Precursors to Solar Eruptions Created in the Lab
The sun could seriously mess with your day. All it would take is an unexpected surge in X-rays or a blast of solar plasma and we could see our high-technology civilization brought down to its knees.
But with the help of a fascinating experiment in a laboratory at the California Institute of Technology (Caltech) in Pasadena, Calif., a micro-solar environment is being recreated in the hope of understanding the precursors to some of the most powerful solar eruptions.
As we are currently witnessing, the solar surface becomes pock-marked with sunspots during periods of intense magnetic activity. These dark spots are symptomatic of powerful loops of magnetic field being forced from the sun’s interior and through the photosphere — colloquially (if erroneously) known as the solar “surface.” These field lines can be visualized as lengths of twisted rubber bands that are kinked.
The sunspot number relates directly to the magnetic health of our nearest star; the greater the sunspot number, the more stressed the magnetic field has become. Every 11-years (on average) the sun moves from a minimum to a maximum state of magnetic stress and it just so happens that in 2013 solar physicists expect this natural cycle to peak.
In the period running up to “solar maximum” we can expect to see more solar explosions known as flares and coronal mass ejections (CMEs) and they often (although not exclusively) occur around these upwellings of magnetism over sunspots. It may come as no surprise to hear that regions of intense sunspot (magnetic) activity are regions of the sun that make space weather forecasters twitchy — active regions often spawn energetic eruptions and form the foundation of space weather.
But what causes these eruptions? And can we predict when they’re going to blow?
This is exactly what Paul Bellan, professor of applied physics at Caltech, and graduate student Eve Stenson hope to find out by focusing on beautiful — yet violent — coronal loops.
As shown here, coronal loops are long, arching magnetic field lines. Inside the constraining magnetic field, superheated solar gases (known as plasma) glows. These loops can be thought of as hosepipes funneling the plasma from the solar interior high into the sun’s atmosphere (or corona). These loops of multimillion degree plasma are known to be the source of solar flares.
Flares occur when stressed magnetic field lines are forced together and then snap. The action of snapping and reconnecting causes intense acceleration of the plasma contained within. Accelerated plasma generates intense radiation, resulting in flares. These flares can occur without warning, often drenching the Earth’s atmosphere in X-rays, potentially causing damage to satellites and creating a dangerously high level of radiation — an obvious hazard for astronauts.
When it comes to flares and CMEs, the more warning we can get the better, so Bellan and Stenson set up an apparatus inside a laboratory vacuum chamber to create their very own coronal loops created by electromagnets and plasma injection gun.
The results are nothing less than spectacular. Although the magnetic loops are created for only a fraction of a second, the researchers are able to inject hydrogen and nitrogen gas into the footpoints of the synthetic coronal loop and then apply a high-voltage current to the loop footpoints. The electric current turns the hydrogen and nitrogen into a plasma — which is accelerated to a speed of six miles per second — and they are able to watch events rapidly unfold.
“All three steps—the magnetic field, and the gas, and the high voltage-happen in just a flash of light inside the chamber,” said Stenson in a Caltech news release. “We use high-speed cameras with optical filters to capture the behavior of the plasmas.”
The two types of plasma are injected at different footpoints and glow different colors so the plasma motion can be observed with the optical filters. In the series of images (below), the red plasma flows into the loop from one footpoint while blue plasma simultaneously flows into the loop from the other end.
This experiment allowed Bellan and Stenson to find two magnetic forces at work: “One force expands the arch radius and so lengthens the loop while the other continuously injects plasma from both ends into the loop,” Bellan explained. “This latter force injects just the right amount of plasma to keep the density in the loop constant as it lengthens.”
The next experiment will be to create two loops right next to each other to see how they interact, helping us understand how the huge coronal loops on the sun behave.
Revealing the nature of mini loops in the lab could ultimately provide us with the necessary tools to understand which regions of the sun may contain loops of plasma ripe to explode — an obvious boon for space weather prediction models.
This research was published in the Aug. 13 edition of the journal Physical Review Letters (DOI: 10.1103/PhysRevLett.109.075001).
Press release: Caltech
Images: Top: Detail of Bellan and Stenson’s mini-coronal loop (Bellan & Stenson, 2012); Middle: Coronal loops as seen by NASA’s Transition Region and Coronal Explorer (NASA); Bottom: A time series (over 4 micro-seconds) of the evolution of the lab-made coronal loop with colored plasma (Bellan & Stenson, 2012).