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We present the seasonal variations of the nocturnal mesospheric potassium (K) layer at Arecibo, Puerto Rico (18.35°N, 66.75°W) from 160 nights of K Doppler lidar observations between December 2003 and January 2010, during which the solar activity is mostly low. The background temperature is also measured simultaneously by the lidar and shows a strong semiannual oscillation with maxima occurring during equinoxes at all altitudes. The annual mean K density profile is approximately Gaussian with a peak altitude of 91.7 km. The K column abundance and the centroid height have strong semiannual variations, with maxima at the solstices. Both parameters are negatively correlated to the mean background temperature with a correlation coefficient

We present the first simultaneous and common-volume observations of mesospheric K and Na densities and temperature conducted by a K Doppler lidar and a Na lidar at the Arecibo Observatory (18.35°N, 66.75°W), Puerto Rico. Measurements reported here were conducted on three nights with 9, 6, and 7 h of observations in December 2003 and January 2004. The behaviors of the Na and K layer and the temperature variations showed that the bottom edges of the Na and K layers matched one another almost exactly. Layer peak and column densities increased/decreased together with temperature for the Na layer, but not for the K layer. The correlation between temperature and density fluctuations was strongly positive at and just below the peak altitude of the nightly mean density profile but transitioned to negative several kilometers above the respective peak of either Na or K. The peak densities of the sporadic Na and K layers occurring at low altitudes did not behave in the same manner especially when the background temperature decreased. We discuss, qualitatively, the impact of dynamical and chemical effects on the behaviors of the Na and K layers. Our analysis suggests that wave-induced dynamical transport can account for the observed correlations between temperature, Na density, and K density fluctuations. Dynamical transport and wave-induced chemical transport have similar effects on the behaviors of the Na and K layers. The resulting differences in Na and K layers are qualitatively consistent with different dependences on temperature of the Na and K chemistries.

We report on the first observation of a descending layer of atomic potassium (K) in the thermosphere. This observation was made with the K Doppler lidar at the Arecibo Observatory in Puerto Rico (18.35°N; 66.75°W) on 12 March 2005. The layer was first observed before 08:00 UT (04:00 AST) centered near 145 km with the vertical extent up to ∼155 km, and then it descended to near 126 km just over 2 h later at dawn. The descent rate of 2.56±0.38 m/s matches the vertical phase speed of the GSWM09-computed semidiurnal tide between 120 and 150 km. This also matches the descent rates of the thermospheric semidiurnal tides measured at Arecibo. Although the K density above 120 km remains less than 1 cm −3 , its presence is unequivocal and has strong similarities to the neutral iron (Fe) layers in the thermosphere over 155 km recently discovered by lidar observations at McMurdo, Antarctica. The thermospheric K layer is plausibly explained by radiative electron recombination with K + within a tidal ion layer, which descends with the downward phase progression of the semidiurnal tide. Based on the production rate of K atoms and using current knowledge of tidal ion layer composition, we calculate an electron density of near 5×10 4 cm −3 and K + concentration of 650 cm −3 at 135 km immediately prior to the layer formation. This discovery of a thermospheric K layer, coupled with the McMurdo discovery of similar Fe layers, may lead to a new approach to studying the thermosphere in the altitude range of ∼100–150 km with resonance fluorescence lidars.

[1] We compare the mesospheric Na and Fe layers by using simultaneous and common-volume lidar measurements made at the Arecibo Observatory (18.3°N, 66.75°W), Puerto Rico, in 2003. The temporal variations of the two species are highly correlated at practically all heights, although not always positively. Positive correlations occur in the bottom and top sides while negative correlation is observed in a relatively narrow region in the middle part. Chemical and dynamical effects are discussed to interpret this particular relationship between Na and Fe layers. It is shown that gas phase chemistry determines the structure of the Na and Fe layers after the metals are ablated from meteoroids entering the atmosphere. The observed region of negative correlation appears to be slightly lower than that of the expected region of negative correlation based on inert response to dynamics. It appears that such a difference may be due to temperature-dependent chemistry. Overall, the observed correlations between Na and Fe layers can be well explained by their responses to wave dynamics.

We present a high-spectral-resolution test and calibration station for precision measurement of ultra-narrow bandwidth optical filters, and how this is used in the processing of daytime measurements from a resonance Doppler potassium lidar at Arecibo. The test station consists of Doppler-free saturation–absorption spectroscopy coupled with a small free-spectral-range Fabry–Perot etalon, which produces a precise measurement of the filter passband over a range of 20 GHz (40 pm) or more with a resolution of under 2 MHz. This setup is used to measure the bandpass function of a Faraday anomalous dispersion optical filter with a band center at 770 nm and full width at half maximum of about 3.64 GHz (∼7.2 pm), which is the principal spectral filter in the Arecibo lidar. This bandpass function is then used to calibrate the Doppler-broadened returns from the K lidar. As the Faraday filter passband is narrow enough, the return lidar signals in both the resonance fluorescence and Rayleigh scattering are affected. We describe a calibration process to deconvolve the measured filter function from the return signals in order to achieve accurate temperature measurements. Our approach is demonstrated with actual lidar measurements.

We present an analysis of two separate and distinct sporadic layer events in the mesosphere and lower thermosphere region above the Arecibo Observatory. These layers were observed in both neutral K and ionic Ca + with lidars, and in electron density with incoherent scatter radar. Temperature profiles were determined from the K lidar. One sporadic event was a high altitude layer, in which both atomic and ion sporadic layers were positioned above 100 km altitude. This represents a relatively common sporadic layer visible in both ions and neutrals at Arecibo. The other observation was less typical, with a lower altitude and more diffuse sporadic E layer, extending from below 90 km to above 95 km, which dissipated coincident with growth of a sporadic neutral K layer. We analyze these separate events using a temperature-dependent chemical model, which employs commonly accepted chemical processes. We find that the model successfully reproduces the high altitude layer of June 12–13, 2002. The result shows a temperature dependence related to the chemical lifetimes of the metallic constituents, and that the neutral layer would not have formed had the temperature profile matched that of the MSIS-90 model. Second, the temperature dependent chemistry model also reproduced K + in close agreement with electron densities on June 14–15, 2002. However, the modeled neutrals do not agree well with the observation above 90 km, and it likely requires inclusion of dynamical forcing and advection. We speculate that model and observational deficiencies, primarily exclusion of dynamics such as advection and wave interactions, are the likely shortcomings in the failure to reproduce the observations.

We show examples of common volume observations of three metals by lidar focusing on the altitude of the topside of the meteoric metal layer as described by Hoffner and Friedman (H&F) [The mesospheric metal layer topside: a possible connection to meteoroids, Atmos. Chem. Phys. 4 (2004) 801–808]. In contrast to H&F, we will focus on time scales of a few hours and less whereas the previous study examined the seasonally averaged climatological state on time scales of several days or weeks, and we examine the entire topside, whereas H&F focused on data at 113 km. The examples, taken under different observation conditions in 1997 and 1998 at Kuhlungsborn, Germany (54°N, 15°E), show that the metal layers can often be observed at altitudes as high as 130 km if the signal is integrated over a period of several hours. Under such conditions it is possible to derive reasonably good metal abundance ratios from nocturnally averaged data, which, in turn, allow the discussion of metal abundance ratios to broaden from a single altitude as discussed in H&F to an altitude range extending as high as 130 km. The examples herein show, for the first time, that it is possible to track the transition in the metal abundance ratios from the main layer to an altitude region that has not been studied in the past by lidar. On shorter time scales, small structures are detectable and observable, sometimes above 120 km, resulting in, on average, a broad but weak topside layer above 105 km. In particular, the example of 26–27 October 1997, obtained during enhanced meteor activity, is an indication that this broad layer may result from meteor ablation occurring in this altitude range during the observation. Ratios of metal densities for Ca, Fe, K, and Na are remarkably consistent above about 110 km and in close agreement with the results of H&F. They are less consistent with ratios measured in individual meteor trails and appear to have little relation to the ratios measured in CI meteorites. Finally, it is the temporal smoothing of descending sporadic metal atom layers on top of an undisturbed background metal layer that is the basis of the summer topside extension as described by H&F.

We use over ten years’ incoherent scatter radar (ISR) data and several years of potassium and sodium resonance lidar data to study the morphologies of ion, potassium, and sodium layering activities in the mesopause region above Arecibo, Puerto Rico. A tidal effect is clearly seen in the layer trajectories of all three species. Semidiurnal ion layer tidal structure is observed in the spring, summer, and fall at above 105 km altitude. Below 100 km altitude, the most consistent ion layers occur at 95 km altitude at 1800 local time (LT) during most time of the year except in the summer. The strongest ion layers occur around sunset hours at 106 km altitude in the summer. Although both potassium and sodium are alkaline metals, their layers exhibit very different characteristics. Throughout the mesopause region, sodium layer trajectories are always downward throughout the year, in response to the diurnal tide in this region. The tidal effect on the potassium layers can be seen mostly in the 90–95 km altitude region. While the mesopause height does not appear to be related to the seasonal variations of the potassium layers centroid height, it does appear to have great influence on that of the sodium layers. The potassium layer abundance is highest in the summer and a secondary peak occurs in the winter. The semiannual variation of the potassium layer abundance may provide an important clue to a better understanding of the potassium chemistry.

A large wave event was observed in the three upper-mesospheric (80–105 km) airglow emissions of O(1S), Na and OH by the Boston University all-sky imager, at the Arecibo Observatory, during the night of 3 May 2003. The airglow structures appeared to be due to a large upward propagating internal gravity wave, which subsequently became unstable near the 95 km height level and produced large-scale vertical motions and mixing. Simultaneous density and temperature lidar measurements indicated the presence of a large temperature inversion of 80 K valley-to-peak between 88 and 96 km during the time of the event. Near-simultaneous temperature profiles, made by the TIMED SABER instrument, provided evidence that the horizontal extent of the inversion was localized to within 500 km of Arecibo during the wave event. As the gravity wave dissipated, an internal bore was generated, apparently due to the deposition of momentum and energy into the region by the original wave. Although mesospheric gravity wave breaking has been reported previously (Swenson and Mende, 21(1994); Hecht et al., 102(1997); Yamada et al., 28(2001), for example), this was the first time that the phenomenon has been associated with the generation of an internal mesospheric bore. The event suggested that the breaking of a large mesospheric gravity wave can lead to the generation of an internal bore, as suggested by Dewan and Picard 106(2001). Such behavior is of particular interest since little is known of their origins.

In the past, many studies have been carried out to demonstrate the influence of meteoroids on the atmospheric metal layer, observed roughly in the altitude range 80–105 km. Even with the capability of present day resonance lidars to measure metal densities within single meteor trails, it has been difficult to prove any influence of meteors on the average metal layer. In contrast to approaches taken earlier, we discuss here the seasonal characteristics of potassium, calcium, calcium ion, iron and sodium above 110 km altitude where the average nocturnal densities are so low that the existence of a baseline level of metal atoms and ions is often overlooked. By comparing simultaneous and common-volume observations of different metal layers at one location, we demonstrate that despite their different seasonal characteristics at lower altitudes remarkably similar seasonal characteristics are observed at higher altitudes. In addition, a qualitative agreement is also found for potassium at different latitudes. A comparison of metal densities at 113 km altitude with known meteor showers indicates a strong influence of shower meteoroids on the topside of the metal layers. Simultaneous observations of K along with Ca, Fe and/or Na permit the calculation of abundance ratios, which at 113 km altitude are quite similar to values measured in single meteor trails by ground based lidars. Furthermore, the increase in densities throughout summer is strong evidence for the influence of sporadic meteoroids on the high metal layers. This increase correlates well with the seasonal variation of sporadic micrometeor input independent of meteor showers. Given these evidences, we contend that there is a direct influence of ablating meteoroids on the topside of the mesospheric metal layer.

We report on the first simultaneous observations of Ca and K metallic layers using the low-latitude lidar systems located at the Arecibo Observatory in Puerto Rico (18.35°N, 66.75°W). We often observe sudden increases in both Ca and K densities during early morning hours on nights where meteor showers take place. During these periods, the Ca/K abundance ratio varied between 2 and 3. On occasion, differences were observed in Ca and K layers, which relate to differences in the chemistry of the two metals. It is known that metallic layers display distinct seasonal variations, but chemistry alone cannot explain the measured differences. Thus, we examined whether or not the seasonal distribution of micrometeoroids, derived from meteor observations using the Arecibo 430 MHz radar, can account for the dissimilar metallic observations. We found that the deposition flux of micrometeoroids, with particle sizes ranging between 0.5 and 100 μm , increased by a factor of two during the summer as compared with the winter, suggesting a seasonal variation of their sporadic activity. In addition, our data support the idea that differential ablation leads to a depletion of Ca atoms in the mesosphere.

We have developed a lidar to study the temperature structure of the nighttime mesopause region over the Arecibo Observatory (18.35°N, 66.75°W) by measuring the lineshape of the fluorescence spectrum of atomic potassium that is deposited in the mesosphere and lower thermosphere (MLT) by meteors. To demonstrate how the potassium lidar can enhance MLT studies at Arecibo, we show recent results for: (1) comparisons with airglow temperature measurements; (2) simultaneous operations with stratospheric and mesospheric temperature profiling by Rayleigh lidar; (3) simultaneous observations of K, Ca + , and E -region electron density profiles; and (4) occurrences of sporadic K layers, and relationships to sporadic E layers.

Highly structured electron densities in the E and F regions over Puerto Rico during the night of February 20/21, 1999 were accompanied by intense coherent VHF radar backscatter from the E region and perturbations in neutral sodium in the mesosphere. Simultaneous observations of the event were made with the VHF Cornell University Portable Radar Interferometer (CUPRI) located near Isabela, PR, the University of Illinois VHF radar located at Salinas, PR, the Arecibo incoherent scatter radar, and the sodium lidar located at the Arecibo Observatory. On this geomagnetically quiet night, regions of very different electron concentrations moved through the region. The F-region peak altitudes of the low density regions differed by about 100 km from the high-density region altitudes. The E region also exhibited an unusual enhancement with a vertical extent of about 6 km and caused intense VHF backscatter. The echoing E regions seen by both VHF radars were highly structured with multiple filaments and Doppler shifts exceeding 300 m/s (directed north and upward) some of the time. The Arecibo incoherent scatter radar recorded a large eastward component of the velocity (∼200 m/s ) during the early portion of the event, which then switched to strongly westward (peaking over 500 m/s and averaging perhaps 400 m/s for about half an hour) before returning eastward. The meridional velocity components were also variable. The Arecibo lidar showed an intense sodium layer that maintained a constant altitude until the strongest VHF echoes began. Then the layer fell 2 km over a time span of about half an hour and the lidar echoes intensified. Because (1) the timing of the events at the different locations is well correlated with the F-region drifts as measured with the Arecibo radar, and (2) because the Pedersen conductivity falls precipitiously at the start of the event, we conclude there was strong coupling between the E and F regions, perhaps even reaching the mesosphere, during this event. However, major problems may remain. How can the E-region cloud track the F-region blob when supposedly it is coupled to the neutrals? Does the neutral wind track the F-region ion velocity? We do not think so, but this is the easiest solution.

Over two campaigns in 1998 and 1999, multiple sporadic sodium events were observed by the Arecibo Observatory sodium density lidar while simultaneously monitoring the plasma density using the incoherent scatter radar. In this paper, we test the theoretical explanation proposed by Cox and Plane (1998) where Na + in a plasma layer is neutralized via an ion–molecule mechanism to form a sporadic sodium layer. A particular challenge is to interpret observations made in a Eulerian frame of observation where the spatial and temporal characteristics of events cannot easily be separated. The reaction scheme in the original mechanism is modi=ed to include the reactions NaO + +N2 → Na + ·N2 +O and NaO + +O2 → Na + +O3, following the results of theoretical quantum calculations. Six unique case studies of sporadic sodium layers are presented here, and excellent agreement between simulation and observations was obtained for =ve of them. c � 2002 Published by Elsevier Science Ltd.

Measurements of the geocoronal Balmer-alpha (Hα) brightness have been made at the Arecibo Observatory during 11 separate periods since 1983 using both a Fabry-Perot interferometer and a tilting filter photometer. The tilting filter photometer is calibrated for absolute sensitivity using a constant brightness source traceable to National Institute of Standards and Techniques (NIST) standards and is used to cross-calibrate the Fabry-Perot interferometer. Since the observational technique has not changed since 1983, and since the data analyses technique are uniform, these data provide a measure of the solar cycle variation of Hα brightness at Arecibo. Unlike earlier studies, we discern no systematic discrepancy between the Hα brightness and estimates of the solar Lyman-beta flux that pumps the resonant fluorescent Hα emission. Rather, we conclude that geocoronal hydrogen abundance is generally (but not always) larger than models suggest, although not systematically so. The largest Hα emission is measured during solar minimum conditions, when brightness is approximately 50% greater than during solar maximum conditions, for measurements at solar depression angles from 15° to 35°. Above about 40° solar depression (corresponding to an illuminated column with a base at approximately 2000 km), no solar cycle variation is evident, and the brightness there is persistently greater than models. Intricacies of the Lyman-beta radiative transport problem make detection of an early morning maximum of hydrogen density near the exobase difficult using brightness data alone, although the Hα brightness is generally asymmetric with respect to local midnight, with brighter emission in the postmidnight sector. Studies of the semidiurnal (nighttime) variation demonstrate that the Hα brightness can vary by a factor of 2 over a period of days.

Measurements of the geocoronal Balmer-alpha (Hα) brightness taken between 1983 and 1994 at the Arecibo Observatory (18.35° N, 66.75° W) occasionally display aperiodic brightness variations that we attribute to a local response of exospheric hydrogen abundance to geomagnetic activity. Approximately twofold Hα brightness enhancements in the days following moderate storm onset conditions are demonstrated, and the absolute brightness during these events is significantly greater than modeled brightness based on quiet geomagnetic conditions. Although there are no direct measurements of the line center solar Lyman beta (Ly β) flux that pumps Hα during these events, the magnitude of the brightness enhancements and the temporal reproducibility of the phenomena (following onset of all storms in our data) make it likely that the enhanced Hα brightness is due to enhanced column abundances of hydrogen in the exosphere above Arecibo and not to sporadic increases in the solar Ly β flux. Increased hydrogen abundance due to thermal expansion of the midlatitude atmosphere following propagation of the auroral heating event cannot account for the entire enhancement. The data also suggest that Hα. brightness has become systematically stronger from 1984 to 1994, independent of solar cycle variability or variations due to geomagnetic activity. An increase of ∼ 3% per year, may be attributable to real change in the geocoronal hydrogen column abundance during the period, and is possibly a consequence of increasing hydrogenous species concentrations in the lower atmosphere. The enhancement of geocoronal Hα brightness between 1984 and 1994 may be the consequence of increasing methane in the troposphere and may signal that the effects of methane deposition in the lower atmosphere have propagated throughout the atmosphere.

The Arecibo Observatory will soon complete a major instrumental upgrade which will provide improved capabilities for observations of the upper atmosphere. As in the past, Arecibo capabilities center on 430-MHz incoherent scatter radar (ISR) measurements of the ionosphere made locally or in conjunction with other national and international incoherent scatter facilities; the upgrade will add the capability for simultaneous two beam incoherent scatter observations and will extend plasma line measurements to ±15 MHz. Aeronomical studies may also be performed using a 46.8-MHz coherent scatter radar, the feed of which is coaxial with the 430-MHz line feed, the newly refurbished medium-frequency (MF) radar facility, and a new digital ionosonde. The original airglow laboratory continues to house the Fabry-Perot interferometers, Ebert-Fastie spectrometer, and tilting filter photometers used for observations of mesospheric, thermospheric, and exospheric airglow, while a new laboratory provides a permanent home for the lidars used for resonance fluorescence observations of atomic metal layers in the mesopause region and for Doppler Rayleigh measurements of the upper stratosphere and lower mesosphere. Finally, the recently upgraded high-frequency, high-power transmitter facility has performed active aeronomical and plasma physical studies alone and in conjunction with other Arecibo and visitor-supplied instruments. Observations performed as the upgrade nears completion have already provided a first look at what lies ahead in areas as diverse and fundamental as the structure and dynamics of the turbopause, the properties of upper atmospheric tides, the electrodynamics of the E and F regions, the dynamics of light ions in the topside, and the physics of plasma turbulence. Examples of these and other observations are presented, and the opportunities for future investigations are discussed.

[1] We present an analysis of longitudinal variation in the solar semidiurnal tide observed in the nocturnal thermal structure of the low-latitude mesopause region (83–103 km), with a focus on two sites: Arecibo, Puerto Rico (18.3°N, 66.8°W) and Maui, Hawaii (20.7°N, 156.3°W). Localized observations made by lidars are combined with longitudinal measurements by Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) and compared with the Global-Scale Wave Model-2002 (GSWM-02) for 6 months: January, April, May, July, August, and October. In winter-spring, lidar-measured amplitudes are larger than those recorded by SABER, whereas in summer the amplitudes are similar and in autumn the lidars observe smaller amplitudes than SABER. GSWM amplitudes are between the lidar and SABER measurements for January, are comparable to the measurements in spring, underpredict for summer, and agree with the lidars in autumn. The phase structure is consistent among lidars and SABER for all of the months except January, when there is a large phase shift, and the local time of the temperature maximum is different between Arecibo and Maui. GSWM shows better agreement with phase measurements in winter-spring than in summer-autumn. Modal analysis of the SABER data indicates that the (2, 2) Hough mode is present throughout the year. In winter, it is joined by the (2, 4) mode, while in summer it is accompanied by the (2, 3) mode. This change in the predominant Hough modes may be related to the winter-summer phase difference.

[1] We present the mean seasonal climatology of the nocturnal temperature structure in the mesopause region (80–105 km) above the Arecibo Observatory, Puerto Rico (18.35°N, 66.75°W) from 106 nights of potassium Doppler lidar observations between December 2003 and September 2006. This first complete range-resolved mesopause climatology for a tropical latitude exhibits several unique features. Compared to higher latitude sites, mesospheric temperature inversion layers in the nocturnal means are much weaker at Arecibo. Seasonally large inversions occur in summer but are almost non-existent during the rest of the year. The Arecibo climatology shows a three-level mesopause: a high altitude in summer (∼100 km), a medium altitude in late autumn and winter (∼96 km), and a low altitude in early spring (∼91 km). The mesopause is cold in the solstices (∼171 K in summer, ∼176 K in winter) and warm around equinoxes, particularly late autumn when it is near 195 K, while the spring mesopause temperature is close to 185 K. The lower thermosphere around 100 km at Arecibo shows a decreasing temperature from spring to summer when it reaches its coldest temperature, which is contrary to the increasing temperature observed at all midlatitude locations. Semiannual variations in the seasonal temperature have amplitudes as large as the annual variations through most of the MLT altitude range at Arecibo. These observed seasonal variations appear to be associated with the semi-annual oscillation, a predominantly tropical phenomenon. This report provides one of the very few observations of the semi-annual oscillation in lower thermosphere temperature.

[1] During June and July 2002, a 30-MHz imaging coherent scatter radar was installed and operated on the island of St. Croix, to view the E region ionosphere above Arecibo, Puerto Rico. During the observing period, 10 events with discernible quasiperiodic echo structure were observed with the coherent scatter radar. In six of those events, simultaneous measurements were made with the Arecibo 430-MHz incoherent scatter radar. The imaging coherent scatter radar allows us to locate and track the echo structures within the volume illuminated by the transmitter, which shows structures that are generally aligned along wavefronts. A slight preference for motion of the structures toward the southwest is evident throughout the period, but the propagation directions and speeds vary greatly. The incoherent scatter radar measurements show a close correspondence between the occurrence of the coherent echoes and the location of the enhanced electron density structures. In particular, the coherent echoes occur when the electron density layers show uplifts.

[1] A number of observations of the sodium density primarily in the mesosphere and lower thermosphere (but also of the electron density structure) have shown what appears to be overturning or convective roll cells near the transition from the mesosphere to the lower thermosphere. The cells are found in the region between 95 and 105 km and occur near the boundary between the region of lower stability in the mesosphere and the region of higher stability in the lower thermosphere. The vertical scale for the rolls is ∼5–6 km, and the timescale is ∼1–3 hours. The rolls occur in a region characterized by large shear in the neutral winds, but the timescales are too long to be explained by simple Kelvin-Helmholtz instability. We present observations from the University of Illinois lidar facility located at the Starfire Optical Range near Albuquerque, New Mexico, from the Arecibo Observatory in Puerto Rico, and from the Maui/MALT Lidar Facility located on Maui in Hawaii that show the overturning structure. possible mechanisms for generating the convective rolls are discussed, Including the relationship of the observed features to the characteristics expected for an inflection point instability.

The study of meteors has developed new relevance in recent years. This has been motivated by scientific interests as mundane as meteor ablation processes and the chemistry and state of the middle atmosphere, as fundamental as the origins and evolution of the earth and its environment and life, and as sublime as the possibility that life exists or existed elsewhere in the solar system or the galaxy. Motivation has also come from new techniques for recording the characteristics of meteor trails, using radar, optics, and ultrasound, and the new information these techniques have provided. This special issue contains a collection of papers that address the “mundane” of these issues, yet the conclusions are of fundamental importance to the study of the earth and its celestial environs. They deal with how and where meteoric material is deposited in the atmosphere and the chemistry that follows in order to form the metal layers in the mesosphere and lower thermosphere. It should be noted that we address issues that involve only the small portion of the total meteoric mass input ( μ g –g particles) that is responsible for the phenomena discussed. We do not address meteorites, bolides, and other large objects.