Wednesday, April 3, 2013

The Sun Part 7: The Corona

The Sun's corona is the outermost layer of the Sun's atmosphere and it is enormous, extending from the chromosphere beneath it to more than a million kilometres into space. This layer, composed mostly of ionized hydrogen atoms, has an average density far less than even that of the chromosphere, and its outermost edge vanishes into the near-vacuum density of interplanetary space. This layer is about a million times less bright than the overwhelming radiance of the Sun's surface. Despite the relative coolness of the chromosphere below (it generally reaches a maximum of about 20,000°C), this layer mysteriously approaches temperatures well over 1,000,000°C. Why it is so hot is a question physicists are busy trying to answer.

Viewing the Corona

Amateur astronomers like us can only see the Sun's corona during a solar eclipse, when the far brighter solar surface is blocked behind the moon, as shown below.

Luc Viatour;Wikipedia

It might be tempting to peer at the Sun during an eclipse, thinking that it should be safe to do so, but it isn't. Even a tiny sliver of exposed photosphere emits enough UV radiation to permanently damage the eye's retina, and there are no pain receptors there so you won't be aware of it. Sunglasses (even UV ones), smoked glass, CD's or black camera film don't work. Astronomy sites recommend that we use only a proper solar filter or Shade #14 Welder's glasses. The next total eclipse depends on where you are on Earth. NASA provides an eclipse website that predicts the next partial and total eclipses around the world.

The Corona is Like a Very Hot, Very Light Gas

Plasma acts like a highly compressed liquid where convection takes place inside the Sun, but in the corona, hydrogen is completely ionized and helium is partly or completely ionized and they act more like very hot very light gases, with important differences. The density here is about a million times less than Earth's atmosphere.

The coolest layer of the Sun, around 3800°C, is found about 500 km above the Sun's surface. This is part of the chromosphere, the lowest layer of the atmosphere. If we go from 500 km outwards to about 2000 km altitude, the temperature rises very sharply, from 3800°C to over 1,000,000°C. This zone is called the transition region. Its actual thickness and altitude are variable and not well defined. This region is hot enough to completely ionize helium atoms and partly ionize various heavier trace elements present as well, such as iron. This zone is filled with extremely low-density, extremely energetic, plasma that ranges from 1.0 x 10-14 g/cm3 at the bottom of the zone to about 1.0 x 10-16 g/cm3 at the top. Compare this to the density of the chromosphere, about 2 x 10-7 g/cm3.

The Corona Transitions Into Solar Wind

At the top of the corona, electrons, protons and a few other particles escape the Sun into surrounding space. These fast moving particles, combined with radiation emitted from the Sun, make up solar wind. Old models relied on thermal energy to accelerate the particles enough to overcome the Sun's intense gravity. At a million degrees Celsius, these particles, although very far apart, collide with tremendous energy, and this force accelerates them on average to about 145 km/s. This represents their average thermal energy. However, that is well below the velocity they would need to escape the Sun's gravity, about 618 km/s. New models take magnetic and electric interactions into account as well as thermal energy. For example, many electrons in the plasma, being relatively light, in fact do reach escape velocity because this plasma is too diffuse to act like thermal plasma (thermal plasmas are explored in The Sun Part 3). As electrons escape, they build up an electric field that further accelerates other ions away from the Sun, allowing them to escape as well. Magnetism also plays a significant role, especially around solar maximum. Open magnetic field lines make solar wind gusty. These open lines are called coronal holes, and they are usually located at the Sun's magnetic poles during quiescence. They appear as dark spots in UV images of the Sun. A large one is easily visible on the lower right part of the NASA STEREO colourized image taken with an extreme UV 19.5 nm imager, below right.

Remember how magnetic field lines loop around a bar magnet but at the ends, a few lines point straight out or in rather than form a loop? See the magnetic field lines around a bar magnet, below left. The Sun's magnetic field is much the same when it is very quiet. Coronal holes form over the North and South poles.

Coronal holes become very mobile, multiple, and unpredictable during high activity, when the Sun's magnetic field lines become disorganized, twisted and tortured. These holes are like open channels through which ions accelerate up to 800 km/s, well above average escape velocity. Although they operate in both directions - into and away from the Sun - the holes contribute significantly to solar particle loss. When Earth is in line with a coronal hole, increased geomagnetic activity, such as more and brighter aurorae, is expected.

The high thermal energy of solar plasma, augmented by the effects of magnetism and electricity, means that the Sun loses, on average, between 4 and 6 billion metric tonnes of matter every hour.

Coronal Loops

Powerful magnetic fields in the corona have a powerful impact on plasma movement. Plasma tends to "stick" to magnetic field lines. An electron, being a charged particle, will experience a force that is perpendicular to both its direction of motion and the magnetic field, called the Lorentz force. Around a strong magnetic field line, the force is enough to deflect the electron into a circular motion. It becomes trapped into rotating around it. Electrons in the solar plasma (and positive ions going the opposite direction) follow helical paths as they travel along the length of a magnetic field line, as shown below right.

This trapping effect leads to magnificent structures called coronal loops, observed by NASA's TRACE satellite, shown below, which decorate the corona, particularly during solar maximum. The glow is from hot and excited ions trapped along magnetic field lines, illuminating them.

We took a sneak preview of coronal loops in the previous article when we compared them to prominences. This is a colourized 17 nm X-ray image - the ions trapped along the magnetic field lines are extremely energetic. They are emitting a lot of X-ray photons, representing a great deal of radiant energy.

These loops are closed magnetic field lines and they are thought to be harbingers of solar flares and coronal mass ejections (CME's), even though research suggests that CME's are the direct result of prominence eruptions rather than coronal loops. Sunspots are often found at the point where coronal loops originate, called footprints. A pair of footprints anchors each magnetic field loop to the photosphere, each footprint being of opposite polarity. Bright coronal loops, often many thousands of kilometers high, populate the corona at solar maximum, each one lasting hours or days. However, even larger but fainter loops may appear when solar activity is low, and these last longer, up to weeks, a pattern reminiscent of prominences.

The corona is full of open and closed magnetic field lines. A magnetic field line represents a one-way flow of magnetic force. Physicists measure this flow more accurately as magnetic flux but as visualization aids, field lines work fine. The closer the lines are drawn, the greater the flux. Most magnetic field lines around the Sun are "empty" of plasma, but coronal loops are loaded with it. The plasma within them is denser than the surrounding corona, and it glows, indicating where the flux lines are.

Imaging the Corona Using Trace Atoms

The plasma glows around the X-ray range of the EM spectrum because of its incredible heat. It is not accurate to call this output its black body spectrum, however, because the corona is not optically thick enough to act like a black body (the photosphere, on the other hand, is). Along with the heat glow, excited electrons in ions glow as well, adding to the total emission. SOHO, for example, has several solar imagers, each tuned to a specific electromagnetic (EM) wavelength. When the Sun is imaged, the light captured can be attributed to a specific excited ion and a corresponding temperature. For example, the 17 nm X-ray image above indicates the presence of excited partly ionized iron atoms that have lost 8 electrons, Fe IX (a neutral iron atom is called Fe I). These occur at temperatures around 60,000°C (corresponding to an energy of around 15,000 kJ/mol. Carbon, helium and especially iron, present in small to trace amounts in the corona, are very useful because they have a variety of possible excited ion emission wavelengths that allow them to "light up" specific regions and structures, when they are imaged at that wavelength. Flares may be extremely hot, approaching 10,000,000°C. Fe XVIII, excited iron ions with 17 missing electrons, exist at this temperature, emitting photons at 9.4 nm. A 9.4 nm imager will illuminate such a flare in great detail.

Coronal Loop Characteristics

Much about coronal loops remains a mystery, but SOHO observations suggest that plasma flows in one direction through a coronal loop. As it follows the loop into the top of the transition region it is intensely heated before it cools once again on the way back down. The size and height of coronal loops vary widely, as shown below. Smaller loops may not reach the bottom of the transition region where temperatures begin to rise. In the largest loops, however, the plasma undergoes a kind of evaporation (into fully ionized) in the upper part of the loop followed by condensation (to partially ionized and neutral) at the footprint. (The top of the transition region is not shown in the diagram; the dotted line shows where it starts.)

Coronal loops (and especially solar flares) tend to occur around the same time and place as CME's do. It's easy to imagine a loop twisting and a CME being pinched off and hurled into space, but there is no evidence that CME's form this way. Most ejections do, however, come from regions where sunspots are grouped together, indicating intense magnetic activity. And, as mentioned in the previous article evidence suggests that most ejections come from hot destabilized prominences. We will explore CME's in more detail later on in this article.

In the hottest zone of a coronal loop, electrons have enough energy to emit X-rays when they collide with other particles. These are the most energetic photons emitted from the Sun into space. The gamma photon emissions that originate from fusion in the core are not nearly as energetic by the time they leave the Sun. Their upper limit is ultraviolet. Loop X-ray emission carries away a tremendous amount of energy, so why is this layer of the corona so incredibly hot, when it seems to have a powerful built-in cooling mechanism?

Coronal Heating Problem: Two Theories

Over a thin layer of the Sun's atmosphere, temperatures skyrocket up to 1,000,000°C, when it seems the temperature should gradually decrease with altitude. This is observation really puzzles researchers. Coronal loops might be involved in coronal heating somehow. Researchers know the plasma within them is heated by coronal heating, but it's possible they play an active role in the heating process themselves. Recent TRACE images show that significant heating occurs near the footprints of coronal loops where they emerge and where they connect back to the photosphere. You can see evidence of this heating in the image of the loops above, taken in the extreme ultraviolet range, as extra brightness near their bases.

To make the coronal loop mystery even more complex, there seems to be no single-fit temperature profile for coronal loops. Temperatures vary wildly along the length of loops and the loops themselves exhibit a wide variety of overall temperatures. There are cool loops (below 1 million °C), warm loops (around 1 million °C and hot loops (above 1 million °C). If you look at a photo gallery of coronal loops (just search "coronal loops" in Google's image search engine), you will notice a wide range in brightness (depending on the wavelength chosen) as well as overall size. They are, not surprisingly, under intense investigation by researchers.

Two theories for coronal heating stand out. One theory, which has not been directly verified, has to do with the Sun's magnetism. As mentioned in the previous article, the Sun's surface (its photosphere) is granular in appearance because it is covered with many tiny magnetic dipoles. These dipoles are much smaller (50 to 1000 km across) than magnetic field loops, but they may be the force behind coronal heating.

Coronal loops are generally linked to increased solar activity during solar maximum in the 11-year solar cycle. The tiny magnetic dipoles associated with granules, on the other hand, are always present, creating a magnetic carpet that covers the star all the time, even during solar minimum, when the surface is fairly quiet. This is important because the corona always stays hot. Its temperature doesn't go up and down with the solar cycle, so the heating mechanism must be a continuous one.

The mechanism proposed for this theory is the same as it is for CME's - magnetic reconnection. The laws of electromagnetism tell us that magnetic field lines cannot cross one another. When two lines come close to each other they are rearranged on the Sun's surface. This process should release energy when electrical currents along these lines suddenly collapse, and this could heat the corona. In this case, magnetic reconnection occurs on a far smaller and more constant scale than it does for large solar flares and CME's.

In order for the theory to work, there must be a great deal of continuous reconnection going on all over the Sun's surface in order to heat the corona so intensely. Theoretical tiny flares associated with the reconnections, called nanoflares, would be far too small to be observed directly and there is a question of whether they would be widespread enough to account for all the heating. The main problem with this theory is that you have to show how cyclic and extremely energetic large-scale phenomena, like coronal flares and mass ejections, can originate from the same mechanism as continuous, lower energy and tiny granule-associated nanoflares do. No one has been able to do that yet.

A second theory is called the wave heating theory. Two kinds of wave occur specifically in plasma: magneto-acoustic waves and Alfvén waves. These waves may carry energy from the Sun's interior outward and into the corona. Magneto-acoustic waves are longitudinal waves, like sound waves and pressure waves, shown below left. Alfvén waves, on the other hand, travel as transverse waves that are a bit like low frequency radio waves, below right.
Longitudinal Wave
Transverse Wave
Alfvén waves are an interesting combination of an electromagnetic wave (photons) and a hydrodynamic wave. This is how they are set up: In an electrically conductive fluid, like plasma, any movement of the fluid produces an electric field and a magnetic field. The electrical current, in turn applies mechanical force to the liquid itself, altering its motion and the magnetic field. If you think of a magnetic field line as a string with tension in it, an Alfvén wave is a disturbance traveling along the string once it is plucked by the perturbation in the magnetic field. Plasma tends to move in the same direction as magnetic field lines move. This is why coronal loops contain plasma, as if magnetic field lines are "sticky" to it.  Many field lines are directed outward, piercing the photosphere. An Alfvén wave, therefore, could reverberate plasma along its length and propagate a great deal of energy outward into the corona. Alfvén waves travel much faster in very strong magnetic fields and in areas of very low density, so it could mean that there is increased energy available to heat the very low-density transition region.

Problems With Waves and Nanoflares

Both Alfvén waves and magneto-acoustic waves have been observed on the Sun. The turbulence in the Sun launches both types of wave. These waves may carry energy some distance up through the solar atmosphere.

Magneto-acoustic waves do not offer as much promise as Alfvén waves in explaining coronal heating because of the problems mentioned earlier as well as the fact that the density of the gas in the corona drops so dramatically. These waves, which are pressure waves, shouldn't be able to carry enough energy to the corona fast enough to heat the gases. Many of these waves would also be reflected back onto the photosphere. In 1997, SOHO observations suggest that they account for no more than 10% of coronal heating.

Alfvén waves detected in 2007 were clocked travelling as fast as 2500 km/s (9 million km/h). It's possible that such waves, hitting the transition region, could be transformed into shock waves that quite suddenly dissipate their energy as heat. This theory might explain why the temperature increases so rapidly across a fairly specific layer in the corona (the transition region), but no precise mechanism has been worked out.

It's even more difficult to explain how the granule/nanoflare magnetic reconnection theory accounts for the steep temperature increase well above the photosphere.

Many physicists believe it's possible that no single mechanism heats the corona. Magnetic reconnection, Alfvén waves and magneto-acoustic waves may all contribute to coronal heating.

Keeping the Heat In: Two Mechanisms

Once heat is supplied to the transition region, additional mechanisms appear to be at work keeping heat in this layer.

First, electrons are ultimately responsible for outward directed thermal conduction in the core. They carry kinetic energy that is transferred from collision to collision between photons, protons and nuclei, re-emitting new photons that carry energy slowly outward. This works because the core is so dense and particles are very close together so they can interact a great deal with each other. The transition region of the corona, though almost as hot as plasma is near the base of the convective zone (, is far, far less dense. There are great distances between particles. Here, thermal conduction isn't outward so much as it is parallel to magnetic field lines. The Lorentz force bends the path of plasma along these lines, which tend to run parallel to and across the Sun's surface. Electrons tend to stay in these lines because there are very few collisions to scatter them. This prevents them from the upward perpendicular movement that would allow heat to dissipate upward and outward through the corona.

Second, helium atoms, which account for somewhere between 5% and 15% of the corona by number, are only partially ionized below the transition region but within it, they are energetic (hot) enough to completely ionize. Partially ionized helium radiates energy away very well. This atom still has one of its two electrons, so it can radiate energy away as black body radiation and it can also emit photons as the electron moves from one energy level down to another one. This emission process has the same dynamics as the ultraviolet Lyman series of emission lines observed for hydrogen (see Atoms Part 2: Atoms and Light). The strongest emission is 30.4 nm. The emitted photon carries energy away with it. When a helium atom is fully ionized, however, it has no electrons left at all. Instead it is an alpha particle, a nucleus consisting of two protons and two neutrons. The atom now has no way to effectively lose energy, except through occasional collisions in the very thin coronal atmosphere. Physicists consider this electron-less state to be a kind of phase transition. It goes a long way in explaining how such a steep temperature gradient can exist in the corona. It doesn't take a very large increase in heat to fully ionize helium once it is partially ionized. Once this happens, heat has almost no way to dissipate from the region (an exception is energy dissipation through X-ray iron emission but iron is present in only trace amounts). Likewise, if the temperature decreases just slightly, helium returns to a partially ionized state where heat can rapidly be further dissipated.

Coronal Mass Ejection

The corona is very hot and, especially near solar maximum, very chaotic. Although ions are few and far between, the corona is nonetheless a gigantic reservoir of material. Billions of tonnes of coronal matter can be hurled from the Sun in just one coronal mass ejection, or CME. CME's remain very mysterious to scientists. Massive loops of coronal material are hurled away from the Sun at speeds of over 1,000,000 km/h. Furthermore, CME's have roughly ten times the energy of the prominences that produced them. Although the mechanism is not well understood, most physicists believe that CME's are caused by magnetic reconnection. This process happens anywhere magnetic fields exist in space. It is very common in the Earths' magnetosphere as well as on the Sun. But, maybe surprisingly, magnetic reconnection on the Sun is not yet understood. Scientists do have a basic idea of what happens - in order to build a CME, two bundles of oppositely directed magnetic flux must come very close together. This isn't hard to fathom because oppositely directed magnetic fields attract each other - that's how magnets work. On the Sun, these magnetic field lines are strongly stretched out by plasma pressure, which means they have a lot of potential energy, like stretched out rubber bands. The lines come close, almost cross, and then cancel and reconnect as new, now relaxed, parallel lines, and they release a tremendous amount of magnetic potential energy doing it, often equivalent to millions of hydrogen bombs. The problem with modeling (and understanding) CME's is that the classical theory used to model how electrically conductive plasma behaves, called magnetohydrodynamics, does not accurately predict how powerful and how fast magnetic reconnection occurs in CME's. It is also difficult to model these complex processes in three dimensions, even with the latest software.

The Halloween 2003 Solar Storm Captured by SOHO

To highlight just how challenging it is to predict and understand solar activity, the Halloween 2003 solar storm, one of the most powerful solar storms ever recorded by modern imagers, occurred two to three years after solar maximum, at a time when the Sun's magnetic activity should have been waning. Between October 19th and November 7th, a total of 17 solar flares erupted. For unknown reasons, the Sun's magnetic field stretched and snapped like a rubber band, triggering a series of CME's that hurled billions of tonnes of material in all directions at speeds up to 8 million km/h. A solar storm becomes a geomagnetic storm when clouds of charged particles from CME's, as well as extra intense solar wind, crash into Earth's magnetic field, disrupting it and causing it to reverberate like a trampoline. Vibrant auroras associated with the 2003 storm were seen as far south as Florida as flights, satellites and telecommunications were disrupted and parts of Sweden lost power. SOHO failed temporarily and NASA's ACE satellite (designed to study the composition of solar wind) was damaged.

A particular solar flare and accompanying CME on October 28, 2003, was the most powerful flare scientifically recorded. It produced an X-ray flux of X45, which saturated X-ray imaging instruments for 11 minutes. Solar flares vary widely in strength. Some are barely detectible while others send powerful radiation all the way to Earth, so scientists rate their strength on a power-ten system like earthquakes. Flares are ranked class A, B, C, M and X with each letter representing a 10-fold increase in strength. Within each letter, there is a finer scale from 1 to 9. X-class flares can go even higher than 9, so an X45 flare, the most powerful ever recorded (and adjusted upward from an original measurement of X28), is 45 times more powerful than an X1 flare, which is 10,000 times more powerful than an A class flare.

Luckily, the bulk of it just missed Earth.

Even so, SOHO managed to get some fascinating footage of the storming Sun, in a variety of wavelengths, highlighting what was happening in various layers of the solar atmosphere. Each movie is 30 seconds long and is compiled from the same two weeks of data.

In the first movie, SOHO's MDI (Michelson Doppler Imager) approximated a full-spectrum visible light image of the Sun from several narrow bandwidths of visible light. You can see several sunspot-dense regions in the photosphere:

At the same time, SOHO's extreme ultraviolet imaging telescope (EIT), set at 30.4 nm, captured this series of images in extreme ultraviolet:

30.4 nm is the emission line of singly ionized helium, mostly present at temperatures between 60,000°C and 80,000°C. This temperature range corresponds to an atmospheric layer of keen interest to scientists - the transition region between the chromosphere and the corona above it. This is the region you are seeing in the movie above.

The EIT also captured images set at 19.5 nm, capturing even more energetic UV radiation:

19.5 nm is the emission line of Fe XII, present at temperatures of over 1.5 million °C. Fe XII is a partly ionized iron ion with 11 of its electrons removed (Fe I is a neutral iron atom). There are relatively few iron ions present in the corona but they can be selectively viewed when their specific emission wavelength of 19.5 nm is picked out of the total emission spectrum. These images highlight activity in the hottest region of the corona, just above the transition region.

The same imager also captured a much larger field of 19.5 nm coronal radiation extending from the solar disk. The Sun's photosphere was blocked out entirely in order to capture this far dimmer radiation, and the Sun's 19.5 nm image is placed overtop to give viewers an idea of how much larger this field of view is:

The movie above shows how incredibly hot and energetic the plasma in solar flares can be.

CME's, flares and solar wind have a far-reaching impact on the planets and moons that orbit the Sun. In fact, the Sun impacts a region as large as 100 astronomical units (AU), or 15 billion km on average. We'll explore the Sun's region of influence, called the heliosphere, next, in the final article, The Sun Part 8.


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