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The heliosphere is a bubble created by solar wind from the Sun. Its closest boundary is 100 AU away, while its farthest boundary is 200-300 AU away. The solar wind is a stream of charged particles that flows from the Sun at a speed of 400-700 km/s. The heliosphere is maintained by the Sun’s magnetic field, which creates the heliospheric current sheet. The heliosphere has other structures, including the termination shock and the heliopause, which have not yet been reached by space probes.
The heliosphere is a large bubble in space created by solar wind from the Sun. At the edge of the heliosphere, the solar wind collides with the gases of the interstellar medium and ceases to be the dominant space climate. The heliosphere is enormous: its closest boundary is about 100 AU (astronomical units, or Earth-Sun distances) away, while its farthest boundary is 200-300 AU away. The heliosphere has an elliptical shape, like a comet’s tail, due to the rapid motion of the Sun through the interstellar medium as it orbits the galactic center.
As stated, the cause of the heliosphere is the solar wind. The solar wind is a continuous stream of charged particles, mainly free electrons and protons, flowing from the Sun at a speed of between 400 and 700 km/s (about 1,000,000 mph). This equals 6.7 billion tons per hour, or an Earth’s mass every 150 million years. While it sounds like a lot, it’s actually very widespread due to the vastness of the space.
In addition to the solar wind, the heliosphere is also maintained by the Sun’s magnetic field, which extends outward at least 100 AU, and is shaped like a ballerina’s dress that spins due to the Sun’s rotation every 27 days. This structure, the heliospheric current sheet, creates a ripple in the entire heliosphere and, together with the heliosphere itself, is the largest structure in the solar system.
In addition to the heliospheric current sheet, the heliosphere has another structure. For example, there is the termination shock, a boundary about 70-90 AU from the Sun where the solar wind transitions from being supersonic to subsonic. This boundary was crossed by the Voyager II space probe in 2007. In fact, the probe crossed it five times, because the boundary fluctuates due to corresponding fluctuations in solar output, including solar flares. In space, the speed of sound is much faster than on Earth (about 100 km/s), so the solar wind is still moving fast at this distance, but not fast enough to exceed the speed of sound.
Further away from the termination shock is the heliopause, where charged particles in the solar wind collide with particles in the interstellar medium, and the arc shock, where the solar wind ceases to have any effect on the interstellar medium. Neither has yet been reached by our space probes, but they will be by 2020. Furthermore, the Interstellar Boundary Explorer, launched in 2008, will provide valuable information on the interstellar boundary.
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