scholarly journals Three-dimensional magnetic field structure of a flux-emerging region in the solar atmosphere

2019 ◽  
Vol 632 ◽  
pp. A112 ◽  
Author(s):  
Rahul Yadav ◽  
Jaime de la Cruz Rodríguez ◽  
Carlos José Díaz Baso ◽  
Avijeet Prasad ◽  
Tine Libbrecht ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Surface ◽  
Free Field ◽  
Opposite Polarity ◽  
Field Vector ◽  
Small Scale ◽  
Maximum Height ◽  
Horizontal Magnetic Field ◽  
Point Separation ◽  
The Magnetic Field

We analyze high-resolution spectropolarimetric observations of a flux-emerging region (FER) in order to understand its magnetic and kinematic structure. Our spectropolarimetric observations in the He I 10830 Å spectral region of a FER were recorded with GRIS at the 1.5 m aperture GREGOR telescope. A Milne–Eddington-based inversion code was employed to extract the photospheric information of the Si I spectral line, whereas the He I triplet line was analyzed with the Hazel inversion code, which takes into account the joint action of the Hanle and the Zeeman effects. The spectropolarimetric analysis of the Si I line reveals a complex magnetic structure near the vicinity of the FER, where a weak (350–600 G) and horizontal magnetic field was observed. In contrast to the photosphere, the analysis of the He I triplet presents a smooth variation of the magnetic field vector (ranging from 100 to 400 G) and velocities across the FER. Moreover, we find supersonic downflows of ∼40 km s−1 appearing near the foot points of loops connecting two pores of opposite polarity, whereas strong upflows of 22 km s−1 appear near the apex of the loops. At the location of supersonic downflows in the chromosphere, we observed downflows of 3 km s−1 in the photosphere. Furthermore, nonforce-free field extrapolations were performed separately at two layers in order to understand the magnetic field topology of the FER. We determine, using extrapolations from the photosphere and the observed chromospheric magnetic field, that the average formation height of the He I triplet line is ∼2 Mm from the solar surface. The reconstructed loops using photospheric extrapolations along an arch filament system have a maximum height of ∼10.5 Mm from the solar surface with a foot-point separation of ∼19 Mm, whereas the loops reconstructed using chromospheric extrapolations reach around ∼8.4 Mm above the solar surface with a foot-point separation of ∼16 Mm at the chromospheric height. The magnetic topology in the FER suggests the presence of small-scale loops beneath the large loops. Under suitable conditions, due to magnetic reconnection, these loops can trigger various heating events in the vicinity of the FER.

scholarly journals Structure of nonlinear whistlers moving through plasma at an angle to the magnetic field

2018 ◽  
Vol 4 (1) ◽  
pp. 25-28
Author(s):  
Геннадий Кичигин ◽  
Gennadiy Kichigin
Keyword(s):  
Magnetic Field ◽  
External Magnetic Field ◽  
Numerical Solutions ◽  
Field Vector ◽  
Magnetized Plasma ◽  
Small Scale ◽  
Motion Direction ◽  
Magnetic Field Change ◽  
The Magnetic Field ◽  
Two Fluid

The paper presents solutions of two-fluid magnetic hydrodynamics equations describing small-scale fast magnetosonic stable waves — nonlinear whist-lers moving in a cold magnetized plasma at an angle α to the external magnetic field. At the fixed angle α, the Alfvén Mach number of the whistlers has a narrow range of allowed values. It has been found that when passing from extremely small Mach numbers to ex-tremely large ones, amplitudes and spatial structure of wave velocity components and whistler magnetic field change significantly. The range of angles of the motion direction of whistlers with respect to direction of the the external magnetic field vector is determined. Within this range, the obtained approximate analytical and numerical solutions are in satisfactory agreement.

scholarly journals On the structure of the magnetic field of sunspots

1968 ◽  
Vol 35 ◽  
pp. 215-229 ◽  
Author(s):  
E. I. Mogilevsky ◽  
L. B. Demkina ◽  
B. A. Ioshpa ◽  
V. N. Obridko
Keyword(s):  
Magnetic Field ◽  
Free Field ◽  
Large Field ◽  
Doppler Velocity ◽  
Small Scale ◽  
High Excitation ◽  
Proposed Model ◽  
Background Magnetic Field ◽  
Definition Of ◽  
The Magnetic Field

The model of the magnetic field of sunspots, taking account of fine structure of magnetic field in solar plasma, is considered. Small-scale subgranules with their own field form magnetic filaments in the external current-free field. The filaments are vertical in the umbra, while in the penumbra they run along the surface with sharp bends. In a number of spot umbra the relation between Doppler velocity and the field is established on polarized spectrograms. The π-component splitting in umbra is interpreted as a result of a weak background magnetic-field existence together with a large field of magnetic filaments. Spectrographic definition of the magnetic field in spot umbra is accomplished on the effect of magnetic-lines intensification and directly on spectrograms of low-excitation (Fe I, Ti I) and high-excitation (Fe II) lines. Magnetic field measured in low-excitation lines exceeds twice the field value obtained in high-excitation lines. This result has been considered in the light of the proposed model of sunspot field.

scholarly journals New Force-Free Models of Magnetic Clouds

2005 ◽  
Vol 13 ◽  
pp. 133-133
Author(s):  
M. Vandas ◽  
E. P. Romashets ◽  
S. Watari
Keyword(s):  
Magnetic Field ◽  
Magnetic Cloud ◽  
Free Field ◽  
Flux Rope ◽  
Field Vector ◽  
Magnetic Clouds ◽  
Field Magnitude ◽  
Flux Ropes ◽  
Magnetic Field Magnitude ◽  
The Magnetic Field

AbstractMagnetic clouds are thought to be large flux ropes propagating through the heliosphere. Their twisted magnetic fields are mostly modeled by a constant-alpha force-free field in a circular cylindrical flux rope (the Lundquist solution). However, the interplanetary flux ropes are three dimensional objects. In reality they possibly have a curved shape and an oblate cross section. Recently we have found two force-free models of flux ropes which takes into account the mentioned features. These are (i) a constant-alpha force-free configuration in an elliptic flux rope (Vandas & Romashets 2003, A&A, 398, 801), and (ii) a non-constant-alpha force-free field in a toroid with arbitrary aspect ratio (Romashets & Vandas 2003, AIP Conf Ser. 679, 180). Two magnetic cloud observations were analyzed. The magnetic cloud of October 18-19, 1995 has been fitted by Lepping et al. (1997, JGR, 102, 14049) with use of the Lundquist solution. The cloud has a very flat magnetic field magnitude profile. We fitted it by the elliptic solution (i). The magnetic cloud of November 17-18, 1975 has been fitted by Marubashi (1997) with use of a toroidally adjusted Lundquist solution. The cloud has a large magnetic field vector rotation and a large magnetic field magnitude increase over the background level. We fitted it by the toroidal solution (ii). The both fits match the rotation of the magnetic field vector in a comparable quality to the former fits, but the description of the magnetic field magnitude profiles is remarkable better. It is possible to incorporate temporal effects (expansion) of magnetic clouds into the new solutions through a time-dependent alpha parameter as in Shimazu & Vandas (2002, EP&S, 54, 783).

INFLUENCE OF INTRINSIC PINNING ON THE RESISTIVE PROPERTIES OF YBa2Cu3O7-δ SINGLE CRYSTALS

2013 ◽  
Vol 27 (30) ◽  
pp. 1350220 ◽  
Author(s):  
R. V. VOVK ◽  
A. V. SAMOILOV ◽  
I. L. GOULATIS ◽  
A. CHRONEOS
Keyword(s):  
Magnetic Field ◽  
Magnetic Flux ◽  
Field Vector ◽  
Small Scale ◽  
Layered Crystal ◽  
Vortex Matter ◽  
Layered Crystal Structure ◽  
Wide Range ◽  
Glass Model ◽  
The Magnetic Field

The dynamics of vortex matter in YBa 2 Cu 3 O 7-δ single crystal with unidirectional twin boundaries is studied experimentally in a wide range of velocities of the magnetic flux in a tilted magnetic field. It is determined that with the orientation of the magnetic field vector in the locality of the ab-plane, the dynamics of the magnetic flux near the melting temperature of the vortex lattice can be described by the Kim–Anderson model and as the temperature is lowered, by the theory of collective pinning on small-scale defects or by the vortex glass model. The intrinsic pinning caused by the layered crystal structure of the material has an impact on the dynamics of magnetic flux and this effect increases with the decreasing of the temperature.

Effects of neutral gas friction and ion viscosity on the Rayleigh-Taylor instability of a stratified plasma in the presence of Hall currents

1974 ◽  
Vol 11 (1) ◽  
pp. 1-10 ◽  
Author(s):  
P. K. Bhatia
Keyword(s):  
Magnetic Field ◽  
Field Vector ◽  
Variable Density ◽  
Taylor Instability ◽  
Horizontal Magnetic Field ◽  
Unstable Stratification ◽  
Neutral Gas ◽  
Rayleigh Taylor Instability ◽  
The Magnetic Field ◽  
Proper Solutions

The effects of neutral gas friction, on the Rayleigh–Taylor instability of an infinitely conducting plasma of variable density, with ion viscosity and Hall currents, are investigated. For an ambient horizontal magnetic field, it is shown that the solution is characterized by a variational principle. Making use of the existence of this, proper solutions are obtained for a semi-infinite plasma, in which the density is stratified exponentially along the vertical, confined between two planes. In the simultaneous presence of the effects of ion viscosity and Hall currents, it is found that the potentially unstable stratification is unstable for all wavenumber perturbations, irrespective of whether or not the effects of neutral gas friction are included. Further, it is found that the growth rate increases with both Hall currents and neutral gas friction, and decreases with ion viscosity. The influence of the Hall currents and the neutral gas friction, therefore, is destabilizing, while that of ion viscosity is stabilizing. In the absence of Hall currents, it is found that the viscous plasma is stable, even for a potentially unstable stratification, for perturbations confined to a cone about the magnetic field vector. The angle of the cone of stable propagation of an inviscid plasma,. however, decreases with both Hall currents and effects of neutral gas friction.

Rayleigh Taylor instability of a viscous Hall plasma with magnetic field

1972 ◽  
Vol 7 (1) ◽  
pp. 117-132 ◽  
Author(s):  
G. Bhowmik
Keyword(s):  
Magnetic Field ◽  
Variational Principle ◽  
Larmor Frequency ◽  
Field Vector ◽  
Stable Configuration ◽  
Horizontal Magnetic Field ◽  
Wave Numbers ◽  
Hall Plasma ◽  
The Stability ◽  
The Magnetic Field

The influence of finite Larmor frequency on the stability of a viscous, finitely conducting liquid in a downward gravitational field under the influence of a uniform magnetic field directed along or normal to gravity, is investigated. The solution in each case is shown to be characterized by a variational principle Based on the variational principle, an approximate solution is obtained for the stability of a layer of fluid of constant kinematic viscosity and an exponentia density distribution. It has been found that finite resistivity and finite Larmor frequency do not introduce any instabifity in a potentially stable configuration. However, for a potentially unstable configuration we find that, for an ideal Hal plasma, the results depend on the orientation of the magnetic field, though the instability persists for all wave-numbers in the presence of non-ideal (finite resistivity and viscosity) effects. For the field aligned with gravity, it is found that a potentially unstable field-free configuration is stabilized if the buoyancy number B ( = gβ/12 V2) is less than unity. For B > 1, the instability arises for wave-numbers exceeding a critical value, which decreases on allowing for Hall terms in the generalized Ohm's law, suggesting a destabilizing influence of finite Larmor frequency. For an ambient horizontal magnetic field, it is found that an ideal plasma is stable, even for B > 0, for perturbations confined to a cone about the magnetic field vector. The angle of the cone of stable propagation, however, decreases on account of finite Larmor frequency.

scholarly journals Structure of nonlinear whistlers moving through plasma at an angle to the magnetic field

2018 ◽  
Vol 4 (1) ◽  
pp. 28-32 ◽  
Author(s):  
Геннадий Кичигин ◽  
Gennadiy Kichigin
Keyword(s):  
Magnetic Field ◽  
External Magnetic Field ◽  
Numerical Solutions ◽  
Field Vector ◽  
Magnetized Plasma ◽  
Small Scale ◽  
Motion Direction ◽  
Magnetic Field Change ◽  
The Magnetic Field ◽  
Two Fluid

The paper presents solutions of two-fluid magnetic hydrodynamics equations describing small-scale fast magnetosonic stable waves — nonlinear whist-lers moving in a cold magnetized plasma at an angle α to the external magnetic field. At the fixed angle α, the Alfvén Mach number of the whistlers has a narrow range of allowed values. It has been found that when passing from extremely small Mach numbers to ex-tremely large ones, amplitudes and spatial structure of wave velocity components and whistler magnetic field change significantly. The range of angles of the motion direction of whistlers with respect to direction of the the external magnetic field vector is determined. Within this range, the obtained approximate analytical and numerical solutions are in satisfactory agreement.

scholarly journals Magnetic topology of the north solar pole

2018 ◽  
Vol 616 ◽  
pp. A46 ◽  
Author(s):  
A. Pastor Yabar ◽  
M. J. Martínez González ◽  
M. Collados
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Vector ◽  
Line Of Sight ◽  
Small Scale ◽  
Quiet Sun ◽  
The North ◽  
Field Distributions ◽  
Disc Centre ◽  
The Magnetic Field

The magnetism at the poles is similar to that of the quiet Sun in the sense that no active regions are present there. However, the polar quiet Sun is somewhat different from that at the activity belt as it has a global polarity that is clearly modulated by the solar cycle. We study the polar magnetism near an activity maximum when these regions change their polarity, from which it is expected that its magnetism should be less affected by the global field. To fully characterise the magnetic field vector, we use deep full Stokes polarimetric observations of the 15 648.5 and 15 652.8 Å FeI lines. We observe the north pole as well as a quiet region at disc centre to compare their field distributions. In order to calibrate the projection effects, we observe an additional quiet region at the east limb. We find that the two limb datasets share similar magnetic field vector distributions. This means that close to a maximum, the poles look like typical limb, quiet-Sun regions. However, the magnetic field distributions at the limbs are different from the distribution inferred at disc centre. At the limbs, we infer a new population of magnetic fields with relatively strong intensities (~600−800 G), inclined by ~30° with respect to the line of sight, and with an azimuth aligned with the solar disc radial direction. This line-of-sight orientation interpreted as a single magnetic field gives rise to non-vertical fields in the local reference frame and aligned towards disc centre. This peculiar topology is very unlikely for such strong fields according to theoretical considerations. We propose that this new population at the limbs is due to the observation of unresolved magnetic loops as seen close to the limb. These loops have typical granular sizes as measured in the disc centre. At the limbs, where the spatial resolution decreases, we observe them spatially unresolved, which explains the new population of magnetic fields that is inferred. This is the first (indirect) evidence of small-scale magnetic loops outside the disc centre and would imply that these small-scale structures are ubiquitous on the entire solar surface. This result has profound implications for the energetics not only of the photosphere, but also of the outer layers since these loops have been reported to reach the chromosphere and the low corona.

scholarly journals Discovery of long-period magnetic field oscillations and motions in isolated sunspots

2020 ◽  
Vol 635 ◽  
pp. A64
Author(s):  
A. B. Griñón-Marín ◽  
A. Pastor Yabar ◽  
H. Socas-Navarro ◽  
R. Centeno
Keyword(s):  
Magnetic Field ◽  
Temporal Evolution ◽  
Opposite Polarity ◽  
Field Vector ◽  
Interesting Property ◽  
Coronal Loops ◽  
Long Period ◽  
Use Of Data ◽  
Long Time ◽  
The Magnetic Field

We analyse the temporal evolution of the inclination component of the magnetic field vector for the penumbral area of 25 isolated sunspots. Compared to previous works, the use of data from the HMI instrument aboard the SDO observatory facilitates the study of a very long time series (≈1 week) with a good spatial and temporal resolution. We used the wavelet technique and we found some filamentary-shaped events with large wavelet power. Their distribution of periods is broad, ranging from the lower limit for this study of 48 min up to 63 h. An interesting property of these events is that they do not appear homogeneously all around the penumbra but they seem to concentrate at particular locations. The cross-comparison of these wavelet maps with AIA data shows that the regions where these events appear are visually related to the coronal loops that connect the outer penumbra to one or more neighbouring opposite polarity flux patches.

scholarly journals Testing magnetohydrostatic extrapolation with radiative MHD simulation of a solar flare

2019 ◽  
Vol 631 ◽  
pp. A162 ◽  
Author(s):  
X. Zhu ◽  
T. Wiegelmann
Keyword(s):  
Magnetic Field ◽  
Solar Flare ◽  
Initial Conditions ◽  
Free Field ◽  
Optimization Method ◽  
Field Vector ◽  
Plasma Pressure ◽  
Nonlinear Force ◽  
The Magnetic Field ◽  
Force Free Field

Context. On the sun, the magnetic field vector is measured routinely solely in the photosphere. By using these photospheric measurements as a boundary condition, we developed magnetohydrostatic (MHS) extrapolation to model the solar atmosphere. The model makes assumptions about the relative importance of magnetic and non-magnetic forces. While the solar corona is force-free, this is not the case with regard to the photosphere and chromosphere. Aims. The model has previously been tested with an exact equilibria. Here we present a more challenging and more realistic test of our model with the radiative magnetohydrodynamic simulation of a solar flare. Methods. By using the optimization method, the MHS model computes the magnetic field, plasma pressure and density self-consistently. The nonlinear force-free field (NLFFF) and gravity-stratified atmosphere along the field line are assumed as the initial conditions for optimization. Results. Compared with the NLFFF, the MHS model provides an improved magnetic field not only in magnitude and direction, but also in magnetic connectivity. In addition, the MHS model is capable of recovering the main structure of plasma in the photosphere and chromosphere.

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