted to constrain the frequency of the heating by fitting to observed emission measures. To date, however, no physically motivated value for the time interval between nanoflares has been obtained. In this paper, we calculate a physically motivated distribution of time intervals between successive “nanoflare” reconnection events in driven magnetohydrodynamic simulations. We show that this distribution follows a power law with a slope near −1, much shallower than previously inferred from observations and determined from loop model comparisons with observations. We show that the energy flux injected into the corona in our model is of order $10^{7}~\text{erg}\,\text{cm}^{-2}\,\text{s}^{-1}$, and that the heating rate due to these reconnection events reaches values of order $1\mbox{--}10~\text{erg}\,\text{cm}^{-3}\,\text{s}^{-1}$. Additionally, we show that this power law slope is dependent only weakly on the amount of magnetic helicity injected into the solar magnetic field by the photospheric motions, but the rate of reconnection is about $45\%$ higher if no magnetic helicity is injected.

Polynomial Reconstruction of the Reconnection Magnetic Field Observed by Multiple Spacecraft

AbstractWe describe methods for polynomial reconstruction of the magnetic field close to a cluster of spacecraft and apply that to reconstruction of the magnetic field observed during a magnetic reconnection event on 10 August 2017 by the Magnetospheric Multiscale spacecraft. Four different models are described, which vary in complexity between a 12‐parameter linear model, which has only linear variation with respect to the spatial coordinates, and a 27‐parameter quadratic model, which has the full quadratic expansion except that the second derivative with respect to the Minimum Directional Derivative minimum gradient coordinate has been neglected. In contrast to previous reconstruction techniques, these reconstructions can be found using only the magnetic field and current density measured at a single time by the cluster of spacecraft. The equations satisfying are satisfied exactly, while the equations specifying the model fields at the spacecraft locations are satisfied for most models in a best least squares sense. For this magnetotail event, the models have very small errors in magnetic field components ( 0.1 nT) at a distance from the nearest spacecraft on the order of the spacecraft separation, , here equal to 20.5 km. The magnetic structures found using the quadratic models are very time dependent, with a stretched field leading to plasmoid formation at one point in time.

EXTraS discovery of an X-ray superflare from an L dwarf

We present the first detection of an X-ray flare from an ultracool dwarf of spectral class L. The event was identified in the EXTraS database of XMM-Newton variable sources, and its optical counterpart, J0331−27, was found through a cross-match with the Dark Energy Survey Year 3 release. Next to an earlier four-photon detection of Kelu-1, J0331−27 is only the second L dwarf detected in X-rays, and much more distant than other ultracool dwarfs with X-ray detections (photometric distance of 240 pc). From an optical spectrum with the VIMOS instrument at the VLT, we determine the spectral type of J0331−27 to be L1. The X-ray flare has an energy of EX, F ∼ 2 × 1033 erg, placing it in the regime of superflares. No quiescent emission is detected, and from 2.5 Ms of XMM-Newton data we derive an upper limit of LX, qui &lt; 1027 erg s−1. The flare peak luminosity (LX, peak = 6.3 × 1029 erg s−1), flare duration (τdecay ≈ 2400 s), and plasma temperature (≈16 MK) are similar to values observed in X-ray flares of M dwarfs. This shows that strong magnetic reconnection events and the ensuing plasma heating are still present even in objects with photospheres as cool as ∼2100 K. However, the absence of any other flares above the detection threshold of EX, F ∼ 2.5 × 1032 erg in a total of ∼2.5 Ms of X-ray data yields a flare energy number distribution inconsistent with the canonical power law dN/dE ∼ E−2, suggesting that magnetic energy release in J0331−27 – and possibly in all L dwarfs – takes place predominantly in the form of giant flares.

Magnetic Reconnection in the Space Sciences: Past, Present, and Future

AbstractMagnetic reconnection converts, often explosively, stored magnetic energy to particle energy in space and in the laboratory. Through processes operating on length scales that are tiny, it facilitates energy conversion over dimensions of, in some cases, hundreds of Earth radii. In addition, it is the mechanism behind large current disruptions in fusion machines, and it can explain eruptive behavior in astrophysics. We have known about the importance of magnetic reconnection for quite some time based on space observations. Theory and modeling employed magnetized fluids, a very simplistic description. While successful at modeling the large‐scale consequences of reconnection, it is ill suited to describe the engine itself. This is because, at its heart, magnetic reconnection in space is kinetic, that is, governed by the intricate interaction of charged particles with the electromagnetic fields they create. This complex interaction occurs in very localized regions and involves very short temporal variations. Researching reconnection requires the ability to measure these processes as well as to express them in models vastly more complex than fluid approaches. Until very recently, neither of these capabilities existed. With the advent of NASA's Magnetospheric Multiscale mission and modern modeling advances, this has now changed, and we have now determined its small‐scale structure in exquisite detail. In this paper, we review recent research results to predict what will be achieved in the future. We discuss how reconnection contributes to the evolution of larger‐scale systems, and its societal impacts in the context of threatening space hazards, customarily referred to as “space weather.”