The Role of Source Geometry and Atmospheric Propagation in Global Bolide Infrasound Detectability

This paper analyzes 623 bolide events from 2007 to 2025 to demonstrate that infrasound detectability is primarily governed by entry geometry, specifically favoring steeper angles and lower-altitude energy deposition, while atmospheric propagation and energy levels act as secondary modulating factors.

The Gist

Imagine the Earth's atmosphere as a giant, invisible ocean of air. When a space rock (a meteoroid) crashes into this ocean at supersonic speeds, it doesn't just make a splash; it creates a massive, rolling shockwave. This shockwave is a sound so low-pitched that our ears can't hear it, called infrasound. It's like the deep rumble of a giant whale that travels for thousands of miles without losing much energy.

This paper is a massive detective story. The authors wanted to answer a simple question: Why do we hear (detect) some of these space rock crashes with our global microphone network, but miss others?

To solve this, they looked at 623 space rock entries recorded by NASA between 2007 and 2025. They then checked if the International Monitoring System (a global network of microphones originally built to listen for nuclear tests) had "heard" them.

Here is what they found, explained with everyday analogies:

1. The "50% Success Rate" Surprise

In the past, scientists thought we only detected about 20% of these events. This study found that with better technology and more microphones, we are actually catching about 50% of them.

• The Analogy: Imagine trying to hear a conversation in a noisy room. Ten years ago, you had a cheap, broken microphone and only one person to listen. Now, you have a high-tech array of microphones and a team of experts. You aren't hearing everything (you still miss half), but you are catching way more than before.

2. The Angle of Entry is the "Master Key"

The biggest discovery is that how the rock enters the atmosphere matters more than how big or loud the explosion is.

• The Steep Diver (Detected): When a rock dives in at a steep angle (like a cannonball dropped straight down), it creates a tight, focused shockwave.
  • The Analogy: Think of a laser pointer. If you shine a laser straight at a mirror, the beam stays tight and hits the target perfectly. This is what happens with steep entries; the sound energy is focused and easily caught by the "mirrors" in the atmosphere (called waveguides) that bounce sound around the globe.
• The Shallow Skier (Missed): When a rock glides in at a shallow angle (like a stone skipping across a pond), the shockwave is stretched out over a long distance.
  • The Analogy: This is like trying to shine a flashlight through a foggy window. The light spreads out, gets weak, and scatters. Even if the rock is huge, the sound energy is spread so thin and at such a weird angle that the atmospheric "mirrors" don't catch it, and it leaks out into space instead of bouncing back to Earth.

3. The Atmosphere is a "Roller Coaster"

Even if the rock dives in perfectly, the atmosphere has to cooperate. The air isn't uniform; it has layers of wind and temperature that act like invisible tunnels or waveguides.

• The Analogy: Imagine sound traveling through the air like a roller coaster car. If the track (the atmosphere) has the right curves (wind and temperature layers), the car (the sound) stays on the track and zooms across the globe. If the track is broken or flat, the car falls off.
• The study found that the "steep divers" are much better at getting onto these roller coaster tracks than the "shallow skiers," regardless of how much energy they have.

4. Energy Isn't Everything

You might think a bigger explosion (more energy) would always be louder. The study says: Not necessarily.

• The Analogy: Imagine two people shouting. One is a giant (high energy) shouting while running away from you at a weird angle through a wall (shallow entry). The other is a smaller person (lower energy) shouting directly at you through an open door (steep entry). You will hear the smaller person much better.
• The authors found that while a massive explosion (like the Chelyabinsk meteor) is loud enough to be heard no matter what, most of the rocks we see are in the "medium" size range. For these, the angle of entry is the deciding factor, not just the size of the boom.

5. The "Where" Matters More Than the "When"

The study also noted that the loudest part of the sound doesn't always happen at the same time as the brightest flash of light.

• The Analogy: Think of a firework. The brightest flash might happen at the very top, but the "boom" you hear might come from the explosion that happened a second earlier or a few miles away. The sound source is often a long, stretched-out line, not a single point.

The Bottom Line

This paper tells us that our global listening system is much better than we thought, but it's not perfect. It acts like a selective filter. It naturally picks up the "steep divers" because their sound waves fit perfectly into the atmospheric tunnels that carry sound around the world. It often misses the "shallow skiers," even if they are big, because their sound waves scatter and get lost.

So, when we look at our list of detected space rocks, we aren't seeing the whole picture. We are seeing the ones that entered at the "right" angle to be heard, while the ones that glided in quietly are still hiding in the data.

Technical Summary

Technical Summary: The Role of Source Geometry and Atmospheric Propagation in Global Bolide Infrasound Detectability

Problem Statement
Global infrasound monitoring serves as a critical complement to optical systems for detecting energetic meteoroid atmospheric entries (bolides), offering persistent, over-the-horizon coverage. However, the physical factors governing whether a specific bolide event generates infrasound detectable at regional to global distances remain insufficiently constrained. Previous studies, often limited by small sample sizes, heterogeneous detection strategies, or regionally constrained observations, have suggested low detection rates (e.g., <20%) and relied heavily on event energy as the primary predictor of observability. There is a need for a systematic, population-level assessment to determine how source geometry (entry angle, trajectory) and atmospheric propagation conditions interact to control detectability across the full spectrum of bolide events.

Methodology
This study conducts a systematic global analysis correlating 623 bolide events reported by the NASA Center for Near-Earth Object Studies (CNEOS) between September 2007 and May 2025 with waveform data from the International Monitoring System (IMS).

• Dataset: The analysis utilizes the CNEOS fireball catalog, which provides event timing, location, peak brightness altitude, estimated impact energy, and velocity vectors.
• Signal Search: A uniform, automated detection framework using the Cardinal multi-frequency array processing software was applied to the global IMS network. The search employed expanded arrival-time windows (150–600 m/s apparent celerity) to account for distributed source regions and complex propagation paths.
• Detection Criteria: Valid detections were defined by back azimuth consistency with the CNEOS-reported location (within ±15°, or ±45° for short ranges), celerity within the predicted infrasonic range (180–330 m/s), and trace velocity within expected bounds (250–500 m/s).
• Propagation Modeling: Atmospheric conditions along source-receiver paths were characterized using the Ground-to-Space (G2S) atmospheric model to assess effective sound speed profiles and the presence of tropospheric, stratospheric, and thermospheric waveguides (ducts).
• Statistical Analysis: The study compares the distributions of entry angle, entry velocity, and peak brightness altitude between the full CNEOS population and the subset of infrasound-detected events using Kernel Density Estimates (KDEs) and Empirical Cumulative Distribution Functions (CDFs). Correlation analyses (Pearson and Spearman) were performed to assess relationships among parameters.

Key Contributions

• Updated Detection Rates: The study identifies 311 infrasound detections from 623 CNEOS events, yielding a global detection rate of approximately 50%. This represents a significant increase over earlier estimates (<20%), attributed to the maturation of the IMS network and advances in automated, multi-frequency array processing.
• Geometric Selectivity: The research demonstrates that infrasound detectability is not uniform across the bolide population but is highly selective. Detected events are preferentially associated with steeper entry angles and lower-altitude energy deposition.
• Decoupling of Energy and Detectability: While higher-energy events are generally more detectable, the study establishes that energy alone does not uniquely determine observability. Within comparable energy ranges, detectability remains strongly dependent on entry geometry.
• Propagation Independence: The analysis shows that favorable atmospheric ducting conditions are not systematically correlated with specific entry geometries, indicating that the observed geometric bias is driven by source-waveguide coupling efficiency rather than coincidental atmospheric conditions.

Results

• Entry Angle: There is a pronounced systematic shift in entry angles; detected bolides occur more frequently at steeper angles compared to the broader CNEOS population. Steep trajectories concentrate energy deposition over shorter path lengths, facilitating more efficient coupling of acoustic energy into atmospheric waveguides (particularly stratospheric ducts).
• Altitude: Detected events tend to reach peak brightness at lower altitudes than non-detected events, further supporting the hypothesis that lower-altitude energy release favors long-range propagation.
• Velocity: Entry velocity distributions between detected and non-detected populations are broadly similar, with only a modest shift toward lower velocities in the detected subset. Velocity alone is not a primary discriminator of detectability.
• Correlations: In the detected subset, a moderate inverse relationship exists between entry angle and velocity, and a positive correlation exists between peak brightness altitude and velocity. However, these correlations are weaker in the full population, suggesting that the detected subset represents a specific configuration of parameters that optimizes acoustic coupling.
• Signal Characteristics: The study notes that shallow-entry events often exhibit greater azimuthal variability and multiple arrivals, consistent with a spatially distributed acoustic source rather than a single point source coincident with peak optical brightness.

Significance
The paper concludes that infrasound detectability is a probabilistic process governed primarily by source geometry (specifically entry angle) and secondarily by atmospheric propagation conditions, with event energy acting as a modulating factor rather than a sole determinant.

• Planetary Defense and Monitoring: The findings imply that global infrasound networks provide an incomplete sampling of the bolide population, potentially underrepresenting shallow, high-altitude entries that may still possess significant kinetic energy. This necessitates caution when using infrasound-derived statistics for impact flux assessments or hazard modeling without correcting for geometric biases.
• Source Characterization: The results challenge the assumption of a point-source model for bolide infrasound, particularly for shallow trajectories, suggesting that effective acoustic sources are often distributed regions evolving along the trajectory.
• Future Applications: The study highlights the potential for elevated sensing platforms (e.g., balloon-borne sensors) to capture acoustic energy in stratospheric waveguides that surface arrays may miss, and suggests that regional networks may be better suited for monitoring shallow-entry events than global systems alone.
• Comparative Planetology: The identified physical principles regarding the interplay between entry geometry and atmospheric waveguides are applicable to impact and entry phenomena on other atmosphere-bearing planetary bodies (e.g., Venus, Mars, Titan), offering a framework for interpreting potential future acoustic observations in extraterrestrial environments.

The authors emphasize that while the study provides robust population-level constraints, a rigorous, station-level sensitivity model incorporating time-dependent noise and full range-dependent propagation is required to define a universal detectability floor, a task reserved for future work.