Motivated by plasma and wave measurements in the cusp auroral region, we have investigated electron-acoustic solitons in a plasma consisting of fluid ions, a cool fluid electron and a hot Boltzmann electron component. A recently described method of integrating the full nonlinear fluid equations as an initial-value problem is used to construct electron-acoustic solitons of arbitrary amplitude. Using the reductive perturbation technique, a Korteweg-de Vries equation, which includes the effects of finite cool-electron and ion temperatures, is derived, and results are compared with the full theory. Both theories admit rarefactive soliton solutions only. The solitons are found to propagate at speeds greater than the electron sound speed (ε0c/ε0ε)½υε, and their profiles are independent of ion parameters. It is found that the KdV theory is not a good approximation for intermediate-strength solitons. Nor does it exhibit the fact that the cool- to hot-electron temperature ratio restricts the parameter range over which electron-acoustic solitons may exist, as found in the arbitrary-amplitude calculations.
