#18H.     The Solar Wind -- History

Not everyone accepted Parker's theory of the solar wind when it was first published in 1958, and it was debated until observations confirmed it. Konstantin Gringauz in 1959 flew "ion traps" on the Soviet Lunik 2 and 3 missions, instruments measuring the total electric charge of arriving ions. He found that the signal fluctuated as the spacecraft spun around its axis, suggesting an ion flow was entering the instrument whenever it faced the Sun. In 1961 Herbert Bridge with Bruno Rossi and the MIT team obtained more detailed observations with an elaborate trap on NASA's Explorer 10, a spacecraft designed to explore the nightside tail of the magnetosphere which however was often immersed in the solar wind. And when Mariner II in 1962 flew towards Venus, it not only detected a continuously flowing solar wind, but also observed in it fast and slow streams, approximately repeating at 27 day intervals, suggesting that their sources rotated with the Sun.

The top graph below shows the variations in the speed of the solar wind as seen by Mariner 2: the values fluctuate from 400 to 700 km/sec. The bottom graph shows variations in the "storminess" of the Earth's magnetic field, which obviously goes up and down in a similar way.

The sources of the fast streams were later found to be the "coronal holes" studied in 1973-4 from the space station Skylab. In sunspot regions field lines form arches which hold back the solar wind, but in the "holes" in between they extend outwards and allow the plasma to accelerate unimpeded. Lower-velocity solar wind comes from regions in between sunspot regions and coronal holes


The polar regions of the Sun also have such outwards-directed field lines, as evidenced by the "polar plumes" seen in the corona during a total eclipse of the Sun. It was therefore expected that the solar wind above the Sun's poles would be relatively fast and smooth-flowing. A mission to that region had been planned since the 1970s, and it led after many delays and cutbacks to the spacecraft Ulysses (Latin name of Odysseus), a joint European-NASA venture. Ulysses passed above the Sun's south pole in September 1994, and above the north pole in 1995.

All planetary orbits lie approximately in the same flat plane as that of the Earth ("plane of the ecliptic"), which is also close to the Sun's equatorial plane. To reach a position above the Sun's pole, Ulysses needed to be flung out of this plane, and it did so by first flying out to the planet Jupiter and then using that planet's gravity as a pivot while swinging into the third dimension. Ulysses has confirmed the existence of a steady fast-flowing solar wind above the poles and has observed many interesting phenomena associated with it.

The Ulysses mission has its own home page on the web, leading to many related files. To reach it, click here.

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Last updated March 13, 1999