Large-scale Magnetic Reconnection Research Articles

TeV Neutrinos and Hard X-Rays from Relativistic Reconnection in the Corona of NGC 1068

The recent discovery of astrophysical neutrinos from the Seyfert galaxy NGC 1068 suggests the presence of nonthermal protons within a compact “coronal” region close to the central black hole. The acceleration mechanism of these nonthermal protons remains elusive. We show that a large-scale magnetic reconnection layer, of the order of a few gravitational radii, may provide such a mechanism. In such a scenario, rough energy equipartition between magnetic fields, X-ray photons, and nonthermal protons is established in the reconnection region. Motivated by recent 3D particle-in-cell simulations of relativistic reconnection, we assume that the spectrum of accelerated protons is a broken power law, with the break energy being constrained by energy conservation (i.e., the energy density of accelerated protons is at most comparable to the magnetic energy density). The proton spectrum is dnp/dEp∝Ep−1 below the break and dnp/dEp∝Ep−s above the break, with IceCube neutrino observations suggesting s ≃ 3. Protons above the break lose most of their energy within the reconnection layer via photohadronic collisions with the coronal X-rays, producing a neutrino signal in good agreement with the recent observations. Gamma rays injected in photohadronic collisions are cascaded to lower energies, sustaining the population of electron–positron pairs that makes the corona moderately Compton thick.

Discovery of extreme, roughly daily superflares on the recurrent nova V2487 Oph

ABSTRACTV2487 Oph is a recurrent nova with detected eruptions in 1900 and 1998. Startlingly, V2487 Oph shows flares, called ‘Superflares’, with up to 1.10 mag amplitude, fast rises of under one-minute, always with an initial impulsive spike followed by a roughly exponential tail, typically one-hour durations, and with random event times averaging once-per-day. The typical flare energy E is over 1038 erg, while the yearly energy budget is 1041 erg. V2487 Oph Superflares obey three relations; the number distribution of flare energies scales as E−2.34 ± 0.35, the waiting time from one flare to the next is proportional to E of the first event, and flare durations scale as E0.44 ± 0.03. Scenarios involving gravitational energy and nuclear energy fail to satisfy the three relations. The magnetic energy scenario, however, can explain all three relations. This scenario has magnetic field lines above the disc being twisted and amplified by the motions of their footprints, with magnetic reconnection releasing energy that comes out as Superflare light. This exact mechanism is already well known to occur in white light solar flares, in ordinary M-type flare stars, and in the many Superflare stars observed all across the H-R diagram. Superflares on Superflare stars have rise times, light-curve shapes, and durations that are very similar to those on V2487 Oph. So we conclude that the V2487 Oph Superflares are caused by large-scale magnetic reconnection. V2487 Oph is now the most extreme Superflare star, exhibiting the largest known flare energy (1.6 × 1039 erg) and the fastest occurrence rate.

In Situ Detection of the Solar Eruption: Lay a Finger on the Sun

This paper is to introduce a proposed deep space mission, which aims to perform in situ detection of the central structure, namely the magnetic reconnection current sheet, which drives the large scale eruption on a star. The main focus of this mission is on the fine physical characteristics of the large scale magnetic reconnection taking place on the Sun, our nearest star, revealing the secret of the most violent energy release process, also known as the solar storm, in the solar system. The scientific goal of this mission, magnetic reconnection, is a kernel process of energy conversion occurring in the magnetized plasma in the universe, and has long been a fairly important research topic or even research area in solar physics, space science, plasma physics, and the related fields. In situ detection enhances the spatial resolution of the instruments used on the Earth 5~20 times, will provide us extra-clear images of the Sun and the related information. We are thus able to study, to learn, and to understand the Sun on an unprecedented platform, and further resolve the long-standing puzzle in the solar community regarding the fine physical property of the kernel process driving the solar eruption, as well as that of the corona heating.

Preflare Processes, Flux Rope Activation, Large-scale Eruption, and Associated X-class Flare from the Active Region NOAA 11875

Abstract We present a multiwavelength analysis of the eruption of a hot coronal channel associated with an X1.0 flare (SOL2013-10-28T02:03) from the active region NOAA 11875 by combining observations from Atmospheric Imaging Assembly/Solar Dynamics Observatory (SDO), Helioseismic and Magnetic Imager/SDO, Reuven Ramaty High Energy Solar Spectroscopic Imager, and Hiraiso Radio Spectrograph. EUV images at high coronal temperatures indicated the presence of a hot channel at the core of the active region from the early preflare phase evidencing the preexistence of a quasi-stable magnetic flux rope. The hot channel underwent an activation phase after a localized and prolonged preflare event occurring adjacent to one of its footpoints. Subsequently, the flux rope continued to rise slowly for ≈16 minutes during which soft X-ray flux gradually built-up characterizing a distinct precursor phase. The flux rope transitioned from the state of slow rise to the eruptive motion with the onset of the impulsive phase of the X1.0 flare. The eruptive expansion of the hot channel is accompanied by a series of type III radio bursts in association with the impulsive rise of strong hard X-ray nonthermal emissions that included explicit hard X-ray sources of energies up to ≈50 keV from the coronal loops and ≈100 keV from their footpoint locations. Our study contains evidence that preflare activity occurring within the spatial extent of a stable flux rope can destabilize it toward eruption. Moreover, sudden transition of the flux rope from the state of slow rise to fast acceleration precisely bifurcated the precursor and the impulsive phases of the flare, which points toward a feedback relationship between early coronal mass ejection dynamics and the strength of the large-scale magnetic reconnection.

THE FORMATION AND LAUNCH OF A CORONAL MASS EJECTION FLUX ROPE: A NARRATIVE BASED ON OBSERVATIONS

We present a data-driven narrative of the launch and early evolution of the magnetic structure that gave rise to the coronal mass ejection (CME) on 2008 December 12. The structure formed on December 7 and launched early on December 12. We interpret this structure as a flux rope based on prelaunch morphology, postlaunch magnetic measurements, and the lack of large-scale magnetic reconnection signatures at launch. We ascribe three separate onset mechanisms to the complete disconnection of the flux rope from the Sun. It took 19 hr for the flux rope to be fully removed from the Sun, by which time the segment that first disconnected was around 40 R {sub ☉} away. This implies that the original flux rope was stretched or broken; we provide evidence for a possible bisection. A transient dark arcade was observed on the Sun that was later obscured by a bright arcade, which we interpret as the strapping field stretching and magnetically reconnecting as it disconnected from the coronal field. We identify three separate structures in coronagraph images to be manifestations of the same original flux rope, and we describe the implications for CME interpretation. We cite the rotation in the central flux rope vectormore » of the magnetic clouds observed in situ by ACE/Wind and STEREO-B as evidence of the kink instability of the eastern segment of the flux rope. Finally, we discuss possible alternative narratives, including multiple prelaunch magnetic structures and the nonflux rope scenario. Our results support the view that, in at least some CMEs, flux rope formation occurs before launch.« less

Transient jet formation and state transitions from large-scale magnetic reconnection in black hole accretion discs

Magnetically arrested accretion discs (MADs), where the magnetic pressure in the inner disc is dynamically important, provide an alternative mechanism for regulating accretion to what is commonly assumed in black hole systems. We show that a global magnetic field inversion in the MAD state can destroy the jet, significantly increase the accretion rate, and move the effective inner disc edge in to the marginally stable orbit. Reconnection of the MAD field in the inner radii launches a new type of transient outflow containing hot plasma generated by magnetic dissipation. This transient outflow can be as powerful as the steady magnetically-dominated Blandford-Znajek jet in the MAD state. The field inversion qualitatively describes many of the observational features associated with the high luminosity hard to soft state transition in black hole X-ray binaries: the jet line, the transient ballistic jet, and the drop in rms variability. These results demonstrate that the magnetic field configuration can influence the accretion state directly, and hence the magnetic field structure is an important second parameter in explaining observations of accreting black holes across the mass and luminosity scales.

DYNAMICS OF STRONGLY TWISTED RELATIVISTIC MAGNETOSPHERES

Magnetar magnetospheres are believed to be strongly twisted due to shearing of the stellar crust by internal magnetic stresses. We present time-dependent axisymmetric simulations showing in detail the evolution of relativistic force-free magnetospheres subjected to slow twisting through large angles. When the twist amplitude is small, the magnetosphere moves quasi-statically through a sequence of equilibria of increasing free energy. At some twist amplitude the magnetosphere becomes tearing-mode unstable to forming a resistive current sheet, initiating large-scale magnetic reconnection in which a significant fraction of the magnetic free energy can be dissipated. This "critical" twist angle is insensitive to the resistive length scale. Rapid shearing temporarily stabilizes the magnetosphere beyond the critical angle, allowing the magnetosphere of a rapidly differentially rotating star to store and dissipate more free energy. In addition to these effects, shearing the surface of a rotating star increases the spindown torque applied to the star. If shearing is much slower than rotation, the resulting spikes in spindown rate can occur on timescales anywhere from the long twisting timescale to the stellar spin period or shorter, depending both on the stellar shear distribution and the existing distribution of magnetospheric twists. A model in which energy is stored in the magnetosphere and released by a magnetospheric instability therefore predicts large changes in the measured spindown rate before soft gamma repeater giant flares.

An observation of solar active region expansion into the heliosphere

According to current models, solar closed-field active regions are isolated from contributing directly to the solar wind except through smallor large-scale magnetic reconnection, including coronal mass ejections. Here we show the first direct evidence of active regions contributing directly to the solar wind by a steady-state, quiescent expansion. Advanced image processing of coronagraph data reveals a large system of nested closed-field magnetic structures expanding and accelerating to heights of at least 14 R within a helmet streamer above a large active region. The system persists for several days, and must therefore be an important contribution to the slow solar wind.

Reconnection current sheet structure in a turbulent medium

Abstract. In the presence of turbulence, magnetic field lines lose their dynamical identity and particles entrained on field lines diffuse through space at a rate determined by the amplitude of the turbulence. In previous work (Lazarian and Vishniac, 1999; Kowal et al., 2009; Eyink et al., 2011) we showed that this leads to reconnection speeds which are independent of resistivity. In particular, in Kowal et al. (2009) we showed that numerical simulations were consistent with the predictions of this model. Here we examine the structure of the current sheet in simulations of turbulent reconnection. Laminar flows consistent with the Sweet-Parker reconnection model produce very thin and well ordered currents sheets. On the other hand, the simulations of Kowal et al. (2009) show a strongly disordered state even for relatively low levels of turbulence. Comparing data cubes with and without reconnection, we find that large scale field reversals are the cumulative effect of many individual eddies, each of which has magnetic properties which are not very different from turbulent eddies in a homogeneous background. This implies that the properties of stationary and homogeneous MHD turbulence are a reasonable guide to understanding turbulence during large scale magnetic reconnection events. In addition, dissipation and high energy particle acceleration during reconnection events take place over a macroscopic volume, rather than being confined to a narrow zone whose properties depend on microscopic transport coefficients.

Fragmentation during merging of plasmoids in the magnetic field reconnection

Context. Application of the magnetic-reconnection theory onto large-scale events, such as solar flares, requires formation of very thin (kinetic-scale) current sheets within the rather thick flare current layer. Hence, some fragmentation/filamentation mechanisms has to be in action.Aims. We aim at identifying fragmentation mechanisms for magnetic field and current density structures. Namely, we focus at detailed study of the processes during the merging of plasmoids that had been formed in the current layer.Methods. A 2.5-D electromagnetic particle-in-cell model is used and its results analysed. Results. It is shown that the merging process of plasmoids is not a simple process as presented in some previous studies. On the contrary, this process leads to a complex fragmentation. We found two types of fragmentation processes: a) fragmentation in the current sheet generated between the merging plasmoids and b) fragmentation at the boundary of plasma outflow from the reconnection between these plasmoids. While the first type of fragmentation is generated by the tearing-mode (plasmoid) instability of the secondary current sheet, the second one looks to be connected with an increase of the plasma β parameter during these processes. Thus, sheared high-β plasma flows produce this additional fragmentation. Conclusions. The fragmentation and energy transport from large to small scales in a large-scale magnetic reconnection seem to be the result of interplay and positive feedback between instabilities driven by high gradients in both magnetic (intense current density) and velocity (high vorticity) fields.

TIME-DEPENDENT MODELS OF FLARES FROM SAGITTARIUS A*

The emission from Sgr A*, the supermassive black hole in the Galactic Center, shows order of magnitude variability ("flares") a few times a day that is particularly prominent in the near-infrared (NIR) and X-rays. We present a time-dependent model for these flares motivated by the hypothesis that dissipation of magnetic energy powers the flares. We show that episodic magnetic reconnection can occur near the last stable circular orbit in time-dependent magnetohydrodynamic simulations of black hole accretion - the timescales and energetics of these events are broadly consistent with the flares from Sgr A*. Motivated by these results, we present a spatially one-zone time-dependent model for the electron distribution function in flares, including energy loss due to synchrotron cooling and adiabatic expansion. Synchrotron emission from transiently accelerated particles can explain the NIR/X-ray lightcurves and spectra of a luminous flare observed 4 April 2007. A significant decrease in the magnetic field strength during the flare (coincident with the electron acceleration) is required to explain the simultaneity and symmetry of the simultaneous lightcurves. Our models predict that the NIR and X-ray spectral indices differ by 0.5 and that there is only modest variation in the spectral index during flares. We also explore implications of this model for longer wavelength (radio-submm) emission seemingly associated with X-ray and NIR flares; we argue that a few hour decrease in the submm emission is a more generic consequence of large-scale magnetic reconnection than delayed radio emission from adiabatic expansion.

A route to explosive large-scale magnetic reconnection in a super-ion-scale current sheet

Abstract. How to trigger magnetic reconnection is one of the most interesting and important problems in space plasma physics. Recently, electron temperature anisotropy (αeo=Te⊥/Te||) at the center of a current sheet and non-local effect of the lower-hybrid drift instability (LHDI) that develops at the current sheet edges have attracted attention in this context. In addition to these effects, here we also study the effects of ion temperature anisotropy (αio=Ti⊥/Ti||). Electron anisotropy effects are known to be helpless in a current sheet whose thickness is of ion-scale. In this range of current sheet thickness, the LHDI effects are shown to weaken substantially with a small increase in thickness and the obtained saturation level is too low for a large-scale reconnection to be achieved. Then we investigate whether introduction of electron and ion temperature anisotropies in the initial stage would couple with the LHDI effects to revive quick triggering of large-scale reconnection in a super-ion-scale current sheet. The results are as follows. (1) The initial electron temperature anisotropy is consumed very quickly when a number of minuscule magnetic islands (each lateral length is 1.5~3 times the ion inertial length) form. These minuscule islands do not coalesce into a large-scale island to enable large-scale reconnection. (2) The subsequent LHDI effects disturb the current sheet filled with the small islands. This makes the triggering time scale to be accelerated substantially but does not enhance the saturation level of reconnected flux. (3) When the ion temperature anisotropy is added, it survives through the small island formation stage and makes even quicker triggering to happen when the LHDI effects set-in. Furthermore the saturation level is seen to be elevated by a factor of ~2 and large-scale reconnection is achieved only in this case. Comparison with two-dimensional simulations that exclude the LHDI effects confirms that the saturation level enhancement is due to the ion anisotropy effects, while the LHDI effects shorten the overall time scale significantly. The results imply that the ion temperature anisotropy is one of the key properties that enable large-scale magnetic reconnection to develop in a super-ion-scale current sheet.

The role of the Weibel instability at the reconnection jet front in relativistic pair plasma reconnection

The role of the Weibel instability is investigated for the first time in the context of the large-scale magnetic reconnection problem. A late-time evolution of magnetic reconnection in relativistic pair plasmas is demonstrated by particle-in-cell simulations. In the outflow regions, powerful reconnection jets pile up the magnetic fields and then a contact discontinuity appears there. Further downstream, it is found that the two-dimensional extension of the relativistic Weibel instability generates electromagnetic fields, which are comparable to the antiparallel or piled-up fields. In a microscopic viewpoint, the instability allows the plasma’s multiple interactions with the discontinuity. In a macroscopic viewpoint, the instability leads to rapid expansion of the current sheet and then the reconnection jet front further propagates into the downstream. Possible application to the three-dimensional case is briefly discussed.

Characterization of waves in the vicinity of an interplanetary directional discontinuity

[1] Directional discontinuities are frequently encountered in the solar wind. This study utilizes the possibility of simultaneous four-point measurements that the Cluster satellites provide for an investigation of the waves near one such interplanetary discontinuity event. In particular, the k-filtering technique has for the first time been applied for the wave characterization in terms of frequency, wave vector, wave power, and polarization at different distances from a discontinuity. The advantages of the k-filtering method are that these parameters can be transformed into the plasma frame of reference to allow comparison with theory and that it is possible to detect more than one wave mode at each frequency in the spacecraft frame of reference. The discontinuity event in this study was chosen because its wave activity had unusually high power and because it also contains signatures of large-scale magnetic reconnection, such as a reconnection exhaust. Two wave modes are found: one which has the features of a shear Alfvén wave with propagation direction ordered by the background magnetic field direction and one which behaves like a compressional Alfvén wave and is ordered by the discontinuity normal. Both wave modes become weaker farther away from the discontinuity, but the compressional Alfvén mode is more suppressed. The shear mode dominates for long wavelengths at all distances from the discontinuity. The wave field is also found to be asymmetric on the two sides of the event.

Multi-scale reconnections in a complex CME

A series of three flares of GOES class M, M and C, and a CME were observed on 20 January 2004 occurring in close succession in NOAA 10540. Types II, III, and N radio bursts were associated. We use the combined observations from TRACE, EIT, H α images from Kwasan Observatory, MDI magnetograms, GOES, and radio observations from Culgoora and Wind/ WAVES to understand the complex development of this event. We reach three main conclusions. First, we link the first two impulsive flares to tether-cutting reconnections and the launch of the CME. This complex observation shows that impulsive quadrupolar flares can be eruptive. Second, we relate the last of the flares, an LDE, to the relaxation phase following forced reconnections between the erupting flux rope and neighbouring magnetic field lines, when reconnection reverses and restores some of the pre-eruption magnetic connectivities. Finally, we show that reconnection with the magnetic structure of a previous CME launched about 8 h earlier injects electrons into open field lines having a local dip and apex (located at about six solar radii height). This is observed as an N-burst at decametre radio wavelengths. The dipped shape of these field lines is due to large-scale magnetic reconnection between expanding magnetic loops and open field lines of a neighbouring streamer. This particular situation explains why this is the first N-burst ever observed at long radio wavelengths.

Large-scale reconnection in a large flare

Some specific features of the large-scale magnetic reconnection in large solar flares are briefly reviewed. In particular, the large-scale structure and dynamics of the 3B/X5.7 flare on 14 July 2000 are interpreted in terms of the topological model. The role of the betatron effect in collapsing magnetic traps, that are created by reconnection in the solar corona, is considered. We discuss some possibilities to observe the collapsing trap acceleration in solar flares.

ASPECTS OF THREE-DIMENSIONAL MAGNETIC RECONNECTION (Invited Review)

In this review paper we discuss several aspects of magnetic reconnection theory, focusing on the field-line motions that are associated with reconnection. A new exact solution of the nonlinear MHD equations for reconnective annihilationis presented which represents a two-fold generalization of the previous solutions. Magnetic reconnection at null points by several mechanisms is summa- rized, including spine reconnection, fan reconnectionand separator reconnection, where it is pointed out that two common features of separator reconnection are the rapid flipping of magnetic field lines and the collapse of the separator to a current sheet. In addition, a formula for the rate of reconnection between two flux tubes is derived. The magnetic field of the corona is highly complex, since the magnetic carpet consists of a multitude of sources in the photosphere. Progress in understanding this complexity may, however, be made by constructing the skeleton of the field and developing a theory for the local and global bifurcations between the different topologies. The eruption of flux from the Sun may even sometimes be due to a change of topology caused by emerging flux break-out .A CD-ROM attached to this paper presents the results of a toy model of vacuum reconnection, which suggests that rapid flipping of field lines in fan and separator reconnection is an essential ingredient also in real non-vacuum conditions. In addition, it gives an example of binary reconnectionbetween a pair of unbalanced sources as they move around, which may contribute significantly to coronal heating. Finally, we present examples in TRACE movies of geometrical changes of the coronal magnetic field that are a likely result of large-scale magnetic reconnection.

Reorganization of the solar corona following a C4.7 flare

Yohkoh X-ray images, multifrequency two-dimentional observations of the Nancay Radioheliograph, Kitt Peak and Mees magnetograms provide a unique set of data with which to study a C4.7 long-duration flare that was observed close to the equator (S07, W11) on 25 Oct. 1994 at 09:49 UT. Linear force-free field extrapolations indicate a very high degree of non-potentiality in the active region. The X-ray flare started with the expansion of spectacular twisted loops. Fifteen minutes after the flare onset sporadic radio (type III) bursts were observed spreading over an area of almost 1 3 of the solar disc and two remote X-ray brightenings appeared over quiet regions of opposite magnetic polarity located in on opposite hemispheres of the Sun. In the close vicinity of these remote brightenings two coronal holes formed. The timing and location of these events combined with the overall magnetic configuration provide evidence for a large-scale magnetic reconnection occurring between the expanding twisted loops and the overlying huge loops which inter-connect quiet solar regions.

Evidence for Large-Scale Solar Magnetic Reconnection from Radio and X-Ray Measurements

Utilizing Yohkoh Soft X-ray Telescope and Nancay radioheliograph data, we present, for the first time, observations of expanding twisted X-ray loops and a series of nonthermal radio bursts that follow the loop expansion in time and space up to ∼12′ distance. The loops were produced during a long-duration C4.7 flare close to disk center on 1994 October 25 at 1049 UT. The series of radio bursts were observed on the southern hemisphere above a weak positive-polarity region. The Kitt Peak magnetogram shows the existence of a weak negative-polarity region on the northern hemisphere at the same heliolongitude. Simultaneously with the nonthermal radio bursts, we observed the appearance of two remote X-ray brightenings and subsequent formation of two coronal holes above these weak (quiet) magnetic regions of opposite polarity, which strongly suggest the involvement of these remote regions in the event. During the 6 hr-long gradual phase of the flare, new X-ray loop connections developed among the active region and the remote quiet regions. A nonthermal radio continuum emission originating from the active region was also observed. We propose that the series of radio bursts, two remote X-ray brightenings, and new coronal loop connections were all signatures of a large-scale reconnection process between the expanding twisted flare loops and overlying transequatorial loops connecting quiet-Sun regions. The reconnection was only partial; the external part of the overlying large-scale fields were pushed out in the solar wind by the expanding twisted loops, leading to the formation of the coronal holes. The interaction between the active region and the large-scale fields seemed to be active during the entire gradual phase of the flare. This scenario may also explain the measurement of high-energy electrons in the interplanetary medium from 74° south heliolatitude as observed by Ulysses. © 1996. The American Astronomical Society. All rights reserved.