Precise control of magnetic fields and optical polarization in a time-orbiting potential trap

Optical polarization data are presented for nebulosities associated with dark clouds and regions of star formation.Illuminating sources are identified, and it is found that these usually show evidence of polarization induced by aligned grains, which we believe indicates the presence of magnetic fields in the gas–dust tori that surround the central objects.

Download Full-text

Radio Astronomy Techniques of Observing Magnetic Fields: The Galaxy

Symposium - International Astronomical Union ◽

10.1017/s0074180900174273 ◽

1993 ◽

Vol 157 ◽

pp. 271-277

Author(s):

Richard Wielebinski

Keyword(s):

Magnetic Fields ◽

Scattered Light ◽

Zeeman Effect ◽

Theoretical Work ◽

Magellanic Clouds ◽

Optical Polarization ◽

Solar Magnetic Fields ◽

The Galaxy ◽

Andromeda Nebula ◽

Ap Stars

Optical techniques were first used to detect magnetic fields in cosmic objects. Hale used the Zeeman effect to detect solar magnetic fields in 1908. Babcock extended the Zeeman technique to show the existence of magnetic fields in Ap stars in 1946. Optical polarization observations were made as early as 1920 by W.F. Meyer who observed the Hubble variable nebula NGC 2261. Polarization observations of the Andromeda nebula were made by Öhman in 1942. However at first the interpretation of these optical polarization observations was in terms of scattered light only. The theoretical work of Davis and Greenstein suggested that optical polarization could also be due to dust grains aligned in magnetic fields. Observations of Hiltner and Hall supported this interpretation. Extensive surveys of starlight polarization were made by many observers giving information about nearby magnetic fields in the Galaxy. Optical polarization observations of galaxies gave some information on the Magellanic Clouds and other nearby objects but due to lack of sensitivity progress was slow.

Download Full-text

Comparison of Radio and Optical Polarization in Extended Extragalactic Radio Sources: II. Implications for the Magnetic Fields

Symposium - International Astronomical Union ◽

10.1017/s0074180900190837 ◽

1990 ◽

Vol 140 ◽

pp. 451-452

Author(s):

K. Meisenheimer ◽

H.-J. Röser

Keyword(s):

Magnetic Fields ◽

Radio Sources ◽

Synchrotron Emission ◽

Optical Polarization ◽

Optical Studies ◽

Extragalactic Source ◽

Extragalactic Radio Sources ◽

Extended Radio ◽

Synchrotron Light

Although optical synchrotron light from an extragalactic source - the jet in M 87 - has been discovered more than 30 years ago (Baade 1956) there are still only a handful extended radio sources with established optical synchrotron emission (see Röser 1989 and references therein, also Meisenheimer et al. 1989b). We outline the relevance of optical studies and summarize some results concerning the magnetic fields in extragalactic radio sources.

Download Full-text

What do we really know about the magnetic fields of the Milky Way?

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309031196 ◽

2008 ◽

Vol 4 (S259) ◽

pp. 515-518

Author(s):

Richard Wielebinski

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Milky Way ◽

Radio Sources ◽

Data Sets ◽

Optical Polarization ◽

Polarization Rotation ◽

Data Bases ◽

Robust Model ◽

The Magnetic Field

AbstractWe have several methods of measuring magnetic fields in the Milky Way. We can study optical polarization, radio polarization, rotation measures of pulsars and extragalactic radio sources as well as include Zeeman results. Each of the above mentioned methods was at times used to make a model of the magnetic fields of the Milky Way. However one or two of the data sets by themselves cannot tell us the whole story. Any model of the magnetic fields must be able to fit all the observational results. At the present time a lot of progress has been made. We have increased our data bases in most of the observational areas. However a robust model of the magnetic field of the Milky Way has not yet emerged. We must possibly wait to the era of SKA.

Download Full-text

MAGNETIC FIELDS IN BLAZAR PC-SCALE JETS — POSSIBLE CONNECTION TO SPIN RATES OF BLACK HOLES?

International Journal of Modern Physics D ◽

10.1142/s0218271808013133 ◽

2008 ◽

Vol 17 (09) ◽

pp. 1545-1552 ◽

Cited By ~ 7

Author(s):

P. KHARB ◽

M. L. LISTER ◽

P. SHASTRI

Keyword(s):

Magnetic Field ◽

Black Holes ◽

Magnetic Fields ◽

High Energy ◽

Optical Polarization ◽

Higher Spin ◽

Field Geometry ◽

Bl Lacs ◽

Simple Picture

We re-examine the differences observed in the pc-scale magnetic field geometry of high and low optical polarization quasars (HPQs, LPRQs) using the MOJAVE sample. We find that, as previously reported, HPQ jets exhibit predominantly transverse B fields while LPRQ jets tend to display longitudinal B fields. We attempt to understand these results along with the different B field geometry observed in the low and high energy peaked BL Lacs (LBLs, HBLs) using a simple picture wherein the spinning central black holes in these AGNs influence the speed and strength of the jet components (spine, sheath). Higher spin rates in HPQs compared to LPRQs and in LBLs compared to HBLs could explain the different total radio powers, VLBI jet speeds, and the observed B field geometry in these AGN classes.

Download Full-text

Magnetic-field strengths from optical polarization data

Symposium - International Astronomical Union ◽

10.1017/s0074180900101263 ◽

1967 ◽

Vol 31 ◽

pp. 381-383

Author(s):

J. M. Greenberg

Keyword(s):

Magnetic Field ◽

Optical Polarization ◽

Polarization Data ◽

Term Model ◽

Source Of Information ◽

Field Strengths

Van de Hulst (Paper 64, Table 1) has marked optical polarization as a questionable or marginal source of information concerning magnetic field strengths. Rather than arguing about this–I should rate this method asq+-, or quarrelling about the term ‘model-sensitive results’, I wish to stress the historical point that as recently as two years ago there were still some who questioned that optical polarization was definitely due to magnetically-oriented interstellar particles.

Download Full-text

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

International Astronomical Union Colloquium ◽

10.1017/s0252921100026038 ◽

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.

Download Full-text

Radio Measurements of Coronal Magnetic Fields

International Astronomical Union Colloquium ◽

10.1017/s0252921100024933 ◽

1994 ◽

Vol 144 ◽

pp. 21-28 ◽

Cited By ~ 4

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.

Download Full-text

Emergence of Twisted Magnetic Flux Related Sigmoidal Brightening

International Astronomical Union Colloquium ◽

10.1017/s0252921100064630 ◽

2000 ◽

Vol 179 ◽

pp. 263-264

Author(s):

K. Sundara Raman ◽

K. B. Ramesh ◽

R. Selvendran ◽

P. S. M. Aleem ◽

K. M. Hiremath

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Magnetic Flux ◽

Ultra Violet ◽

Active Regions ◽

Morphological Properties ◽

Energy Storage And Conversion ◽

The Sun ◽

X Rays ◽

The Magnetic Field

Extended AbstractWe have examined the morphological properties of a sigmoid associated with an SXR (soft X-ray) flare. The sigmoid is cospatial with the EUV (extreme ultra violet) images and in the optical part lies along an S-shaped Hαfilament. The photoheliogram shows flux emergence within an existingδtype sunspot which has caused the rotation of the umbrae giving rise to the sigmoidal brightening.It is now widely accepted that flares derive their energy from the magnetic fields of the active regions and coronal levels are considered to be the flare sites. But still a satisfactory understanding of the flare processes has not been achieved because of the difficulties encountered to predict and estimate the probability of flare eruptions. The convection flows and vortices below the photosphere transport and concentrate magnetic field, which subsequently appear as active regions in the photosphere (Rust & Kumar 1994 and the references therein). Successive emergence of magnetic flux, twist the field, creating flare productive magnetic shear and has been studied by many authors (Sundara Ramanet al.1998 and the references therein). Hence, it is considered that the flare is powered by the energy stored in the twisted magnetic flux tubes (Kurokawa 1996 and the references therein). Rust & Kumar (1996) named the S-shaped bright coronal loops that appear in soft X-rays as ‘Sigmoids’ and concluded that this S-shaped distortion is due to the twist developed in the magnetic field lines. These transient sigmoidal features tell a great deal about unstable coronal magnetic fields, as these regions are more likely to be eruptive (Canfieldet al.1999). As the magnetic fields of the active regions are deep rooted in the Sun, the twist developed in the subphotospheric flux tube penetrates the photosphere and extends in to the corona. Thus, it is essentially favourable for the subphotospheric twist to unwind the twist and transmit it through the photosphere to the corona. Therefore, it becomes essential to make complete observational descriptions of a flare from the magnetic field changes that are taking place in different atmospheric levels of the Sun, to pin down the energy storage and conversion process that trigger the flare phenomena.

Download Full-text

Solar Filaments as Tracers of Subsurface Processes

International Astronomical Union Colloquium ◽

10.1017/s0252921100064459 ◽

2000 ◽

Vol 179 ◽

pp. 177-183

Author(s):

D. M. Rust

Keyword(s):

Magnetic Fields ◽

Magnetic Flux ◽

Coronal Mass Ejections ◽

Interplanetary Space ◽

Intermediate Stage ◽

Coronal Plasma ◽

Magnetic Helicity ◽

The Sun ◽

New Paradigm ◽

Solar Filaments

AbstractSolar filaments are discussed in terms of two contrasting paradigms. The standard paradigm is that filaments are formed by condensation of coronal plasma into magnetic fields that are twisted or dimpled as a consequence of motions of the fields’ sources in the photosphere. According to a new paradigm, filaments form in rising, twisted flux ropes and are a necessary intermediate stage in the transfer to interplanetary space of dynamo-generated magnetic flux. It is argued that the accumulation of magnetic helicity in filaments and their coronal surroundings leads to filament eruptions and coronal mass ejections. These ejections relieve the Sun of the flux generated by the dynamo and make way for the flux of the next cycle.

Download Full-text