The Ion Composition Boundary: A New Type of Plasma Boundary in the Inner Heliosheath

Voyager 1 observed a shock-like discontinuity in the magnetic field strength and proton density at 2020.4, where the compression ratio was 1.35 and 1.36, respectively, and there was no change in the magnetic field direction [Burlaga et al., 2023]. After the jump, the magnetic field strength remained at a higher level until recently, creating a magnetic hump or pileup region. A magnetic pileup boundary or ion composition boundary has been routinely observed between the cometary bow shock and the comet’s ionopause. Solar wind protons are reflected from and heavy cometary pickup ions (mainly water group ions) are transmitted through this boundary, also known as protonopause.  A similar ion composition boundary is expected in the inner heliosheath. As the solar wind approaches the very local interstellar medium (VLISM), the density of interstellar pickup protons and pickup He⁺ is gradually increasing. At the ion composition boundary, solar wind protons are reflected from the potential barrier. However, heavier pickup He⁺ ions with higher kinetic energy are able to cross this boundary, separating solar wind protons from heavier interstellar pickup ions. The electrons carry the magnetic field across the ion composition boundary without change in the magnetic field direction. To maintain pressure balance, the transmitted pickup ions and the magnetic field are compressed, creating a magnetic pileup region. Such an ion composition boundary or magnetic pileup boundary was first suggested by Sauer et al. [1995] in the magnetosheath of comets, Mars, and Venus. We suggest that the magnetic pileup boundary observed by Voyager 1 in 2020 is associated with an ion composition boundary in the solar wind. We present multi-fluid simulations of the ion composition boundary in the inner heliosheath. We show that an ion composition boundary is formed when the generalized sonic Mach number has reached 1 from below. The generalized sonic Mach number can be increased by either accelerating the plasma or reducing the sonic speed. As the solar wind approaches the heliopause, interstellar He⁺ pickup ions gradually reduce the sonic speed until the generalized sonic Mach number reaches 1 and a new type of plasma boundary, the ion composition boundary is formed. Our results imply that Voyager 1 is still in the inner heliosheath, has not crossed the heliopause, and has not entered the VLISM yet. We predict that Voyager 1 will continue to observe a stronger magnetic field until the heliopause is reached, which is expected to be a tangential discontinuity with a rotation in the magnetic field. The heliospheric ion composition boundary could be verified by New Horizons or other interstellar missions in the future, such as Interstellar Probe.