IRIS Captures First-Ever Clear Images of Nanojets in Solar Corona

IRIS Captures First-Ever Clear Images of Nanojets in Solar Corona
The solar corona, the outermost part of the Sun’s atmosphere, is shaped and mysteriously heated to millions of degrees by the magnetic field of our star. It has long been hypothesized that the heating results from a myriad of tiny magnetic energy outbursts called nanoflares, driven by the fundamental process of magnetic reconnection. Solar astronomers using NASA’s Interface Region Imaging Spectrograph (IRIS) mission have now discovered very fast and bursty nanojets, the telltale signature of reconnection-based nanoflares resulting in coronal heating.

Antolin et al spotted nanojets in the solar corona. Image credit: NASA’s Goddard Space Flight Center / Scientific Visualization Studio.

Antolin et al spotted nanojets in the solar corona. Image credit: NASA’s Goddard Space Flight Center / Scientific Visualization Studio.

Nanoflares are small explosions on the Sun, approximately nine orders of magnitude lower than solar flares, but they are difficult to spot.

They are very fast and tiny, meaning they are hard to pick out against the bright surface of the Sun.

On April 3, 2014, during what’s known as a coronal rain event when streams of cooled plasma fall from the corona to the Sun’s surface looking almost like an enormous waterfall, solar astronomers noticed bright jets appearing near the end of the event.

These telltale flashes are nanojets — heated plasma traveling so fast that they appear on images as bright thin lines seen within the magnetic loops on the Sun.

Nanojets are considered key evidence of the presence of nanoflares.

Each nanojet is believed to be initiated by a process known as magnetic reconnection where twisted magnetic fields explosively realign.

One reconnection can set off another reconnection, creating an avalanche of nanojets in the solar corona, a process that could create the energy that is heating the corona.

“Misaligned magnetic field lines can break and reconnect, producing nanoflares in avalanche-like processes,” said lead author Dr. Patrick Antolin, a researcher at Northumbria University.

“However, no direct and unique observations of such nanoflares existed to date, and the lack of a smoking gun had cast doubt on the possibility of solving the coronal heating problem.”

IRIS gathers its high resolution images by focusing in on a small portion of the Sun at a time.

So observing specific events is a combination of educated guesswork and looking at the right place at the right time.

Once the nanojets were identified against the backdrop of the coronal rain, Dr. Antolin and colleagues coordinated with NASA’s Solar Dynamics Observatory and the NASA/JAXA Hinode observatory to get a complete view of the Sun, and confirm whether they were detecting nanojets, and assess their effects on the corona.

The researchers combined the many observations with advanced simulations to recreate the events they saw on the Sun.

The models showed that the nanojets were a telltale signature of magnetic reconnection and nanoflares, contributing to coronal heating in the simulations.

“From coordinated multi-band high-resolution observations, we discovered evidence of very fast and explosive nanojets, the tell-tale signature of reconnection-based nanoflares resulting in coronal heating,” Dr. Antolin said.

“Using state-of-the-art numerical simulations, we have demonstrated that the nanojet is a consequence of the slingshot effect from the magnetically tensed, curved magnetic field lines reconnecting at small angles.”

“Nanojets are therefore the key signature to look for reconnection-based coronal heating in action.”

“More studies will need to be done to establish the frequency of nanojets and nanoflares all over the Sun, and how much energy they contribute to heating the solar corona,” the scientists said.

“Going forward, missions like the NASA/ESA Solar Orbiter and NASA’s Parker Solar Probe can give more detail into the processes that heat the solar corona.”

A paper on the findings was published this week in the journal Nature Astronomy.

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P. Antolin et al. Reconnection nanojets in the solar corona. Nat Astron, published online September 21, 2020; doi: 10.1038/s41550-020-1199-8

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