Abstract:Magnetic field reconnection is believed to play a fundamental role in magnetized plasma systems throughout the universe 1 , including planetary magnetospheres, magnetars, and accretion discs around black holes. This letter presents extreme ultraviolet (EUV) and X-ray observations of a solar flare showing magnetic reconnection with a level of clarity not previously achieved. The multi-wavelength EUV observations from SDO/AIA show inflowing cool loops and newly formed, outflowing hot loops, as predicted. RHESSI X-ray spectra and images simultaneously show the appearance of plasma heated to >10 MK at the expected locations. These two data sets provide solid visual evidence of magnetic reconnection producing a solar flare, validating the basic physical mechanism of popular flare models. However, new features are also observed that need to be included in reconnection and flare studies, such as 3D non-uniform, non-steady, and asymmetric evolution. Main Text:The early concept of magnetic reconnection was proposed in the 1940s 1 to explain energy release in solar flares, the most powerful explosive phenomena in the solar system. The reconnection process reconfigures the field topology and converts magnetic energy to thermal energy, mass motions, and particle acceleration. The theories and related numerical simulations, especially 3D modelling, are still subjects of extensive research to obtain a full understanding of the process under different conditions. Meanwhile, observational studies have made progress in finding evidence of reconnection and deriving its physical properties to constrain and improve the theories.In-situ measurements of the magnetic field, plasma parameters, and particle distributions have shown the existence of magnetic reconnection in laboratory plasmas 2, 3 , fusion facilities, and magnetospheres of planets 4,5 . Such in-situ measurements are still not possible in the extremely hot solar atmosphere. Instead, observations are obtained through remote sensing of emissions across the entire electromagnetic spectrum from radio to X rays and gamma rays.However, in the corona 6 the magnetic field pressure dominates the plasma pressure (low plasma beta) and the magnetic flux is "frozen into" the highly conductive plasma. As a result, the emitting plasma trapped in coronal loops outlines the geometry of the magnetic field and their structural changes reflect the changes of the field connectivity (in general). Considerable pieces of evidence for features likely linked to reconnection in solar flares 7,8 and coronal mass ejections (CMEs 9 ) have been obtained so far. These include signatures of plasma inflow/outflow 10-14 , hot cusp structures 15 , current sheets [16][17][18] , fast-mode standing shocks 19 , and plasmoid ejection 20 .However, most evidence has been indirect and fragmented. Detailed observations of the complete picture are still missing due to the highly dynamic flare/CME process and limited observational capabilities.The launch of the Solar Dynamic Observatory (SDO 21 ) in 2010 sign...

The unusually large NOAA active region 2192, observed in October 2014, was outstanding in its productivity of major two-ribbon flares without coronal mass ejections. On a large scale, a predominantly north-south oriented magnetic system of arcade fields served as a strong, also lateral, confinement for a series of large two-ribbon flares originating from the core of the active region. The large initial separation of the flare ribbons, together with an almost absent growth in ribbon separation, suggests a confined reconnection site high up in the corona. Based on a detailed analysis of the confined X1.6 flare on October 22, we show how exceptional the flaring of this active region was. We provide evidence for repeated energy release, indicating that the same magnetic field structures were repeatedly involved in magnetic reconnection. We find that a large number of electrons was accelerated to non-thermal energies, revealing a steep power law spectrum, but that only a small fraction was accelerated to high energies. The total non-thermal energy in electrons derived (on the order of 10 25 J) is considerably higher than that in eruptive flares of class X1, and corresponds to about 10% of the excess magnetic energy present in the active-region corona.

The first evidence of transverse oscillations of a multistranded loop with growing amplitudes and internal coupling observed by the Atomspheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is presented. The loop oscillation event occurred on 2011 March 8, triggered by a CME. The multiwavelength analysis reveals the presence of multithermal strands in the oscillating loop, whose dynamic behaviors are temperature-dependent, showing differences in their oscillation amplitudes, phases and emission evolution. The physical parameters of growing oscillations of two strands in 171Å are measured and the 3-D loop geometry is determined using STEREO-A/EUVI data. These strands have very similar frequencies, and between two 193Å strands a quarter-period phase delay sets up. These features suggest the coupling between kink oscillations of neighboring strands and the interpretation by the collective kink mode as predicted by some models. However, the temperature dependence of the multistarnded loop oscillations was not studied previously and needs further investigation. The transverse loop oscillations are associated with intensity and loop width variations. We suggest that the amplitude-growing kink oscillations may be a result of continuous non-periodic driving by magnetic deformation of the CME, which deposits energy into the loop system at a rate faster than its loss.

Solar magnetized "tornadoes", a phenomenon discovered in the solar atmosphere, appear as tornado-like structures in the corona but root in the photosphere. Like other solar phenomena, solar tornadoes are a feature of magnetized

The Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) has been providing high-cadence, high-resolution, full-disk UV-visible/extreme ultraviolet (EUV) images since 2010, with the best time coverage among all the solar missions. A number of codes have been developed to extract plasma differential emission measures (DEMs) from AIA images. Although widely used, they cannot effectively constrain the DEM at flaring temperatures with AIA data alone. This often results in much higher X-ray fluxes than observed. One way to solve the problem is by adding more constraint from other data sets (such as soft X-ray images and fluxes). However, the spatial information of plasma DEMs are lost in many cases. In this Letter, we present a different approach to constrain the DEMs. We tested the sparse inversion code and show that the default settings reproduce X-ray fluxes that could be too high. Based on the tests with both simulated and observed AIA data, we provided recommended settings of basis functions and tolerances. The new DEM solutions derived from AIA images alone are much more consistent with (thermal) X-ray observations, and provide valuable information by mapping the thermal plasma from ∼0.3 to ∼30 MK. Such improvement is a key step in understanding the nature of individual X-ray sources, and particularly important for studies of flare initiation.

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Su et al. (2012) proposed a new explanation for filament formation and eruption, where filament barbs are rotating magnetic structures driven by underlying vortices on the surface. Such structures have been noticed as tornado-like prominences when they appear above the limb. They may play a key role as the source of plasma and twist in filaments. However, no observations have successfully distinguished rotational motion of the magnetic structures in tornado-like prominences from other motions such as oscillation and counter-streaming plasma flows. Here we report evidence of rotational motions in a tornado-like prominence. The spectroscopic observations in two coronal lines were obtained from a specifically designed Hinode/EIS observing program. The data revealed the existence of both cold and million-degree-hot plasma in the prominence leg, supporting the so-called "the prominence-corona transition region". The opposite velocities at the two sides of the prominence and their persistent time evolution, together with the periodic motions evident in SDO/AIA dark structures, indicate a rotational motion of both cold and hot plasma with a speed of ∼5 km s −1 .

For a solar flare occurring on 2010 November 3, we present observations using several SDO/AIA extreme-ultraviolet (EUV) passbands of an erupting flux rope followed by inflows sweeping into a current sheet region. The inflows are soon followed by outflows appearing to originate from near the termination point of the inflowing motion -an observation in line with standard magnetic reconnection models. We measure average inflow plane-of-sky speeds to range from ∼150 − 690 km s −1 with the initial, high-temperature inflows being the fastest. Using the inflow speeds and a range of Alfvén speeds, we estimate the Alfvénic Mach number which appears to decrease with time. We also provide inflow and outflow times with respect to RHESSI count rates and find that the fast, hightemperature inflows occur simultaneously with a peak in the RHESSI thermal light curve. Five candidate inflow-outflow pairs are identified with no more than a minute delay between detections. The inflow speeds of these pairs are measured to be ∼10 2 km s −1 with outflow speeds ranging from ∼10 2 -10 3 km s −1 -indicating acceleration during the reconnection process. The fastest of these outflows are in the form of apparently traveling density enhancements along the legs of the loops rather than the loop apexes themselves. These flows could possibly either be accelerated plasma, shocks, or waves prompted by reconnection. The 1 arXiv:1111.1945v2 [astro-ph.SR] 26 Jun 2012 measurements presented here show an order of magnitude difference between the retraction speeds of the loops and the speed of the density enhancements within the loops -presumably exiting the reconnection site.