Reciprocatory magnetic reconnection in a coronal bright point

Coronal bright points (CBPs) are small-scale and long-duration brightenings in the lower solar corona. They are often explained in terms of magnetic reconnection. We aim to study the sub-structures of a CBP and clarify the relationship among the brightenings of different patches inside the CBP. The event was observed by the X-ray Telescope (XRT) aboard the Hinode spacecraft on 2009 August 22$-$23. The CBP showed repetitive brightenings (or CBP flashes). During each of the two successive CBP flashes, i.e., weak and strong flashes which are separated by $\sim$2 hr, the XRT images revealed that the CBP was composed of two chambers, i.e., patches A and B. During the weak flash, patch A brightened first, and patch B brightened $\sim$2 min later. During the transition, the right leg of a large-scale coronal loop drifted from the right side of the CBP to the left side. During the strong flash, patch B brightened first, and patch A brightened $\sim$2 min later. During the transition, the right leg of the large-scale coronal loop drifted from the left side of the CBP to the right side. In each flash, the rapid change of the connectivity of the large-scale coronal loop is strongly suggestive of the interchange reconnection. For the first time we found reciprocatory reconnection in the CBP, i.e., reconnected loops in the outflow region of the first reconnection process serve as the inflow of the second reconnection process.Comment: 13 pages, 8 figure

H-Alpha and Hard X-Ray Observations of a Two-Ribbon Flare Associated with a Filament Eruption

We perform a multi-wavelength study of a two-ribbon flare on 2002 September 29 and its associated filament eruption, observed simultaneously in the H-alpha line by a ground-based imaging spectrograph and in hard X-rays by RHESSI. The flare ribbons contain several H-alpha bright kernels that show different evolutional behaviors. In particular, we find two kernels that may be the footpoints of a loop. A single hard X-ray source appears to cover these two kernels and to move across the magnetic neutral line. We explain this as a result of the merging of two footpoint sources that show gradually asymmetric emission owing to an asymmetric magnetic topology of the newly reconnected loops. In one of the H-alpha kernels, we detect a continuum enhancement at the visible wavelength. By checking its spatial and temporal relationship with the hard X-ray emission, we ascribe it as being caused by electron beam precipitation. In addition, we derive the line-of-sight velocity of the filament plasma based on the Doppler shift of the filament-caused absorption in the H-alpha blue wing. The filament shows rapid acceleration during the impulsive phase. These observational features are in principal consistent with the general scenario of the canonical two-ribbon flare model.Comment: 15 pages, 5 figures, accepted for publication in Ap

NMR Determination of an Incommensurate Helical Antiferromagnetic Structure in EuCo2As2

We report $^{153}$Eu, $^{75}$As and $^{59}$Co nuclear magnetic resonance (NMR) results on EuCo$_2$As$_2$ single crystal. Observations of $^{153}$Eu and $^{75}$As NMR spectra in zero magnetic field at 4.3 K below an antiferromagnetic (AFM) ordering temperature $T_{\rm N}$ = 45 K and its external magnetic field dependence clearly evidence an incommensurate helical AFM structure in EuCo$_2$As$_2$. Furthermore, based on $^{59}$Co NMR data in both the paramagnetic and the incommensurate AFM states, we have determined the model-independent value of the AFM propagation vector ${\bf k}$ = (0, 0, 0.73 $\pm$ 0.07)2$\pi$/$c$ where $c$ is the $c$ lattice parameter. Thus the incommensurate helical AFM state was characterized by only NMR data with model-independent analyses, showing NMR to be a unique tool for determination of the spin structure in incommensurate helical AFMs.Comment: 6 pages, 4 figures, accepted for publication in Phys.Rev.