Winds Across the Void

Overview

The Sun’s hot and stormy atmosphere is forever expanding in all directions, filling the solar system with a ceaseless flow — called the solar wind — that contains electrons, protons and other ions and magnetic fields. Early spacecraft measurements showed that there are two kinds of wind, a fast one moving at about 750 thousand meters per second, and a slow one with about half that speed.

The twinkling, or scintillation, of radio sources suggested that a fast wind is streaming out at high solar latitudes. Measurements from the Helios 1 and 2 spacecraft indicated that the electrons, protons and helium nuclei in the solar wind have different temperatures; in the high-speed wind the more massive particles are hotter. The Ulysses spacecraft has made measurements all around the Sun, at a distance comparable to that of the Earth and near a minimum in the Sun’s 11-year activity cycle. Ulysses’ velocity data conclusively prove that a uniform fast wind pours out at high latitudes near the solar poles, and that a capricious, gusty, slow wind emanates from the Sun’s equatorial regions. Polar plumes are not a major source of the high-speed wind. It originates at the boundaries of the magnetic network in coronal holes. SOHO data, also taken near activity minimum, have shown that the fast wind blows at a wide range of latitudes near the base of the corona, and that the slow wind is associated with the narrow stalks of equatorial coronal streamers.

The high-speed wind is accelerated very close to the Sun, within just a few solar radii, and the slow component obtains full speed much further away, often spurting out of helmet streamers in magnetically-driven blobs. SOHO has additionally demonstrated that heavier particles in polar coronal holes move faster than light particles in coronal holes; the oxygen ions have agitation speeds 60 times greater than those of protons. Magnetic waves might preferentially accelerate the heavier ions in coronal holes.

Ulysses has detected magnetic fluctuations, attributed to Alfvén waves, far above the Sun’s poles; they may block cosmic rays trying to enter these regions. The Ulysses and SOHO spacecraft have determined the rough shape and content of the heliosphere, a vast Sun-centered bubble extending far into space.