Metal layers at high altitudes

Introduction Conclusions References Tables Figures Back Close Abstract Introduction Conclusions References Tables Figures Back Close Abstract In the past, many studies have been carried out to demonstrate the influence of me-teoroids 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 mete-5 ors 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 demon-10 strate 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 15 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 20 as observed during meteor showers is a strong evidence for the influence of sporadic meteoroids on the high metal layers.