# In Situ Analysis of Jupiter's Broadband Kilometric Auroral Radio Emissions With Juno

Brieuc Collet, L. Lamy, C K Louis, V. Hue, T K Kim

► To cite this version:

Brieuc Collet, L. Lamy, C K Louis, V. Hue, T K Kim. In Situ Analysis of Jupiter's Broadband Kilometric Auroral Radio Emissions With Juno. Geophysical Research Letters, 2025, 52 (9), 10.1029/2024gl114447. http://dx.doi.org/10.5194/glr-114447

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# In situ analysis of Jupiter's broadband kilometric auroral radio emissions with Juno

B. Collet¹, L. Lamy²,¹, C. K. Louis ², V. Hue³, T.K. Kim ⁴

¹Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France

²LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris,
Meudon, France

³Aix-Marseille Université, CNRS, CNES, Institut Origines, LAM, Marseille, France

⁴Center for Space Plasma and Aeronomic Research (CSPAR), The University of Alabama in Huntsville,
Huntsville, AL 35805, USA

## Key Points:

* From a survey of 60 orbits, we identify 7 broadband-kilometric (bKOM) sources of Jovian auroral radio emissions crossed by Juno.

* Four bKOM sources are associated with dawn storm-type ultraviolet aurorae, three of which are colocated with auroral cavities.

* The bKOM sources are driven by the Cyclotron Maser Instability by conics and sometimes shell electron distribution functions.
**Abstract**

Among the zoo of Jovian auroral radio emissions, the broadband kilometric (bKOM) component has been the least studied. Taking advantage of Juno in situ measurements within the auroral regions, we surveyed the Juno/Waves radio observations over the 60 first orbits to identify 7 bKOM source candidates. These were mostly detected during dawn storm auroral episodes (4/7) and three were found to be colocated with auroral cavities. By applying a growth rate analysis based on JADE-E electron measurements, we show that the observed waves are driven by the Cyclotron Maser Instability from two free energy sources. The main emission, produced slightly above the electron gyrofrequency ($f_{ce}$), is amplified by conics-type electron distribution functions (EDF) with 2 to 30 keV electron characteristic energies. Sporadic bursts, produced slightly below $f_{ce}$, are driven by shell-type EDF of 0.1 to 10 keV. These results are analyzed comparatively to those obtained previously for Jovian hectometric and decametric emissions.

**Plain Language Summary**

Jupiter's auroral radio emissions have made possible to characterize this planet using radio telescopes on Earth. The mechanism generating these waves has been theorized as a wave-plasma instability. Thanks to Juno, we can now characterize this mechanism in situ at Jupiter. In this letter, we focus on the least studied component of these emissions: the kilometric radio emissions. For this purpose, a total of seven source crossings were identified in the first 60 orbits completed by the Juno spacecraft. Three of these crossings were identified in auroral cavities that bear resemblance to those observed at Earth. We then verified that these waves were generated by the Cyclotron Maser Instability and we found that the source of free energy differs from other planetary radio auroral emissions sources.

**1 Introduction**

The magnetosphere of Jupiter produces the most powerful radio auroral emissions in the Solar system, whose understanding is a main objective of the Juno mission (Bagenal et al., 2017). They are emitted on the eXtraordinary (X) mode, with a hollow cone beaming, along high latitude magnetic field lines at frequencies $f$ close to the local electron cyclotron frequency over a wideband spectrum ranging from a few 10s kHz up to 40 MHz. These emissions were usually divided into the DecAMetric (DAM, $f > 3$ MHz), the Hec-tOMetric (HOM, $0.3 < f < 3$ MHz) and the broadband KilOMetric components (bKOM, $f < 0.3$ MHz and occasionally up to 1 MHz), thus probing sources of increasing altitudes above the ionosphere, and into emissions driven by Io, Europa, and Ganymede or associated with the Jovian aurorae (Clarke et al., 1996; Prangé et al., 1996; Clarke et al., 2004; Badman et al., 2014).

The HOM/DAM components have been the most studied so far, thanks to remote long-term ground- and space-based observations and to regular in situ measurements within their source region by Juno, in polar orbit around Jupiter since mid-2016. Juno first showed that HOM and DAM sources are colocated along M-shells in the range $12R_J < M < 58R_J$ ($1 R_J = 71,492$ km = a Jovian radius), where the magnetospheric plasma corotation breaks down, and mapping to the equatorial edge of the main ultraviolet aurorae (Louis et al., 2019). Pioneer case studies of Juno electron/radio in situ measurements within the HOM/DAM source region associated (auroral and moon-induced) validated the electron Cyclotron Maser Instability (CMI) as their generation mechanism (Louarn et al., 2017, 2018; Louis et al., 2020). Building up on this approach, Collet et al. (2024), hereafter C24, systematically analyzed all HOM/DAM sources and their relationship with the ultraviolet aurorae over 2016-2022. Overall, C24 identified 3 types of CMI-free energy sources driven by low energy electrons of 0.2-5 keV. They consist of loss cone (prominent), conics (marginal), or shell-type (marginal) electron distribution functions.
Much less is known about bKOM emissions, which can only be observed from space. This component has been investigated thanks to radio observations of Voyager (Leblanc & Daigne, 1985; Warwick et al., 1979, and references therein), Ulysses (Kimura et al., 2008, and references therein), Cassini (Zarka et al., 2021; Fischer et al., 2025) and most recently Juno (Imai et al., 2019; Louis et al., 2019, 2021; Louis, Jackman, et al., 2023; Fischer et al., 2025). Overall, bKOM appears as time variable bursts distinct from HOM/DAM ones, typically weaker by 1-2 orders of magnitude and more sensitive to the solar wind (Desch & Kaiser, 1980; Barrow et al., 1988; Zarka et al., 2021; Louis, Jackman, et al., 2023). Juno-based studies showed that bKOM sources lie along M-shells ranging from 14 to 66 $R_J$ and are associated with all the components of the ultraviolet aurorae (Imai et al., 2019; Louis et al., 2019), as opposed to HOM/DAM sources. However, bKOM sources have not been investigated in situ yet so that their generation conditions and source of free energy remains unexplored.

In this study, we extend the work of C24 to bKOM sources, to attempt building a comprehensive view of Jupiter as an auroral radio source. This work relies on the analysis of Juno radio (Waves) and electron (JADE-E) high resolution measurements over 2016-2024 (section 2). We first survey radio observations to identify 7 bKOM source candidates along the Juno trajectory (section 3). We then analyze in detail the two most representative cases (section 4) with a CMI growth analysis of electron distribution functions. We then conclude on the statistical properties of validated bKOM sources (section 5).

## 2 Dataset

Juno is a spin-stabilized (2 rpm) spacecraft that explores the Jovian auroral regions with polar orbits since mid-2016. For the present work, we analyzed the time interval ranging from August 2016 to April 2024, corresponding to 60 Juno orbits (hereafter referred to by perijoves or PJ). Figure S1 shows the magnetic projection of this trajectory 400 km above 1 bar level using the JRM33 internal field model Connerney et al. (2020) complemented by a current sheet model Connerney et al. (2022) (hereafter referred to as JRM33+CON20).

Juno/Waves measures electric fields in the Jovian environment from 50 Hz to 41 MHz with an electric dipole antenna in four channels: LFR-Low (50 Hz-20 kHz), LFR-High (10-150 kHz), HFR-Low (150 kHz-3 MHz) and LFR-High (3-41 MHz) (Kurth et al., 2017). The bKOM spectral range overlaps the LFR-High and HFR-Low ones. Juno/Waves operates either in the Survey mode, where the whole frequency range is sampled every 1 sec with 18 channels per decade, or in the Burst mode, where the data are sampled at high resolution as waveform snapshots. These can be used to reconstruct (through Fast Fourier Transform) time-frequency arrays with a frequency range and resolution that depends on the receiver, namely 1 s × 61 Hz for LFR-High and 1 s × 1.709 kHz for HFR-Low.

Electrons are sampled by Juno/JADE-E (McComas et al., 2013) which is a swept-energy electron spectrometer that samples energies ranging from 0.1 to ~100 keV with 3 sensors. One of these being not operational, only 2/3 of the sky is sampled at each time step, with a total of 64 anodes each sampling a 7.5° field-of-view. These measurements are obtained either in high-resolution mode, with a 1 s temporal cadence, or in low resolution mode, with a cadence increasing up to 30 s to 300 s. In high resolution, JADE-E samples all directions within a Juno rotation. Electron measurements are significantly depleted for pitch angles $\theta = (v, B)$ (with $v$ the electron velocity and $B$ the local magnetic field) near 90° in strong magnetic field regions because of an instrumental, energy-dependent, masking of incident electrons (McComas et al., 2013; Allegrini et al., 2017).

For our purpose, we also systematically used measurements of the magnetic field amplitude $B$ provided by Juno/MAG (Connerney et al., 2017) and occasionally processed
images of the ultraviolet aurorae obtained either by Juno/UVS (Gladstone et al., 2017) and the Hubble Space Telescope (Lamy et al., 2015) (see supplementary material).

## 3 Radio survey of bKOM source candidates

Extending the work carried on by C24 for HOM/DAM sources, we first surveyed Juno/Waves observations between 1 and 400 kHz, to identify bKOM source candidates. The latter were searched for in time-frequency measurements either in Survey or Burst mode. In this section, JADE-E and MAG measurements of the electron density $n_e$ and the magnetic field amplitude $B$ were used to derive the electron plasma frequency $f_{pe}$ (proportional to $\sqrt{n_e}$) and electron cyclotron frequency $f_{ce}$ (proportional to $B$). This is worth reminding that the values of $n_e$ derived from JADE-E represent a lower limit of the actual electron density (Sulaiman et al., 2022). The source candidates were picked up from the selection criteria originally used in Louis et al. (2019) by tracking radio emissions below $f_{ce}+1\%$. Whenever Burst mode measurements were available, we used them in priority to achieve the finest spectral resolution. In addition, we restricted ourselves to time intervals for which the JADE-E time resolution was of either 1 s (highest) or 30 s.

These selection criteria are more selective than those used in Louis et al. (2019); Imai et al. (2019). Overall, our survey was restricted to a total of 7 bKOM source candidates, 4 in the south (PJ1S, PJ4S, PJ11S, PJ12S) and 3 in the north (PJ5N, PJ7N, PJ9N) whose time intervals and characteristics are listed in Table 1. Figures S2 to S8 show the time-frequency spectrograms for these 7 intervals. Due to the lack of high resolution data for the Waves and JADE-E instruments, we have not considered PJ7N and PJ12S in the rest of the study.

Fig. S1 illustrates that the 5 remaining bKOM source candidates are located along magnetic flux tubes mapping from 30 to 52 $R_J$, corresponding to the statistical position of the main auroral oval (sMAO). This result is confirmed with HST/UVS UV images in Fig. S2 to S6 indicating the colocation with the main aurora and the characteristic bright spot of dawn storms (PJ5N, PJ9N, PJ11S). They are located on the dawn sector (local time between 02 and 07 hours) which the trajectory favors during sMAO crossings.

As opposed to the case of HOM/DAM sources, the $f_{pe}/f_{ce}$ ratio observed for the 5 bKOM source candidates is closer to and can overcome the CMI quenching threshold of ~0.1 (Hilgers, 1992). Indeed, for PJ5N, PJ1S, and PJ9N, this condition is not met during the complete studied intervals. In three cases, we notice sudden electron depletions (hereafter referred to as cavities) which have not been observed for any HOM/DAM source and are reminiscent of the auroral cavities on Earth (see Section 5). Particularly, during PJ5N, we identified 5 small-scale cavities (a few tens of km long) associated with radio emissions near $f_{ce}$ that we interpret as distinct, successive, radio source crossings. It is worth noting that these cavities coincide with whistler-mode waves seen below $f_{pe}$ interpreted as auroral hisses. Such emissions have been studied extensively at Jupiter and are thought to be generated by weakly energetic electron beams associated with field-aligned currents (FACs) (Ebert et al., 2021; Sulaiman et al., 2022).

Interestingly, the three cases of auroral cavities (PJ1S, PJ9N, PJ5N), as well as the case of PJ11S (with no cavity), were all detected during episodes of auroral dawn storms. These dawn storms are thought to result from magnetotail reconfiguration, most probably internally driven, generating intense and widespread UV emissions fixed on the dawn sector (Bonfond et al., 2021; Kimura et al., 2015).

In Figure S9, we investigated the solar wind parameters, as numerically predicted at Jupiter with the MS-FLUKKS MHD model (Kim et al., 2016), matching these events. Interestingly, PJ7N, PJ11S, and PJ12S coincided with a significant magnetospheric com-
<table><caption>**Table 1.** Catalog of bKOM source crossings identified from Juno/Waves observations over PJ1-60. Columns 1 and 2 indicate the perijove and the event time interval. Columns 3-4 give the bKOM frequency and the $\frac{f_p}{f_{ce}}$ ratio (derived from JADE-E and MAG measurements) during the source crossings. Columns 5-6-7 give the radial distance from Jupiter, the magnetic latitude, and M Shell (JRM33+CON20). Columns 8-9 indicate whether dawn storms (from Bonfond et al., 2021) or if high solar wind parameters were identified less than a day from the source crossings. Column 10 illustrates the direction of field aligned current (FAC). Column 11 gives the source peak spectral flux density as observed by Waves. Column 12 gives the latitudinal extent (defined as the $r \cos \theta_{SIII}$ length with $r$ the radial distance and $\theta_{SIII}$ the latitude) of the source crossings. Columns 13 and 14 indicate whether the electron or radio measurements were in High Resolution mode. The last column indicates the category of unstable population. Column 15 indicates the presence of cavity. Column 16 indicates the identified CMI unstable population (see Section 4). “N/A” indicates that the resolution of either JADE-E or Waves measurements was not sufficient to obtain the corresponding property.</caption><thead><tr><th>PJ</th><th>Time</th><th>f<sub>ce</sub><br>(kHz)</th><th>f<sub>e</sub>/k<sub>e</sub></th><th>R<br>(R<sub>J</sub>)</th><th>MLat<br>(°)</th><th>M Shell<br>(R<sub>J</sub>)</th><th>Dawn storm</th><th>Solar Wind</th><th>FAC</th><th>Intensity<br>W m<sup>-2</sup> Hz<sup>-1</sup> (×10<sup>3</sup> k<sub>no</sub>)</th><th>Lat. size<br>(×10<sup>3</sup> k<sub>no</sub>)</th><th>JADE-E</th><th>Waves</th><th>Cavities</th><th>Unstable populations</th></tr></thead><tbody><tr><td>1S</td><td>2016-08-27<br>21.15 to 22.40</td><td>12</td><td>0.03 to 0.3</td><td>10 to 11</td><td>-35 to -30</td><td>40 to 50</td><td>X</td><td>X</td><td></td><td>3×10<sup>-11</sup></td><td>650</td><td>LR</td><td>Survey</td><td>X</td><td>N/A</td></tr><tr><td>4S</td><td>2017-02-02<br>15.30 to 17.15</td><td>100 to 300</td><td>0.01 to 0.07</td><td>4 to 6</td><td>-59 to -48</td><td>30 to 51</td><td></td><td></td><td>↑</td><td>4×10<sup>-11</sup></td><td>170</td><td>HR</td><td>Burst</td><td></td><td>Shell Comics</td></tr><tr><td>5N</td><td>2017-03-27<br>02.00 to 03.30</td><td>30 to 150</td><td>0.01 to 0.5</td><td>6 to 8.3</td><td>33 to 51</td><td>22 to 52</td><td>X</td><td></td><td>↑</td><td>1×10<sup>-11</sup></td><td>400</td><td>HR</td><td>Burst</td><td></td><td>Conics</td></tr><tr><td>9N</td><td>2017-10-24<br>10.20 to 11.30</td><td>20 to 30</td><td>0.05 to 0.5</td><td>8.5</td><td>35 to 40</td><td>20 to 35</td><td>X</td><td></td><td>↓</td><td>1×10<sup>-11</sup></td><td>450</td><td>LR+HR</td><td>Survey</td><td>X</td><td>Conics</td></tr><tr><td>11S</td><td>2018-02-07<br>17.30 to 20.00</td><td>30 to 120</td><td>0.003 to 0.05</td><td>5.5 to 8</td><td>-54 to -45</td><td>52</td><td>X</td><td>X</td><td>↑ during f < f<sub>ce</sub></td><td>1×10<sup>-10</sup></td><td>320</td><td>LR</td><td>Burst</td><td></td><td>N/A</td></tr><tr><td>12S</td><td>2018-04-01<br>15.40 to 18.10</td><td>18 to 43</td><td>N/A</td><td>8 to 10</td><td>-49 to -40</td><td>50 to 53</td><td></td><td>X</td><td></td><td>2 × 10<sup>-10</sup></td><td>600</td><td>None</td><td>Survey</td><td>N/A</td><td>N/A</td></tr><tr><td>7N</td><td>2017-07-10<br>17.15 to 17.55</td><td>15</td><td>N/A</td><td>10</td><td>24 to 28</td><td>14 to 19</td><td></td><td>X</td><td></td><td>5 × 10<sup>-11</sup></td><td>500</td><td>None</td><td>Survey</td><td>N/A</td><td>N/A</td></tr></tbody></table>

pression (high density, pressure, and magnetic field intensity) supporting a partial correlation of the bKOM occurrence with the solar wind activity.

## 4 Growth rate analysis of bKOM sources

Hereafter, we analyze the candidate bKOM source crossings to check whether they are consistent with a local generation by the CMI and which electrons can drive it.

The CMI is a resonant instability which amplifies radio waves at a frequency given by Eq.1

$$f = \frac{f_{ce}}{\Gamma} + \frac{k_{\parallel} v_{\parallel}}{2\pi} \quad (1)$$

with $\Gamma = (1 - \frac{v^2}{c^2})^{-1/2}$ the Lorentz factor, $k$ the wave number and $\parallel$ designates the direction parallel to **B**. This resonance equation transposes into a resonance circle in the EDF phase plane. The CMI requires (1) a highly magnetized and tenuous plasma, so that $\frac{f_{pe}}{f_{pe}} \le 0.1$ and (2) weakly relativistic electrons whose distribution function $F_e$ (EDF) presents a population inversion characterized by a positive gradient of the electronic distribution function in the direction perpendicular to the magnetic field $\frac{\partial F_e}{\partial v_{\perp}} > 0$ (Wu & Lee, 1979).

Three main types of EDF present such a population inversion, and are therefore CMI-unstable, namely loss cone, conics, and shell-type distributions. Loss cone-type EDF stands as the prominent source of free energy for HOM/DAM, but could not be investigated for the candidate bKOM sources as the loss cone angle is not resolved by JADE-E at such large distances (beyond 2 $R_J$). C24 identified the other two types of EDF as secondary sources for HOM/DAM: conics-type EDFs which are characterized by enhanced
$F_e$ either in the downward or upward directions and generating waves at $f > f_{ce}$; and shell-type EDFs which are characterized by an enhancement of $F_e$ in a circular shell resulting from the adiabatic evolution of electrons accelerated in the parallel direction generating waves at $f < f_{ce}$.

Hereafter, we analyze the candidate source crossings with the CMI growth rate analysis method developed by C24. This method computes the normalized wave growth $\gamma = \frac{\omega_i}{2\pi f_{ce}}$ (with $\omega_i$ the growth rate) on a set of resonance circles from EDFs sampled by JADE-E each second. This method allows one to retrieve the most unstable circle per category of unstable electrons.

Among the 7 bKOM source candidates of table 1, only 2 were sampled with both JADE-E and Waves high resolution measurements simultaneously (PJ5N and PJ4S). While we analyzed each case (presented in Figures S10 to S12), we show here details about these 2 cases as representative examples.

## 4.1 The case of PJ4S

Figure 1 shows radio, electron, and magnetic measurements along the Juno trajectory for PJ4S (panels a-d), already partially studied by Imai et al. (2019), together with the results of our growth rate analysis (panels a and e-f).

Figure 1a (resp b) shows a Waves low resolution (resp high resolution) dynamic spectrum. On 1b, the bKOM source candidate corresponds to the emission observed at $f < f_{ce} + 1\%$ between 15:20 and at least 15:26 (after which only Survey mode low resolution measurements were available), with an excursion below $f_{ce}$ from 15:23 to 15:26. Figure 1a indicates that the source region might extend until 15:40.

Figure 1c-d show JADE-E measurements of the electron flux as a function of energy (for all pitch angles) and pitch angle (for 1-5 keV electrons). We notice an inverted V structure from 15:15 to 15:20, peaking near 70 keV at 15:15. The black line in panel b) indicates the expected (unresolved) loss cone angle. The lower flux values observed near 90° are the result of the instrumental shadowing mentioned earlier. From 15:22 to 15:37, we observe depletions in the upward and downward directions at pitch angles larger than the loss cone angle. This distribution, reminiscent of the conics discussed by Louarn et al. (2018), is delimited by larger pitch angles, near 30° and 150°.

Figure 1e displays the perturbation in the azimuthal magnetic field component deduced from JRM33+CON22 and MAG measurements, whose variations probe FAC: the red (resp. blue) line denotes regions where FACs are upward (resp. downward) directed. We observe an upward FAC from 15:22 to 15:35 and a mostly downward FAC from 15:35 to 15:40.

Figure 1f shows the maximal growth rate derived separately for 3 categories of resonance circles. The green diamonds track shell type resonance circles and the orange “+” (purple “X”) symbols correspond to circles intercepting the upward (downward) part of the EDF, in the shown cases, we identified those unstable populations as conics. The characteristic electron energies of these CMI-unstable resonance circles are shown in Figure 1g and the associated predicted emission frequencies are superimposed to the radio observations in Figure 1b.

Shell-type EDFs yield the largest growth rates throughout the interval (up to $10^{-3}$), and the predicted frequencies associated with the most CMI-unstable circles (equivalent to energies between 0.3 and 8 keV) match well the observed emissions below $f_{ce}$ from 15:23 to 15:26. However, the lower growth rates associated with the shell-type EDFs seen before 15:23 do not match any observed radio emissions. To account for this discrepancy, C24 also proposed that the electron-wave interaction time might be too short to achieve a detectable radio flux.
The unstable upward and downward resonance circles (purple and orange) display remarkably similar growth rates, reaching a few $10^{-4}$ from 15:25 to 15:27 and from 15:29 to 15:37. These circles clearly intercept the conics-type part of the EDF, where $\frac{\partial F_e}{\partial v_\perp} \ge 0$, from 15:24 to 15:29 and from 15:30 to 15:33 coinciding with the largest growth rates. These bidirectional CMI-unstable electrons predict emission frequencies between $f_{ce}$ and $f_{ce}+1\%$ after 15:25, mostly after Waves stopped operating in Burst mode. The associated characteristic electron energies additionally match the trend of the inverted V observed in panel b) from 15:15 to 15:30.

From 15:30 to 15:36, contrary to what was observed in $f \le f_{ce}$ HOM/DAM sources, the resonant circles associated with conics follow an energy trend (1 to 7 keV) different from the one of shell type EDFs (0.1 to 1 keV). This suggests a different source of acceleration for these 2 populations.

Overall, the observed radio emissions indicate a first, shell-driven, CMI source from 15:22 to 15:28 responsible for radio wave amplification below $f_{ce}$ followed by a second, conics-driven, CMI source from 15:30 to 15:37 amplifying waves between $f_{ce}$ and $f_{ce}+1\%$. Although not seen in panel a), this second source is visible in the low resolution dynamic spectrum of Figure S2.

For the shell-driven source, growth rates are about $\gamma \approx 5 \times 10^{-4}$ with electrons of a few keV. Assuming a group velocity of $0.1c$ and a source size of $170 \times 10^6$ m (deduced from the latitudinal size of this interval), we obtain a gain of $3 \times 10^{10}$. Assuming a galactic noise of $2 \times 10^{-21} \text{ W.m}^{-2}\text{Hz}^{-1}$ (Manning & Dulk, 2001), we achieve a theoretical wave intensity of about $6 \times 10^{-11} \text{ W.m}^{-2}\text{Hz}^{-1}$ which is consistent with the observed wave intensity about $6.3 \times 10^{-11} \text{ W.m}^{-2}\text{Hz}^{-1}$. We note that, here, the radio source size is an overestimation as it is estimated from the interval where $f < f_{ce}$ which does not account for multiple filamentary sources. We also note that the use of $v_g = c$ is a limitation of this intensity comparison since $v_g$ depends strongly on the thermal effects of the plasma.

## 4.2 The case of PJ5N within a cavity

Figure 2, with a format identical to Figure 1, presents the case of PJ5N with a bKOM source candidate colocated with an auroral cavity. Despite similar cavities in PJ5N were already noticed by Ebert et al. (2021), the bKOM generation mechanism was not addressed.

In Figure 2a and b, radio emissions near $f_{ce}$ are visible only from 02:27:57 to 02:32:05. Interestingly, the flux density drops over the full spectral range over these limits. Emission was detected slightly below $f_{ce}$ from 02:28:50 to 02:29:05, and below $f_{ce}+1\%$ from 02:28:30 to 02:29:00 and from 02:29:30 to 02:29:45. The spin-modulated pattern of the emission change from 02:30:30 to 02:32:00 for emissions between $f_{ce}+1\%$ and $f_{ce}+2\%$, suggesting those are generated on a different mode than the ones observed shown here before and above.

Figure 2c displays the JADE-E electron spectrogram processed slightly differently than for PJ4S. Indeed, during this time interval, electron measurements were particularly noisy, likely due to instrumental sub-second variations (whose influence on CMI growth rate computation was addressed by Louarn et al., 2018) and displayed unphysical data points resulting from the background subtraction step. To attempt mitigating these effects, we averaged the electron data over 3 sec. The spectrogram reveals a sudden depletion of electrons between 02:27:38 and 02:32:00 while $\frac{f_{pe}}{f_{ce}}$ simultaneously drops from 0.25 to 0.07. This cavity, delimited by the vertical dashed blue lines coincides with the bright radio emission region noticed in Fig.2.a, with a striking correspondence at 02:32:00. A monoenergetic signature is visible between 02:30:00 and 02:32:00, with a peak energy
<figure><img src="image_2.png" /><figcaption>**Figure 1.** In situ measurements and growth rate analysis as a function of time for the PJ4S bKOM source. a) Low resolution (Survey mode) dynamic spectrum (Waves) b) High resolution (Burst mode) dynamic spectrum (Waves) where frequencies are plotted relatively to $f_{ce}$. c) Electron flux as a function of energy (JADE-E). d) Electron flux as a function of pitch angle for electrons between 1 and 5 keV (JADE-E). e) Azimuthal component of the magnetic field (MAG). The red (blue) lines indicate upward (downward) FAC. f) Maximal CMI growth rate derived separately for circles intercepting either shell (green diamonds), upward electrons (blue circle), or downward electrons (orange crosses) types of resonance circles. g) Electron characteristic energy associated with the resonance circles investigated in panel e). The associated predicted emissions frequencies are displayed by identical symbols on top of Panel b) for growth rates $\ge 7 \times 10^{-5}$ (black dashed line in Panel f) ).</figcaption></figure>
flux decreasing from 30 to 10 keV. This signature probably extends to energies beyond
the JADE-E energy range limit.

Figure 2d shows that the above-mentioned monoenergetic signature corresponds to bidirectional electron beams tracked between 02:30:30 and 02:32:00. From 02:27:30 to 02:30:30, we also observe weaker bi-directional beams and intermittent depletions at pitch angles smaller than 30° or larger than 150°.

Figure 2e displays scarce magnetic data, lacking from 02:29:00 to 02:31:00 and from 02:32:00 to 02:34:00 (see Figure S3). Still, we could infer from the $B_\Phi$ drop that the presence of an upward FAC coincides with the auroral cavity. This is similar to what is usually observed within auroral cavities at Earth (Roux et al., 1993).

In Figures 2f-g, we excluded shell-type resonance circles. Indeed, despite the data averaging mentioned above, the poor data quality displayed frequent shell-type structures which we considered as unphysical. For the remaining two categories of resonance circles, we again observe similar growth rates, as large as $\gamma \ge 10^{-4}$ within the cavity, corresponding to electron energies of 5-30 keV, the lower energy limit of the inverted V feature. The inferred predicted wave frequencies ($f = f_{ce}+0.7$ to 6%) correspond fairly well to the observed emissions. The emission below $f_{ce}+1\%$ near 02:29:00 is partially predicted. This is not the case for the second emission below $f_{ce} + 1\%$ near 02:29:45, probably because of the low quality EDF analyzed here. Overall, the emissions predicted at $f_{ce}+2\%$ and above could, in fact, be a superposition of remote and local waves. While Figure 2c does not highlight any CMI-unstable feature, a close inspection of EDF, such as those displayed in Figure S13 indicates that the free energy source lies mostly in the conics part of the EDF, which superimposes to multiple intensifications and depletions. We did not detect any unstable EDFs from 02:30:30 to 02:32:00, preventing CMI wave amplification there.

Growth rates of $\gamma \approx 10^{-4}$ were observed for about 2.5 min, corresponding to a source size of $3 \times 10^6$ m along the Juno trajectory. Using the same assumptions as in the previous section, we achieve a gain of $3 \times 10^{11}$ and a predicted wave spectral flux density of $3 \times 10^{-11} \text{ W.m}^{-2}.\text{Hz}^{-1}$, comparable to the $1 \times 10^{-11} \text{ W.m}^{-2}.\text{Hz}^{-1}$ measured flux density.

# 5 Summary and discussion

From the survey of Waves observations from PJ1 to PJ60, we have identified the crossing of 7 bKOM sources summarized in Table 1 at frequencies between 12 and 300 kHz, corresponding to distances from Jupiter between 11 and 4 $R_J$. This limited number of sources can be attributed to the fact that the auroral oval is scarcely traversed at $f_{ce}$ corresponding to the bKOM range (see Fig. S1). The sources were hosted along flux tubes mapping from $M = 20$ to $50 \, R_J$. The source region corresponded to $f_{pe}/f_{ce}$ ratios largely varying between 0.003 and 0.3. It coincided with an auroral cavity in three of the cases (PJ1S, PJ5N, PJ9N) and with upward FAC in 4 cases (PJ4S, PJ5N, PJ9N, PJ11S). 4 of the sources (PJ1S, PJ5N, PJ9N, PJ11S) were colocated with the bright dawn storm region of the main UV auroral emission. As the auroral oval is only crossed in the dawn sector for $f_{ce}$ in the bKOM range, we can only conclude on a partial correlation with this sector and dawn storms. Similarly, only four (PJ1S, PJ7N, PJ11S, PJ12S) were detected during solar wind compressions confirming a partial correlation.

By analyzing the growth rate of these sources, of which we show two representative examples here —one within a cavity and one outside a cavity, we have identified similar energy sources, namely conics-type EDF and intermittent shell-type EDF. The former has characteristic energies of 2 to 50 keV driving emission above $f_{ce}$, up to a few % and the latter drives emissions below $f_{ce}$, down to -0.5%, from electron beams of 0.1 to 10 keV.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Same as Figure 1 for PJ4S. The vertical blue dashed lines delimit the auroral cavity.</figcaption></figure>
Overall, these characteristics somewhat differ from those of the HOM/DAM source regions. First of all, the bKOM sources appear to be directly connected to the main aurora, mostly during episodes of dawn storms. In contrast, HOM/DAM sources were rather observed at the equatorward edge of the main aurora and/or in the zone of diffuse aurora. Then, as opposed to HOM/DAM sources, bKOM emissions were generally observed within auroral cavities, with a clear example of signal extinction at the boundary (PJ5N) indicating strong refraction effects. This is rather reminiscent of the source region of Auroral Kilometric Radiation (AKR) at Earth (Louarn & Le Quéau, 1996; Zarka et al., 1986; Roux et al., 1993). Similarly, it suggests that the acceleration region and the bKOM source coincided.

Indeed, while shell-driven CMI emissions are regularly observed for both bKOM and HOM/DAM sources, they rather appear as a secondary free energy source, as opposed to AKR or Saturn's Kilometric Radiation (SKR) at Saturn (Ergun et al., 2000; Mutel et al., 2010). The bKOM radiation is instead mostly supplied by conics-type EDF (see also Figs S10-S11-S12), such as some HOM/DAM emissions which are also supplied by loss cone-type EDF (not observable here). Some of these conics were observed on a large pitch angle range reminiscent of trapped electrons studied by Louarn et al. (1990) for AKR amplification. Conics population can be generated by low-frequency turbulence (Louarn et al., 2018). The measured EDF were highly variable with time, with succeeding intensifications and depletions which further indicate the co-existence of several acceleration processes such as FACs (supported by inverted Vs and cavities), low-frequency turbulence (supported by conics) and other short-lived phenomena generating beams and mono energetic accelerations. As in the case of AKR source crossed by Viking (Ungstrup et al., 1990), upward electron beams may indicate relaxed EDFs after CMI amplification, then hinting for insufficient time resolution to track the unstable electrons. Modeling the EDF temporal evolution as a function of the position of the electron acceleration site will be needed to better understand the acceleration processes at work.

Overall, the statistical study of bKOM, HOM, and DAM sources has revealed that these are all driven by weakly energetic electrons of 0.5 to 30 keV. This range is strikingly similar to that driving AKR at Earth (Ergun et al., 2000) or SKR at Saturn (Lamy et al., 2018). However, for several bKOM (and HOM/DAM) source crossings, monoenergetic signatures reached energies beyond the JADE-E range, and the Jovian main aurorae are known to be supplied by highly energetic electrons of several 100s keV. An important perspective of this work will be to study the CMI-instability of such highly energetic electrons whose EDF can be tracked with JEDI. This will primarily require modifying our expression of the X-mode CMI growth rate, which relies on the weakly relativistic assumption that the amplification is supported by a cold electron population.

## Open Research Section

The Juno data used in this manuscript are reachable from the Planetary Data System at Kurth, W. S. and Piker C.W (2024) for Waves data, at Allegrini, F. and Wilson, R.J. and Ebert, R.W. and Loeffler, C. (2024) for JADE-E data and at Connerney, J. (2024) for MAG data. All the UVS data used in this study are publicly available on the PDS Atmospheres Node Data Set Catalog (https://pds-atmospheres.nmsu.edu/cgi-bin/getdir.pl?dir=DATA\&volume=jnouvs_3001). The Juno/Waves estimated flux density Collection (Version 02) is available at Louis, Zarka, et al. (2023); Louis et al. (2021)

## Acknowledgments

The authors thank the Juno radio and plasma teams, especially the JADE and Waves instrument teams, J. Szalay, Philippe Louarn (IRAP), R. Prangé (LESIA) and B. Bonfond, who provided useful discussions on JADE-E data analysis, CMI instability and UV emission correlation, E. Penou (IRAP), for his help with the particle analysis through
the CLWeb software, and the LESIA laboratory for allowing access to its internal servers. We thank the APIS service at LESIA/Paris Astronomical Data Centre (Observatoire de Paris, CNRS) for providing value-added data derived from UV observations of the ESA/NASA Hubble Space Telescope. The French authors acknowledge support from CNES and CNRS/INSU national programs of planetology (PNP) and heliophysics (PNST, also funded by CEA). V. Hue acknowledges support from the French government under the France 2030 investment plan, as part of the Initiative d'Excellence d'Aix-Marseille Université – A*MIDEX AMX-22-CPJ-04. In accordance with AGU's AI tool policy, the authors acknowledge the use of Deepl tool to help to improve the English in the manuscript. All suggestions provided by the AI tool were reviewed and edited by the authors for correctness and consistency

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