Radial Trends in Plasma Parameters Across the Heliosphere

Though the solar wind is characterized by spatial and temporal variability across a wide range of scales, long-term averages of in-situ measurements reveal clear radial trends: changes in average values of basic plasma parameters (e.g., density, temperature, speed, and magnetic field) with distance from the Sun. To establish our current understanding of such trends, data from twelve heliospheric spacecraft --- 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 --- were compiled into a dataset spanning over three orders of magnitude in solar distance. To avoid introducing artifacts into this composite dataset, special attention was given to solar cycle, spacecraft heliocentric elevation, and instrument calibration. The radial trend in each parameter was found to be generally well described by a power-law fit, though up to two break-points were identified in each fit. These radial trends are soon to be publicly released to benefit research groups in the validation of global heliospheric simulations and in the development of new deep-space missions such as Interstellar Probe.