A team of solar physicists from the Bay Area Environmental Research Institute, University of Oslo and Lockheed Martin Solar and Astrophysics Laboratory has built a model that accurately explains the formation of spicules — small jets of plasma lasting a few minutes that form in the solar chromosphere. The research is published in the journal Science.
In the lower solar atmosphere, the chromosphere is permeated by violently driven jets called spicules.
Spicules occur thousands of times per day, erupt as fast as 60 miles per second, and can reach lengths of 6,000 miles before collapsing.
Their origin is poorly understood, although they are expected to play a role in heating the million-degree corona and are associated with so-called Alfvén waves, a strong kind of magnetic wave solar scientists suspect is key to heating the Sun’s atmosphere and propelling the solar wind.
To better grasp how spicules form, Dr. Juan Martínez-Sykora, a solar physicist at Lockheed Martin and the Bay Area Environmental Research Institute, and co-authors used state-of-the-art numerical models to develop simulations that spontaneously produced numerous spicules.
“The model began with a basic understanding of how plasma moves in the Sun’s atmosphere. Constant convection, or boiling, of material throughout the sun generates islands of tangled magnetic fields,” the researchers said.
“When boiling carries them up to the surface and farther into the Sun’s lower atmosphere, magnetic field lines rapidly snap back into place to resolve the tension, expelling plasma and energy.”
“Out of this violence, a spicule is born. But explaining how these complex magnetic knots rise and snap was the tricky part.”
“Usually magnetic fields are tightly coupled to charged particles,” Dr. Martínez-Sykora said.
“With only charged particles in the model, the magnetic fields were stuck, and couldn’t rise beyond the Sun’s surface. When we added neutrals, the magnetic fields could move more freely.”
“Neutral particles provide the buoyancy the gnarled knots of magnetic energy need to rise through the Sun’s boiling plasma and reach the chromosphere. There, they snap into spicules, releasing both plasma and energy. Friction between ions and neutral particles heats the plasma even more, both in and around the spicules.”
The team’s model revealed something else about how energy moves in the solar atmosphere: it turns out this whip-like process also naturally generates Alfvén waves.
The scientists found that the properties of their simulations matched observations of real spicules by NASA’s Interface Region Imaging Spectrograph (IRIS) and the Swedish 1-m Solar Telescope in La Palma, the Canary Islands.
“Numerical models and observations go hand in hand in our research,” said co-author Dr. Bart De Pontieu, a researcher at Lockheed Martin Solar and Astrophysics Laboratory.
The simulations indicate spicules could play a big role in energizing the Sun’s atmosphere, by constantly forcing plasma out and generating so many Alfvén waves across the Sun’s entire surface.
“This is a major advance in our understanding of what processes can energize the solar atmosphere, and lays the foundation for investigations with even more detail to determine how big of a role spicules play,” said IRIS mission scientist Dr. Adrian Daw, from NASA’s Goddard Space Flight Center.
“A very nice result on the eve of our launch anniversary.”
J. Martínez-Sykora et al. 2017. On the generation of solar spicules and Alfvénic waves. Science 356 (6344): 1269-1272; doi: 10.1126/science.aah5412