Creating a Heliosphere
Plasma movement inside the Sun means there are great flows of moving charge. These moving charges generate intense and mobile magnetic fields that surround the Sun. The magnetic fields not only supply forces that push on and accelerate charged particles in the plasma but they also extend outward into space, having an impact. The Sun also contains an inner magnetic dynamo that greatly intensifies its magnetic field. We explored how the Sun's dynamo works in the second part of The Sun Part 5. Basically, plasma experiences mechanical shear in the region between the calm inner radiation zone and the outer turbulent convection zone. This shear force drags and distorts magnetic field lines, bringing them together and greatly amplifying the existing magnetic field. The Sun's magnetic field is so powerful that its influence extends well past Pluto, although its exact range is unknown. It creates a bubble of influence called the heliosphere. The heliosphere ends at an outer boundary called the heliopause, shown in the NASA artist's rendition below.
This bubble of influence is filled with solar wind, the charged particles and radiation that stream in all directions at an average velocity of around 400 m/s from the Sun's surface and corona. It might look like the space between objects in our solar system is just an empty vacuum but it isn't. It contains particles and it exerts pressure. The flow of particles from the Sun eventually meets up with interstellar space. Interstellar space too, is not a vacuum. It contains ions, gas atoms and dust (small molecules), as well as radiation from distant stars, all of which exert pressure against the heliosphere bubble, especially along its leading edge. Interstellar space bumps up against the outer edge of the heliosphere, slowing solar wind down and eventually overcoming it altogether.
The actual shape of the heliosphere (everything inside the heliopause labeled in the diagram above) appears to be more complex than the above image lets on. Because the Sun rotates (once very 27 days), its magnetic field corkscrews outward. If you could see the magnetic field lines above the poles, they would spiral upward like a slinky. The NOAA site models the real-time density and speed of plasma as if you are looking down at the North Pole of the Sun from a great distance. This information comes from the two stereo satellites of the NASA STEREO mission.
Below is a computer model image of the heliospheric current sheet. The Sun is shown in the middle. Earth is the tiny third planet out.
This image models the equatorial plane of the Sun's magnetic field. Rather than a smooth plane, it is ruffled like a ballerina's skirt because of the effect of the Sun's rotation on the plasma streaming from it.
The Sun Experiences Two Kinds Of Movement Through Interstellar Space
If you look again at the first heliosphere (blue/black) image, you can see red arrows coming in from the left. That is interstellar space bumping up against the heliosphere. This happens because the Sun isn't stationary. The Sun experiences two kinds of movement. It moves through space, following a kind of wavy orbit around the gravitational center of the Milky Way, shown below as a NASA artist's rendition.
The Sun is about 26,000 light years from the center of the Milky Way, and this means its orbital speed is very high, about 782,000 km/h. Even at this rate, it takes the Sun 226 million years to complete a single orbit. As the Sun orbits it moves up and down through the galactic plane. One complete up-down cycle takes 64 million years. Like the bow shock which is thought, until recently, to be part of the heliosheath (you can see it labeled in the blue/black heliosphere image far above), the galaxy itself has a bow shock. There is new evidence that the galaxy, in fact, has a powerful magnetosphere of its own. Scientists recently identified giant geysers of magnetized gamma-ray-emitting gas spewing from the galactic center, representing a tremendous amount of magnetic energy. There is an artist's image of these gamma ray geysers, or bubbles, later in this article.
The Milky Way galactic bow shock faces the Virgo Cluster as our galaxy moves toward it, and it should cause superheated gas and cosmic rays to stream behind. Cosmic rays are high-energy particles, mostly free protons, which usually come from supernovae but may also come from galactic bow shock. Cosmic ray energy varies widely from 109 eV to 1021 eV. A (subatomic) proton with 1021 eV of energy has as much kinetic energy as a 90-km/h baseball. We will explore cosmic rays in more detail later on in this article. It's not known how energetic galactic bow shock cosmic rays are, if they exist.
Life on Earth is generally not impacted by cosmic rays. Almost all but the most energetic cosmic rays are blocked out by the heliosphere and Earth's atmosphere, but cosmic radiation does become more significant with altitude and it splits ozone in the ozone layer, which protects life from damaging ultraviolet radiation. DNA is particularly vulnerable to high-energy radiation so even small increase of cosmic rays striking the surface could have disastrous effects on life.
Scientists speculate that once every 64 million years, when the solar system emerges from the denser galactic disk, the solar system may be more exposed to these cosmic rays, leading to more penetrating the Sun's and Earth's protective magnetospheres. Marine fossil records offer some support. They show that biodiversity increases and decreases on a roughly similar 62 million-year cycle. Scientists are continuing to look for links between the Sun's galactic environment and past extinction events. This line of research though relatively new, highlights how important the heliosphere is in protecting life on Earth from cosmic radiation. We will revisit this idea too, later on in this article.
The Sun's orbiting motion would not be the cause of heliospheric bow shock. This is because all the other stars and gas (at this radius) are generally moving at the same rate. They all move together in the same giant swirl, so the Sun is not invading "new" interstellar space. If it were, there would indeed be bow shock. At 782,000 km/h, the forces would be disastrous. The Sun also experiences movement relative to its neighbours and the neighbouring interstellar medium, and here, the Sun is invading "new" interstellar space. For decades, researchers believed this movement created bow shock at the leading edge of the heliosphere. Let's first examine what bow shock is.
The Edge of the Helioshpere: Bow Shock?
Solar wind and interstellar space can both be technically called cold plasmas, thanks to their significant but highly diffuse ionized content. Like other fluids, air and water for example, the Sun's heliosphere, until recently, was expected to experience bow shock along its leading edge as the Sun moves through interstellar space, in the same way that a high speed jet creates a sonic boom in the air when it accelerates past the speed of sound, past Mach 1.
Pressure waves get compressed ahead of the jet (and the Sun). In a sonic boom, they are forced to merge into one giant shock wave that travels at the speed of sound. When a jet flies faster than Mach 1, the pressure wave can't keep up. It creates a boom carpet on the ground that trails the jet (with a width that depends on the jet's altitude). Along this carpet, windows rattle and people hear a cracking boom that lasts about a tenth of a second, much louder than the crack of a bullwhip or fireworks, which are also (smaller) sonic booms.
Sound is a mechanical disturbance traveling through a medium, which can be gas, liquid or solid. Space acts like an extremely diffuse gas. There may be enough particles to technically carry a disturbance but a listening instrument would have to be extremely sensitive to pick such "space sounds" up. Instead, scientific instruments in space "listen" to electromagnetic waves - which don't require a medium to travel through - such as radio waves, microwaves and light, for example The bow shock of the heliosphere would work a bit differently than a sonic boom but it follows the same basic idea. The heliospheric shock wave should trail the Sun the same way as sonic boom does, but from here the mechanism differs. The disturbance of a sound wave in air moves through relatively stationary gas molecules. In solar wind, the particles themselves are moving, and their movement is influenced by the magnetic field they are embedded in.
The charged particles in the solar wind follow helical paths around magnetic field lines that extend from the Sun through interplanetary space. The velocity of these gyrating or corkscrewing particles can be treated the same as thermal velocity in a gas. In an ideal gas, the average thermal velocity is close to the speed of sound through that gas. At bow shock, particles in the supersonic interstellar wind bunch up at the front of the shock, as heliospheric pressure waves bunch up against each other. The velocity of the solar wind (the vector component of the particle velocity that is forward-directed) is forced below the speed at which the particles are gyrating. In this way, it quickly goes from supersonic to subsonic, and it is the quick change in density that is the bow shock itself, just like the air/jet example. Many examples of visible bow shock have been observed throughout the galaxy, leading researchers to believe the Sun should be no exception. This Hubble image of a star in the Orion Nebula below is a great example of a star's heliosheath experiencing bow shock.
You can see very clearly where the bow shock is, to the left of the star where glowing plasma and gases are colliding and piling up.
Continuing with this model, behind the bow shock, closer to the Sun, a point is reached where the pressure from interstellar plasma no longer exceeds the pressure from solar interplanetary plasma. It equals it. This point is called the heliopause. Just outside the heliopause, solar winds are no longer strong enough to push back the interstellar medium, and its outer edge should be marked by three changes - a drop in particle energy, a change in the direction of the magnetic field and an increase in the number of cosmic rays. The solar wind is essentially stopped. Termination shock is located just inside the heliopause, where the solar wind (the forward vector velocity component) hasn't yet been stopped but it abruptly slows to subsonic speed as it begins to feel the effects of interstellar wind pushing back on it.
Scrolling down you will see another NASA image of the heliosheath. Still operational, Voyager 1 approached the edge of the heliosphere last year (2012). The approach was signaled by a sharp increase in cosmic rays hitting it (many researchers agree that it passed termination shock in 2004). As Voyager 1 exits the heliosphere, scientists expect to see the direction of the magnetic field around it to change from the Sun's magnetic field to the direction of the interstellar magnetic field, but there is some current debate that it may have already exited the heliosphere. A sudden drop in lower energy particles detected along with a sudden rise in high-energy cosmic rays points to an exit, but the expected change in magnetic field didn't happen yet. Unfortunately, Voyager 1's magnetic field detector stopped working in 1990, so researchers have to infer the change in direction from other instruments onboard. Nevertheless, some experts mark the exit date at August 2012. Many other researchers believe Voyager 1 might be in a new unknown region, one that is not modeled for yet.
NASA's IBEX Imager Dispels the Heliospheric Bow Shock Model
NASA's Interstellar Boundary Explorer (IBEX), launched in 2008, has imaged the entire interstellar boundary by imaging the glow from energetic neutral atoms (ENA). ENA imaging is an ingenious way to see things that are otherwise invisible. It works like this:
The new ENA shoots off in a straight line, shown below right, with almost the same velocity as the original energetic (fast) hydrogen ion. This atom, no longer bound by the magnetic field, glows momentarily much like a hot wire glows, and that glow is picked up by the imager.
ENA imagers can "see" planetary magnetospheres and the heliospheric boundary, where these collisions are most frequent, as there is a higher abundance of neutral atoms, especially hydrogen, in these regions.
The IBEX ENA imager found some unexpected things. First, it "saw" a narrow ribbon brighter than anything else in the heliopause region of space. The image below shows what it looked like in 2009, shown painted on the heliopause, with the locations of Voyager 1 and 2 included for reference. Six months later the image changed significantly.
No one knows yet exactly what process is creating the ENA ribbon, and the boundary itself is not a smooth bubble at all. It is an ever-changing three-dimensional ribbon that is far less uniform than once thought. IBEX has even discovered temporary knots in it. The first batch of data came from the solar minimum; researchers are eager to see how more intense and variable solar wind from the present solar maximum effects the boundary as it reaches it.
IBEX also measured the speeds of interstellar particles entering the heliospheric boundary and it found something even more surprising - the Sun is moving significantly slower than once thought - 83,700 km/h rather than around 95,000 km/h.
This motion should not be confused with the Sun's much higher orbital speed, mentioned earlier. Every star in the Milky Way experiences a very regular orbital motion and velocity, which can be calculated by using a reference star closer to the galactic center. Many stars also experience at least some random motion among other stars, called proper motion. Most stars are fairly fixed in their regular orbital loops but some are not. Barnard's star is an extreme example. It is shooting off radially outward from the galactic center at a right angle to all of its stellar neighbours, including the Sun. To calculate the Sun's random (proper) motion speed prior to the IBEX data, scientists used the average motion of other stars in the Sun's vast neighbourhood and measured the Sun's motion relative to it, a less reliable and less direct method.
No Bow Shock For the Sun
The Sun's new adjusted speed is not sufficient to create bow shock at the boundary. The forward vector component of the solar wind particles is no longer high enough to be supersonic. It should create a bow wave instead, where pressure waves have enough time to spread out ahead of the moving heliosheath, like a wave front at the bow of a ship. This way, particles avoid a rapid drop in pressure (shock). No bow shock changes how the solar system is modeled. All the models that include the effects of bow shock must now be tossed out. Keep in mind that the two images of the heliosphere in this article are now outdated. No bow shock means that the heliosphere is not compressed and weakened nearly as much as once thought. It may protect the solar system from cosmic rays much better than predicted. The heliopshere has just become even more interesting!
In addition to a new intensified interest in the mechanics of the heliosheath, scientists also wonder how protection from cosmic ray flux, which varies as the Sun enters new regions of the galaxy along its rotation period, might have influenced evolution on Earth, and how the heliospheres of other stars might influence life in their solar systems, if life elsewhere exists. Should a star's heliosheath be examined, along with the presence of liquid water, for example, when scientists look for habitable alien worlds? These are questions that researchers, with increasing imaging sophistication, may be able to test.
The Heliosphere And The 11-Year Solar Cycle
When the magnetic field diminishes before it suddenly changes polarity during solar maximum, the heliosphere doesn't just shut off. Other complicated magnetic field structures, which are not fully understood, appear to fill the void while a new oppositely directed field builds up. The heliosphere, however, is influenced by solar wind. Just how much its shape and size change along with variations in solar weather are questions the IBEX imager might soon answer.
Sun As Protector
The heliopause is the outermost boundary of the solar wind. Although the Sun itself bombards Earth with radiation and high-speed particles, the Sun's powerful magnetosphere protects Earth from an even greater threat - galactic radiation. The heliosphere deflects about 90% of galactic cosmic rays and Earth's magnetosphere deflects most of the rest of it.
Cosmic "Rays" And Gamma "Rays"
Along with our magnetosphere, our atmosphere also protects us. It blocks cosmic rays with energies below 1GeV. 1 GeV is one billion, or 1 x 109, electron volts. This is the bottom of the cosmic ray energy scale. I mentioned earlier that cosmic rays generally range in energy from 109 to 1021 eV and possibly even higher. The most energetic cosmic rays discovered so far are protons accelerated by incredibly strong magnetic fields associated with black holes in the centers of active galaxies. Cosmic rays are particles with mass that have enormous velocity, so they carry incredible momentum. You may be tempted to confuse these with gamma rays, which also have very high energy. "Ray" is not an especially good name for either phenomenon. Robert Millikan first coined the term "cosmic ray" in the 1920's, believing that what he observed in his experiments was EM radiation. Cosmic rays are actually energetic particles with mass, most of which are protons (hydrogen nuclei, 99%) but alpha particles (helium nuclei, 1%) and electrons (1%) also make up primary rays. When these ultra-fast particles collide with other particles in space or in atmospheres, they create new particles upon collision and these are called secondary cosmic rays. Gamma rays (and X-rays), on the other hand, are high-energy massless photons of electromagnetic (EM) radiation. They don't interact with magnetic fields like cosmic rays do, having no charge to them. But photons, too, have momentum even though they do not have mass. The momentum of a photon is not simply mass x velocity. It is better called relativistic momentum where relativistic mass (energy has a mass equivalent) replaces classical mass.
Gamma rays have the highest energy of all EM photons, but even the highest energy gamma rays (1.6 x 1013 eV, detected in 2011 coming from an active galaxy called Markarian 421) do not approach the extremes of cosmic ray energy.
To get a sense of scale, only gamma ray photons have energies that approach 1 GeV (1 billion electron volts), equivalent to the lowest energy cosmic ray particles. A photon of visible light has around 2 eV of energy, while UVA and UVB ultraviolet photons, between 3 and 4.5 eV respectively, have enough energy to cause sunburn and skin damage. Cosmic EM radiation (not cosmic rays) consists not only of gamma rays but also of photons of all energies. That's why astronomers can capture distant stars and gases, using UV, ultraviolet, X-ray, visible light and radio wave imagers, for example. Cosmic radiation (both particles and photons) comes from many sources in the universe, such as the centers (black holes) of active galaxies, supernovae, quasars and gamma ray bursts. It consists of cosmic EM radiation (gamma rays, X-rays, etc.) as well as electrons and the nuclei of elements (protons, alpha particles and a few even larger nuclei) blasted out into space at almost the speed of light.
Cosmic Radiation and Astronauts
All of these kinds of radiation have tremendous energy and momentum, so they are very dangerous to astronauts outside of Earth's protective atmosphere, where radiation (both particles and gamma rays) easily penetrates spacecraft and people. Some online articles suggest that electromagnetic shielding might someday be used to protect astronauts from gamma rays while on an eventual manned trip to Mars, but, as I understand it, this would be challenging. EM shielding generally consists of a metal mesh or foam where the holes are smaller than the wavelength of the EM radiation; the EM radiation gets reflected away from the surface. If you look at a microwave oven door you can see a fine mesh. This mesh keeps microwaves in but it lets visible light (smaller wavelengths) through so you can see inside the oven. Gamma ray wavelengths are extremely short, less than a trillionth of a metre, so small they can get right into atoms. However, relatively dense and thick materials, like our atmosphere, block gamma radiation very effectively and we will explore how that works later on in this article.
Cosmic ray (particle) shielding is also still a work in progress. One (ingenious and perhaps gross) idea is to use the astronaut's poop, solving two "problems" at once. At least this is on the table for a planned 2018 private mission. The water content in poop supplies a large (particle-absorbing) nuclei-to-volume ratio. I suspect it would absorb some gamma rays as well. Magnetic shielding has been studied more extensively, and this might be a more reliable option for astronauts. A current-carrying ring or bubble could enclose a space capsule, inducing a particle-deflecting magnetic field, but the ring would have to generate a very intense magnetic field in order to deflect very energetic cosmic protons, etc., and astronauts inside the bubble would be subjected to prolonged intense magnetic field exposure with unknown consequences. Cosmic radiation shielding is a challenging problem that is under intense investigation as NASA prepares to ultimately send astronauts to Mars. Unless shielding can be improved, these people will be subjected to radiation similar to what Apollo astronauts experienced (between 1.2 and 1.4 mSv/day - about the same as a CT scan) and for a much longer period of time (and the destination, Mars, has very little protective atmosphere and magnetosphere).
The Heliosphere - A Bubble of Calm in a Violent Cosmic Sea
The center of the Milky Way glows with gamma rays, as galactic wind particles slam into interstellar dust and gas. In 2010, NASA's Fermi Gamma Ray Space Telescope (GLAST) discovered two gigantic gamma ray bubbles (shown below in a NASA artist's rendition) coming from the center of the Milky Way. These gamma ray bubbles, extending 30,000 light years from the center, likely come from a past violent episode of black hole consumption. There is a black hole (Sagittarius A*) with a mass nearly 4 million times the mass of the Sun lurking at the center of the Milky Way. Usually, it gives off only faint radiation as it consumes matter, but starting 30,000 years ago, it experienced at least one, and possibly a series of outbursts of activity.
The Sun is about 26,000 light years from the galactic center, so what do these bubbles of gamma rays mean for us? Apparently, our heliosphere has proven itself to be a good protector, as these gamma rays have already reached us causing no apparent damage (they've reached, and been detected by, GLAST, which is in low Earth orbit).
How the Heliosphere Protects Earth Against Cosmic Rays and Gamma Rays
Nearly all cosmic rays are deflected by the magnetic field of the Sun (as well as Earth's magnetic field). The magnetic field lines bend their trajectories away because they are charged particles acted on by the Lorentz force.
Gamma rays, unlike plasma particles, do not interact with the magnetic field of the heliosphere (with the exception of vacuum birefringence in very strong magnetic fields such as around magnetars. In order to be attenuated, they must be absorbed by the nuclei in atoms or plasma. There are three mechanisms that do this, based on the energy of the gamma photons.
When a lower energy (< 50keV) gamma ray photon strikes an atomic nucleus, it causes an electron to be ejected from the atom. The electron has the same energy as the gamma photon minus the binding energy of that electron, resulting in an overall dampening effect. This is called the photoelectric effect.
The photoelectric effect becomes less significant for gamma photons with energies between 100 keV and 10 MeV. Here, Compton scattering dampens the energy of gamma photons. The gamma photon strikes a nucleus and an electron is ejected, but in this case, another new gamma photon is also released. Its energy is lower than original gamma photon and its trajectory is different. This is why it's called "scattering."
Cosmic radiation gamma photons tend to have far higher energies, exceeding 10 million eV (or 10MeV). Here, energy absorption (attenuation) comes mostly through pair production. A gamma photon interacts with the electric field of a nucleus, which converts it into the rest mass of an electron-positron pair. Any energy the gamma photon had that exceeded the pair's rest mass becomes the new particle's kinetic energy as well as the recoil velocity of the nucleus. The electron strikes other particles and the positron strikes another atom's electron, annihilating with it and emitting two new gamma photons (with a minimum energy of 0.5 MeV), causing Compton scattering and so on.
Any of the free electrons released or produced in these processes has enough energy to cause significant ionization of atoms struck by the radiation. Cosmic gamma photons strike mostly bare protons in heliospheric plasma, so pair production is the significant energy absorption mechanism.
Earth's Atmosphere Offers the Best Gamma Radiation Protection
A thick slab of lead, for example, has large nuclei and a high density, so it offers good gamma ray shielding, especially to lower energy gamma photons, but Earth's atmosphere, because it is so thick (there are a lot of atoms to go through), is actually much more effective. In fact, it offers almost 100% protection from cosmic gamma rays. It's almost totally opaque to them. Earth, however, is vulnerable to a gamma ray burst. A gamma ray burst is the most powerful explosion in the universe, and it is made up of the highest energy photons known in the universe.
Luckily, two factors are on our side. First, gamma ray bursts are only created when very massive stars collapse as supernova explosions or when two stars collide, and only a few occur in the Milky Way every million years. Second, these bursts shoot very narrow beams of radiation, especially in the case of polar jets. A beam of radiation has to be directed right at Earth in order to do any damage. The image below shows how a massive star could collapse into a black hole with a jet of gamma rays (gamma ray burst) emanating from the center of its axis of rotation.
|Nicolle Rager Fuller of the NSF;Wikipedia|
The Indirect Threat Of a Gamma Ray Burst
Ground level effects on Earth from a gamma ray burst would be indirect. Such a burst, if close enough, would have significant effects on the chemistry of the atmosphere. High-energy gamma photons would break apart molecules of nitrogen (N2) and oxygen (O2) and temporarily ionize them. The atoms would then recombine into significant amounts of nitrous oxides, NO and NO2 (which would then be split by a gamma photon into NO). These molecules do three kinds of damage.
First, they deplete the ozone layer (O3) by reacting with it. The ozone layer is built up by UV light from the Sun striking O2 molecules and splitting them into two O atoms. Each O atom quickly reacts with two more O2 molecules, creating two O3 molecules. O3 is unstable so it soon splits back into an O2 and an O. This cyclic process maintains a balance between O2 and O3 in the upper stratosphere, maintaining the ozone layer. NO catalyzes the oxygen recombination reaction, reducing O3 abundance (at ground level, in contrast, NO2 contributes to ozone pollution). With the ozone layer stripped away, deadly UV radiation will flood Earth's surface. Increased UV radiation, persisting for perhaps many years before the ozone layer is replenished, would be the single most deadly threat of a gamma ray burst.
Second, NO2 is a brown gas so it may also significantly reduce sunlight striking the ground (it absorbs in the visible range so it will provide no UV protection). Reduced solar insolation, as it is called, might be enough to lead to an extended period of climate cooling.
Third, the only way the atmosphere can return to normal and remove nitrous oxides is by precipitating nitric acid (HNO3). Some experts think the amount (and rate) of acid deposited might be harmful to some life such as thin-skinned amphibians but plants might not be as seriously damaged and they might even benefit from the increased availability of nitrogen.
B. C. Thomas at the Department of Physics and Astronomy believes that the atmosphere, based on current modeling, would recover from a typical gamma ray burst in a little over a decade, so Earth itself would recover quite quickly, leaving the question of how life would recover from increased UV radiation. This kind of radiation damages DNA molecules and it may lower metabolic rates, photosynthetic capacity and delay or arrest cell division. In multicellular organisms it could delay development, cause birth effects and increase cancer rates. Some researchers believe that Earth may already have been hit with a gamma ray burst, triggering the Ordovician mass extinction, 440 million years ago, when roughly two thirds of marine life (all animal life was marine then) went extinct.
Even an event several thousand light years from Earth could destroy half the ozone layer and possibly trigger a mass extinction.
The Sun Conclusion
The universe, while seemingly almost timeless to us, is a very violent place. The Sun exists among countless other stars, where physical forces are continuously at play on an incredibly large scale.
The Sun is made of space gas. Mostly hydrogen and a little helium were squeezed hard enough and in enough quantity to create an incredibly complex ongoing spherical nuclear fusion explosion. Life on Earth evolved to make use of that energy.
The physics of the universe makes stars like our Sun inevitable and perhaps even life, relying on their energy, inevitable as well. We humans are very transient delicate beings. Eventually, all Earth life will perish as the Sun evolves into a white dwarf, and this remnant may last long enough to bear witness to the end of the universe itself.