Magnetics Business & Technology - November/December 2019 - 30

VISIONS

New Survey Technique for Interstellar Magnetic Fields Developed by University of Wisconsin
A new, more accessible
and much cheaper approach to surveying the
topology and strength of
interstellar magnetic fields
which weave through
space in our galaxy and
beyond, representing one
of the most potent forces
in nature, been developed
by researchers at the University of Wisconsin-Madison. It could revolutionize how scientists study
magnetic effects on star The formation of stars in the turbulent
billows of gas and dust of the Orion Moformation.
lecular Cloud, depicted in an illustration
Together with gravity, based on data from the European Space
magnetic fields play a Agency's Planck satellite. Photo credit:
major role in many of ESA and The Planck Collaboration

astrophysical processes
ranging from star formation to stirring the massive dust and gas clouds that permeate
interstellar space, that underpin the structure and composition of
stars, planets and galaxies. On the galactic scale, magnetic fields
dominate the acceleration and propagation of cosmic rays and
play an important role in transferring heat and polarized radiation.
What's more, the polarized radiation that arises from galactic
magnetic fields exceeds by orders of magnitude that of the Cosmic Microwave Background which is the relic radiation of the first
moments of the universe. The next milestone in understanding the
origin of the universe, some scientists believe, requires measuring
the backgrounds polarized radiation but unraveling the topology
of the intervening magnetic fields between it and Earth will be a
necessary step to reliably obtain those data.
Despite their importance and pervasive influence, though, interstellar magnetic fields represent one of the final frontiers of astrophysics. Little is known about them, in large part, because they
are exceedingly difficult to study.
"There are very limited ways to study magnetic fields in
space," explains Alexandre Lazarian, a UW-Madison
professor of astronomy and an authority on the interstellar medium, the seemingly empty spaces between
the stars that are, in fact, rich in matter and feature
twisted, folded and tangled magnetic fields composed
of fully or partially ionized plasmas entrained
Alexandre Lazarian
on magnetic fields. "Our understanding of all
these (astrophysical) processes suffers from
our poor knowledge of magnetic fields."
In the journal Nature Astronomy, August 2019 issue, an international team led by Lazarian demonstrated new methodology capable of tracing the orientations of magnetic fields in the swirl of
interstellar space. Their proof-of-concept builds on a series of theoretical and numerical studies published over the last two years
by Lazarian and his students which lay out a radical new approach
to mapping the tangle of magnetic fields in space.
Until now, much of the detailed mapping of magnetic fields in dif-

30

fuse environments such as clouds of dust and gas in space involved infrared polarimetry with instruments deployed either on
satellites or balloons flown high in the stratosphere.
The new method, known as the Velocity Gradient Technique and
informally as the "Wisconsin technique," uses previously collected
observational data from a variety of ground-based telescopes.
This transcends the need to put instruments in space, a costly
and limited resource for astronomers. Building on studies of turbulence in magnetic fields in conducting fluids, Lazarian and his students devised the new statistical approach to measure the topology of magnetic fields using routine spectroscopic observations
taken from the ground.
For the most part, infrared light is absorbed by Earth's atmosphere, which is why conventional magnetic field measurements
require telescopes positioned on long-duration, high-altitude balloon flights, or above it on satellites. In recent years, many new
measurements of interstellar magnetic fields, for instance, were
gathered using the Planck satellite, a European space observatory with infrared capabilities which operated from 2009 to 2013.
Applying the new Wisconsin technique to several interstellar molecular clouds whose magnetic fields had been previously measured by the Planck satellite, Lazarian and his students were able
to generate high-resolution maps using existing ground-based
observations.
"The technique provides magnetic field maps of resolution comparable to maps obtained with the Planck mission," says Lazarian,
"and it utilizes spectroscopic observations collected by researchers for other purposes. Given that the technique utilizes data from
ground-based telescopes and interferometers, the resolution of
magnetic field maps can be significantly improved."
In addition to determining the direction of the interstellar magnetic
fields, the new methodology can determine the strength of the
field at a fine scale, down to each pixel on a map. "This demonstrates that the Wisconsin technique can revolutionize studies of
magnetic effects on star formation by using existing ground-based
telescopes without waiting for new space-based polarization missions with a higher resolution in some distant future," Lazarian
says.
The new technique, Lazarian adds, also opens a unique window
to the development of three-dimensional magnetic field maps,
work that has already been demonstrated in a corresponding paper published in the Astrophysical Journal by Lazarian and his
student, Diego Gonzales Casanova.
To contrast the capabilities of the new technique with traditional
polarimetry, Lazarian and his group, including UW-Madison physics graduate student Yue Hu and astronomy graduate student Ka
Ho Yuen, key authors of the new Nature Astronomy report, deployed their new methodology to produce the first magnetic field
map of the Smith Cloud, a mysterious cloud of atomic hydrogen
that seems to be crashing onto the disk of the Milky Way. Previous efforts to map the cloud's magnetic field were frustrated by its
weak infrared emission, obscuring dust and galactic atomic hydrogen along the same line of sight.
For more info, see and www.news.wisc.edu and www.nature.com.

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