Saturday, November 21, 2009

Fjordman: A History of Geology and Planetary Science Part 3

The third and final part of Fjordman’s history of geology and planetary science has been posted at Atlas Shrugs. Part one can be read here and part two here.

Large sunspots may under certain conditions be seen by the unaided eye, but the modern study of them began around 1610 with the introduction of the telescope. Sunspots are strongly magnetic and appear darker because they are slightly cooler than the regions that surround them. Based on daily observation records between 1826 and 1843, the German amateur astronomer Heinrich Schwabe (1789-1875), a pharmacist living in the town of Dessau, in 1843 announced that sunspots vary in number in a cycle of roughly ten to eleven years. He was originally looking for a yet-unknown planet moving inside the orbit of Mercury. His article caught the eye of Alexander von Humboldt, who in 1851 published Schwabe’s table updated to 1850. After that many scientists became interested in the 11-year sunspot cycle.. It has later been established that periods with many sunspots correspond to high solar activity.

The Swiss astronomer Rudolf Wolf (1816-1893) had studied at the universities of Zürich, Vienna and Berlin, where the German astronomer Johann Franz Encke was one of his teachers. Wolf became director of the Bern Observatory in Switzerland in 1847 and in 1848 devised the “Zürich sunspot number” to gauge the number of sunspots.

A gentleman of independent means, the English amateur astronomer Richard Carrington (1826-1875), devoted himself to the study of sunspots. Carrington found by observing their motions that the Sun rotates faster at the equator than near the poles. Another early pioneer in the study of sunspot cycles was the German astronomer Gustav Spörer (1822-1895).

The Anglo-Irish geophysicist Edward Sabine (1788-1883) in 1852 found an association between the sunspot cycle and the occurrence of large magnetic storms. On September 1, 1859, Richard Carrington in England through his telescope, which projected an 11-inch-wide image of the Sun on a screen, observed what we now know was a huge solar flare, a magnetic explosion on the Sun. Only 17 hours later this event triggered a large magnetic storm on the Earth. Just before dawn the next day, auroras occurred even in Cuba and Hawaii. Spark discharges shocked telegraph operators in several regions and set telegraph paper on fire.
- - - - - - - - -
Unusual solar activity can cause geomagnetic storms (disturbances in the Earth’s magnetosphere) and interrupt electromagnetic communications, for instance by affecting the ionosphere. A powerful solar flare of the strength observed by Carrington could potentially cause quite serious damage today due to our much more extensive reliance on electromagnetic equipment and communications in the twenty-first century as compared to the mid-nineteenth.

Early estimates of stellar surface temperatures made using Newton’s law of cooling gave far too high temperatures. More accurate values were obtained by using the radiation laws of the Slovenian physicist Joseph Stefan in 1879 and the German physicist Wilhelm Wien in 1896. Stefan calculated the temperature of the Sun’s surface to about 5400 °C, which was the most sensible value by date. Stefan’s Law or the Stefan-Boltzmann Law, named after Stefan and his Austrian student Ludwig Boltzmann, suggests that the amount of radiation given off by a body is proportional to the fourth power of its temperature as measured in Kelvin units.

The part of the Sun that we normally see has a temperature of more than 5500 degrees C, almost 5800 K. Temperatures in the core, where nuclear fusion occurs, reach over 15 million K. The lowest layer of the atmosphere is called the photosphere. The next zone is the chromospheres, where the temperature rises to 20,000 K. The corona, the Sun’s outer atmosphere, is remarkably hot. In the part nearest the surface the temperature is 1 million to 6 million K, but it can reach tens of millions of degrees when a flare occurs. Sunspots are cooler regions where magnetic energy builds up and is often released in solar flares and discharges of charged particles known as coronal mass ejections. These events can trigger space storms that affect the Earth. The flow of coronal gas into space is known as the solar wind. The corona is visible during total solar eclipses as a large halo of white, glowing gas, but the relative rarity of such eclipses present logistical difficulties for detailed observations.

The technical problems associated with producing an artificial eclipse to study the Sun were solved by the French solar physicist Bernard Lyot (1897-1952), an expert in optics who had studied engineering in Paris in addition to mathematics, physics and chemistry. As an astronomer, Lyot found that the lunar surface behaves like volcanic dust and that Mars has sandstorms. In 1930 he invented an instrument known as the coronagraph, a telescope equipped with an occulting disk sized in such a way as to block out the solar disk, which is more difficult than it sounds. By 1931 he was obtaining photographs of the corona. He found new spectral lines in the corona and made the first motion pictures of solar prominences.

In the 1930s, Lyot boldly inferred a coronal temperature of around 600,000 K. This claim was met with skepticism at the time. Acceptance of these very high temperatures came through the spectroscopic work of the German astrophysicist Walter Grotrian (1890-1954) and the Swedish astrophysicist Bengt Edlén (1906-1993) soon after, but an explanation for how the Sun’s upper atmosphere could be so much hotter than its surface took a long time to work out.

The Swedish physicist Hannes Alfvén (1908-1995) was one of the founders of plasma physics and magnetohydrodynamics, the study of plasmas in magnetic fields. Alfvén was born in Norrköping, Sweden. Both his parents were practicing physicians. He studied at Uppsala University and became a research physicist and professor in Stockholm. He made many discoveries in solar and space plasma physics and his work on cosmic rays led him to propose in 1937 the existence of a galactic magnetic field. The one discovery for which he is best known is the magnetohydrodynamic wave commonly called the Alfvén wave, whose existence for decades was difficult to prove. Finally in 2009, pictures taken by a team using the Swedish Solar Telescope in Spain’s Canary Islands revealed that “corkscrew” waves — Alfvén waves — were pushing heat from the Sun’s surface to its outer atmosphere, the corona.

Read the rest at Atlas Shrugs.