Electrons surfing on Alfvén waves

The answer to a fundamental question of auroral physics: can Alfvén waves accelerate electrons?

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The aurora borealis captured in Alaska by photographer Jean Beaufort.

Shimmering curtains and hazy glows of northern and southern lights (auroras) have inspired us to look upward for millennia. Even so, for most of recorded history, the complexity of events leading up to these breathtaking displays and the distance over which they unfold have been vastly under-estimated. While auroral light is produced just a few hundred kilometers above the ground, the process starts at the sun. Violent eruptions caused by solar flares and coronal mass ejections produce variations in the outflowing gas of ions and electrons (plasma) called the solar wind and often precede auroras. On the other end of this process, electrons rain down from space, plunge into the upper atmosphere, and strike atoms. Atoms gain energy from these collisions and then relinquish it by emitting photons (Fig. 1). A collection of these photons, produced by a shower of electrons, generates an aurora.

Fig. 1: Precipitating electrons transfer energy to atoms in the upper atmosphere through collisions. Atoms relax to lower energy states by giving off photons. Image: Austin Montelius, College of Liberal Arts and Sciences, University of Iowa.

Amidst the beauty of this stellar Rube Goldberg process remain questions about intervening steps. Principally, how does solar activity cause precipitating electrons? A survey of literature reveals multiple causes are likely at work. Hazy diffuse auroras are attributed to electrons leaking from reservoirs of energized electrons around Earth, e.g. the Van Allen radiation belts. Stationary auroral arcs indicate a steady stream of electrons pulled earthward by quasi-static electric potentials. However, the bright and writhing auroras associated with geomagnetic storms suggests something else. Satellite data collected by flying above these bright auroras often show powerful Alfvén waves (Fig. 2), disturbances that travel along magnetic field lines like waves on a string, descending toward Earth. Where do these Alfvén waves come from, and can they be a source of auroral electrons? 

Fig. 2: Alfvén waves travel along geomagnetic field lines connected to auroras. Alfvén waves are launched by reconnecting field lines stretched by the flowing solar wind. Experiments sought to answer the question: can Alfvén waves cause electron precipitation that causes auroras? Image: Austin Montelius, College of Liberal Arts and Sciences, University of Iowa.

Like waves on a string, launched by shaking the string, Alfvén waves are produced by shaking magnetic field lines. The solar wind shakes Earth’s magnetic field in a number of ways, through eruption of solar plasma, or when the passing solar wind stretches out the geomagnetic field. Stretched magnetic field lines eventually reconnect and snap back toward earth, like a rubber band, launching a spectrum of Alfvén waves on their way (see video).

It is perhaps more interesting to ask whether Alfvén waves cause auroras or, rather, are simply an associated phenomenon. This question has existed in the literature for over 40 years without a direct test. However, the evidence from rocket and satellite data that Alfvén waves participate in electron precipitation is suggestive. Alfvén waves are often found traveling earthward carrying an amount of energy similar to that given off by auroral light. Alfvén wave intensity and auroral activity often increase and decrease together. The magnetic field lines carrying Alfvén waves commonly thread through auroral displays. And yet, these measurements fall short of being able to conclude that Alfvén waves push electrons toward earth.

To discern these secrets of the sky, we went underground to a basement lab at UCLA. Inside a 20-m long cylindrical vacuum chamber wrapped in magnetic coils, millisecond flashes of plasma provide the environment for microsecond bursts of Alfvén waves. The experiment, called the Large Plasma Device, or LAPD for short, is part of an NSF/DOE collaborative research facility at UCLA, and is one of the premiere labs for Alfvén wave research in the world (Fig. 3). Perhaps you’ve never heard of Alfvén waves, but it turns out they’re pretty common in magnetized plasmas. They’re often present wherever there’s electrons, ions, and magnetic fields, which includes most of the universe.

Fig. 3: Panoramic view of the 20-m long Large Plasma Device (LAPD) where experiments where performed. Image: Basic Plasma Science Facility.

The goal of experiments was simple: launch Alfvén waves and simultaneously measure the electron velocity distribution. Even though the goal can be simply stated, every aspect of accomplishing it required time, care, and the ingenuity of advisors and collaborators. It took time to determine relevant plasma conditions (e.g. density, temperature, and magnetic field), to launch and measure Alfvén waves, to measure electron velocity distributions, and develop theory to help interpret the data.

With data finally in hand, we correlated changes in the electron velocity distribution with the fluctuating Alfvén wave. We found a small fraction of electrons, less than 1/1000, move at a velocity allowing them to be picked up by the Alfvén wave and accelerated, like a surfer on a wave in the ocean, continuously speeding up as they move along with the wave (Fig. 4). Our data show it is exactly these electrons that gain energy from the Alfvén wave. Electrons surfing on plasma waves is a process known as Landau damping, first predicted by Russian physicist Lev Landau. We found the energy gained per electron per second by surfing on Alfvén waves in the experiment is similar to what’s expected for auroral electrons. This result provides an answer to the 40-year-old hypothesis: Alfvén waves do accelerate electrons, and Alfvén waves can cause electron precipitation. This means Alfvén waves can in fact be responsible for some of the most spectacular auroral displays.

Fig. 4: Experiments showed electrons can surf on Alfvén waves above auroras. Image: Austin Montelius, College of Liberal Arts and Sciences, University of Iowa.

For laboratory plasma astrophysics experiments such as this, a common question is “how do you know what you measure in the lab is the same thing that happens in space?” Similarity analysis identifies a key set of dimensionless ratios relevant to the dynamics of space plasma, and by scaling the plasma down to laboratory scales while preserving the dimensionless ratios, the same dynamics will be present. For example, the ratio of the Alfvén wave speed to the electron thermal speed is the same in the experiment as in the region where auroral electrons are accelerated, and by preserving this ratio, similarity analysis shows that magnetic forces dominate over gas dynamics in both settings.

I’m also commonly asked by friends and family if there’s anything useful about my work. Curiosity and innovation are inseparable. Sometimes we have the benefit of knowing what innovations our curiosity will spur, and sometimes we don’t. I'd hate to live in a world where the only topics of research are those that map immediately to market. There’s a long history of innovations produced from curiosities. Curiosity about the role of Alfvén waves in generating auroras has been around long enough to see daily life become dependent on a global network of satellites for navigation and communication. Never before have the dynamics of near-Earth space been more relevant, and there are many opportunities for exciting research in laboratory astrophysics.

So, what’s next? This project developed a number of tools to explore the energization of particles in plasma. We used new lab techniques and analysis tools to determine how electrons gain energy in order to unveil the underlying acceleration mechanism. There are plenty of places in space and astrophysical plasmas where we’d like to know more about the origin of energetic particles from the origin of cosmic rays, to heating the solar corona, and the source of fast electrons and ions in the Van Allen radiation belts.

Jim Schroeder

Assistant Professor of Physics, Wheaton College (IL)