ESA’s Solar Orbiter may have taken another step forward in solving the eighty-year-old mystery of why the Sun’s outer atmosphere is so hot. On March 3, 2022, just months before the end of Solar Orbiter’s nominal mission, the spacecraft’s Extreme Ultraviolet (EUI) sensor provided data for the first time showing that a magnetic phenomenon called reconnection is constantly occurring at small scales.

At that time, the spacecraft was approximately midway between Earth and the Sun. This made it possible to coordinate observations with NASA’s Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS) missions. Data from the three tasks were then combined in analysis.

Magnetic reconnection happens when the magnetic field changes itself to a more stable configuration. This is the primary mechanism for releasing energy in superheated gases known as plasma, and is thought to be the primary mechanism for powering large-scale solar flares. This makes it a direct cause of space weather and a prime candidate for the mysterious warming of the Sun’s outer atmosphere.

It has been known since the 1940s that the Sun’s outer atmosphere, called the corona, is much hotter than the Sun’s surface. The surface glows at about 5,500 °C, while the corona is a diluted gas of about 2 million °C. How the Sun injects energy into its atmosphere to heat it to such high temperatures has remained a great mystery ever since.

In the past, magnetic reconnection was commonly observed during large-scale explosion events. However, the new result represents an ultra-high resolution observation of persistent small-scale (about 390 km in diameter) reconnection in the corona. It turned out to be a long-term “soft” sequence compared to the sudden bursts of energy that reconnection is usually associated with.

The incident on 03/03/2022 took place within an hour. Temperatures around the magnetic field point, known as the zero point, where the magnetic field intensity drops to zero, were held at about 10 million°C, causing the material to flow out in separate “droplets”. It moves from the zero point at a speed of about 80 km/h. In addition to this continuous output, an explosion event also occurred around this zero point and lasted for four minutes.

The Solar Orbiter results show that magnetic reconnection is constantly occurring at scales too small to be detected before, in both soft and explosive ways. This is important because it means that reconnection can constantly transfer mass and energy to the overlying corona, helping to heat it.

These observations also suggest that smaller and more frequent magnetic reconnections await discovery. The goal now is to observe them in the future at even higher spatio-temporal resolution with EUI around Solar Orbiter’s closest approaches to estimate how much of the corona’s heat can be transferred in this way. Solar Orbiter’s last close pass to the Sun occurred on April 10, 2023. At that time, the spacecraft was only 29 percent of the Earth’s distance from the Sun.