A four-year dataset of 1,436 daily measurements shows Schumann resonance amplitude rising from 0.345 to 0.366 as geomagnetic storm intensity crosses Kp 7, a 6.2 percent increase tied directly to changes in the ionosphere above Earth. Findings published in Journal of Atmospheric and Solar-Terrestrial Physics in April 2026 quantify this shift using continuous ELF recordings taken at 10 minute resolution from southern Spain. The signal sits at 7.8 hertz because electromagnetic waves circle the planet inside a cavity bounded by the ground and a charged atmospheric layer 60 to 100 kilometers high. Lightning injects energy into this cavity at a global rate of roughly 40 flashes per second, forcing waves to bounce and interfere until stable standing patterns form. Each reflection loses a small fraction of energy depending on how conductive the upper boundary is, which sets the amplitude that instruments detect. The entire system operates like a planetary-scale waveguide where any change in boundary conditions directly alters signal strength at the Sierra Nevada station in Spain .
The measurements come from a station at 37.03 degrees north that logs magnetic field variations in two perpendicular directions with sensitivity below a picotesla, capturing signals generated thousands of kilometers away. Data coverage reaches 98.3 percent across the 1,461 day period, leaving only minor gaps that were excluded if fewer than six valid records existed in a day. Each day aggregates roughly 279 individual measurements, producing stable averages that filter out transient spikes while preserving large-scale changes. Quality filters reject any amplitude outside 0.01 to 5.0 units and frequencies outside 5 to 12 hertz, ensuring only physically valid resonance states remain. This produces a clean dataset where daily changes on the order of 0.005 units can be resolved without instrument drift contaminating the signal. The result is one of the highest continuity ground-based ELF records currently available in Europe.
Storm intensity is defined using the Kp index, which compresses global magnetometer readings into a scale from 0 to 9 based on disturbances in Earth’s magnetic field. Each unit increase represents stronger electric currents flowing through the ionosphere as charged solar particles collide with atmospheric gases. The dataset assigns each day to a category based on the maximum Kp recorded within the previous 48 hours, capturing delayed atmospheric response after a storm hits. Quiet days below Kp 5 dominate the record with 1,124 entries, while moderate storms contribute 290 days and strong storms above Kp 7 appear only 22 times. This imbalance reflects how rare extreme solar-driven disturbances are compared to stable geomagnetic conditions. The classification still provides enough high-intensity events to measure consistent amplitude changes tied to space weather forcing.
Amplitude increases follow a strict step pattern that does not reverse across categories, rising from 0.3448 during quiet periods to 0.3508 during moderate storms and 0.3661 during strong storms. The increase of 0.0213 units between quiet and strong conditions reflects reduced energy loss inside the cavity rather than increased lightning input. Measurement uncertainty remains narrow, with 95 percent confidence intervals overlapping only slightly between categories, confirming a real shift rather than random variation. The monotonic structure appears again in a five-bin breakdown where amplitude climbs steadily through Kp 5, 6, 7, and 8 levels, reaching 0.3784 at the highest intensity. The top bin contains only three days, but the intermediate steps with at least 19 days each form a consistent staircase. This pattern ties amplitude directly to geomagnetic intensity rather than isolated extreme events.
Lightning remains the dominant energy source, so the analysis removes its influence using a global lightning index built from satellite-corrected stroke density maps. Daily lightning totals are summed across a 0.5 degree grid to produce a single number representing global storm activity. The correlation between lightning and Kp measures at minus 0.061, showing that strong geomagnetic storms do not coincide with more lightning and often occur during slightly quieter atmospheric conditions. Time-of-day effects are stripped out by normalizing each measurement against hourly averages, eliminating the daily cycle driven by continental heating. Seasonal shifts are also removed by adjusting each value relative to monthly means, accounting for migration of storm belts between hemispheres. After these corrections, the amplitude increase tied to Kp remains intact, isolating the ionosphere as the source of change.
The physical driver sits in the ionosphere, where geomagnetic storms inject high-energy particles that increase electron density and electrical conductivity. Conductivity determines how easily currents flow, which directly controls how much electromagnetic energy is lost during each reflection inside the cavity. A more conductive layer reflects waves more efficiently, reducing dissipation and allowing amplitude to build. At the same time, solar wind pressure compresses the ionosphere downward by a few kilometers, shrinking the cavity height. A smaller cavity forces waves to complete shorter paths, increasing their resonant frequency slightly. The dataset records a frequency shift of 0.0082 hertz per unit increase in Kp, matching the expected change from a compressed boundary. These two mechanisms act together, raising amplitude while nudging frequency upward during storm periods.
Directional measurements reveal that not all parts of the ionosphere respond equally, with the north-south magnetic channel showing a stronger frequency shift than the east-west channel. The north-south correlation reaches 0.22 while the east-west channel shows only 0.025, indicating a directional dependence in how the boundary changes. This asymmetry traces back to geomagnetic currents that flow along field lines aligned roughly north to south, especially during auroral activity. These currents alter conductivity unevenly across different directions, changing how waves reflect depending on their orientation. The amplitude increase appears in both channels, meaning the energy retention effect is global, but the frequency shift exposes structural differences in the ionosphere. The measurement system captures this with resolution below 0.01 hertz, enough to detect subtle directional distortions during storms.
Statistical testing accounts for autocorrelation in the data, where one day’s measurement influences the next due to slow atmospheric recovery. Residual autocorrelation reaches 0.44, reducing effective sample size to roughly 450 independent observations. A Newey-West regression corrects for this by adjusting error estimates across seven lag periods, ensuring that significance levels remain valid. The Kp coefficient strengthens as more controls are added, rising from 0.0045 in a simple model to 0.0051 when lightning and seasonal factors are included. This increase shows that background variability masks part of the signal rather than creating it. The final model reaches a significance level of 6.2 × 10⁻⁷, confirming that the amplitude increase is not a statistical artifact. The effect explains about 4.6 percent of total variance, with roughly 3 percent directly attributable to geomagnetic forcing.
Storm windows extending four days after a Kp 5 event show elevated amplitude averaging 0.353 compared to 0.343 during quiet periods. This persistence reflects the time required for the ionosphere to return to baseline conductivity after particle injection stops. Charged particles remain trapped along magnetic field lines, continuing to influence the cavity even after the initial disturbance passes. The effect size measured by Cohen’s d reaches 0.19, small but consistent across hundreds of observations. These extended impacts show that the system does not respond instantly but instead holds altered conditions for several days. The delay creates a measurable footprint of space weather inside Earth’s electromagnetic environment.
Current verified measurements place the baseline amplitude range between 0.344 and 0.366 across geomagnetic conditions recorded between 2013 and 2017, with frequency shifts remaining below 0.1 hertz even during strong storms. The Kp index remains nearly independent of lightning activity, confirming that these changes originate in the ionosphere rather than the lower atmosphere. Continuous monitoring stations operating at mid-latitudes record these variations with precision below 0.5 percent, establishing a stable observational baseline. The resonance system remains active at all times due to constant lightning input, with geomagnetic storms acting as a secondary modifier layered on top of this dominant driver. These measurements define the current operational envelope of the Earth-ionosphere cavity under varying space weather conditions.
The next procedural step is integrating Kp data directly into real-time Schumann resonance monitoring pipelines so amplitude shifts linked to ionospheric compression can be separated from lightning-driven variability. Extending the dataset beyond 2017 across a full solar cycle provides the required range of storm intensities to confirm persistence of the 3 percent variance contribution. Expanding station coverage to higher latitudes tests whether the directional frequency shift strengthens closer to auroral zones where geomagnetic currents peak. Continuous comparison between stations aligned at different geomagnetic angles isolates how field geometry shapes resonance behavior. These steps move the system from passive observation toward a calibrated indicator of ionospheric state under active geomagnetic forcing.
Source:
Cann, K. (2026). Geomagnetic storm enhancement of Schumann resonance amplitude: a dose-response analysis using the Sierra Nevada ELF archive.
Preprint (ESSOAr)
https://doi.org/10.22541/essoar.15002103/v1
