Part 1 of 2
Text Box: Reviewed 14 November 2007   
Requests for clarifications to eugenesittampalam (at) – 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)
Text Box: Last month the European-built Ulysses spacecraft finished its first complete orbit of the sun, a 7-year reconnaissance of particles and magnetic fields high over the sun’s north and south poles. Now researchers are hoping to extend Ulysses operations into a second orbit... 
They would also like to complement its long-range view of the sun with images and measurements "closer to the region where the solar wind is actually heated and gains its maximum energy," says Richard Marsden, the European Space Agency’s (ESA’s) project scientist for Ulysses. ... 
Launched in October 1990 and operated jointly by ESA and NASA, Ulysses "went into totally new, unexplored territory," says Peter Wenzel, head of ESA’s Solar System Division. ...
Throughout its circuit, Ulysses' nine instruments sampled the magnetic field and the stream of electrons, protons, and other particles in the solar wind. "Ulysses has provided us with a map of the solar wind at all latitudes," says Marsden. 
Each kind of observation turned up surprises. ...
The key event in the solar cycle is the reversal of the solar magnetic field shortly after the maximum, in 2001 or 2002, which is still a mystery to astronomers. "This is going to be a very exciting period," says Marsden. ... 
Ulysses Laps Sun, Inspires New Missions, Alexander Hellemans, Science 280, 668 (1 May 1998)
Text Box: The Flipping Core!
No, not really. No body of matter actually flips here like a coin, as we shall see in this simple model of the solar interior.
Nuclear fission is the primary reaction in the Sun and stars. 
Fusion, though considerable, is only the consequential secondary. (See The Cosmos.)

The Sun, like all matter, breathes. The Sun's breathing cycle, as we now know, has a 22-year period.
Over the inhalation half cycle, the Sun – strictly, the whole of the heliosphere – dilates, enhancing fission.
Over the exhalation half cycle, the Sun contracts, aiding fusion (the net remaining fission over the full cycle).
This is the basic reason for sunspots, which indicate increased solar activity, to peak every half cycle, or 11 years.

Fission occurs all across the exterior of the solid core, peaking at the relatively rarefied corona with the final-stage neutron decay.
Fusion, too, occurs across this cross section but peaks at the dense innermost fluid core layers. (The chromosphere would thus be "boiling hot" in relation to the "condensed" photosphere, as observations now overwhelmingly confirm.) 

Under the recoil of exhalation, the solid core within gets an opposing boost in spin.
And, by viscous drag, the inner fluid layers start to increasingly corotate with the solid core (see figures below).
The opposing spins of the fluid core layers thus give rise to a pair of antiparallel magnetic dipoles, or a quadrupole.

At peak exhalation in our Sun, however, the magnetic effect of the inner fluid layers tend to predominate.
The net magnetic field of the Sun would thus have its north pole pointing up, that is, above the ecliptic. 

Under the recoil of inhalation, the spin of the solid core wanes.
And the magnetic effect of the outer fluid layers take over dominance – flipping the poles.

In reality, though, the fluid layers reverse spin directions only gradually. 
The layer by layer change thus takes the net magnetic effect through zero from one peak to the opposite peak every 11 years.

In a stellar body, as in the Sun, the solid core spin axis is generally not perfectly aligned (antiparallel) with the fluid outer core. 
Not surprisingly, close observations now do reveal these two distinct dipoles, or quadrupole, in the Sun.
Text Box: Splitting of the sun's global oscillation frequencies by large-scale flows can be used to investigate how rotation varies with radius and latitude within the solar interior. The nearly uninterrupted observations by the Global Oscillation Network Group (GONG) yield oscillation power spectra with high duty cycles and high signal-to-noise ratios. Frequency splittings derived from GONG observations confirm that the variation of rotation rate with latitude seen at the surface carries through much of the convection zone, at the base of which is an adjustment layer leading to latitudinally independent rotation at greater depths. A distinctive shear layer just below the surface is discernible at low to mid-latitudes. ...
Differential Rotation and Dynamics of the Solar Interior, Science 272, 1300-1302 (31 May 1996)
Text Box: The solar wind is a supersonic outflow of coronal plasma into interplanetary space, and is the agent that carries solar disturbances to the Earth. Direct measurements of the wind speed over a range of distances – from the orbit of Mercury to beyond the outermost planets and now over the solar poles – show that the acceleration is largely complete by 70 solar radii (Ro). ... Our results indicate that the acceleration of the polar wind is almost complete by 10Ro, much closer to the Sun than had been expected. This suggests that the acceleration of the solar wind and the heating of the solar corona occur in essentially the same region, and thus that the underlying mechanisms may be strongly linked.
...recent observations by the Ulysses spacecraft have shown that the speed distribution over the north pole is indistinguishable from that under the south pole. ...
It also suggests that the mechanisms by which the corona is heated and the solar wind accelerated are not as independent as had been suggested.
Rapid acceleration of the polar solar wind, R. R. Grail (UCSD) et al., Nature 379, 429-432 (1 Feb 1996)
Text Box: The 1-metre solar telescope at the Royal Swedish Academy of Sciences' observatory on the Canary Island of La Palma, installed in the spring of this year, is providing astronomers with images of sunspots showing unprecedented detail… Here, fundamental physical processes are occurring in the solar photosphere on scales of less than 100 km – a challenging phenomenon for theoreticians to tussle with. This week, p xiii;
Fine details of the filamentary structure of sunspots are revealed in new observations. These high-resolution measurements herald the quality of data to be expected from a new generation of solar telescopes. ... The Sun under a microscope, John H. Thomas (Depts of Physics & Astronomy and Mechanical Engineering, University of Rochester, NY), pp 134-135;
Sunspot umbrae – the dark central regions of the spots – are surrounded by brighter filamentary penumbrae, the existence of which remain largely inexplicable.  ...but discriminating between theoretical models has been difficult because the structure of the filaments has not hitherto been resolved. Here we report observations of penumbral filaments that reveal dark cores inside them. We cannot determine the nature of these dark cores, but their very existence provides a crucial test for any model of penumbrae. ..
Dark cores in sunspot penumbral filaments, Cöran B. Scharmer et al. (The Institute for Solar Physics of the Royal Swedish Academy of Sciences, Stockholm, Sweden), pp 151-152; Nature, vol 420  (14 Nov 2002)
Text Box: The Solar Neutrino Problem
For the final solution to this longstanding problem, which arises from our poor understanding today of both solar physics and neutrino physics, please click on The Neutrino below.
          Go to Part 2 of 2
A Synopsis The Cosmos The Spin
ADDENDA The Cosmological Redshift The Neutrino
Two-Slit Tests The Galaxy Nuclear Reactions
NASA Tests Gravity The Sun
KamLAND Test Anti-Gravity The Pulsar
UCLA Test Relativity Superconductivity
Q and A Mass-Energy Fusion Energy
 Eugene Sittampalam
 14 November 2007