119258360
SLOW MAGNETOACOUSTIC WAVES OBSERVED ABOVE A QUIET-SUN REGION IN A DARK CAVITY
Waves play a crucial role in diagnosing the plasma properties of various structures in the solar corona and coronal heating. Slow magnetoacoustic (MA) waves are one of the important types of magnetohydrodynamic waves. In past decades, numerous slow MA waves were detected above active regions and coronal holes, but were rarely found elsewhere. Here, we investigate a “tornado”-like structure consisting of quasi-periodic streaks within a dark cavity at about 40–110 Mm above a quiet-Sun region on 2011 September 25. Our analysis reveals that these streaks are actually slow MA wave trains. The properties of these wave trains, including phase speed, compression ratio, and kinetic energy density, are similar to those of the reported slow MA waves, except that the period of these waves is about 50 s, much shorter than the typical reported values (3–5 minutes).
17103243
An analytical model probing the internal state of coronal mass ejections based on observations of their expansions and propagations
[1] In this paper, a generic self-similar flux rope model is proposed to probe the internal state of CMEs in order to understand the thermodynamic process and expansion of CMEs in interplanetary space. Using this model, three physical parameters and their variations with heliocentric distance can be inferred based on coronagraph observations of CMEs' propagation and expansion. One is the polytropic index Γ of the CME plasma, and the other two are the average Lorentz force and the thermal pressure force inside CMEs. By applying the model to the 8 October 2007 CME observed by STEREO/SECCHI, we find that (1) the polytropic index of the CME plasma increased from initially 1.24 to more than 1.35 quickly and then slowly decreased to about 1.34; it suggests that there be continuously heat injected/converted into the CME plasma and the value of Γ tends to be , a critical value inferred from the model for a force-free flux rope; (2) the Lorentz force directed inward while the thermal pressure force outward, and both of them decreased rapidly as the CME moved out; the direction of the two forces reveals that the thermal pressure force is the internal driver of the CME expansion, whereas the Lorentz force prevented the CME from expanding. Some limitations of the model and approximations are discussed meanwhile.
119228154
Full halo coronal mass ejections: Do we need to correct the projection effect in terms of velocity?
[1] The projection effect is one of the biggest obstacles in learning the real properties of coronal mass ejections (CMEs) and forecasting their geoeffectiveness. To evaluate the projection effect, 86 full halo CMEs (FHCMEs) listed in the Coordinated Data Analysis Workshop CME catalog from 1 March 2007 to 31 May 2012 are investigated. By applying the Graduated Cylindrical Shell model, we obtain the deprojected values of the propagation velocity, direction, and angular width of these FHCMEs and compare them with the projected values measured in the plane-of-sky. Although these CMEs look full halo in the view angle of SOHO, it is found that their propagation directions and angular widths could vary in a large range, implying projection effect is a major reason causing a CME being halo, but not the only one. Furthermore, the comparison of the deprojected and projected velocities reveals that most FHCMEs originating within 45° of the Sun-Earth line with a projected speed slower than 900 km s−1 suffer from large projection effect, while the FHCMEs originating far from the vicinity of solar disk center or moving faster than 900 km s−1have small projection effect. Thus, for the latter class of FHCMEs, it is not necessary to correct the measured velocities.
115149137
CONTRACTING AND ERUPTING COMPONENTS OF SIGMOIDAL ACTIVE REGIONS
It has recently been noted that solar eruptions can be associated with the contraction of coronal loops that are not involved in magnetic reconnection processes. In this paper, we investigate five coronal eruptions originating from four sigmoidal active regions, using high-cadence, high-resolution narrowband EUV images obtained by the Solar Dynamic Observatory (SDO). The magnitudes of the flares associated with the eruptions range from GOES class B to class X. Owing to the high-sensitivity and broad temperature coverage of the Atmospheric Imaging Assembly (AIA) on board SDO, we are able to identify both the contracting and erupting components of the eruptions: the former is observed in cold AIA channels as the contracting coronal loops overlying the elbows of the sigmoid, and the latter is preferentially observed in warm/hot AIA channels as an expanding bubble originating from the center of the sigmoid. The initiation of eruption always precedes the contraction, and in the energetically mild events (B- and C-flares), it also precedes the increase in GOES soft X-ray fluxes. In the more energetic events, the eruption is simultaneous with the impulsive phase of the nonthermal hard X-ray emission. These observations confirm that loop contraction is an integrated process in eruptions with partially opened arcades. The consequence of contraction is a new equilibrium with reduced magnetic energy, as the contracting loops never regain their original positions. The contracting process is a direct consequence of flare energy release, as evidenced by the strong correlation of the maximal contracting speed, and strong anti-correlation of the time delay of contraction relative to expansion, with the peak soft X-ray flux. This is also implied by the relationship between contraction and expansion, i.e., their timing and speed.
67817261
Critical Height for the Destabilization of Solar Prominences: Statistical Results from STEREO Observations
At which height is a prominence inclined to be unstable, or where is the most probable critical height for the prominence destabilization? This question was statistically studied based on 362 solar limb prominences well recognized by Solar Limb Prominence Catcher and Tracker from 2007 April to the end of 2009. We found that there are about 71% disrupted prominences (DPs), among which about 42% of them did not erupt successfully and about 89% of them experienced a sudden destabilization process. After a comprehensive analysis of the DPs, we
41231566
Quantitative Analysis of CME Deflections in the Corona
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 Astrophys. J.652, 1305, 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.
54198577
Freeze-out radii extracted from three-pion cumulants in pp, p-Pb and Pb-Pb collisions at the LHC
In high-energy collisions, the spatio-temporal size of the particle production region can be measured using the Bose-Einstein correlations of identical bosons at low relative momentum. The source radii are typically extracted using two-pion correlations, and characterize the system at the last stage of interaction, called kinetic freeze-out. In low-multiplicity collisions, unlike in high-multiplicity collisions, two-pion correlations are substantially altered by background correlations, e. g. mini-jets. Such correlations can be suppressed using three-pion cumulant correlations. We present the first measurements of the size of the system at freeze-out extracted from three-pion cumulant correlations in pp, p-Pb and Pb-Pb collisions at the LHC with ALICE. At similar multiplicity, the invariant radii extracted in p-Pb collisions are found to be 5-15% larger than those in pp, while those in Pb-Pb are 35-55% larger than those in p-Pb. Our measurements disfavor models which incorporate substantially stronger collective expansion in p-Pb as compared to pp collisions at similar multiplicity. (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
118387621
Underground Prototype Water Cherenkov Muon Detector with the Tibet Air Shower Array
We are planning to build a 10,000 m2 water‐Cherenkov‐type muon detector (MD) array under the Tibet air shower (AS) array. The Tibet AS+MD array will have the sensitivity to detect gamma rays in the 100 TeV region by an order of the magnitude better than any other previous existing detectors in the world. In the late fall of 2007, a prototype water Cherenkov muon detector of approximately 100 m2 was constructed under the existing Tibet AS array. The preliminary data analysis is in good agreement with our MC simulation. We are now ready for further expanding the underground water Cherenkov muon detector.
15483575
Energy Loss of Particles in Pulsar Radio Emission Region
Extremely relativistic particles play an important role in various radiative mechanisms of pulsar radio as well as high-energy emission. It is thus very essential to estimate the Lorentz factor of these particles in the radiation regions both observationally and theoretically. In this paper, according to the frequency dependence of component separation and considering some radiation processes, we obtain the Lorentz factor and its variation in the radiation region observationally for seven multi-frequency observed pulsars. It is found that the Lorentz factor decays substantially in the emission region, and the decay can not be well understood by all known energy loss processes. This result hints that there are some unknown processes which are responsible to loosing energy effectively.
2020440
The Distribution of Two-dimensional Eccentricity of Sunyaev-Zel'dovich Effect and X-Ray Surface Brightness Profiles
With the triaxial density profile of dark matter halos and the corresponding equilibrium gas distribution, we derive two-dimensional Sunyaev-Zel'dovich (SZ) effect and X-ray surface brightness profiles for clusters of galaxies. It is found that the contour map of these observables can be well approximated by a series of concentric ellipses with scale-dependent eccentricities. The statistical distribution of their eccentricities (or, equivalently, axial ratios) is analyzed by taking into account the orientation of clusters with respect to the line of sight and the distribution of the axial ratios and the concentration parameters of dark matter halos. For clusters of mass 1013 h-1 M☉ at redshift z = 0, the axial ratio is peaked at η ~ 0.9 for both SZ and X-ray profiles. For larger clusters, the deviation from circular distributions is more apparent, with η peaked at η ~ 0.85 for M = 1015 h-1 M☉. To be closer to observations, we further study the axial-ratio distribution for mass-limited cluster samples with the number distribution of clusters at different redshifts described by a modified Press-Schechter model. For a mass limit of value Mlim = 1014 h-1 M☉, the average axial ratio is η ~ 0.84, with a tail extended to η ~ 0.6. With the fast advance of high-quality imaging observations of both SZ effect and X-ray emissions, our analyses provide a useful way to probe cluster halo profiles and therefore to test theoretical halo formation models.
118660389
TWO-PHASE ICM IN THE CENTRAL REGION OF THE RICH CLUSTER OF GALAXIES A1795: A JOINT CHANDRA, XMM-NEWTON, AND SUZAKU VIEW
Based on a detailed analysis of the high-quality Chandra, XMM-Newton, and Suzaku data of the X-ray bright cluster of galaxies A1795, we report clear evidence for a two-phase intracluster medium (ICM) structure, which consists of a cool (with a temperature T-c approximate to 2.0-2.2 keV) and a hot (T-h approximate to 5.0-5.7 keV) component that coexist and dominate the X-ray emission at least in the central 80 kpc. A third weak emission component (T-3 approximate to 0.8 keV) is also detected within the innermost 144 kpc and is ascribed to a portion of interstellar medium of the cD galaxy. Deprojected spectral analysis reveals flat radial temperature distributions for both the hot phase and cool phase components. These results are consistent with the ASCA measurements reported by Xu et al. and resemble the previous findings for the Centaurus Cluster. By analyzing the emission measure ratio and gas metal abundance maps created from the Chandra data, we find that the cool phase component is more metal-enriched than the hot phase one in the 50-100 kpc region, which agrees with that found in M87. The coexistence of the cool phase and hot phase ICM cannot be realized by bubble uplifting from active galactic nuclei (AGNs) alone. Instead, the two-phase ICM properties are better reconciled with a cD corona model. In this model, the cool phase may be ascribed to the plasmas confined in magnetic loops, which are surrounded by the intruding hot phase ICM and have been polluted by metals synthesized in the cD galaxy. AGN feedback energy released in the innermost 10 kpc can serve as the heating source to prevent the loop-interior gas from cooling down to temperatures much lower than the observed value. The total gravitating mass profile exhibits a hierarchical structure, regardless of the ICM temperature modeling.