Imaging coronal magnetic-field reconnection in a solar flare

Abstract

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...

Review on Current Sheets in CME Development: Theories and Observations

“…Similar downward flows over the top of flare loops were also observed in both HXR (25∼50 keV and 12∼25 keV) and SXR (6∼12 keV) by RHESSI in 3 events studied by Sui and Holman (2003) and Sui et al (2004). In these events, high temperature loops or the loops observed in high energy bands (25∼50 keV or 12∼25 keV) were seen above the low temperature loops or the loops observed in low energy bands (6∼12 keV).…”

“…In this process, magnetic reconnection between two systems yielded the ejection of the newly formed loops at speed between 90 and 440 km s −1 . Ignoring the projection effect and assuming the outflow speed to be identified with the local Alfvén speed, Su et al (2013) eventually ended up with that M A is between 0.05 and 0.5. These results are similar to those obtained by Takasao et al (2012) who noticed that the inflow speed was between 12 and 90 km s −1 , the outflow speed between 220 and 460 km s −1 , and M A between 0.055 and 0.2 accordingly.…”

“…Su et al (2013) studied a flare on 17 August 2011 that was created by collision of two coronal loop systems and observed in both EUV and X-ray emissions. The multi-wavelength EUV observations from SDO/AIA showed inflowing cool loops and newly formed, outflowing hot loops.…”

“…10). Usually, the value of M A could be obtained via different approaches (see also Ciaravella et al 2002;Ko et al 2003;Lin et al 2005;Bemporad et al 2006;Ciaravella and Raymond 2008;Savage et al 2010;Liu et al 2010Liu et al , 2013Liu 2013;Su et al 2013), and then estimate l min according to (1) is straightforward. For example, the minimum of the distances of every two adjacent blobs observed in the 2002 January 8 event is 3.062 × 10 5 km (see Fig.…”

Magnetic reconnection between a solar filament and nearby coronal loops

“…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.…”