heliosphere

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heliosphere

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Written by Erik Gregersen

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heliosphere, the region surrounding the Sun and the solar system that is filled with the solar magnetic field and the protons and electrons of the solar wind.

The solar magnetic field in the heliosphere has a dipole structure. The magnetic field lines that are carried outward from the Sun by the solar wind remain attached to the Sun’s surface. Because of the Sun’s rotation, the lines are drawn into a spiral structure. In one hemisphere (either the northern or the southern), the magnetic field lines are directed inward, and in the other they are directed outward. Between these two different hemispheres is a structure called the heliospheric current sheet.

The solar wind flows outward through the solar system into the interstellar medium (ISM) and begins to feel the effects of the ISM at the termination shock, where the solar wind starts to lose speed. The region beyond the termination shock in which the solar wind slows is called the heliosheath. Neutral atoms in the heliosheath form a “ribbon” that is probably caused by solar wind particles being reflected back into the solar system by the magnetic field in the ISM. At the outward boundary of the heliosheath is the heliopause, where the outward pressure of the solar wind balances the pressure of the incoming ISM. The heliopause is usually considered to be the boundary of the solar system and is about 123 astronomical units (AU; 1 astronomical unit = 150 million km) from the Sun. (By comparison, Neptune, the outermost planet, is 30 AU from the Sun.)

Nicolaus Copernicus. Nicolas Copernicus (1473-1543) Polish astronomer. In 1543 he published, forward proof of a Heliocentric (sun centered) universe. Coloured stipple engraving published London 1802. De revolutionibus orbium coelestium libri vi.

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It was originally thought that the heliosphere was stretched into a teardrop shape by its encounter with the ISM. The U.S. space probes Voyager 1 and 2 crossed the termination shock at a distance of 94 and 84 AU from the Sun in 2004 and 2007, respectively. Since the two Voyagers are traveling out of the solar system in different directions, this implied that the heliosphere has an asymmetrical shape. However, subsequent observations of atoms in the heliosheath by the Cassini spacecraft orbiting Saturn and the Interstellar Boundary Explorer orbiting Earth showed the heliosphere was actually a sphere. Voyager 1 crossed the heliopause on August 25, 2012.

Erik Gregersen

solar wind

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solar wind

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Related Topics:: aurora; interplanetary medium; solar wind power satellite; interplanetary magnetic field; reconnection

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solar wind, flux of particles, chiefly protons and electrons together with nuclei of heavier elements in smaller numbers, that are accelerated by the high temperatures of the solar corona, or outer region of the Sun, to velocities large enough to allow them to escape from the Sun’s gravitational field. The solar wind is responsible for creating the tail of Earth’s magnetosphere and the tails of comets, both of which face away from the Sun. At a distance of one astronomical unit (AU; the mean distance between Earth and the Sun, about 150 million km [93 million miles]), during a relatively quiet period, the wind contains approximately 1 to 10 protons per cubic centimetre moving outward from the Sun at velocities of 350 to 700 km (about 220 to 440 miles) per second; this creates a positive ion flux of 10⁸ to 10⁹ ions per square centimetre per second, each ion having an energy equal to at least 15 electron volts. During solar flares, the proton velocity, flux, plasma temperature, and associated turbulence increase substantially.

There are two solar winds: a fast, uniform, and steady wind, blowing at 800 km (500 miles) per second, and a slow, gusty, and sporadic wind, with about half the speed of the fast one. The two winds originate at different places on the Sun and accelerate to terminal velocity at different distances from it. The distribution of the two solar wind sources depends on the 11-year solar activity cycle.

When the solar wind encounters Earth’s magnetic field, a shock wave results, the nature of which is not fully understood. As the solar wind spreads out into an increasing volume, its density and pressure become less. Eventually the pressure of the solar wind becomes comparable to that of the interstellar medium. The termination shock, where the solar wind slows because it encounters the interstellar medium, has been measured at about 94 and 84 AU by the Voyager 1 and 2 spacecraft, respectively.

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This article was most recently revised and updated by Richard Pallardy.