Potential Field Line Resonances in Ganymede's Magnetosphere Observed by Juno

On June 7, 2021 Juno performed a close flyby of Ganymede, providing a unique opportunity to improve the current understanding of the magnetosphere. Here, we study the high-resolution (64 Hz signal) magnetic field spectrum employing three different approaches: 1) analysis of a ~3.5 minute near Juno's closest flyby to the moon, 2) analysis of 12 sliding windows of ~2 min in this period and, and 3) analysis of a ~63 s window selected based on the dominant frequencies observed in the evolutionary spectra of the entire flyby. A fundamental period of ~16 s is observed in all three methods. In the specific case of the spectrum of the 63 s window, this shows a harmonic structure which could potentially be related to the frequencies associated with resonances of a dipole field. If this were the case, we estimate that the plasma density near the equator at 1.685 RG (selected L-shell) is equal to 22 amu/cm3, a much lower value than the local density of the plasma sheet near Ganymede (~100 amu/cm3). These two aspects may indicate that Juno crossed closed magnetic field lines of Ganymede's magnetosphere for a short period of time only, if at all.