Dying comets probe the Sun

22/01/2012

Updating Magic Universe

Debris traces the solar magnetic field

What started as a bonanza for comet spotters becomes a new tool for exploring levels in the Sun’s atmosphere that have been hard to see up to now. The SOHO spacecraft (Solar and Heliospheric Observatory) has identified more than 1400 small “sungrazing” comets that fly close to the Sun and evaporate. In July last year, the comet observers using SOHO’s Large Angle and Spectrometric Coronagraph (LASCO) team alerted colleagues operating the newer SDO (Solar Dynamics Observatory) to a larger-than-usual sungrazer heading for its doom.

As he reports in the current issue of Science magazine, Karel Schrijver from the Lockheed Martin Advanced Technology Center in California tracked Comet 2011 N3 SOHO by extreme ultraviolet light with his Atmospheric Imaging Assembly on SDO, which observes highly ionized atoms. What he learned about the comet and about the Sun I’ll tell below as a concise update for Magic Universe. Meanwhile the word is that SDO also observed Comet Lovejoy last month, when it survived a close encounter with the Sun, passing behind it and reappearing on the other side.

Here are a few relevant paragraphs from my story about Comets and Asteroids in Magic Universe.

The big comet count came from another instrument on SOHO, called LASCO, developed under US leadership. Masking the direct rays of the Sun, it kept a constant watch on a huge volume of space around it, looking out primarily for solar eruptions. But it also saw comets when they crossed the Earth-Sun line, or flew very close to the Sun.

A charming feature of the SOHO comet watch was that amateur astronomers all around the world could discover new comets, not by shivering all night in their gardens but by checking the latest images from LASCO. These were freely available on the Internet. And there were hundreds to be found, most of them small ‘sungrazing’ comets, all coming from the same direction. They perished in encounters with the solar atmosphere, but they were related to larger objects on similar orbits that did survive, including the Great September Comet (1882) and Comet Ikeya-Seki (1965).

SOHO is seeing fragments from the gradual break-up of a great comet, perhaps the one that the Greek astronomer Ephorus saw in 372 BC,’ explained Brian Marsden of the Center for Astrophysics in Cambridge, Massachusetts. ‘Ephorus reported that the comet split in two. This fits with my calculation that two comets on similar orbits revisited the Sun around AD 1100. They split again and again, producing the sungrazer family, all still coming from the same direction.’

The progenitor of the sungrazers must have been enormous, perhaps 100 kilometres in diameter or a thousand times more massive than Halley’s Comet. Not an object you’d want the Earth to tangle with. Yet its most numerous offspring, the SOHO-LASCO comets, are estimated to be typically only about 10 metres in diameter.

Update January 2012

In July 2011 a larger than usual sungrazer spotted by SOHO was tracked across the face of the Sun by a newer spacecraft, the Solar Dynamics Observatory, SDO. Named as Comet 2011 N3 SOHO, it evaporated to the point of invisibility after 20 minutes, but not before the event had transformed the game from comet-spotting fun to highly productive cometary and solar physics.

Led by Karel Schrijver from the Lockheed Martin Advanced Technology Center in California, the SDO team was able to gauge the size of the comet. Initially it was up to 50 metres wide. This opened the way to investigating the sungrazers in much more detail. It should become possible to learn more about the composition of these comets, according to how they boil and rupture in the intense heat.

As for solar physics, the miniature tail of the dying comet lit up magnetic field lines at altitudes high in the solar atmosphere that otherwise are almost impossible to detect. Seeing the lines traced by sungrazers at different heights above the Sun will make it possible to trace more accurately the links between the magnetism near the visible surface and the vast field that reaches out into space and influences the Earth.

References

Karel Schrijver et al., Science 20 January 2012, vol. 335, pp. 324-328 DOI: 10.1126/science.1211688

NB: Movies are available at http://www.sciencemag.org/content/335/6066/324/suppl/


The Sun and auroras for beginners

22/01/2012

Pick of the pics

Our Explosive Sun by Pål Brekke

In “Our Explosive Sun”, the picture has this caption. “A unique image of the planets close to the Sun observed with the LASCO telescope on SOHO. An occulting disk inside the telescope blocks the bright light from the solar disk creating an artificial solar eclipse. Mercury, Venus, Jupiter, Saturn, and the Pleiades are visible. Just outside the occulting disk one can see enormous ejections of gas from the hidden Sun. The horizontal streaks from the planets are artifacts from the digital camera (ESA/NASA).”

It’s one of my favourite images from the Space Age. The Large Angle and Spectrometric Coronagraph (LASCO) took it on 15 May 2000. Four planets and the Pleiades star cluster were almost in line with the Sun – which chose this theatrical moment to blast off a huge puff of gas in a coronal mass ejection (CME). So I’m not surprised to find the picture in Our Explosive Sun by Pål Brekke, a colourful book that’s just been published by Springer.

Pål Brekke (NRS)

Pål (pronounced Paul) is a Norwegian solar physicist who worked in the SOHO team for more than a decade, latterly as Deputy Project Scientist. We’ve known each other well from the time when I was writing a lot for the European Space Agency. Pål’s now a Senior Advisor at the Norwegian Space Centre.

Let’s be clear that Our Explosive Sun is a book for beginners, be they amateur astronomers, aurora watchers, high school students, or interested non-experts of any description. There’s plenty of elementary information about our mother star and the Solar System, and about how to observe the Sun safely or photograph the Northern Lights. Making the book distinctive are a mass of extraordinarily vivid and up to date illustrations, plus the occasional insights you get only from a true expert.

For example, in warning of the dangers that solar explosions will pose to astronauts flying to the Moon or Mars, Pål reminds us that the lunar flights of Apollos 16 and 17, in April and December 1972, were lucky to miss a big burst of deadly solar protons in August of that year. And in explaining the distances of stars, he notes that in about 40 years time an astronomer with a supertelescope on a planet in the Pleiades star cluster might in principle see Galileo turning his own telescope on the Pleaides for the first time, from a distance of 440 light-years.

It’s a pity perhaps that Pål doesn’t mention cosmic rays, which provide one of the great markers of solar variations both currently and in the past. And his remarks on solar activity and climate change are brief and rather cautious, e.g.: One thousand years ago, it was warmer on Greenland than today. … Human-driven climate change will work in addition to natural climate variability mainly caused by the Sun.

References

Pål Brekke, Our Explosive Sun: A Visual Feast of Our Source of Light and Life, Springer 2012. [Hardcover]

Amazon UK: http://www.amazon.co.uk/Our-Explosive-Sun-Visual-Source/dp/146140570X


Sun still sulks

20/06/2010

Pick of the pics and Climate Change: News and Comments

The Sun still sulks

Two magnetograms from the ESA-NASA SOHO spacecraft contrast the Sun’s liveliness of exactly 10 years ago (20 June 2000) on the left with its feeble performance today (20 June 2010) on the right. In these images made with Stanford’s Michelson Doppler Interferometer, north magnetic polarity is white, south magnetic polarity is black.

Solstice sunrise over Stonehenge 2005. Credit: User: Solipsist.

As many thousands flock to Stonehenge for tomorrow’s summer solstice, this is a moment to ask for the umpteenth time what the Sun is up to. The mean sunspot number in June 2000 was 119, today it is 28, with the spots clustered in the northern region showing most magnetic activity. Since 2004 there have been 803 days with no sunspots at all (35 in 2010, 260 in 2009). During a typical sunspot minimum there are fewer than 500 spotless days.

In the current issue of the Royal Astronomical Society’s magazine Astronomy and Geophysics, Nigel Weiss of Cambridge considers the long-term variability of the Sun and alternative theories about it, especially concerning “grand maxima” in activity like that in the 20th Century, and “grand minima” like the Maunder Minimum of 300 years ago associated with the Little Ice Age. Weiss’s conclusion is that there’s a 40 % chance the current grand maximum will be followed by a grand minimum.

As for the climatic implications, Weiss and I agreed to differ some years ago. Although we both say that the Intergovernmental Panel on Climate Change underestimates the influence of the Sun, Weiss thinks it can’t compete with man-made global warming. His article ends:

Even if the Sun does enter a new Maunder-like grand minimum, any cooling effect will be small compared with the warming produced by anthropogenic greenhouse gases.

Contrast that with Henrik Svensmark’s conclusion in an article for the Danish newspaper Jyllands-Posten.

That the Sun might now fall asleep in a deep minimum was suggested by solar scientists at a meeting in Kiruna in Sweden two years ago. So when Nigel Calder and I updated our book The Chilling Stars, we wrote a little provocatively that “we are advising our friends to enjoy global warming while it lasts.”

In fact global warming has stopped and a cooling is beginning. Mojib Latif from the University of Kiel argued at the recent UN World Climate Conference in Geneva that the cooling may continue through the next 10 to 20 years. His explanation was a natural change in the North Atlantic circulation, not in solar activity. But no matter how you interpret them, natural variations in climate are making a comeback.

The outcome may be that the Sun itself will demonstrate its importance for climate and so challenge the theories of global warming. No climate model has predicted a cooling of the Earth – quite the contrary. And this means that the projections of future climate are unreliable. A forecast saying it may be either warmer or colder for 50 years is not very useful, and science is not yet able to predict solar activity.

So in many ways we stand at a crossroads. The near future will be extremely interesting. I think it is important to accept that Nature pays no heed to what we humans think about it. Will the greenhouse theory survive a significant cooling of the Earth? Not in its current dominant form. Unfortunately, tomorrow’s climate challenges will be quite different from the greenhouse theory’s predictions. Perhaps it will become fashionable again to investigate the Sun’s impact on our climate.

References

N. Weiss, “Modulation of the Sunspot Cycle”, Astronomy and Geophysics, Vol. 51, pp. 3.9-3.15, 2010

H. Svensmark: “While the Sun sleeps” (in Danish), Jyllands-Posten, 9 September, 2009

For a related post on this blog see http://calderup.wordpress.com/2010/05/12/puzzling-sun/

Postscript on the Song of the Sun

I see that Sheffield solar physicists now generate music from observations of the magnetic coronal loops. Read about it (and hear it): http://www.telegraph.co.uk/science/space/7840201/Music-of-the-sun-recorded-by-scientists.html

For an earlier Song of the Sun, using its internal vibrations seen by SOHO’s MDI, click on the second item here (but beware – it’s about 18 MB with visuals) http://www.esa.int/esaSC/SEMLAJWO4HD_index_0.html


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