A simple magnetic field configuration could trigger solar eruptions

Richard Carrington made the first record of a solar flare in 1859 from his observatory outside London. Although it wasn’t clear at the time, research since then has shown that such eruptions can have an enormous impact on Earth’s space neighborhood. The tons of plasma spewed into space may breach Earth’s protective magnetic field and eventually emerge as auroras and geomagnetic storms.

Combined observations and theory have led physicists since Carrington to understand that solar eruptions, including flares and coronal mass ejections such as the one shown in figure 1, are powered by magnetic fields in the Sun’s corona.

Figure 1.

A remaining challenge is pinpointing a specific mechanism that triggers those eruptions. As a precondition for eruption, many models invoke complex magnetic field structures such as twisted ropes of flux to create an instability that drives an eruption, but astronomers have rarely observed such topologies in the preeruption corona.

Now Chaowei Jiang at the Harbin Institute of Technology’s campus in Shenzhen, China, and colleagues have demonstrated through three-dimensional simulations that magnetic field lines that are arranged in a simple looped configuration near sunspots are sufficient to initiate a solar eruption. ¹

Tangled up in plasma

In the 1960s, solar physicists converged on an explanation for how small solar flares form. They postulated that the magnetic field lines that permeate the Sun’s plasma atmosphere interact with one another in a process called magnetic reconnection (see Physics Today, September 2013, page 12 ). When oppositely directed field lines approach each other, they can break and reconnect in a lower-energy configuration. The field lines are bent tightly immediately after reconnection. Then they abruptly straighten and release energy, much like the snapping of a rubber band.

Because the plasma conducts electricity, the field lines cannot move with respect to the plasma in which they are embedded. As a result, when the field lines move, so does the plasma. The straightening field lines then launch jets of charged particles in opposite directions away from the reconnection locus, as illustrated in figure 2. (See the article by Forrest Mozer and Philip Pritchett, Physics Today, June 2010, page 34 .) That process generates the kinetic energy in a solar eruption.

Figure 2.

But that picture does not explain what initiates the reconnection process. The Sun’s corona remains quiescent as long as two magnetic forces in the plasma are balanced. The magnetic pressure-gradient force generally points outward, while the magnetic tension force generally points inward and confines an eruption. A fundamental question that therefore remains—and also is a central point of controversy—is how the preeruption force balance is abruptly destroyed.

A theory proposed in the 1980s suggested that a single bipolar loop of magnetic flux, pinned at either end to sunspots on the solar surface, could disrupt the stability of the corona and lead to an eruption. Portions of the loop would squeeze together slowly and rearrange quickly in a way that initiated magnetic reconnection. ² Increased outward magnetic pressure would then cause the entire configuration to expand and disrupt the equilibrium. The field lines would twist around each other as the eruption progressed.

Simulations in two dimensions have provided support for that simple configuration’s ability to generate enough energy to disrupt the corona’s stability. ³ Because it’s so difficult to observe the details of the Sun’s corona, 3D simulations are vital to providing evidence in support of theoretical models. However, scientists have struggled to get such simulations to reproduce the phenomenon. The simple field-line configuration model had “never been proven to initiate, alone, an eruption in any 3D simulation,” according to Guillaume Aulanier at the Plasma Physics Laboratory in Paris. ⁴

Instead, many of the current 3D simulations favored by the solar-physics community today rely on the preexistence of a twisted flux rope or other multipolar field-line topologies that are more complex than simple bipolar loops. Tightly twisted lines of a flux rope create an ideal instability in the plasma and disrupt the magnetic force balance. That disruption then drives rapid magnetic reconnection.

Twisted plasma loops are indeed visible in observations of coronal mass ejections—but only after the eruption is underway. Those observations have fueled debate over whether complex flux topologies exist before the eruption or are formed during it. ⁵ Solving that puzzle could provide clues to the mechanism that initiates the eruption.

Shearing and twisting

Revisiting the simple field-line configuration proposed decades ago, Jiang and his colleagues now show in 3D that an eruption may proceed without any complex preeruption field-line structure. Using China’s National Supercomputing Center in Tianjin, the researchers designed a magnetohydrodynamic simulation driven by slow rotational flow of the plasma at the Sun’s surface.

They found that horseshoe-like loops of flux ballooned outward while remaining connected to the solar surface. The horseshoe’s pinned sides sheared against one another, forming a sheet of electrical current. As the current sheet became thinner, the field lines pushed toward each other and then abruptly rearranged themselves. That reconnection converted magnetic energy to kinetic energy and drove the eruption, as illustrated in figure 3. A rope of magnetic flux—the multicolored circular structure that becomes increasingly twisted over time—appeared only after the eruption had initiated and thus dispelled the notion that it was a necessary preexisting condition.

Figure 3.

Closer analysis of the forces experienced by each field line in the erupting flux revealed the real culprit: Strong curvature in the newly reconnected field lines created a slingshot effect that resulted in extremely strong magnetic tension forces directed away from the Sun. Those forces provided an efficient means of driving particles and radiation into space and showed that the eruptions were driven from below by the reconnection jet (see figure 2).

Key to the simulation’s success was its high numerical accuracy and resolution. Jiang and colleagues accomplished that goal by building height-dependent gravitational acceleration and grid-dependent numerical resistivity into the code. The result was a highly conducting plasma on par with the ones used in laboratory experiments.

The researchers also imposed boundary conditions to ensure that an untwisted loop remained pinned at the solar surface long enough for a current sheet to form. But the simulation needed high accuracy to maintain the field-line-pinning condition so it could create a current sheet in the plasma’s evolution before the eruption. Earlier 3D simulations failed to reach the requisite accuracy. If the field lines’ connection points slipped at all, the simulation resulted in energy loss that prohibited the current sheet from forming. “It is only by fulfilling such a stringent requirement that the fast, turbulent reconnection can arise and impulsively convert the magnetic energy into an explosion,” says Jiang.

Regardless of the precise magnetic configuration, the researchers argue, the key mechanism remains the same. A thin current sheet forms slowly in the central part of the field and then rapid magnetic reconnection triggers and drives the explosion via a jet from below.

“The model that this work supports has been around since the 1980s,” says Peter Wyper at Durham University. “But only as a conjectured cartoon. No simulations until now have got it to work, so other mechanisms have become favored instead. This work suggests that it may play a more crucial role after all.”

Already, observations provide support for Jiang’s simulations. The morphology of the simulations is consistent with eruptions captured by NASA’s Solar Dynamics Observatory. Additionally, many observations of the preeruption corona show thin, hot structures that could be a proxy for the slowly formed current sheet before eruption. The finding could lead to a universal model of solar eruptions and provide better understanding of how the phenomenon influences space weather.