Our understanding of the process of fast reconnection has undergone a dramatic change in the last 10 years driven, in part, by the availability of high-resolution numerical simulations that have consistently demonstrated the break-up of current sheets into magnetic islands, with reconnection rates that become independent of Lundquist number, challenging the belief that fast magnetic reconnection in flares proceeds via the Petschek mechanism that invokes pairs of slow-mode shocks connected to a compact diffusion region. The reconnection sites are too small to be resolved with images but these reconnection mechanisms, Petschek and the plasmoid instability, have reconnection sites with very different density and velocity structures and so can be distinguished by high-resolution lineprofiles observations. Using IRIS spectroscopic observations we obtain a survey of typical line profiles produced by small-scale events thought to be reconnection sites on the Sun. Slit-jaw images are used to investigate the plasma heating and re-configuration at the sites. A sample of 15 events from two active regions is presented. The line profiles are complex with bright cores and broad wings extending to over 300 km s −1 . The profiles can be reproduced with the multiple magnetic islands and acceleration sites that characterise the plasmoid instability but not by bi-directional jets that characterise the Petschek mechanism. This result suggests that if these small-scale events are reconnection sites, then fast reconnection proceeds via the plasmoid instability, rather than the Petschek mechanism during small-scale reconnection on the Sun.

Magnetic reconnection, the rearrangement of magnetic field topology, is a fundamental physical process in magnetized plasma systems all over the universe 1,2 . Its process is difficult to be directly observed. Coronal structures, such as coronal loops and filament spines, often sketch the magnetic field geometry and its changes in the solar corona 3 . Here we show a highly suggestive observation of magnetic reconnection between an erupting solar filament and its nearby coronal loops, resulting in changes in connection of the filament. X-type structures form when the erupting filament encounters the loops. The filament becomes straight, and bright current sheets form at the interfaces with the loops. Many plasmoids appear in these current sheets and propagate bi-directionally. The filament disconnects from the current sheets, which gradually disperse and disappear, reconnects to the loops, and becomes redirected to the loop footpoints. This evolution of the filament and the loops suggests successive magnetic reconnection predicted by theories 1 but rarely detected with such clarity in observations. Our results on the formation, evolution, and disappearance of current sheets, confirm three-dimensional magnetic reconnection theory and have implications for the evolution of dissipation regions and the release of magnetic energy for reconnection in many magnetized plasma systems.Magnetic reconnection 1,2 is considered to play an essential role in the rapid release of the magnetic energy and its conversion to other forms (thermal, kinetic and particle) in magnetized plasma systems (such as accretion disks, solar and stellar coronae, planetary magnetospheres, and laboratory plasmas) throughout the universe. It shows the reconfiguration of the magnetic field geometry. In solar physics, numerous theoretical studies of the magnetic reconnection have been undertaken to explain flares 5 , filament eruptions 6 , et al. In two dimensional (2D) models, reconnection occurs at an X-point where anti-parallel magnetic field lines converge and reconnect 1,5,6 . So far, many observations of magnetic reconnection signatures, e.g., cusp-shaped post-flare loops 7 , loop-top hard X-ray source 3,8 , reconnection inflows 3,9 and outflows 3,10,11 , flare supra-arcades downflows 12,13 , current sheets, and plasmoid ejections 11 , have been reported by using remote sensing data. However, to directly observe the details of magnetic reconnection process is difficult, because of the small spatial scale and the fast temporal evolution of the process.A solar filament is a relatively cool and dense plasma structure in the corona suspended above a magnetic polarity inversion line (PIL), with ends rooted in regions with opposite magnetic polarity. Its spine, a narrow ribbon-like structure through the full filament, consists of horizontal and parallel threads 14 when viewed from above. In the region around a filament, the plasma-beta, i.e., the ratio of thermal to magnetic energy density, is below unity. Because of the high electric conductivity, the...

Aims. We identify high-frequency Alfvén waves propagating upward in the solar chromosphere and transition region from observation by Solar Optical Telescope (SOT) onboard Hinode. Methods. The spicule shape is enhanced through application of a normal radial gradient filter and an un-sharp mask on the images taken by SOT. The displaced position of the spicule is at each height obtained by tracing the maximum intensity after image processing. The dominant wave period is obtained by the FFT method applied to the time variations of the displaced position at a certain height. The phase speed is estimated with the help of a cross-correlation analysis of two temporal sequences of the displaced positions at two heights along the spicule. Results. We find in four cases that the spicules are modulated by high-frequency (≥0.02 Hz) transverse fluctuations. Such fluctuations are suggested to be Alfvén waves that propagate upward along the spicules with phase speed ranges from 50 to 150 km s −1 . Three of the modulated spicules show clear wave-like shapes with short wavelengths less than 8 Mm. Conclusions. Our work identified directly upward propagation of Alfvén waves in the solar chromosphere and transition region. In addition to the recently reported Alfvén waves with very long wavelength and wave period, we find here four examples of Alfvén waves with shorter wavelengths and periods. These findings shed new light on the wave origin and on coronal and solar-wind heating.