Magnetic Field Diagnostics Research Articles

Diagnostics of Coronal Magnetic Field in Terms of Radio Burst and Fine Structures

A complex solar radio burst was observed on 19 October 2001 with the spectrometers of NAOC (National Astronomical Observatory of China) and Nobeyama Radioheliograph (NoRH). Basing on the analysis of brightness temperature spectra of radio sources and various fine spectral structures, we get a diagnosis of magnetic field of radio active region.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Coherent population trapping method of studying magnetohydrodynamic instabilities in a plasma, confined in a magnetic trap

In magnetic traps of electron cyclotron resonance (ECR) sources of multicharged ions not utilizing the minimum |B| magnetic structure, the magnetohydrodynamic instability is likely to develop. A local laser induced fluorescence-type magnetic field diagnostics based on coherent population trapping effect may help in the study of the instability. Theoretical estimates presented in the article show that parameters of plasmas in ECR sources of multicharged ions satisfy the conditions at which the diagnostic works.

Magnetic field diagnostics of plasmas based on coherent population trapping: theory and experiment.

We propose a local, all-optical method of magnetic field diagnostics for plasmas. This method is based on the phenomenon of coherent population trapping in a multilevel medium. The proposed technique allows one to detect both the strength and the orientation of the field. The feasibility of the method was tested in a proof-of-principle experiment in a low-pressure neon discharge.

Effect of Random Inhomogeneities on Electron Cyclotron Maser Emission

The spectral properties of solar radio spikes are believed to be tightly related to the spike source inhomogeneity: in particular, the spikes display a rather large scatter of spectral bandwidth of the emission lines both within one event and from one event to another. This paper studies the effect of random magnetic and density inhomogeneities on the spectral properties of the electron cyclotron maser emission produced by superthermal electrons, which is likely responsible for solar radio spikes. Magnetic irregularities with a broad distribution over spatial scales are shown to provide considerable spectral broadening that is not necessarily small compared to the natural bandwidth, even for small magnetic inhomogeneities. The developed renormalization scheme allows us to describe quantitatively the spectral shape of the emission line when the broadening is large. Moreover, the presence of stronger magnetic inhomogeneities gives rise to the splitting of the emission line into two peaks, in agreement with the observed noninteger harmonic ratio between spectrally separated spike bands. Typical values of the spectral bandwidth are found to be fairly consistent with spike observations. The developed theory suggests new possibilities for random magnetic field diagnostics. The estimated magnitudes of the random magnetic field at the spike source look rather reasonable.

The molecular Zeeman effect and diagnostics of solar and stellar magnetic fields

Recent advances in the computation of the Zeeman splitting of molecular lines have paved the way for their use as diagnostics of solar and stellar magnetic fields. A systematic study of their diagnostic capabilities had not been carried out so far, however. Here we investigate how molecular lines can be used to deduce the magnetic and thermal structure of sunspots, starspots and cool stars. First, we briefly describe the Stokes radiative transfer of Zeeman-split molecular lines. Then, we compute Stokes spectra of TiO, OH, CH and FeH lines and investigate their diagnostic capabilities. We also compare the synthetic profiles with observations. Spectra of TiO, OH and FeH are found to be interesting diagnostics of sunspot magnetic fields. This is also true for cool stars, where, however, the OH Stokes V profiles may require very high S/ N data to be reliably employed. Finally we investigate the potential of various molecular bands for high-contrast imaging of the solar surface. The violet CN and CH bands turn out to be most promising for imaging the photosphere, the TiO bands are excellent for imaging sunspot umbrae, while the UV OH band can be used for imaging both the photosphere and sunspots.

Diagnostics of a strong magnetic field according to the results of observation of radiation due to transitions between levels of hyperfine structure of an ensemble of excited particles

It is shown that measurement of linearly polarized radiation due to transitions between levels of hyperfine structure of an ensemble of particles makes it possible to determine the value of a magnetic field applied to the ensemble.

Coherent population trapping in a gas of excited atoms

Coherent population trapping in a gas discharge was studied for transitions between excited levels of neon atoms. Resonances corresponding to the arising of coherent population trapping in the Λ and V schemes for Zeeman sublevels of the lower and upper working states were observed in the presence of a longitudinal magnetic field. The effect of nonlinear polarization plane rotation under coherent population trapping conditions was studied. The possibility of using the results obtained in this work for diagnostics of local magnetic fields and other plasma parameters in gas discharges was considered.

Progress toward a practical magnetic field diagnostic for low-field fusion plasmas based on dual mode correlation reflectometry

Previously, the proof of principle of measurement of magnetic field strength, |B| internal to a plasma by cross correlation of ordinary (O) and extraordinary (X) mode fluctuation reflectometer signals has been demonstrated in a linear plasma device. It was found that dual mode (O-X) reflectometry data could be interpreted by a one-dimensional numerical model to determine |B| in the vicinity of the reflectometer cutoff positions. Radial correlation properties of turbulence are also measured simultaneously with |B|. This technique is potentially well suited to measurement of |B| in low-field fusion devices (e.g., B<0.6 T) where standard magnetic field diagnostics are expected to be difficult to implement. However, transfer of this method to toroidal magnetic fusion devices presents a number of potential difficulties such as the effects of magnetic shear, steep density gradients, and limited time for cross correlations. In addition, recent simulations suggest that two-dimensional modeling may be required to interpret experimental data when density fluctuation levels are high. Dual mode correlation reflectometry experiments on the National Spherical Torus Experiment are presented, and progress toward the implementation of this technique as a practical diagnostic is discussed.

Diagnostics of a weak magnetic field upon pulse excitation of an ensemble of particles

A method of diagnostics of weak magnetic fields, based on the spectropolarimetry of damped radiation from an ensemble of particles excited by a rectangular pulse, is suggested. This method can be used without any assumptions about the dynamics of excitation.

MAGNETIC DIAGNOSTICS FROM ASTEROSEISMOLOGY

Recent results on the properties of stellar oscillations in presence of rotation and magnetic field are presented. Their implication on the diagnostics of magnetic field in pulsating magnetic stars are given and illustrated with results on roAp stars.

Astrophysical Spectropolarimetry and Magnetic Field Diagnostics

This paper highlights the increasing interest of astrophysical spectropolarimetry by showing how remote sensing techniques based on the Hanle and Zeeman effects are allowing us to investigate the magnetism of the extended solar atmosphere. Particular emphasis is given to the development of new diagnostic windows on the weakest magnetic fields of the photosphere, chromosphere and corona. Spectropolarimetry in the He i 10830 Å multiplet is allowing us to infer the three-dimensional geometry of the magnetic fields that confine the plasma of solar prominences. Multilevel modelling of the Hanle and Zeeman effects in the Ca II IR triplet and other chromospheric lines is helping us to decipher the strength and topology of chromospheric magnetic fields in regions where photospheric magnetograms show the well-known ‘salt and pepper’ patterns of mixed polarities. The Hanle effect in molecular lines, as well as in atomic lines of rare-earth elements like Ce II, offers a novel diagnostic tool for empirical investigations of ‘turbulent’ magnetic fields in relatively deep regions of the ‘quiet’ solar photosphere.

Diagnostics of Solar Magnetic fields based on the Hanle effect

The Hanle effect was first used in solar physics to determine weak magnetic fields in prominences (see Leroy 1981). More recently the development of a new generation of very accurate spectro-polarimeters has open a new window for the diagnostics of weak magnetic fields in the solar photosphere and chromosphere, based on the Hanle effect. I present here such recent research programs performed with the solar telescope THEMIS and with other polarimeters such as ZIMPOL and the LOCARNO polarimeter.

Diagnostics of a magnetic field according to broadening of a line due to the 6s 1 S 0-6p 1 P 1 transition in the barium atom

It was shown that the collisional broadening of lines in an emission spectrum of an ensemble of particles with prevailing Coulomb interaction makes it possible to carry out the diagnostics of an external magnetic field.

Sunspot and starspot interiors as seen from molecular lines

Molecular lines represent powerful diagnostics of stellar and solar magnetic fields. They are currently successfully used for studying the thermal and magnetic structure of sunspots. The present calculations show how molecular lines can been used for testing stellar surface images and measuring magnetic fields inside spatially unresolved starspots. Particularly, observational capabilities for the registration of the Stokes V signal from starspots in the TiO band at 7055 Å are estimated.

Diagnostics of a magnetic field using spectropolarimetry of emission from a system under conditions of stationary excitation

A method of diagnostics of a magnetic field H is proposed, based on spectropolarimetry of emission of an atomic system placed into this field and disturbed from a stationary state by a field pulse H′ superimposed onto the field H.

The molecular Zeeman effect and diagnostics of solar and stellar magnetic fields

The molecular Zeeman effect and diagnostics of solar and stellar magnetic fields

New possibilities for the diagnostics of solar magnetic fields

Abstract The polarized spectrum that is produced by coherent scattering processes has a structural richness comparable to that of the intensity spectrum but different in appearance and physical origin. The amplitudes of the polarization features are influenced by magnetic fields via the Hanle effect in a way that is very different different from the ordinary Zeeman effect. While the main contribution to Zeeman-effect observations comes from the strong fields of the photospheric magnetic flux tubes, the Hanle effect is sensitive to weak magnetic fields, turbulent fields of mixed polarities, and chromospheric fields. As different spectral lines respond very differently to the Hanle effect, the scattering polarization offers novel and rich diagnostic opportunities. In the present overview we illustrate some of these new effects and indicate what can be learnt from them.

Diagnostics of a strong magnetic field of a plasma by spectropolarimetry of its emission

The conditions under which spectropolarimetry of the emission of a stationary plasma can provide information on the magnitude of a strong magnetic field applied to the plasma are obtained.

Magnetic Field Diagnostics Based on Far‐Infrared Polarimetry: Tests Using Numerical Simulations

The dynamical state of star-forming molecular clouds cannot be understood without determining the structure and strength of their magnetic fields. Measurements of polarized far-infrared radiation from thermally aligned dust grains are used to map the orientation of the field and estimate its strength, but the accuracy of the results has remained in doubt. In order to assess the reliability of this method, we apply it to simulated far-infrared polarization maps derived from three-dimensional simulations of supersonic magnetohydrodynamical turbulence, and we compare the estimated values to the known magnetic field strengths in the simulations. We investigate the effects of limited telescope resolution and self-gravity on the structure of the maps. Limited observational resolution affects the field structure such that small-scale variations can be completely suppressed, thus giving the impression of a very homogeneous field. The Chandrasekhar-Fermi method of estimating the mean magnetic field in a turbulent medium is tested, and we suggest an extension to measure the rms field. Both methods yield results within a factor of 2 for field strengths typical of molecular clouds, with the modified version returning more reliable estimates for slightly weaker fields. However, neither method alone works well for very weak fields, missing them by a factor of up to 150. Taking the geometric mean of both methods estimates even the weakest fields accurately within a factor of 2.5. Limited telescope resolution leads to a systematic overestimation of the field strengths for all methods. We discuss the effects responsible for this overestimation and show how to extract information on the underlying (turbulent) power spectrum.

Magnetic field diagnostics based on far-infrared polarimetry: Tests using numerical simulations

Magnetic field diagnostics based on far-infrared polarimetry: Tests using numerical simulations