Locally generated low-altitude Alfvén waves deliver Jupiter’s brightest auroras

Auroral acceleration by inertial Alfvén waves is commonly invoked to explain the preponderance of broadband electron energy distributions observed above Jupiter’s main auroral zones. These distributions extend to 100s keV and MeV energies and are associated with the brightest auroras. Jupiter’s low-altitude auroral zones represent a highly-magnetized, density-depleted plasma regime that is conducive to inertial Alfvén wave acceleration. However, despite the robust theoretical foundation, observational evidence remains lacking. Here, we perform a detailed analysis of auroral electron distributions above Jupiter’s auroral zones. We demonstrate the two types of distributions – monoenergetic and broadband – are separated by length scales where the perpendicular wave number is respectively less than or comparable to the inverse of the electron inertial length. Furthermore, in contrast to the longstanding acceptance that Alfvén waves exclusively originate remotely from Jupiter’s equatorial plasma sheet, we demonstrate they can be locally generated at Jupiter’s low altitudes via a beam-plasma instability. From this new understanding, we find locally-generated Alfvén waves are directly responsible for accelerating the most intense auroral electrons at Jupiter.