We present multi-spacecraft measurements of a quasi-parallel shock crossing obtained by the Cluster satellites. Observations commonly show the presence of Short Large-Amplitude Magnetic Structures (SLAMS) at the Earth's quasi-parallel bow shock. These structures are thought to play an important role for decelerating, heating and deflecting the incident solar wind. We investigate the electric potential over SLAMS in the Normal Incidence Frame NIF as a possible means for achieving thermalisation in the foreshock region of the Earth's quasi-parallel bow shock. We show that SLAMS exhibit a substantial electric potential on the order of a few hundred Volt (V). Thus at the Earth's quasi-parallel bow shock, these structures may be important for dissipation that transforms the ram energy of the solar wind bulk flow into thermal energy. SLAMS might be responsible for returning particles, that have been reflected at the shock and whose guiding centre motion is pointing upstream, back to the shock.
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