Lightning at Jupiter pulsates with a similar rhythm as in-cloud lightning at Earth

The study investigates whether Jovian lightning exhibits millisecond-scale fine structure analogous to terrestrial intracloud (IC) lightning initiation and stepping. Earlier spacecraft observations (Voyager, Galileo, Cassini, New Horizons) detected Jovian lightning but lacked temporal resolution to resolve individual strokes or sub-millisecond processes. Juno’s Waves instrument discovered rapid whistlers (<20 kHz) and Jupiter dispersed pulses (JDPs, <150 kHz) with durations below a few milliseconds, suggesting fast lightning activity. It remained unclear if these signals reflect step-like channel extensions characteristic of IC initiation on Earth. The authors aim to analyze nearly five years of high-cadence Juno Waves data to characterize interpulse intervals and determine whether Jovian lightning initiation processes mirror those of terrestrial IC lightning.

- Voyager 1 (1979) detected seconds-long whistler-like signals (7–1 kHz) with cadence up to ~1 lightning/s, implying upper limits of ~40 ms for underlying processes. Optical instruments on Voyager, Galileo, Cassini, and New Horizons had exposure times (seconds to minutes) insufficient to resolve individual strokes. - Galileo Probe (10 Hz–100 kHz) detected distant radio sferics with long pulses (~hundreds of microseconds) in a dry, stable descent region unlikely to host local thunderstorms; signals likely originated thousands of kilometers away. - Juno Waves discovered rapid whistlers (<20 kHz) with sub-ms to tens of ms scales and JDPs (<150 kHz) with sub-ms to several ms scales; rapid whistlers are upward-propagating and observed at cadences of a few per second. - Juno SRU enabled 1 s exposures panned at 1 pixel per 2.7 ms, observing ~5.4 ms-long optical lightning with inter-flash separations of tens of ms; MWR detected sferics at 600 MHz and 1.2 GHz with spatial distributions similar to JDPs and whistlers but 0.1 s integration limited millisecond resolution. - Prior modeling places Jovian lightning in water-ice clouds (250–270 K) with breakdown fields comparable to Earth’s thunderclouds, suggesting potentially similar initiation physics.

- Dataset: Nearly five years of Juno Waves LFR data during the nominal mission (PJ1–PJ34; Aug 27, 2016 to Jun 7, 2021), focusing on observations within 5.5 Jovian radii. - Instruments: - LFR-hi: 16.384 ms electric field waveform snapshots at 375 kHz sampling; nominal time resolution ~125 µs. - LFR-lo: 122.88 ms electric and magnetic field waveform snapshots at 50 kHz sampling; nominal time resolution ~940 µs. - Snapshots acquired once per second near perijove; LFR-lo electric and magnetic snapshots overlap, LFR-hi and LFR-lo do not. - Event identification and selection: - Visual inspection of 326,466 LFR-hi snapshots to identify JDPs (ordinary-mode propagation through low-density regions, ne < ~250 cm−3). Found 3,182 snapshots with ≥1 JDP; selected 375 snapshots containing sequences of ≥3 JDPs with well-resolved interpulse intervals and with consistent dispersion and cutoff (to ensure common source/path). Shortest separations discerned ~170 µs. - Visual inspection of 158,716 LFR-lo snapshots to identify rapid whistlers (<20 kHz, whistler-mode). Found 4,502 snapshots with ≥1 whistler; selected 120 snapshots with ≥3 whistlers showing consistent dispersion/upper cutoffs and separations; extracted 482 inter-whistler intervals. Shortest separations discerned ~1.3 ms. - Spectral processing: - FFT spectrograms computed with 128-sample Hann windows and 75% overlap. - For LFR-hi (JDPs): frequency step 2.93 kHz; time step 85 µs; nominal time resolution 125 µs. - For LFR-lo (whistlers): frequency step 391 Hz; time step 640 µs; nominal time resolution 940 µs. - Measurements: - Determined time delays between centers of neighboring spectral traces within each group (JDPs and whistlers separately), yielding 2,576 JDP interpulse intervals and 482 inter-whistler intervals. - Mapped detection locations: JDPs projected vertically to 1-bar level; whistlers projected along magnetic field lines. - Probability density function (PDF): - Constructed PDFs of interpulse intervals for JDPs and whistlers, normalized by total observation time and corrected for detection bias in LFR-lo using a calibration factor Kw = 0.21 to ensure continuity between regimes. - Accounted for finite snapshot-length truncation (JDPs > ~5 ms) by combining with whistler-based intervals for longer delays; fitted power-law PDF f(δ) ∝ δ^B above set thresholds. - Consistency checks: - Calculated average delay per group to avoid bias from long trains; assessed regularity using criterion of standard deviation < 0.5 × group mean. - Terrestrial comparison (supporting context): - Broadband ground-based magnetic measurements (SLAVIA) at Czech sites (200 MHz sampling; 5 kHz–90 MHz) provided examples of IC initiation pulse trains for qualitative comparison of time scales. - Step-length estimation: - Used typical interpulse intervals (few ms) and assumed average leader propagation speeds in water clouds of 10^5–10^6 m/s to estimate average step lengths of several hundred to a few thousand meters.

- Typical timescales: Jovian lightning radio pulse sequences exhibit characteristic interpulse intervals around 1 ms. - JDP interpulse intervals (n=2,576): range 0.17–11.72 ms; mean 1.37 ± 1.27 ms; median 0.95 ms. Shortest resolvable separation ~170 µs. - Rapid whistler intervals (n=482): range 1.8–94 ms; shortest resolvable separation ~1.3 ms. - Distribution: The combined probability density of intervals follows a power law f(δ) ∝ δ^B. - For whistlers alone above 4 ms: B = −1.87 ± 0.07. - Combined JDP+whistler PDF above 1 ms: B = −1.89 ± 0.03. - JDP-only between 1 and 4 ms: B ≈ −1.85 ± 0.08. - Results exclude a random (Poisson) process at these timescales; delays below 100 ms reflect interlinked processes. - Regularity: About one quarter of groups (90/374) displayed nearly regular spacing (group SD < 0.5 × group mean), with 643 intervals ranging 0.20–5.22 ms (mean 1.28 ± 0.80 ms; median 1.10 ms). Group-average delays ranged 0.43–6.66 ms (median 1.49 ms), consistent with the full dataset. - Occurrence and counts: - JDP groups occurred up to 25 pulses per 16.384 ms snapshot; the most frequent group size was five pulses. - Whistler groups occurred most often at the minimum selection threshold of three per 122.88 ms snapshot, decreasing approximately exponentially up to 13 per snapshot. - Spatial/altitude distribution: - Sources located predominantly at middle to high latitudes. - Whistler groups observed when Juno was closer than ~110,000 km; JDP groups detected up to ~260,000 km. - Detection statistics (selection outcomes): - LFR-hi: 326,466 snapshots → 3,182 with ≥1 JDP → 375 with ≥3 JDPs → 2,576 intervals; shortest 170 µs. - LFR-lo: 158,716 snapshots → 4,502 with ≥1 whistler → 120 with ≥3 whistlers → 482 intervals; shortest 1.3 ms. - Physical interpretation: Millisecond spacing and power-law statistics are consistent with step-like extensions of lightning channels following initiation, analogous to terrestrial intracloud initial breakdown processes.

The millisecond-scale cadence and power-law distribution of Jovian lightning radio pulses indicate non-Poissonian, interlinked processes akin to terrestrial intracloud (IC) lightning initiation and stepping. The presence of nearly regular interpulse intervals in a substantial fraction of groups suggests step-like leader extensions. Multi-stroke cloud-to-ground (CG) analogies are unlikely: Jovian groups would require stroke separations ~30 times shorter than on Earth and contradict models predicting ~10 s intervals for subsequent discharges in Jovian water clouds. Prior studies place Jovian lightning in water-ice clouds with breakdown fields comparable to Earth’s, supporting similar initiation physics. Assuming average leader speeds in water clouds of 10^5–10^6 m/s and observed ms-scale spacing, average step lengths are inferred to be several hundred to a few thousand meters. Instrumental constraints of other Juno instruments (MWR integration, SRU exposure) preclude resolving ms-scale repetition, while Galileo Probe’s unique sferic observations are broadly compatible but attenuated at high frequencies due to ionospheric or propagation effects. Overall, the findings strongly suggest that Jovian lightning initiation mechanisms resemble terrestrial IC processes, extending understanding of atmospheric electricity under Jovian conditions.

Using nearly five years of high-time-resolution Juno Waves observations, the study identifies millisecond-scale repetitive radio pulses from Jovian lightning with power-law-distributed interpulse intervals (exponent ~−1.9). About a quarter of pulse trains show near-regular spacing. These properties point to step-like leader extensions during lightning initiation, closely analogous to terrestrial intracloud processes in water-ice clouds. The work provides the first robust statistical characterization of Jovian lightning fine structure at millisecond scales and constrains physical models of discharge initiation at Jupiter. Future research should combine multi-instrument Juno observations with improved temporal resolution, refine propagation modeling through the Jovian ionosphere to retrieve intrinsic source waveforms, and pursue coordinated ground-based or future in situ measurements to directly quantify leader velocities and step dynamics.

- Finite snapshot lengths limit detectable maximum intervals within JDP records, artificially reducing PDF above ~5 ms; mitigation by combining with whistler intervals introduces cross-band normalization assumptions (calibration factor Kw). - Detection sensitivity differences between LFR-hi and LFR-lo and varying background levels bias event selection and interval statistics. - Ionospheric dispersion and instrument resolution prevent resolving intrinsic pulse rise times and direct measurement of step velocities. - Spatial/altitude coverage and selection of sequences with consistent dispersion/cutoffs may bias source region sampling. - Comparisons to terrestrial processes rely on assumed similarity of leader speeds and cloud conditions; no direct optical or in situ corroboration at ms scales from other instruments (SRU, MWR) due to their temporal limitations. - Galileo Probe observations are unique and not directly comparable; high-frequency attenuation complicates cross-mission comparisons.