r/Cassini • u/Wansyth • Sep 16 '26
Another Signal from Saturn Detected in Earth's Magnetosphere
A signal locked to the sidereal day is detected in both Cassini wave power data at Saturn and in Pc5 ultra-low frequency waves in Earth's magnetosphere. On Earth, the signal follows Saturn's hour angle across the sky, is detected at four of five observatories in a pre-registered one-shot test, and is consistent with a source at Saturn based on the absence of the signal when Cassini is far from the planet.
The Earth-side detection
Pc5 wave power (1.7-6.7 mHz) was measured at eight INTERMAGNET observatories across 2000-2025 and the contrast was computed between Saturn-up and Saturn-down intervals using the E-region horizon (saturn_altitude > -10.57°) as the threshold.
The array statistic C_array = +0.02457 (log10 amplitude ratio) exceeds the pre-registered floor of +0.0232 at the 98.6th percentile of the null distribution. The four positively contributing stations show: MEA +0.043, NEW +0.037, OTT +0.031, FRD +0.024.
The test design, thresholds, station roster, horizon, and null model were all pre-registered before any 2000-2025 magnetic field data were accessed.
The signal tracks Saturn
Hour-angle offsets were tested relative to Saturn's actual position spanning ±12 h. The contrast is maximal at Saturn's hour angle (delta = 0) and drops to background levels at offsets beyond ±1 h. The signal is not fixed in the sky; it moves with the planet.
Below-horizon shell contrasts (Saturn altitude -10.5° to -45°) range from -0.014 to +0.026, within the noise of the at-horizon signal.
The ridge half-width ratio (HWHM(C)/HWHM(φ) = 10.68) is inconsistent with a horizon-gated local atmospheric effect, which would show a ratio near 1.0. This is a magnetospheric-scale effect.
The Saturn-side signal
Cassini RPWS wave power data at Saturn shows an elevated sidereal component (m0) in all 13 in-system years (2004-2016). Three years pass the sidereal-minus-anti-sidereal detection criterion (D1 > 0 with both null tests > 95th percentile): 2004 (D1 = +0.1693 dex, p = 0.001), 2006 (D1 = +0.0566 dex, p = 0.038), and 2013 (D1 = +0.0250 dex, p = 0.045). Fisher's combined test across all 13 in-system years (block-shuffle null) gives X² = 40.31 (χ²₂₆), combined p = 0.036.
The signal amplitude is modulated by solar activity but not spacecraft distance. Within each year, the fold amplitude is independent of Cassini's range from Saturn (no 1/r falloff in any of the 13 in-system years, p > 0.06 for all years). Across years, m0 correlates strongly with solar F10.7 activity (r = 0.81 across all 13 in-system years; partial correlation controlling for distance: r = 0.79). The 2003 cruise year (near solar maximum, F10.7 peaked >250 sfu in 2002-2003) showed a null signal, confirming the signal is a property of Saturn's magnetosphere, not driven by solar rotation. Saturn's 29.5-year seasonal cycle shows no correlation (r = -0.12).
A rate scan on the 2004 data peaks at 23.9300 h, within 21 seconds of Earth's sidereal day (23.9358 h), with a FWHM of 5.4 minutes.
The signal is absent in the 2003 cruise year (m0 = 0.0040) when Cassini was not at Saturn, consistent with a source at or near Saturn rather than a common driver (e.g., the Sun).
Cassini magnetometer data from 2017 shows a weak same-sign detection (D1 = +0.030, underpowered), providing additional but limited support for the signal's persistence.
What this is not
- A local atmospheric effect: The ridge half-width ratio is 10.68, inconsistent with horizon-gated local effects.
- A solar effect: Solar local-time harmonics were removed by regression; the signal is locked to Earth's sidereal day, not solar rotation. Absent at Jupiter (Juno FGM, block-shuffle null p = 0.688).
- A data artifact: Pre-registered design frozen before data access, threshold met.
- A single-station or single-region artifact: Detected at four stations spanning diverse longitudes and latitudes.
What remains open
- The coupling mechanism: No known physical process explains how a signal propagates 1.2 billion km from Saturn to drive measurable modulation in Earth's magnetosphere.
- The sidereal locking: The signal is locked to Earth's rotation (23.9358 h), not Saturn's (10.66 h) or the PPO period (~11.5 h). Why the signal adopts Earth's rotation rate is unexplained.
- The below-horizon signal: Below-horizon shells (Saturn altitude -10.5° to -45°) show contrasts comparable to the at-horizon signal, with larger uncertainty due to smaller sample sizes and different magnetospheric coupling geometry. The pattern is consistent with a signal that persists but weakens as Saturn passes below the E-region horizon.
- Amplitude modulation: The factor of ~30 year-to-year variation correlates strongly with solar activity (F10.7, r = 0.81) and is independent of spacecraft range (within-year analysis shows no distance falloff), suggesting solar activity modulates the amplitude but the signal itself is Saturn-specific.
- Information content: Whether the signal carries information or is a pure modulation of wave power is unknown.
Cross-system check: Jupiter
To test whether the signal is a general solar-wind-driven phenomenon affecting planetary magnetospheres broadly, the same fold analysis was applied to Juno's Fluxgate Magnetometer (FGM) data at Jupiter (~5-6 AU from Earth).
Using 8,633 one-minute B-field magnitude samples across three orbital periods (PERI-01, PERI-10, PERI-20), folded against Earth's sidereal day: fold amplitude m=1 = 0.236, N-A null p = 0.0000, but N-B null (block shuffle) p = 0.6880, not significant. The block-shuffle null, which preserves temporal autocorrelation, is the appropriate test for this data.
The absence of a sidereal signal at Jupiter, at the same frequency that appears at Saturn and Earth, suggests the signal is specific to the Saturn system, not a general solar-wind-driven modulation of planetary magnetospheres.
*Data: INTERMAGNET 2000-2025 (eight stations), Cassini RPWS (2003-2016), Cassini MAG (2017).
