The solar wind throughout the solar cycle

Abstract

The existence of a solar corpuscular radiation (SCR) was conjectured by Biermann (1951) based on the fact that the ion tails of comets always point radially away from the Sun. Earlier it had been thought that this was due to the solar radiation pressure, but when the relevant cross sections were measured it became clear that these were far too small. This is visible in Figure 3.1, where stars can be seen shining through the ion tail of comet Hale-Bopp, one of the more spectacular sights in the sky of the 20th century. Parker (1958) provided the first theoretical description of the SCR in terms of a supersonic magnetized fluid. He coined the term " solar wind " in order to set it apart from other ideas of a (subsonic) solar breeze that were around at the time. The solar wind was ultimately observed in the early 1960s by the Soviets and independently with the American Mariner 2 mission to Venus (Gringauz et al., 1961; Neugebauer and Snyder, 1962). An excellent account of these early developments is given by Parker (2001). The first generation of solar wind instruments were Faraday cups with stepped-potential retarding grids or curved-plate electrostatic analyzers, both providing energy-per-charge (E/q) spectra. This revealed the basic constituents of protons with an admixture of a few percent (by number) of alpha particles (Neugebauer and Sny-der, 1966). It is remarkable how much could be gleaned from the first few months of observations: The existence of alternating high-speed streams with (slow) inter-stream solar wind, the rough proportionality of the proton temperature with the bulk speed, the approximate equality of the proton and alpha thermal speeds, i.e., the mass-proportionality of their kinetic temperatures, and more (cf. Neugebauer and von Steiger, 2001). With E/q sensors of increasing sophistication, Bame et al. (1970) discovered 1 Appeared in: André Balogh, Louis J. Lanzerotti, and Steven T. Suess, The Heliosphere through the Solar Activity Cycle, Springer Praxis Publishing, 2008, pp. 41-78. 2 CHAPTER 3. THE SOLAR WIND THROUGHOUT THE SOLAR CYCLE Figure 3.1: Comet Hale-Bopp as seen in April 1997. The straight, blueish ion tail, as opposed to the curved, white dust tail, points radially away from the Sun. Since it is transparent to starlight this must be due to solar corpuscular radiation (c W. Pacholka). heavy ions 1 such as oxygen, silicon, and iron in charge states that gave direct proof of the million-degree temperatures in the solar corona. Around the same time the noble gases Ne and Ar were first measured using the foil collection technique on the Apollo lunar missions (Geiss et al., 2004, and references therein). Bame et al. (1977) also found that the solar wind from the newly discovered coronal holes (Krieger et al., 1973) was structure-free and thus a distinctly different state of the phenomenon. With the plasma instruments on the two Helios missions (1974–1982) the distribution functions of protons and alpha particles could be characterized in full detail and as a function of heliocentric distance from 0.3 to 1 AU. Anisotropies perpendicular to the magnetic field direction were found to be common at small distances; with increasing distance they could be seen to isotropize gradually while at the same time another anisotropy along the magnetic filed directed away from the Sun would build up (Marsch, 1991, and references therein). A second generation of solar wind instruments combined an electrostatic E/q an-alyzer with either magnetic deflection to form a Wien filter thus providing a mass-per-charge (m/q) measurement such as the ICI sensor on the ISEE-3 mission (Coplan et al., 1978), or with a solid state detector to provide an energy measurement such as the ULECA sensor on ISEE-3 (Hovestadt et al., 1978). Both techniques allowed to break free from the assumption that all components of the solar wind flow a the same velocity, which had always to be made when interpreting E/q spectra. In addition, the resolution of the m/q spectra was independent of the kinetic temperature, while the usefulness of E/q spectra was seriously limited at times of high temperatures (i.e., in high-speed streams). Thus second-generation sensors contributed to new and improved determinations of abundances (Bochsler et al., 1986; Schmid et al., 1988) and charge 1 Sometimes these are called " minor " ions, but that term should be avoided considering their important role in modern solar wind research.