The solar corona is heated by braided magnetic loops, microflares and jets according to the new MHD simulations led by G. Cozzo (CfA; INAF-OAPA))

The solar corona is a structure that extends for several solar radii and is filled with low-density plasma at temperatures of millions of degrees. What heats coronal plasma to such extreme temperatures? It appears to be a combination of complex magnetic phenomena.

X-ray and UV observations of the Sun reveal an atmosphere rich in bright structures that host million-degree plasma, known as active regions. These regions often show long luminous arcs, extending for many Earth radii, in which the plasma is confined by the magnetic field and heated to very high temperatures. These structures, called coronal loops, were already interpreted in terms of plasma–magnetic field interaction in 1973 by Prof. Giuseppe Vaiana, a Sicilian-born astrophysicist and one of the pioneers of X-ray solar astrophysics. Only now, more than 50 years later, are we beginning to understand in detail the complex mechanisms that allow the coronal magnetic field to heat the plasma to millions of degrees.

Surprisingly, the key to heating such vast structures—and therefore the entire solar corona—lies in highly localized and very rapid phenomena known as nanoflares. If we think of the magnetic field as a bundle of lines that, with some differences, behave like elastic strings, nanoflares arise from magnetic reconnection: this occurs when magnetic field lines come together and rearrange, releasing energy. This impulsive energy release leaves a recently observed signature: micro-jets of plasma launched at hundreds of km/s in directions perpendicular to the magnetic field lines. These were identified for the first time about ten years ago in observations from NASA’s Solar Dynamics Observatory.

Magnetohydrodynamic simulations led by astrophysicist G. Cozzo (Harvard–Smithsonian Center for Astrophysics and INAF – Palermo Astronomical Observatory), presented in two papers, have made it possible to investigate the role of nanoflares in coronal heating. A first set of simulations focuses on the magnetic flux tubes that define coronal loops and on the fact that they are anchored in the solar photosphere, where plasma motions twist the magnetic field lines. This makes the magnetic tubes unstable, causing them to fragment and favoring reconnection events that generate heating pulses. These pulses produce intense electric currents that, as they dissipate, heat the plasma to temperatures up to 10 million degrees. The process continues as long as photospheric plasma motions keep twisting the magnetic field lines, triggering a cascade of events capable of heating the entire magnetic structure.

In a second set of simulations, the team reproduced the formation of the jets generated by reconnection events: these last only about ten seconds and release an energy of about 10²⁴ erg, confirming their role in heating coronal plasma.

The simulations are described in the papers “3D MHD simulations of coronal loops heated via magnetic braiding I. Continuous driving” and “3D MHD simulations of coronal loops heated via magnetic braiding II. Automatic detection of reconnection outflows and statistical analysis of their properties”, published in XXX.

The author of the study, G. Cozzo, comments: “The concept of nano-jets can be made easier to understand through an analogy between the acceleration of nano-jets following magnetic reconnection and the release of an arrow from a bowstring that has been drawn tight. The process of drawing the bow is equivalent to the tangling of magnetic field lines, during which they assume increasingly complex configurations, accumulating energy and magnetic tension. At the moment of reconnection, the bowstring is ‘released’ and quickly returns to a simpler configuration. During this relaxation process, the plasma around the field line is accelerated, just like the arrow pushed by the string, in a direction transverse to the magnetic field”.

These phenomena, however, are difficult to observe and understand—another reason to rely on simulations. Cozzo continues: “The fundamental purpose of these simulations is to reproduce both the mechanism that loads energy into magnetic loops—achieved through twisting motions of the field lines at their footpoints—and the subsequent release of magnetic energy into heat and kinetic energy. This is done by incorporating into the equations solved by the computer the so-called magnetic resistivity of the plasma, similar to the resistive behavior of the filaments in old light bulbs”.

The cover image shows a coronal loop observed in ultraviolet by NASA’s SDO/AIA. The image of planet Earth gives a sense of the scale of the structure.