The heliopause is the boundary between the solar system and interstellar space.
The heliosphere is the plasma cavity of space maintained by the Sun via its energetic emissions. The heliopause is the plasmic film which defines the heliospheric bubble: an energetic shell roughly 18 billion kilometers from the Sun, which is 119 times farther away from the Sun than Earth.
The heliopause is a definite boundary rather than a wimping out of solar energy. The level of free-flying protons – the contents of cosmic rays – surges past the heliopause by a factor of 20.
The heliopause is an energetic membrane that breathes with the 11-year solar energy cycle, as well as somewhat varying in character locally. This was shown by the distinct readings of 2 NASA probes – readings taken 24 billion kilometers and 6 years apart.
The Sun’s magnetic field lines up nearly perfectly with the galactic field: a strange harmonic consistency considering the dissonance in high-energy radiation between the 2 plasma fields.
Sources:
Christopher Crockett, “Voyager 2 reveals the dynamic, complex nature of the solar system’s edge,” Science News (4 November 2019).
Hannah Devlin, “Nasa’s Voyager 2 sends back its first message from interstellar space,” The Guardian (4 November 2019).