 Solar activity and the solar cycle
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 Podcast
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Intro
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Some recent news items related to solar activity, and its cyclic nature. Also, are brought up in discussions of climate change from time to time, so I thought this'd be a timely subject. Also see a related blog post this week...
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 Why do we care about solar activity?
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Solar physics 101
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The Sun
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The Sun is a yellow dwarf star, and is about halfway through its 10 billion year life on the main sequence. Sun generates energy by way of nuclear fusion reactions -- burning hydrogen into helium, in the process converting about 4 million tons of matter into energy every second -- generating heat & light for the solar system
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The Sun rotates faster at the equator than at the poles -- differential rotation. The actual period of this rotation is about 25 days at the equator and 35 days at the poles -- but due to the Earth's motion in its orbit around the Sun, the apparent rotation period of the Sun at its equator is about 28 days.
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As is the case for all stars on the main sequence, the Sun's luminosity has been slowly increasing over its lifetime, will continue to do so for the next few billion years. But this isn't anything that changes quickly enough for any of us to care in our daily lives. It does mean that the Earth will be uninhabitable in 500 - 700 million years, but I'll save that discussion for a future episode.
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Sun's structure
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"Surface" features (i.e., on photosphere)
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Sunspots
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One interesting (and surprisingly influential) type of feature on the Sun's surface -- sunspot
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Region on the Sun's surface (photosphere) that is cooler than its surroundings. On its own, a sunspot would be blindingly bright (brighter than an electric arc), with a temperature of 4000-4500 K, but the surrounding photosphere has a temperature of about 5800 K, so they look dark by comparison.
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Sunspots can be from 4,000 to 30,000 km (2,500 to 11,000 miles) in diameter, and come in a variety of shapes -- circular to complex.
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Typically come in pairs or groups.
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Generally restricted to a band between 35 N and 35 S w.r.t. solar equator.
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Anatomy of a sunspot
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Sunspots may exist for anywhere from a few days to a few months, but they all eventually decay.
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Sunspots also played an important role in the historical debate over the nature of the solar system.
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Faculae
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Essentially the inverse of sunspots -- bright granular structures on the Sun's surface that are slightly hotter than the surrounding photosphere. A sunspot always has an associated facula, though faculae can also exist apart from sunspots.
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Faculae are produced by concentrations of magnetic field lines, and are most commonly found in the vicinity of sunspots.
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Prominences
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A large bright feature extending outwards from the photosphere into the corona, often in a loop.
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A prominence forms over about a day, and may persist for several months. Some break apart and give rise to coronal mass ejections.
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CME -- Coronal Mass Ejections
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Huge bubbles of gas ejected from the Sun, often from a prominence, over the course of several hours.
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Solar flares
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Large explosions on the surface of the Sun.
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Flares are very fast things, coming and going in a matter of minutes, and are thought to be formed as magnetic fields get twisted and sheared and release energy via magnetic reconnection.
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Solar flares are characterized by their brightness in X-rays, but they also radiate charged particles, and radio frequencies up to Gamma-rays.
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Solar wind
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The solar wind is a stream of charged particles (mostly high-energy electrons and protons) which are ejected from the upper atmosphere of the sun.
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Consists primarily of high-energy electrons and protons that are able to escape the sun's gravity.
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Accelerated by thermal energy, and some other mechanism (not definitively known, but likely magnetic).
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The solar cycle
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Approx. 11 year period (11.1), but varies from ~9 to ~14 years
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Solar max: peak in number of sunspots
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Solar min: minima of number of sunspots
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Even though sunspots are somewhat darker than rest of Sun's "surface," still come with enough activity around them (the faculae) that total solar irradiance is higher when sunspot count is higher
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Over the course of solar cycle, there's a variation of about 0.1% in total intensity -- but this varies with wavelength, is as high as 1.5% variation in UV wavelengths
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Solar cycle at its core is a magnetic phenomena, but was first seen in sunspot counts
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May have been seen by naked eye (!) by Chinese astronomers as early as 350 BC
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First telescope observations -- 4 astronomers in 1610 / 1611
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Discovery of sunspot cycle
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Because sunspots come in many sizes and different levels of grouping, starting with Wolf solar astronomers have found it useful to define a standard sunspot number index, which continues to be used today. The sunspot number is computed based on the number of individual sunspots seen, the number of sunspot groups, and a scaling factor that is supposed to account for visibility and telescope size.
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In 1904, Maunder plotted the first sunspot "butterfly diagram," which showed that sunspot groups moved from higher latitudes at the beginning of a sunspot cycle to lower latitudes at the end of it.
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The physical basis of the solar cycle was explained by George Ellery Hale, who in 1908 showed that sunspots were strongly magnetized, and in 1919 went on to show that the magnetic polarity of sunspot pairs:
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Half a century later, the father-and-son team of Harold Babcock and Horace Babcock showed that the solar surface is magnetized even outside of sunspots, and that this dipole also reverses its polarity in sync with the sunspot cycle.
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Hale's observations showed that the solar cycle is really a magnetic cycle with an average duration of 22 years. But since nearly all manifestations of the solar cycle are insensitive to magnetic polarity, it is more common to speak of the "11-year solar cycle".
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Magnetic field lines would normally repel each other, which by itself would cause sunspots to disperse rapidly. But recent satellite data has shown detailed internal structure below sunspots -- there's a powerful downdraft underneath each sunspot, forming a rotating vortex that concentrates magnetic field lines.
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Sunspots are self-perpetuating storms, similar in some ways to terrestrial hurricanes.
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Other stars' cycles
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The direct observation of starspots is not yet possible over extended periods, but some effects have been measured
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Many stars have magnetic cycles with periods ranging from 6 to 14 years, some being regular (like the Sun), some irregular, and some not cyclic at all.
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These stars exhibit variations in brightness which vary from 0.1% up to 4% (recall that the measured solar variation over a decade is 0.1%), and equally variable magnetic activity.
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Stars about the same age as the Sun, or older, have the same correspondence between increases in magnetic activity and increases in brightness. But the opposite is true for younger stars. It is believed that starspots dominate on these younger stars, and faculae are absent or very weak.
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Other, relatively minor cycles
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Solar size
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Diameter of Sun varies cyclically, on about a 100 year period
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Minor player: only changes solar output by about 0.001%
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Impacts of the solar cycle and solar activity at Earth
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One of the offshoots of solar activity is generation of C-14 and Be-10 in Earth's atmosphere
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Direct impacts
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Changes in amount of sunlight reaching the earth -- fraction of a percent, but has an impact over the long run
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CMEs
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Solar flares
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Indirect impacts -- GCR
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Wrapup
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Been some ink spent on press release that start of next cycle may be upon us.
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Same individual (David Hathaway of NASA's Marshal Space Flight Center) said the same thing in the fall of 2006.
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New cycle always starts with a high-latitude, reversed polarity (w.r.t. previous cycle) sunspot.
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What has been seen since 11th Dec. is a high-latitude, reversed polarity facula -- so, not quite there.
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Right now, we're in solar minimum -- next cycle will start at some point in the next year or so, but in its own good time.
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A number of groups are attempting to predict solar cycles -- tend to be all over the place, but according to a number of them, next cycle should be about 30% stronger than the last one. If this pans out, expect some good Northern Lights (and occasional power grid problems) in about 2013!
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Sources and other links
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