A Reconnecting Current Sheet Imaged in a Solar Flare
Magnetic reconnection changes the magnetic field topology and powers explosive events in astrophysical, space, and laboratory plasmas. For flares and coronal mass ejections (CMEs) in the solar atmosphere, the standard model predicts the presence of a reconnecting current sheet, which has been the subject of considerable theoretical and numerical modeling over the last 50 years, yet direct, unambiguous observational verification has been absent. In this Letter, we show a bright sheet structure of global length (>0.25 R ?) and macroscopic width ((5-10)?103?km) distinctly above the cusp-shaped flaring loop, imaged during the flare rising phase in EUV. The sheet formed due to the stretch of a transequatorial loop system and was accompanied by various reconnection signatures. This unique event provides a comprehensive view of the reconnection geometry and dynamics in the solar corona.

title: TWO DISTINCT PHASES OF HARD X-RAY EMISSIONS IN A SOLAR ERUPTIVE FLARE
abstract: We present a detailed analysis of the evolution of an M7.6 flare that occurred near the south-east limb on October 24, 2003 utilizing a multi-wavelength data set. Preflare images at TRACE 195 Å show that the bright and complex system of coronal loops already existed at the flaring site. The X-ray light curves clearly reveal two phases of flare evolution. The emission during the first phase is seen in GOES and RHESSI measurements at energies below 25 keV, while the second phase is evident in all the X-ray energies as high as 300 keV. The first phase is gradual whereas the second phase shows impulsive emission with several individual hard X-ray bursts. The first phase starts with the appearance of an X-ray loop-top (LT) source in RHESSI images below 25 keV. About 5 minute later, the TRACE 195 Å images show an intense emission that is cospatial with RHESSI LT source. This hot and diffuse TRACE emission is attributed to the existence of 15-20 MK plasma, heated directly from the primary energy source. Both X-ray and TRACE LT sources exhibit an altitude decrease for ∼ 11 minute. The high energy X-ray emission between 25-300 keV dominates the second phase, which correlates well with TRACE 1600 Å and Hα flare ribbons. The first phase seems to be mostly dominated by hot thermal emission from flaring loops with temperatures T > 25 MK. The second phase is found to be dominantly non-thermal in Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea IGAM/Institute of Physics, University of Graz, Universitätsplatz 5, A-8010 Graz, Austria Department of Astronomy and Space Science, Kyung Hee University, Yongin 446-701, Korea Physics Department, New Jersey Institute of Technology, 161 Warren Street, Newark, NJ 07102 Astronomical Institute, Moscow State University, Universitetskij Prospekt 13, Moscow 119992 Radio Astronomy Centre, Tata Institute of Fundamental Research, Udhagamandalam (Ooty) 643 001, India

title: He I D3 Observations of the 1984 May 22 M6.3 Solar Flare
abstract: The He I D3 line has a unique response to a flare impact on the low solar atmosphere and can be a powerful diagnostic tool for energy transport processes. Using images obtained from the recently digitized films of the Big Bear Solar Observatory, we report D3 observations of the M6.3 flare on 1984 May 22, which occurred in an active region with a circular magnetic polarity inversion line (PIL). The impulsive phase of the flare starts with a main elongated source that darkens in D3, inside of which bright emission kernels appear at the time of the initial small peak in hard X-rays (HXRs). These flare cores subsequently evolve into a sharp emission strand lying within the dark halo; this evolution occurs at the same time as the main peak in HXRs, reversing the overall source contrast from ?5% to 5%. The radiated energy in D3 during the main peak is estimated to be about 1030 erg, which is comparable to that carried by nonthermal electrons above 20?keV. Afterward, the flare proceeds along the circular PIL in the counterclockwise direction to form a dark circular ribbon in D3, which apparently mirrors the bright ribbons in H? and He I 10830 ?. All of these ribbons last for over one hour in the late gradual phase. We suggest that the present event resembles the so-called black-light flare that was proposed based on continuum images, and that D3 darkening and brightening features herein may be due to thermal conduction heating and the direct precipitation of high-energy electrons, respectively.

title: The Variation of Relative Magnetic Helicity around Major Flares
abstract: We have investigated the variation of magnetic helicity over a span of several days around the times of 11 X-class flares which occurred in seven active regions (NOAA 9672, 10030, 10314, 10486, 10564, 10696, and 10720) using the magnetograms taken by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO). As a major result we found that each of these major flares was preceded by a significant helicity accumulation, (1.8–16) × 1042 Mx2 over a long period (0.5 to a few days). Another finding is that the helicity accumulates at a nearly constant rate, (4.5–48) × 1040 Mx2 hr−1, and then becomes nearly constant before the flares. This led us to distinguish the helicity variation into two phases: a phase of monotonically increasing helicity and the following phase of relatively constant helicity. As expected, the amount of helicity accumulated shows a modest correlation with time-integrated soft X-ray flux during flares. However, the average helicity change rate in the first phase shows even stronger correlation with the time-integrated soft X-ray flux. We discuss the physical implications of this result and the possibility that this characteristic helicity variation pattern can be used as an early warning sign for solar eruptions.

title: CORONAL THICK TARGET HARD X-RAY EMISSIONS AND RADIO EMISSIONS
abstract: A distinctive class of hard X-ray (HXR) sources located in the corona was recently found, which implies that the collisionally thick target model (CTTM) applies even to the corona. We investigated whether this idea can be independently verified by microwave radiations which have been known as the best companion to HXRs. This study is conducted on the GOES M2.3 class flare which occurred on 2002 September 9 and was observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager and the Owens Valley Solar Array. Interpreting the observed energy-dependent variation of HXR source size under the CTTM, the coronal density should be as high as 5 Multiplication-Sign 10{sup 11} cm{sup -3} over a distance of up to 12''. To explain the cutoff feature of the microwave spectrum at 3 GHz, however, we require a density no higher than 1 Multiplication-Sign 10{sup 11} cm{sup -3}. Additional constraints must be placed on the temperature and magnetic field of the coronal source in order to reproduce the microwave spectrum as a whole. First, a spectral feature called the Razin suppression requires a magnetic field in a range of 250-350 G along with high viewing angles around 75 Degree-Sign . Second, to avoid excess fluxesmore » at high frequencies due to the free-free emission that was not observed, we need a high temperature {>=}2 Multiplication-Sign 10{sup 7} K. These two microwave spectral features, Razin suppression and free-free emissions, become more significant at regions of high thermal plasma density and are essential for validating and determining additional parameters of the coronal HXR sources.« less

title: Pre-Flare Activity and Magnetic Reconnection during the Evolutionary Stages of Energy Release in a Solar Eruptive Flare
abstract: In this paper, we present a multi-wavelength analysis of an eruptive white-light M3.2 flare that occurred in active region NOAA 10486 on 2003 November 1. The excellent set of high-resolution observations made by RHESSI and the TRACE provides clear evidence of significant pre-flare activities for ~9 minutes in the form of an initiation phase observed at EUV/UV wavelengths followed by an X-ray precursor phase. During the initiation phase, we observed localized brightenings in the highly sheared core region close to the filament and interactions among short EUV loops overlying the filament, which led to the opening of magnetic field lines. The X-ray precursor phase is manifested in RHESSI measurements below ~30 keV and coincided with the beginning of flux emergence at the flaring location along with early signatures of the eruption. The RHESSI observations reveal that both plasma heating and electron acceleration occurred during the precursor phase. The main flare is consistent with the standard flare model. However, after the impulsive phase, an intense hard X-ray (HXR) looptop source was observed without significant footpoint emission. More intriguingly, for a brief period, the looptop source exhibited strong HXR emission with energies up to ~50-100 keV and significant non-thermal characteristics. The present study indicates a causal relation between the activities in the pre-flare and the main flare. We also conclude that pre-flare activities, occurring in the form of subtle magnetic reorganization along with localized magnetic reconnection, played a crucial role in destabilizing the active region filament, leading to a solar eruptive flare and associated large-scale phenomena.

title: Instanton-inspired model of QCD phase transitions and bubble dynamics
abstract: We have reinvestigated the collision of gluonic bubbles in an SU(2) model of QCD which was studied by Johnson, Choi, and Kisslinger in the context of the instanton-inspired model of QCD phase transition bubbles with plane-wave approximation. We discuss treacherous points of the instanton-inspired model that cause the violation of causality due to the presence of imaginary gluon fields. By constructing a new slightly modified Lorentzian model where we have three independent real gluon fields, we reanalyzed the process of bubble collisions. Our numerical results show some indication of forming a bubble wall in the colliding region.

title: The Higgs Boson Sector of the Next-to-MSSM with CP Violation
abstract: We perform a comprehensive study of the Higgs sector in the framework of the next-to-minimal supersymmetric standard model with CP-violating parameters in the superpotential and in the soft-supersymmetry-breaking sector. Since the CP is no longer a good symmetry, the two CP-odd and the three CP-even Higgs bosons of the next-to-minimal supersymmetric standard model in the CP-conserving limit will mix. We show explicitly how the mass spectrum and couplings to gauge bosons of the various Higgs bosons change when the CP-violating phases take on nonzero values. We include full one-loop and the logarithmically enhanced two-loop effects employing the renormalization-group (RG) improved approach. In addition, the LEP limits, the global minimum condition, and the positivity of the square of the Higgs-boson mass have been imposed. We demonstrate the effects on the Higgs-mass spectrum and the couplings to gauge bosons with and without the RG-improved corrections. Substantial modifications to the allowed parameter space happen because of the changes to the Higgs-boson spectrum and their couplings with the RG-improved corrections. Finally, we calculate the mass spectrum and couplings of the few selected scenarios and compare to the previous results in literature where possible; in particular, we illustrate a scenario motivated by electroweak baryogenesis.

title: Classical stability of black D3-branes
abstract: We have investigated the classical stability of charged black D3-branes in the type-IIB supergravity under small perturbations. For s-wave perturbations it turns out that black D3-branes are unstable when they have small charge density. As the charge density increases for given mass density, however, the instability decreases down to zero at a certain finite value of the charge density, and then black D3-branes become stable all the way down to the extremal point. It has also been shown that such critical value at which its stability behavior changes agrees very well with the predicted one by the thermodynamic stability behavior of the corresponding black hole system through the Gubser-Mitra conjecture. Unstable mode solutions we found involve non-vanishing fluctuations of the self-dual five-form field strength. Some implications of our results are also discussed.

title: New measurements of the transverse beam asymmetry for elastic electron scattering from selected nuclei
abstract: Here we have measured the beam-normal single-spin asymmetry A{sub n} in the elastic scattering of 1-3 GeV transversely polarized electrons from 1H and for the first time from 4He, 12C, and 208Pb. For 1H, 4He and 12C, the measurements are in agreement with calculations that relate An to the imaginary part of the two-photon exchange amplitude including inelastic intermediate states. Surprisingly, the 208Pb result is significantly smaller than the corresponding prediction using the same formalism. Our results suggest that a systematic set of new An measurements might emerge as a new and sensitive probe of the structure of heavy nuclei.

title: Relativistic corrections to the exclusive decays of C-even bottomonia into S-wave charmonium pairs
abstract: Within the nonrelativistic quantum chromodynamics (NRQCD) factorization formalism, we compute the relativistic corrections to the exclusive decays of bottomonia with even charge conjugation parity into S-wave charmonium pairs at leading order in the strong coupling constant. Relativistic corrections are resummed for a class of color-singlet contributions to all orders in the charm-quark velocity v(c) in the charmonium rest frame. Almost every process that we consider in this work has negative relativistic corrections ranging from -20 to -35%. Among the various processes, the relativistic corrections of the next-to-leading order in v(c) to the decay rate for chi(b2) -> eta(c)(mS) + eta(c)(nS) with m, n = 1 or 2 are very large. In every case, the resummation of the relativistic corrections enhances the rate in comparison with the next-to-leading-order results. We compare our results with available predictions based on the NRQCD factorization formalism. The NRQCD predictions are significantly smaller than those based on the light-cone formalism by 1 or 2 orders of magnitude.