SIGMOID-TO-FLUX-ROPE TRANSITION LEADING TO A LOOP-LIKE CORONAL MASS EJECTION
Sigmoids are one of the most important precursor structures for solar eruptions. In this Letter, we study a sigmoid eruption on 2010 August 1 with EUV data obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO). In AIA 94 A (Fe XVIII; 6 MK), topological reconfiguration due to tether-cutting reconnection is unambiguously observed for the first time, i.e., two opposite J-shaped loops reconnect to form a continuous S-shaped loop, whose central portion is dipped and aligned along the magnetic polarity inversion line (PIL), and a compact loop crossing the PIL. A causal relationship between photospheric flows and coronal tether-cutting reconnections is evidenced by the detection of persistent converging flows toward the PIL using line-of-sight magnetograms obtained by the Helioseismic and Magnetic Imager on board SDO. The S-shaped loop remains in quasi-equilibrium in the lower corona for about 50 minutes, with the central dipped portion rising slowly at ~10 km s–1. The speed then increases to ~60 km s–1 about 10 minutes prior to the onset of a GOES-class C3.2 flare, as the S-shaped loop speeds up its transformation into an arch-shaped loop, which eventually leads to a loop-like coronal mass ejection. The AIA observations combined with Hα filtergrams as well as hard X-ray imaging and spectroscopy are consistent with most flare loops being formed by reconnection of the stretched legs of less-sheared J-shaped loops that envelopes the rising flux rope, in agreement with the standard tether-cutting scenario.

title: Simulation of photoelectric X-ray polarimetry and reconstruction of the photoelectron azimuthal angle
abstract: Sensitive X-ray polarimetry in the keV energy range can be achieved by measuring the azimuthal angle distribution of emitted electrons after the photoelectric absorption of X-rays in a micropattern gas detector. However, the initial direction of the electron is not readily measurable due to the randomization of its motion during energy loss. By using the Geant4, Maxwell and Garfield packages, we simulated the detected electron tracks following photoelectric absorption, electron drift and diffusion in the gas, and proposed a technique capable of reconstructing the initial direction of the emitted photoelectron. The technique allows us to measure the angular modulation of flux predicted for a polarized X-ray beam. We calculated the modulation factors in 2–10 keV with a gas mixture of neon and CO2, and discussed how electron diffusion along the drift will dilute the track and suppress the modulation. These results are useful for the design of the X-ray polarimeter.

title: The Relationship between the Sudden Change of the Lorentz Force and the Magnitude of Associated Flares
abstract: The rapid and irreversible change of photospheric magnetic fields associated with flares has been confirmed by many recent studies. These studies showed that the photospheric magnetic fields respond to coronal field restructuring and turn to a more horizontal state near the magnetic polarity inversion line (PIL) after eruptions. Recent theoretical work has shown that the change in the Lorentz force associated with a magnetic eruption will lead to such a field configuration at the photosphere. The Helioseismic Magnetic Imager has been providing unprecedented full-disk vector magnetograms covering the rising phase of the solar cycle 24. In this study, we analyze 18 flares in four active regions, with GOES X-ray class ranging from C4.7 to X5.4. We find that there are permanent and rapid changes of magnetic field around the flaring PIL, the most notable of which is the increase of the transverse magnetic field. The changes of fields integrated over the area and the derived change of Lorentz force both show a strong correlation with flare magnitude. It is the first time that such magnetic field changes have been observed even for C-class flares. Furthermore, for seven events with associated coronal mass ejections (CMEs), we use an estimate of themore » impulse provided by the Lorentz force, plus the observed CME velocity, to estimate the CME mass. We find that if the timescale of the back reaction is short, i.e., in the order of 10 s, the derived values of CME mass ({approx}10{sup 15} g) generally agree with those reported in literature.« less

title: Plasmoid ejection and secondary current sheet generation from magnetic reconnection in laser-plasma interaction.
abstract: Reconnection of the self-generated magnetic fields in laser-plasma interaction was first investigated experimentally by Nilson et al.  by shining two laser pulses a distance apart on a solid target layer. An elongated current sheet (CS) was observed in the plasma between the two laser spots. In order to more closely model magnetotail reconnection, here two side-by-side thin target layers, instead of a single one, are used. It is found that at one end of the elongated CS a fanlike electron outflow region including three well-collimated electron jets appears. The (>1 MeV) tail of the jet energy distribution exhibits a power-law scaling. The enhanced electron acceleration is attributed to the intense inductive electric field in the narrow electron dominated reconnection region, as well as additional acceleration as they are trapped inside the rapidly moving plasmoid formed in and ejected from the CS. The ejection also induces a secondary CS.

title: BBU effect in an ERL-FEL two-purpose test facility
abstract: Both the Energy Recovery Linac (ERL) and Free Electron laser (FEL) are considered to be candidates of the fourth generation light source. It is proposed to combine FEL into an ERL facility to integrate the advantages of both ERL and FEL, and to realize a compact two-purpose light source. A test facility to verify this principle is being designed at the Institute of High Energy Physics, Beijing. One main concern is the beam breakup (BBU) instability which limits the available beam current. To this end, we developed a numerical simulation code to calculate the BBU threshold, which is found to have only a small reduction even in a high-FEL-bunch-charge operation mode, compared with that in the case with ERL bunches only. However, even with ERL beam current far below BBU threshold, we observed a fluctuation of the central orbit of the ERL bunches in the presence of FEL beam. We then present a physical model of BBU and understand the mechanism of the orbit-fluctuation in an ERL-FEL two-purpose machine. We found that by choosing an appropriate FEL bunch repetition rate, the central orbit fluctuation amplitude can be well controlled.

title: Extended radio emission after the soft X-ray maximum of the NOAA 9077 AR solar flare on July 10, 2000
abstract: An extended radio emission after a soft X-ray (SXR) maximum was detected in the active region NOAA 9077 by several observatories for the solar flare after 21:42 UT on July 10, 2000. Also some radio fine structures before the enduring radio emission were observed with the 1.0-2.0 GHz spectrometer of Beijing Astronomical Observatory (BAO) in the same time. We apply a shear-driven quadrupolar reconnection model (SQR) to analyze the fine structures and the related radio emission. We find that the footpoint shear motion of the flux loop is accompanied with the emerging up of the loop during the reconnection process. We tentatively interpret the extended radio emission as the nonthermal radiation caused by a new reconnection process between emerging flux loop and pre-existing overarching loop after the soft X-ray maximum.

title: Quantitative Analysis of CME Deflections in the Corona
abstract: In this paper, ten CME events viewed by the STEREO twin spacecraft are analyzed to study the deflections of CMEs during their propagation in the corona. Based on the three-dimensional information of the CMEs derived by the graduated cylindrical shell (GCS) model (Thernisien, Howard, and Vourlidas in  2006), it is found that the propagation directions of eight CMEs had changed. By applying the theoretical method proposed by Shen et al. (Solar Phys.269, 389, 2011) to all the CMEs, we found that the deflections are consistent, in strength and direction, with the gradient of the magnetic energy density. There is a positive correlation between the deflection rate and the strength of the magnetic energy density gradient and a weak anti-correlation between the deflection rate and the CME speed. Our results suggest that the deflections of CMEs are mainly controlled by the background magnetic field and can be quantitatively described by the magnetic energy density gradient (MEDG) model.

title: The experimental study on beam-photoelectron instability in BEPC
abstract: A vertical coupled bunch instability explained as the photoelectron instability (PEI) was observed in the photon factory at KEK. To further investigate the instability, the experiments on the PEI have been carried out in Beijing Electron Positron Collider (BEPC) with the collaboration between MEP, China and KEK, Japan. Many interesting phenomena have been observed for different sets of parameters.