The Solar Wind

See also From Core to Corona: Layers of the Sun

Origin of the Solar Wind:

The Sun is a massive, luminous ball of gas. It is composed of about 90% hydrogen (by number) and 10% helium with a small fraction of heavier elements, such as carbon, oxygen, and iron. Within a region about 0.3 solar radii from the center of the Sun, the temperature and density are high enough (~10^7 K and 100 g/cm^3) to promote fusion of protons into helium nuclei. The energy released from this process diffuses outward as "hard" X-rays which are degraded into radiation of longer wavelengths by continuous absorption and emission of the photons by the gas surrounding the core.

Above a distance of about 0.7 solar radii from the center, diffusion is not fast enough to transport energy out of the Sun so it is carried away by convection. As the hot gas rises, it becomes less dense until it is finally transparent, and the transported energy is radiated out into space. The region where is energy radiation occurs is the photosphere which is regarded as the visible surface of the Sun.

Above the photosphere lies a layer of transparent gas called the chromosphere (from the Greek chroma, meaning color). The chromosphere is so much dimmer than the photosphere that it can be seen only with the aid of a coronagraph (a device that uses a disk to occult the photosphere) or during a total eclipse of the Sun when it appears as a reddish fringe just beyond the Moon.

Above the chromosphere lies the corona (meaning "crown" in Latin). Like the chromosphere it is visible only with a coronagraph or during a total solar eclipse. It appears as a pearly white structure, parts of which can extend several solar radii from the edge of the Sun. The coronal temperature ranges from 1 to 2x10^6 K, and it is therefore a highly ionized plasma. As an example, spectral lines from iron with up to 12 electrons removed have been observed, and the lightest elements are fully ionized. Despite the corona's high temperature, it is not very dense (more of a "hot vacuum" by terrestrial standards) and a black body placed into the coronal gas and shielded from the solar radiation would reradiate the heat absorbed from the gas at an equilibrium temperature range of 600 to 2000 K (300 to 1700 deg. C) since the corona is optically thin.

The corona is a highly structured region of plasma. This structure is imposed by the solar magnetic field which extends from the solar surface out into the corona. Since the corona is a plasma (i.e. a collection of positively charged nuclei and negatively charged electrons), it is an excellent electrical conductor. As a result of this conductivity, the coronal plasma can move along but not across magnetic field lines. There are two types of magnetic field lines, "closed" and "open". Closed field lines are anchored at two points in the photosphere and extend into the corona as a loop or arch (a visible manifestation of these magnetic field loops can be seen in the motion of solar prominences). Open field lines are anchored at only one point in the photosphere and extend into interplanetary space. It is in these open field regions that the corona can expand outward in the form of the solar wind.

The Solar Wind Itself:

The expanding coronal gas or solar wind fills interplanetary space. The solar magnetic fields embedded in the plasma are carried into space by the solar wind to form the interplanetary magnetic field (IMF). Beyond some 15-20 solar radii, the solar magnetic field is dominated by the solar wind flow which expands almost radially away from the Sun. Because of solar rotation, the point where the open field line is anchored to the Sun moves and as a result the interplanetary magnetic field has the form of a spiral (This spiral pattern is not unlike the one seen for water emanating from a rotating garden sprinkler). At the orbit of the Earth, one astronomical unit (AU) or about 1.5x10^8 km from the Sun, the interplanetary magnetic field makes an angle of about 45 degrees to the radial direction. Further out the field is nearly transverse (i.e. about 90 degrees) to the radial direction.

At 1 AU the average speed of the solar wind is about 400 km/s. This speed is by no means constant. The solar wind can reach speeds in excess of 900 km/s and can travel as slowly as 300 km/s. The average density of the solar wind at 1 AU is about 7 protons/cm^3 with large variations. The solar wind confines the magnetic field of Earth and governs phenomena such as geomagnetic storms and aurorae. The solar wind confines the magnetic fields of other planets as well.

As the solar wind expands, its density decreases as the inverse of the square of its distance from the Sun. At some large enough distance from the Sun (in a region known as the heliopause), the solar wind can no longer "push back" the fields and particles of the local interstellar medium and the solar wind slows down from 400 km/s to perhaps 20 km/s. The location of this transition region (called the heliospheric termination shock) is unknown at the present time, but from direct spacecraft measurements must be at more than 50 AU. In fact, in 1993 observations of 3 kHz radiation from Voyagers 1 and 2 have been interpreted as coming from a radio burst at the termination shock. This burst is thought to have been triggered by an event in the solar wind observed by Voyager 2. From the time delay between this triggering event and the observation of the 3 kHz radiation, the distance of the termination shock has been put between 130 and 170 AU.

Bibliography:

Barnes, Aaron, "Acceleration of the Solar Wind", Reviews of Geophysics, 30, pg 43-55, 1992

Gurnett, 1993 (. . .)

Kaler, James B. Stars. New York: Scientific American Library, 1992.

Villanueva, Louis, A Study of the Solar Wind from the Voyager Spacecraft, 1977-1992, Thesis (MIT,1994).

See also From Core to Corona: Layers of the Sun

Back to Plasmas - the Fourth State of Matter

Text contributed by Louis Villanueva, California State University at Hayward.