NASA’s IRIS Reveals the Heating Mechanism of Solar Corona

Scientists have recently described the first ever distinct images of nanojets (bright thin lights that move in a perpendicular direction to the magnetic structures in the Sun’s atmosphere, known as the corona) in a process that exposes the presence of one of the promising coronal heating candidates: nanoflares.

In pursuit of understanding why the Sun’s atmosphere is so much hotter than the surface, and to help differentiate between a host of theories about what causes this heating, researchers turn to NASA’s Interface Region Imaging Spectrograph (IRIS) mission. Video Credit: NASA’s Goddard Space Flight Center/Scientific Visualization Studio.

Details of the team’s observations have been reported in a paper published in the Nature Astronomy journal.

In a quest to understand why the atmosphere of the Sun is relatively hotter than the surface, and to help distinguish between scores of theories about what causes this heating, scientists turned to the Interface Region Imaging Spectrograph (IRIS) mission of NASA. With the help of a high-resolution imager, IRIS was finely modified to zoom in on particular hard-to-see events on the Sun.

Nanojets and Nanoflares

Small explosions on the Sun, called nanoflares, are hard to identify. These nanoflares are tiny and extremely fast, which means they are difficult to spot against the bright surface of the Sun.

On April 3^rd, 2014, during the so-called coronal rain event when streams of cooled plasma discharge from the corona to the surface of the Sun looking virtually like a massive waterfall, scientists observed bright jets appearing close to the end of the event.

Such telltale flashes are referred to as nanojets—that is, heated plasma moving so fast that they materialize on images as bright thin lines observed within the magnetic loops on the Sun.

Nanojets are said to be a “smoking gun,” a crucial proof of the occurrence of nanoflares. Every nanojet is supposed to be started by a process called magnetic reconnection in which twisted magnetic fields explosively realign. One reconnection can trigger another reconnection, producing an avalanche of nanojets in the Sun’s corona—a process that could generate the energy that is heating the corona.

The Solar Dynamic Observatory

In the above visualization, the Solar Dynamic Observatory offers a complete view of the Sun prior to zooming into IRIS’s close-up view of the nanojets, which temporarily illuminate in the magnetic loops.

IRIS collects its high resolution images by concentrating in on a small part of the Sun at a time. Therefore, visualizing particular events is a combination of educated guesswork and looking at the correct place at the correct time.

As soon as the nanojets were detected against the setting of the coronal rain, scientists coordinated with NASA’s Solar Dynamics Observatory (SDO) and the Hinode observatory, a collaboration among the European Space Agency (ESA), Japan Aerospace Exploration Agency, and NASA to obtain a full view of the Sun, and validate whether they were identifying nanojets, and evaluate their influences on the corona.

The team integrated the various observations with sophisticated simulations to reproduce the events they witnessed on the Sun. The models demonstrated that the nanojets were indeed a telltale signature of nanoflares and magnetic reconnection, adding to coronal heating in the simulations.

Additional studies are required to determine the frequency of nanoflares and nanojets all over the Sun, and the amount of energy they contribute to heating the Sun’s corona. In the near future, missions such as Parker Solar Probe and Solar Orbiter can provide more information about the processes that heat the solar corona.

Journal Reference

Antolin, P., et al. (2020) Reconnection nanojets in the solar corona. Nature Astronomy. doi.org/10.1038/s41550-020-1199-8.