Peter Jenniskens

Observations of Persistent Leonid Meteor Trails 2. Photometry and Numerical Modeling

During the 1998 Leonid meteor shower, multi-instrument observations of persistent meteor trains were made from the Starfire Optical Range on Kirtland Air Force Base, New Mexico, and from a secondary site in nearby Placitas, New Mexico. The University of Illinois Na resonance lidar measured the Na density and temperature in the trains, while various cameras captured images and videos of the trains, some of which were observed to persist for more than 30 min. The Na density measurements allow the contribution of Na airglow to the observed train luminescence to be quantified for the first time. To do this, persistent train luminescence is numerically modeled. Cylindrical symmetry is assumed, and observed values of the Na density, temperature, and diffusivity are used. It is found that the expected Na luminosity is consistent with narrowband CCD all-sky camera observations, but that these emissions can contribute only a small fraction of the total light observed in a 0.5–1 μ bandwidth. Other potential luminosity sources are examined, in particular, light resulting from the possible excitation of monoxides of meteoric metals (particularly FeO) and O2(b1∑g+) during reactions between atmospheric oxygen species and meteoric metals. It is found that the total luminosity of these combined processes falls somewhat short of explaining the observed brightness, and thus additional luminosity sources still are needed. In addition, the brightness distribution, the so-called hollow cylinder effect, remains unexplained.

Characteristics of Fe ablation trails observed during the 1998 Leonid Meteor Shower

Eighteen Fe ablation trails were observed during the 17/18 Nov 1998 Leonid meteor shower with an airborne Fe lidar aboard the NSF/NCAR Electra aircraft over Okinawa. The average altitude of the 18 trails from the high velocity (72 km/s) Leonid meteors, 95.67±0.93 km, is approximately 6.7 km higher than previously observed for slower (∼30 km/s) sporadic meteors. This height difference is consistent with the assumption that meteors ablate when atmospheric drag reaches a critical threshold. The average age of the Fe trails, determined by a diffusion model, is 10.1 min. The youngest ages were observed below 92 km and above 98 km where chemistry and diffusion dominate, respectively. The average abundance of the trails is 10% of the abundance of the background Fe layer. Observations suggest that the 1998 Leonid shower did not have a significant impact on the abundance of the background Fe layer.

Amorphous Water Ice

At low pressure and temperature, water ice can occur in a number of metastable states. These forms can persist over extended periods of time, up to the timescale of the solar system, due to activation energy barriers that prevent restructuring from one form into another. In this regime of temperature (T < 220 K) and pressure (P < 0.2 GPa), there is no thermodynamic equilibrium. All structural transitions are time dependent and irreversible.

Precisely reduced meteoroid trajectories and orbits from the 1995 Leonid meteor outburst

Abstract The discovery of enhanced Leonid activity in 1994 led to the mounting of a multi-station photographic campaign on two continents during the Leonids of 1995. The goal was to determine orbits and trajectories of outburst Leonids. The outburst did recur as predicted, resulting in 23 precisely reduced Leonid orbits and 30 Leonid trajectories. Individual velocities and radiant positions, as well as the average orbit are presented. It is found that the distribution of Leonid radiants contains a dense cluster of seven radiant positions. This clustering is tentatively associated with the visually observed outburst component. These seven meteors provide the first orbital elements of outburst Leonids and put strong constraints on theoretical models of the origin and evolution of the Leonid stream.

Surface Heating from Remote Sensing of the Hypervelocity Entry of the NASA GENESIS Sample Return Capsule

An instrumented aircraft and ground-based observing campaign was mounted to measure the radiation from the hypervelocity (11.0 km/s) reentry of the Genesis Sample Return Capsule prior to landing on the Utah Test and Training Range on September 08, 2004. The goal was to validate predictions of surface heating, the physical conditions in the shock layer, and the amount and nature of gaseous and solid ablation products as a function of altitude. This was the first hypervelocity reentry of a NASA spacecraft since the Apollo era. Estimates of anticipated emissions were made. Erroneous pointing instructions prevented us from acquiring spectroscopic data, but staring instruments measured broadband photometric and acoustic information. A surface-averaged brightness temperature was derived as a function of altitude. From this, we conclude that the observed optical emissions were consistent with most of the emitted light originating from a gray body continuum, but with a surface averaged temperature of 570 K less than our estimate from the predicted heat flux. Also, the surface remained warm longer than expected. We surmise that this is on account of conduction into the heat shield material, ablative cooling, and finite-rate wall catalycity. Preparations are underway to observe a second hypervelocity reentry (12.8 km/s) when the Stardust Sample Return Capsule returns to land at U.T.T.R. on January 15, 2006.

Dust Trails of 8P/Tuttle and the Unusual Outbursts of the Ursid Shower

Abstract We calculate the position of dust trails from comet 8P/Tuttle, in an effort to explain unusual Ursid meteor shower outbursts that were seen when the comet was near aphelion. Comet 8P/Tuttle is a Halley-type comet in a 13.6-year orbit, passing just outside of Earths orbit. We find that the meteoroids tend to be trapped in the 12:14 mean motion resonance with Jupiter, while the comet librates in a slightly shorter period orbit around the 13:15 resonance. It takes 6 centuries to decrease the perihelion of the meteoroid orbits enough to intersect Earths orbit, during which time the meteoroids and comet separate in mean anomaly by 6 years, thus explaining the 6-year lag between the comets return and Ursid outbursts. The resonances also prevent dispersion along the comet orbit and limit viewing to only one year in each return. We identified past dust trail encounters with dust trails from 1392 (Dec. 1945) and 1378 (Dec. 1986) and predicted another outburst on 2000 December 22 at around 7:29 and 8:35 UT, respectively, from dust trails dating to the 1405 and 1392 returns. This event was observed from California using video and photographic techniques. At the same time, five Global-MS-Net stations in Finland, Japan, and Belgium counted meteors using forward meteor scatter. The outburst peaked at 8:06±07 UT, December 22, at zenith hourly rate ∼90 per hour, and the Ursid rates were above half peak intensity during 4.2 h. We find that most Ursid orbits do scatter around the anticipated positions, confirming the link with comet 8P/Tuttle and the epoch of ejection. The 1405 and 1392 dust trails appear to have contributed similar amounts to the activity profile. Some orbits provide a hint of much older debris being present as well. This work is the strongest evidence yet for the relevance of mean motion resonances in Halley-type comet dust trail evolution.

A mechanism for forming deep cracks in comets

Abstract A low temperature structural change in amorphous water ice, the high density to low density amorphous transition, is proposed as a mechanism for the formation of deep cracks in cometary nuclei. Such cracks are proposed to underlie active vents in the cometary crust and to play a role in the splitting of cometary nuclei.

INELASTIC NEUTRON SCATTERING STUDY OF HIGH DENSITY AMORPHOUS WATER ICE

Abstract The various forms of amorphous ices with trapped impurities play an important role in astrophysics. Such ices observed spectroscopically as frost on interstellar dust in dense molecular clouds and in comets require a detailed understanding of the spectroscopic properties in laboratory conditions. Using the neutron inelastic scattering technique on ISIS at Rutherford-Appleton Laboratory, the recovered high density amorphous ice is measured along with its subsequent high temperature metastable phases: low density amorphous ice, cubic and hexagonal ices under the same conditions in the energy transfer region from 2 to 500 meV (i.e. 16–4025 cm −1 ). The results show that the spectra of low density amorphous ice have similar features to cubic and hexagonal ices. However, the spectrum for the high density amorphous ice is significantly different from the others in the translational and librational regions (

The Dynamical Evolution of a Tubular Leonid Persistent Train

The dynamical evolution of the persistent train of a bright Leonid meteor was examined for evidence of the source of the luminosity and the physical conditions in the meteor path. The train consisted of two parallel somewhat diffuse luminous tracks, interpreted as the walls of a tube. A general lack of wind shear along the trail allowed these structures to remain intact for nearly 200 s, from which it was possible to determine that the tubular structure expanded at a near constant 10.5 ms−1, independent of altitude between 86 and 97 km. An initial fast decrease of train intensity below 90 km was followed by an increase in intensity and then a gradual decrease at longer times, whereas at high altitudes the integrated intensity was nearly constant with time. These results are compared to a model that describes the dynamical evolution of the train by diffusion, following an initial rapid expansion of the hot gaseous trail behind the meteoroid. The train luminosity is produced by O (1S) emission at 557 nm, driven by elevated atomic O levels produced by the meteor impact, as well as chemiluminescent reactions of the ablated metals Na and Fe with O3. Ozone is rapidly removed within the train, both by thermal decomposition and catalytic destruction by the metallic species. Hence, the brightest emission occurs at the edge of the train between outwardly diffusing metallic species and inwardly diffusing O3. Although the model is able to account plausibly for a number of characteristic features of the train evolution, significant discrepancies remain that cannot casily be resolved.

Leonid Storm Flux Analysis from One Leonid Mac Video AL50R

A detailed meteor flux analysis is presented of a seventeen-minute portion of one videotape, collected on November 18, 1999, during the Leonid Multi-instrument Aircraft Campaign. The data was recorded around the peak of the Leonid meteor storm using an intensified CCD camera pointed towards the low southern horizon. Positions of meteors on the sky were measured. These measured meteor distributions were compared to a Monte Carlo simulation, which is a new approach to parameter estimation for mass ratio and flux. Comparison of simulated flux versus observed flux levels, seen between 1:50:00 and 2:06:41 UT, indicate a magnitude population index of r = 1.8 ± 0.1 and mass ratio of s = 1.64 ± 0.06. The average spatial density of the material contributing to the Leonid storm peak is measured at 0.82 ± 0.19 particles per square kilometer per hour for particles of at least absolute visual magnitude +6.5. Clustering analysis of the arrival times of Leonids impacting the earths atmosphere over the total observing interval shows no enhancement or clumping down to time scales of the video frame rate. This indicates a uniformly random temporal distribution of particles in the stream encountered during the 1999 epoch. Based on the observed distribution of meteors on the sky and the model distribution, recommendations are made for the optimal pointing directions for video camera meteor counts during future ground and airborne missions.