NASA's legendary Solar Dynamics Observatory (SDO) has observed a new type of explosion on the Sun. Called "forced reconnection," this newly-discovered phenomenon was theorized 15 years ago. The discovery is expected to help scientists understand a major mystery about the Sun's atmosphere, improve space weather predictions, and lead to breakthroughs in the controlled fusion and lab plasma experiments.
A type of magnetic reconnection called a "spontaneous reconnection" has been previously witnessed both on the Sun and around Earth, as well as a realignment of tangled magnetic field lines on the Sun in a process called "magnetic reconnection".
This newly-seen magnetic explosion was sparked by a nearby eruption– in the scorching upper reaches of the Sun's atmosphere, a big loop of material spewed by a solar surface eruption called a "prominence" started falling back to the surface. Before it returned, it was caught in a snarl of magnetic field lines, triggering a magnetic solar explosion.
"This was the first observation of an external driver of magnetic reconnection," said Dr. Abhishek Srivastava, a solar scientist at the Indian Institute of Technology. "This could be very useful for understanding other systems."
"For example, Earth’s and planetary magnetospheres, other magnetized plasma sources, including experiments at laboratory scales where plasma is highly diffusive and very hard to control," he explained.
The previously-observed spontaneous reconnection needs a region with just the right conditions in order for it to occur, including having a thin sheet of plasma that only weakly conducts electric current.
On the other hand, forced reconnection can occur in a wider range of places, including plasma with even lower resistance to conducting an electric current, but this can only happen if there is some type of eruption to trigger it.
Such an eruption squeezes the plasma and magnetic fields that reconnect them.
The Sun's magnetic field lines are invisible but they still have an impact on the material around them– a soup of very hot charged particles called plasma.
Researchers were able to examine this plasma using observations from SDO, looking particularly at a wavelength of light showing particles heated up to 2 million °C (3.6 million °F)
This enabled NASA to directly observe the forced reconnection event for the first time in the Sun's uppermost atmospheric layer called the solar corona.
In a sequence of images taken more than an hour, prominence in the corona could be seen falling back into the photosphere. It then ran into a snarl of magnetic field lines that caused them to reconnect in a distinct X-shape.
Scientists are continuing to look for more of such events.
With further observations, they can begin to study the mechanics behind the reconnection.
"Our thought is that forced reconnection is everywhere, but we have to continue to observe it, to quantify it if we want to prove that."
"On the Observations of Rapid Forced Reconnection in the Solar Corona" – Srivastava, A. K. et al. – Astrophysical Journal – DOI: 10.3847/1538-4357/ab4a0c
Using multiwavelength imaging observations from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory on 2012 May 3, we present a novel physical scenario for the formation of a temporary X-point in the solar corona, where plasma dynamics are forced externally by a moving prominence. Natural diffusion was not predominant; however, a prominence driven inflow occurred first, forming a thin current sheet, thereafter enabling a forced magnetic reconnection at a considerably high rate. Observations in relation to the numerical model reveal that forced reconnection may rapidly and efficiently occur at higher rates in the solar corona. This physical process may also heat the corona locally even without establishing a significant and self-consistent diffusion region. Using a parametric numerical study, we demonstrate that the implementation of the external driver increases the rate of the reconnection even when the resistivity required for creating normal diffusion region decreases at the X-point. We conjecture that the appropriate external forcing can bring the oppositely directed field lines into the temporarily created diffusion region first via the plasma inflows as seen in the observations. The reconnection and related plasma outflows may occur thereafter at considerably larger rates.
Featured image credit: NASA Goddard
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