Bulk Velocity and Thermal Properties of the Solar Wind in the Inner Heliosphere

Abstract:

Despite over 50 years of satellite exploration beyond near-Earth space, only sparse in-situ observations have been made of the solar wind in the inner heliosphere (distances of less than 1 AU). In recent years, the Fast Imaging Plasma Spectrometer (FIPS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft was able to measure the bulk and thermal speeds of protons in the solar wind at distances of 0.3 – 0.5 AU. Using data from FIPS, we build upon the work of Gershman et al. (2012) and present a detailed study of the bulk velocity and thermal properties of the solar wind for the time period of March 2011 to March 2015. We compare and contrast the FIPS observations at 0.3 – 0.5 AU with measurements from the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE) satellite at 1 AU. Of particular interest is the radial evolution of proton thermal speeds inside Corotating Interaction Regions (CIRs), Interplanetary Coronal Mass Ejections (ICMEs), and quiescent solar wind. This study represents the most complete survey of solar wind plasma inside 0.5 AU since the Helios mission in the early 1980’s. Furthermore, our results provide a preview of the exciting science possible with the upcoming launches of Solar Orbiter and Solar Probe Plus in 2018.