Flux Sheet Research Articles

Formation of Ellerman bombs due to 3D flux emergence

Aims. We investigate the emergence of a “sea-serpent” magnetic field into the outer solar atmosphere and the connection between undulating fieldlines and formation of Ellerman bombs. Methods. We perform 3D numerical experiments solving the time-dependent and resistive MHD equations. Results. A sub-photospheric magnetic flux sheet develops undulations due to the Parker instability. It rises from the convectively unstable sub-photospheric layer and emerges into the highly stratified atmosphere through successive reconnection events along the undulating system. Brightenings with the characteristics of Ellerman bombs are produced due to reconnection, which occurs during the emergence of the field. At an advanced stage of the evolution of the system, the resistive emergence leads to the formation of long, arch-like magnetic fields that expand into the corona. The enhancement of the magnetic field at the low atmosphere and episodes of emergence of new magnetic flux are also discussed.

Probing magnetic turbulence by synchrotron polarimetry: statistics and structure of magnetic fields from Stokes correlators

We describe a technique for probing the statistical properties of cosmic magnetic fields based on radio polarimetry data. Second-order magnetic field statistics like the power spectrum cannot always distinguish between magnetic fields with essentially different spatial structure. Synchrotron polarimetry naturally allows certain 4th-order magnetic field statistics to be inferred from observational data, which lifts this degeneracy and can thereby help us gain a better picture of the structure of the cosmic fields and test theoretical scenarios describing magnetic turbulence. In this work we show that a 4th-order correlator of physical interest, the tension-force spectrum, can be recovered from the polarized synchrotron emission data. We develop an estimator for this quantity based on polarized-emission observations in the Faraday-rotation-free frequency regime. We consider two cases: a statistically isotropic field distribution, and a statistically isotropic field superimposed on a weak mean field. In both cases the tension force power spectrum is measurable; in the latter case, the magnetic power spectrum may also be obtainable. The method is exact in the idealized case of a homogeneous relativistic-electron distribution that has a power-law energy spectrum with a spectral index p=3, and assumes statistical isotropy of the turbulent field. We carry out tests of our method using synthetic data generated from numerically simulated magnetic fields. We show that the method is valid, that it is not prohibitively sensitive to the value of the electron spectral index, and that the observed tension-force spectrum allows one to distinguish between, e.g., a randomly tangled magnetic field (a default assumption in many studies) and a field organized in folded flux sheets or filaments.

Comparison of the thin flux tube approximation with 3D MHD simulations

The structure and dynamics of small vertical photospheric magnetic flux concentrations has been often treated in the framework of an approximation based upon a low-order truncation of the Taylor expansions of all quantities in the horizontal direction, together with the assumption of instantaneous total pressure balance at the boundary to the non-magnetic external medium. Formally, such an approximation is justified if the diameter of the structure (a flux tube or a flux sheet) is small compared to all other relevant length scales (scale height, radius of curvature, wavelength, etc.). The advent of realistic 3D radiative MHD simulations opens the possibility of checking the consistency of the approximation with the properties of the flux concentrations that form in the course of a simulation. We carry out a comparative analysis between the thin flux tube/sheet models and flux concentrations formed in a 3D radiation-MHD simulation. We compare the distribution of the vertical and horizontal components of the magnetic field in a 3D MHD simulation with the field distribution in the case of the thin flux tube/sheet approximation. We also consider the total (gas plus magnetic) pressure in the MHD simulation box. Flux concentrations with super-equipartition fields are reasonably well reproduced by the second-order thin flux tube/sheet approximation. The differences between approximation and simulation are due to the asymmetry and the dynamics of the simulated structures.

Numerical simulation of wave propagation in magnetic network

AbstractWe present 2-D numerical simulations of wave propagation in the magnetic network. The network is modelled as consisting of individual magnetic flux sheets located in intergranular lanes. They have a typical horizontal size of about 150 km at the base of the photosphere and expand upward and become uniform. We consider flux sheets of different field strengths. Waves are excited by means of transverse motions at the lower boundary, to simulate the effect of granular buffeting. We look at the magneto-acoustic waves generated within the flux sheet and the acoustic waves generated in the ambient medium due to the excitation. We calculate the wave energy fluxes separating them into contributions from the acoustic and the Poynting part and study the effect of the different field strengths.

Inferring the chromospheric magnetic topology through waves

AbstractThe aim of this work is to examine the hypothesis that the wave propagation time in the solar atmosphere can be used to infer the magnetic topography in the chromosphere as suggested by Finsterle et al. (2004). We do this by using an extension of our earlier 2-D MHD work on the interaction of acoustic waves with a flux sheet. It is well known that these waves undergo mode transformation due to the presence of a magnetic field which is particularly effective at the surface of equipartition between the magnetic and thermal energy density, the β = 1 surface. This transformation depends sensitively on the angle between the wave vector and the local field direction. At the β = 1 interface, the wave that enters the flux sheet, (essentially the fast mode) has a higher phase speed than the incident acoustic wave. A time correlation between wave motions in the non-magnetic and magnetic regions could therefore provide a powerful diagnostic for mapping the magnetic field in the chromospheric network.

Magnetic properties of G-band bright points in a sunspot moat

We present simultaneous spectropolarimetric observations of four visible (630 nm) and three infrared (1565 nm) spectral lines from the German Vacuum Tower Telescope, together with speckle-reconstructed filtergrams in the G-band and the Ca ii H line core from the Dutch Open Telescope. After alignment of the data sets, we used the G-band intensity to locate bright points (BPs) in the moat of a regular sunspot. With the cospatial and cotemporal information provided by the polarimetric data, we characterize the magnetic, kinematic, and thermal properties of the BPs. We find that (a) 94% of the BPs are associated with magnetic fields; (b) their field strengths range between 500 and 1400 G, with a rather flat distribution; (c) the contrast of BPs in the G-band depends on the angle between the vector magnetic field and the line of sight; (d) the BPs harbor downflows of magnetized plasma and exhibit Stokes V profiles with large area and amplitude asymmetries; (e) the magnetic interior of BPs is hotter than the immediate field-free surroundings by about 1000 K at equal optical depth; and (f) the mean effective diameter of BPs in our data set is 150 km, with very few BPs larger than 300 km. Most of these properties can be explained by the classical magnetic flux tube model. However, the wide range of BP parameters found in this study indicates that not all G-band BPs are identical to stable long-lived flux tubes or sheets of kG strength.

Supertubes in field theories

To a domain wall or string object, Noether charge and topological spatial objects can be attracted, forming a composite Bogomolny-Prasad-Sommerfield (BPS) object. We consider two field theories and derive a new BPS bound on composite linear solitons involving multiple charges. Among the BPS objects 'supertubes' appear when the wall or string tension is canceled by the bound energy, and could take an arbitrary closed curve. In our theories, supertubes manifest as Chern-Simons solitons, dyonic instantons, charged semilocal vortices, and dyonic instantons on the vortex flux sheet.

Braze Pressure Welding (BPW) - A New Method for Joining Steel Pipes -

Braze Pressure Welding (BPW) with high frequency induction heating is a newly developed pressure welding technique using interlayer metals for welding the general steel pipes for pipe arrangement in buildings. BPW enables to make joints by solid-state welding in air using a flux sheet which is also developed for this technique. In this method, the interlayer metal is expected to play the primary role in making high performance joints. At a joining temperature, the melted interlayer removes contaminations from the joining surfaces of the base metals. Then the liquid filler is discharged from the joining interface by the joining pressure, and forms fillets at the gaps around the joint. In this stage, the joining pressure is relatively small, so the welding deformation can be restrained to a minimum. The fillets have the effects both on relaxing the stress concentration at the joint and on increasing the joining area, which contribute to the strengthening of the joint. Thus, the high quality pipe joining without the dispersion in joint properties due to welder’s skill can be performed. In this report, the concept and merits of BPW are explained, and the results of the investigation on the cleaning effect of the interlayer metal and the effects of fillet formation on strengthening the joint are discussed. The practical application of BPW in Japan is also introduced.

Ellerman Bombs and Jets Associated with Resistive Flux Emergence

Using two-dimensional (2D) magnetohydrodynamic simulations we study the effects of resistive processes in the dynamics of magnetic flux emergence and its relation to Ellerman bombs and other dynamic phenomena in the Sun. The widely accepted scenario of flux emergence is the formation and expansion of Ω-shaped loops due to the Parker instability. Since the Parker instability has the largest growth rate at finite wavelength λp ~ 10H-20H, where H is the scale height (≈200 km in the solar photosphere), a number of magnetic loops may rise from the initial flux sheet if it is sufficiently long. This process is shown in our numerical simulations. The multiple emerging loops expand in the atmosphere and interact with each other, leading to magnetic reconnection. At first reconnection occurs in the lower atmosphere, which allows the sinking part of the flux sheet to emerge above the photosphere. This reconnection also causes local heating that may account for Ellerman bombs. In the later stage, reconnection between the expanding loops occurs at higher levels of the atmosphere and creates high-temperature reconnection jets, and eventually a large (λp) coronal loop is formed. Cool and dense plasma structures, which are similar to Hα surges, are also formed. This is not because of magnetic reconnection but due to the compression of the plasma in between the expanding loops.

Three-Dimensional Magnetohydrodynamic Simulation of Nonlinear Magnetic Buoyancy Instability of Flux Sheets with Magnetic Shear

Abstract A series of three-dimensional magnetohydrodynamic simulations is used to study the nonlinear evolution of the magnetic buoyancy instability of a magnetic flux sheet with magnetic shear. A horizontal flux sheet that is initially placed below the solar photosphere is susceptible to both the interchange instability and the Parker instability (the undular mode of the magnetic buoyancy instability). The growth rate in the linear stage of the instability in the numerical simulation is consistent with that predicted by linear theory. In the nonlinear stage, the development depends on the initial perturbation as well as the initial magnetic field configuration (i.e., the presence of magnetic shear). When an initial perturbation is assumed to be periodic, the emerging flux rises to the corona and the magnetic field expands like a potential field, as observed in 2D simulations. When an initial non-periodic perturbation or random perturbations are assumed, the magnetic flux expands horizontally when the magnetic field emerges a little into the photosphere. The distribution of the magnetic field and gas tends to be in a new state of magnetohydrostatic equilibrium. When magnetic shear is present in the initial magnetic flux sheets, the interchange mode is stabilized so that the emerging loop is higher than in the no magnetic shear case. We discuss how the presented results are related to the emerging flux observed on the Sun.

Solar magnetic elements at 0 $\farcs$1 resolution

Small magnetic structures can be seen in G-band filtergrams as isolated bright points, strings of bright points and dark micro-pores. At a resolution of 0.1, new forms of magnetic structures are found in strong field areas: elongated ribbons and more circular flowers. In this paper we study the temporal evolution of these small scale magnetic structures. In strong-field regions the time-evolution is more that of a magnetic fluid than that of collections of flux-tubes that keep their identity. We find that the granular flow concentrates the magnetic field into flux sheets that are visible as thin bright features in the filtergrams. Weak upflows are found in the flux sheets and downflows in the immediate surroundings. The flux sheets often become unstable to a fluting instability and the edges buckle. The sheets tend to break up into strings of bright points, still with weak upflows in the magnetic elements and zero velocity or downflows between them. Where there are larger flux concentrations we find ribbons, flowers and micro-pores. There is a continuous transition between these forms and they evolve from one form to another. The appearance is mostly determined by the horizontal size - larger structures are dark (micro-pores), narrower structures are ribbon shaped and the flowers are the smallest in extent. All these structures have darker inner parts and a bright edge. The plasma is found to be at rest in the ribbons, with small concentrations of weak upflow sites. Narrow sheets with downdrafts are found right at the edges of the magnetic field concentrations.

Radiative properties of magnetic elements

For the understanding of more complex numerical simulation results, a basic facular model is created consisting of a magnetic flux sheet embedded in a plane parallel atmosphere. The atmosphere within the flux sheet is similar to the external one but shifted in the downward direction to result in a Wilson depression of 150 km. It is shown that the horizontal spatial extension of the contrast enhancement produced by this model increases from center to limb from a few tenths to up to 1 �� as a consequence of enhanced radiation from the limbward surface outside (behind) the magnetic flux concentration. For a conceivable explanation of this radiative effect it is noted that a plasma parcel on the solar surface sideways of the flux sheet sees a more transparent sky in the direction towards the flux sheet compared to a direction away from it because of the rarefied atmosphere within the flux sheet. This facular model also produces a dark lane at the disk-center side (in front) of the flux sheet even though no flow is present. It it is due to the deep layers of the flux sheet that have a lower temperature gradient and are cooler than the surrounding atmosphere at equal geometrical depth. This implies that limb observations off er ag limpse of the cool bottom of magnetic elements. The center-to-limb variation of the size and the dark-lane effect derived from this basic model is recovered in a self-consistent, two-dimensional non-stationary simulation of a magnetic flux concentration. These findings are in excellent agreement with and provide an interpretation of recent observations of faculae at very high resolution by Lites et al. (2004, Sol. Phys., 221, 65).

Understanding facular granules and lanes

Recent high resolution observations by Lites et al. (2004) show details of facular granules at , including dark facular lanes. For an interpretation of these data a basic facular model is constructed, consisting of a magnetic flux sheet embedded in a plane parallel atmosphere. While the maximum contrast originates from the “hot wall” of the flux-sheet depression, the model explains the wide brightening limbward of the facular magnetic field as due to a radiative transfer effect caused by the reduced opacity of the rarefied flux-sheet atmosphere. This model produces a dark, narrow lane centerward of the facular granule even in the absence of granular flow as a consequence of the cool deep layers of the magnetic flux sheet. These results carry over to a self-consistent simulation of a flux concentration in dynamic interaction with convective motion, where the dark lane deepens and broadens.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Observations at $0{\hbox{.\!\!^{\prime\prime}}}1$ Resolution of the Dynamic Evolution of Magnetic Elements

We present observations of the dynamic evolution of photospheric magnetic elements in the G-band, magnetograms and Dopplergrams. The observations were obtained with the Swedish 1m Solar Telescope on La Palma at close to the diffraction limit of $0{\hbox{.\!\!^{\prime\prime}}}1$ . In the most quiet regions we observe individual bright points in the G-band with corresponding magnetic signal in the magnetograms. Where the filling factor of the magnetic field is larger, the bright points interact when advected by the granular and super-granular flow-fields, flux sheets form and fragment. The plage region of the decaying active region is filled with more complex topologies like ribbon structures with darker interior and bright, knotted edges. These change into flower-like shape when small in extent and into micro-pores when the flux region is larger in extent. The magnetic elements in the plage region are associated with upflows with strong downflows in the immediate vicinity in the low-field region. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Ion irradiation effects on magnetostriction of Tb–Fe thin film

Giant magnetostrictive TbFe 2 thin films were prepared by the ion plating process. The films were treated by Ar plasma irradiation and heat-treatment. The plasma irradiation was carried out at low plasma energy by a high heat flux sheet plasma system. Both treatments lead to the increase in the maximum magnetostriction and the magnetostrictive susceptibility at low magnetic field. The plasma irradiation effects seem to be due to the heat generation during irradiation. The magnetostrictive characteristics are affected by the increase in in-plane anisotropy magnetization by treatments.

Fingerprints of collisionless reconnection at the separator, I, Ambipolar‐Hall signatures

Plasma, electric, and magnetic field data on the Polar spacecraft have been analyzed for the 29 May 1996 magnetopause traversal searching for evidence of in situ reconnection and traversal of the separator. In this paper we confine our analysis to model‐free observations and intrasensor coherence of detection of the environs of the separator. (1) We illustrate the first documented penetration of the separator of collisionless magnetic reconnection in temporal proximity to successful Walén tests with opposite slopes. (2) We present the first direct measurements of E∥ at the magnetopause. (3) We make the first empirical argument that E∥ derives from the electron pressure gradient force. (4) We document the first detection of the electron pressure ridge astride the magnetic depression that extends from the separator. (5) We provide the first empirical detection of the reconnection rate at the magnetopause with the locally sub‐Alfvénic ion inflow, MAi ≃ 0.1, and trans‐Alfvénic exhaust at high electron pressure of MiA ≃ 1.1–5. (6) We exhibit the first empirical detection of supra‐Alfvénic electron flows parallel to B in excess of 5 in narrow sheets. (7) We illustrate the detection of heat flux sheets indicative of separatrices near, but not always in superposition, with the supra‐Alfvénic parallel electron bulk flows. (8) We present the first evidence that pressure gradient scales are short enough to explain the electron fluid's measured cross‐field drifts not explained by E × B drift but predicted by the measured size of E∥. (9) We illustrate that the size of the observed E∥ is well organized with the limit implied by Vasyliunas's analysis of the generalized Ohm's law of scale length , indicative of the intermediate scale of the diffusion region. (10) We document the first detection of departure from electron gyrotropy not only at the separator crossing but also in its vicinity, an effect presaged by Vasyliunas [1975]. (11) We make the first reports of very large values of electron βe ≃ 680 localized at the separator, which imply that the electron thermal gyroradius exceeds the electron inertial length by more than an order of magnitude there. This clearly delineates that the environs of the reversed field region in this data contain non‐MHD scales. The ambipolar association and the measured E∥ data imply the presence of the nonideal ρs scale in these layers surrounding the null point. The high βe signals the possible demagnetization of the thermal electrons in any structures with spatial scales of the electron skin depth, which is theoretically anticipated to surround the magnetic null line of the separator proper. This possibility is supported by the large number of temporally unaliased spectra at high βe that are inconsistent with gyrotropy.

Solar photospheric magnetic reconnection

We investigate photospheric magnetic reconnection due to an encounter of oppositely directed vertical magnetic flux sheets, performing 2.5-dimensional magnetohydrodynamic (MHD) numerical simulations. We construct the initial flux sheets adopting the thin flux tube approximation. Since actual solar resistivity possesses a maximum at the temperature-minimum region, we adopt a resistivity model in which the resistivity is described as a function of height with a maximum (where the magnetic Reynolds number = 1000) at a middle height of our simulation box. Owing to the resistivity, the Sweet-Parker type reconnection occurs at the middle. The inflow speed (υ i ∼ 160 m/s) is nearly equal to the speeds implied by observations of canceling magnetic features on the photosphere. Thus photospheric reconnection seems to be a cancellation mechanism. It is shown that upward propagating MHD slow mode waves are generated by an upward reconnection jet. Moreover, when we incline the initial field lines 30° from the vertical direction in the other flux sheet, Alfven waves are also generated as a result of the reconnection. The energy flux carried by the slow modes and Alfven waves are 1010 and 108 erg/cm2/s, respectively, and the durations are 40 s. Since in models of solar spicules upward propagating slow waves or Alfven waves are usually assumed as the initial perturbations, we compare the energy of both waves. It is found that the wave energies due to the reconnection are comparable to those assumed in spicule models. Thus the photospheric magnetic reconnection might be one of the causes of solar spicules.

Intensification of Magnetic Fields by Conversion of Potential Energy

A strong superequipartition magnetic field strength on the order of 10 T (105 G) has been inferred at the bottom of the solar convection zone. We show that the "explosion" of weak magnetic flux tubes, which is caused by a sudden loss of pressure equilibrium in the flux loop rising through the superadiabatically stratified convection zone, provides a mechanism that leads to a strong field: the flow of high-entropy material out of the exploded loop leads to a significant intensification of the magnetic field in the underlying flux sheet at the bottom. In contrast to the amplification by differential rotation, this process converts the potential energy of the stratification into magnetic energy and thus is not dynamically limited by the back-reaction on the flow field via the Lorentz force.

Understanding Small Solar Magnetic Structures: Comparing Numerical Simulations to Observations

We present direct comparisons of small magnetic structures observed in the solar photosphere with the results from numerical simulations of those structures. We compare diagnostic signatures derived from emergent Stokes polarization spectra from both the observed and model atmospheres, the former recorded with the National Solar Observatory/High Altitude Observatory Advanced Stokes Polarimeter, the latter from a fully dynamic MHD simulation of a magnetic flux sheet in a convective atmosphere. We focus on the asymmetries in the Stokes V spectra and find, first and foremost, that the asymmetries from the observed Stokes I and V in and around solar pores and azimuth centers (ACs) are quantitatively comparable to those derived from the simulation. We also find enhanced Stokes V asymmetry on the periphery of pores and ACs. We interpret this as a consequence of strong downdrafts in the surroundings of these magnetic structures, accompanied by the expansion of the magnetic field lines with height above these field-free downdrafts (the effect). The magnetic canopy can be present whether or not there is a continuum signature (i.e., a dark pore). Not surprisingly, the patterns and magnitudes of asymmetries scale with the size of the magnetic element. In the interior of the pores and ACs, we find evidence for mixed up- and downflows, with little spatial correlation between the zero-crossing shift of the V profile and the V amplitude. Finally, we report on asymmetries observed in the linear polarization lin(λ) = [Q(λ)2 + U(λ)2]1/2, finding further support for the presence of the magnetic canopy from those diagnostics. We additionally present expectations for spectropolarimetric observations at significantly higher spatial resolution.

A formation mechanism of magnetic elements in regions of mixed polarity

We present 2-D, fully compressible radiation-MHD simulations of the solar photospheric and subphotospheric layers that run for 2 hours of solar time starting from a magnetic configuration with mixed polarities. In the atmospheric layers the simulation reveals a correlation between field strength and inclination, with a nearly vertical strong-field magnetic component and a more horizontal weak-field component, in agreement with the observations. Our simulation also shows that magnetic flux is converted from one of these states to the other. In particular, magnetic flux sheets can also be formed when a new downflow lane starts due to granule fragmentation. The dynamics of the granulation and field-line reconnection are found to play a role in the initial stages of a magnetic element's formation. The simulation predicts that during or shortly after their formation magnetic elements could be associated with oppositely polarized flux at a small spatial scale.