Proton auroras during the transitional stage of substorm onset

Abstract Optical auroral measurements repeatedly reveal the existence of a transitional stage between a quiescent preexisting arc and its significant auroral expansion in a substorm onset. Such a transitional stage of substorm onset (TSSO) is characterized by a gradual intensification and the emergence of auroral beads, along the preexisting arc. However, existing studies on TSSO are limited to electron auroras which are dominant in optical luminosity. In this study, we collect 12 substorm events to investigate the proton auroral features during the late growth phase and the TSSO. Our major results include: (1) we confirm the previous notion that the onset electron auroral arc is usually located at the poleward “shoulder” of the main proton auroral band. (2) While the electron auroral arc typically intensifies by a few times or even an order of magnitude during the TSSO, the concurrent proton aurora at the same location as the electron auroral arc shows much less noticeable variations. The proton auroral variations averaged over the arc band, as well as that integrated over the entire latitudinal range, are mostly within 10% of their mean late-growth-phase levels during the TSSO. Substantial intensifications of proton auroras occur after the poleward expansion of electron auroras. Even considering the spatial spreading of proton auroras, we estimate that the variation of ion precipitation fluxes on top of the ionosphere would be typically < 30% during the TSSO. The above observations impose implications and quantitative constraints on the possible mechanisms of TSSO and substorm onset. We assert that there is no significant ion energization or large-scale magnetic field reconfiguration during the TSSO. Instead, it is likely that the underlying mechanism of the TSSO might be certain kind of ky-dominated instability wave mode. The instability wavelength is comparable to the ion gyroradius, so that the ions are demagnetized, suppressing ion flux variations with the instability. Enhancing upward electric fields in the auroral acceleration region during the TSSO may also play a partial role in weakening the ion precipitation flux variation