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The heliopause is the boundary between the heliosphere and the interstellar medium outside the solar system. As the solar wind approaches the heliopause, it slows suddenly, forming a shock wave called the termination shock of the solar wind.

In astronomy, the heliopause is the boundary where the Sun's solar wind is stopped by the interstellar medium.

The solar wind blows a "bubble" known as the heliosphere in the interstellar medium (the hydrogen and helium gas that permeates the galaxy). The outer border of this "bubble" is where the solar wind's strength is no longer great enough to push back the interstellar medium. This is known as the heliopause, and is often considered to be the outer border of the solar system.

Inside the heliopause is a boundary called the "termination shock" where supersonic solar wind particles are slowed to subsonic speeds by the interstellar medium. The layer between the termination shock and the heliopause is known as the heliosheath.

Outside the heliopause, the interaction between the interstellar medium and the heliopause produces the bow shock, a turbulent region in front of the Sun's progress through the interstellar medium.

According to one hypothesis^[1], there exists a region of hot hydrogen known as the hydrogen wall between the bow shock and the heliopause. The wall is composed of interstellar material interacting with the edge of the heliosphere.

Another hypothesis suggests that that the heliopause could be smaller on the side of the solar system facing the orbital motion through the galaxy. It may also vary depending on the current velocity of the solar wind and the local density of the interstellar medium. It is known to lie far outside the orbit of Pluto. The current mission of the Voyager 1 and 2 spacecraft is to find and study the termination shock, heliosheath, and heliopause. Thus far, Voyager 1 has reached the termination shock and "Voyager 2" is close according to NASA announcements made on May 23-24, 2006 ^[2]. It is anticipated that both missions may ultimately reach the heliopause itself. In the mean time, the Interstellar Boundary Explorer (IBEX) mission will attempt to image the heliopause within two years of its 2008 launch.

Voyager 1 & 2 and Pioneer 10 & 11 approaching the heliosheath

When particles emitted by the sun bump into the interstellar ones, they slow down while releasing energy. Many particles accumulate in and around the heliopause, highly energised by their negative acceleration, creating a shock wave.

An alternative definition is that the heliopause is the magnetopause between the solar system's magnetosphere and the galaxy's plasma currents.

1. ^ The Properties of the Local Interstellar Medium and the Interaction of the Stellar Winds
2. ^ Voyager Enters Solar System's Final Frontier

• Fermi glow
• Solar flare
• Voyager Program

• Voyager Interstellar Mission Objectives
• UI's Don Gurnett Says Voyager 1 Is Approaching Edge Of Solar System December 8, 2003 Univ. of Iowa Press release
• Heliopause Seems to Be 23 Billion Kilometres, article from Universe Today

• The heliospheric hydrogen wall and astrospheres
• Heliosphere, has a diagram.
• Global Structure of the Heliosphere
• Publications in Refereed Journals

The Sun v • d • e
Structure: Solar Core - Radiation Zone - Convection Zone
Atmosphere - Photosphere - Chromosphere - Transition region - Corona
Extended Structure: Termination Shock - Heliosphere - Heliopause - Heliosheath - Bow Shock
Solar Phenomena: Sunspots - Faculae - Granules - Supergranulation - Solar Wind - Spicules
Coronal loops - Solar Flares - Solar Prominences - Coronal Mass Ejections
Moreton Waves - Coronal Holes
Other: Solar System - Solar Variation - Solar Dynamo - Heliospheric Current Sheet - Solar Radiation - Solar Eclipse