Variations in the pickup ion density structure in response to the growth of the Kelvin--Helmholtz instability along the heliopause

Abstract

Features of the response of pickup ions (PUIs) to the Kelvin--Helmholtz instability (KHI) on the heliopause (HP) are examined by means of two-dimensional hybrid simulations. We assume the supersonic neutral solar wind as the source of PUIs gyrating about the magnetic field in the outer heliosheath. These PUIs become energetic neutral atoms (ENAs) via charge exchange with interstellar hydrogen, and a portion of these ENAs are detected by spacecraft such as the Interstellar Boundary Explorer (IBEX}. To evaluate the possibility of identifying the KHI on HP from ENA observations, we assume that an imprint of the KHI may be displayed in spatial and temporal variations in the observed ENA profile. As an alternative to ENA, the column density of PUIs integrated across the HP is calculated. The KH-inducing vortex forces not only background protons but also PUIs to roll up deep in the inner heliosheath. The KH vortex also results in the emission of magnetosonic pulses that sweep PUIs in the outer heliosheath and lead to their local confinement. These effects elongate the PUIs spatial distribution in the direction normal to the HP. The appearance of the local confined structure in the PUIs column density is consequently confirmed, and this feature can be confirmed as the KHI evolution. Although the simulation cannot be quantitatively compared with the observations currently available because its resolution is too small, we expect that the derived properties will be useful for diagnosing the nature of HP fluctuation in future missions.