Last Friday afternoon, Gerhard Haerendel of the Max–Planck Institute for Extraterrestrial Physics presented at Dartmouth’s Physics and Astronomy Department weekly colloquium. Haerendel began his talk, entitled “Fascinating Plasma Structures,” with a moving story, and delved into his recent research.

One of the largest structures ever created by man was a faint prospect above the horizon at Boulder, Colorado on the morning of December 27, 1984. Owing to the dourness of the weather, few were able to witness this ephemeral flight of a cloud of unusual composition., Haerendel, however, assisted in what was actually a highly ambitious project for plasma physics: the release of barium directly into the solar wind, a current of charged particles that spills forth from the sun.

After the virtually instantaneous ionization of barium in the presence of the sun’s ultraviolet rays, the interaction of the two plasmas was monitored by video cameras at Boulder and onboard two aircraft in the Pacific Ocean. An American satellite was also stationed inside the Earth’s magnetosphere to monitor the barium particles after they had descended. Haerendel, who is visiting Dartmouth as a Harris Fellow for four weeks, had pegged the experiment’s chances of success at around 30 percent but now confesses, “actually, it was zero [percent].”

While the attempt to recover the particles for analysis was unsuccessful, tracking of the barium vapor after its initial release revealed that the cloud actually developed into an artificial comet with a volume estimated at one percent of the Earth’s, complete with a well–defined head and tail. Surprisingly, instead of being “blown back” by the strength of the solar wind, the head of the serendipitous comet proceeded to drift at 90 degrees to the incident wind. The origins of this phenomenon may be governed by simple physical relationships, namely the Lorentz force that acts on charged particles moving in a magnetic field.

One of the more thrilling manifestations of the power of plasma is the aurora, which is caused by the emissions of high–altitude atoms that have been energized by contact with charged particles. Haerendel’s research into auroras dealt with the formation of auroral arcs, which are characterized by repeated peaks in measured energy over time accompanied by upward electrical currents above the aurora. Specifically, Haerendel investigated magnetic fractures, which help generate the magnetic energy needed to accelerate the electrons associated with arc–sustaining currents. To illustrate magnetic fracture, Haerendel bent a meter stick and showed how, when released, the meter stick would overshoot its rest position before returning to it, as magnetic field lines do when passed through regions of shear stress with periodic release.

Finally, Haerendel delved into some of his most recent research on the formation of plasma droplets falling from solar prominences, relatively cold (~8,000 K) and dense structures in the corona (which is usually between 1,000,000 and 2,000,000 K). Remarkably, these prominences are protected from the intense surrounding heat by the properties of magnetic fields. Videos showing prominences in close profile reveal “rain”–like plasma, as if the prominences were like “waterfalls.” The formation of these droplets, according to Haerendel, is driven by magnetic tension, which squeezes droplets together, and gravity, which accelerates and elongates the drop downward. As with falling water and air resistance, plasma droplets are exposed to magnetic drag as they fall. Once again, Haerendel was able to explain a beautiful plasma structure in simple physical terms.

Speaking before an audience with representatives from all stages of academic careers, Haerendel emphasized the important role of interested students in aiding future scientific developments. According to Haerendel, there is nothing better than to “solve problems” and allow new minds creative freedom.

Video archives of the colloquia of the Dartmouth Department of Physics & Astronomy, including that of September 30th, are available online at https://physics.dartmouth.edu/tags/colloquium-archives-june-2017.