Rāṣṭradharmī 🥷

The Sun is not travelling through empty space. As it orbits the Milky Way, it moves through the local interstellar medium, while continuously releasing a stream of charged particles called the solar wind. This magnetized wind pushes against the surrounding interstellar environment, creating an enormous bubble around the Sun and its planets: the heliosphere. The solar wind travels outward at hundreds of kilometres per second. Far beyond Neptune, it encounters the pressure of the interstellar medium and slows abruptly at the termination shock. It then flows through a turbulent region called the heliosheath, before reaching the heliopause, where the solar wind's outward pressure is balanced by the surrounding interstellar pressure. Beyond this boundary, neutral hydrogen can accumulate in a region known as the hydrogen wall. The heliosphere may also have an extended tail, although its precise shape remains an active area of research. Only two spacecraft have directly crossed the heliopause. Voyager 1 crossed in August 2012 at approximately 122 AU, and Voyager 2 followed in November 2018 at approximately 119 AU. By November 2026, they had travelled roughly 172 AU and 144 AU from the Sun, respectively. Their measurements provide our first direct observations of the boundary between the Sun's plasma environment and interstellar space. The heliosphere does not stop every galactic cosmic ray, but it does reduce the number reaching the inner Solar System. It is a dynamic magnetic environment that helps shape the radiation conditions around our planetary neighbourhood.