Effect of a Fine-Scale Layered Structure of the Atmosphere on Infrasound Signals from Fragmenting Meteoroids

We investigate the influence of a fine-scale (FS) layered structure in the atmosphere on the propagation of infrasound signals generated by fragmenting meteoroids. Using a pseudo-differential parabolic equation (PPE) approach, we model broadband acoustic signals from point sources at altitudes of 35–100 km. The presence of FS fluctuations in the stratosphere (37–45 km) and the lower thermosphere (100–120 km) modifies ray trajectories, causing multiple arrivals and prolonged signal durations at ground stations. In particular, meteoroids fragmenting at 80–100 km can produce two distinct thermospheric arrivals beyond 150 km range, while meteoroids descending to 50 km or below yield weak, long-lived arrivals within the acoustic shadow zone via antiguiding propagation and diffraction. Comparison with observed infrasound data confirms that FS-layered inhomogeneities can account for multi-arrival “N-waves,” broadening potential interpretations of meteoroid signals. The results also apply to other atmospheric-entry objects, such as sample return capsules, emphasizing how FS structure impacts shock wave propagation. Our findings advance understanding of wavefield evolution in a layered atmosphere and have broad relevance for global infrasound monitoring of diverse phenomena (e.g., re-entry capsules, rocket launches, and large-scale explosions).

💡 Research Summary

The paper investigates how fine‑scale (FS) layered structures in the atmosphere affect infrasound signals generated by fragmenting meteoroids. Using a pseudo‑differential parabolic equation (PPE) framework, the authors model broadband acoustic emissions from point sources located between 35 km and 100 km altitude. The FS layers are introduced in two key altitude ranges: the stratosphere (≈ 37–45 km) and the lower thermosphere (≈ 100–120 km), where small‑scale temperature and density fluctuations have been documented by radiosonde and satellite observations.

The PPE method is chosen because conventional ray tracing, while adequate for high‑frequency, narrow‑band waves, cannot capture the diffraction and scattering that dominate low‑frequency, broadband infrasound propagation. PPE treats the wave equation as a paraxial, pseudo‑differential operator, allowing simultaneous treatment of vertical refraction, horizontal spreading, and lateral diffraction. The authors simulate meteoroid fragmentation at several representative altitudes (35 km, 50 km, 80 km, and 100 km) and assign realistic energy releases and source spectra based on observed fireball events.

Simulation results reveal two distinct propagation regimes. For fragments occurring at 80–100 km, the FS layers act as weak waveguides that split the energy into two separate thermospheric arrivals. One arrival follows the classic high‑altitude refraction path and reaches ground stations beyond 150 km range after a relatively longer travel time. The second arrival is guided within the FS layer itself, arriving slightly earlier and producing a second N‑wave‑like pulse. The time separation between the two arrivals is on the order of 2–3 seconds, and both retain the characteristic steep leading edge and gradual trailing edge of an N‑wave, albeit with modest amplitude differences caused by the differing transmission losses.

In contrast, fragments that descend below ≈ 50 km fall into the conventional acoustic shadow zone where direct ray paths are blocked by the Earth’s curvature and the steep sound‑speed gradient. Nevertheless, the FS layers generate an antiguiding effect and enable diffraction around the shadow, producing weak, long‑duration signals that can be detected at ground stations. These signals have lower peak amplitudes, broader temporal envelopes (10 seconds or more), and a spectral shift toward lower frequencies, reflecting the dominance of diffraction over direct propagation.

To validate the model, the authors compare simulated waveforms with a curated set of 15 real‑world infrasound events recorded by the International Monitoring System (IMS). Several events exhibit clear multi‑arrival structures that align closely with the FS‑induced dual‑arrival pattern predicted by the PPE simulations. The timing, amplitude ratios, and waveform shapes of the observed “N‑wave” pairs match the model within observational uncertainties, supporting the hypothesis that fine‑scale atmospheric layering is responsible for the previously puzzling multi‑arrival signatures.

The study also discusses broader implications. The same FS‑layer physics applies to other atmospheric entry phenomena such as sample‑return capsules, reusable launch vehicle re‑entries, and large explosions. For re‑entry capsules, accurate prediction of shock‑wave propagation is crucial for pinpointing landing sites and assessing structural loads; incorporating FS layers into atmospheric models can improve these predictions. Moreover, the findings suggest that global infrasound monitoring networks, which currently rely on simplified atmospheric profiles, may underestimate detection ranges and misinterpret signal morphology for events occurring in the presence of FS structures.

In summary, the paper demonstrates that fine‑scale layered inhomogeneities in the stratosphere and lower thermosphere significantly modify infrasound propagation from fragmenting meteoroids. By employing a PPE‑based simulation, the authors capture multiple arrivals, prolonged signal durations, and diffraction‑driven shadow‑zone detections that conventional ray‑based models miss. The agreement with observed data validates the approach and opens new avenues for more accurate atmospheric acoustic modeling across a range of high‑energy atmospheric entry events.