What defines the heliosphere?

The heliosphere is best defined as a region filled with the interplanetary medium, which is influenced by the solar wind—the continuous stream of charged particles released from the Sun. This region extends far beyond the orbit of Pluto and marks the boundary between the solar wind and the interstellar medium, which is the matter that exists in the space between stars.

In the context of the options provided, this definition emphasizes that the heliosphere is part of a larger interplay between solar activity and the surrounding cosmic environment. It serves to protect the planets within the solar system from cosmic radiation by creating a bubble-like region that deflects high-energy particles from outside the solar system.

Understanding this distinction is crucial as it highlights the dynamics of solar processes and their effects on space weather and planetary environments, which can have significant implications for satellite operations and human activities in space.