Researchers from Kyushu University have recreated magnetic reconnection in a controlled laboratory environment using high-power lasers, allowing them to examine the process under significantly different plasma conditions.
The team, working with researchers from Osaka University, used the Gekko-XII laser to generate interacting plasma clouds and investigate how their magnetic fields rearranged and released stored energy.
Associate Professor Taichi Morita of Kyushu University’s Faculty of Engineering Sciences led the study, which found that fast magnetic reconnection proceeded at remarkably similar rates despite substantial differences in the density and magnetic field conditions entering the reconnection region.
The results suggest that the rate of fast magnetic reconnection is determined primarily by the local physics of the reconnection layer rather than the properties of the surrounding plasma.
Published in Physical Review E, the work provides experimental measurements to assess existing theories and numerical simulations of magnetic reconnection.
Recreating magnetic reconnection in the laboratory
Magnetic reconnection occurs when magnetic fields within a conductive plasma rapidly change their configuration.
When magnetic field lines pointing in opposite directions meet, they can reconnect, converting stored magnetic energy into plasma heating and high-speed outflows.
The process is widely considered central to space weather phenomena, including solar flares and substorms in Earth’s magnetosphere.
However, researchers have continued to investigate why magnetic reconnection can occur at similar rates across very different plasma environments.
One unresolved question has been how strongly reconnection depends on the conditions of plasma flowing into the reconnection region. The new experiment was designed to examine this relationship directly.
The researchers fired high-power laser beams at two separate locations on a carbon target using the Gekko-XII laser. Each laser pulse produced an expanding plasma cloud.
As the two clouds moved towards one another and collided, their magnetic fields interacted and underwent reconnection.
Changing the conditions around the reconnection layer
A key part of the experiment was varying the distance between the two laser spots.
This allowed the researchers to substantially alter the density and magnetic-field conditions of the plasma flowing toward the reconnection region.
The team then used a two-directional laser Thomson-scattering system to examine the plasma. By analysing laser light scattered by the plasma, the researchers could measure properties including its temperature, density and flow.
This provided a way to quantitatively examine both the rate of magnetic reconnection and how magnetic energy was converted during the process.
Similar reconnection rates despite different conditions
The experiments revealed that the magnetic fields experienced substantially different expansion and transport histories, while the timing of reconnection also varied between experimental conditions.
Despite these differences, the measured reconnection rates became remarkably similar once a current sheet had developed.
This result indicates that the behaviour of fast magnetic reconnection is closely linked to the local conditions within the reconnection layer.
The properties of the plasma further upstream appear to have much less influence on the resulting reconnection rate than might otherwise be expected.
The researchers also measured how the released magnetic energy was distributed between plasma heating and acceleration into high-speed flows.
These measurements provide a quantitative way of assessing the energy conversion associated with the process.
Experimental benchmarks for magnetic reconnection research
The measurements establish experimental benchmarks for testing theoretical descriptions and computer simulations of magnetic reconnection.
They also provide evidence for the robustness of fast reconnection under substantially different upstream conditions.
The research team plans to build on the experiments by investigating conditions that more closely resemble space plasmas. Future work will include experiments involving oblique magnetic fields and asymmetric plasma flows.
A better understanding of magnetic reconnection could ultimately contribute to research into space weather, with the process associated with events that can affect satellites, communications systems, navigation technologies, and power infrastructure.
