The Faraday Sky

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Mapping The Milky Way’s Magnetic Fields – The Faraday Sky
by Tammy Plotner on December 9, 2011

Click to view full size image 1 or image 2

The Faraday Sky (corrected for the number density of electrons n_e): Red and blue colors indicate regions of the sky where the magnetic field (B||) points toward and away from the observer, respectively. The North celestial pole is at the top left and the South Pole is at the bottom right.

OVERLAY: The Faraday Sky ( uncorrected for the number density of electrons n_e): The band of the Milky Way (the plane of the galactic disk) extends horizontally in this panoramic view. The center of the Milky Way lies in the middle of the image.

<<Kudos to the scientists at the Max Planck Institut and an international team of radio astronomers for an incredibly detailed new map of our galaxy’s magnetic fields! This unique all-sky map has surpassed its predecessors and is giving us insight into the magnetic field structure of the Milky Way beyond anything so far seen. What’s so special about this one? It’s showing us a quality known as Faraday depth – a concept which works along a specific line of sight. To construct the map, data was melded from 41,000 measurements collected from a new image reconstruction technique. We can now see not only the major structure of galactic fields, but less obvious features like turbulence in galactic gas.

So, exactly what does a new map of this kind mean? All galaxies possess magnetic fields, but their source is a mystery. As of now, we can only guess they occur due to dynamo processes… where mechanical energy is transformed into magnetic energy. This type of creation is perfectly normal and happens here on Earth, the Sun, and even on a smaller scale like a hand-crank powered radio – or a Faraday flashlight! By showing us where magnetic field structures occur in the Milky Way, we can get a better understanding of galactic dynamos.

For the last century and a half, we’ve know about Faraday rotation and scientists use it to measure cosmic magnetic fields. This action happens when polarized light goes through a magnetized medium and the plane of polarization revolves. The amount of turn is dependent on the strength and direction of the magnetic field. By observation of the rotation we can further understand the properties of the intervening magnetic fields. Radio astronomers gather and examine the polarized light from distant radio sources passing through our galaxy on its way to us. The Faraday effect can then be judged by measuring the source polarization at various frequencies. However, these measurements can only tell us about the one path through the Milky Way. To see things as a whole, one needs to know how many sources are scattered over the visible sky. This is where the international group of radio astronomers played an important role. They proved data from 26 different projects which gave a grand total of 41,300 pinpoint sources – at an average of about one radio source per square degree of sky.

Although that sounds like a wealth of information, it’s still not really enough. There’s huge areas, particularly in the southern sky, where only a few measurements exist. Because of this lack of data, we have to interpolate between existing data points and that creates its own problems. First, the accuracy varies and more precise measurements should help. Also, astronomers are not sure of exactly sure of how reliable a single measurement can be – they just have to take their best guess based on what information they have. Still, other problems exist. There are measurement uncertainties due to the complex nature of the process. A small error can increase by tenfold and this could convolute the map if not corrected. To help fix these problems, scientists at MPA developed a new algorithm for image capture, named the “extended critical filter”. In its creation, the team utilizes tools provided by the new discipline known as information field theory – a powerful tool that blends logical and statistical methods to applied fields and stacks it up against inaccurate information. This new work is exciting because it can also be applied to other imaging and signal-processing venues in alternate scientific fields.

The good news is the galactic dynamo theory seems to be spot on. It has predicted symmetrical structures and the new map reflects it. In this projection, the magnetic fields are lined up parallel to the plane of the galactic disc in a spiral. This direction is opposite above and below the disc and the observed symmetries in the Faraday map arise from our location within the galactic disc. Here we see both large and small structures tied in with the turbulent, dynamic Milky Way gas structures. This new map algorithm has a great side-line, too… it characterizes the size distribution of these structures. Larger ones are more definitive than smaller ones, which is normal for turbulent systems. This spectrum can then be stacked against computer models of dynamics – allowing for intricate testing of the galactic dynamo models.

This incredible new map is more than just another pretty face in astronomy. By providing information of extragalactic magnetic fields, we’re enabling radio telescope projects such as LOFAR, eVLA, ASKAP, Meerkat and the SKA to rise to new heights. With this will come even more updates to the Faraday Sky and reveal the mystery of the origin of galactic magnetic fields.>>

[bystander]

An improved map of the Galactic Faraday sky
ASTRON JIVE | George Heald | 2011 Dec 07

New all-sky map shows the magnetic fields of the Milky Way with the highest precision
Max Planck Institute for Astrophysics | 2011 Dec 05

An improved map of the Galactic Faraday sky - N. Oppermann et al

• arXiv.org > astro-ph > arXiv:1111.6186 > 26 Nov 2011

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[b][color=#0000FF]The Faraday effect or Faraday rotation[/color][/b]

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<<In physics, the Faraday effect or Faraday rotation is a Magneto-optical phenomenon, that is, an interaction between light and a magnetic field in a medium. The Faraday effect causes a rotation of the plane of polarization which is linearly proportional to the component of the magnetic field in the direction of propagation.

Discovered by Michael Faraday in 1845, the Faraday effect was the first experimental evidence that light and electromagnetism are related. The theoretical basis of electromagnetic radiation (which includes visible light) was completed by James Clerk Maxwell in the 1860s and 1870s. This effect occurs in most optically transparent dielectric materials (including liquids) under the influence of magnetic fields.

The Faraday effect causes left and right circularly polarized waves to propagate at slightly different speeds, a property known as circular birefringence. Since a linear polarization can be decomposed into the superposition of two equal-amplitude circularly polarized components of opposite handedness and different phase, the effect of a relative phase shift, induced by the Faraday effect, is to rotate the orientation of a wave's linear polarization.

The Faraday effect has a few applications in measuring instruments. For instance, the Faraday effect has been used to measure optical rotatory power and for remote sensing of magnetic fields. The Faraday effect is used in spintronics research to study the polarization of electron spins in semiconductors. Faraday rotators can be used for amplitude modulation of light, and are the basis of optical isolators and optical circulators; such components are required in optical telecommunications and other laser applications.

The discovery is well documented, because Faraday's daily notebook has been published. In 1845 he undertook a series of experiments explicitly intended to find some effect on light from electric and magnetic fields, and succeeded.

• On 13 Sept. 1845, in paragraph #7504, under the rubric Heavy Glass, he wrote: -
  
  • ...BUT, when the contrary magnetic poles were on the same side, there was an effect produced on the polarized ray, and thus magnetic force and light were proved to have relation to each other....
  He summarized the results of his experiments on 30 Sept. 1845, in paragraph #7718, famously writing: -
  
  • ...Still, I have at last succeeded in illuminating a magnetic curve or line of force, and in magnetizing a ray of light....

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Faraday rotation in the interstellar medium

The effect is imposed on light over the course of its propagation from its origin to the Earth, through the interstellar medium. Here, the effect is caused by free electrons and can be characterized as a difference in the refractive index seen by the two circularly polarized propagation modes. Hence, in contrast to the Faraday effect in solids or liquids, interstellar Faraday rotation has a simple dependence on the wavelength of light (λ), namely:

where the overall strength of the effect is characterized by RM, the rotation measure. This in turn depends on the axial component of the interstellar magnetic field B||, and the number density of electrons n_e, both of which vary along the propagation path.>>

[bystander]

Scientists Chart Milky Way's Magnetic Fields
National Research Laboratory | 2012 Feb 03

The Milky Way’s Magnetic Personality
Universe Today | Tammy Plotner | 2012 Feb 06