A Magnetic Map of the Silent Sun

Title: Mapping the Hidden Magnetic Field of the Quiet Sun

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

Status: Published in the Astrophysical Journal Letters [Closed Access, Pre-print available on Arxiv]

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.


An example of the Zeeman splitting of spectral lines of light coming from a sunspot due to its strong magnetic field. Phases of strong solar activity can exhibit magnetic fields as high as 4000 Gauss, which is several thousand times stronger than the Earth’s magnetic field, which is under 1 Gauss.

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).


The map of the quiet Sun’s magnetic field. Dark blue regions overlap with granules and have weak fields, while their boundaries (red) have stronger magnetic fields. Figure 5 in the paper.

Revealing the Hidden Field

Granules representing plasma convective cells on the Sun’s surface. Each individual granule is roughly the size of Texas.

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

About Sumeet Kulkarni

I'm a third-year PhD candidate at the University of Mississippi. My research revolves around various aspects of gravitational wave astrophysics as well as noise characterization of the LIGO detectors. It involves a lot of coding, and I like to keep tapping my fingers on a keyboard even in my spare time, creating tunes instead of bugs. I run a science cafe featuring monthly public talks for the local community here in Oxford, MS, and I also love writing popular science articles. My other interests include reading, cooking, cats and coffee.

1 Comment

  1. coriolis force thats what im looking at
    the proper motion of the sun as its locomotion takes it through the milky way galaxy
    and as it does the intergalactic gas moves across the equator region and the north goes clockwise and the south goes anticlock or is it the other way around and then they move over the tropics of the sun 30 degree latitude north and south
    creating tornadoes cyclones hurricanes in the suns high corona sphere coroilis wise
    as for the state of texas how are ya all doing down there in texas
    solar corona 1300 kilometres diameter granule x 20,000 kilometres deep convection plasma cells at 46 4000 gauss that proper motion coriolis force that crested those tornados cyclones hurricanes in the gramules thats them there 20,000 x 1300 kilometres cigar shaped columnar jets ready to erupt if you put a certain flame heat to the base tip of the cigar colum
    blowing out cigar smoke rings into the heliosphere
    plus that magnetosphere size is it
    earth gauss 1 = 8000 kilometres to 45,000 kilometres radius
    approximately 46 gauss solar minimum = 400,000 kilometres radius = 2 light seconds
    approximately 4600 gauss solar maximum = 400,000,000 kilometres = 3.5 light minutes radius
    or does it go further magnetosphere sun wise


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