Not very often, I'd suppose, because if all of us managed to look at the sun that way, many of Earth's and the universe's beauties would become explicable. And, at least in my opinion, that would only make them more beautiful.
[caption id="attachment_22709" align="aligncenter" width="512" caption="An ultraviolet image of the sun taken at the time of emission of the CME (NASA)"]
[/caption]Last week, there was a great flurry of activity after our nearest star unleashed a massive coronal mass ejection (CME) that ranked in the 'X' compartment on the scale of flare strength. What's more, the flare was pointed directly at Earth, prompting astrophysicists to let loose warnings about electricity grids, satellite operations, GPS reception and communication networks being severely disrupted.
However, the stream of high-temperature high-energy particles belched by the sun - that's what makes a CME - didn't quite have the effect it should have had. When scientists started to ask why, they found a simple explanation: the magnetic field embedded in the ejection was oriented in such a way to the earth's own that it was fizzled out on contact.
[caption id="attachment_22710" align="aligncenter" width="545" caption="Streams of particles along the magnetic flux lines on the sun's surface are visible"]
[/caption]The sun's magnetic field is extremely strong and, because of the star's high-temperature surface, such fields influence the formation and paths of charged particles that exist in a plasma state (the fourth state of matter). Sometimes, these particles are shot out in the form of a flare when magnetic energy stored in the sun's corona suddenly jumps a few orders of magnitude, accelerating the heavier ions to speeds near that of light. Needless to say, this process is also linked to a big jump in temperature.
More often than not, the radiation emitted during such a flare is across the electromagnetic spectrum and not quite just localized to the visible region. Because of this, most flares are observable only by special instruments and not the naked eye.
[caption id="attachment_22711" align="aligncenter" width="545" caption="An illustration of solar wind and its impact with Earth's magnetic field"]
[/caption]Now, what causes this jump in magnetic activity? To understand that, observe the sun's core: at a temperature of 15,000,000 kelvin, it is the hottest place in the solar system. By comparison, the surface of the sun exists at a meagre 6,000 kelvin. Because of the temperature gradient between the two, there is a mass convection going on at all times: gases move from the hotter core to the cooler surface. However, they don't travel in a straight line. This is because of two mutually-acting reasons.
- When stars are formed, they have an accretion process during which they suck in gases from around them because of the steadily building gravitational force, and to conserve momenta, they're made to go around. Therefore, stars are born spinning.
- They do have the option to stop, but that would violate the law of conservation of energy. To keep from misbehaving like that, they conserve their angular momentum by keeping themselves revolving about an axis.
This energy conservation manifests itself in the gases in the convection current as well. Instead of moving straight - during which they're not conserving the star's angular momentum - they move out in a spiral. The speed at which they move out to a particular point is dependent mostly on the temperature gradient between the core and the surface at that point. Because of this uncertainty, gases at different parts of the sun's surface are moving at different velocities, as a result giving the sun a differential rotation: the speed of rotation at each solar latitude is different.
[caption id="attachment_22712" align="aligncenter" width="545" caption="Stars near the centre of the Milky Way galaxy take less time to orbit it than stars farther away. This difference in orbital periods gives rise to a spiral pattern much as differential rotation on the sun gives rise to spiral convection."]
[/caption]Consequently: though the sun's magnetic field possesses an overall shape and size that is measurable and fairly fixed, it's local magnetic fields are the troublesome ones. For example (and this being a conveniently chosen example), when one band of convection is shearing against another band moving at different speeds, a zone of high mechanical and thermodynamic stress is created.
When this stress no longer becomes bearable, a "wound" is punctured on the sun's surface. This creates a no-convection region that limits the amount of energy coming in from the core to the wound, cooling it. As the temperature falls, so does visibility, and a sunspot appears on the surface.
CMEs and flare activity are particularly energetic around sunspots because there, the magnetic field is very strong and oriented in such a way that it connects the sun's corona to its interior. In fact, at a sunspot, the component of the magnetic field perpendicular to the sun's surface is more pronounced than the other, inclined, component, providing a sort of rails on which charged particles can be accelerated and shot out.
[youtube http://www.youtube.com/watch?v=uecMk8ZZ1uE]
The rate of formation of sunspots affects solar storm activity, and this rate is dependent on whether the sun is in its solar maximum or solar minimum. During solar maximum, which lasts for a period of 11 years on average, the magnetic field at the solar equator is rotating slightly faster than that at the solar poles, producing enough heliomagnetic stress to influence the formation of hundreds of sunspots. Needless to say, solar storms can get really vicious when our star is in its solar maximum or is in the process of entering it - like now. The solar minimum, on the other hand, lasts for about 12 months, seeing decreased sunspot activity.
When such storms come in contact with the Earth's magnetic field, which they don't often do because they're pointed in other directions, they're called geomagnetic storms. Yes, this results in the disruption of our communication network, but it also gifts us the auroras that are a treat to watch. The storm that hit us a few days ago as a result of the massive 'X' class CME last weak fared poorly, being relegated to the 'G' class.
When the charged and heated solar wind reaches Earth, the particles come in contact with the geomagnetic field, transferring a bulk of their energy to it and increasing the movement of plasma through the magnetosphere. Some of this energy transferred manifests as increased electric currents in the ionosphere, which is what disrupts our communication.
[caption id="attachment_22713" align="aligncenter" width="519" caption="The magnetosphere engulfs all these layers in a bullet shaped envelope and extends far out into space, almost as much as 127,000 km to 160,000 km in the direction away from the sun."]
[/caption]However, the principle effect of the currents is increase the magnetic force in Earth's magnetosphere, pushing the boundary between the layer and the malevolent solar wind out, as if into space. A geomagnetic storm may last from a few minutes to a few days.
The auroras they create, however, last for only a few hours each time. During the geomagnetic storm, there is an immigration of particles from the magnetosphere and the ionosphere to the atmospheric thermosphere. Here, the incidence of oxygen and nitrogen atoms is relatively higher than it is at the ground and collision rates are sparse enough to admit ionization of their atoms.
Once an atom is ionized, it's said to be in the excited state, and returns to the ground state either by gaining or by losing an electron. The jump from excited to ground states is characterized by a loss of energy that shows up as green, red or blue light as a curtain in the night sky, usually in the latitudinal region between 3° and 6°.
[caption id="attachment_22706" align="aligncenter" width="545" caption="Images of auroras. The image on the second row and first coloumn is an incidence of aurora australis."]
[/caption]CMEs and flares are not the only reasons a geomagnetic storm may occur. For example, there is a solar proton storm, which causes the acceleration of protons emitted by the sun to near-light speeds either by increased heliomagnetic activity or by the shock of CME-release. There are also geomagnetically induced currents that are caused by variations in interplanetary space weather or by changes in the dynamo action of Earth's core.
Such is the relationship between the gorgeous Sun and our home planet, one that extends far beyond the telltale gravitational pull, one that is from core to core. Now that the sun is entering a period of solar maximum, geomagnetic activity will be on the rise, and with that, stronger greener, redder and bluer auroras, annoying signal and network disruptions, and grander flares, coronal mass ejections, and heliomagnetic activity that let us admire the Sun for what it is and understand the solar system better.
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