scholarly journals Sunspots at centimeter wavelengths

2010 ◽  
Vol 6 (S273) ◽  
pp. 265-271
Author(s):  
Mukul R. Kundu ◽  
Jeongwoo Lee
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Radio Observation ◽  
Coronal Magnetic Field ◽  
Dimensional Structure ◽  
Solar Array ◽  
Cosmic Radio ◽  
Owens Valley ◽  
Halo Structure ◽  
The Core

AbstractThe early solar observations of Covington (1947) established a good relation between 10.7 cm solar flux and the presence of sunspots on solar disk. The first spatially resolved observation with a two-element interferometer at arc min resolution by Kundu (1959) found that the radio source at 3 cm has a core-halo structure; the core is highly polarized and corresponds to the umbra of a sunspot with magnetic fields of several hundred gauss, and the halo corresponds to the diffuse penumbra or plage region. The coronal temperature of the core was interpreted as due to gyroresonance opacity produced by acceleration of electrons gyrating in a magnetic field. Since the opacity is produced at resonant layers where the frequency matches harmonics of the gyrofrequency, the radio observation could be utilized to measure the coronal magnetic field. Since this simple interferometric observation, the next step for solar astronomers was to use arc second resolution offered by large arrays at cm wavelengths such as Westerbrock Synthesis Radio Telescope and the Very Large Array, which were primarily built for cosmic radio research. Currently, the Owens Valley Solar Array operating in the range 1-18 GHz and the Nobeyama Radio Heliograph at 17 and 34 GHz are the only solar dedicated radio telescopes. Using these telescopes at multiple wavelengths it is now possible to explore three dimensional structure of sunspot associated radio sources and therefore of coronal magnetic fields. We shall present these measurements at wavelengths ranging from 1.7 cm to 90 cm and associated theoretical developments.

Calculated Coronal Magnetic Fields and Their Comparison with the Coronal Structures as Observed during Five Solar Eclipses

1994 ◽  
Vol 144 ◽  
pp. 559-564
Author(s):  
P. Ambrož ◽  
J. Sýkora
Keyword(s):  
Magnetic Field ◽  
Solar Cycle ◽  
Magnetic Fields ◽  
Solar Corona ◽  
Coronal Magnetic Field ◽  
Coronal Magnetic Fields ◽  
Solar Eclipses ◽  
Coronal Structures

AbstractWe were successful in observing the solar corona during five solar eclipses (1973-1991). For the eclipse days the coronal magnetic field was calculated by extrapolation from the photosphere. Comparison of the observed and calculated coronal structures is carried out and some peculiarities of this comparison, related to the different phases of the solar cycle, are presented.

Radio Measurements of Coronal Magnetic Fields

1994 ◽  
Vol 144 ◽  
pp. 21-28 ◽  
Author(s):  
G. B. Gelfreikh
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Solar Corona ◽  
Active Regions ◽  
High Sensitivity ◽  
Coronal Magnetic Field ◽  
Transverse Magnetic ◽  
Radio Sources ◽  
Radio Waves ◽  
Solar Active Regions

AbstractA review of methods of measuring magnetic fields in the solar corona using spectral-polarization observations at microwaves with high spatial resolution is presented. The methods are based on the theory of thermal bremsstrahlung, thermal cyclotron emission, propagation of radio waves in quasi-transverse magnetic field and Faraday rotation of the plane of polarization. The most explicit program of measurements of magnetic fields in the atmosphere of solar active regions has been carried out using radio observations performed on the large reflector radio telescope of the Russian Academy of Sciences — RATAN-600. This proved possible due to good wavelength coverage, multichannel spectrographs observations and high sensitivity to polarization of the instrument. Besides direct measurements of the strength of the magnetic fields in some cases the peculiar parameters of radio sources, such as very steep spectra and high brightness temperatures provide some information on a very complicated local structure of the coronal magnetic field. Of special interest are the results found from combined RATAN-600 and large antennas of aperture synthesis (VLA and WSRT), the latter giving more detailed information on twodimensional structure of radio sources. The bulk of the data obtained allows us to investigate themagnetospheresof the solar active regions as the space in the solar corona where the structures and physical processes are controlled both by the photospheric/underphotospheric currents and surrounding “quiet” corona.

scholarly journals Nonlinear force-free modeling of magnetic fields in flare-productive active regions

2015 ◽  
Vol 11 (S320) ◽  
pp. 167-174
Author(s):  
M. S. Wheatland ◽  
S. A. Gilchrist
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Boundary Data ◽  
Numerical Solutions ◽  
Free Field ◽  
Active Regions ◽  
Coronal Magnetic Field ◽  
Nonlinear Force ◽  
The Magnetic Field ◽  
Force Free Field

AbstractWe review nonlinear force-free field (NLFFF) modeling of magnetic fields in active regions. The NLFFF model (in which the electric current density is parallel to the magnetic field) is often adopted to describe the coronal magnetic field, and numerical solutions to the model are constructed based on photospheric vector magnetogram boundary data. Comparative tests of NLFFF codes on sets of boundary data have revealed significant problems, in particular associated with the inconsistency of the model and the data. Nevertheless NLFFF modeling is often applied, in particular to flare-productive active regions. We examine the results, and discuss their reliability.

scholarly journals Optical and Radio Observations of Large Scale Magnetic Fields on the Sun

1971 ◽  
Vol 43 ◽  
pp. 609-615 ◽  
Author(s):  
G. Daigne ◽  
M. F. Lantos-Jarry ◽  
M. Pick
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Interplanetary Medium ◽  
Coronal Magnetic Field ◽  
The Sun ◽  
Radio Observations ◽  
Active Centers ◽  
Field Patterns

It is possible to deduce information concerning large scale coronal magnetic field patterns from the knowledge of the location of radioburst sources.As the method concerns active centers responsible for corpuscular emission, the knowledge of these structures may have important implications in the understanding of corpuscular propagation in the corona and in the interplanetary medium.

scholarly journals Distortion of magnetic fields in the pre-stellar core Barnard 68

2008 ◽  
Vol 4 (S259) ◽  
pp. 107-108 ◽  
Author(s):  
Ryo Kandori ◽  
Motohide Tamura ◽  
Ken-ichi Tatematsu ◽  
Nobuhiko Kusakabe ◽  
Yasushi Nakajima ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Near Infrared ◽  
Flux Ratio ◽  
Field Structure ◽  
Wide Field ◽  
Stellar Core ◽  
The Core ◽  
Cloud Cores ◽  
The Stability

AbstractMagnetic fields are believed to play an important role in controlling the stability and contraction of molecular cloud cores. In the present study, magnetic fields of a cold pre-stellar core, Barnard 68, have been mapped based on wide-field near-infrared polarimetric observations of background stars. A distinct “hourglass-shaped” magnetic field is identified toward the core, as the observational evidence of magnetic field structure distorted by mass accumulation in a pre-stellar core. Our findings on the geometry of magnetic fields as well as the mass-to-magnetic flux ratio are presented.

Exact equilibria for nonlinear force-free magnetic fields with its applications to astrophysics and fusion plasmas

2014 ◽  
Vol 80 (2) ◽  
pp. 173-195 ◽  
Author(s):  
S. M. Moawad
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Solar Physics ◽  
Coronal Magnetic Field ◽  
Fusion Plasmas ◽  
Magnetic Structures ◽  
Magnetic Flux Ropes ◽  
Solar Prominences ◽  
Confinement Fusion ◽  
Nonlinear Force

AbstractKnowledge of the structure of coronal magnetic field originating from the photosphere is relevant to the understanding of many solar activity phenomena, e.g. flares, solar prominences, coronal loops, and coronal heating. In most of the existing literature, these loop-like magnetic structures are modeled as force-free magnetic fields (FFMF) without any plasma flow. In this paper, we present several exact solution classes for nonlinear FFMF, in both translational and axisymmetric geometries. The solutions are considered for their possible relevance to astrophysics and solar physics problems. These are used to illustrate arcade-type magnetic field structures of the photosphere and twisted magnetic flux ropes through the coronal mass ejections (CMEs), as well as magnetic confinement fusion plasmas.

scholarly journals Computing nonlinear force free coronal magnetic fields

2003 ◽  
Vol 10 (4/5) ◽  
pp. 313-322 ◽  
Author(s):  
T. Wiegelmann ◽  
T. Neukirch
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Boundary Conditions ◽  
Random Noise ◽  
Coronal Magnetic Field ◽  
Optimization Method ◽  
Numerical Code ◽  
Coronal Magnetic Fields ◽  
Nonlinear Force ◽  
Free Fields

Abstract. Knowledge of the structure of the coronal magnetic field is important for our understanding of many solar activity phenomena, e.g. flares and CMEs. However, the direct measurement of coronal magnetic fields is not possible with present methods, and therefore the coronal field has to be extrapolated from photospheric measurements. Due to the low plasma beta the coronal magnetic field can usually be assumed to be approximately force free, with electric currents flowing along the magnetic field lines. There are both observational and theoretical reasons which suggest that at least prior to an eruption the coronal magnetic field is in a nonlinear force free state. Unfortunately the computation of nonlinear force free fields is way more difficult than potential or linear force free fields and analytic solutions are not generally available. We discuss several methods which have been proposed to compute nonlinear force free fields and focus particularly on an optimization method which has been suggested recently. We compare the numerical performance of a newly developed numerical code based on the optimization method with the performance of another code based on an MHD relaxation method if both codes are applied to the reconstruction of a semi-analytic nonlinear force-free solution. The optimization method has also been tested for cases where we add random noise to the perfect boundary conditions of the analytic solution, in this way mimicking the more realistic case where the boundary conditions are given by vector magnetogram data. We find that the convergence properties of the optimization method are affected by adding noise to the boundary data and we discuss possibilities to overcome this difficulty.

scholarly journals On the Nature of Magnetars

2004 ◽  
Vol 218 ◽  
pp. 265-266
Author(s):  
Ya. N. Istomin
Keyword(s):  
Magnetic Field ◽  
Neutron Star ◽  
Magnetic Fields ◽  
Electromagnetic Fields ◽  
Gamma Ray ◽  
X Ray ◽  
Surface Magnetic Field ◽  
The Core ◽  
Neutron Star Crust ◽  
High Magnitude

The electromagnetic fields of magnetodipole radiation can penetrate to the conducting matter of a neutron star crust and create there electric currents and tangential magnetic fields of high magnitude. The solution obtained here has the form of surface magnetic field discontinuities propagating through the crust to the core. This model explains the phenomena of magnetars — Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars.

scholarly journals Distortion of magnetic fields in Barnard 68

2019 ◽  
Vol 72 (1) ◽  
Author(s):  
Ryo Kandori ◽  
Motohide Tamura ◽  
Masao Saito ◽  
Kohji Tomisaka ◽  
Tomoaki Matsumoto ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Near Infrared ◽  
Critical Mass ◽  
Polarized Light ◽  
Evolutionary Status ◽  
Core Mass ◽  
The Core ◽  
Magnetic Field Modeling ◽  
Inclination Angles

Abstract The magnetic field structure, kinematical stability, and evolutionary status of the starless dense core Barnard 68 (B68) are revealed based on the near-infrared polarimetric observations of background stars, measuring the dichroically polarized light produced by aligned dust grains in the core. After subtracting unrelated ambient polarization components, the magnetic fields pervading B68 are mapped using 38 stars and axisymmetrically distorted hourglass-like magnetic fields are obtained, although the evidence for the hourglass field is not very strong. On the basis of simple 2D and 3D magnetic field modeling, the magnetic inclination angles on the plane-of-sky and in the line-of-sight direction are determined to be 47° ± 5° and 20° ± 10°, respectively. The total magnetic field strength of B68 is obtained to be $26.1 \pm 8.7\, \mu \mbox{G}$. The critical mass of B68, evaluated using both magnetic and thermal/turbulent support, is $M_{\rm cr} = 2.30 \pm 0.20\, {M}_{\odot }$, which is consistent with the observed core mass of $M_{\rm core}=2.1\, M_{\odot }$, suggesting a nearly critical state. We found a relatively linear relationship between polarization and extinction up to AV ∼ 30 mag toward the stars with deepest obscuration. Further theoretical and observational studies are required to explain the dust alignment in cold and dense regions in the core.

Sign in / Sign up

Export Citation Format

Share Document