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Y. -M. Wang

Publications and source records attributed to Y. -M. Wang.

At least 19 recordsLinked to original sources
arXiv2025

Coronal hole picoflare jets are progenitors of both fast and Alfvénic slow solar wind

Solar wind, classified by its bulk speed and the Alfvénic nature of its fluctuations, generates the heliosphere. The elusive physical processes responsible for the generation of the different types of this wind are a topic of active debate. Recent observations reveal intermittent jets, with kinetic energy in the picoflare range, emerging from dark areas of a polar coronal hole threaded by open magnetic field lines. These could substantially contribute to solar wind. However, their ubiquity and direct links to solar wind have not been established. Here, we report a unique set of remote-sensing and in situ observations from the Solar Orbiter spacecraft that establish a unified picture of fast and Alfvénic slow wind, connected to the similar widespread picoflare jet activity in two coronal holes. Radial expansion of coronal holes ultimately regulates the speed of the emerging wind.

astro-ph.SR↗
arXiv2022

Undetected Minority-polarity Flux as the Missing Link in Coronal Heating

During the last few decades, the most widely favored models for coronal heating have involved the in situ dissipation of energy, with footpoint shuffling giving rise to multiple current sheets (the "nanoflare" model) or to Alfv{é}n waves that leak into the corona and undergo dissipative interactions (the wave heating scenario). As has been recognized earlier, observations suggest instead that the energy deposition is concentrated at very low heights, with the coronal loops being filled with hot, dense material from below, which accounts for their overdensities and flat temperature profiles. While an obvious mechanism for footpoint heating would be reconnection with small-scale fields, this possibility seems to have been widely ignored because magnetograms show almost no minority-polarity flux inside active region (AR) plages. Here, we present further examples to support our earlier conclusions (1) that magnetograms greatly underrepresent the amount of minority-polarity flux inside plages and "unipolar" network, and (2) that small loops are a major constituent of \ion{Fe}{9} 17.1 nm moss. On the assumption that the emergence or churning rate of small-scale flux is the same inside plages as in mixed-polarity regions of the quiet Sun, we estimate the energy flux density associated with reconnection with the plage fields to be on the order of 10$^7$ erg cm$^{-2}$ s$^{-1}$, sufficient to heat the AR corona.

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arXiv2021

Magnetograph Saturation and the Open Flux Problem

Extrapolations of line-of-sight photospheric field measurements predict radial interplanetary magnetic field (IMF) strengths that are factors of ~2--4 too low. To address this "open flux problem,'' we reanalyze the magnetograph measurements from different observatories, with particular focus on those made in the saturation-prone Fe I 525.0 nm line by the Mount Wilson Observatory (MWO) and the Wilcox Solar Observatory (WSO). The total dipole strengths, which determine the total open flux, generally show large variations among observatories, even when their total photospheric fluxes are in agreement. However, the MWO and WSO dipole strengths, as well as their total fluxes, agree remarkably well with each other, suggesting that the two data sets require the same scaling factor. As shown earlier by Ulrich et al., the saturation correction $δ^{-1}$ derived by comparing MWO measurements in the 525.0 nm line with those in the nonsaturating Fe I 523.3 nm line depends sensitively on where along the irregularly shaped 523.3 nm line wings the exit slits are placed. If the slits are positioned so that the 523.3 and 525.0 nm signals originate from the same height, $δ^{-1}$ ~ 4.5 at disk center, falling to ~2 near the limb. When this correction is applied to either the MWO or WSO maps, the derived open fluxes are consistent with the observed IMF magnitude. Other investigators obtained scaling factors only one-half as large because they sampled the 523.3 nm line farther out in the wings, where the shift between the right- and left-circularly polarized components is substantially smaller.

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arXiv2021

Coronal Mass Ejections and the Solar Cycle Variation of the Sun's Open Flux

The strength of the radial component of the interplanetary magnetic field (IMF), which is a measure of the Sun's total open flux, is observed to vary by roughly a factor of two over the 11 yr solar cycle. Several recent studies have proposed that the Sun's open flux consists of a constant or "floor" component that dominates at sunspot minimum, and a time-varying component due to coronal mass ejections (CMEs). Here, we point out that CMEs cannot account for the large peaks in the IMF strength which occurred in 2003 and late 2014, and which coincided with peaks in the Sun's equatorial dipole moment. We also show that near-Earth interplanetary CMEs, as identified in the catalog of Richardson and Cane, contribute at most $\sim$30\% of the average radial IMF strength even during sunspot maximum. We conclude that the long-term variation of the radial IMF strength is determined mainly by the Sun's total dipole moment, with the quadrupole moment and CMEs providing an additional boost near sunspot maximum. Most of the open flux is rooted in coronal holes, whose solar cycle evolution in turn reflects that of the Sun's lowest-order multipoles.

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arXiv2021

Observations of Slow Solar Wind from Equatorial Coronal Holes

Because of its distinctive compositional properties and variability, low-speed ($\lesssim 450$ km s$^{-1}$) solar wind is widely believed to originate from coronal streamers, unlike high-speed wind, which comes from coronal holes. An alternative scenario is that the bulk of the slow wind (excluding that in the immediate vicinity of the heliospheric current sheet) originates from rapidly diverging flux tubes rooted inside small coronal holes or just within the boundaries of large holes. This viewpoint is based largely on photospheric field extrapolations, which are subject to considerable uncertainties and do not include dynamical effects, making it difficult to be certain whether a source is located just inside or outside a hole boundary, or whether a high-latitude hole will be connected to Earth. To minimize the dependence on field-line extrapolations, we have searched for cases where equatorial coronal holes at central meridian are followed by low-speed streams at Earth. We describe 14 examples from the period 2014--2017, involving Fe XIV 21.1 nm coronal holes located near active regions and having equatorial widths of $\sim$3$^\circ$--10$^\circ$. The associated in situ wind was characterized by speeds $v\sim 300$--450 km s$^{-1}$ and by O$^{7+}$/O$^{6+}$ ratios of $\sim$0.05--0.15, with $v$ showing the usual correlation with proton temperature. In addition, consistent with other recent studies, this slow wind had remarkably high Alfvénicity, similar to that in high-speed streams. We conclude that small coronal holes are a major contributor to the slow solar wind during the maximum and early post-maximum phases of the solar cycle.

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arXiv2021

Further Evidence for Looplike Fine Structure inside "Unipolar" Active Region Plages

Earlier studies using extreme-ultraviolet images and line-of-sight magnetograms from the Solar Dynamics Observatory (SDO) have suggested that active region (AR) plages and strong network concentrations often have small, looplike features embedded within them, even though no minority-polarity flux is visible in the corresponding magnetograms. Because of the unexpected nature of these findings, we have searched the SDO database for examples of inverted-Y structures rooted inside "unipolar" plages, with such jetlike structures being interpreted as evidence for magnetic reconnection between small bipoles and the dominant-polarity field. Several illustrative cases are presented from the period 2013--2015, all of which are associated with transient outflows from AR "moss." The triangular or dome-shaped bases have horizontal dimensions of $\sim$2--4 Mm, corresponding to $\sim$1--3 granular diameters. We also note that the spongy-textured Fe IX 17.1 nm moss is not confined to plages, but may extend into regions where the photospheric field is relatively weak or even has mixed polarity. We again find a tendency for bright coronal loops seen in the 17.1, 19.3, and 21.1 nm passbands to show looplike fine structure and compact brightenings at their footpoints. These observations provide further confirmation that present-day magnetograms are significantly underrepresenting the amount of minority-polarity flux inside AR plages and again suggest that footpoint reconnection and small-scale flux cancellation may play a major role in coronal heating, both inside and outside ARs.

astro-ph.SR↗
arXiv2021

Small-scale Flux Emergence, Coronal Hole Heating, and Flux-tube Expansion: A Hybrid Solar Wind Model

Extreme-ultraviolet images from the Solar Dynamics Observatory often show looplike fine structure to be present where no minority-polarity flux is visible in magnetograms, suggesting that the rate of ephemeral region (ER) emergence inside "unipolar" regions has been underestimated. Assuming that this rate is the same inside coronal holes as in the quiet Sun, we show that interchange reconnection between ERs and open field lines gives rise to a solar wind energy flux that exceeds 10$^5$ erg cm$^{-2}$ s$^{-1}$ and that scales as the field strength at the coronal base, consistent with observations. In addition to providing Ohmic heating in the low corona, these reconnection events may be a source of Alfv{é}n waves with periods ranging from the granular timescale of $\sim$10 minutes to the supergranular/plume timescale of many hours, with some of the longer-period waves being reflected and dissipated in the outer corona. The asymptotic wind speed depends on the radial distribution of the heating, which is largely controlled by the rate of flux-tube expansion. Along the rapidly diverging flux tubes associated with slow wind, heating is concentrated well inside the sonic point (1) because the outward conductive heat-flux density and thus the outer coronal temperatures are reduced, and (2) because the net wave energy flux is dissipated at a rate proportional to the local Alfv{é}n speed. In this "hybrid" solar wind model, reconnection heats the lower corona and drives the mass flux, whereas waves impart energy and momentum to the outflow at greater distances.

astro-ph.SR↗
arXiv2014

Angular Momentum Exchange by Gravitational Torques and Infall in the Circumbinary Disk of the Protostellar System L1551 NE

We report the ALMA observation of the Class I binary protostellar system L1551 NE in the 0.9-mm continuum, C18O (3-2), and 13CO (3-2) lines at a ~1.6 times higher resolution and a ~6 times higher sensitivity than those of our previous SMA observations, which revealed a r ~300 AU-scale circumbinary disk in Keplerian rotation. The 0.9-mm continuum shows two opposing U-shaped brightenings in the circumbinary disk, and exhibits a depression between the circumbinary disk and the circumstellar disk of the primary protostar. The molecular lines trace non-axisymmetric deviations from Keplerian rotation in the circumbinary disk at higher velocities relative to the systemic velocity, where our previous SMA observations could not detect the lines. In addition, we detect inward motion along the minor axis of the circumbinary disk. To explain the newly-observed features, we performed a numerical simulation of gas orbits in a Roche potential tailored to the inferred properties of L1551 NE. The observed U-shaped dust features coincide with locations where gravitational torques from the central binary system are predicted to impart angular momentum to the circumbinary disk, producing shocks and hence density enhancements seen as a pair of spiral arms. The observed inward gas motion coincides with locations where angular momentum is predicted to be lowered by the gravitational torques. The good agreement between our observation and model indicates that gravitational torques from the binary stars constitute the primary driver for exchanging angular momentum so as to permit infall through the circumbinary disk of L1551 NE.

astro-ph.SR↗
arXiv2013

Implications of LHCb measurements and future prospects

During 2011 the LHCb experiment at CERN collected 1.0 fb-1 of sqrt{s} = 7 TeV pp collisions. Due to the large heavy quark production cross-sections, these data provide unprecedented samples of heavy flavoured hadrons. The first results from LHCb have made a significant impact on the flavour physics landscape and have definitively proved the concept of a dedicated experiment in the forward region at a hadron collider. This document discusses the implications of these first measurements on classes of extensions to the Standard Model, bearing in mind the interplay with the results of searches for on-shell production of new particles at ATLAS and CMS. The physics potential of an upgrade to the LHCb detector, which would allow an order of magnitude more data to be collected, is emphasised.

hep-ex↗
arXiv2013

$B \to K \ell^{+}\ell^{-}$ decay at large hadronic recoil

We predict the amplitude of the $B\to K \ell^+\ell^-$ decay in the region of the dilepton invariant mass squared $0<q^2\leq m_{J/ψ}^2$, that is, at large hadronic recoil. The $B\to K$ form factors entering the factorizable part of the decay amplitude are obtained from QCD light-cone sum rules. The nonlocal effects, generated by the four-quark and penguin operators combined with the electromagnetic interaction, are calculated at $q^2<0$, far below the hadronic thresholds. For hard-gluon contributions we employ the QCD factorization approach. The soft-gluon nonfactorizable contributions are estimated from QCD light-cone sum rules. The result of the calculation is matched to the hadronic dispersion relation in the variable $q^2$, which is then continued to the kinematical region of the decay. The overall effect of nonlocal contributions in $B\to K\ell^+\ell^-$ at large hadronic recoil is moderate. The main uncertainty of the predicted $B\to K \ell^+\ell^-$ partial width is caused by the $B\to K$ form factors. Furthermore, the isospin asymmetry in this decay is expected to be very small. We investigate the deviation of the observables from the Standard Model predictions by introducing a generic new physics contribution to the effective Hamiltonian.

hep-ph↗
arXiv2012

How much charm can PANDA produce?

We consider the production of charmed baryons and mesons in the proton-antiproton binary reactions at the energies of the future $\bar{P}$ANDA experiment. To describe these processes in terms of hadronic interaction models, one needs strong couplings of the initial nucleons with the intermediate and final charmed hadrons. Similar couplings enter the models of binary reactions with strange hadrons. For both charmed and strange hadrons we employ the strong couplings and their ratios calculated from QCD light-cone sum rules. In this method finite masses of $c$ and $s$ quarks are taken into account. Employing the Kaidalov's quark-gluon string model with Regge poles and adjusting the normalization of the amplitudes in this model to the calculated strong couplings, we estimate the production cross section of charmed hadrons. For $p\bar{p}\to Λ_c\barΛ_c$ it can reach several tens of $nb$ at $p_{lab}= 15 {GeV}$, whereas the cross sections of $Σ_c$ and $D$ pair production are predicted to be smaller.

hep-ph↗
arXiv2011

Form Factors and Strong Couplings of Heavy Baryons from QCD Light-Cone Sum Rules

We derive QCD light-cone sum rules for the hadronic matrix elements of the heavy baryon transitions to nucleon. In the correlation functions the $Λ_c,Σ_c$ and $Λ_b$ -baryons are interpolated by three-quark currents and the nucleon distribution amplitudes are used. To eliminate the contributions of negative parity heavy baryons, we combine the sum rules obtained from different kinematical structures. The results are then less sensitive to the choice of the interpolating current. We predict the $Λ_{b}\to p$ form factor and calculate the widths of the $Λ_{b}\to p\ellν_l$ and $Λ_{b}\to p π$ decays. Furthermore, we consider double dispersion relations for the same correlation functions and derive the light-cone sum rules for the $Λ_cND^{(*)}$ and $Σ_cND^{(*)}$ strong couplings. Their predicted values can be used in the models of charm production in $p\bar{p}$ collisions.

hep-ph↗
arXiv2011

$B \to π\ell ν_l$ Width and $|V_{ub}|$ from QCD Light-Cone Sum Rules

We employ the $B\toπ$ form factors obtained from QCD light-cone sum rules and calculate the $B\to π\ell ν_l$ width ($\ell=e,μ$) in units of $1/|V_{ub}|^2$, integrated over the region of accessible momentum transfers, $0\leq q^2\leq 12.0 ~GeV^2$. Using the most recent BABAR-collaboration measurements we extract $|V_{ub}|=(3.50^{+0.38}_{-0.33}\big|_{th.}\pm 0.11 \big|_{exp.})\times 10^{-3}$. The sum rule results for the form factors, taken as an input for a $z$-series parameterization, yield the $q^2$-shape in the whole semileptonic region of $B\to π\ellν_\ell$. We also present the charged lepton energy spectrum in this decay. Furthermore, the current situation with $B\to τν_τ$ is discussed from the QCD point of view. We suggest to use the ratio of the $B\to πτν_τ$ and $B\to π\ell ν_l ~(\ell =μ,e) $ widths as an additional test of Standard Model. The sensitivity of this observable to new physics is illustrated by including a charged Higgs-boson contribution in the semileptonic decay amplitude.

hep-ph↗
arXiv2011

Morphology, dynamics and plasma parameters of plumes and inter-plume regions in solar coronal holes

Coronal plumes, which extend from solar coronal holes (CH) into the high corona and - possibly - into the solar wind (SW), can now continuously be studied with modern telescopes and spectrometers on spacecraft, in addition to investigations from the ground, in particular, during total eclipses. Despite the large amount of data available on these prominent features and related phenomena, many questions remained unanswered as to their generation and relative contributions to the high-speed streams emanating from CHs. An understanding of the processes of plume formation and evolution requires a better knowledge of the physical conditions at the base of CHs, in plumes and in the surrounding inter-plume regions (IPR). More specifically, information is needed on the magnetic field configuration, the electron densities and temperatures, effective ion temperatures, non-thermal motions, plume cross-sections relative to the size of a CH, the plasma bulk speeds, as well as any plume signatures in the SW. In spring 2007, the authors proposed a study on "Structure and dynamics of coronal plumes and inter-plume regions in solar coronal holes" to the International Space Science Institute (ISSI) in Bern to clarify some of these aspects by considering relevant observations and the extensive literature. This review summarizes the results and conclusions of the study. Stereoscopic observations allowed us to include three-dimensional reconstructions of plumes. Multi-instrument investigations carried out during several campaigns led to progress in some areas, such as plasma densities, temperatures, plume structure and the relation to other solar phenomena, but not all questions could be answered concerning the details of plume generation process(es) and interaction with the SW.

astro-ph.SR↗
arXiv2011

The Evolution of Dark Canopies Around Active Regions

As observed in spectral lines originating from the chromosphere, transition region, and low corona, active regions are surrounded by an extensive "circumfacular" area which is darker than the quiet Sun. We examine the properties of these dark moat- or canopy-like areas using \ion{Fe}{9} 17.1 nm images and line-of-sight magnetograms from the {\it Solar Dynamics Observatory}. The 17.1 nm canopies consist of fibrils (horizontal fields containing EUV-absorbing chromospheric material) clumped into featherlike structures. The dark fibrils initially form a quasiradial or vortical pattern as the low-lying field lines fanning out from the emerging active region connect to surrounding network and intranetwork elements of the opposite polarity. The area occupied by the 17.1 nm fibrils expands as supergranular convection causes the active region flux to spread into the background medium; the outer boundary of the dark canopy stabilizes where the diffusing flux encounters a unipolar region of the opposite sign. The dark fibrils tend to accumulate in regions of weak longitudinal field and to become rooted in mixed-polarity flux. To explain the latter observation, we note that the low-lying fibrils are more likely to interact with small loops associated with weak, opposite-polarity flux elements in close proximity, than with high loops anchored inside strong unipolar network flux. As a result, the 17.1 nm fibrils gradually become concentrated around the large-scale polarity inversion lines (PILs), where most of the mixed-polarity flux is located. Systematic flux cancellation, assisted by rotational shearing, removes the field component transverse to the PIL and causes the fibrils to coalesce into long PIL-aligned filaments.

astro-ph.SR↗
arXiv2010

Charm-loop effect in $B \to K^{(*)} \ell^{+} \ell^{-}$ and $B\to K^*γ$

We calculate the long-distance effect generated by the four-quark operators with $c$-quarks in the $B\to K^{(*)} \ell^+\ell^-$ decays. At the lepton-pair invariant masses far below the $\bar{c}c$-threshold, $q^2\ll 4m_c^2$, we use OPE near the light-cone. The nonfactorizable soft-gluon emission from $c$-quarks is cast in the form of a nonlocal effective operator. The $B\to K^{(*)}$ matrix elements of this operator are calculated from the QCD light-cone sum rules with the $B$-meson distribution amplitudes. As a byproduct, we also predict the charm-loop contribution to $B\to K^*γ$ beyond the local-operator approximation. To describe the charm-loop effect at large $q^2$, we employ the hadronic dispersion relation with $ψ=J/ψ,ψ(2S), ...$ contributions, where the measured $ B\to K^{(*)}ψ$ amplitudes are used as inputs. Matching this relation to the result of QCD calculation reveals a destructive interference between the $J/ψ$ and $ψ(2S)$ contributions. The resulting charm-loop effect is represented as a $q^2$-dependent correction $ΔC_9(q^2)$ to the Wilson coefficient $C_9$. Within uncertainties of our calculation, at $q^2$ below the charmonium region the predicted ratio $ΔC_9(q^2)/C_9$ is $\leq 5% $ for $B\to K \ell^+\ell^-$, but can reach as much as 20% for $B\to K^*\ell^+\ell^-$, the difference being mainly caused by the soft-gluon contribution.

hep-ph↗
arXiv2009

Evidence For Mixed Helicity in Erupting Filaments

Erupting filaments are sometimes observed to undergo a rotation about the vertical direction as they rise. This rotation of the filament axis is generally interpreted as a conversion of twist into writhe in a kink-unstable magnetic flux rope. Consistent with this interpretation, the rotation is usually found to be clockwise (as viewed from above) if the post-eruption arcade has right-handed helicity, but counterclockwise if it has left-handed helicity. Here, we describe two non--active-region filament events recorded with the Extreme-Ultraviolet Imaging Telescope (EIT) on the {\it Solar and Heliospheric Observatory} ({\it SOHO}), in which the sense of rotation appears to be opposite to that expected from the helicity of the post-event arcade. Based on these observations, we suggest that the rotation of the filament axis is in general determined by the net helicity of the erupting system, and that the axially aligned core of the filament can have the opposite helicity sign to the surrounding field. In most cases, the surrounding field provides the main contribution to the net helicity. In the events reported here, however, the helicity associated with the filament ``barbs'' is opposite in sign to and dominates that of the overlying arcade.

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