The Trans-Heliospheric Survey

Radial trends in plasma parameters across the heliosphere^★

¹ Department of Physics & Astronomy, University of Delaware, 104 The Green, 217 Sharp Lab, Newark, DE 19716, USA
e-mail: bmaruca@udel.edu
² Bartol Research Institute, 104 The Green, 217 Sharp Lab, Newark, DE 19716, USA
³ Center for Space Physics, Boston University, 725 Commonwealth Ave, Room 506, Boston, MA 02215, USA
⁴ Space Science and Engineering, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA
⁵ Department of Mechanical Engineering, 44 Cummington Mall, Boston University, 44 Cummington Mall, Boston, MA 02215, USA
⁶ NASA Headquarters Science Mission Directorate, 300 Hidden Figures Way SW, Washington, DC 20546, USA
⁷ Mullard Space Science Laboratory, University College London, Holmbury House, Holmbury St. Mary, Dorking RH5 6NT, UK
⁸ Department of Astrophysical Sciences, Princeton University, Peyton Hall, 4 Ivy Lane, Princeton, NJ 08544, USA
⁹ Heliophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
¹⁰ Laboratory for Atmospheric and Space Physics, 1234 Innovation Drive, Boulder, CO 80303, USA
¹¹ School of Chemical and Physical Sciences, Victoria University of Wellington, Gate 7, Kelburn Parade, Kelburn, Wellington 6012, New Zealand
¹² University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA

Received: 19 January 2023
Accepted: 6 April 2023

Abstract

Context. Though the solar wind is characterized by spatial and temporal variability across a wide range of scales, long-term averages of in situ measurements have revealed clear radial trends: changes in average values of basic plasma parameters (e.g., density, temperature, and speed) and a magnetic field with a distance from the Sun.

Aims. To establish our current understanding of the solar wind's average expansion through the heliosphere, data from multiple spacecraft needed to be combined and standardized into a single dataset.

Methods. In this study, data from twelve heliospheric and planetary 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 heliocentric distance. To avoid introducing artifacts into this composite dataset, special attention was given to the solar cycle, spacecraft heliocentric elevation, and instrument calibration.

Results. 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.

Conclusions. These radial trends are publicly released here to benefit research groups in the validation of global heliospheric simulations and in the development of new deep-space missions such as Interstellar Probe.