Understanding lightning processes on other planets provides valuable insights into atmospheric dynamics and fundamental plasma physics. While Jovian lightning has been known since the Voyager 1 mission in 1979, the limited time resolution of previous observations (Voyager 1, 2; Galileo; Cassini; New Horizons) hindered detailed analysis of the fine structure of individual lightning events. These missions primarily detected long-duration signals, providing only limited information about the underlying processes. The Galileo Probe, while detecting radio frequency signals, entered a dry, stable atmospheric region, suggesting the signals originated from distant storms. The Juno mission, with its close proximity to Jupiter and advanced instrumentation, offers a unique opportunity to address this gap in our knowledge. This study leverages high-resolution data from the Juno Waves instrument to investigate the fine temporal structure of Jovian lightning and compare it to terrestrial lightning phenomena. The central research question focuses on whether Jovian lightning exhibits step-like structures, similar to the progression seen in terrestrial intracloud lightning initiation. Understanding the similarities and differences between Jovian and terrestrial lightning processes can advance our comprehension of atmospheric electricity in diverse planetary environments. This research is crucial because it directly addresses a fundamental question in planetary science: the nature of lightning and its dynamics across different planetary bodies. This comparison will reveal potential similarities in the fundamental physical processes involved in lightning initiation and propagation across different atmospheric environments.

Prior research on Jovian lightning relied heavily on data from Voyager 1 and subsequent missions. These missions, while detecting Jovian lightning whistlers, offered limited temporal resolution, preventing detailed characterization of the individual lightning processes. The maximum lightning detection rate reported was one lightning discharge per second, with the temporal scale of lightning processes estimated to be no more than 40 ms. Optical observations from these missions suffered from excessively long exposure times, preventing the resolution of individual lightning strokes. The Galileo Probe, while having the ability to distinguish between intracloud (IC) and cloud-to-ground (CG) lightning on Earth during calibration, encountered a dry, stable atmospheric environment during its descent, resulting in only distant lightning signals being recorded, dominated by low frequencies and long pulses (hundreds of microseconds). The Juno mission's Waves instrument has provided higher-resolution data, detecting Jovian rapid whistlers and Jupiter dispersed pulses (JDPs) at much shorter timescales (milliseconds to tens of milliseconds). The shorter-timescale data acquired by Juno significantly advances the understanding of Jovian lightning beyond previous capabilities. These previous studies, while offering initial detections and characterizations of Jovian lightning, lack the temporal resolution needed to investigate the fine structure and detailed dynamics of the lightning processes.

This research utilizes five years of data from the Juno Waves instrument's Low Frequency Receiver (LFR), specifically the high-frequency (LFR-hi) and low-frequency (LFR-lo) components. The LFR-hi records 16.384 ms electric field waveforms at a 375 kHz sampling rate, while the LFR-lo records 122.88 ms electric and magnetic field waveforms at 50 kHz. Data acquisition occurs once per second during Juno's close approaches to Jupiter. The analysis focuses on identifying sequences of Jupiter Dispersed Pulses (JDPs) in LFR-hi data and rapid whistlers in LFR-lo data. The selection criteria emphasized well-distinguishable interpulse intervals and consistent dispersion and frequency cutoff characteristics within each group of pulses, ensuring that the observed signals likely originated from the same lightning source. A total of 326,466 LFR-hi snapshots and 158,716 LFR-lo snapshots were visually inspected. 375 snapshots with sequences of at least three JDPs and 120 snapshots with at least three whistlers were selected for detailed analysis. The time delays between neighboring pulses were calculated for both JDPs and whistlers. The probability density function (PDF) of the interpulse intervals was calculated separately for JDPs and whistlers and then combined and normalized to create a comprehensive PDF, accounting for the different detection capabilities of the LFR-hi and LFR-lo receivers. The spatial distribution of lightning events was mapped using vertical projections for JDPs and magnetic field-aligned projections for whistlers. Power spectrograms were generated from the electric and magnetic field waveforms using the Fast Fourier Transform (FFT) with a von Hann window. Finally, the estimated average leader propagation speed in Jovian water clouds was utilized to estimate the lengths of the step-like extensions of the lightning channels, considering the observed interpulse intervals.

The analysis of Juno Waves data revealed millisecond-scale time separations between individual electromagnetic pulses associated with Jovian lightning. The shortest observed time separation was approximately 170 µs for JDPs and 1.3 ms for whistlers. The distribution of interpulse intervals follows a power law with an exponent of approximately -1.9 for delays above 1 ms, indicating a non-random process. Approximately one-quarter of the observed pulse sequences exhibited highly regular interpulse intervals, further supporting the idea of a step-like process. The average time separation between pulses within JDP groups was 1.37 ± 1.27 ms, while the average interpulse interval for whistlers was 1.49 ms. The spatial distribution of these repetitive lightning signals is concentrated at middle and higher latitudes, similar to the distribution of rapid whistlers observed earlier in the mission. The number of pulses within a single snapshot ranged up to 25 for JDPs and up to 13 for whistlers. The most frequent groups of JDPs consisted of five pulses. These findings strongly suggest the presence of step-like extensions of Jovian lightning channels, analogous to terrestrial intracloud lightning initiation. By considering average leader propagation speeds similar to terrestrial intracloud lightning, the estimated average length of each step in the lightning channel extension is in the range of hundreds to thousands of meters. The relatively low charge transfer indicated by the observations is consistent with the stepping process, which usually involves smaller charge transfers than the main return stroke in cloud-to-ground lightning.

The millisecond-scale time separations between pulses in Jovian lightning closely resemble the time scales observed during the initiation of terrestrial intracloud lightning. The power law distribution of interpulse intervals and the occurrence of highly regular pulse sequences strongly support the hypothesis of step-like channel extension. The findings contradict the scenario of multi-stroke lightning flashes as the primary source of the observed signals, given the much shorter intervals between pulses compared to terrestrial multi-stroke lightning. The similarity in time scales between Jovian and terrestrial intracloud lightning initiation suggests analogous physical processes are at play. The breakdown fields calculated for Jovian water clouds are comparable to those observed in terrestrial thunderclouds, further supporting the parallel between the two phenomena. The estimated lengths of the lightning channel steps, based on assumed leader propagation velocities, provide a valuable insight into the spatial scale of lightning initiation processes on Jupiter. The study emphasizes the crucial role of high-resolution data in unveiling the intricate details of planetary lightning, highlighting the advancements made possible by the Juno mission.

This study provides compelling evidence that Jovian lightning exhibits step-like channel extensions similar to terrestrial intracloud lightning initiation. The millisecond-scale time separations between pulses, the power law distribution of interpulse intervals, and the frequent occurrence of regular pulse sequences all support this conclusion. The results highlight the significance of high-temporal-resolution measurements for advancing our understanding of planetary lightning processes. Future research should focus on further investigation of the properties of individual pulses, the influence of atmospheric conditions on the step-like process, and more detailed comparisons with terrestrial lightning data. The advancements made in understanding Jovian lightning could inform and advance models of atmospheric electricity and lightning formation on both terrestrial and extra-terrestrial planets.

The study's interpretation relies on the assumption that the average leader propagation velocity in Jovian water clouds is similar to terrestrial intracloud lightning. While this assumption is supported by theoretical considerations and prior studies, it remains a limitation. Additionally, the analysis relies on the visual inspection of waveforms and spectrograms, introducing potential subjective bias. Finally, the inability to directly measure the velocity of individual steps limits the precision of step length estimations. The study acknowledges that the high-frequency portion of the signal might have been attenuated due to the long propagation path below the ionosphere, potentially affecting the completeness of the findings.