In situ and remote observations of the ultraviolet footprint of the moon Callisto by the Juno spacecraft

Abstract

Jupiter exhibits peculiar multiwavelength auroral emissions resulting from the electromagnetic interactions of Io, Europa, and Ganymede with the magnetospheric plasma flow. Characterizing the faint auroral footprint of the fourth Galilean moon, Callisto, has always been challenging because of its expected weakness and its proximity to Jupiter's bright main aurora. Here, we report on unusual magnetospheric conditions that led to an equatorward shift of Jupiter's main auroral oval unveiling the auroral footprints of the four Galilean moons in a single observation. Remote observations by the Juno spacecraft reveal a double-spot structure, characteristic of the footprints of the other three moons, with a maximum ultraviolet brightness of 137 $\pm$ 15 kR. Concurrent observations within Callisto's flux tube reveal field-aligned electrons with a characteristic energy of 10 keV, depositing an energy flux of 55 mW.m^-2 in Jupiter's atmosphere. The electron properties are consistent with the triggering of radio emissions with intensities lower than 5 × 10^-18 W.m^-2.Hz^-1.

Fig. 1. Juno-UVS observations of the northern auroral region of Jupiter during PJ22.

a Juno trajectory plotted in magnetic coordinates. The gray area indicates where the in situ measurements shown in Fig. 3 have been made. b False color UV map of the auroral structures observed onto Jupiter’s northern auroral region resulting from co-adding consecutive Juno-UVS data from 02:54:00 to 03:09:02. The colors represent various UV spectral bands: red, green, and blue tend to correspond to high-, medium-, and low-energy electron precipitation, respectively, while white indicates a mixture of energies^,.The magnetic footpaths of the Juno spacecraft and the Galilean moons are shown as orange and green lines, respectively. The auroral footpaths of Io, Europa, and Ganymede (green solid lines) were calculated using JRM33 + CON2020^, while the Callisto footpath (green dashed line) was derived using JRM33 + KK2005 (see “UV maps” in Methods). The white triangles along the Juno footpath highlight Juno’s magnetic footprints with a 10 min time step. Juno’s magnetic footprints at the beginning and end of the Juno-UVS data integration time are represented by orange dots. The white boundaries show the statistical position of the main oval emissions. Orange crosses indicate the statistical location of the Main Alfvén Wing (MAW) spots of Io, Europa, and Ganymede. The footprints of the four Galilean moons are outlined by orange lines. The un-annotated Juno-UVS observation is displayed in Supplementary Fig. 3.

Fig. 2. Time evolution of the Callisto footprint.

a Juno-UVS spin-by-spin observations of the Callisto UV footprint. The leading and trailing spots, identified as Transhemispheric electron beam (TEB) and Main Alfvén Wing (MAW) spots, are highlighted by orange and  red lines, respectively, when observed. The dashed green line represents Callisto’s footpath derived using the JRM33 + KK2005 magnetic field model. Each frame is centered around the same S_III longitude/latitude, shown as a white dotted grid. In this reference frame, the Callisto footprint gradually drifts over time. Conversely, nearby auroral emissions, co-rotating with Jupiter’s magnetosphere, are fixed over time. b Evolution of the System-III (S_III) equatorial longitude of the TEB (orange) and MAW (red) spots of Callisto as a function of time.

Fig. 3. JADE-E and Waves in situ observations.

a Electron energy-time spectrogram. b Pitch angle (PA)-time spectrogram for electrons with energies  between 1.1 and 30.2 keV. The size of the loss cone is indicated by the dashed gray lines. c Partial electron downward energy flux (EF Down, in black) and characteristic energy (EC Down, in blue). Electrons with energy between 1.1 and 30.2 keV have been considered. d Frequency-time spectrogram of the ratio of the electric (E) to magnetic (cB) spectral densities. Color bands above (a) indicate on which auroral structure on Jupiter’s northern pole Juno’s magnetic footprint maps. Juno’s M-Shell, altitude (Alt), magnetic latitude (MLat), and longitude separation to Callisto as a function of time are indicated below the panel (d).

Fig. 4. Electron energy and velocity distributions within the Callisto flux tube.

a Electron energy distribution. Colored curves represent the distribution measured with a 1 s resolution during the flux tube crossing. The gray curves highlight the electron distributions measured during 30 s prior to the crossing, outside of the Callisto flux tube. b Electron distribution function in the velocity space $\left({\mathrm{v}}_{\mathrm{\mid \mid }},{\mathrm{v}}_{\mathrm{\perp }}\right)$ at the beginning of the flux tube crossing. The size of the loss cone is indicated by the red line. The gray circle indicates the resonance circle that maximizes the growth rate, enabling wave amplification. Note that at ${\mathrm{v}}_{\mathrm{\mid \mid }}$ = 0 m.s^-1, the iso-energy contours decrease, which is due to spacecraft shadowing.