Generation of twist on magnetic flux tubes at the base of the solar convection zone

Using two-dimensional magnetohydrodynamics calculations, we investigate a twist gen- eration mechanism on a magnetic flux tube at the base of the solar convection zone based on the idea of Choudhuri, 2003, Sol. Phys., 215, 31 in which a toroidal mag- netic field is wrapped by a surrounding mean poloidal field. During generation of the twist, the flux tube follows four phases. (1) It quickly splits into two parts with vortex motions rolling up the poloidal magnetic field. (2) Owing to the physical mechanism similar to that of the magneto-rotational instability, the rolled-up poloidal field is bent and amplified. (3) The magnetic tension of the disturbed poloidal magnetic field re- duces the vorticity, and the lifting force caused by vortical motion decreases. (4) The flux tube gets twisted and begins to rise again without splitting. Investigation of these processes is significant because it shows that a flux tube without any initial twist can rise to the surface in relatively weak poloidal fields.Comment: 10 pages, 6 figur

Competition between hidden order and antiferromagnetism in URu_2Si_2 under uniaxial stress studied by neutron scattering

We have performed elastic neutron scattering experiments under uniaxial stress sigma applied along the tetragonal [100], [110] and [001] directions for the heavy electron compound URu2Si2. We found that antiferromagnetic (AF) order with large moment is developed with sigma along the [100] and [110] directions. If the order is assumed to be homogeneous, the staggered ordered moment mu_o continuously increases from 0.02 mu_B (sigma=0) to 0.22 mu_B (0.25 GPa). The rate of increase partial mu_o/partial sigma is ~ 1.0 mu_B/GPa, which is four times larger than that for the hydrostatic pressure (partial mu_o/partial P sim 0.25 mu_B/GPa). Above 0.25 GPa, mu_o shows a tendency to saturate, similar to the hydrostatic pressure behavior. For sigma||[001], mu_o shows only a slight increase to 0.028 mu_B (sigma = 0.46 GPa) with a rate of ~ 0.02 mu_B/GPa, indicating that the development of the AF state highly depends on the direction of sigma. We have also found a clear hysteresis loop in the isothermal mu_o(sigma) curve obtained for sigma||[110] under the zero-stress-cooled condition at 1.4 K. This strongly suggests that the sigma-induced AF phase is metastable, and separated from the "hidden order" phase by a first-order phase transition. We discuss these experimental results on the basis of crystalline strain effects and elastic energy calculations, and show that the c/a ratio plays a key role in the competition between these two phases.Comment: 9 pages, 7 figures, to appear in Physical Review

Increasing d-wave superconductivity by on site repulsion

We study by Variational Monte Carlo an extended Hubbard model away from half filled band density which contains two competing nearest-neighbor interactions: a superexchange $J$ favoring d-wave superconductivity and a repulsion $V$ opposing against it. We find that the on-site repulsion $U$ effectively enhances the strength of $J$ meanwhile suppressing that of $V$, thus favoring superconductivity. This result shows that attractions which do not involve charge fluctuations are very well equipped against strong electron-electron repulsion so much to get advantage from it.Comment: 4 pages, 3 figure

Detection of flux emergence, splitting, merging, and cancellation of network field. I Splitting and Merging

Frequencies of magnetic patch processes on supergranule boundary, namely flux emergence, splitting, merging, and cancellation, are investigated through an automatic detection. We use a set of line of sight magnetograms taken by the Solar Optical Telescope (SOT) on board Hinode satellite. We found 1636 positive patches and 1637 negative patches in the data set, whose time duration is 3.5 hours and field of view is 112" \times 112". Total numbers of magnetic processes are followed: 493 positive and 482 negative splittings, 536 positive and 535 negative mergings, 86 cancellations, and 3 emergences. Total numbers of emergence and cancellation are significantly smaller than those of splitting and merging. Further, frequency dependences of merging and splitting processes on flux content are investigated. Merging has a weak dependence on flux content only with a power- law index of 0.28. Timescale for splitting is found to be independent of parent flux content before splitting, which corresponds to \sim 33 minutes. It is also found that patches split into any flux contents with a same probability. This splitting has a power-law distribution of flux content with an index of -2 as a time independent solution. These results support that the frequency distribution of flux content in the analyzed flux range is rapidly maintained by merging and splitting, namely surface processes. We suggest a model for frequency distributions of cancellation and emergence based on this idea.Comment: 32 pages, 10 figures, 1 table, accepted to Ap