A twisting, whirling streamer of plasma escaping the sun in the aftermath of a coronal mass ejection (CME) has been captured on video by the European Space Agency’s Solar Orbiter, and could provide vital clues about how magnetic energy that drives the solar wind and giant eruptions on the sun is released.
The twirling, helical streamer was spotted by Solar Orbiter on Oct. 12, 2022. It lasted for more than three hours, extending up to 2 million kilometers (1.3 million miles) in length, transporting plasma and magnetic energy away from the sun.
Solar Orbiter used its Metis instrument, which features a coronagraph to block the glare of the body of the sun, to allow Solar Orbiter to see just the sun’s fainter outer atmosphere, called the corona, in both visible and ultraviolet light. It’s the wispy corona, filled with streamers, that we can see from Earth during a total solar eclipse.
Helical structures have been caught on camera in the solar corona before, but they have never been observed in such detail or for so long. How the streamer developed is now offering solar physicists clues as to what drives the solar wind and CMEs.
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Researchers led by Paolo Romano of the National Institute for Astrophysics at the Astrophysical Observatory of Catania in Italy have studied the twisting streamer and traced it all the way back to its origin in the lower corona.
The picture that they present is this: At the base of the corona, magnetic energy becomes stored in taut magnetic field lines. The corona is full of holes, like Swiss cheese, where the magnetic field lines are open — this, they extend out into space rather than bending back onto the sun’s surface (called the photosphere) in a closed loop.
The solar wind blows through these coronal holes. A phenomenon known as interchange reconnection takes place, which is the magnetic reconnection (where magnetic field lines break and then reconnect, releasing energy) between open and closed magnetic fields in the lower part of the corona. The most powerful reconnection events can liberate enough energy to produce solar flares and exploding magnetic filaments that result in CMEs.
On a smaller scale, interchange reconnection powers a constant and global population of jets that inject energy into the corona and through the coronal holes. These jets energize magnetic waves in the solar plasma called Alfvén waves, which in turn push plasma around and through the coronal holes, in the process ejecting matter on the solar wind that blows through those holes.
So where does the twisting streamer come into play? Based on observations by Solar Orbiter and its partner in crime, NASA’s Parker Solar Probe — which gets so close to the sun as to actually dip into the corona — solar physicists have become convinced that the magnetic structure of escaping plasma from a massive interchange reconnection event is in the form of a “twisted flux rope.”
A flux rope is a tube of magnetic energy that has erupted in a solar filament, which is a lace or channel of plasma held by closed magnetic fields. Interchange reconnection between the filament and the surrounding open magnetic field lines releases the energy that powers a CME, the resulting ejection being a powerful jet that bursts through the solar corona and into space, dragging plasma with it to form the CME. This flux tube forms the bright center of a CME.
And that’s exactly what Solar Orbiter witnessed: the twisting flux tube in the aftermath of a powerful interchange reconnection event that released a giant CME into space.
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According to computer simulations, the twisting nature of the flux tube is a natural consequence of a long-duration interchange reconnection. Solar Orbiter’s observations with Metis revealed more, however. The inclination, or tilt, of the twisting features in the escaping flux tube decreases with height above the sun, indicating that the magnetic field lines are growing more radially aligned with the sun as they propagate out into the solar system. Perturbations in the magnetic field carried by the streamer might then become amplified as they carry on deep into space.
Romano’s team speculate that it could be these perturbations that create the magnetic switchbacks, where the direction of the magnetic field in the solar wind suddenly reverses course before continuing onward in a kind of zig-zagging motion, as seen by both Solar Orbiter and Parker Solar Probe.
For decades, the sun’s greatest mysteries — how the solar corona is heated to more than a million degrees Celsius, for example, and where the energy comes from to drive the solar wind and CMEs — were closely held secrets. Now that Solar Orbiter and Parker Solar Probe are getting as close to the sun as it’s about possible to get, at long last the sun is giving up its greatest enigmas.
The academic research paper describing Solar Orbiter’s observations of the twisting streamer was published March 26 in The Astrophysical Journal.
