Radial Trends in Plasma Parameters Across the Heliosphere

Though the solar wind is characterized by spacial and temporal variability, long-term averages of in-situ measurements reveal clear radial trends: changes in average values of plasma parameters (e.g., density, temperature, and magnetic field) with distance from the Sun. Ahead of the Interstellar Probe mission, we establish our best understanding of these radial trends using data from 12 existing missions: Parker Solar Probe (PSP), Helios 1 and 2, Mariner 2 and 10, Ulysses, Cassini, Pioneer 10 and 11, New Horizons, and Voyager 1 and 2. To create this composite dataset, we give special attention to variations in solar cycle, spacecraft heliocentric elevation, and instrument calibration. Owing to the non-contemporaneity of these missions, this analysis focuses on long averages that span many solar-wind streams and show the solar wind's "typical" evolution. These results reveal radial variations in the solar wind's heating and turbulence, offer improved constraints on global simulations of the heliosphere, and provide baseline expectations for observations from Interstellar Probe.