Top panel: cross sections for collisions of H⁺ with H vs. collision energy in the center-of-mass frame E_cm. Momentum transfer cross section from Krstić & Schultz (1999), Glassgold et al. (2005), and Schultz et al. (2008), and charge-exchange cross section from Hodges & Breig (1991) and Schultz et al. (2008). Dashed line: Langevin m.t. cross section. Experimental data for the c.e. cross section: Newman et al. (1982) (filled triangles). The empty circle shows the value of the m.t. cross section measured by Brennan & Morrow (1971). Bottom panel: Collisional rate coefficient for momentum transfer and charge exchange. The filled and empty circles show the rate coefficient adopted by Kulsrud & Cesarsky (1971) and Zweibel & Shull (1982), respectively.