Auroral substorms are the result of a complex process involving coupling between the solar wind, magnetosphere and ionosphere. In this study, the objective is to understand the processes that precede what is perhaps the most fundamental aspect, the brightening of a preexisting arc at onset. The notion that this brightening provides an indicator of substorm onset remains one of the few aspects of substorms that is not disputed. In order to understand the processes that lead up to onset, auroral arcs that form during substorm growth phases are examined using ground‐based (CANOPUS) data with in situ measurements acquired by the FAST satellite. Four cases have been studied and are discussed in detail here. Consistent with other reports, the data show that the growth phase arcs persist for 30 min or more. During this time, the arcs often show little or no fluctuations, although a gradual intensification is clear in the ground‐based optical measurements. Two important results emerge from this study. First, FAST observations show that growth phase arcs develop within the region of proton precipitation but generally poleward of most intense precipitation, a topic that has been addressed (and disputed) using ground‐based observations of luminosity, a secondary effect. Second, we show that FAST observations before the growth phase arc is established show scattered plasma sheet electrons with no significant structure. However, once the growth phase is well developed, FAST confirms that the arcs are excited via inverted V precipitation, implying that an inverted V potential drop gradually develops in conjunction with the evolution of the growth phase.
