Metal layers at high altitudes: A possible connection to meteoroids

Abstract

Abstract. 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, for potassium at different latitudes a qualitative agreement is also found. 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 permits the calculation of abundance ratios. We find that these ratios at 113 km altitude are quite similar to values measured in single meteor trails by ground based lidars. Given these evidences, we contend that there is a direct influence of ablating meteoroids on the topside of the mesospheric metal layer. Furthermore, the increase in densities throughout summer with similar abundance ratios as observed during meteor showers is a strong evidence for the influence of sporadic meteoroids on the high metal layers.

Sporadic and thermospheric enhanced sodium layers observed by a lidar chain over China

“…The high occurrence of TeSLs in summer reflected the sodium concentration in the topside layer, which agreed with the summer topside extension of the metal layer described by Höffner and Friedman []. The absence of TeSLs at Hefei and Wuhan in early summer (i.e., May and June) should be noted.…”

“…Previous studies revealed a pronounced summer maximum for the incoming meteors [e.g., Singer et al , ; Lau et al , ]. The meteor not only provided a cause‐and‐effect explanation for the Es seasonal dependence [ Haldoupis et al , ] but also responded for the summer topside extension of the metal layer [ Höffner and Friedman , , ]. Thus, it could also contribute to the summer enhancement of the occurrence rate of the SSLs/TeSLs, which were strongly correlated with Es.…”

“…Recently, Höffner and Friedman [, ] found that the mean K and Ca layers at 54°N and 18°N exhibited similar topside extensions (∼ 120 to 130 km), and the topside metal layers as well as the high‐altitude sporadic metal layer (i.e., the lower thermospheric enhanced metal layer) above 105 km have attacked increasing attention. Collins et al [] first reported a high‐altitude SSL near 109 km at Poker Flat, Alaska, during an active aurora.…”

Lower thermospheric‐enhanced sodium layers observed at low latitude and possible formation: Case studies

“…The next one is the “topside layer,” which is deemed as a high‐altitude extension of the neutral metal layer. For instance, Höffner and Friedman [] found that the mean K and Ca layers at 54°N exhibited similar summer topside extensions (∼120 km). Because the metal density enhancement in summer correlated well with the seasonal/annual variation of sporadic micrometeoroid input, independent of meteor showers, the authors suggested a direct link between ablating meteoroids and the topside metal layers.…”

A case study on occurrence of an unusual structure in the sodium layer over Gadanki, India

“…In this context, it is also to be noted that high altitude sporadic metal layers (or high altitude metal layers) have been reported from several observational sites (e.g. Friedman et al 2013, Höffner andFriedman 2004;Chu et al 2011;Xue et al 2013). Gao and Mathews (2014a,b) and references therein report on high altitude radar and optical meteors indicative of sputtering as a source of metal ions above the traditional meteor zone.…”