Authors: Sanjiv K. Tiwari, Julia K. Thalmann , Navdeep K. Panesar, Ronald L. Moore, Amy R. Winebarger
First Author’s Institution: Lockheed Martin Solar and Astrophysics Laboratory (Palo Alto, CA) and the Bay Area Environmental Research Institute (Petaluma, CA)
Status: Published in the Astrophysical Journal Letters [open access on arXiv]
The coronal heating problem is one of the biggest unsolved mysteries in solar physics. The solar corona is the region of the Sun’s atmosphere that extends past the surface – or photosphere – of the Sun. It is a diffuse cloud of plasma that is heated to temperatures several hundred times that of the photosphere, which isn’t what you’d expect as you move further away from a hot object. The coronal heating problem has been hotly debated since the 1940s and is thought to be related to the Sun’s magnetic field. However, no theory has yet been able to explain why the corona is so much hotter than the photosphere, and the possibility remains that multiple processes may be at work.
To study coronal heating, solar physicists use various structures that operate on smaller scales. The authors of today’s paper focus on the heating processes of coronal loops, which occur when plasma in the corona flows along the solar magnetic field (Figure 2). Coronal loops are rooted in strong concentrations of magnetic field such as sunspots – dark patches in the solar photosphere. Sunspots are composed of two regions, a dark umbra and a surrounding penumbra (Figure 3), that are surrounded by a bright region called a plage. Understanding how coronal loops are linked to their sunspot footprints and the surrounding magnetic field is critical to determining the heating mechanisms at work.
Using data from the Solar Dynamics Observatory (SDO), the authors selected two regions with both sunspots and coronal loops, known as active regions. Although we can see coronal plasma trace magnetic field lines in coronal loops, there is still no way to directly measure the coronal magnetic field. The authors used simulations to reconstruct and determine the strength of the coronal magnetic field based on the magnetic field at the base of the loops (which can be directly measured using instruments like SDO). One of the example active regions, along with its simulated coronal magnetic field, is shown in Figure 3.
From these observations, the authors found that the loops present in their sample active regions are rooted in both sunspots and plage, and that the brightest loops have one foot at the edge of a sunspot umbra and another foot in a plage or a sunspot penumbra. Although they find no visible loops with both footprints in sunspot umbrae, the simulated coronal magnetic field shows field lines that connect sunspot umbrae. This indicates that the coronal loops that connect sunspot umbrae are too cool to be visible in extreme ultraviolet wavelengths (the type of light observed by SDO). The relationship between loop brightness and footprint location is shown in Figure 4.
A key property of sunspots is that they suppress convection (or the ‘boiling’ behavior in the solar photosphere indicated by the presence of ‘granules’ surrounding the sunspot in Figure 2), with sunspot umbrae suppressing convection the most. The authors find that since the brightest loops are partially rooted in non-umbral regions (i.e. the regions with more convective activity), convection is a major driving force of coronal loop heating. However, if a loop has both footprints in non-umbral regions, they will not be as bright. Therefore the strong magnetic field present in the umbral footprint, along with enhanced convection in the non-umbral footprint, is necessary for generating the brightest (and hottest) loops.
Through observations and simulations of two active regions, today’s paper shows that both convection and a strong magnetic field in the footprints of coronal loops are necessary for heating to occur. This provides yet another clue to solving the decades-old coronal heating problem. With several new instruments like the ground-based solar telescope, DKIST, and the Parker Solar Probe coming online soon, this mystery may be solved sooner than you think.