SunScapes: Our Magnetic Star

Solar Mosaic In this image, dozens of ultraviolet photographs of the Sun are stitched into a mosaic, with the coldest regions shaded blue and the hottest red and white.

More than simply beautiful, these photographs from the National Aeronautics and Space Administration (NASA) allow astronomers to study in further detail the Sun’s magnetic field.As the star at the center of our solar system, the Sun makes life on Earth possible. But not all of its effects on our lives are beneficial: the Sun sporadically disrupts our weather patterns and interrupts our communication and navigation systems. It generates radiation that may harm astronauts and airline passengers. It can even push power grids to failure. All of these and other phenomena, including the many spectacular solar events shown in the following photographs, are controlled by the Sun’s variable magnetic field.

On Earth, there is a magnetic field that remains relatively stable over time. Explorers, surveyors, pilots and ship captains use this field to orient their compasses and to find their way around the globe. Even animals, like some migratory birds, can use the Earth’s magnetic field to navigate with their own internal compasses. But on the Sun, the magnetic field is much more erratic; there are many magnetic poles whose positions and strengths constantly change. Indeed, a compass on the Sun would merely point to the pole closest and strongest at that moment.

As troublesome as it is for us, the Sun’s magnetic field is also useful, as it makes direct observations of the Sun possible. All elements caught in the Sun’s magnetic field emit extreme ultraviolet light, invisible and exceptionally dangerous. Orbiting telescopes can detect such light and represent it as safe, visible, recognizable colors. More than simply beautiful, these photographs from the National Aeronautics and Space Administration (NASA) allow astronomers to study in further detail the Sun’s magnetic field.

Rotational Chaos From surface to core, the Sun is a star made of very hot gas that rotates around its axis. Astronomers detect sound waves on the Sun by measuring the minute variations in the surface brightness, and in turn use those sound waves to measure the Sun’s average rotational speed at any one interior or surface point. That speed is represented in this image by color, with the fast gas near the equator shown in red and slow gas near the poles in blue. This differential rotation wreaks havoc on the Sun’s magnetic field—literally twisting the field into knots—causing the disruptions and irregularities that affect life on Earth.

Sunspot Connections Sprinkled across the solar surface are cold patches of intense magnetic field—called sunspots—which appear darker than surrounding, hotter areas. In this composite image, dark sunspots can be seen in each "layer" where the field is strong and coherent. The bottom green panel shows the visible surface at about 5,000 °C (9,000 °F). The blue image (not to scale) shows the emission up to some 6,500 kilometers (about 4,000 miles) above the solar surface. The yellow image shows the Sun in the light of hydrogen gas. The top image shows the highest domain of the solar atmosphere, called the corona, over 1.5 million km (roughly 1 million miles) above the solar surface, where the temperature exceeds 1 million °C (1.8 million °F).

The Yellow Sun?

The Sun’s magnetic field heats its atmospheric gases to millions of degrees Celsius. Paradoxically, the gasses closest to the Sun are the coolest and densest and the ones farther away are hotter and more diffuse. At these different temperatures, the Sun’s atmosphere glows at different wavelengths or colors, most of which are invisible to the human eye. Indeed, the Sun emits all the colors of the spectrum of light, but gives off more yellow light than any other. In fact, when we look up at the Sun, our “yellow light” sensitive eyes only see the coolest part of its atmosphere, the gas glowing at visible wavelengths, itsphotosphere, as seen in this photograph. The hotter parts of the atmosphere farther away from the surface glow at wavelengths visible only to special optical systems and detectors. The following photographs, all taken on February 8th, 2001 by telescopes outfitted with such systems, show the Sun at a variety of wavelengths ranging from the visible, through the ultraviolet to the X-ray part of the spectrum. False colors were used in those images that depict the Sun at wavelengths outside of the visible spectrum, giving visual form to things we could never see with our own eyes.

The Yellow Sun? When we look up at the Sun, our “yellow light” sensitive eyes only see the coolest part of its atmosphere, the gas glowing at visible wavelengths, its photosphere, as seen in this photograph.

Sunspotted Magnetic maps of the Sun, like this one created with polarized filters that are tuned to very narrow color ranges, show regions of north magnetic polarity as bright, and regions of south polarity as dark. Comparisons with images of the Sun taken in visible light show that sunspots correspond to compact, strong clusters of magnetic field.

In the Light of Calcium Within the first 6,500 km (about 4,000 miles) above the Sun's visible surface, the temperature within the magnetic field nearly doubles from 5,500 to 10,000 °C (about 10,000 to 18,000 °F) and the gas glows in the light of calcium (in blue).

In the Light of Hydrogen Within the first 6,500 km (about 4,000 miles) above the Sun's visible surface, the temperature within the magnetic field nearly doubles from 5,500 to 10,000 °C (about 10,000 to 18,000 °F) and the gas glows in the light of hydrogen (in red).

In the Light of Helium The Sun observed from space in the light of ionized helium (He II) at about 28,000 °C(50,000 °F).

In the Light of Iron The Sun observed from space in the light of ionized iron (Fe IX/X) at about 1.1 million °C(2 million °F).

In the Light of Iron The Sun observed from space in the light of ionized iron (Fe XII) at 1.7 million °C(3 million °F).

In the Light of Iron The Sun observed from space in the light of ionized iron (Fe XIV) at 2.2 million °C(4 million °F).

X-Rayed An X-ray photograph of the Sun, revealing emission from gases at 3.3 to 5.6 million °C(6 to 10 million °F).

Solar Wind An X-ray photograph of the Sun, with the bright center obscured by an opaque disk. The glow we see—sunlight scattered off the solar wind—reveals streams of gas moving away from the Sun at speeds that range from about 300 to 1,000 km per second (about 200 to 600 miles per second).

The Solar Dynamo

The Sun’s magnetic field is generated deep within its interior by a process referred to as thesolar dynamo. Though it is not yet fully understood, the solar dynamo is driven by the Sun’s differential solar rotation and the seething, convective motions that occur in the outermost layers of the star. These processes ensure that charged particles remain in constant motion, which generates magnetic fields that float to the surface. Magnetic regions of all sizes pop up all over the Sun, with the strongest fields clustering in fairly narrow belts on either side of the equator.

Hot Gas Rising This high-resolution image reveals the multitude of convective cells, each about the size of Texas, that cover most of the solar surface. In these cells, hot, buoyant gas from the interior rises to the surface where it expands and cools. The cooler, denser gas slides towards the edges and eventually sinks down into the cooler, darker, network of lanes. The large dark spot is a sunspot, a point of particularly strong magnetic field.

Magnetic Cycles The number of sunspots and their associated magnetic fields goes up and down in a steady cycle, peaking every 11 years. The composite image in blue shows ten magnetic maps of the Sun approximately one year apart, from one maximum of activity almost to the next. As activity fades, the large magnetic regions disappear and only small ones continue to emerge.

Magnetic Cycles The composite image in red shows the X-ray emission from the corona. As the cycle fades out, the X-ray emission becomes weaker and more diffuse, brightening again as the next cycle starts.

Thrown For A Loop

Most of the solar corona is made up of glowing loops of hot material that outline a magnetic field extending from one surface pole to its partner. The heating that is associated with the magnetic field warms gas at the base of the loops up to 550,000 degrees Celsius (1 million ºF) or more. This causes it to "evaporate" into the high regions of the atmosphere, where it is supported by the pressure of the gas below, balancing the Sun’s gravitational pull.

This image of a magnetically active region shows "loops" that connect one magnetic pole to another.

Thrown For A Loop This image of a magnetically active region shows "loops" that connect one magnetic pole to another.

Coronal Motion Nothing is static on the Sun, least of all its corona. Each of these three columns of images of magnetically active regions spans only two-and-a-half hours, yet each image looks completely different. These looplike structures repeatedly change position, density and temperature, resulting in a beautiful yet extremely complex and variable atmosphere.

X Points the Way Where magnetic fields from two or more magnetically active regions are comparable in strength but opposite in direction, the fields deflect each other and a so-called X point forms. The field strength goes to zero at the crossing point of the X. Solar physicists think that at such points much of the action in the corona occurs.

Solar Explosions

The activity and variability of the Sun’s magnetic field can cause large, eruptive, explosive events called coronal mass ejections (CMEs). In a CME, a total volume as large as that of the Mississippi River can be ejected into space in a matter of minutes, at speeds of hundreds of kilometers per second. Most of those erupting rivers of gas cannot escape the Sun's gravitational pull and soon fall back to the surface. Those that do escape can cause disruptions in Earth’s atmosphere and in our communication and navigation systems.

In filament eruptions like this one, relatively dense material of some 5,500 degrees Celsius (10,000 ºF) lifts rapidly from just above the solar surface, caught in the stretching, twisted magnetic field.

Solar Explosions In filament eruptions like this one, relatively dense material of some 5,500 degrees Celsius (10,000 ºF) lifts rapidly from just above the solar surface, caught in the stretching, twisted magnetic field.

Near Eruption Embedded within the hot, glowing corona is relatively cool, absorbing gas (colored black) that is lifted by the untwisting magnetic field. If this eruption had had a little more momentum, much of the relatively cool mass would have been thrown into space in a coronal mass ejection.

Eruption Arcs This image in green shows a row of magnetic arcs, glowing as it cools down from many millions of degrees Celsius to a mere one or two million degrees, after a large amount of relatively cool gas in the form of a filament erupted through it.

Eruption Arcs Another cooling formation is shown in the gold colored image seen from the side

Eruption Emissions In this image, the Sun is blocked from view by an opaque disk built into the telescope (the white circle shows the size of the Sun) so that the faint emissions from a coronal mass ejection can be seen clearly.

Aurorae Charged particles that escape the Sun’s gravitational pull can interact with other magnetic fields, like the ones on Earth and on Saturn. On Earth, these particles collide with atoms of oxygen, nitrogen and hydrogen in the upper atmosphere to produce colorful displays in the sky called aurorae, named Aurora Borealis (the Northern Lights) or Aurora Australis (the Southern Lights).

Aurorae Charged particles that escape the Sun’s gravitational pull can interact with other magnetic fields, like the ones on Earth and on Saturn. On Saturn, the variable aurorae are seen as blue rings that change shape and brightness in a few days.

The images of the Sun are primarily from the NASA Transition Region and Coronal Explorer (TRACE) and the Extreme Ultraviolet Imaging Telescope (EIT), the Large Area Solar Coronal Observatory (LASCO) and the Michelson Doppler Imager (MDI) telescopes on the ESA/NASA Solar and Heliospheric Observatory (SOHO). Additional material is from the Japanese-American YOHKOH satellite, the Big Bear Solar Observatory, the National Solar Observatory at Kitt Peak, the Swedish Solar Observatory and courtesy of Åke Nordlund (Nordita, Kopenhagen). 

Further information is available at http://www.lmsal.com/

The images of the Earth and Saturn are courtesy of NASA, the ESA and J.Clarke (Boston University).