The Sun Part 6: Photosphere and Chromosphere

Turbulent and chaotic magnetic activity in the convection zone of the Sun is behind the appearance of sunspots, prominences, solar flares, coronal loops, and coronal mass ejections, all of which are surface and atmospheric features that signal intensifying solar activity as the Sun approaches solar maximum. Other features such as granules and spicules populate the Sun continuously, regardless of its activity. Exactly how all these features form and work remains, in large part, a mystery, one that scientists are actively working on.

Below is a NASA diagram of the Sun's layers.

The photosphere is the Sun's surface. Since a star doesn't have a solid boundary like a planet, moon or meteor does, this boundary is defined a bit differently, as you will see. The chromosphere is a layer of atmosphere just above the surface. It too is unlike any terrestrial atmosphere. It is a turbulent, hot and unpredictable mixture of plasma and atoms. Physicists have an arsenal of imagers and telescopes trained on it in an attempt to pry open some of its fascinating mystery.

In the previous article, we examined why the Sun experiences periods of relative calm followed by periods of intense solar storms. In this article and the next one, we will investigate the visible evidence of solar activity on the surface of the Sun and in its atmosphere.

Sunspots

As the Sun approaches solar maximum, sunspots, intense magnetic knots that spiral outward, tend to show up while the magnetic dipole field as a whole wanes. Magnetic field lines have been wound up and twisted by the Sun's differential rotation, and now the stress has reached its limit. They unfurl upward like uncoiling springs, puncturing the surface of the Sun, called the photosphere. These lines usually come in pairs and when they do, they have opposite polarities like the north and south poles of a magnet. This is why you often, but not always, see identical pairs of sunspots, like the pair shown below. Sunspots appear dark because they are zones where magnetic activity inhibits the motion of convection. Cooler zones that are between 2700°C and 4200°C form in contrast to the average surface temperature of 5500°C around them.

SiriusB;wikipedia

Sunspots look almost black but they are actually very bright, brighter than the moon's surface. The reason why they appear so dark has to do with black body thermodynamics. Very hot objects glow. They give off light, like a hot briquette does. The colour of the light tells you what the temperature of the object is. As the temperature of a hot object falls, its colour changes. This is called black body radiation and you can explore it in my article Atoms and Heat.  If we compare a change in colour (which is photon wavelength) to the change in luminosity or brightness, the luminosity decreases on a scale of about 4 to 1. See how peak visible luminosity (the two black dots) differs between 5400°C and 3000°C in the graph below right. That's approximately the temperature difference between a sunspot and the surrounding surface. The difference in luminosity is quite drastic.

Why Sunspots are "Cool"

Where the magnetic field is extra intense, additional force is exerted on the charged particles in the plasma, disrupting its convective motion. Recent SOHO data reveals powerful downdrafts spiraling downward underneath a sunspot, like a hurricane on Earth.

The process works like an eddy current brake where a magnetic field creates eddy currents in a rotating metal disk. This mechanism makes use of Lenz's law. The eddy currents in the disk set up a magnetic field opposite to one that is already present, and the disk resists rotation. Modern roller coasters use a linear version of these brakes. Inside a sunspot, where magnetic field lines puncture the photosphere, strong eddy currents disrupt the upward motion of convection, so heat from the interior is not transferred upward as efficiently and the sunspot cools.

Below is a close-up SOT telescopic image of two sunspots, roughly 20,000 km across. Notice the fine granular structure of the surrounding surface. These granules (each less than 1000 km across) are the tops of relatively tiny convection cells. Each granule lasts only about 20 minutes which means the surface continually changes over time. Because convection is inhibited in the black-appearing sunspots, they look smooth in comparison. The flow of plasma upward in the granular convection cells is very fast - up to 25,000 km/h. It creates shock waves when it reaches the far less dense photosphere above. The shock of these pressure waves regularly sends waves rippling across the Sun's surface.

In the previous article, we explored the convective layer. This layer of the Sun, underneath the photosphere, is where the granules are created. It is a layer of fluid plasma that literally boils like candy boiling in a pot, except that this fluid has incredibly low density. At the top surface of the layer, the photosphere, the density is only 4 x 10^-7 g/cm³ on average. That is thousands of times less dense than air at sea level on Earth, which is 1.5 x 10^-3 g/cm³.

Convection is driven by a difference in temperature. The base of the convective layer, where it transitions from the radiative zone, is very hot, about 1,500,000°C. Thermal columns, shown as loops in the image below, carry the hot plasma (shown as red) to the surface of the Sun where it cools off tremendously (blue), to about 5500°C, before it sinks back down and reheats.

modified from Eryian Con-struct;Wikipedia

These thermal columns extend right up to the photosphere lending it the granular appearance you saw in the photograph above. The plasma is full of charged particles in motion, so the turbulence produces magnetic dipoles as well as magnetic fields all over the surface.

The Photosphere

The surface of the Sun marks the layer in which photons of visible light, ultraviolet (UV) light and infrared light escape into space and become visible to imagers, telescopes and the naked eye. As you go downward underneath this layer, photons are increasingly trapped as they are absorbed by electrons and re-emitted. This means that the photosphere is also the boundary underneath which the Sun is completely opaque to electromagnetic radiation (http://solarscience.msfc.nasa.gov/surface.shtml). Even though the plasma is thousands of times less dense than air just beneath the photosphere, light doesn't shine through it as it would through air because free electrons scatter photons, making the plasma opaque at distances over a few hundred kilometres. Opacity means that photons cannot travel through, and it depends on the energy of the photon and the material. Human tissues, except bone, are transparent to X-rays for example. Although we think of Earth's atmosphere as being transparent to light it isn't. As we will see, even visible light is scattered in it to some extent and, luckily for us, Earth's atmosphere is completely opaque to the Sun's extreme UV photons and all of its X-rays.

The photosphere is only about 100 km thick, a very thin boundary compared to the 1.4 million km diameter of the Sun. It is easy to figure out where the surface of a rocky planet like Earth is, but for stars, the boundary is less obvious. The Sun's atmosphere is composed of the layers we can see through with various instruments.

As you move inward through the photosphere and deep into the core of the Sun, you find a trend toward photons becoming more and more trapped and having more and more energy. Average photon energy increases from visible to ultraviolet to X-rays and finally to the extremely energetic gamma rays created in the core's fusion reaction. Photons lose a tiny bit of energy each time they collide with another particle such as an electron in the plasma, so what starts out as gamma rays eventually ends up as much lower energy photons leaving the Sun's surface. The Sun's output is a continuous spectrum that ranges from high frequency X-rays to very low frequency radio waves, but the vast majority of photons range from 100 nm (ultraviolet) to 1 mm (infrared), peaking in radiance at around 580 nm, which is the wavelength of yellow light. The higher energy X-rays I mentioned don't originate from fusion in the core. These energetic photons come from coronal loops, flares and coronal mass ejections, violent structures in the atmosphere that we will explore in the next article.

The Sun is Yellow, No, White!

The Sun's output peaks at yellow visible light. Is that why the Sun appears yellow in the sky? The answer is no, and it is a very good question. The Sun's spectrum is a good approximation of the Sun's surface temperature, about 5500°C, which we explored in The Sun Part 5. Most of the photon emissions from the surface are visible yellow light, but the Sun as a hot body also glows, like a filament in a light bulb. This (black body) glow is called incandescence, and it overlaps and overwhelms its emission spectrum. This may seem confusing because the Sun in the sky looks yellow to us. The Sun's incandescence is the glowing light it gives off because it is very hot, and the Sun is white-hot. This is the Sun's true colour, how it looks from the International Space Station. It looks yellow instead of white in the sky because white sunlight (made up of a balanced spectrum of all visible colours) is scattered a bit more toward the blue end of the spectrum by gas atoms in the atmosphere than red light is. It looks yellow as a result of this light scattering effect, rather than as a result of its peak emission wavelength being yellow light.

Note: The Sun appears yellow in the two sunspot images shown earlier because either attenuation filters are used on solar telescopes, blocking most of the overwhelming brightness and maximizing yellow light (as with the whole Sun image), or the images are taken in black and white and then recolourized (the sunspot close-up). Click here to learn how to view the Sun yourself safely, using either Welder's glass or by using filters with your own telescope.

The Photosphere is Very Weakly Ionized Plasma

The photosphere is white-hot and though it is hot, this temperature is cool enough to allow almost all the hydrogen and all the helium to exist in atomic form as gases. Helium is a very tightly bound atom with the highest ionization energy of all the elements. It will completely ionize (lose both its electrons) only at energies above 5251 kJ/mol. It will partially ionize (lose one electron) at energies above 2372 kJ/mol. The photosphere is not hot enough even to partially ionize helium atoms, but about 3% of hydrogen atoms are ionized. Hydrogen will ionize at much lower energies, above 1312 kJ/mol and it is fully ionized as it only has one electron to lose.

Faculae and Supergranules

When the Sun approaches solar maximum, it actually tends to brighten slightly (about 0.1% overall), despite the increasing appearance of dimmer sunspots. This is because extra bright areas, called faculae, are also showing up across the surface. Like sunspots, these are associated with areas of increased magnetic activity, but the magnetic flux in this case is concentrated into much smaller bundles. They do not interfere with convection the same way that sunspots do. In fact, their mechanism is unrelated to sunspot production. These bright spots are regions where the magnetic field is extra intense and this force reduces the density of the photosphere gas so much that it makes it transparent. You can see right through into deeper layers of the granules where the plasma is hotter and brighter.

Supergranules are much larger than granules, about 35,000 km across. These structures last about a day, much longer than the smaller granules do. The origin of supergranules is not known but some researchers believe they are a higher-order pattern of granulation, part of a convection hierarchic structure, in other words. They are the focus of current research. The SOHO image of the Sun below reveals a web-like pattern or network across the Sun's surface that outlines supergranule cells.

This red, white and blue image is a Doppler-shift SOHO image. Red supergranules are moving into the Sun while blue supergranules are moving outward toward the surface. It highlights where bundles of magnetic field lines in the chromosphere are concentrated by the motions of supergranules in the photosphere underneath.

The chromosphere is the innermost part of the Sun's atmosphere. Here, additional solar features, such as filaments, spicules and prominences can be observed.

The chromosphere gets its "chromo" or colour name from hydrogen. The temperature rises in this irregular layer from about 5500°C to about 20,000°C. At these temperatures most hydrogen atoms are in an excited state and they emit (mostly) red light. If you look at the Sun through an H-alpha (660 nm) filter on a telescope, which isolates this colour, you can see all the features of this layer, as in the NASA photo below.

Although this light is in the visible range, you can't see it without an H-alpha filter. It is overwhelmed by the much brighter photosphere beneath it. If you want to see the photosphere features mentioned earlier, such as sunspots, you just need a good telescope that filters out some of the Sun's brightness, because most of this light, too, is in the visible range.

The chromosphere is much more enigmatic than researchers thought a few decades ago. Now, they don't restrict their observations of the Sun's chromospheric features to an H-alpha filter. As we will see, some structures in this layer exhibit a far more complex and puzzling temperature range, which extends far beyond 20,000°C.  At these higher more energetic temperatures, ultraviolet and X-ray imaging are used instead, because these structures are invisible to the naked eye.

Prominences

If you look closely at the red Sun image above, you will see dark thread-like filaments. These are slightly cooler clouds of gas suspended by magnetic field loops. Denser larger filaments of material projecting well above the surface of the Sun are called prominences. An example is shown in the orange/red image below. Prominences, which can be as huge as 800,000 km across, are often shaped like loops anchored in the photosphere. They tend to form over a day and then last for several months. Scientists are not sure why and how they form. When the Sun is quiescent, prominences tend to be relatively cool. These ones are virtually indistinguishable from dark filaments, when viewed directly top-down. They do not move or change much for days or weeks on end before dissolving unceremoniously. Others seem much more unstable. These ones tend to form during periods of higher activity, lasting only hours or minutes before erupting without warning into solar flares or coronal mass ejections, CME's.

The plasma in the prominence flows along magnetic flux lines. As these magnetic flux lines become increasingly twisted and disorganized (usually by the action of the solar dynamo), the prominence will grow increasingly unstable. It may eventually erupt, ejecting a significant amount of plasma from the Sun. The shockwave of this fast-traveling mass of plasma and radiation may cause a geomagnetic storm  if it shoots off in Earth's direction.

Prominences are a bit of a mystery because, while most observations place them at around 6000°C to 10,000°C, well within the temperature range of the chromosphere, these structures can be gigantic, extending well into the outer coronal layer, where temperatures spike to 1,000,000°C. New research suggests that prominences might be inner structures inside larger formations, like a filament inside a light bulb, except in this case, the "filament" plasma is much cooler than the hollow croissant-shaped "light bulb" or cavity of plasma that surrounds it. Scientists are curious about the vast range in temperature and about how some prominences seem to exist for weeks, appearing quite stable, before they suddenly erupt into massive CME's. Understanding how prominences work and how instability develops in them could help researchers predict potentially catastrophic CME's before they are observed on their way to Earth.

The image of a prominence below was taken by NASA's SDO observatory. Jupiter and Earth are juxtaposed on the image to show just how large one of these can be.

This image was taken in the extreme ultraviolet range, about 30.4 nm. The ultraviolet emissions are from highly ionized iron and from singly ionized helium and it corresponds to a temperature of about 50,000°C. There is a tiny amount of iron and other heavy elements in the corona and in the Sun in general. They came in the gas/dust cloud from which the Sun formed. In this image you are seeing the structure of the corona. Normally we can't see the corona because its black body radiation peaks in the invisible extreme UV range and because the Sun's visible output (from the photosphere) is a billion times brighter than the extreme UV output from the corona. A special filter is required to block it from the imager.

Prominences can also be easily observed in visible light using an H-alpha filter (660 nm). Below is a time-lapse movie of an H-alpha prominence shot in 2010:



Prominences look a bit like coronal loops (one is shown below for comparison which we will explore in detail in the next article), but there are key differences between them. Prominences tend to be much cooler than coronal loops. The plasma in a prominence (and at least the inner "filament" within a large unstable prominence) is about the same temperature as the rest of the chromosphere, so they are visible using a hydrogen alpha filter. Coronal loops, on the other hand are not visible in the visible spectrum at all, while even the hottest prominences have a visible component. NASA's TRACE images coronal loops filled with intensely hot (1,000,000°C) completely ionized plasma using a 17 nm (X-ray) filter. An example is shown below.

Perhaps even more mysterious than prominences, coronal loops are filled with mystery all their own, which we will dive into in the next article.

Spicules

Finally, spicules, or solar flux tubes as they are sometimes called, are tiny jets of material that erupt all over the Sun's surface and up through the chromosphere all the time, ejecting material outward into the upper atmosphere at speeds of around 90,000 km/h. They look like orange fur (along with a prominence to the left) in the NASA image below.

Like many solar surface features, spicules are under intense investigation right now. Careful observations using SOHO, TRACE and Sweden's SST telescope, along with computer modelling revealed in 2004 that spicules are probably formed by plasma energy waves that leak into the low atmosphere where they turn into shock waves. Spicules have an interesting regular periodicity. A new one appears in the same spot every 5 minutes. The waves that cause them have the same 5-minute periodicity. These waves, created by interacting electric and magnetic fields in moving plasma, resonate inside the Sun. Most of the waves are damped down by inner plasma flows where they are trapped and echo back and forth, but some of them escape, creating shock waves in the process, and these shock waves hurl jets of plasma out with them, creating spicules. The shock waves propel hundreds of thousands of spicules across the Sun's surface at any given time.

Though individual spicules are small, they add up, carrying hundreds of times more mass into the atmosphere than what would be needed to explain solar wind using ballistics mechanics. More recent (2011) evidence suggests that spicules could be related to prominences.  Plasma shot upward by spicules may rain back down when it cools in the upper solar atmosphere, and as it does, it might be captured in the tangled trap of a prominence's magnetic flux lines. In the 1980's, researchers, observing that spicule plasma appeared to be as cool as the rest of the chromospheric plasma, discarded spicules as a possible cause of mysterious coronal heating, something we will investigate in the next article. However, in 2011, NASA's SDO measured some spicule plasma temperatures at over 1 million degrees Celsius, bringing spicules back into the spotlight as serious coronal heating contenders. Other research from 2011 suggests that spicules in the chromosphere and granules in the photosphere beneath them might be related to each other somehow and that the spicules might be generating a continuous supply of waves called Alfvén waves. These waves have been modeled to have enough energy to heat the corona and power solar wind as well.

We will revisit these special Alfvén waves in the next article when we explore the outer layers of the Sun's atmosphere and one of the Sun's most intriguing puzzles - the coronal heating mystery, next, In The Sun Part 7.