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Register a new userThe horizontal internetwork magnetic field: numerical simulations in comparison to observations with Hinode
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Observations with the Hinode space observatory led to the discovery of predominantly horizontal magnetic fields in the photosphere of the quiet internetwork region. Here we investigate realistic numerical simulations of the surface layers of the Sun with respect to horizontal magnetic fields and compute the corresponding polarimetric response in the Fe I 630 nm line pair. We find a local maximum in the mean strength of the horizontal field component at a height of around 500 km in the photosphere, where it surpasses the vertical component by a factor of 2.0 or 5.6, depending on the initial and boundary conditions. From the synthesized Stokes profiles we derive a mean horizontal field component that is, respectively, 1.6 and 4.3 times stronger than the vertical component. This is a consequence of both the intrinsically stronger flux density of, and the larger area occupied by the horizontal fields. We find that convective overshooting expels horizontal fields to the upper photosphere, making the Poynting flux positive in the photosphere, while this quantity is negative in the convectively unstable layer below it.
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Three-dimensional magnetohydrodynamic simulations of the surface layers of the Sun intrinsically produce a predominantly horizontal magnetic field in the photosphere. This is a robust result in the sense that it arises from simulations with largely different initial and boundary conditions for the magnetic field. While the disk-center synthetic circular and linear polarization signals agree with measurements from Hinode, their center-to-limb variation sensitively depends on the height variation of the horizontal and the vertical field component and they seem to be at variance with the observed behavior.
We use sequences of images and magnetograms from Hinode to study magnetic elements in internetwork parts of the quiet solar photosphere. Visual inspection shows the existence of many long-lived (several hours) structures that interact frequently, and may migrate over distances ~7 Mm over a period of a few hours. About a fifth of the elements have an associated bright point in G-band or Ca II H intensity. We apply a hysteresis-based algorithm to identify elements. The algorithm is able to track elements for about 10 min on average. Elements intermittently drop below the detection limit, though the associated flux apparently persists and often reappears some time later. We infer proper motions of elements from their successive positions, and find that they obey a Gaussian distribution with an rms of 1.57+-0.08 km/s. The apparent flows indicate a bias of about 0.2 km/s toward the network boundary. Elements of negative polarity show a higher bias than elements of positive polarity, perhaps as a result of to the dominant positive polarity of the network in the field of view, or because of increased mobility due to their smaller size. A preference for motions in X is likely explained by higher supergranular flow in that direction. We search for emerging bipoles by grouping elements of opposite polarity that appear close together in space and time. We find no evidence supporting Joys law at arcsecond scales.
We have been monitoring yearly variation in the Suns polar magnetic fields with the Solar Optical Telescope aboard {it Hinode} to record their evolution and expected reversal near the solar maximum. All magnetic patches in the magnetic flux maps are automatically identified to obtain the number density and magnetic flux density as a function of th total magnetic flux per patch. The detected magnetic flux per patch ranges over four orders of magnitude ($10^{15}$ -- $10^{20}$ Mx). The higher end of the magnetic flux in the polar regions is about one order of magnitude larger than that of the quiet Sun, and nearly that of pores. Almost all large patches ($ geq 10^{18}$ Mx) have the same polarity, while smaller patches have a fair balance of both polarities. The polarity of the polar region as a whole is consequently determined only by the large magnetic concentrations. A clear decrease in the net flux of the polar region is detected in the slow rising phase of the current solar cycle. The decrease is more rapid in the north polar region than in the south. The decrease in the net flux is caused by a decrease in the number and size of the large flux concentrations as well as the appearance of patches with opposite polarity at lower latitudes. In contrast, we do not see temporal change in the magnetic flux associated with the smaller patches ($ < 10^{18}$ Mx) and that of the horizontal magnetic fields during the years 2008--2012.
A fully three-dimensional (3D) magnetohydrodynamical (MHD) model is applied to simulate the evolution of the large-scale magnetic field in cluster galaxies interacting with the intra-cluster medium (ICM). As the model input we use a time dependent gas velocity field resulting from 3D N-body sticky-particle simulations of a galaxy. The modeled clouds are affected by the ram pressure due to their rapid motion through the ICM in the central part of a cluster. Numerical simulations have shown that after the initial compression phase due to ram pressure a process of gas re-accretion onto the galactic disk takes place. We find that the gas re-accretion leads to an increase of the total magnetic energy without any dynamo action. The simulated magnetic fields are used to construct the model maps of high-frequency (Faraday rotation-free) polarized radio emission. We show that the evolution of the polarized intensity shows features that are characteristic for different evolutionary stages of an ICM-ISM interaction. The comparison of polarized radio continuum emission maps with our model permits to determine whether the galaxy is in the compression or in the re-accretion phase. It also provides an important constraint upon the dynamical modeling of an ICM-ISM interactions.
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