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.

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