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Meteors

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We endeavor to create the first comprehensive characterization of meteoroids and meteoroid plasmas (i.e. meteors) to understand their effects on the lower ionosphere and their threat to orbiting spacecraft. Meteoroids are naturally occurring objects in space that travel between 11 and 72.8 km/s and originate primarily from comets and asteroids. On average, over 100 billion meteoroids enter Earth’s atmosphere daily with masses larger than 1 microgram. These include shower meteoroids, which are associated with a parent body, as well as sporadic meteoroids, which form the background population. Although meteoroids have a profound effect on our space environment and produce plasma densities that are orders of magnitude greater than the background ionosphere, we understand very little about their fundamental properties. These include meteoroid mass and density that depends on orbit and velocity, the formation and distribution of irregularities in the lower ionosphere, the mass deposition rate into our atmosphere, the effects of meteoroid fragmentation on plasma formation, and the effect of the background electric and magnetic fields on plasma expansion and distribution. We seek to answer these questions by probing into the plasma that surrounds the meteoroid, known as the head echo, and behind the meteoroid, called the trail, in order to assess the threat to spacecraft. Our approach includes both experiment and modeling.

Radar studies

When a meteoroid enters the Earth's atmosphere, heats up, and ablates, a plasma forms. The plasma traveling with the meteoroid is called a meteor head, while the plasma that is left behind is called a meteor trail. By transmitting pulsed electromagnetic waves with a high-power large-aperture (HPLA) radar and measuring the waves that are scattered back, one hopes to characterize the plasma and its evolution and infer many properties of the parent meteoroid such as velocity, mass, density, and composition. The image below shows typical radar data for a meteor taken with the Jicamarca Incoherent Scatter Radar outside Lima, Peru. It plots signal to noise ratio (SNR) as a function of range and time the pulse was transmitted. The diagonal line on the left is the signal from a meteor head, while the high-SNR portion to the right is signal due to the accompanying trail.

Meteor head and trail seen with the Jicamarca Incoherent Scatter Radar

Smaller "meteor radars" have an even longer history of observing trails. They receive what are called specular trail reflections, resulting when the radar beam is perpendicular to the meteor trail. Although the techniques are different, both radar methods provide unique insight into the meteoroid population.

Estimating Mass, Density, and Size

Since meteoroids themselves normally burn up before reaching the Earth's surface, it is impossible to directly measure their mass, density, and size. Instead we infer these values from radar measurements.

Modeling of Surrounding Plasmas

Particle-in-cell simulations provide insight into the dynamics of the plasmas that we see as meteors.

 

Orbit Determination

Hyperbolic Orbit
A hyperbolic meteoroid trajectory shown intersecting with Earth's orbit.

Once we know the position and velocity of a meteor with respect to the radar, we can determine its orbital elements and where it came from. While many meteors come from comet trails or asteroids, others may come from interstellar sources. High Power Large Aperture (HPLA) radars can measure a meteor's position and velocity with enough accuracy to determine whether an meteoroid's origin is interstellar or not. To further increase certainty, we model effects such as solar radiation pressure, Poynting-Robertson drag, and Lorentz forces, which disproportionately affect small particles.

Interstellar Meteors
Specific mechanical energy for a set of approximately 17,000 meteors observed by the ALTAIR radar on Kwajalein Atoll. Of this specially selected sample, about 20% of the meteors correspond to interstellar meteoroids (ME > 0) with 95% confidence. Overall, we expect less than 5% of all meteors to come from interstellar sources.

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