The heliosphere is the region of space governed by the Sun. It is filled with plasma from the Sun in the form of the solar wind. Outside the heliosphere is plasma from other stars (interstellar space). The heliopause marks the division between the solar plasma and the interstellar plasma. This is where the pressure of the solar wind equals the pressure of the interstellar medium. The Heliosphere
NASA art

Before reaching the heliopause the solar wind is slowed from supersonic to subsonic velocity, and this creates a shockwave shown on the above diagram as the termination shock. The exact positions of both the heliopause and the termination shock vary with time as the pressure of the solar wind changes. At sunspot minimum, the density of the solar wind is reduced and these boundaries contract inward toward the Sun. Conversely, during times of intense coronal mass ejection (CMEs), the density, velocity and magnetic field strength of the interplanetary medium increase. The increased dynamic pressure and the magnetic pressure of the solar wind during these times pushes the heliopause away from the Sun.

Heliospheric density graph

The Voyager spacecraft have crossed the termination shock several times as it moves in and out from the Sun.

The heliopause has been found to generate high power VLF radio waves (2 - 3 kHz), particularly when CMEs crash against it. However, these signals can only be detected beyond Saturn because only then does the interplanetary plasma frequency (proportional to the square root of the plasma density) drop low enough to allow signal propagation at these very low frequencies.

The International Heliophysical Year (2007 - 2009) is a United Nations sponsored scientific program to promote studies of physical phenomena within the heliosphere. Following 50 years after the very successful International Geophysical Year, it expands the domain of study from the Earth to the extended solar system. More details can be found at the official IHY website. IHY logo

ASAAustralian Space Academy