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ScienceWeek
ASTROPHYSICS: ON THE SUN AND THE HELIOSPHERE
The following points are made by E. J. Smith et al (Science 2003 302:1165):
1) The space between the Sun and nearby stars is filled with ionized and neutral gas, dust, magnetic fields, and charged particles. The Sun excludes this pristine interstellar medium from a large volume called the heliosphere that completely encloses the planetary system. The Sun's influence extends to such great distances because the solar wind fills the heliosphere and exerts an outward pressure on the interstellar medium. The solar wind magnetic field keeps the low-energy interstellar plasma and magnetic field from penetrating into the heliosphere. However, interstellar neutrals, dust, and high-energy cosmic rays enter the heliosphere and their properties are altered by the solar wind, solar gravity, solar radiation, and charge exchange. The interaction between the Sun and the interstellar medium takes place not only at the outer boundary, but throughout the heliosphere.
2) Many space missions have explored the heliosphere near the solar equator, but only the Ulysses spacecraft has traveled from the equator to above the Sun's polar caps and extended our knowledge to a full three dimensions. The Ulysses orbit is inclined 80.2 deg to the solar equator, with a minimum solar distance (perihelion) beyond the orbit of Earth at 1.3 AU, a maximum distance (aphelion) of 5.3 AU, and a period of 6.3 years. The spacecraft, experiments, and observational results over the first complete orbit during 1992-1998 at sunspot minimum are described in (1-4). The second orbit, completed during the recent maximum in solar activity, produced the results described by the authors.
3) The solar/sunspot cycle is driven by changes in the Sun's magnetic field (5). At solar minimum, two opposing magnetic poles occupy the north and south polar caps. As solar maximum approaches, the poles decrease in strength and eventually vanish, while very strong magnetic fields develop in sunspots and active regions. The polar cap fields reappear after many months but with their signs reversed; the field that was outward is now inward, and vice versa. The magnetic fields in the solar wind originate at the solar surface and must respond to the changing solar field.
4) At solar minimum, polar cap fields are a major source of the heliospheric magnetic field (HMF), dividing the heliosphere into two hemispheres in which the field has the same signs as the magnetic poles. The field remains in each hemisphere and does not cross into the other hemisphere or return to the Sun. Such fields with one end on the Sun and the other end extending out into the heliosphere are said to be "open". The heliospheric current sheet (HCS), which serves as the magnetic equator, separates the fields in the two hemispheres. The Sun's magnetic poles are generally not aligned with the rotation axis; as the Sun rotates, the HCS wobbles up and down, allowing fields above and below it to be observed, and the HMF appears to be divided into two "magnetic sectors". Because one end of the field rotates with the Sun while the other end is carried off by the solar wind, the field lines are not radial but form spirals.
References (abridged):
1. E. J. Smith, R. G. Marsden, D. E. Page, Science 268, 1005 (1995)
2. E. J. Smith, R. G. Marsden, Geophys. Res. Lett. 22, 3297 (1995)
3. R. G. Marsden, E. J. Smith, J. F. Cooper, C. Tranquille, Astron. Astrophys. 316, 279 (1996)
4. A. Balogh, R. G. Marsden, E. J. Smith, Eds., The Heliosphere Near Solar Minimum: The Ulysses Perspective (Springer-Praxis, London, 2001)
5. P. V. Foukal, Solar Astrophysics (Wiley, New York, 1990)
Science http://www.sciencemag.org
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THE PHYSICS OF THE SUN AND TERRESTRIAL CLIMATE
The Sun, a *main-sequence star 1.4 million kilometers in diameter, is composed predominantly of hydrogen and helium (approximately 70 percent hydrogen by mass, 28 percent helium by mass, and 2 percent heavier elements by mass) and it generates its energy via nuclear fusion processes, particularly via the *proton-proton chain reaction. As a result, the Sun is losing mass at a rate of approximately 4 million metric tons per second.
The generation of energy occurs in the "central core", which has a temperature of approximately 15 million kelvins, is approximately 400,000 kilometers in diameter, and contains approximately 60 percent of the mass of the Sun in 2 percent of its volume.
Outside the core is the "radiative zone", an envelope of unevolved material through which energy from the core is diffusively transported by successive absorption and emission of radiation in collisions between atomic particles. It has been estimated that it may take from 1 million years to as long as 10 to 20 million years for the energy generated in the core to reach the surface.
The radiative zone extends to within 200,000 kilometers of the surface. In the surface layer (the "convective zone"), where the temperature is only 1 million kelvins, convection is the most important mode of energy transport.
The following points are made by Eugene N. Parker (Physics Today June 2000):
1) The Sun is essentially a thermonuclear core enclosed in an opaque shroud that insulates the high temperature of the core from the cold Universe outside. The core is brighter than 10 supernovas at maximum light, but the enclosing shroud turns back all but one part in 2 x 10^(11) of the thermal radiation. The outward journey of the energy from the core takes approximately 1 million years, which illustrates the immense opacity and thermal capacity of the shroud.
2) Approximately 10^(-5) of the outflowing energy from the core of the Sun is diverted into magnetic fields that produce a variety of exotic effects, including *coronal mass ejection, *solar flares, the million degree corona, the *solar wind, and x-ray emission. These phenomena are of interest to the physicist because they represent unanticipated manifestations of classical physics, extrapolations to astronomical scales of basic principles traditionally studied in terrestrial laboratories.
3) The total luminosity of the Sun varies with time, and systematic monitoring of several Sun-type stars during the past 4 decades reveals magnetic activity cycles comparable to that of the Sun. The luminosities of some of those stars have been monitored for approximately 15 years, and the data show approximately the same variation as the magnetic activity.
4) The Earth contains a great deal of information about past solar activity. The rate of production of carbon-14 depends directly on the intensity of *cosmic rays, and such rays are partially excluded from the Solar System by the outward sweep of magnetic fields in the solar wind. Thus the cosmic ray intensity and carbon-14 production vary oppositely to the general level of solar activity.
5) The carbon-14 record indicates that over the last 70 centuries the Sun has been without normal activity for 10 centuries and hyperactive for 8 centuries. The other 52 centuries were variable but more or less normal. The most recent quiescent period was from 1645 to 1715, the period called the "Maunder Minimum". The 12th century "Medieval Maximum" is the most recent epoch of hyperactivity. The empirical relation between the total luminosity and magnetic activity, based on many Sun-type stars, suggests that the Sun was fainter during the *Maunder Minimum by 0.4 +- 0.2 percent, and perhaps brighter by a comparable amount during the Medieval Maximum. The mean annual temperature in the northern temperate zone was lower than normal by 1 to 2 degrees centigrade during the Maunder Minimum and higher by 1 to 2 degrees centigrade during the Medieval Maximum. The fractional change in temperature is comparable to the fractional change in solar brightness, with the implication that the Sun is the driver of the climate. The consequences for agriculture were severe during both periods, the Maunder Minimum being disastrous in northern Europe and China, and the Medieval Maximum disastrous in the semi-arid regions. These periods of abnormal activity of the Sun are without explanation, as are the variations within the so-called "normal centuries".
6) The general level of solar activity doubled or tripled from 1900 to 1950, an estimate based on sunspot numbers and on the intensity of geomagnetic activity. This increase suggests an increase in solar luminosity by perhaps one part in 2000, and the author suggests it is interesting to note that the mean temperature in the northern temperate zone, as well as the surface sea water temperatures, rose during the same period. "Warmer seas, of course, reduce the rate at which atmospheric carbon dioxide is absorbed into the oceans. It appears that the global warming since 1950 is in part a consequence of the continuing increase in solar brightness, seriously aggravated by the extravagant burning of fossil fuel. So the mystery of the variations in the total luminosity of the Sun is part of the complicated picture of global warming." [Editor's note: See report #3 in this issue for another approach to millennial-scale climate changes.]
Physics Today http://www.physicstoday.org
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Notes:
main-sequence star: The Main Sequence is a region on the *Hertzsprung-Russell diagram where most stars lie, including our own Sun. The evolution of a star can be diagrammed as a movement along the Main Sequence and an eventual branching off the Main Sequence to regions associated with various types of old stars.
Hertzsprung-Russell diagram: The Hertzsprung-Russell diagram is a plot of stellar absolute magnitude against spectral type, and is perhaps the most useful diagrammatic aid in astrophysics. It allows the portrayal of the evolution of a star as occurring along various paths in the diagram.
proton-proton chain reaction: A chain of nuclear reactions inside a star that converts hydrogen to helium, with the associated release of energy. In the reaction, 4 hydrogen nuclei (protons) fuse to form one nucleus of helium, with the production of a number of intermediate nuclei such as deuterium and isotopes of lithium, beryllium, and boron. The proton-proton reaction is the most important stellar reaction at temperatures below 18 million kelvins, and thus operates chiefly in stars of less than 2 solar masses.
coronal mass ejection: The corona is the Sun's faint outer atmosphere, where the temperature is 2 million degrees kelvin or more, the corona consisting of a low-density hot gas that glows with a pale white color.
solar flares: A solar flare is a sudden release of energy in the corona of the Sun, the phenomenon usually lasting up to several hours (in rare cases, up to more than a day).
solar wind: The solar wind is the steady flow of charged particles, consisting primarily of protons and electrons, from the solar corona into interplanetary space. The solar-wind particles have energies high enough to enable the particles to escape the Sun's gravitational field, but the wind is influenced by the Sun's magnetic field, and the particles can be trapped by planetary magnetic fields.
cosmic rays: Highly energetic particles moving at close to the speed of light and continuously bombarding the Earth's atmosphere from all directions. The energies of the particles are enormous and range from 10^(8) to over 10^(19) electronvolts.
Maunder Minimum: Named after the astronomer Edward W. Maunder (1851-1928), who first noted the absence of reports of sunspots in the period 1645 to 1715.
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