Jupiter's brightest auroras are powered by locally generated Alfvén waves.

Jupiter displays the brightest auroras in the solar system, driven by intensely varying energetic electrons that accelerate into the atmosphere. Here, we provide a comprehensive formalism that accurately describes the bimodality of observed auroral electron acceleration regimes. We demonstrate that the occurrence of the two dominant auroral regimes-broadband and monoenergetic-is dictated by specific characteristic plasma length scales, namely, the relationship between the transverse length scale and the electron inertial length. Furthermore, in contrast to the long-standing acceptance that Alfvén waves responsible for auroral electron acceleration exclusively originate at distant equatorial regions, we demonstrate that they can be locally generated at Jupiter's low altitudes. From this understanding, we find that locally generated, low-altitude Alfvén waves can be directly responsible for accelerating the most intense auroral electrons at Jupiter. These findings establish that the low-altitude acceleration region can itself become a source of Alfvén waves rather than merely a sink for wave energy, thus altering the standard picture of global magnetospheric energy transfer in strongly magnetized planets.