dipper@utastro.UUCP (Debbie Byrd) (07/08/85)
The word corona means "crown" -- and the sun's corona really looks like a crown. More -- after this. July 8 The Solar Corona On today's date in the year 1842, there was a total eclipse of the sun. It became famous for some of the earliest scientific descriptions of the sun's corona. The solar corona is just the tenuous outermost atmosphere of the sun. The corona is visible to us only during a total eclipse, when the black disk of the moon blots out the main body of the sun -- and the corona appears as a lovely encircling shell of wispy white light. Some early astronomers believed that the solar corona was only an optical illusion. But the first photographs of a total eclipse proved that the corona is real. In 1869, scientists used a device called a spectroscope to split the white light of the corona into its component rainbow of colors. Instead of all colors being present, only a few puzzling sharp bright discrete colors were found. Fifty years later this evidence was understood to mean that the sun's outer atmosphere has a much higher temperature than the photosphere, which is the visible surface of the sun. In fact, the temperature of the corona is about two million degrees centigrade, compared to only about six thousand degrees for the photosphere. In other words, as you go up from the visible surface of the sun to the sun's outer atmosphere, things get hotter -- not colder as you might expect. Some combination of magnetic effects and shock waves appears to be transmitting energy from beneath the sun's surface up into the tenuous corona. Script by Deborah Byrd and Harlan Smith. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin
msb@lsuc.UUCP (Mark Brader) (07/18/85)
dipper@utastro.UUCP (Debbie Byrd) writes: > July 8 The Solar Corona > > On today's date in the year 1842, there was a total eclipse of the > sun. It became famous for some of the earliest scientific descriptions > of the sun's corona. > The reason there were no earlier scientific descriptions is that the present visible (excited) form of the corona has only existed since about that time, or perhaps a few decades previously. Apparently the sun's level of activity tends to change at intervals on the order of a century, and the present level is exceptionally high. Periods of high activity are characterized by the presence of sunspots and the appearance of auroras. (Whether there is always a (2x11)-year cycle when there are sunspots is unknown due to the scarcity of pre-telescopic sunspot observations.) But even in past periods of high activity there are no descriptions of eclipses resembling the way they appear now with the corona. The present level, as I said, must therefore be exceptionally high. In periods of low activity there are almost no sunspots at all. This most recently happened from 1645 to 1715. (There were then almost no auroras in Scandinavia where they are now commonplace.) This may be why, when Schwabe described the 11-year sunspot cycle about 1840, he was not believed at first. When he was seen to be correct, scientists fell into believing that it had always been that way, rather than that Schwabe had observed something new, and the 1645-1715 "Maunder minimum" had to be rediscovered more than once in historical records (Halley, Newton...) before IT was believed. Anyway, the 1645-1715 period coincides with the coldest part of the "Little Ice Age", when places like Britain had what we here call a winter. Evidence is that this is not a coincidence, and therefore the present global temperature is unusually high (but may stay that way in future due to man's CO2 output). My source for all this is the article "The Case of the Missing Sunspots", by John A. Eddy, in Scientific American, May 1977, p.80. Eddy was the second rediscoverer, after Maunder (mentioned above). Anyone know of further developments since then? Mark Brader
cramer@kontron.UUCP (Clayton Cramer) (07/25/85)
> dipper@utastro.UUCP (Debbie Byrd) writes: > > July 8 The Solar Corona > > > > On today's date in the year 1842, there was a total eclipse of the > > sun. It became famous for some of the earliest scientific descriptions > > of the sun's corona. > > > > The reason there were no earlier scientific descriptions is that the > present visible (excited) form of the corona has only existed since about > that time, or perhaps a few decades previously. Apparently the sun's > level of activity tends to change at intervals on the order of a century, > and the present level is exceptionally high. > > Periods of high activity are characterized by the presence of sunspots > and the appearance of auroras. (Whether there is always a (2x11)-year cycle > when there are sunspots is unknown due to the scarcity of pre-telescopic > sunspot observations.) But even in past periods of high activity there are > no descriptions of eclipses resembling the way they appear now with the corona. > The present level, as I said, must therefore be exceptionally high. > > In periods of low activity there are almost no sunspots at all. This most > recently happened from 1645 to 1715. (There were then almost no auroras > in Scandinavia where they are now commonplace.) This may be why, when Schwabe > described the 11-year sunspot cycle about 1840, he was not believed at first. > When he was seen to be correct, scientists fell into believing that it had > always been that way, rather than that Schwabe had observed something new, > and the 1645-1715 "Maunder minimum" had to be rediscovered more than once > in historical records (Halley, Newton...) before IT was believed. > > Anyway, the 1645-1715 period coincides with the coldest part of the "Little > Ice Age", when places like Britain had what we here call a winter. Evidence > is that this is not a coincidence, and therefore the present global temperature > is unusually high (but may stay that way in future due to man's CO2 output). > > My source for all this is the article "The Case of the Missing Sunspots", > by John A. Eddy, in Scientific American, May 1977, p.80. Eddy was the > second rediscoverer, after Maunder (mentioned above). Anyone know of > further developments since then? > > Mark Brader I just *know* I'm going to get flamed for this. When I was a kid (about 8 or 9), I became very interested in the subject of the sunspot period. I grabbed the birth years of 100 randomly selected Nobel laureates and plotted them versus sun spot records. Fifty per cent of the Nobel laureates were born in the 20% of the years when sun spot activity was at its lowest. Of course, only being 8 or 9, I didn't have a sufficient grasp of statistics to do a more detailed analysis, nor quite the skills to get a Federal grant to study this issue. :-) Makes you wonder if the explosion of knowledge and technology in the last few centuries might be connected to the increase in solar activity? Just a thought. When the above ideas revolutionize biophysics, I'm sure I won't get any credit for sparking it.:-)
josh@polaris.UUCP (Josh Knight) (07/25/85)
In article <713@lsuc.UUCP> msb@lsuc.UUCP (Mark Brader|LSUC|Toronto) writes: > My source for all this is the article "The Case of the Missing Sunspots", > by John A. Eddy, in Scientific American, May 1977, p.80. Eddy was the > second rediscoverer, after Maunder (mentioned above). Anyone know of > further developments since then? The Maunder minimum is observed in ice cores, I believe from both Greenland and Antartica. Solar activity also produces the C14 (i.e. the radioactive isotope of Carbon used for dating) in the earth's atmosphere and the calibration of the Carbon dating scheme with continuous/overlapping tree ring records back some 5000 years can be used to infer solar activity over that time. I'm not sure (can't read the papers right now, just have references listed in my thesis without article titles), but I think one of the articles: Science Vol. 192, p. 1189, 1976, by J.A. Eddy or Science Vol. 198, p. 824, 1977, by J.A. Eddy, P.A. Gilman and D.E. Trotter (probably the first) talks about what can be inferred about solar activity in the long term. I seem to remember that there were controversial findings of 11 year cycles in tree rings by an acknowledged expert on the topic (tree rings, weather etc.) and these would most likely be before the Maunder Minimum. None of this is "since" the SciAm article, but more recent papers are more likely to cite the Science articles (and therefore be traceable via the science citation index) than the SciAm article. -- Josh Knight, IBM T.J. Watson Research josh at YKTVMH on BITNET, josh.yktvmh.ibm-sj on CSnet, ...!philabs!polaris!josh