Part 1 of 2
Text Box: Reviewed 14 November 2007 Requests for clarifications to eugenesittampalam (at) gmail.com – most welcome!
Text Box: Close to the surface of the sun, the corona (the sun’s outer atmosphere) suddenly attains temperatures above 1,000,000 Kelvin (K), much hotter than the 6000 K of the photosphere below. In the recent history of solar physics, no single problem has been as stubborn as that of explaining this coronal heating. The corona supplies the energy and momentum that sustains the solar wind... How the Sun’s Corona Gets Hot, H. S. Hudson and T. Kosugi (Institute of Space and Astronautical Science of Japan, Kanagawa, Japan), Science 285, 849 (6 Aug 1999)
Text Box: This vast mantle of gas, dubbed the chromosphere in its lowest reaches and the corona at the higher altitudes, is the scene of processes that heat it to millions of degrees, unleash huge jets and arcing filaments of gas, and launch great bubbles of matter called coronal mass ejections (CMEs) into space. This turmoil is felt throughout the solar system, because it generates a relentless "solar wind" that distorts Earth's magnetic field and blasts material off comets, producing their tails. Now solar physicists are getting their clearest look yet at what drives solar weather, courtesy of the European Space Agency's Solar and Heliospheric Observatory (SOHO). From its vantage point 1.5 million kilometers sunward of Earth, SOHO's 11 instruments have been watching the sun's every move, from deep-seated pulsations in its visible surface – the photosphere – to the far reaches of the corona at distances of several solar radii. ...but SOHO's visible light telescope, the Large Angle and Spectrometric Coronagraph Experiment (LASCO), has observed particles in the polar regions being accelerated to high velocities close to the surface. "The surprising result is that the acceleration of that wind seems to occur below two solar radii – this is against all theories," says LASCO team leader Guenter Brueckner of the U.S. Naval Research Laboratory in Washington, D.C. ... SOHO Probes the Sun’s Turbulent Neighborhood, Alexander Hellemans, Science 277, 479 (25 Jul 1997)
Text Box: MADISON, WISCONSIN – The most eloquent reaction of all came from Robert Bless, the University of Wisconsin astronomer who hosted last week's American Astronomical Society meeting here. When a reporter told him that an instrument aboard the Solar and Heliospheric Observatory (SOHO) satellite had detected 100-million-degree oxygen ions in the sun's atmosphere, he silently dropped his jaw. That temperature is tens of times higher than has ever been measured before in the corona, the sun's halo of ionized gases. ... In a coronal hole above the sun's north pole, early results showed that "the higher we went, the higher the oxygen temperature went up," says Kohl [John Kohl of the Harvard-Smithsonian Center for Astrophysics (CfA), principal investigator for the Ultraviolet Coronagraph Spectrometer (UVCS)]. It still hadn't peaked at the limit of the measurements 0.9 solar radii above the surface, where the temperature soared to 100 million degrees. ... Putting Some Sizzle in the Corona, James Glanz, Science 272, 1738 (21 Jun 1996)
Text Box: Coronal mass ejections are rapid reconfigurations of coronal structures that involve the ejection into the solar wind of immense quantities (even by solar standards) of coronal material. Detectable only from space-borne coronagraphs – instruments that eclipse the Sun's bright disk – they appear as gigantic loop-like bubbles subtending up to a quarter of the Sun's circumference (see Fig. 1 [not reproduced here]). The bubbles lift off into space, leaving behind only bright legs rooted to the Sun. As coronal mass ejections are space-age latecomers to the solar terrestrial scene (they were discovered in the early 1970s), space physicists are only now appreciating their central relevance to the field. ... Replacing the solar flare myth, Nancy Crooker (Center for Space Physics, Boston University, MA), Nature 367, 595-596 (17 Feb 1994)
Text Box: CMEs are erupting bubbles of solar gases containing tens of millions of tons of solar material as well as a portion of the solar magnetic field, and they expand quickly as they blow out into space. ... Through LASCO, CMEs look like bright bubbles emerging from the corona, explains Simon Plunkett, a solar physicist with the Naval Research Laboratory who works with the SOHO satellite. If a CME is aimed either directly toward or directly away from Earth, it appears as an expanding white ring, a "halo CME." ... Forecasting the Storms and Showers of Space, Gary Taubes, Science 286, 2438-2440 (24 Dec 1999)
Text Box: Two very large CMEs took place in January and April this year, captured in spectacular photographs by LASCO, which blocks light coming directly from the sun to get a clear view of the corona. The largest CMEs launch up to a billion tons of gas into space in one go. "They are really carrying more matter than we thought before, and are explaining 50% of the slow solar wind," says [Ester] Antonucci [of Turin University in Italy]. ... SOHO Probes the Sun’s Turbulent Neighborhood, Alexander Hellemans, Science 277, 479 (25 Jul 1997)
Text Box: Spatially resolving the surfaces of nearby stars promises to advance our knowledge of stellar physics. Using optical long-baseline interferometry, we constructed a near-infrared image of the rapidly rotating hot star Altair with a resolution of <1 milliarcsecond. The image clearly reveals the strong effect of gravity darkening on the highly distorted stellar photosphere. Standard models for a uniformly rotating star cannot explain our findings, which appear to result from differential rotation, alternative gravity-darkening laws, or both. Imaging the Surface of Altair, John D Monnier (University of Michigan) et al., Science 317, 342-345 (2007) Rapidly rotating stars are flattened spheres with poles that are hotter than the equator. Seeing the Surfaces of Stars, Andreas Quirrenbach (University of Heidelberg), Perspectives, Science 317, 325-326 (2007)
Text Box: The rotation of the Sun is not that of a rigid body; at its surface, the gas near the poles has a lower angular velocity than that near the equator. This latitudinal variation persists to the base of the convection zone, below which the angular velocity becomes approximately uniform. ... Here we report observations of rotationally split modes made over a three-year period with the Birmingham Solar Oscillations Network. Our results indicate that there is a substantial region inside the Sun that is rotating more slowly than the surface. This situation seems likely to be transient – the minimum-energy state would have all the deeper regions rotating with the same angular velocity – and is at variance with our current ideas about the rotational evolution of main-sequence stars. We have no solution to the dynamical problem this poses. ... Indeed, we cannot exclude the possibility of relatively rapid core rotation surrounded by a shell of slowly rotating material... Slow rotation of the Sun's interior, Y. Elsworth (University of Birmingham, UK) et al., Nature 376, 669-672 (24 Aug 1995)