by Lori Ann White
Step outside after dark on a clear night and look up. Identify the phase of the moon, marvel at the flash of a meteor, trace familiar constellations, and wonder what else might be out there.
This, in a nutshell, is the life of a scientist in an observational field like astronomy. Astronomers and scientists in the related fields of astrophysics and cosmology face certain constraints in their research: instead of conducting hands-on experiments to test their hypotheses, they ask questions and look for answers in the light that comes from the stars. It's even more challenging when what's being studied can't be seen directly; for example, magnetic fields, which are—at best—twice-removed from direct observation.
Susan Clark, who will officially begin her appointment as KIPAC's newest faculty member September 1, 2021, is quite familiar with such challenges. While the main thrust of her research is reaching a broad understanding of astrophysical magnetism—including planetary and stellar fields, the immensely powerful fields of pulsars and active galactic nuclei, and the huge fields throughout galaxy clusters—one of her favorite areas of research is mapping the vast lines of magnetic force that weave through our own galaxy.
"The Milky Way has a magnetic field and there is so much about it we don’t understand," she says. "It's difficult to observe and difficult to simulate."
To do so, she and others who research cosmic magnetic fields must understand how the fields affect charged particles of diffuse dust and gas that makes up the interstellar medium (ISM), as well as how those particles leave a mark on the light we see, whether that light is emitted by the particles, passes through them, or bounces off them.
Many of the basic interactions have been filled in. Scientists have discovered that the tiny dust grains of the ISM align themselves to the galactic magnetic fields like iron filings to a bar magnet, although perpendicular to the field lines, instead of in parallel (this mechanism is called "radiative torque alignment", see e.g. here for details). Starlight, in turn, passes through this dust, which absorbs only the radiation polarized in a complementary fashion. What's missing from the remaining light captured by detectors on Earth reveals the orientation of the dust grains, and in turn, the orientation of the magnetic field. In addition, the heated dust grains emit infrared radiation which is also polarized, providing additional information about the magnetic field.
These and other probes of cosmic magnetism, such as the synchrotron radiation emitted by charged particles as they accelerate in a magnetic field, do have shortcomings: each technique only probes a certain portion of the magnetic field, and combining them all to create a fuller picture is very difficult.
However, during Clark's tenure as a Hubble Fellow at the Institute for Advanced Study in Princeton, New Jersey, she and her colleagues have identified a new way to fill these magnetic maps—neutral hydrogen.
Hydrogen is the most plentiful element in the Universe, and various forms of it still make up the majority of the ISM. Clark and her colleagues used huge radio telescopes like Arecibo in Puerto Rico to study the structure of clouds of neutral hydrogen.
"If you map the sky at high enough resolution, you see all of this complex spatial structure—patterns in the gas—thin, long, strands of neutral hydrogen that are very well-aligned with the magnetic field," she said, although exactly why this occurs is still under investigation.
In addition to using neutral hydrogen to broaden magnetic field maps, Clark also discovered a way to use it to give a three-dimensional structure to the maps—a much trickier proposition, since the night sky as seen from our vantage point on Earth is inherently a two-dimensional phenomenon. But by determining relative positions of the tendrils based on the Doppler shifts of their emissions, Clark could identify different field lines at different distances and determine how tangled they might be, and how the accumulation of these differences could depolarize dust emissions passing through them.
This knowledge has immediate ramifications already for one major area of KIPAC research: the polarization of the cosmic microwave background (CMB). KIPAC researchers like Chao-lin Kuo with the BICEP experiment have been measuring CMB polarization for years in an effort to identify the imprint of primordial gravitational waves, but the effort has been stymied by galactic "foregrounds"—similar signals from the dust in our own galaxy. The better Clark understands how the Milky Way's magnetic fields affect dust clouds, the easier it is to remove their contributions, leaving the CMB's signal behind. (Tracing CMB polarization In order to search for axionic dark matter in the Milky Way was also covered in this August 2020 KIPAC blogpost.)
Clark recognizes that her quest to unlock the secrets of the Milky Way's magnetic fields is an ambitious one—but for her, that's a feature, not a bug.
"I definitely realized early on that this was a notoriously difficult problem," she says. "That piqued my interest."
And that also explains why working at Stanford is a draw.
"Stanford is the perfect place to tackle ambitious projects," says Clark. "I'm thrilled to be joining such a dynamic and stimulating research environment.
Another draw is teaching and KIPAC’s commitment to outreach. "I love to teach anything related to radiative processes or fluid dynamics, the ISM, introductory astrophysics—that's often a real hook for people to get excited about science if they weren't already," she says.
Clark has already helped some disadvantaged students get excited about science through the Princeton Prison Teaching Initiative. Clark taught at three different correctional facilities during 2018 and 2019, receiving the Unsung Hero Award in 2019 for her efforts.
"It's an incredible program, and one of the most rewarding classroom environments I've been in," she says. "I'm very fortunate to have had that teaching experience." It gave her an opportunity to put her teaching philosophy into practice.
"You want to reach everyone, including the people who think of themselves as afraid of math," she says. "In our US culture we've normalized statements like, 'Oh, I'm just not a math person.' I think changing that mindset will help make STEM subjects more inclusive.
"You don't have to be a math person, you don't have to be a science person—you become one."
Interstellar Magnetism (Susan E. Clark, 2019; IAS Website)
Mapping the Magnetic Interstellar Medium in Three Dimensions Over the Full Sky with Neutral Hydrogen (Clark & Hensley, 2019; arXiv)
The physical nature of neutral hydrogen intensity structure (Clark, Peek, & Miville-Deschênes, 2019; arXiv)