David D. Meisel

The micrometeoroid mass flux into the upper atmosphere : Arecibo results and a comparison with prior estimates

Radar micrometeor observations at Arecibo Observatory have enabled direct estimates of the meteoroid mass flux into the upper atmosphere. We report mass flux determinations from November 1997/1998 observations that are based on the observed number of meteor events per day in the 300-m diameter Arecibo beam and on particle mass determinations from that fraction of all particles for which deceleration is measured. The average mass of the Arecibo micrometeoroids that manifest observable deceleration is ∼0.32/0.76 µgm/particle with a resultant annual whole-Earth mass flux of 1.6 × 106/2.7 × 106 kg/yr over the ∼10−5−10² µgm mass range for 1997/1998, respectively. The annual whole-earth mass flux per decade of particle mass is calculated and compared with that of Ceplecha et al. [1998] (3.7 × 106 kg/yr) and with that derived by Love and Brownlee [1993] (LB) from small particle impact craters on the orbital Long Duration Exposure Facility (LDEF). We also give the LDEF results as significantly modified using the Arecibo-determined average particle velocity of ∼50 km/sec—much larger than the effective value of 12 km/sec used by LB. This modification results in a net LDEF mass flux of 1.8×106 kg/yr—7% of the value we determined from reanalysis of the LB data using their original 12 km/sec mean impact speed. These results may provoke some debate.

Three‐dimensional radar observation of a submillimeter meteoroid fragmentation

A meteor observed with the naked eye is colloquially called a shooting star. The streak of light is generated by an extra-terrestrial particle, a meteoroid, entering the Earth’s atmosphere. The term meteor includes both luminosity detectable by optical means and ionization detectable by radar. The radar targets of meteor head echoes have the same motion as the meteoroids on their atmospheric flight and are relatively independent of aspect angle. They appear to be compact regions of plasma created at around 100 km altitude and have no appreciable duration. This thesis reviews the meteor head echo observations carried out with the tristatic 930 MHz EISCAT UHF radar system during four 24h runs between 2002 and 2005, and a 6h run in 2003 with the monostatic 224 MHz EISCAT VHF radar. It contains the first strong observational evidence of a submillimeter-sized meteoroid breaking apart into two distinct fragments. This discovery promises to be useful in the further understanding of the interaction processes of meteoroids with the Earth’s atmosphere and thus also the properties of interplanetary/interstellar dust. The tristatic capability of the EISCAT UHF system makes it a unique tool for investigating the physical properties of meteoroids and the meteor head echo scattering process. The thesis presents a method for determining the position of a compact radar target in the common volume of the antenna beams and demonstrates its applicability for meteor studies. The inferred positions of the meteor targets are used to estimate their velocities, decelerations, directions of arrival and radar cross sections (RCS) with unprecedented accuracy. The head echoes are detected at virtually all possible aspect angles all the way out to 130° from the meteoroid trajectory, limited by the antenna pointing directions. The RCS of individual meteors simultaneously observed with the three receivers are equal within the accuracy of the measurements with a very slight trend suggesting that the RCS decreases with increasing aspect angle. A statistical evaluation of the measurement technique shows that the determined Doppler velocity agrees with the target range rate. This demonstrates that no contribution from slipping plasma is detected and that the Doppler velocities are unbiased within the measurement accuracy. The velocities of the detected meteoroids are in the range of 19-70 km/s, but with very few detections at velocities below 30 km/s. The thesis compares observations with a numerical single-body ablation model, which simulates the physical processes during meteoroid flight through the atmosphere. The estimated meteoroid masses are in the range of 10-9 - 10-5.5 kg.

Radio and Meteor Science Outcomes From Comparisons of Meteor Radar Observations at AMISR Poker Flat, Sondrestrom, and Arecibo

Radio science and meteor physics issues regarding meteor “head-echo” observations with high power, large aperture (HPLA) radars, include the frequency and latitude dependency of the observed meteor altitude, speed, and deceleration distributions. We address these issues via the first ever use and analysis of meteor observations from the Poker Flat AMISR (PFISR: 449.3 MHz), Sondrestrom (SRF: 1,290 MHz), and Arecibo (AO: 430 MHz) radars. The PFISR and SRF radars are located near the Arctic Circle while AO is in the tropics. The meteors observed at each radar were detected and analyzed using the same automated FFT periodic micrometeor searching algorithm. Meteor parameters (event altitude, velocity, and deceleration distributions) from all three facilities are compared revealing a clearly defined altitude “ceiling effect” in the 1,290 MHz results relative to the 430/449.3 MHz results. This effect is even more striking in that the Arecibo and PFISR distributions are similar even though the two radars are over 2,000 times different in sensitivity and at very different latitudes, thus providing the first statistical evidence that HPLA meteor radar observations are dominated by the incident wavelength, regardless of the other radar parameters. We also offer insights into the meteoroid fragmentation and “terminal” process.

Investigation of a resonance Lidar for measurement of thermospheric metastable helium

Abstract Expected signal returns for a ground-based resonance Lidar system used for profiling metastable thermospheric helium are presented, scaleable to specific system configurations. The signal estimate is dependent on the calculation of the effective backscatter cross-section for the He 10 830 A resonant transition as well as an estimate of thermospheric metastable helium densities obtained from a recent model. The peak backscatter cross-section is found to be 2.7(±0.3) × 10 −16 m 2 with an effective backscatter cross-section (assuming a 1 GHz rms laser linewidth centered at a wavelength of 10 830.32 A) of 2.6(±0.3) × 10 −16 m 2 . Measurements using the metastable He 3188 A and 3889 A lines are evaluated. Challenges in experimental design (i.e., laser characteristics, near infrared single photon detection, and background noise), as well as potential operation from a space-borne platform, are also discussed. With current technology, profiles with relatively high temporal and vertical resolution are shown to be attainable under twilight conditions.

Dynamical and orbital properties of the Aricebo micrometeors

Using the Arecibo Observatory (AO) 430 MHz Radar we have developed a Doppler technique to measure very precise micrometeor instantaneous velocities directly from the meteor head-echo. In addition, a large number of these observations show deceleration. With the velocity, the deceleration, the assumption of a spherical shape, and a mean micrometeoroid mass density (3 g cm ) we have obtained estimates of in-atmosphere particle sizes. Therefore we can produce a more realistic orbital analysis than previously obtained for micrometeors. We first use an MSIS standard atmosphere model and the measured deceleration in order to obtain the meteor extra-atmospheric speeds, assuming that sputtering is the only mass-loss mechanism that these particles undergo prior and during the time we detect them. So far, over 7000 detections obtained during the Leonids 1997 (L97) and 1200 during the Leonids 1998 (L98) observation campaigns have been analyzed. Out of these detections, we present elements without correction for perturbations (i.e. radiation pressure, perturbation by the Jovian planets and photoelectric charging effects) of over 500 events from 1997 and 200 from 1998.

Direct determination of the micrometeoric mass flux into the upper atmosphere

The advent of radar micrometeor observations at Arecibo Observatory (AO) has enabled direct estimates of the meteoric mass flux into the upper atmosphere. These observations yield on average ∼3200 events per day in the 300 m diameter Arecibo beam. Doppler velocity estimates are found for approximately 50% of all events and of these, approximately 55% (26.5% of the total) also yield measurable (linear) decelerations. Assuming spherical particles of canonical density 3 gm/cc, the meteoric masses obtained range from a few micrograms to a small fraction of a nanogram. This approach yields an average mass of 0.31 microgram/particle for the 26.5% of all particles that manifest observable deceleration. The 45% with velocities, but not decelerations, correspond to particle masses larger than a few micrograms. However if we assume that all observed particles average 0.31 micrograms each, we find a mass flux of about 1.4× 10 −5 kg/km 2 -day over the whole Earth. Detailed annual whole-Earth mass flux per decade of particle mass is calculated and compared with those of Ceplecha et al. [1]. Our results fall below those of Ceplecha et al. for observed mass fluxes however inclusion of those particles for which we cannot explicitly determine mass yield similar fluxes. Many of the particles we observe show evidence of catastrophically disintegrating in the meteor zone. We thus suggest that the majority of micrometeoroid mass is deposited in the 80–115 km altitude region where ionospheric and atmospheric manifestations such as sporadic E and neutral atomic metal layers are well documented. We further suggest that the “background” diurnal micrometeor mass flux is sufficient to dominate the average lower atmosphere mass influx from the annual meteor showers.