For much of the 1800’s, scientists suspected that the sun emitted some form of matter that streamed toward Earth, because it helped explain how the sunspot cycle could be synchronized with the appearance of the aurora near Earth.

This idea received a little more detail in 1903, when Norwegian astronomer Kristian Birkeland (1867-1917) suggested aurora might be caused by a stream of electrons traveling from sun to Earth. British geophysicist Sydney Chapman (1888-1970) again raised the idea of solar electron streams in a 1918 paper on magnetic storms. British physicist Frederick Lindemann (1886-1957), at Oxford University, adjusted this theory, by pointing out that negative charge accumulated on Earth would disrupt the process. Instead, Lindemann suggested that any cloud or stream expelled from the sun must be electrically neutral, containing equal charge from ions and electrons.

Such ideas remained only theories, however, until 1943 when German astronomer Cuno Hoffmeister (1892-1968) actually provided observations. His careful studies of comet tails uncovered an odd aberration: Instead of pointing directly away from the sun, they were inclined at several degrees from this orientation. Eight years later, Ludwig Biermann (1907-1986) at the University of Gottingen, described this deflection as an interaction between the particles in the comet’s tail and the stream of charged particles coming from the sun, what is now called the solar wind.

If the only thing acting on comets was the pressure from sunlight, comet tails would always point directly away from the sun. What Hoffmeister’s observations and Beirmann’s calculations showed was that comet tails, like million-mile-long windsocks, pointed in the direction that the solar wind blew them. The pressure from sunlight alone was simply not enough to account for the magnitude or direction of this aberration. The force could only be provided by a stream of particles traveling away from the sun at speeds of hundreds of miles per second.

Despite this work by Hoffmeister and Biermann, the concept of a solar wind remained controversial by many researchers in the 1950s. Meanwhile, in 1959, Russian space scientist Konstantin Gringauz (1918- 1993) flew instruments called ion traps on the Soviet Lunik 2 and 3 missions. The instruments measured the total electric charge of incoming charged particles, or ions. Gringauz reported that the signal fluctuated as the spacecraft spun around its axis, suggesting ions were entering the instrument only when it faced the sun. A more careful analysis of the data failed to find the necessary signal, and the idea of the solar wind still was not proven.

In 1961, American Herbert Bridge (1919-1995) and Italian Bruno Rossi (1905- 1993), with a team of scientists at MIT, obtained more detailed observations with an elaborate ion trap on NASA's Explorer 10. Still the data did not convince everyone, as the signal was intermittent. We now know that this was because Explorer 10 was right along the boundary where the solar wind hits Earth’s magnetic environment.

While such tantalizing, but incomplete evidence trickled in, the Swedish physicist Hans Alfven (1908-1995) and the American solar physicist Eugene Parker (1927 - ) published extensively in 1957-1958, on the behavior of the sun’s atmosphere, the corona. Their analysis of the fundamental physics around the sun suggested the corona was an unstable, heated gas of charged particles, called plasma, which simply could not remain trapped by the sun’s gravity. Instead, it was destined to expand into interplanetary space at hundreds of miles per second in a continuous wind of particles. This solar wind would flow past comet tails and the planets themselves, and interact with such objects by exerting both magnetic and thermal pressure on them. Parker discovered through the mathematics of plasmas that the solar wind would travel faster than the speed of sound and form a spiral pattern as it expanded past the orbits of the inner planets.

Despite all of this previous work, the discovery of the solar wind is almost universally credited to the Mariner 2 probe which flew past Venus on December 14, 1962. It not only detected a continuously flowing solar wind, but also observed fast and slow streams within it, repeating at approximately 27-day intervals, suggesting their sources moved with the sun’s own 27-day rotation cycle.

Over the next few decades, observations made by a variety of satellites rounded out our picture of the solar wind. It is, indeed, shaped like a pinwheel, as Parker predicted. The pinwheel is now called the Parker spiral and it is made of fast and slow streams. Observations made by the Ulysses spacecraft in the late-1990s also show that the solar wind is fastest at the solar poles and slowest near the solar equator. Fast streams also originate at areas in the sun’s atmosphere called coronal holes. On top of this, bursts of solar material, called coronal mass ejections, sometimes expand out into space. Eventually the solar wind collides with gases in the interstellar medium far beyond the orbit of Pluto, an interaction now being studied with the Voyager spacecraft. The entire region from this point back to the sun is called the heliosphere and the planets in our solar system orbit within this solar wind.

Modern satellites such as NASA’s Advanced Composition Explorer, or ACE, can measure the density, temperature and magnetism of the solar wind every five minutes. Such data continues to provide insights about the detailed behavior of this wind millions of miles from Earth.

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