NASA’s Solar Dynamics Observatory has observed a magnetic explosion the
likes of which have never been seen before. In the scorching upper reaches of
the Sun’s atmosphere, a prominence — a large loop of material launched by an
eruption on the solar surface — started falling back to the surface of the Sun.
But before it could make it, the prominence ran into a snarl of magnetic field
lines, sparking a magnetic explosion.
Scientists have
previously seen the explosive snap and realignment of tangled magnetic field
lines on the Sun — a process known as magnetic reconnection — but never one
that had been triggered by a nearby eruption. The observation, which confirms a
decade-old theory, may help scientists understand a key mystery about the Sun’s
atmosphere, better predict space weather, and may also lead to breakthroughs in
the controlled fusion and lab plasma experiments.
“This was the
first observation of an external driver of magnetic reconnection,” said
Abhishek Srivastava, solar scientist at Indian Institute of Technology (BHU),
in Varanasi, India. “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.”
Previously a type of magnetic reconnection known as spontaneous
reconnection has been seen, both on the Sun and around Earth. But this new
explosion-driven type — called forced reconnection — had never been seen
directly, thought it was first theorized 15 years ago. The new observations
have been published in the Astrophysical Journal.
The
previously-observed spontaneous reconnection requires a region with just the right
conditions — such as having a thin sheet of ionized gas, or plasma, that only
weakly conducts electric current — in order to occur. The new type, forced
reconnection, can happen in a wider range of places, such as in plasma that has
even lower resistance to conducting an electric current. However, it can only
occur if there is some type of eruption to trigger it. The eruption squeezes
the plasma and magnetic fields, causing them to reconnect.
While the Sun’s jumble of magnetic field lines are invisible, they
nonetheless affect the material around them — a soup of ultra-hot charged
particles known as plasma. The scientists were able to study this plasma using
observations from NASA’s Solar Dynamics Observatory, or
SDO, looking specifically at a wavelength of light showing particles heated 1-2
million kelvins (1.8-3.6 million F).
The observations
allowed them to directly see the forced reconnection event for the first time
in the solar corona — the Sun’s uppermost atmospheric layer. In a series of
images taken over an hour, a prominence in the corona could be seen falling
back into the photosphere. En route, the prominence ran into a snarl of
magnetic field lines, causing them to reconnect in a distinct X shape.
Spontaneous
reconnection offers one explanation for how hot the solar atmosphere is —
mysteriously, the corona is millions of degrees hotter than lower atmospheric
layers, a conundrum that has led solar scientists for decades to search for
what mechanism is driving that heat. The scientists looked at multiple
ultraviolet wavelengths to calculate the temperature of the plasma during and
following the reconnection event. The data showed that the prominence, which
was fairly cool relative to the blistering corona, gained heat after the event.
This suggests forced reconnection might be one way the corona is heated
locally. Spontaneous reconnection also can heat plasma, but forced reconnection
seems to be a much more effective heater — raising the temperature of the
plasma quicker, higher, and in a more controlled manner.
While a
prominence was the driver behind this reconnection event, other solar eruptions
like flares and coronal mass ejections, could also cause forced reconnection. Since
these eruptions drive space weather — the bursts of solar radiation that can
damage satellites around Earth — understanding forced reconnection can help
modelers better predict when disruptive high-energy charged particles might
come speeding at Earth.
Understanding
how magnetic reconnection can be forced in a controlled way may also help
plasma physicists reproduce reconnection in the lab. This is ultimately useful
in the field of laboratory plasma to control and stabilize them.
The scientists
are continuing to look for more forced reconnection events. With more
observations they can begin to understand the mechanics behind the reconnection
and often it might happen.
Image & info via NASA SDO: https://www.nasa.gov/feature/goddard/2019/nasa-s-sdo-sees-new-kind-of-magnetic-explosion-on-sun
Credits: NASA’s
Goddard Space Flight Center
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