Heliopause

The Sun continuously releases charged particles and carries a magnetic field outward in the solar wind, creating a vast region called the heliosphere. The solar wind interacts with the gas, dust, plasma and magnetic field of the space between stars. The heliopause marks the boundary where the outward solar influence gives way to the surrounding interstellar medium. Because the boundary is defined by interacting flows and fields, it is a diffuse physical transition, not a rigid shell.

Farther from the Sun, the solar wind encounters increasing resistance from the interstellar environment. Before reaching the heliopause, it crosses the termination shock, where its outward flow slows; beyond that lies the heliosheath, an outer region still strongly influenced by solar particles and magnetic fields. At the heliopause, the solar wind’s influence is sufficiently diminished that the spacecraft enter interstellar space as operationally identified through in-situ measurements. The shape and position of the boundary can vary with solar activity and location.

Voyager 1 crossed the heliopause on 25 August 2012, at about 122 astronomical units from the Sun, becoming the first spacecraft to enter interstellar space. Its particle instruments recorded a rise in cosmic rays and a decline in particles from the heliosphere. Because its plasma instrument had stopped working, the crossing was corroborated later when plasma-wave measurements indicated denser surrounding plasma. Voyager 2 crossed on 5 November 2018; its working plasma instrument provided direct measurements alongside changes in particle data.

Voyager measurements show that the environment near and just beyond the heliopause is complex. The interstellar plasma is denser than the plasma inside the heliosphere, and observations indicate that material and fields can be disturbed around the boundary. Voyager 2 measurements also found evidence of compression near the heliopause and a small leakage of heliospheric particles across it. The two probes crossed at different locations and times, so their observations do not imply a perfectly uniform surface or a single fixed distance from the Sun.

The heliopause is a natural laboratory for studying how a star’s outflow interacts with interstellar matter. Measurements made by spacecraft crossing it help scientists map the extent of the heliosphere, examine the transfer of particles and magnetic fields across its edge, and understand the environment surrounding our solar system. The Voyager spacecraft remain the only probes to have made direct in-situ measurements after crossing this boundary.