Rapid Reconnection Research Articles

EPISODIC JETS AS THE CENTRAL ENGINE OF GAMMA-RAY BURSTS

Most Gamma-ray bursts (GRBs) have erratic light curves, which demand that the GRB central engine launches an episodic outflow. Recent Fermi observations of some GRBs indicate a lack of the thermal photosphere component as predicted by the baryonic fireball model, which suggests a magnetic origin of GRBs. In view that powerful episodic jets have been observed along with continuous jets in other astrophysical black hole systems, here we propose an intrinsically episodic, magnetically-dominated jet model for GRB central engine. Accumulation and eruption of free magnetic energy in the corona of a differentially-rotating, turbulent accretion flow around a hyperaccreting black hole lead to ejections of episodic, magnetically dominated plasma blobs. These blobs are accelerated magnetically, collide with each other at large radii, trigger rapid magnetic reconnection and turbulence, efficient particle acceleration and radiation, and power the observed episodic prompt gamma-ray emission from GRBs.

FAST MAGNETIC RECONNECTION AND SPONTANEOUS STOCHASTICITY

Magnetic field lines in astrophysical plasmas are expected to be frozen-in at scales larger than the ion gyroradius. The rapid reconnection of magnetic-flux structures with dimensions vastly larger than the gyroradius requires a breakdown in the standard Alfvén flux-freezing law. We attribute this breakdown to ubiquitous MHD plasma turbulence with power-law scaling ranges of velocity and magnetic energy spectra. Lagrangian particle trajectories in such environments become "spontaneously stochastic," so that infinitely many magnetic field lines are advected to each point and must be averaged to obtain the resultant magnetic field. The relative distance between initial magnetic field lines which arrive at the same final point depends upon the properties of two-particle turbulent dispersion. We develop predictions based on the phenomenological Goldreich & Sridhar theory of strong MHD turbulence and on weak MHD turbulence theory. We recover the predictions of the Lazarian & Vishniac theory for the reconnection rate of large-scale magnetic structures. Lazarian & Vishniac also invoked "spontaneous stochasticity," but of the field lines rather than of the Lagrangian trajectories. More recent theories of fast magnetic reconnection appeal to microscopic plasma processes that lead to additional terms in the generalized Ohm's law, such as the collisionless Hall term. We estimate quantitatively the effect of such processes on the inertial-range turbulence dynamics and find them to be negligible in most astrophysical environments. For example, the predictions of the Lazarian & Vishniac theory are unchanged in Hall MHD turbulence with an extended inertial range, whenever the ion skin depth δi is much smaller than the turbulent integral length or injection-scale Li.

A NEW MECHANISM FOR DISSIPATION OF ALTERNATING FIELDS IN POYNTING-DOMINATED OUTFLOWS

Reconnection of alternating magnetic fields is an important energy transformation mechanism in Poynting dominated outflows. We show that the reconnection is facilitated by the Kruskal-Schwarzschild instability of current sheets separating the oppositely directed fields. This instability, which is a magnetic counterpart of the Rayleigh-Taylor instability, develops if the flow is accelerated. Then the plasma drips out of the current sheet providing conditions for rapid reconnection. Since the magnetic dissipation leads to the flow acceleration, the process is self-sustaining. In pulsar winds, this process could barely compete with the earlier proposed dissipation mechanisms. However, the novel mechanism turns out to be very efficient at AGN and GRB conditions.

Search for an onset mechanism that operates for both CMEs and substorms

Abstract. Substorms and coronal mass ejections have been cited as the most accessible examples of the explosive energy conversion phenomenon that seems to characterize one of the behavior modes of cosmic plasmas. This paper addresses the question of whether these two examples – substorms and CMEs – support or otherwise the idea that explosive energy conversion is the result of a single process operating in different places and under different conditions. As a candidate mechanism that might be common to both substorms and CMEs we use the Forbes catastrophe model for CMEs because before testing it appears to have the potential, suitably modified, to operate also for substorms. The essence of the FCM is a sudden onset of an imbalance of the forces acting on an incipient CME. The imbalance of forces causes the CME to start to rise. Beneath the rising CME conditions develop that favor the onset of magnetic reconnection which then releases the CME and assists its expulsion. Thus the signature of the FCM is a temporally ordered sequence in which there is first the appearance of force imbalance which leads to upward (or outward) motion of the CME which leads to magnetic reconnection under it which expedites rapid expulsion. We look for the FCM signature in the output of two global magnetospheric MHD simulations that produce substorm-like events. We find the ordered sequence of events as stated but with a significant difference: there is no plasmoid prior to the onset of rapid reconnection, that is, there is no counterpart to the incipient CME on which an imbalance of forces acts to initiate the action in the FCM. If this result – that rapid tailward motion precedes the rapid reconnection of substorm expansion – is ultimately verified by other studies, it suggests that a description of the cause of substorm expansion should identify the cause of the preceding rapid tailward motion, since this leads necessarily to rapid reconnection, whatever the reconnection mechanism turns out to be. Clearly then, it is important to identify the cause of the preceding tailward motion.

Titan’s influence on Saturnian substorm occurrence

Substorms play an important role in the energization and transport of plasmas in planetary magnetospheres, including the shedding of the mass added by moons in the case of Jupiter and Saturn. Mass shedding occurs through rapid reconnection in the near tail resulting in dipolarization on the magnetospheric side of the reconnection point and plasmoid formation down tail. Observations of these sudden reconnection events in Saturn’s near‐tail region provide additional insight into this process. Saturnian substorms, at least on occasion, have a plasmoid formation phase leading to a traveling compression region. Changes in the field strength across reconnection events suggest that open flux has been removed from the tail. The timing of tail reconnection events appears to be controlled by both the orbital phase of Titan, and the variable stretching of the near‐tail field as Saturn rotates.

Features and Properties of Coronal Mass Ejection/Flare Current Sheets

Solar eruptions occur when magnetic energy is suddenly converted into heat and kinetic energy by magnetic reconnection in a current sheet ( CS). It is often assumed that CSs are too thin to be observable because the electric resistivity n(e) in CSs is taken to be very small. In this work, we show the implications for the CS thickness d estimated from observations of three eruptions by the UVCS and the LASCO experiments on SOHO. We infer the effective n(e) causing the rapid reconnection, which predicts much faster reconnection in a thick CS than that caused by the classical and anomalous resistivities. We find that in these events CSs are observable and have extremely large values of d and n(e), implying that large-scale turbulence is operating within CSs. We also discuss the properties of the so-called hyperresistivity caused by the tearing mode and the relation to our results.

A reply

We thank Drs Arthur and McGlennan for their interest in our study. We felt that inspired oxygen concentration (Fio2) at disconnection would not necessarily reflect alveolar Po2 as the Fio2 can change several times in the anaesthetic room and there may not be time for equilibration. Expired oxygen concentration would perhaps have been more useful in this regard, but it is not available in our anaesthetic rooms. In addition, patients with co-morbidity which decreases functional residual capacity or impairs oxygen transfer (such as obesity or chronic airways disease) may be vulnerable to quite rapid desaturation after short periods of apnoea, even if they have been ventilated with a high Fio2 before transfer. We, too, were surprised at the length of time patients were disconnected from monitoring and ventilation, but think our local infrastructure probably reflects practice elsewhere. All the authors have worked in several anaesthetic departments and do not consider this department to be exceptional with regard to the transfer process. Also, all participants in the study have worked or trained at other sites. All would consider the theatres here to be well run, with excellent anaesthetic assistance. Studies of this nature often reveal that widely held assumptions about practice are not valid. We agree that vigilance and attention to basic principles by the anaesthetist are essential to ensuring rapid reconnection to monitoring and breathing systems after transfer to theatre. Unforeseen problems cannot, however, be pre-empted. Human nature dictates that even the most vigilant practitioners are vulnerable to distraction and a key design element of many complex systems is to eliminate the potential for human factors to cause harm. If the transfer process is removed completely, all potential for harm in the process is eliminated. These concerns should certainly be considered in any discussion about the place of anaesthetic rooms in our practice.

Magnetic reconnection in nontoroidal plasmas

Magnetic reconnection is a major issue in solar and astrophysical plasmas. The mathematical result that the evolution of a magnetic field with only point nulls is always locally ideal limits the nature of reconnection in nontoroidal plasmas. Here it is shown that the exponentially increasing separation of neighboring magnetic field lines, which is generic, tends to produce rapid magnetic reconnection if the length of the field lines is greater than about 20 times the exponentiation, or Lyapunov, length.

Rapid Penumbral Decay Associated with an X2.3 Flare in NOAA Active Region 9026

We present observations of rapid penumbral decay associated with a major flare in solar NOAA Active Region 9026 on 2000 June 6. Within 1.5 hr, an X2.3 flare accompanied by an 11° long filament eruption and a full-halo coronal mass ejection (CME) originated near the neutral line of a large δ-spot region, which was associated with significant changes in white-light structure and field topology: an increase of moving features (MMFs), flux emergence and cancellation, and, in particular, the rapid disappearance of two penumbral segments located in opposite-polarity regions on the north and south sides of the δ-spot. The rapid penumbral decay is believed to be the result of field topology change that was caused by rapid reconnection during the flare, rather than part of overall long-term evolution. We present a possible explanation of this event, using a magnetic model for solar flares, considering its complex multipolar δ-configuration and associated filament eruption and CME, i.e., previously closed field lines opened up and reconnected at a null point above the neutral line of this δ-spot. The breakout caused an energy release from a highly sheared field in the umbrae and a transition of the arcades from low lying to high lying, which led to an increase of the inclination angle of the field lines in the peripheral penumbrae; i.e., the field turned from more inclined to more vertical and toward the inner umbrae. Once the field in the penumbrae was vertical enough, the Evershed flow ceased, the manifestation of which in white-light structure is the disappearance of peripheral penumbrae. We also discuss other possible flare models for this event and compare them in several observational features. The present observations provide further evidence that highly energetic events have a distinct associated photospheric field signature and support the findings of recent analyses of photospheric line-of-sight magnetograms from the Big Bear Solar Observatory (BBSO) and the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) that show rapid and permanent changes of photospheric fields associated with flares.

Possible two-step solar energy release mechanism due to turbulent magnetic reconnection

In this paper, a possible two-step solar magnetic energy release process attributed to turbulent magnetic reconnection is investigated by magnetohydrodynamic simulation for the purpose of accounting for the closely associated observational features including canceling magnetic features and different kinds of small-scale activities such as ultraviolet explosive events in the lower solar atmosphere. Numerical results based on realistic transition region physical parameters show that magnetic reconnections in a vertical turbulent current sheet consist of two stages, i.e., a first slow Sweet–Parker-like reconnection and a later rapid Petschek-like reconnection, where the latter fast reconnection phase seems a direct consequence of the initial slow reconnection phase when a critical state is reached. The formation of coherent plasmoid of various sizes and their coalescence play a central role in this complex nonlinear evolution. The “observed” values of the rate of cancellation flux as well as the approaching velocity of magnetic fragments of inverse polarity in present simulation are well consistent with the corresponding measurements in the latest observations. The difference between our turbulent magnetic reconnection two-step energy release model and other schematic two-step models is discussed and then possible application of present outcome to solar explosives is described.

A new look at driven magnetic reconnection at the terrestrial subsolar magnetopause

The physics of steady driven magnetic reconnection at Earth's subsolar magnetopause is addressed. Three‐dimensional, global magnetohydrodynamics (MHD) simulations of the magnetopause are compared with analytical solutions of the resistive MHD equations [Sonnerup and Priest, 1975] corresponding to magnetic field annihilation driven by an incompressible stagnation point flow. The simulations demonstrate that under steady southward interplanetary magnetic field conditions and when the plasma resistivity is spatially uniform, subsolar magnetopause reconnection occurs in long, thin Sweet‐Parker current sheets via a flux pileup mechanism [Sonnerup and Priest, 1975; Priest and Forbes, 1986] (rather than in Petschek slow shock configurations). Magnetic energy piles up upstream of the magnetopause current sheet to accommodate the sub‐Alfvénic solar wind inflow. The scaling of the pileup with Lundquist number, S, is consistent (approximately ∝ S1/4) with that predicted by the analytical, incompressible stagnation point flow solutions (though there are small corrections due to plasma compressibility in the simulations). Since there is a finite energy in the magnetosheath available to drive the magnetic pileup (and associated rapid magnetic reconnection), we expect the pileup to saturate and the reconnection rate to drop as the upstream plasma pressure drops to accommodate the pileup. Thus we expect the reconnection to stall, the rate vanishing in the limit S → ∞. We discuss the role of Hall electric fields in allowing the magnetic pileup to saturate before the reconnection begins to stall, permitting Alfvénic reconnection to occur in thin current sheets in the limit S → ∞.

Topological model for the large solar flare of July 14, 2000

Using observational data obtained with the Yohkoh, SOHO, and TRACE satellites, it is shown that the three-dimensional structure of the large solar flare of July 14, 2000 was determined by the topology of the large-scale magnetic field of the active region giving rise to the flare. The locations and shapes of chromospheric ribbons and brightness centers on these ribbons are explained. The observed behavior of the flare is attributed to rapid magnetic reconnection in the corona. The electric field accelerating particles in the reconnecting current sheets is estimated.

Hall Magnetohydrodynamics of Reversed Field Current Layers

The dynamics of inhomogeneous, reversed field current layers in the Hall limit (i.e., characteristic length scales ≲ the ion inertial length) are briefly reviewed. We demonstate that a density inhomogeneity along the current direction can dramatically redistribute the magnetic field and plasma via magnetic shock-like or rarefaction waves. The relative direction between the density gradient and current flow plays a critical role in the evolution of the current sheet. Specifically a current sheet can become very thin rapidly when the density gradient is directed opposite to the current. We also show that magnetic reconnection proceeds asymmetrically in a three dimensional system. A localized magnetic field perturbation induces a wave structure that propagates in the direction of the electron drift (i.e., opposite to the current). The propagating wave structure is a Hall phenomenon associated with magnetic field curvature. The interaction between the propagating wave structure and an evolving current layer can lead to rapid magnetic field line reconnection. We apply our results to laboratory and space plasma processes.

Zeit zum Nachdenken

Fifteen years after the fall of the Berlin Wall, one can hardly find its traces. Few memorials, fragments of the Wall, surveillance towers, markings or art pieces commemorate the Wall today. But the Wall was not a couple of historical episodes, it was a physical incision that shaped the urban development of the City of Berlin for nearly thirty years. Dense and vital inner-city districts turned into periphery just because of their proximity to the Wall, this unapproachable corridor through the city that extended up to several hundred meters in some places. After the fortifications had been removed, this linear void became a new element in the urban tissue. Ideas about the urban potential of this space were only discussed for a short period. For the city government, the rapid reconnection of the two city-halves and the „kritische Rekonstruktion” were more important issues. In some areas, the space left by the wall was quickly filled with reconstructions of the pre-war blocks—like elsewhere in the inner city...

Why stellar astronomers should be interested in the sun

By all accounts, the Sun is a garden-variety star with an average age, a standard size, a regular temperature, norormal mass, an ordinary structure, and a typical chemical composition. Only one feature makes it special - the Sun is our star. It is located in the center of our solar system, and therefore, is responsible for all life on Earth. Astronomically speaking, the Sun is the only star in the sky that we can study up-close and personal. The unaided human eye does a better job of resolving the Sun than the finest telescope does for any other star. Stellar astronomers issue a press release whenever they can lay a few pixels of some state-of-the-art instrument across a nearby supergiant. The resolution of the Sun, however, is something we can see routinely in the magnificent images that are downloaded every day from the Transition Region and Coronal Explorer (TRACE) spacecraft. In a very real sense, the Sun is the Rosetta Stone of the Stars. Observations of the Sun deflecting starlight ushered in a new way of thinking about gravity. Zeeman effect observations of the Sun showed that stellar atmospheres were controlled by magnetic fields. The discovery of solar helium founded the science of stellar spectroscopy. Measurements of the solar mass, radius, and temperature allowed scientists to probe the interiors of stars for the first time. tim ancient age of the Sun implied that stars shine as a result of thermonuclear fusion. Observations of solar flares flamulated developments in rapid magnetic reconnection theory. The study of solar coronal holes led to a deeper understanding of the role that mass loss plays in the evolution of stars. Detailed analysis of the solar activity cycle inspired the development of Magneto-Hydrodynamic (MHD) dynamo theory. The detection and understanding; of the solar corona uncovered one of the longest unsolved mysteries in all of astrophysics — the coronal-heating problem. And the list goes on. The Sun is indeed a Laboratory for Astrophysics, but it is more than that. The Sun is a Laboratory for all of Physics. This paper describes 20th century physics discoveries that are directly attributable to solar observations. It is in the form of a Top Ten List, and was inspired originally by a talk given by Dr. Eugene N. Parker; subsequent discussions with Dr. Parker have molded it into its present form.

Onset of collisionless magnetic reconnection in thin current sheets: Three-dimensional particle simulations

Three-dimensional (3D) particle-in-cell simulations of collisionless magnetic reconnection are presented. The initial equilibrium is a double Harris-sheet equilibrium and periodic boundary conditions are assumed in all three directions. No magnetic seed island is imposed initially, and no flow conditions are imposed. The current sheet width is assumed to be one ion inertial length calculated with the density in the center of the current sheet. The ion to electron mass ratio is mi/me=150, which suppresses the growth of the drift kink instability. Two different runs have been performed: a simulation with exactly antiparallel magnetic field and a simulation with a constant guide field of the same magnitude as the antiparallel field superimposed. In the antiparallel case the inductive field of the waves excited by the lower hybrid drift instability (LHDI) leads to rapid acceleration of the electrons in the center of the current sheet and subsequently to a current sheet thinning. The current increase in the center is balanced by reverse currents in the gradient region. In the thin current sheet rapid reconnection sets in which self-organizes into a two-dimensional structure with a single X line. However, ∼15% of the total flux is reconnected while reconnection is still patchy and 3D. In the guide field case the growth rate of the LHDI is reduced, but leads nevertheless after a considerably longer time to electron acceleration in the current sheet center and to a thinning of the current layer, followed by single X line reconnection. It is suggested that electron acceleration due to LHDI in current sheets of the order of the ion scale results in rapid onset of reconnection.

Hall magnetohydrodynamics of neutral layers

New analytical and numerical results of the dynamics of inhomogeneous, reversed field current layers in the Hall limit (i.e., characteristic length scales ≲ the ion inertial length) are presented. Specifically, the two- and three-dimensional evolution of a current layer that supports a reversed field plasma configuration and has a density gradient along the current direction is studied. The two-dimensional study demonstrates that a density inhomogeneity along the current direction can dramatically redistribute the magnetic field and plasma via magnetic shock-like or rarefaction waves. The relative direction between the density gradient and current flow plays a critical role in the evolution of the current sheet. One important result is that the current sheet can become very thin rapidly when the density gradient is directed opposite to the current. The three-dimensional study uses the same plasma and field configuration as the two-dimensional study but is also initialized with a magnetic field perturbation localized along the current channel upstream of the plasma inhomogeneity. The perturbation induces a magnetic wave structure that propagates in the direction of the electron drift (i.e., opposite to the current). The propagating wave structure is a Hall phenomenon associated with magnetic field curvature. The interaction between the propagating wave structure and the evolving current layer can lead to rapid magnetic field line reconnection. The results are applied to laboratory and space plasma processes.

SMES-UPS for large-scaled SC magnet system of LHD

The LHD is an SC experimental fusion device of heliotron type. Eight sets of the helium compressors with total electric power of 3.5 MW are installed in the cryogenic system. The analytical studies of the SMES-UPS for the compressors under the deep voltage sag are reported in this paper. The amplitude and frequency of the voltage decrease gradually by the regenerating effect of the induction motors. The SMES-UPS system proposed in this report has the following functions; (1) variable frequency control, (2) regulations by ACR and AVR, and (3) rapid isolation and synchronous reconnection from the loads to grid line. We have demonstrated that SMES was useful for the large-scaled cryogenic system of the experimental fusion device.

Sudden Disappearance of a Small Sunspot Associated with the 2002 February 20 M2.4 Flare

In this Letter, we present the evidence of rapid disappearance of a small sunspot associated with a M2.4 flare, which occurred in NOAA Active Region 9830, at 2106 UT, on 2002 February 20. The sunspot, in the size of about 45 arcsec2, completely disappeared in about 60 minutes, coinciding with the onset of the flare. The magnetic flux of this sunspot disappeared in the similar timescale. Furthermore, we found that the disappearing sunspot is located near the edge of the main hard X-ray source observed by RHESSI. This hard X-ray source is located in the magnetic neutral line dividing the disappearing spot and the opposite polarity flux. Two possible explanations are offered: (1) the disappearing sunspot is due to the rapid magnetic reconnection and subsequent submergence; (2) the disappearing sunspot represents impulsive emergence and detachment of a magnetic bubble.

Three-dimensional Hall magnetic reconnection

New numerical results of three-dimensional magnetic reconnection in the Hall limit (L<c/ωpi where c/ωpi is the ion inertial length) are presented. The reconnection process is initiated with a magnetic field perturbation localized along the current channel in a reversed field plasma configuration. The perturbation induces a magnetic wave structure that propagates opposite to the current, and leads to the asymmetric thinning of the plasma layer, strong plasma flows in the direction of the current, and rapid magnetic reconnection. The propagating wave structure is a Hall phenomenon associated with magnetic field curvature. The reconnection rate is independent of a (weak) guide field and the boundary conditions (i.e., periodic or outflowing).