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Teaching supercomputers to whistle

Speed read
  • Scientists examine microscopic processes affecting energy dissipation in solar wind.
  • Supercomputer models track a trillion particles per simulation.
  • Predicting high energy events in the heliosphere will protect the services satellites provide. 

You’ve whistled while you’ve worked. You’ve wet your whistle. You may have even whistled Dixie. But what do you know about Whistler turbulence?

Whistler turbulence is a high-frequency disturbance of solar plasma particles at a microscopic scale. Understanding Whistler turbulence will help us understand how energy is transferred in the space environment between the sun and Earth.

<strong>Solar spectrum. </strong>Energy is contained inside the small red region representing an initial spectrum of relatively long wavelength fluctuations. As the simulation proceeds, energy cascades to smaller wavelengths with a preference to cascade approximately in the perpendicular direction. Courtesy Joseph Wang.

“Most people don’t realize that the entire solar system, including the Earth, is actually inside the sun’s extended atmosphere,” says Joseph Wang, associate professor of Astronautics in the Department of Astronautical Engineering at the University of Southern California. “This environment is rich with dynamic processes of space weather. At times the solar wind can be relatively calm, while at other times it can contain storms which are hazardous to spacecraft.”

To study the sun’s atmosphere (heliosphere), Wang’s team uses supercomputers at NASA and the National Center for Atmospheric Research (NCAR). They employ a 3D electromagnetic simulation that uses the Particle-in-Cell (PIC) algorithm. Armed with these tools, they aim to understand the microscopic processes that influence energy dissipation in the solar wind.

With the current super-computing capabilities, we can trace the dynamics of hundreds of millions to 1 trillion plasma particles in a typical simulation.~Joseph Wang

Increasing the safety of work done in space is one of the anticipated gains from Wang’s research. Grasping the underlying physics of the heliosphere will also help scientists to develop space weather forecasting models that anticipate when high energy events like coronal mass ejections are approaching the Earth.

“Our inability to predict the effect of high energy events on the near earth space environment can result in damage or even destruction of satellites that provide services on which modern society has come to depend,” says Wang.

Defense, telecommunications, and global positioning systems are examples of these satellite services we now rely on.

Harnessing NASA’s Pleiades and NCAR’s Yellowstone supercomputers allows Wang's team to trace the dynamics of up to 1 trillion plasma particles in a typical simulation. Impossible on a normal desktop machine, Wang’s simulations model around 70 billion ions and electrons in three spatial directions. The PIC algorithm requires 8,192 processors running for around 50 hours.

Wang’s PIC simulations concur with physical observations, he says. In particular, Whistler’s turbulence is shown to transfer energy through a mechanism known as a forward cascade (wave interactions that move from large wavelength modes to smaller ones).

<strong>Tiny dancer. </strong>The heliospheric current sheet results from the influence of the sun's rotating magnetic field on the plasma in the interplanetary medium (solar wind). The wavy spiral shape has been likened to a ballerina's skirt. Courtesy Werner Heil.

Thanks to the supercomputer simulations, Wang is able to confirm that energy is transferred into Whistler waves which propagate across magnetic field lines, rather than along these lines. These findings corroborate observations made by spacecraft such as Cluster.

Even though it is well known by scientists that Whistler waves interact strongly with electrons in the plasma, because ion motions are much slower than Whistler frequencies they have not been expected to interact with the ions. 

“Our recent work has discovered that Whistler turbulence can heat ions as well as electrons, because the frequency of whistlers moving across magnetic field lines can become relatively slow. This provides a potential explanation for a widely debated question of how ions get heated in the solar wind.”

The answer to how Whistler waves perform may be blowing in the (solar) wind. Turns out, the best way to find the answer is to ask a supercomputer.

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