Wednesday, February 13, 2008

Getting to Know the Sun (II)

Sunspots, like weather systems (which, incidentally arise from solar heating effects) are not subject to a high-order accuracy of predictions. Unlike billiard balls and planets, whose positions can be predicted to near perfection - sunspots are large thermodynamic and magnetic systems. To be truthful - solar researchers at this stage are not even sure of the interplay between sunspot magnetism and thermodynamics, far less predicting how they combine to yield flares or other effects (sprays, surges, prominences). The precise role of each of these phenomena to each other is also an ongoing source of contention between solar researchers.[1]

Consider the sunspot itself. It appears to be just a simple dark blemish on the solar surface, or photosphere. In fact, the darkness is purely an illusion resulting from its lower temperature (about 6,000 F) compared to the surrounding photosphere (11,000 F). The sunspot's "single" state is also somewhat illusory - since nearly all sunspots occur in pairs of differing polarity, like the north and south poles of a magnet. (Solar astronomers call them "plus" and "minus" poles).

A question of long standing seems to be how a cooler gas[2] can be immersed in a much hotter gas without itself reaching a higher temperature. This seems to defy one of the laws of thermodynamics. Investigations have only recently disclosed the underlying reason: the powerful magnetic fields - up to 5,000 times stronger than the Earth's. These incredibly powerful magnetic fields trap the cooler gases inside a confined region (tube) which actually "floats" on the hotter, less dense medium of the photosphere.

In the photo (via link) below is a typical, large sunspot pair, as captured in a catadioptric telescope with solar filter. The image embodies how spots are (currently) theorized to be at opposite ends of a "magnetic tube", spanning opposite magnetic polarities. In this case the leader spot (lower right) is at one end of the tube, and the (partly bifurcated) followe (upper left) is at the other.


http://groups.msn.com/PlanetStahl/mathphysics.msnw?action=ShowPhoto&PhotoID=41



From the photo, the leader spot is at the positive polarity end of the magnetic loop containing the cooler gases, while the followe is at the negative polarity end. The key aspect to note in the above is that sunspots are pictured as sections of magnetic loops seen in projection. The darkest regions occur where the trapped gases are coolest and the magnetic fields strongest. Sunspots themselves display two distinct regions: a dark, central umbra and an outer, lighter penumbra - with the latter at the (somewhat) higher temperature.

Why are sunspots so important? For one thing, they may signify that the Sun is a variable star - or at least not as well-behaved as humans surmise. For example, if the so-called "Maunder Minimum" is a genuine effect, then the reduced numbers of sunspots may well have ushered in the "little ice age" from 1645-1715. And if a "little ice age" can be ushered in by sunspots (or rather their absence) then perhaps a major ice age could be. In any case, the answer won't be found unless sunspots are studied closely, to reveal any long term periodicity in their behavior. (Currently, a new consensus is emerging that no new ice ages will occur once the carbon dioxide concentration exceeds 450 parts per million - which may happen in the next fifty years)

As it is, astronomers know that the Sun exhibits an 11-year average sunspot cycle. This means that every eleven years - on average, sunspot numbers reach a peak - what is called the "sunspot maximum". Superimposed on this is a 22-year cycle, for reversal of magnetic polarities to occur in the "leading" spot. To fix ideas, consider again the photo above: Then the "leader" spot and the "follower" will exchange polarities in the next cycle, assuming that their orientation is in the direction of the Sun's rotation (east to west). The 22-year cycle means that 22 years (again, on average) must elapse before leader spots again have negative polarities at sunspot maximum. (In the intervening, or 11-year maximum, the leaders will all have positive polarity).

For some reason, very large sunspots with "complex" magnetic structures(that is, multiple +/- polarities in the same sunspot)can become unstable and given rise to extremely violent flares. This happened in August, 1972- with a giant flare hurling lethal protons at the Earth that would have killed any astronauts. It also occurred in March, 1989, when a giant sunspot group went unstable and precipitated the flare which downed the Ottawa power grid.

A 1992 movie with Charlton Heston, entitled Solar Crisis, depicted a future time in which the Earth is threatened by an impending "mega" flare that is forecast to tear away the atmosphere and shower deadly radiation on everything. Could such a fantastic flare actually occur - and could it be predicted as accurately as in the movie? This is doubtful on both counts.

For one thing, the energy of flares is limited by how much "free" (extractable) energy can be stored in magnetic tubes. A good analogy is a rubber band. If a rubber band is wound up over and over it gains free potential energy (available for future motion). Release it now and it will rapidly twist apart to its original state. Magnetic tubes on the Sun do something very similar. They are twisted up by the motions of the Sun's turbulent surface[3] - and store free magnetic energy as they twist. The more twisted they are, the more free magnetic energy they acquire - to power flares, and the particle bursts that accompany flares.

Flare energy is limited because: 1- there is a limit to the size a magnetic tube can attain, 2- there is a limit to how much twist a tube can acquire. Each of these limits the amount of total magnetic energy available for release. On that basis, it is unlikely that humans will ever see flares greater than those which occurred in August, 1972 and March, 1989. Obviously, if the Sun is a variable star - its physical conditions could change. One of these conditions is its rate of "differential" rotation[4] , which is believed to be responsible for the origin of sunspot magnetic fields. On that assumption, a higher rate of differential rotation could portend much more violent flares, precisely because magnetic tubes can acquire stronger magnetic fields (greater twist).

A much more credible risk from the Sun would be an inherent variability leading to temperature differences, and weather changes on Earth. At the present time, all the evidence indicates that whatever major solar variability exists is on a very long time scale, say ten thousand years. However, as seen earlier, there is nothing to rule out shorter term variability being superimposed on the longer term. In effect, the Sun could very well undergo smaller percentage changes in its energy output over smaller time scales - say 100 years or less[5]. The only way to determine if short term variability exists is to have solar satellites in place for detailed observations over extended times. At the present time, such satellites are not in any budget. Indeed, the last major solar satellite effort was the "Solar Max", which is now defunct. The SOT, or Solar Optical Telescope, was to be launched in 1985-86, but that was killed in a single budget cut - much to the consternation of solar researchers such as myself.

If advanced extraterrestrials were to pay us a visit, they would probably be astounded at a so-called intelligent species that sends all sorts of satellites and vehicles into apace - none of which is dedicated to continuously observing the home planet's nearest star! I would surmise that a truly intelligent species would place solar observations (e.g. of its own star) as having the highest priority of any space-based research - certainly over space shuttles, planetary probes and spy satellites. There wouldn't be any mystery to a solar priority, since after all the Sun is the 'hub of life' for all living organisms anywhere in the solar system. If the Sun's nuclear reactions 'turned off' for just a day[6] - all life on Earth would be frozen out of existence before the Sun's nuclear furnace switched on again. In other words, for all their technological prowess, humans would not survive even a day without the benefit of the Sun's life giving warmth and light.

For a physical perspective, it is useful to regard the Earth as situated inside the Sun. Technically, this is accurate - since Earth is literally bathed in the "solar wind" - a rapidly moving, tenuous gas continuously cast off from the Sun's corona, or outermost atmosphere. Our total understanding promises to be significantly enhanced with the Ulysses spacecraft - which will include passes over the solar polar regions.[7]

We now understand that the Earth - and all the other planets, originated as hot 'globs' of gas spun off from the bloated infant Sun. As each of these gaseous globs separated, then cooled and assumed its own orbit, it carried away some of the Sun's rotational motion. The legacy of this 'planetary secession' is that the Sun now rotates much more slowly (about once in 27 days) than it did in that early epoch.

The ironic end of the story is that one day in the distant future, an aged and inflated 'red giant' Sun will reclaim most of its original pieces. No life of any form will survive since the Earth will effectively be engulfed. All records of human habitation will be completely obliterated in raging firestorms long before the planet itself is reduced to a cinder. If any single, cogent reason can be advanced to induce humans to colonize other worlds, this is it.

[1] This was also a source of controversy at the 75th AGU Conference mentioned in the earlier footnote. At the May 26th, 1994 Thursday morning session, for example, solar physicist Hal Zirin and space physicist A.J. Hundhausen totally disagreed on whether solar flares led to coronal mass ejections or the other way around. Zirin steadfastly maintained that flares caused the mass ejections, while Hundhausen just as strongly maintained the reverse was the more accurate hypothesis. Such stark disagreements actually show the glaring need for a higher spatial resolution for solar-observing instruments. At least this was the general consensus that emerged from the Space Weather sessions I attended.
[2] I am using the term "gas" here, but actually the Sun's material is plasma. A gas - like oxygen at room temperature, has all its electrons. A plasma, on the contrary, is missing one or more electrons because of the high temperature it is subjected to. We say it is "once ionized" (one electron lost) "twice ionized" and so on. The significance of electrons being lost is that the gas becomes electrically conducting. Thus, a hydrogen plasma in the Sun will conduct electrical currents.
[3] In fact, the Sun has no definite surface, because it is a gaseous body. The 'surface' usually referred to, is that from which visible light emanates: the photosphere ('light-sphere'). It is very dense relative to higher layers, and is where most of the turbulent motions occur that can twist magnetic tubes.
[4] Differential rotation applies to a rotating fluid or gas. It means that the rate of rotation depends on the position of a point on the surface. In the case of the Sun, the fastest rotation is at the lowest latitudes, near the equator.
[5] There are some who claim variability arises from external changes in the Earth's orbit, rather than from internal changes in the Sun. Most often, the "Milankovitch Effect" is mentioned - whereby the Earth supposedly makes significant orbital changes, leading to much higher or lower mean global temperatures - depending on whether it is much nearer or farther from the Sun. At the present time, the solar research community gives very little credence to the "Milankovitch Effect", since there is no compelling evidence that it is significant enough to affect climate.
[6] Of course, there would be a long delay between the 'turn off' and when it was noticed on Earth! The average time needed for photons in the Sun's core (where nuclear reactions occur) to filter to the solar surface and escape - is about 1 million years. Thus, if all nuclear reactions shut down today - then the Sun would only 'black out' in one million more years (on average).
[7] See the papers on Ulysses findings: Science: Vol.268, May 19, 1995.

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