What is the chemical composition of the solar wind?

(by Amara Graps)

The solar wind is a collection of streams of energetic particles that originate on the Sun. You can think of the particles of the solar wind as nothing less than the solar corona itself (Noyes, 1982). This is because the distant corona expands outwards due to not enough restraining force from gravity, or from the pressure of the interstellar gas, to confine the distant corona. The solar wind escapes through the coronal holes at supersonic speeds. As the outer corona disperses, it must be replaced by gases welling up from below (lower corona).

The composition of the solar wind is a mixture of materials found in the solar plasma, composed of ionized hydrogen (electrons and protons) with an 8% component of helium (alpha particles) and trace amounts of heavy ions and atomic nuclei: C, N, O, Ne, Mg, Si, S, and Fe ripped apart by heating of the Sun's outer atmosphere, that is, the corona (Feldman et al., 1998).

SOHO also identified traces of some elements for the first time such as P, Ti, Cr and Ni and an assortment of solar wind isotopes identified for the first time: Fe 54 and 56; Ni 58,60,62 (Galvin, 1996).

Note that although the solar wind is electrically balanced, the solar wind consists almost exclusively of charged particles (stripped away nuclei from atoms) and is an excellent electrical conductor. These electrically conducting particles is technically known as a plasma, so it may be misleading to think of the solar wind as like Earth "winds".

Ulysses spacecraft measurements show that the solar wind has a "fast" component and a "slow" component (Lang, 1996). There are some compositional differences in the two wind streams in addition to differences in their speeds (Ulysses Results Web page).

The slow wind has a speed of about 400 km/sec and the fast component is about twice that speed (Lang, 1996). Ulysses determined that the fast speed component originates from the coronal holes near the Sun's poles. (But note that we were near the solar minimum part of the Sun's magnetic cycle during those measurements, and that this situation may be different for another time in the Sun's cycle.)

The SWICS ("Solar Wind Ion Composition Experiment") instrument on Ulysses measured speed and composition of the solar wind. In particular, it measured alpha particles and "heavy" ions from magnesium and oxygen. It found that the abundances of Mg/O is larger in the slow wind than in the fast wind. It also found that the the temperature is very high in the source location of the the low speed solar wind which indicates a very hot coronal source. Also the compositional abundance of Mg/O was found to be higher in the solar wind than in the solar photosphere, which indicates that there are some ionization and energy transport processes in the chromosphere influencing the solar wind composition.

You may wish to further pursue SWICS results by browsing the Ulysses Web pages (URL below). And if you search in the ADS abstracts with keywords: "Solar Wind Composition" for the field of "Title Worlds", you will turn up a rich selection of research results.


Overview and Current Knowledge of the Solar Wind and the Corona

Galvin, A.B. et al, 1996: "Solar Wind Composition: First Results from SOHO and Future Expectations", Bulletin of the American Astronomical Society, Vol. 28, p.897

Feldman U., et al, 1998: "Coronal Composition above the Solar Equator and the North Pole as Determined from Spectra Acquired by the SUMER Instrument on SOHO", The Astrophysical Journal 505, pp. 999-1006

Noyes, Robert, _The Sun, Our Star_, 1982, pp 196-206.

Lang, Kenneth, "Unsolved Mysteries of the Sun- Part 1", Sky and Telescope, August 1996, p. 42.

Ulysses Mission

SWICS experiment, J. Geiss, University of Bern; G. Gloeckler, University of Maryland; figure from J. Geiss, G. Gloeckler and R. von Steiger, Origin of the Solar Wind From Compostion Data, Space Science Rev. 72, 49-60, 1995.

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Special Thanks to A. Graps.