A new four‐plasma categorization scheme for the solar wind

Abstract

Abstract A three‐parameter algebraic scheme is developed to categorize the solar wind at 1 AU into four plasma types: coronal‐hole‐origin plasma, streamer‐belt‐origin plasma, sector‐reversal‐region plasma, and ejecta. The three parameters are the proton‐specific entropy S p = T p / n p 2/3 , the proton Alfvén speed v A , and the proton temperature T p compared with a velocity‐dependent expected temperature. Four measurements are needed to apply the scheme: the proton number density n p , the proton temperature T p , the magnetic field strength B , and the solar wind speed v sw . The scheme is tested and is found to be more accurate than existing categorization schemes. The categorization scheme is applied to the 1963–2013 OMNI2 data set spanning four solar cycles and to the 1998–2008 ACE data set. The statistical properties of the four types of plasma are examined. The sector‐reversal‐region plasma is found to have statistically low alpha‐to‐proton density ratios and high Alfvén Mach numbers. The statistical relations between the proton and alpha‐particle‐specific entropies and oxygen and carbon charge‐state‐density ratios S p , S α , O 7+ /O 6+ , and C 6+ /C 5+ from ACE are examined for the four types of plasma: the patterns observed imply a connection between sector‐reversal‐region plasma and ejecta and a connection between streamer‐belt‐origin plasma and coronal‐hole‐origin plasma. Plasma occurrence rates are examined and solar cycle patterns are found for ejecta, for coronal‐hole‐origin plasma, and for sector‐reversal‐region plasma.