Causes of anomalous heating in the Sun's upper atmosphere revealed

The results of new research point to the cause of heating in the Sun's outer layers. Using observations with large telescopes and satellites, an attempt has been made to answer this question.

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The causes of the anomalous heating of the upper solar atmosphere have recently been revealed.

Researchers at the Yunnan Observatories of the Chinese Academy of Sciences ( CAS ) published a recent study of the Sun's upper atmosphere in The Astrophysical Journal. As a result, a complete physical picture of the anomalous heating occurring in the chromosphere and solar corona.

Numerous investigations carried out in the field of Solar Physics (a branch of astrophysics dedicated to the study of the Sun) have pointed to magnetic activity as the cause of this heating. However, both observations with telescopes and satellites, as well as theoretical research, have been inconclusive in this regard so far.

How did they arrive at the result?

Chinese astrophysicists analyzed long-term observations of the chromosphere visible in the Ca II K line, that is, of the light emitted by ionized calcium (Ca II) in the violet part of the solar spectrum (for wavelengths of 393.4 nm) of this layer that follows the photosphere.

The green coronal spectral lines, emitted by highly ionized metals present in the solar corona, were also monitored.

For the first time, the heating of the solar atmosphere is approached as a consequence of the magnetic configuration of the Sun's outer layers, and not the magnetic activity that occurs in them.

The heating “mechanism” in a nutshell

The chromosphere is a thin layer of the solar atmosphere 10 thousand km thick, composed of hydrogen and helium, as well as metals in neutral and ionized states.

The main source of heat in the calm chromosphere are networks of magnetic fields, whose configuration prevents particles charged with thermal energy from being able to “escape” (be ejected). On the other hand, in the phase of greatest magnetic activity (active phase), the chromosphere owes its temperature to two types of magnetic regions: active and ephemeral (the first, with a lot of activity, and the second, of short duration).

A similar mechanism has been described for the behaviour of the solar corona, but in which ephemeral magnetic fields would provide more heat during the active stage. Therefore, the active corona receives more heat than during the calm corona.

As a result of this investigation, it was confirmed that the magnetic fields of the active and ephemeral regions present a butterfly-shaped spatial distribution, both in the chromosphere and in the corona in their active phases.

For the Sun's two outermost layers, phases of active heating are synchronized with solar cycles, in which the magnetic field increases to a maximum and then reaches a minimum, over 11 years.

News reference:
Li, KJ et al. How Are the Abnormally Hot Chromosphere and Corona Heated by the Solar Magnetic Fields? The Astrophysical Journal, vol. 962, no. 2, 2024.