An image of a Coronal Mass Ejection shows both open and closed magnetic field lines. Image courtesy Solar and Heliospheric Observatory (SOHO), NASA

New theory unravels magnetic instability

Theorists Giovanni Lapenta of Plasma Theory (T-15) and Dana Knoll of Fluid Dynamics (T-3) presented their findings last Friday at the American Geophysical Union meeting in San Francisco.

The theory is based on a 19th century mathematical observation called Kelvin-Helmholtz instability. "What we are trying to determine is why magnetic field lines loop out from the surface of the sun, reconnect and then fall back, and why these systems, which look very stable, are in fact quite unstable," said Lapenta.

According to Lapenta, reconnection rates based on resistivity are orders of magnitude too slow to explain observed coronal reconnections. One possible mechanism that provides fast reconnection rates is known as "driven" reconnection - where external forces drive field lines together in a way that is independent of resistivity.

Lapenta and Knoll believe that related work focused on magnetic field line reconnection in Earth's magnetopause has shown that the Kelvin-Helmholtz instability can cause compressive actions that push field lines together and drive reconnection. "We propose that the same mechanism at work in the magnetopause could conceivably be at work in the solar corona and elsewhere," said Lapenta.

In this theory, motion on the visible surface of the sun - the photosphere - leads to twisting deformation waves that move through the chromosphere, a layer of solar atmosphere just above the photosphere, growing larger as they move and emerging with a rapid increase of speed through the sun's corona, or outer atmosphere. This rapid change in speed, or velocity shear, injected into the corona can cause magnetic loops to reconnect, Lapenta said.

"We have conducted a series of simulations and shown that indeed reconnection can be achieved through local compression driven by Kelvin-Helmholtz and that the reconnection rate is not sensitive to resistivity," said Lapenta.

From this beginning point, Lapenta hopes to study the processes tied to motion on the surface of the sun to better understand why these "velocity shears" occur and how they move away from the sun and lead to coronal mass ejections and other solar events, and to apply this knowledge to better understanding the magnetic fields around the Earth and the disc-shaped rotating masses, or accretion discs, that form around some black holes.

-- Kevin Roark

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