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Register a new userFirst Detection of Plasmoids from Breakout Reconnection on the Sun
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Transient collimated plasma ejections (jets) occur frequently throughout the solar corona, in active regions, quiet Sun, and coronal holes. Although magnetic reconnection is generally agreed to be the mechanism of energy release in jets, the factors that dictate the location and rate of reconnection remain unclear. Our previous studies demonstrated that the magnetic breakout model explains the triggering and evolution of most jets over a wide range of scales, through detailed comparisons between our numerical simulations and high-resolution observations. An alternative explanation, the resistive-kink model, invokes breakout reconnection without forming and explosively expelling a flux rope. Here we report direct observations of breakout reconnection and plasmoid formation during two jets in the fan-spine topology of an embedded bipole. For the first time, we observed the formation and evolution of multiple small plasmoids with bidirectional flows associated with fast reconnection in 3D breakout current sheets in the solar corona. The first narrow jet was launched by reconnection at the breakout current sheet originating at the deformed 3D null, without significant flare reconnection or a filament eruption. In contrast, the second jet and release of cool filament plasma were triggered by explosive breakout reconnection when the leading edge of the rising flux rope formed by flare reconnection beneath the filament encountered the preexisting breakout current sheet. These observations solidly support both reconnection-driven jet models: the resistive kink for the first jet, and the breakout model for the second explosive jet with a filament eruption.
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Blobs, or quasi-spherical emission regions containing relativistic particles and magnetic fields, are often assumed ad hoc in emission models of relativistic astrophysical jets, yet their physical origin is still not well understood. Here, we employ a suite of large-scale two-dimensional particle-in-cell simulations in electron-positron plasmas to demonstrate that relativistic magnetic reconnection can naturally account for the formation of quasi-spherical plasmoids filled with high-energy particles and magnetic fields. Our simulations extend to unprecedentedly long temporal and spatial scales, so we can capture the asymptotic physics independently of the initial setup. We characterize the properties of the plasmoids that are continuously generated as a self-consistent by-product of the reconnection process: they are in rough energy equipartition between particles and magnetic fields; the upper energy cutoff of the plasmoid particle spectrum is proportional to the plasmoid width w, corresponding to a Larmor radius ~0.2 w; the plasmoids grow in size at ~0.1 of the speed of light, with most of the growth happening while they are still non-relativistic (first they grow); their growth is suppressed once they get accelerated to relativistic speeds by the field line tension, up to the Alfven speed (then they go). The largest plasmoids, whose typical recurrence interval is ~2.5 L/c, reach a characteristic size w ~ 0.2 L independently of the system length L, they have nearly isotropic particle distributions and they contain the highest energy particles, whose Larmor radius is ~0.03 L. The latter can be regarded as the Hillas criterion for relativistic reconnection. We briefly discuss the implications of our results for the high-energy emission from relativistic jets and pulsar winds.
Context. Coronal mass ejections (CMEs) on the Sun are the largest explosions in the Solar System that can drive powerful plasma shocks. The eruptions, shocks, and other processes associated to CMEs are efficient particle accelerators and the accelerated electrons in particular can produce radio bursts through the plasma emission mechanism. Aims. Coronal mass ejections and associated radio bursts have been well studied in cases where the CME originates close to the solar limb or within the frontside disc. Here, we study the radio emission associated with a CME eruption on the back side of the Sun on 22 July 2012. Methods. Using radio imaging from the Nanc{c}ay Radioheliograph, spectroscopic data from the Nanc{c}ay Decametric Array, and extreme-ultraviolet observations from the Solar Dynamics Observatory and Solar Terrestrial Relations Observatory spacecraft, we determine the nature of the observed radio emission as well as the location and propagation of the CME. Results. We show that the observed low-intensity radio emission corresponds to a type II radio burst or a short-duration type IV radio burst associated with a CME eruption due to breakout reconnection on the back side of the Sun, as suggested by the pre-eruptive magnetic field configuration. The radio emission consists of a large, extended structure, initially located ahead of the CME, that corresponds to various electron acceleration locations. Conclusions. The observations presented here are consistent with the breakout model of CME eruptions. The extended radio emission coincides with the location of the current sheet and quasi-separatrix boundary of the CME flux and the overlying helmet streamer and also with that of a large shock expected to form ahead of the CME in this configuration.
Magnetic reconnection is thought to drive a wide variety of dynamic phenomena in the solar atmosphere. Yet the detailed physical mechanisms driving reconnection are difficult to discern in the remote sensing observations that are used to study the solar atmosphere. In this paper we exploit the high-resolution instruments Interface Region Imaging Spectrograph (IRIS) and the new CHROMIS Fabry-Perot instrument at the Swedish 1-m Solar Telescope (SST) to identify the intermittency of magnetic reconnection and its association with the formation of plasmoids in so-called UV bursts in the low solar atmosphere. The Si IV 1403A UV burst spectra from the transition region show evidence of highly broadened line profiles with often non-Gaussian and triangular shapes, in addition to signatures of bidirectional flows. Such profiles had previously been linked, in idealized numerical simulations, to magnetic reconnection driven by the plasmoid instability. Simultaneous CHROMIS images in the chromospheric Ca II K 3934A line now provide compelling evidence for the presence of plasmoids, by revealing highly dynamic and rapidly moving brightenings that are smaller than 0.2 arcsec and that evolve on timescales of order seconds. Our interpretation of the observations is supported by detailed comparisons with synthetic observables from advanced numerical simulations of magnetic reconnection and associated plasmoids in the chromosphere. Our results highlight how subarcsecond imaging spectroscopy sensitive to a wide range of temperatures combined with advanced numerical simulations that are realistic enough to compare with observations can directly reveal the small-scale physical processes that drive the wide range of phenomena in the solar atmosphere.
Plasmoids -- magnetized quasi-circular structures formed self-consistently in reconnecting current sheets -- were previously considered to be the graveyards of energetic particles. In this paper, we demonstrate the important role of plasmoids in shaping the particle energy spectrum in relativistic reconnection (i.e., with upstream magnetization $sigma_{rm up} gg 1$). Using two dimensional particle-in-cell simulations in pair plasmas with $sigma_{rm up}=10$ and $100$, we study a secondary particle energization process that takes place inside compressing plasmoids. We demonstrate that plasmoids grow in time, while their interiors compress, amplifying the internal magnetic field. The magnetic field felt by particles injected in an isolated plasmoid increases linearly with time, which leads to particle energization as a result of magnetic moment conservation. For particles injected with a power-law distribution function, this energization process acts in such a way that the shape of the injected power law is conserved, while producing an additional non-thermal tail $f(E)propto E^{-3}$ at higher energies followed by an exponential cutoff. The cutoff energy, which increases with time as $E_{rm cut}proptosqrt{t}$, can greatly exceed $sigma_{rm up} m_e c^2$. We analytically predict the secondary acceleration timescale and the shape of the emerging particle energy spectrum, which can be of major importance in certain astrophysical systems, such as blazar jets.
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N. Nishizuka Institute of Space andn Astronautical Science
, Japan Aerospace Exploration Agency
2013
We propose the particle acceleration model coupled with multiple plasmoid ejections in a solar flare. Unsteady reconnection produces plasmoids in a current sheet and ejects them out to the fast shocks, where particles in a plasmoid are reflected upstream the shock front by magnetic mirror effect. As the plasmoid passes through the shock front, the reflection distance becomes shorter and shorter driving Fermi acceleration, until it becomes proton Larmor radius. The fractal distribution of plasmoids may also have a role in naturally explaining the power-law spectrum in nonthermal emissions.
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