Haosheng Lin

The Granular Magnetic Fields of the Quiet Sun

We report new observations that combine high-precision infrared polarimetry and high-resolution imagery in the visible to demonstrate that most of the quiet solar surface contains a measurable magnetic field. We found that when observed at 1 arcsec2 resolution, 68% of the observed area contains magnetic flux higher than 5×1015 Mx (corresponding to an apparent average field of 1 G). The majority of these magnetic features have magnetic flux below 5×1016 Mx. Their magnetic field strengths range from below 200 to 1000 G, which means that their filling factors are on the order of 1%. The spatial distribution and time evolution of these magnetic features are closely associated with the solar granulation. The properties of these weak granular magnetic features we observed differ from those of the intranetwork fields described in earlier observations. We also observed the formation and disappearance of a kilogauss magnetic feature associated with the development of intergranular lanes, which may be evidence of convective collapse.

A NEW PRECISE MEASUREMENT OF THE CORONAL MAGNETIC FIELD STRENGTH

Magnetism dominates the structure and dynamics of the solar corona. Current theories suggest that it may also be responsible for coronal heating. Despite the importance of the magnetic field in the physics of the corona and despite the tremendous progress made recently in the remote sensing of solar magnetic fields, reliable measurements of the coronal magnetic field strength and orientation do not exist. This is largely due to the weakness of coronal magnetic fields, previously estimated to be on the order of 10 G, and the difficulty associated with observing the extremely faint solar corona emission. Using a very sensitive infrared spectropolarimeter to observe the strong near-infrared coronal emission line Fe XIII λ10747 above active regions, we have succeeded in measuring the weak Stokes V circular polarization profiles resulting from the longitudinal Zeeman effect of the magnetic field of the solar corona. From these measurements, we infer field strengths of 10 and 33 G from two active regions at heights of h = 0.12 R☉ and h = 0.15 R☉, respectively. We expect that this measurement technique will allow, in the near future, the routine precise measurement of the coronal magnetic field strength with application to many critical problems in solar coronal physics.

He I 10830 Å Line Polarimetry: A New Tool to Probe the Filament Magnetic Fields

In this paper, we present a new Stokes polarization observation of a solar filament using the neutral helium line at 10830 A. Similar to the prominence Hanle effect, the polarization of the filament is due to the resonant scattering and magnetic depolarization of the photospheric radiation; it differs only in the scattering geometry. Since this represents one of the first filament polarization observations, we also present a classical derivation of a set of diagnostic formulae that relate the filament polarization signals to the vector magnetic field. We measured the full Stokes profiles (I, Q, U, V) by scanning the slit of the spectrograph parallel to the axis of a small filament. In one section of the filament, the polarization signals showed that the axial component of the magnetic field reverses direction on either side of the filament axis. This axial field reversal is not predicted by any of the existing magnetic field models of the filament, nor was it observed by previous prominence Hanle effect observations. We propose that a tilted magnetic field loop across the filament axis can explain the observed axial field reversal. This observation also serves to demonstrate that measurements of the polarization of the He I 10830 A radiation from filaments is a useful new tool for the diagnostics of filament magnetic field structures.

OBSERVATIONAL TEST OF CORONAL MAGNETIC FIELD MODELS. I. COMPARISON WITH POTENTIAL FIELD MODEL

Recent advances have made it possible to obtain two-dimensional line-of-sight magnetic field maps of the solar corona from spectropolarimetric observations of the Fe XIII 1075 nm forbidden coronal emission line. Together with the linear polarization measurements that map the azimuthal direction of the coronal magnetic field projected in the plane of the sky containing Sun center, these coronal vector magnetograms allow for direct and quantitative observational testing of theoretical coronal magnetic field models. This paper presents a study testing the validity of potential-field coronal magnetic field models. We constructed a theoretical coronal magnetic field model of active region AR 10582 observed by the SOLARC coronagraph in 2004 by using a global potential field extrapolation of the synoptic map of Carrington Rotation 2014. Synthesized linear and circular polarization maps from thin layers of the coronal magnetic field model above the active region along the line of sight are compared with the observed maps. We found that the observed linear and circular polarization signals are consistent with the synthesized ones from layers located just above the sunspot of AR 10582 near the plane of the sky containing the Sun center.

NEW OBSERVATION OF FAILED FILAMENT ERUPTIONS: THE INFLUENCE OF ASYMMETRIC CORONAL BACKGROUND FIELDS ON SOLAR ERUPTIONS

Failed filament eruptions not associated with a coronal mass ejection (CME) have been observed and reported as evidence for solar coronal field confinement on erupting flux ropes. In those events, each filament eventually returns to its origin on the solar surface. In this Letter, a new observation of two failed filament eruptions is reported which indicates that the mass of a confined filament can be ejected to places far from the original filament channel. The jetlike mass motions in the two failed filament eruptions are thought to be due to the asymmetry of the background coronal magnetic fields with respect to the locations of the filament channels. The asymmetry of the coronal fields is confirmed by an extrapolation based on a potential field model. The obvious imbalance between the positive and negative magnetic flux (with a ratio of 1:3) in the bipolar active region is thought to be the direct cause of the formation of the asymmetric coronal fields. We think that the asymmetry of the background fields can not only influence the trajectories of ejecta, but also provide a relatively stronger confinement for flux rope eruptions than the symmetric background fields do.

Bright rings around sunspots

There are two possible explanations for why sunspots are dark: the partial suppression by the sunspot magnetic fields of convective energy transport from the underlying layers, or the removal of energy from the sunspot by enhanced hydromagnetic wave radiation. Both processes would reduce the energy emitted radiatively. The first explanation is currently favoured, and predicts that the blocked energy should show up as a bright ring around the spot, with the actual brightness of the ring sensitive to details of solar convective transport and sunspot structure. Previous searches for these bright rings were inconclusive because of the presence of bright, vertical magnetic flux tubes near the spots, and a lack of sufficient precision in the observations. Here we report high-photometric-precision observations of bright rings around eight sunspots. The rings are about 10 K warmer than the surrounding photosphere and extend at least one sunspot radius out from the penumbra. About 10% of the radiative energy missing from the sunspots is emitted through the bright rings. We also report observations of a second set of sunspots, for which simultaneous magnetic field measurements demonstrate that the rings are not associated with vertical flux tubes.

Pair Analysis of Field Galaxies from the Red-Sequence Cluster Survey

We study the evolution of the number of close companions of similar luminosities per galaxy ( -->Nc) by choosing a volume-limited subset of the photometric redshift catalog from the Red-Sequence Cluster Survey (RCS-1). The sample contains over 157,000 objects with a moderate redshift range of -->0.25 ? z? 0.8 and -->MRc ? ? 20. This is the largest sample used for pair evolution analysis, providing data over nine redshift bins with about 17,500 galaxies in each. After applying incompleteness and projection corrections, -->Nc shows a clear evolution with redshift. The -->Nc value for the whole sample grows with redshift as -->(1 + z)m, where -->m = 2.83 ? 0.33 in good agreement with -->N-body simulations in a ?CDM cosmology. We also separate the sample into two different absolute magnitude bins: -->-25 ? MRc ? ? 21 and -->-21 m-value. Furthermore, we study the evolution of the pair fraction for different projected separation bins and different luminosities. We find that the -->m-value becomes smaller for larger separation, and the pair fraction for the fainter luminosity bin has stronger evolution. We derive the major merger remnant fraction -->frem = 0.06, which implies that about 6% of galaxies with -->-25 ? MRc ? ? 20 have undergone major mergers since -->z = 0.8.

Solar Site Survey for the Advanced Technology Solar Telescope. I. Analysis of the Seeing Data

The site survey for the Advanced Technology Solar Telescope concluded recently after more than 2 years of data gathering and analysis. Six locations, including lake, island, and continental sites, were thoroughly probed for image quality and sky brightness. The present paper describes the analysis methodology employed to determine the height stratification of the atmospheric turbulence. This information is crucial, because daytime seeing is often very different between the actual telescope aperture ( ∼30 m) and the ground. Two independent inversion codes have been developed to simultaneously analyze data from a scintillometer array and a solar differential image monitor. We show here the results of applying them to a sample subset of data from 2003 May that was used for testing. Both codes retrieve a similar seeing stratification through the height range of interest. A quantitative comparison between our analysis procedure and actual in situ measurements confirms the validity of the inversions. The sample data presented in this paper reveal a qualitatively different behavior for the lake sites (dominated by high-altitude seeing) and the rest (dominated by near-ground turbulence).

Solar site testing for the Advanced Technology Solar Telescope

The location of the Advanced Technology Solar Telescope (ATST) is a critical factor in the overall performance of the telescope. We have developed a set of instrumentation to measure daytime seeing, sky brightness, cloud cover, water vapor, dust levels, and weather. The instruments have been located at six sites for periods of one to two years. Here we describe the sites and instrumentation, discuss the data reduction, and present some preliminary results. We demonstrate that it is possible to estimate seeing as a function of height near the ground with an array of scintillometers, and that there is a distinct qualitative difference in daytime seeing between sites with or without a nearby lake.

A Classical Theory of Coronal Emission Line Polarization

We present a classical theory of formation for polarized, magnetic dipole emission lines in the magnetized solar corona. Because of the small Einstein A-coefficients of forbidden lines and the expected magnetic field strengths in the corona, coherences between different magnetic substates can be neglected, so the observed Stokes vector for these lines is the result of the incoherent superposition of the Stokes vectors emitted in the de-excitation of the individual magnetic substates of the transitions upper level. Using classical electrodynamics and a weak-field expansion formalism, we could derive the main polarization properties of the transition J = 1 to J = 0, in the collisionless regime. In particular, we derived the correct amount of atomic alignment in the upper level, induced by the anisotropic, unpolarized illumination from the photosphere, and the dependence of Stokes Q and U linear polarization on the magnetic field direction in the plane of the sky. The influence of atomic alignment on the V profile is also correctly reproduced. This work provides a classical interpretation of the physical process that generates atomic alignment in the radiating ion and how the associated Van Vleck effect in resonance-scattering linear polarization and the alignment contribution to Zeeman effect circular polarization come about.