Space Sci Rev (2007) 130: 207–219 DOI 10.1007/s11214-007-9187-1 On the Differences in Composition between Solar Energetic Particles and Solar Wind R.A. Mewaldt · C.M.S. Cohen · G.M. Mason · A.C. Cummings · M.I. Desai · R.A. Leske · J. Raines · E.C. Stone · M.E. Wiedenbeck · T.T. von Rosenvinge · T.H. Zurbuchen Received: 5 February 2007 / Accepted: 4 April 2007 / Published online: 26 May 2007 © Springer Science+Business Media, Inc. 2007 Abstract Although the average composition of solar energetic particles (SEPs) and the bulk solar wind are similar in a number of ways, there are key differences which imply that solar wind is not the principal seed population for SEPs accelerated by coronal mass ejection (CME) driven shocks. This paper reviews these composition differences and considers the composition of other possible seed populations, including coronal material, impulsive flare material, and interplanetary CME material. Keywords Sun: particle emission · Sun: solar wind · Sun: coronal mass ejections (CMEs) · Sun: abundances · Sun: flares · Acceleration of particles 1 Introduction As solar cycle 23 began, two classes of solar energetic particle (SEP) events were recog- nized that could be distinguished by their composition, spatial, and temporal characteristics, R.A. Mewaldt ( ) · C.M.S. Cohen · A.C. Cummings · R.A. Leske · E.C. Stone California Institute of Technology, Pasadena, CA 91125, USA e-mail: rmewaldt@srl.caltech.edu G.M. Mason Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA M.I. Desai Southwest Research Institute, San Antonio, TX 78238, USA J. Raines · T.H. Zurbuchen University of Michigan, Ann Arbor, MI 48109, USA M.E. Wiedenbeck Jet Propulsion Laboratory, Pasadena, CA 91109, USA T.T. von Rosenvinge NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA