GoKawiilData
Observations for this study are from the Juno spacecraft of NASA32. The energetic particle data are provided by the JEDI47, which measures ions and electrons from about 30 keV to 1 MeV with an energy resolution of around 20%. JEDI consists of three identical sensor heads (JEDI90, JEDI180 and JEDI270) distributed around the spacecraft to optimize pitch angle coverage over a 160° × 12° field of view with an angular resolution of about 18°. The first two energy bins of JEDI used in this study are contaminated and are not included in the analysis, resulting in four energy bins covering approximately 100 keV to 1 MeV, as shown in Fig. 2. Lower-energy ion and electron observations are obtained from the JADE48. JADE consists of two electron sensors (JADE-E) and an ion sensor (JADE-I), both measuring ions with energy per charge from 10 eV q−1 to 46.5 keV q−1 across 64 energy channels and electrons with energy per charge from 30 eV q−1 to 32 keV q−1, with a time resolution that is mode dependent and corresponds to about 2 min in the presented event. Magnetic field vector data are sourced from the Magnetic Field Investigation (MAG) instrument49, which uses two fluxgate magnetometers to provide measurements with a temporal resolution of 1 s. All data are presented in the JSO coordinate system, a Jupiter-centred frame in which the x-axis points to the Sun, the y-axis is in the anti-direction of the orbital motion of Jupiter and the z-axis completes the right-handed system50.
Data post-processing and density calculations
The raw instrument data were processed to generate the products used in this analysis. The JEDI energy-time spectrograms (Fig. 1c) were created by averaging data from all three sensors and all look directions. During the observation period, the instrument operated in a low-resolution mode, binning counts into six logarithmically spaced energy channels from about 30 keV to 1 MeV and into 300 s time bins. The count rates associated with the transient event, ranging from about 20 to 60 counts per second, are considered statistically significant. The electron energy efficiency correction detailed in ref. 51 was applied, although its effect is minimal in the low-radiation environment near the magnetospheric boundary of Jupiter. For JADE, proton densities were derived from JADE-I data using a numerical integration method on SPECIES=3 data52. Although JADE-I is not optimized for solar wind measurements53, this method has been shown to be consistent with forward-modelled Maxwellian fits for similar events28. The omnidirectional differential number intensities for JADE-E were calculated by averaging the observed intensities over 48 look directions, which are binned onboard in the low rate science mode of the instrument48.
Bow shock and foreshock transient characterization
In Extended Data Fig. 1, a magnified timeseries of the foreshock transient interval (11:30–13:30 UTC) is shown. Energetic particle intensification and plasma density depletion begin at about 12:30 UTC, with a localized compression marking the trailing edge of the structure at approximately 12:50 UTC, typical features of foreshock transients4,5,22,54,55.
To better characterize the global environment during this encounter, we use the local magnetic field conditions and the shock normal vector estimated in ref. 56. Using this, we obtain a normal vector of [0.77, 0.45, −0.44], consistent with the duskward Juno location. The orientation of the magnetic field with respect to this normal is shown in Extended Data Fig. 1e, suggesting that the shock orientation transitions from an oblique or quasi-parallel to a quasi-perpendicular one. Specifically, during the formation and observation of the transient itself, the orientation becomes even more quasi-parallel. This shock geometry (with θ Bn ≲ 60°) is expected to produce substantial populations of foreshock suprathermal particles57,58,59 associated with the formation of foreshock transients4,5,22,54.
Particle data further support this interpretation. The presence of diffuse, isotropic suprathermal ions and electrons indicates that the spacecraft is residing within the foreshock region60. Specifically, the pitch angle distributions (PADs) of ions and especially electrons show a clear isotropic population of accelerated particles. These PADs demonstrate that particles are distributed across all pitch angles, a signature of well-scattered populations within the foreshock. This is in agreement with characteristics of accelerated electrons observed during foreshock transients at Earth5,18,19. An illustration of the environment and associated transient is shown in Extended Data Fig. 2.
Focusing on the foreshock transient (12:30–12:50 UT), the electron PAD signature shows a progression as the transient passes through the spacecraft. This signature suggests that particles are accelerated in the approaching region, peaking within the transient and ceasing as the spacecraft exits the structure and the field rotates to a quasi-perpendicular regime after 12:50 UT. This strongly supports a local acceleration mechanism because if the source was external, energetic electrons would be observable over wider intervals. Instead, their strict localization to the transient structure implies they are generated in situ rather than being remote-sensed. Regarding the broader spatial context, based on spacecraft speed (about 4 km s−1) and the interval duration, we estimate that Juno was residing approximately 1R J upstream of the bow shock (Fig. 2, inset). This serves as an approximate estimate, as the bow shock at planetary flanks can change location rapidly. This estimate is consistent with observations at Earth, in which transients are observed at around 1−4R E (refs. 19,61,62).
To determine the exact geometry and scale of the observed foreshock transient, we first established its orientation using minimum variance analysis (MVA) on the magnetic field vector data in the JSO coordinate system63. This technique identifies the principal axes of the variance of the magnetic field by finding the eigenvalues (λ max ≥ λ int ≥ λ min ) and the corresponding eigenvectors of the covariance matrix of the magnetic field over the interval containing the transient crossing. The eigenvector associated with the minimum eigenvalue (n MVA ) is interpreted as the normal direction to the boundary of the transient, assuming a quasi-planar structure. The validity of this normal was confirmed by ensuring a large ratio of the intermediate to minimum eigenvalues (λ int /λ min ≫ 1). With the boundary normal established, we then estimated the scale size of the transient, L, along this direction using the single-spacecraft timing method. The scale is calculated as L = |v sw ⋅ n MVA | × Δt, where v sw is the upstream solar wind velocity and Δt is the measured duration of the passage of the spacecraft through the structure. Finally, the convection electric field −V × B points towards the transient sheet, which allows particles to concentrate and form the observed transient. The overall methodological approach we followed is a standardized process typically done when single spacecraft in situ observations are available4,5,18,28. Specifically, for our case, we used a typical upstream solar wind velocity of v sw = [400, 0, 0] km s−1 in JSO coordinates, which is in agreement with estimations of velocity during that interval, and calculated the scale as L = |v sw ⋅ n MVA | × Δt, where Δt was taken as a 15-min duration of the passage of the spacecraft during the transient event. It should be noted that this 15-min interval, while relatively conservative, provides a realistic range of values for the spatial scale analysis (described below). The outcome of this analysis is provided in Extended Data Table 1.
... continue reading