High-altitude sporadic metal atom layers observed with Na and Fe lidars at 30°N

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Abstract

The seasonal/annual characteristics of the high-altitude sporadic metal atom layers are presented on the basis of extensive Na and Fe lidar measurements at 30°N during the past several years. It is found that the extremely high sporadic Na (Nas) and Fe (Fes) layers above 105 km occurred mostly during summer. They had long durations (a few hours) and broad layer widths (much larger than 2 km). Their absolute peak densities could be comparable to or even larger than those of the corresponding main layers on a few nights. By using all the raw data profiles including sporadic layers, we have constructed the contour plots of Na and Fe densities versus month and altitude at 30°N. The Na and Fe layers both exhibit evidence for summer topside extension, which is consistent with the earlier observations for K and Ca at different latitudes. The summer topside extension of mean metal atom layers might represent a universal phenomenon that is alike for different atom species, different geographic locations and different measurement years. The extremely high sporadic metal atom layers above 105 km occurring during summer give rise to the phenomenon.

Introduction

Free metal atom layers are globally present in the mesopause region from 80 to 110 km. They come from meteoric ablation. In terms of lidar measurements, there are two types of metal atom layers. One is a permanent main layer that is generally confined to an altitude range from 80 to 105 km with a peak density around 90 km. Its structure characteristics are usually obtained by taking nightly average of lidar-observed density profiles. The main layer displays variations with time scales from a few minutes to seasonal span. It also shows latitudinal difference (Plane, 2003; Gardner et al., 2005; Yi et al., 2009). The structure and seasonal variation of main metal layer are a basis for testing the metal layer models established in terms of gas-phase chemistry. The other layer form is called “sporadic metal layers”. A sporadic metal layer refers to a transient phenomenon that a large density enhancement (with a density equal to or larger than twice that of the main metal layer) occurs in a narrow altitude range (Clemesha et al., 1978; Hansen and von Zahn, 1990; Clemesha et al., 1999; Yi et al., 2002).

As early as 1969–1970, Gibson and Sandford (1971) noted an “unusual enhancement” of Na atom near 103 km in their Na lidar measurements. Eight years later, the sporadic metal layers were unambiguously observed by Clemesha et al. (1978). The observed sporadic metal layers display very complicated characteristics (Clemesha, 1995). They usually occur in the upper part of the main metal layers and often show a downward motion with time. Their strengths tend to increase with decreasing occurrence altitude. In particular, the strong sporadic layers tend to arise at low altitudes close to the peaks of the corresponding main layers (Hansen and von Zahn, 1990; Yi et al., 2007). According to the currently limited observations of steerable (3-point) and airborne lidars, the sporadic Na layers (Nas layers) had horizontal dimensions between 100 and 2000 km (Batista et al., 1991; Kane et al., 1991). It is believed that this represents a case of the simultaneous local production of Na atoms on a large horizontal area. The simultaneous and common-volume Fe and Na lidar observations indicated that the sporadic Fe (Fes) and Nas layers occurred in overlapping altitude ranges and moved following almost the same track (Yi et al., 2007). On occasion, the Fes and Nas layers exactly simultaneously reached their maximum peak densities at nearly the same altitude. The observational results suggest that Fes and Nas layers come from the same source substance and may be formed via the same or similar atomization process. The narrow-band Na lidar observations revealed that the majority of the Nas layers had considerably larger temperatures than the mean (Qian et al., 1998). The average temperature enhancement was ∼13 K. However, the maximum temperatures and Nas densities were only weakly correlated. Interestingly, in terms of the steerable narrow-band Na lidar observations during the 1998 Leonid shower, the temperatures within the durable trails associated with bright fireballs (∼3 min after ablation) were also 20–50 K warmer than that of the background Na layer (Chu et al., 2000). Up to now, although several formation mechanisms of the sporadic layers have been suggested, none of them can satisfactorily explain all the layer characteristics (Clemesha, 1995).

In terms of observations at various latitudes (23°S, 30°N and 69°N) during different periods, most Nas layers with peak densities comparable to or much larger than that of the corresponding main Na layer occurred in an altitude range between 90 and 100 km (Hansen and von Zahn, 1990; Clemesha, 1995; Yi et al., 2007). At higher altitudes (above 100 km), the reported sporadic metal layers are scarce and usually weak. The tendency that sporadic metal layers strengthen with decreasing occurrence altitude appears to be universal (independent of observational site and period) (Yi et al., 2007). The sporadic metal layers occasionally observed at altitudes above 105 km attracted some particular attention (Collins et al., 1996; Yi et al., 2002; Gong et al., 2003). It is suggested that the high-altitude sporadic metal layers might belong to a different population and have different origin (Hansen and von Zahn, 1990; Kane et al., 1993; Clemesha, 1995). According to our lidar observations, there exist also many weak atom density enhancements at altitudes above 100 km. Some of them have absolute peak densities much less than that of the corresponding main metal layer, thus were not registered as sporadic metal layers. Here we will investigate these high-altitude sporadic metal atom layers including those weak atom density enhancements above 100 km and show their seasonal/annual variability based on the extensive Na and Fe lidar measurements at Wuhan (30.5°N, 114.4°E), China during the past several years.

Section snippets

Observational results

Our Na and Fe resonance fluorescence lidars came into operation in March 2001 and December 2003, respectively. They can carry out simultaneous and common-volume measurements and also independent operations. Their technical details have been described by Yi et al., 2002, Yi et al., 2007. The data used in this study comprise ∼1360 h of Na measurements on 178 different nights and ∼660 h of Fe measurements on 103 nights up to the end of 2008. The altitude resolution is 96 m which coincides with the

Summary

We have presented the seasonal/annual variations of the high-altitude sporadic metal atom layers based on ∼1360 h of Na measurements on 178 different nights and ∼660 h of Fe measurements on 103 nights during the past several years at 30°N. It is found that the extremely high Nas and Fes layers above 105 km occurred mostly during summer. They have generally long durations and broad layer widths compared with those below 105 km. Their peak densities are usually small, but occasionally (on a few

Acknowledgements

The research is supported jointly by the National Natural Science Foundation of China through Grants 40674085 and 40731055, National High Technology Research and Development Program of China (No.: 2006AA12Z147), and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT0643). The authors appreciate Changming Yu for his technical support in collecting lidar data.

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