The Sun travels through a partially ionized very local interstellar medium (VLISM) as it emits the solar wind inflating a bubble called the heliosphere. Charged particles from the solar wind and the VLISM are separated on two sides of the heliopause, but interstellar neutral (ISN) atoms enter the heliosphere. However, the ISN atom distribution evolves as the ISN gas flows through the outer heliospheric regions due to collisions with other species. The ISN H is by far the dominant species in the VLISM, whereas inside the heliosphere, the neutral H is a very sensitive indicator of the various interaction processes between the Sun and the VLISM.
ISN H atoms are observed using multiple observation techniques. Direct sampling of these atoms is possible using neutral atom detectors, e.g., IBEX-Lo. The line-of-sight-integrated density distributions of ISN H are also inferred from the Lyman-α backscattered light, observed at 1 au (SWAN/SOHO) and on spacecraft farther away from the Sun (Voyager 1, New Horizons). Moreover, ionized ISN atoms form a suprathermal population of pickup ions (PUIs) in the solar wind measured by charged particle detectors. For example, recent SWAP observations on New Horizons of H PUIs showed that the ISN H density might be significantly higher than previously thought.
Several processes need to be included to interpret these observations. First, charge exchange and elastic collisions modify the ISN H distribution beyond the heliopause. Inside the heliosphere, they are further ionized by charge exchange and electron impact collisions as well as by photoionization. Lastly, their ballistic trajectories are modified due to time-, distance-, and radial velocity-dependent solar radiation pressure.
The proposed International Team aims to estimate the spatial distribution of the ISN H inside the heliosphere and its velocity distribution functions at key positions observed by the currently operating spacecraft. The team plans to develop a new model to replace a widely used, two-component approximation with primary and secondary components modeled with Maxwell distributions at the termination shock (TS). This approximation is likely responsible for discrepancies between results of the ISN H studies from different experiments. The team will also work towards an estimation of the ISN H properties in the pristine VLISM. Good models of the ISN H distribution are necessary to interpret energetic neutral atom (ENA) observations as well as to understand the global structure of the heliosphere.
The team members have expertise in different aspects of the ISN H atom physics in the heliosphere, from theoretical consideration of the mentioned effects, through global heliosphere modeling, to the interpretation of observational data. Such a diverse team gives a unique opportunity to discuss H atoms propagation from the VLISM to the heliosphere and the coupling of these crucial effects on the existing observations. The team results will help with the planning of operations of future space instruments, e.g., on IMAP.