Magnetic Field Determines Whether Solar Flares Erupt or Fizzle

New 3-D simulations of plasma on the sun suggest why some solar flares shoot into space, while others collapse back onto the sun’s surface.

Eruptions on the sun regularly release hot, charged material into space, which can spark geomagnetic storms if they smack into the Earth. These outbursts, called coronal mass ejections, are associated with sunspots where magnetic fields from the inside of the sun rise to the surface.

That magnetic movement forms plasmoids, clouds of plasma that are bound by magnetic fields. Plasmoids have been invoked to explain natural phenomena like ball lightning and magnetic bubbles in the Earth’s magnetosphere.

With 3-D computer models of plasma and magnetic flows in the sun, researchers at the University of St Andrews in Scotland showed that plasmoids with a twisted tube shape naturally form when magnetic fields shift. The team presented their results April 18 at the Royal Astronomical Society’s National Astronomy Meeting in Llandudno, Wales.

As plasma moves around in the sun’s lower atmosphere, it brings loops of magnetic field lines closer together in a fanlike shape, trapping the plasma in a magnetic sheath.

Whether the plasma escapes into space or remains trapped on the sun depends on the strength of the interaction between the emerging field and the pre-existing field in the sun’s corona, the simulations show. Unless the sheath is removed somehow, the plasmoids remain trapped in their magnetic prison.

But if the magnetic field lines that make up the sheath break and reconnect with the other magnetic fields in the surrounding solar corona, they could allow the plasmoids to erupt outward at speeds exceeding a million miles per hour.

The simulations track 90 minutes of real time, from the initial emergence of the magnetic fields until the plasmoids reach the sun’s upper atmosphere. The animation above shows the temperature of the plasma in a successful eruption. In the animation below, the magnetic sheath held the plasmoid back.

Image: Vasilis Archontis