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solar neutrinos

the radial component of the magnetic field is either toward or away from the sun. The region in which such an orientation is maintained is called a sector. There are either away or toward sectors depending on the direction of the radial component of the magnetic field. At the transition from away to toward or vice versa, the magnitude of the magnetic field is fairly constant. Normally two or four sectors are observed during a solar rotation. The sectors are dependent on the solar latitude and disappear at higher latitudes where the Interplanetary Magnetic Field (IMF) has the same sign as the appropriate solar pole.

solar maximum The time at which the solar cycle reaches its highest level as defined by the 12-month smoothed value of the sunspot number. During solar maximum there are more active regions and sunspots on the sun, as well as more solar flares leading to greater numbers of geomagnetic storms at the Earth. The most recent solar maximum occurred in July 1989. The next is expected to occur sometime in the year 2000. See solar cycle.

solar maximum mission (SMM) The SMM spacecraft was launched on February 14, 1980 near the height of the solar cycle, to examine solar flares in more physically meaningful detail than ever before. SMM recorded its final data in November 1989.

solar minimum The time at which the solar cycle reaches its lowest point as described by the 12-month smoothed value of the sunspot number. During solar minimum, there may be no sunspots or solar flares. The most recent minimum occurred around October 1996. See solar cycle.

solar nebula The cloud of gas and dust out of which our solar system formed. About 4.6×109 years ago, a slowly rotating cloud of gas and dust began to collapse under its own gravitational influence. As the cloud collapsed, it began to form a flattened disk with a central bulge. The central bulge collapsed down to eventually form the sun. In the outer part of the disk, at least two other blobs of gas and dust collapsed down to form the planets Jupiter and Saturn. Elsewhere, small material condensed out of the cloud and

began to accrete to form the other planets, most of the moons, and the small icy and rocky debris that became the comets and asteroids. This scenario helps to explain the counterclockwise orbital motion of all the planets, the counterclockwise orbital motion of most of the larger moons, and the counterclockwise rotation direction of most of the planets by proposing that the solar nebula was rotating in a counterclockwise direction.

Alternately, the disk of gas and dust that surrounded the newly formed sun at the time of solar system formation. The planets were formed from the material in the nebula. If we take the current masses of all the planets, and add enough hydrogen and helium to make the composition identical to that of the sun, the resulting mass is the mass of a minimum mass solar nebula.

solar neutrinos Low-energy electron neutrinos released in nuclear reactions in the sun and detectable from Earth. Fewer are seen than predicted by standard physics and astrophysics. The reactions of the proton-proton chain produce neutrinos of energy less than 1 MeV at the deuterium-production stage and more energetic ones in connection with the reactions involving beryllium and boron. The first experiment, using Cl37 as a detector, was constructed by Raymond Davis in the Homestake Gold Mine in Lead, South Dakota beginning in 1968. By 1971 it was clear that only about a third of the expected neutrinos were being captured, but the experiment was sensitive only to the Be and B products, thus explanations focused on mechanisms that might cool the center of the sun and reduce production of these high energy particles without affecting the main reaction chain. A second experiment, at Kamioka, Japan, confirmed the deficiency of B8 neutrinos starting in 1989, but also showed that the ones being seen were definitely coming from the direction of the sun. Two experiments of the 1990 (SAGE in the Caucasus Mountains and GALLEX under the Alps) look at the lower energy neutrinos from the main reaction chain. They are also deficient by a factor of about two. The full pattern is best accounted for if the solar model is the standard one, but electron neutrinos can change into mu or tau neutrinos en route to us. A possible mechanism, called MSW (for its inven-

© 2001 by CRC Press LLC

solar P-angle

tors Mikheyev, Smirnov, and Wolfenstein), attributes the change to a catalytic effect of atomic nuclei, so that it happens only inside the sun and other stars, not in empty space.

solar P-angle The heliographical latitude, in degrees, of the center of the solar disk as would be seen from the center of the Earth for a given date.

solar quiet current system The daily variations in the geomagnetic field during quiet solar conditions of a few tens of nanoteslas are the result of the solar quiet (Sq) current system

flowing in the E region of the ionosphere. This system is due to the electric field generated in the manner of a dynamo by tidal winds produced by the solar heating of the atmosphere at E region heights. A part of the electric field originating in the high latitude magnetosphere is also transferred to the E region by field aligned currents. The current system is concentrated in a band of a few hundred kilometers wide near the magnetic dip equator where it is called equatorial electrojet. The equatorial electrojet corresponds to an east-west electric field of a few tenths of mv/m and a vertical polarization field of 10 mv/m. The strength and pattern of the Sq current system depend upon longitude, season, year, and solar cycle.

solar radiation The radiation from the sun covers the full electromagnetic spectrum. Approximately 40% of the sun’s radiative output lies in the visible wavelengths (380 to 700 nm). Ultraviolet radiation (<380 nm) contributes an additional 7% to this ouput and infrared radiation accounts for more than half of the sun’s power. While of great scientific interest, the X-ray radiation (below 0.1 nm) and millimeter radio emission are negligible in the total solar luminosity. The spectral distribution of the solar radiation peaks in the blue-green at 450 nm and is distributed approximately like a black body spectrum at a temperature of 5762 K. The black body approximation is particularly good at wavelengths greater than 400 nm but at shorter wavelengths the solar radiative output falls significantly below what the Planck spectrum would predict. This is due to the increasing number of Fraunhofer absorption lines at

these wavelengths. At visible wavelengths, the smooth continuum spectrum is modified by a number of absorption lines due to the presence of ions in the solar atmosphere. In the ultraviolet and X-ray range, however, the spectrum changes to one dominated by emission lines. Both the Fraunhofer absorption lines and the optically thin emission lines provide important diagnostic information about the sun’s atmosphere.

solar rotation The sun rotates on its axis once in about 26 days. This rotation was first detected by observing the motion of sunspots. The sun’s rotation axis is tilted by about 7.25◦ from the axis of the Earth’s orbit so we see more of the sun’s north pole in September of each year and more of its south pole in March. The sun does not rotate rigidly. The equatorial regions rotate faster (taking only about 24 days) than the polar regions (which rotate once in more than 30 days).

solar spectral irradiance The spectral distribution of the sun’s radiation as observed from the Earth’s orbit. The integral of this over wavelength is known as the total solar irradiance or solar constant.

solar system The collection of planets, moons, asteroids, comets, and smaller debris (such as meteoroids or interplanetary dust particles) which are gravitationally bound to the sun. Our solar system includes the nine major planets, the 65 currently known moons orbiting the planets, the particles comprising the ring systems around the Jovian planets, the myriad of asteroids and comets, and all the smaller debris resulting from these larger objects. The edge of our solar system is the heliopause, defined as the point where the galactic magnetic field counterbalances the sun’s magnetic field.

solar system formation See nebular hypothesis, tidal formation of solar system.

solar ultraviolet measurements of emitted radiation (SUMER) An ultraviolet spectrometer aboard the satellite SOHO designed for the investigation of plasma flow characteristics, turbulence and wave motions, plasma densities and temperatures, and structures and events associ-

© 2001 by CRC Press LLC

solar wind

ated with solar magnetic activity in the chromosphere, the transition zone, and the corona.

solar wind The supersonically expanding outer atmosphere of the sun; the stream of fully ionized atomic particles, usually electrons, protons, and α particles (helium nuclei about 4% by number) with a trace of heavier nuclei emitted from the sun outward into space, with a density of only a few particles per cubic centimeter. The solar wind emanates from coronal holes in the sun’s outer atmosphere and extends outward into the solar system along the field lines of the solar magnetic field. The existence of the solar wind was first predicted in 1958 by E.N. Parker, who argued on theoretical grounds that it is not possible for a stellar atmosphere as hot as the solar corona to be in complete hydrostatic equilibrium out to large distances in the absence of an unreasonably large interstellar confining pressure. The solar wind has two components distinguished by their average speeds. The fast solar wind has speeds of around 700 to 800 kms−1 while the typical speed of the slow solar wind is around 300 to 400 kms−1.

Representative properties of the solar wind, as observed in the ecliptic plane at the heliocentric distance of one astronomical unit, are summarized in the table below. The solar wind velocity, density, etc. are highly variable; the quantities represented in the table can and do vary by factors of two or more, and the values presented should be regarded as illustrative only. The temperatures given are kinetic temperatures.

Representative Solar Wind Properties

(Ecliptic Plane, 1 AU)

 

Property

Typical Value

 

velocity (V )

400 km/s

 

number density (Np)

5 protons/cm3

 

H+ temperature (Tp)

105K

 

e− temperature (Te)

1.5×105K

 

He++ temperature (Tα)

4×Tp

 

magnetic field (B)

5×10−5 gauss

 

composition

96% H+, 4% He++

The flow is hypersonic; for example, in the ecliptic plane at 1 AU the flow velocity is many

hundreds of kilometers per second, whereas typical sound and Alfvén speeds are of order 50 km/s. Because the flow is hypersonic, bow shock waves are formed when the solar wind is deflected by planetary obstacles. If the obstructing planet has a strong magnetic field, as is the case for the Earth and the outer planets, the solar wind is excluded from a sizable magnetospheric cavity that surrounds the planet.

As the solar wind expands, it carries with it a magnetic field whose field lines are rooted in the sun. Because the sun is rotating, the field lines are wrapped into a spiral pattern, so that field lines are nearly radial near the sun but nearly transverse to the radial direction at large heliocentric distances; near the orbit of Earth the field typically lies in the ecliptic plane at about 45◦ from the radial direction and the total intensity of the interplanetary magnetic field is nominally 5 nT.

Except for a brief period near the time of maximum sunspot activity, the general solar field has an underlying dipole character, with a single polarity at high northern latitudes and the opposite polarity at high southern latitudes; when this condition obtains, solar wind originating at polar latitudes typically has very high speed (of order 800 km/s) and its magnetic field is oriented inward or outward in accord with the predominant polarity of the region of origin. In contrast, at equatorial latitudes the solar magnetic field can have either polarity (the solar dipole is, in effect, tilted with respect to the rotation axis), so that as the sun rotates a fixed observer near the ecliptic sees alternating inward and outward magnetic field. A spatial “snapshot” of this alternating polarity structure is called the interplanetary magnetic sector pattern.

The solar wind is frequently disrupted by transient disturbances originating at the sun, especially at times of high sunspot activity. Such events may involve explosive ejection of mass from the corona, with associated shock waves, magnetic field disturbances, energetic particle enhancements, etc.

When transient disturbances in the solar wind impact planetary magnetic fields, they may cause such magnetospheric phenomena as magnetic storms and enhanced aurorae. Transient solar wind disturbances can also reduce the local galactic cosmic ray intensity by sweeping cos-

© 2001 by CRC Press LLC

solar year

mic rays into the outer heliosphere, and can induce magnetic fields in objects containing conductive material. As the solar wind interacts with the comae of comets, it pushes the material back into the comet tails.

As the solar wind flows outward, its ram pressure declines as r−2, and eventually becomes comparable to the local interstellar pressure due to interstellar matter or the galactic magnetic field or cosmic rays.

At such distances the interstellar medium is an effective obstacle to the solar wind, which must adjust by passing through a shock to become subsonic; the shocked wind is swept away by and eventually merged with the interstellar gas. Thus, the termination of the solar wind is thought to be characterized by two boundaries, the termination shock where the flow becomes subsonic, and the “heliopause,” the boundary between the (shocked) solar material and exterior interstellar material. Spacecraft have not yet (June 1998) reached either of these boundaries, which are expected to be at distances of order 100 to 200 AU from the sun.

solar year The amount of time between successive returns to the Vernal equinox. For precision we refer to tropical year 1900 as the standard because the solar year is lengthening by about 1 millisecond per century.

solar zenith angle The zenith angle is the angle between the overhead point for an observer and an object such as the sun. The solar zenith angle is zero if the sun is directly overhead and 90◦ when the sun is on the horizon.

solid Earth tides Astronomical bodies such as the sun and moon deform the heights of gravitational equipotentials around the Earth. The disturbance in the height of an equipotential due to an external body such as the moon takes the form of a second order zonal spherical harmonic, which rotates around the Earth with a period slightly greater than one day (because as the Earth rotates, the moon itself moves in orbit). If the Earth’s surface were inviscid and fluid on a diurnal timescale, then its surface would move so as to coincide with an equipotential (although the redistribution of mass at the surface would itself adjust the equipotentials). This is what

happens with the world’s oceans, leading to high and low tides approximately twice per day (as there are two tidal bulges rotating around the Earth). For the viscoelastic solid Earth, the relaxation to an equipotential is incomplete: the proportion of actual readjustment to theoretical readjustment can be found from the appropriate Love number h2, which (using the PREM Earth model) is 0.612. Due to irregularities such as the fact that the plane of the Earth’s orbit around the sun, that of the moon’s orbit around the Earth and the Earth’s equatorial plane are not all parallel, and that the orbits are elliptical and have different periods, there are in fact many different frequencies in the solid Earth tides. See Love numbers.

solitary wave A wave with a single crest; wavelength is thus undefined.

soliton A spatially localized wave in a medium that can interact strongly with other solitons but will afterwards regain its original form. In hydrodynamical systems whose description involves non-linear equations, one can find solitary waves that are non-vanishing only in small regions of space, stable, and which can travel with constant velocity. They were first experimentally evidenced in 1842 by J. Scott Russel.

In non-linear field theories, equivalent stable bound state solutions called solitons can also exist both at the classical and quantum level. Their most remarkable property is that they do not disperse and thus conserve their form during propagation and collision.

Topological solitons, such as topological defects, require non-trivial boundary conditions and are produced by spontaneous symmetry breaking; non-topological solitons require only the existence of an additive conservation law. See cosmic topological defect, non-topological soliton, sine-Gordon soliton.

solstice Dates on which the day in one hemisphere (Northern or Southern) is of greatest length. Dates on which the sun is at one of two locations on the ecliptic which are most distant from the celestial equator. Because the Earth poles are inclined by 23◦27 to its orbital plane, northern and southern hemispheres typi-

© 2001 by CRC Press LLC

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