Authors: J. C. Trelles Arjona, M. J. Martínez González, and B. Ruiz Cobo
First Author’s Institution: Instituto de Astrofísica de Canarias (IAC),Vía Láctea s/n, E-38205 San Cristóbal de La Laguna, Tenerife, Spain
The Solar Cycle sees our Sun alternate between phases of tempest and tranquility driven by its magnetic field roughly every 11 years. It has been difficult to measure the magnetic strength when the Sun goes silent – until now.
The Sun’s magnetism holds the key to solving a well-known mystery: what makes the temperature of its outermost atmosphere, or the corona several hundred times hotter than its surface? When the Sun is not silent, we can observe and measure magnetic forces at work that produce sunspots, giant solar flares, and coronal mass ejections – fiery processes that can inject heat into the corona.
Over the past few years, the Sun has been at the quiet end of its cycle, displaying little or no surface activity. Yet the solar corona remains heated to over a million degrees even when the Sun is silent. Without the telltale signs of periods of high Solar activity, measuring the surface magnetism that may be driving this heating is extremely difficult.
In a new study published in the Astrophysical Journal, astronomers have achieved this using special techniques to analyze sunlight.
Solar Magnetic Cartography
Sunlight seen through a spectrograph reveals dark lines in its rainbow-like continuous spectrum. These lines represent individual elements that absorb light in the Sun’s atmosphere. In the presence of a magnetic field, these lines split into two, a phenomenon called the Zeeman effect. Figure 1 shows Zeeman splitting by the strong magnetic field in a sunspot.
However, in the quiet Sun’s weaker magnetic fields, the Zeeman splitting is small and there are other physical processes that can contaminate its measurement. In order to distinguish these from magnetic fields, the researchers studied high-resolution polarized light images from the GREGOR Solar Telescope, by restricting the electromagnetic oscillations of sunlight to certain orientations. This enabled the astronomers to obtain a high-resolution map of the variation in the magnetic field of the quiet Sun covering an area spanning roughly 112 times the landmass of the contiguous United States (Figure 2).
Revealing the Hidden Field
The most striking outcome of the map is that the magnetic field variation closely matches solar granules, features on the Sun’s surface representing convective plasma cells (Figure 3). The field is weak within a granule and stronger along the boundaries. On average, the researchers found the magnetic field of the quiet Sun to be 46 Gauss, comparable to that of a refrigerator magnet. While these fields are much weaker than what is observed in a solar maximum, they are still shown to be sufficient to pump energy to heat up the solar corona through small-scale nanoflares.
Detailed studies of the Sun’s magnetism, both when it is roaring and relaxed, are vital to make better models of the solar cycle and possibly predict the intensity of future solar storms which can threaten catastrophic damage to our telecommunication systems.
Astrobite edited by: Pratik Gandhi
Featured image credit: NSO/AURA/NSF