What makes the Sun’s atmosphere hotter than its surface? We may now know

It has to do with high-frequency magnetic waves.
Loukia Papadopoulos
Representational image of the Sun's atmosphere.jpg
Representational image of the Sun's atmosphere.


A joint scientific team led by the Royal Observatory of Belgium (ROB) and the KU Leuven may have cracked the mystery of how the Sun’s outer atmosphere stays so hot despite the fact that it is far away from the celestial object’s core and primary heat source.

This is according to a report by the institutions published this week.

A long-standing mystery

The researchers have found that high-frequency magnetic waves could play an essential role in keeping the Sun’s atmosphere at millions of degrees. They may finally be able to answer a long-standing mystery: What makes the Sun’s atmosphere hotter than its surface?

Under normal circumstances, temperature decreases as you move away from a heat source. However, this is not true for the Sun. The Sun's only heat source is safely located in its core. Yet the corona, the outermost layer of the solar atmosphere, is about 200 times hotter than the photosphere, the Sun’s surface.

“Over the past 80 years, astrophysicists have tried to solve this problem and now more and more evidence is emerging that the corona can be heated by magnetic waves,” said Professor Tom Van Doorsselaere at KU Leuven.

A powerful observation tool

These new insights come from the observations of a powerful tool: the Extreme Ultraviolet Imager (EUI) telescope onboard Solar Orbiter, a spacecraft of the European Space Agency that is currently investigating the Sun from behind. The EUI telescope, operated by ROB, generates images of the solar corona with unprecedented resolution, revealing fast oscillations in the smallest magnetic structures of the solar corona.

It is currently speculated that the energy of these high-frequency waves contributes to the heating of the solar atmosphere.

The main question the scientists were exploring was whether the energy originating from these new, fast oscillations outweighed the energy coming from similar but slower oscillations that were already known. After performing a meta-analysis (a statistical method of using multiple scientific studies to derive common unknown facts), Dr. Daye Lim, lead author of the new study, put forth a theory that high-frequency waves give a more significant contribution to the total heating generated by waves than low-frequency waves.

“Since her results indicated a key role for fast oscillations in coronal heating, we will devote much of our attention to the challenge of discovering higher-frequency magnetic waves with EUI,” said Dr. David Berghmans, the principal investigator of EUI.

The study is published in Astrophysical Journal Letters.

Study abstract:

Transverse oscillations that do not show significant damping in solar coronal loops are found to be ubiquitous. Recently, the discovery of high-frequency transverse oscillations in small-scale loops has been accelerated by the Extreme Ultraviolet Imager on board Solar Orbiter. We perform a meta-analysis by considering the oscillation parameters reported in the literature. Motivated by the power law of the velocity power spectrum of propagating transverse waves detected with CoMP, we consider the distribution of energy fluxes as a function of oscillation frequencies and the distribution of the number of oscillations as a function of energy fluxes and energies. These distributions are described as a power law. We propose that the power-law slope (δ = −1.40) of energy fluxes depending on frequencies could be used for determining whether high-frequency oscillations dominate the total heating (δ < 1) or not (δ > 1). In addition, we found that the oscillation number distribution depending on energy fluxes has a power-law slope of α = 1.00, being less than 2, which means that oscillations with high energy fluxes provide the dominant contribution to the total heating. It is shown that, on average, higher energy fluxes are generated from higher-frequency oscillations. The total energy generated by transverse oscillations ranges from about 1020 to 1025 erg, corresponding to the energies for nanoflare (1024–1027 erg), picoflare (1021–1024 erg), and femtoflare (1018–1021 erg). The respective slope results imply that high-frequency oscillations could provide the dominant contribution to total coronal heating generated by decayless transverse oscillations.

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