We analyze the solar wind interaction with Mars through examination of Mars Global Surveyor Magnetometer/Electron Reflectometer (MGS MAG/ER) data. We focus on data in the postshock, dayside plasma flow, especially the Magnetic Pileup Boundary (MPB). First, we present the data to characterize the features of the MPB. Next, we argue the evidence that the MPB is formed through a series of processes, beginning with electron impact ionization of planetary neutrals in the exosphere of Mars. These new ions form an unstable population of energetic ions in the post-shock flow. Ion cyclotron waves are established to scatter the ion distribution into one more stable. This removes thermal pressure from the ions. In order to maintain a constant total pressure in the flow, the magnetic field intensifies, transferring the lost ion thermal pressure into magnetic energy. Also, we develop a model of the electron impact ionization process in the martian exosphere. This model calculates the evolution of the electron distribution function as the flow encounters exospheric planetary neutrals. It reproduces the electron spectrum observed by the ER in the MPB very well. Therefore, we conclude that electron impact ionization is the process responsible for the onset of magnetic pileup in the postshock flow at Mars.