Very-long-baseline interferometry (VLBI) and polarimetry

Very-long-baseline interferometry (VLBI) and polarimetry

What are the technological capabilities that allow the detection of magnetic fields from space objects located million light-years away? The example of the detection of the magnetic fields of M87's black hole.

A world-wide network of sensors is required to achieve high resolutions: the larger the distance between the sensors, the higher the resolution.

Presentation "VLBI Polarimetry Techniques I" - Notes and figures

• Origin of polarization: https://youtu.be/J_oXou6QGpI?t=483

  • Unlike scalar waves (such as sound), EM waves have polarization i.e. directionality of oscillations (Figure 1). By convention, we refer to polarization as the orientation of the electric field.

  • Ordinary light is made of photons with random E field orientations (Figure 2). 

  • Certain media may select specific orientations of the electric field (Figure 2). This is the case, for instance, for polarized glasses.

• Two polarization modes: 

  • Linear polarization: The electric field has a preferred direction of oscillations

  • Circular polarization: The electric field rotates around the direction of propagation (which is represented by the "Poynting vector")

• Description of polarization using the Stokes parameters

  • In order to fully describe polarization, the four Stokes parameters, I, Q, U and V, are needed. These describe how much polarized versus unpolarized light we have, how much circular polarization we have and what is the strength and orientation of the linear and circular polarization.

• Different analyses can be performed in space. 

  • For instance, electrons in very strong magnetic fields are rotating around magnetic field lines with acceleration, thereby emitting synchrotron light. This can be analyzed.

• Measuring Polarization: https://youtu.be/J_oXou6QGpI?t=1442

  • How polarizers register polarization.

• Polarization and interferometry/ https://youtu.be/J_oXou6QGpI?t=1684

• VLBI in a nutshell: https://youtu.be/J_oXou6QGpI?t=2024

  • We increase step-by-step the number of radiotelescopes on a map and we appreciate how a representation is reconstructed and enhanced in each step.

  • First, we place one radiotelescope and we get a poor image. Same for two. When we place three telescopes, we have three baselines, meaning three different combinations for a plane wave, or three plane waves. These will interfere and will be added coherently to offer a better reconstruction (Figure 4).

  • By using the Earth rotation and performing "Earth rotation synthesis" we obtain a high-fidelity image (Figure 4). 

Reference to very long baseline interferometry from Scientific American: https://twitter.com/sciam/status/1116071802944593920

The Very Long Baseline Array (VLBA): a 5000-mile-wide or 9000-Km-wide telescope.

"With an instrument this powerful, a person on the East Coast could read a newspaper on the West Coast." 

Excerpts from Wikipedia article: "The Very Long Baseline Array (VLBA) is a system of ten radio telescopes which are operated remotely from their Array Operations Center located in Socorro, New Mexico, as a part of the National Radio Astronomy Observatory (NRAO). These ten radio antennas work together as an array that forms the longest system in the world that uses very long baseline interferometry. The longest baseline available in this interferometer is about 8,611 kilometers (5,351 mi)." 

Excerpts from the article "Globe Spanning Telescope" by  Ray Nelson (from magazine "Popular Mechanics"):

"For decades, astronomers have patched together networks of radio telescopes to achieve the high resolution —ability to distinguish fine detail— of long baseline astronomy. But now they have a full-time dedicated tool, one with tremendous speed and flexibility and more resolving power than anything on Earth or in space. With an instrument this powerful, a person on the East Coast could read a newspaper on the West Coast. An astronomer using the array can peer deep into the core of a quasar some 12 billion light-years distant (...)."

"Closer to home, the array helps confirm satellite orbit locations for the Global Positioning System, or GPS, and aids in earthquake research. Using distant quasars as beacons, the antenna positions are triangulated for measurements within a centimeter, sometimes even within millimeters. This helps corroborate GPS satellite status and gives geodetic scientists a new way to gauge tectonic plate movement. All the antennas are controlled from headquarters at the Array Operations Center in Socorro, New Mexico."

"For some research, the Very Long Baseline Array can work with other radio observatories, such as the Very Large Array in New Mexico or the 330-foot-diameter dish now being built in Green Bank, West Virginia, a highly specialized instrument for close-up zooming. When Japan launches a 33-foot radio dish into orbit next year, the world's first space radio telescope will use the Very Long Baseline Array's correlator in Socorro as its "brain," and will tie in with the array for joint observations. Maybe they'll call it Very, Very Long Baseline astronomy."

It is noted that to obtain this order of resolution, a global infrastructure is required. Given that this global infrastructure exists, adequate safeguards against dual-use are required, to forbid the use for nefarious purposes of global scale.

Figure 4:  Image from the article "Globe Spanning Telescope" by  Ray Nelson (from magazine "Popular Mechanics").