BOSTON UNIVERSITY
Department of Astronomy
Center for Space Physics

Meteor Physics

Shooting stars and their invisible plasma wakes — from atomic-scale ablation to kilometer-long plasma trails, instabilities, exotic radar echoes, and upper-atmosphere wind measurements.

Overview

A bright meteor streaks across a star-filled night sky over a dark landscape.
An optical meteor lighting up the night sky. As a meteoroid plunges into the atmosphere at 11–72 km/s it heats to thousands of degrees and vaporizes; collisions excite both the ablated metal atoms and the surrounding air, and their radiative de-excitation produces the visible streak we see as a shooting star. The brightest events — fireballs or bolides — can momentarily outshine the planets. The same collisions that generate this glow also strip electrons from the ablated atoms, seeding the plasma trail whose evolution and radar signatures our group models. Image: NASA / Bill Ingalls — a bright Perseid meteor, 13 Aug 2015 (public domain).

Each clear night, hundreds of meteors are visible to the naked eye from any dark location on Earth — the familiar shooting stars caused by dust-grain to pea-sized meteoroids ablating at 11–72 km/s in the upper atmosphere (70–120 km altitude). As the meteoroid heats and vaporizes, collisions between ablated atoms and ambient air molecules strip electrons, creating an elongated cylinder of ionized plasma: the meteor trail. This plasma glows in visible light and strongly scatters radar signals, making meteors both beautiful and scientifically useful.

Our group studies the plasma physics of meteor trails from first principles: how they form, how the plasma diffuses and evolves under gravity and magnetic forces, why some trails develop plasma instabilities that produce dramatically enhanced radar echoes, and how the trail motion can be used to measure winds at altitudes that are otherwise nearly impossible to probe. We combine atomic-scale ablation simulations, massively parallel PIC plasma codes, and high-power radar observations to connect microphysics to observable signatures.

Key Research Topics

Meteor Trail Formation and Diffusion

When ablated atoms ionize, the initial plasma column is only centimeters wide, then diffuses outward driven by ambipolar diffusion, geomagnetic forces, and neutral wind shear. We developed analytic theory (Dimant & Oppenheim 2006) and 3D kinetic PIC simulations (Tarnecki & Oppenheim 2021, Oppenheim & Dimant 2015) that predict how the trail evolves from microseconds to minutes. A striking day-to-night asymmetry in trail measurements provided evidence for our new theory of plasma trail evolution (Oppenheim et al. 2008, GRL).

Plasma Instabilities and Nonspecular Echoes

Shortly after formation, many meteor trails develop plasma instabilities driven by field-aligned electron drift. These instabilities generate field-aligned irregularities that scatter radio waves from directions far from the geometrically specular point — the so-called nonspecular trail echoes (NSTEs). Our group pioneered the theoretical explanation for NSTEs (Oppenheim, Dyrud et al. 2003; Dyrud et al. 2002, 2005) and continues to study how the magnetic aspect angle, trail altitude, and electron density govern their detectability (Green & Oppenheim 2025).

Radar Head Echoes and Meteoroid Properties

The plasma sheath surrounding an ablating meteoroid creates a compact, bright radar target — the head echo — that moves with the meteoroid at tens of km/s. High-power radars such as Millstone Hill, ALTAIR, and Jicamarca detect these echoes and measure meteoroid speeds, directions, and deceleration. We develop plasma models of the head echo plasma distribution (Dimant & Oppenheim 2017; Sugar et al. 2018, 2021) and collaborate directly with radar observatories to interpret their measurements (Akharman et al. 2025).

Meteor Winds: Atmospheric Remote Sensing

Meteor trail plasma drifts with the neutral wind in the mesosphere and lower thermosphere (80–105 km) — an altitude region nearly inaccessible to balloons, aircraft, and most satellites. By tracking the Doppler shift and spatial drift of non-specular trail echoes with incoherent scatter radar, our group developed a new technique for measuring MLT wind profiles with unprecedented temporal and spatial resolution. Observations at Jicamarca revealed winds exceeding 500 km/hr and intense vertical shears (Oppenheim et al. 2009, 2014).

Atomic-Scale Ablation Simulations

The plasma that forms a meteor trail ultimately originates from individual atoms and molecules being stripped from the meteoroid surface. We use molecular dynamics simulations (Guttormsen, Fletcher & Oppenheim 2020) to model this ablation process at the atomic scale — tracking how mineral surface layers respond to hypervelocity atmospheric impact, what initial velocity distributions the ablated atoms carry, and how meteoroid composition and rotation affect plasma production (Hedges et al. 2025). These simulations feed directly into our larger-scale plasma evolution models.

Meteoroid Masses, Densities, and Populations

Understanding the meteoroid mass influx to Earth matters for atmospheric chemistry, climate, and planetary science. We developed techniques for determining meteoroid mass and bulk density from radar head echo scattering at multiple frequencies and from trail deceleration measurements (Close et al. 2004, 2012; Bass et al. 2008). Collaboration with Sigrid Close (Stanford) on ALTAIR and Arecibo datasets produced the most detailed radar-based meteoroid mass–velocity distributions available.

Radar range-time-intensity map showing two diagonal head-echo streaks labeled 1 and 2 and a broad speckled non-specular trail echo.
The two faces of a meteor in high-power radar data. This range–time–intensity (RTI) map shows the radar signal-to-noise ratio (color bar, dB) as a function of altitude and time. The two narrow, nearly vertical streaks (1 and 2) are head echoes — compact, dense plasma sheaths that travel downward with their meteoroids at tens of km/s; their slope on this plot directly measures the meteoroid speed and deceleration. The broad, speckled cloud of enhanced backscatter filling roughly 96.5–98.5 km is the non-specular trail echo: radio scattering from field-aligned plasma irregularities that grow, via plasma instabilities, in the ionized trail left behind after each meteoroid passes. Unlike classical specular trail reflections, non-specular echoes are generally strongest when the radar line of sight is nearly perpendicular to the geomagnetic field, and they encode the turbulent wake we simulate with our kinetic plasma codes. Observation courtesy of the Oppenheim group, Boston University.
Aerial view of the Jicamarca Radio Observatory main antenna, a vast square field of crossed dipoles in the Peruvian desert.
The 50 MHz Jicamarca Radio Observatory (JRO), Peru. Radar maps like the one at left are recorded at facilities such as JRO, whose main array — roughly 300 m on a side and built from thousands of crossed dipoles — forms one of the most powerful 50 MHz radars on Earth. Sitting almost directly under the magnetic equator, it looks nearly perpendicular to the geomagnetic field, the geometry that most favors detection of non-specular trail echoes. Our group has used JRO to observe meteor trails and measure lower-thermosphere winds. Photo: Jicamarca Radio Observatory, via Wikimedia Commons.

Our Computational and Observational Approach

Simulation Approaches: Atoms to Plasma

Step 1 — Atomic scale: Molecular dynamics (LAMMPS) simulations model how the meteoroid mineral surface ablates under hypervelocity impact, producing the initial velocity distribution of ablated atoms and the plasma ionization fraction.

Step 2 — Plasma formation: Direct Simulation Monte Carlo (DSMC) simulations follow the neutral gas dynamics around the ablating meteoroid — how sputtered atoms collide with and disturb the surrounding air — and track the neutrals as they ionize into plasma. Coupled with electromagnetic finite-difference time-domain (FDTD) simulations of the scattered radar wave, this lets us model meteor plasma formation and the resulting head echoes from first principles.

Step 3 — Plasma scale: Massively parallel 2D and 3D PIC simulations then evolve the plasma self-consistently, capturing trail diffusion, instability growth, and the electromagnetic signatures detectable by radar. Hybrid codes (fluid ions / kinetic electrons) allow efficient simulation of the macroscopic trail structure while retaining electron kinetic effects.

Four-panel 3D particle-in-cell simulation of a meteor plasma trail diffusing in three dimensions, showing nested density iso-contours and a cross-sectional plane. ▶ Play the 3-D simulation movie
A meteor trail diffusing in 3-D — frames from our kinetic plasma simulation. (Click the image to play the movie.) These snapshots come from a fully three-dimensional particle-in-cell (PIC) simulation of a meteor trail run with our EPPIC code. Colored surfaces are three nested iso-contours of plasma density (red, green, blue) drawn parallel to the initial trail, together with a cross-sectional density plane (blue) cut perpendicular to it. (a) The pencil-thin initial plasma column — peak density ~160× the background — with the meteoroid path and geomagnetic field B0 marked. (b) After 4.6 ms, field-aligned ridges and troughs have grown along B0; these density irregularities are what scatter radio waves into the non-specular trail echoes seen by radar. (c) At 76 ms with no neutral wind, the trail spreads by anisotropic ambipolar diffusion — far faster across the field than along it. (d) The same instant with a 100 m/s wind, which shears the trail and drives turbulent diffusion and mixing. Simulation by the Oppenheim group; see Oppenheim & Dimant (2015) and Tarnecki & Oppenheim (2021).

Radar Collaborations

We have worked closely with major incoherent scatter radar observatories: Millstone Hill (MIT Haystack), Jicamarca (Peru), ALTAIR (Kwajalein), and PFISR (Alaska). These instruments detect individual meteors in real time at multiple frequencies and aspect angles, providing the observational constraints that validate our simulations. Recent Millstone Hill observations (Akharman et al. 2025) directly test our head echo plasma models.

Optical Meteors (A Promising Direction)

Optical meteor observations complement radar measurements by providing independent constraints on meteoroid entry angle, velocity, and ablation altitude, and the visible glow of a meteor is itself a probe of the plasma chemistry in the trail. Our group's recent work has centered on radar and simulation rather than optical data, but pairing optical measurements with our plasma models is an appealing future direction: it would help close the loop on the meteor ablation process, from the first atoms evaporating off the surface to the final diffusion of the plasma trail. We would welcome students and collaborators interested in pursuing it.

Selected Publications

Meteor Physics as a Research Area

Meteor physics combines phenomena visible to the naked eye, sophisticated plasma physics, powerful radar observatories, and connections to planetary science and atmospheric chemistry — work at the interface of simulation and observation.

Research in the group runs through the BU Astronomy PhD program: bu.edu/astronomy/graduate.

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