ESA and NASA join forces to answer the Sun's heating riddle

A 65-year old perplexing question may have finally been answered: Why is the Sun's atmosphere hotter than its surface?
Amal Jos Chacko
An artistic impression of Solar Orbiter and Parker Solar Probe.
An artistic impression of Solar Orbiter and Parker Solar Probe.

Solar Orbiter: ESA/ATG medialab; Parker Solar Probe: NASA/Johns Hopkins APL 

For over six decades, scientists have been baffled by a cosmic mystery of scorching proportions: Why is the Sun's atmosphere, known as the corona, hotter than its surface?

This enigma contradicting conventional wisdom that things cool down the farther they are from a heat source has puzzled solar physicists until now, revealed the European Space Agency.

Corona, the sun’s atmosphere, consists of plasma, an electrically charged gas, and sizzles at over 1.8 million degrees Fahrenheit (a million degrees Celsius). However, the sun’s surface temperature is only at around 10864 degrees Fahrenheit (6000 degrees Celsius).

How is this possible?

Unlocking the secret

Addressing this longstanding question, scientists have long suspected that turbulence in the solar atmosphere might be responsible for heating the plasma in the corona.

However, investigating this theory posed a significant challenge. To unravel the mysteries of the Sun, scientists needed both remote sensing and in-situ measurements, but gathering all the necessary data with a single spacecraft was impossible.

Remote sensing happens when a spacecraft parks itself at a distance and uses cameras to analyze the Sun and its atmosphere, showing large-scale results; while the spacecraft flies through the region being investigated and takes various measurements that provide great specific detail.

Enter the European Space Agency (ESA) -led Solar Orbiter spacecraft and NASA's Parker Solar Probe.

The Solar Orbiter was designed to perform both remote sensing and in-situ measurements while maintaining a safe distance from the Sun. Parker Solar Probe, on the other hand, focused primarily on in-situ measurements and dared to get even closer to the Sun.

The missing piece of the puzzle was aligning Parker Solar Probe with Solar Orbiter to capture large-scale consequences of in-situ measurements.

Daniele Telloni, a researcher at the Italian National Institute for Astrophysics (INAF), found that on 1 June 2022, the two spacecraft would align. He realized the alignment was just short of a 45-degree roll and some steering away from bringing the Parker Solar Probe into view.

Although it was first unclear if the spacecraft operations team would authorize this maneuver, with spacecraft designed to point only in specific directions in the excruciating heat of the sun, the approval soon came through once the nature of the potential scientific payoff became apparent.

The revelation and implications

By capturing simultaneous measurements of the solar corona's large-scale configuration and the plasma's microphysical properties, the team made the first combined observational and in-situ estimate of the coronal heating rate.

The results strongly support the hypothesis that turbulence plays a pivotal role in transferring energy, akin to the stirring of morning coffee.

Turbulence stimulates random movements in magnetized fluid, transferring energy to smaller scales, some of which transform into heat.

The presence of magnetized fluid in the solar corona allows stored magnetic energy to contribute to this process, culminating in the heating of individual particles, primarily protons.

While further research is required to completely unravel the solar heating mystery, Daniele's work marks a significant step forward. Solar physicists now have their first measurement of the process responsible for heating the Sun's atmosphere, shedding light on a question that perplexed scientists for decades.