Studying The Local Interstellar Medium through Measurements of Interstellar Hydrogen Inside the Heliosphere

The Interstellar Medium (ISM) incorporates all the matter that fills up the space between the stars of a galaxy. Interstellar matter consists mainly of hydrogen and helium gas, either in atomic or ionized form, as well as some heavier atoms, molecules and dust particles. It varies in temperature and density, forming structures and interacting with stars as a part of the stellar evolution. The Sun’s magnetic field and solar wind forms the heliosphere, effectively shielding us from our interstellar surroundings as we travel through the interstellar medium. However, the neutral component of the ISM, mainly in the form of hydrogen atoms, are not directly affected by the Sun’s magnetic field and can therefore enter the heliosphere where they can be observed through their interactions with solar Lyman-alpha photons that then produce the Lyman-alpha background radiation in the heliosphere. The SWAN instrument, on the SOHO satellite, measures this radiation and can provide a good picture of the interstellar hydrogen inside the heliosphere. In this work I introduce our current understanding of the Local ISM and ways of observing and modelling it. By modelling the intensity signal observed by SWAN, I also show an example of analysing interstellar parameters using SWAN observations of interstellar hydrogen inside the heliosphere