Small-scale Flux Tubes Research Articles

Mass Loss on the Red Giant Branch: Plasmoid-driven Winds above the RGB Bump

Abstract The onset of cool massive winds in evolved giants is correlated with an evolutionary feature on the red giant branch (RGB) known as the “bump.” Also at the bump, shear instability in the star leads to magnetic fields that occur preferentially on small length-scales. Pneuman has suggested that the emergence of small-scale flux tubes in the Sun can give rise to enhanced acceleration of the solar wind as a result of plasmoid acceleration (the so-called “melon-seed mechanism”). In this paper, we examine Pneuman’s formalism to determine if it may shed some light on the process that drives mass loss in stars above the RGB bump. Because we do not currently have detailed information for some of the relevant physical parameters, we are not yet able to derive a detailed model: instead, our goal in this paper is to explore a “proof of concept.” Using parameters that are known to be plausible in cool giants, we find that the total mass-loss rate from such stars can be replicated. Moreover, we find that the radial profile of the wind speed in such stars can be steep or shallow depending on the fraction of the mass-loss rate that is contained in the plasmoids: this is consistent with empirical data that indicate that the velocity profiles of winds from cool giants span a range of steepnesses.

Black hole magnetosphere with small-scale flux tubes – II. Stability and dynamics

ABSTRACT In some Seyfert galaxies, the hard X-rays that produce fluorescent emission lines are thought to be generated in a hot corona that is compact and located at only a few gravitational radii above the supermassive black hole. We consider the possibility that this X-ray source may be powered by small-scale magnetic flux tubes attached to the accretion disc near the black hole. We use three-dimensional, time-dependent, special relativistic, force-free simulations in a simplified setting to study the dynamics of such flux tubes as they get continuously twisted by the central compact star/black hole. We find that the dynamical evolution of the flux tubes connecting the central compact object and the accretion disc is strongly influenced by the confinement of the surrounding field. Although differential rotation between the central object and the disc tends to inflate the flux tubes, strong confinement from surrounding field quenches the formation of a jet-like outflow, as the inflated flux tube becomes kink unstable and dissipates most of the extracted rotational energy relatively close to the central object. Such a process may be able to heat up the plasma and produce strong X-ray emission. We estimate the energy dissipation rate and discuss its astrophysical implications.

Black hole magnetosphere with small-scale flux tubes

There is observational evidence that the X-ray continuum source that creates the broad fluorescent emission lines in some Seyfert Galaxies may be compact and located at a few gravitational radii above the black hole. We consider the possibility that this compact source may be powered by small scale flux tubes near the black hole that are attached to the orbiting accretion disk. As a first step, this paper investigates the salient features of black hole magnetospheres that contain small scale, disk-hole linking "closed" flux tubes, using the force-free approximation in an axisymmetric setting. We find that the extent of the closed zone is a result of the balance between the black hole spin induced twist in the closed zone and the confinement pressure of the external (open) field of the disk. The maximal extent of the closed zone, for a typical external confinement, is usually a few gravitational radii. The pressure competition between the closed zone and the external confinement could in principle lead to interesting dynamics and dissipation relevant for the compact X-ray corona.

Photospheric plasma and magnetic field dynamics during the formation of solar AR 11190

Context.The Sun features on its surface typical flow patterns called the granulation, mesogranulation, and supergranulation. These patterns arise due to convective flows transporting energy from the interior of the Sun to its surface. The other well known elements structuring the solar photosphere are magnetic fields arranged from single, isolated, small-scale flux tubes to large and extended regions visible as sunspots and active regions.Aims.In this paper we will shed light on the interaction between the convective flows in large-scale cells as well as the large-scale magnetic fields in active regions, and investigate in detail the statistical distribution of flow velocities during the evolution and formation of National Oceanic and Atmospheric Administration active region 11190.Methods.To do so, we employed local correlation tracking methods on data obtained by the Solar Dynamics Observatory in the continuum as well as on processed line-of-sight magnetograms.Results.We find that the flow fields in an active region can be modelled by a two-component distribution. One component is very stable, follows a Rayleigh distribution, and can be assigned to the background flows, whilst the other component is variable in strength and velocity range and can be attributed to the flux emergence visible both in the continuum maps as well as magnetograms. Generally, the plasma flows, as seen by the distribution of the magnitude of the velocity, follow a Rayleigh distribution even through the time of formation of active regions. However, at certain moments of large-scale fast flux emergence, a second component featuring higher velocities is formed in the velocity magnitudes distribution.Conclusions.The plasma flows are generally highly correlated to the motion of magnetic elements and vice versa except during the times of fast magnetic flux emergence as observed by rising magnetic elements. At these times, the magnetic fields are found to move faster than the corresponding plasma.

СПЕКТРАЛЬНІ ДОСЛІДЖЕННЯ МАГНІТНИХ ПОЛІВ І ТЕРМОДИНАМІЧНИХ УМОВ В АКТИВНИХ ОБЛАСТЯХ НА СОНЦІ

The main results of studies of magnetic fields and thermodynamic conditions in active regions in the Sun obtained at the Astronomical Observatory of the Taras Shevchenko National University of Kyiv (AO KNU) during the last 10 years (2010–2019) are presented. The true diameter of the extremely small(spatially unresolved) magnetic flux tubes was estimated on a base of data from Hinode Space Observatory; this diameter was found to be within the range of 15–20 km, which is much smaller than the spatial resolution limit of the largest solar telescopes to date (≈ 60 km). According to the observations made at AO KNU, it was shown that there are three types of magnetic fields in a solar faculae: strong (kilogauss range) fields in small-scale flux tubes, areas of weak and moderate background fields of regular polarity, and areas of sub-telescopic fields of mixed polarity. The magnetic flux of mixed polarity exceeds the absolute flux of the entire field of regular polarity by at least 2 times. In the sunspot umbra observed at GST of AO KNU and ATsU-5 of GAO NASU, thin spectral effects of Zeeman splitting were found, which indicate presence of very strong magnetic fields at least of 5,8 kG. The polarity of the magnetic field in sub-telescopic structures with such very strong fields is found to be the same as in the background field, and the Doppler velocity is about 2 km/s (plasma lifting). Super- strong magnetic fields in the range of 5–5,7 kG were also detected from observations of the Big Bear Observatory (BBSO), USA. A unique observational material also obtained on AO KNU concerning the area of a seismic source of extremely powerful solar flare on October 28, 2003 of X17.2 / 4B class was analyzed. This solar flare had a Balmer decrement with a record intensity ratio I (Hβ) / I (Hα) = 1,68 of the Hβ and Hα lines, which is unprecedented for all flares observed. In this flare, indications of the existence of particularly strong magnetic fields in the range of several tens of kilogauss were found. The semi-empirical model of this flare has an interesting feature, namely three discrete layers with high plasma concentration and temperature, including a very thick and thin layer in the chromosphere with the following parameters: hydrogen concentration nH = 1018 cm–3, thickness Δh = 3–5 km and height h ≈ 1200 km above the level of the photosphere. In active solar prominences, rather strong magnetic fields (up to 4 kG) were detected at altitudes of 3–14 Mm. The modeling of spectral line profiles within the two-component model showed that the true magnitude of local magnetic fields can be even greater by at least 3 times. A theoretical MHD model is proposed, according to which such high strengths occur in force-free magnetic ropes with a characteristic scale of 300 km.

MULTI-WAVELENGTH HIGH-RESOLUTION OBSERVATIONS OF A SMALL-SCALE EMERGING MAGNETIC FLUX EVENT AND THE CHROMOSPHERIC AND CORONAL RESPONSE

State-of-the-art solar instrumentation is revealing magnetic activity of the Sun with unprecedented resolution. Observations with the 1.6m New Solar Telescope of the Big Bear Solar Observatory are making next steps in our understanding of the solar surface structure. Granular-scale magnetic flux emergence and the response of the solar atmosphere are among the key research. As part of a joint observing program with NASA's IRIS mission, the NST observed active region NOAA 11810 in photospheric and chromospheric wavelengths. Complimentary data are provided by SDO and Hinode space-based telescopes. The region displayed a group of solar pores, in the vicinity of which we detect a small-scale buoyant horizontal magnetic flux tube causing abnormal granulation and interacting with the pre-existing ambient field in upper atmospheric layers. Following the expansion of distorted granules at the emergence site, we observed a sudden appearance of an extended surge in the HeI data. IRIS catched ejection of a hot plasma jet associated with the HeI-surge. The SDO/HMI data reveal emerging magnetic looplike structures. Hinode/Ca II H and IRIS filtergrams detail the connectivities of the newly emerged magnetic field in the lower solar chromosphere. From these data we fid that the orientation of the emerging flux tube was almost perpendicular to the overlying ambient field. Nevertheless the interaction of emerging magnetic field lines with the pre-existing overlying field generates high-temperature emission regions and boosts the surge/jet production. The localized heating is detected before and after the first signs of the surge/jet ejection. We compare the results with previous observations and theoretical models, and propose a scenario for the activation of plasma jet/surges and confined heating. Such process may play a significant role in the mass and energy flow from the interior to the corona.

Observations of Unresolved Photospheric Magnetic Fields in Solar Flares Using Fe i and Cr i Lines

The structure of the photospheric magnetic field during solar flares is examined using echelle spectropolarimetric observations. The study is based on several Fe I and Cr I lines observed at locations corresponding to brightest H$\alpha$ emission during thermal phase of flares. The analysis is performed by comparing magnetic field values deduced from lines with different magnetic sensitivities, as well as by examining the fine structure of $I\pm V$ Stokes profiles splitting. It is shown that the field has at least two components, with stronger unresolved flux tubes embedded in weaker ambient field. Based on a two-component magnetic field model, we compare observed and synthetic line profiles and show that the field strength in small-scale flux tubes is about $2-3$ kG. Furthermore, we find that the small-scale flux tubes are associated with flare emission, which may have implications for flare phenomenology.

ORIGIN OF MACROSPICULE AND JET IN POLAR CORONA BY A SMALL-SCALE KINKED FLUX TUBE

We report an observation of a small scale flux-tube that undergoes kinking and triggers the macrospicule and a jet on November 11, 2010 in the north polar corona. The small-scale flux-tube emerged well before the triggering of macrospicule and as the time progresses the two opposite halves of this omega shaped flux-tube bent transversely and approached towards each other. After $\sim$ 2 minutes, the two approaching halves of the kinked flux-tube touch each-other and internal reconnection as well as energy release takes place at the adjoining location and a macrospicule was launched which goes upto a height of 12 Mm. Plasma starts moving horizontally as well as vertically upward along with the onset of macrospicule and thereafter converts into a large-scale jet which goes up to $\sim$ 40 Mm in the solar atmosphere with a projected speed of $\sim$ 95 km s$^{-1}$. We perform 2-D numerical simulation by considering the VAL-C initial atmospheric conditions to understand the physical scenario of the observed macrospicule and associated jet. The simulation results show that reconnection generated velocity pulse in the lower solar atmosphere steepens into slow shock and the cool plasma is driven behind it in form of macrospicule. The horizontal surface waves also appeared with the shock fronts at different heights, which most likely drove and spread the large-scale jet associated with the macrospicule.

A Statistical Study on the Reconnection in Boundary Layers of Small-scale Magnetic Flux Tubes in Solar Winds

With the data from WIND satellite in 1995—2005, the small-scale magnetic flux tubes determined with two methods, i.e., the artificial recognition and program selection respectively, are compared. It is found that there are magnetic reconnections in the boundary layers of 41% small-scale magnetic flux tubes determined with the program selection method, which is similar to the re- sult of small-scale magnetic flux tubes determined with the method of artificial recognition. The features of magnetic reconnections, such as the magnetic shear angle, magnetic field strength, and duration of the dissipation region of recon- nection at the small-scale flux tubes’ boundaries determined from both methods have the same statistical tendency. This shows that there is no essential differ- ence in the properties of reconnections in the boundary layers of the small-scale magnetic flux tubes determined with the two methods. Hence the data yielded by both methods can be used as the samples for statistically studying the events of reconnection in the front and back boundary layers of small-scale flux ropes. There are totally 71 magnetic reconnection events selected in this paper. Our statistical result shows that in 50 events (70%) the decrease of magnetic field strength in the dissipation region of reconnection is larger than 20%, and in 47 events (66%) the magnetic shear angle is larger than 90 degrees. These indicate that the magnetic reconnections in boundaries of the small-scale magnetic flux tubes are more likely to be anti-parallel. The statistics has been performed sep- arately in the reconnections of the front and back boundary layers of small-scale magnetic flux tubes. The results show that the features of reconnections in the front and back boundaries are similar to each other, which is different from the various properties in the front and back boundaries of the magnetic clouds, and this means that the expansion in the large-scale magnetic flux tubes, such as magnetic clouds, does not happen in the small-scale magnetic flux tubes.

THE RELATIONSHIP BETWEEN VERTICAL AND HORIZONTAL MAGNETIC FIELDS IN THE QUIET SUN

Vertical magnetic fields have been known for decades to exist in the internetwork region of the Sun, while the properties of horizontal magnetic fields have only recently been extensively investigated with Hinode. Vertical and horizontal magnetic fields in the internetwork region are considered to be separate entities and have thus far not been investigated in a unified way. We discover a clear positional association between the vertical and horizontal magnetic fields in the internetwork region with Hinode. Essentially, all of the horizontal magnetic patches are associated with the vertical magnetic patches. Alternatively, half of the vertical magnetic patches accommodate the horizontal magnetic patches. These horizontal patches are located around the borders of the vertical patches. The intrinsic magnetic field strength as obtained with the Stokes V line ratio inside the horizontal patches is weak, and is in the subequipartition field regime (B < 700 G), while the field strength outside the horizontal patches ranges from weak to strong (kG) fields. Vertical magnetic patches are known to be concentrated on mesogranular and supergranular boundaries, while the horizontal magnetic patches are found only on mesogranular boundaries. These observations provide us with new information on the origin of the vertical and horizontal internetwork magnetic fields, in a unified way. We conjecture that internetwork magnetic fields are formed by the emergence of small-scale flux tubes with bipolar footpoints, and the vertical magnetic fields of the footpoints are intensified to kG fields due to convective collapse. Resultant strong vertical fields are advected by the supergranular flow, and eventually form the network fields.

Intensity contrast from MHD simulations and HINODE observations

Changes in the solar surface area covered by small-scale magnetic elements are thought to cause long-term changes in the solar spectral irradiance, which are important for determining the impact on Earth's climate. To study the effect of small-scale magnetic elements on total and spectral irradiance, we derive their contrasts from 3-D MHD simulations of the solar atmosphere. Such calculations are necessary since measurements of small-scale flux tube contrasts are confined to a few wavelengths and suffer from scattered light and instrument defocus, even for space observations. To test the contrast calculations, we compare rms contrasts from simulations with those obtained with the broad-band filter imager mounted on the Solar Optical Telescope (SOT) onboard the Hinode satellite and also analyse centre-to-limb variations (CLV). The 3-D MHD simulations include the interaction between convection and magnetic flux tubes. They have been run with non-grey radiative transfer using the MURaM code. Simulations have an average vertical magnetic field of 0G, 50G, and 200G. Emergent intensities are calculated with the spectral synthesis code ATLAS9 and are convolved with a theoretical point-spread function to account for the properties of the observations' optical system. We find reasonable agreement for simulated and observed intensity distributions in the visible continuum bands. Agreement is poorer for the CN and G-Bands. The analysis of the simulations exhibits a potentially more realistic centre-to-limb behaviour than calculations based on 1-D model atmospheres. We conclude that starting from 3-D MHD simulations represents a powerful approach to obtaining intensity contrasts for a wide wavelength coverage and for different positions on the solar disk. This also paves the way for future calculations of facular and network contrast as a function of magnetic fluxes.

Nonlinear Numerical Simulations of Magneto-Acoustic Wave Propagation in Small-Scale Flux Tubes

We present results of nonlinear, two-dimensional, numerical simulations of magneto-acoustic wave propagation in the photosphere and chromosphere of small-scale flux tubes with internal structure. Waves with realistic periods of three to five minutes are studied, after horizontal and vertical oscillatory perturbations are applied to the equilibrium model. Spurious reflections of shock waves from the upper boundary are minimized by a special boundary condition. This has allowed us to increase the duration of the simulations and to make it long enough to perform a statistical analysis of oscillations. The simulations show that deep horizontal motions of the flux tube generate a slow (magnetic) mode and a surface mode. These modes are efficiently transformed into a slow (acoustic) mode in the v A<c S atmosphere. The slow (acoustic) mode propagates vertically along the field lines, forms shocks, and remains always within the flux tube. It might effectively deposit the energy of the driver into the chromosphere. When the driver oscillates with a high frequency, above the cutoff, nonlinear wave propagation occurs with the same dominant driver period at all heights. At low frequencies, below the cutoff, the dominant period of oscillations changes with height from that of the driver in the photosphere to its first harmonic (half period) in the chromosphere. Depending on the period and on the type of the driver, different shock patterns are observed.

Small-scale magnetic field diagnostics outside sunspots: Comparison of different methods

We analyse different observational data related to the problem of intrinsic magnetic field strength in small-scale fluxtubes outside sunspots. We conclude that the kG range of fluxtube fields follows from not only classical line ratio method, but also from other old and new techniques. For the quiet regions on the Sun, the most probable mode of such fields has a magnetic field strength of 1.2–1.5 kG assuming the rectangular field profile. To best interpret the observations, a weak background field between fluxtubes should be assumed, and its magnetic field strength is expected to increase with the filling factor of fluxtubes. The alternative point of view about subkilogauss fluxtube fields is critically examined, and possible sources of different conclusions are presented.

Pre-flare changes in the turbulence regime for the photospheric magnetic field in a solar active region

Observations of the total magnetic field in the active region NOAA 6757 have been used to study the turbulence regime from 2.5 h before the onset of a 2B/X1.5 flare until two minutes after its maximum. The curvature of the exponent ζ(q) for the structure functions of the Bz field increases monotonically before the flare (i.e., the multifractal character of the Bz field becomes more complex) but straightens at the flare maximum and coincides with a linear Kolmogorov dependence (implying a monofractal structure for the Bz field). The observed deviations of ζ(q) from a Kolmogorov line can be used for short-term forecasting of strong flares. Analysis of the power spectra of the Bz field and the dissipation of magnetic-energy fluctuations shows that the beginning of the flare is associated with the onset of a new turbulence regime, which is closer to a classical Kolmogorov regime. The scaling parameter (cancellation index) of the current helicity of the magnetic field, kh, remains at a high level right up until the last recording of the field just before the flare but decreases considerably at the flare maximum. The variations detected in the statistical characteristics of the turbulence can be explained by the formation and amplification of small-scale flux tubes with strong fields before the flare. The dissipation of magnetic energy before the flare is primarily due to reconnection at tangential discontinuities of the field, while the dissipation after the flare maximum is due to the anomalous plasma resistance. Thus, the flare represents an avalanche dissipation of tangential discontinuities.

Small-scale magnetic flux tube diagnostics in a solar flare

I ±V profiles of the Fei 5247 and 5250 lines in the 2B flare of June 16, 1989 have been analyzed. A bright knot of the flare outside the sunspot where the central intensity of Hα reached a peak value of 1.4 (relative to the continuum) has been explored. The Fei 5250/Fei 5247 magnetic line ratio based on the StokesV peak separations of these lines at five evolutionary phases of the flare (including the start of the flare, the flash phase, the peak and 16 min after the peak) has been analyzed. It was found that the StokesV peak separation for the Fei 5250 line was systematically larger than that of the Fei 5247 line. This is evidence for the presence in the flare of small-scale flux tubes with kG fields. The flux tube magnetic field strength was about 1.1 kG at the start of the flare and during the flash phase, 1.55 kG during the peak, and 1.38 kG 16 min after the peak. The filling factor,α, appears to decrease monotonically during the flare.