On the influence of neutral turbulence on ambipolar diffusivities deduced from meteor trail expansion

Abstract

Abstract. By measuring fading times of radar echoes from underdense meteor trails, it is possible to deduce the ambipolar diffusivities of the ions responsible for these radar echoes. It could be anticipated that these diffusivities increase monotonically with height akin to neutral viscosity. In practice, this is not always the case. Here, we investigate the capability of neutral turbulence to affect the meteor trail diffusion rate.

<i>Letter to the Editior</i>Testing the hypothesis of the influence of neutral turbulence on the deduction of ambipolar diffusivities from meteor trail expansion

“…Observing the decay of echoes from meteor trails enables us to estiCorrespondence to: C. M. Hall (chris.hall@tgo.uit.no) mate the ambipolar diffusion coefficient of ions in the trail. It might be anticipated that this diffusivity would exhibit an exponential increase with height, however Hall (2002) noticed that this was often not the case, as had also been observed by, for example, Dyrud et al (2001) in results from other meteor radar. While the latter paper concentrated on the less-than-expected diffusivities in the upper part of the echo height regime (i.e.…”

“…While the latter paper concentrated on the less-than-expected diffusivities in the upper part of the echo height regime (i.e. some km above the altitude of peak echo occurrence of, for example, ∼90 km for a ∼30-MHz radar) and the mechanism for this behaviour, Hall (2002) examined the more-than-expected values in the lower part of the regime. By examining time scales of diffusion processes in the atmosphere, Hall (2002) found a possibility that the fading of radar echoes might be augmented by neutral air turbulence, giving rise to an overestimation of diffusivity.…”

“…some km above the altitude of peak echo occurrence of, for example, ∼90 km for a ∼30-MHz radar) and the mechanism for this behaviour, Hall (2002) examined the more-than-expected values in the lower part of the regime. By examining time scales of diffusion processes in the atmosphere, Hall (2002) found a possibility that the fading of radar echoes might be augmented by neutral air turbulence, giving rise to an overestimation of diffusivity. In other terms, it seemed possible that the diffusivity estimated from the radar might be a combination of both molecular and turbulent processes.…”

“…In practice, however, this is more the exception: unexpectedly large diffusivities are obtained in the lower part of the regime, and unexpectedly low values are obtained in the upper part; only in the few kilometres on either side of the maximum in echo occurrence (viz., 90 km for a 30-MHz radar) does the diffusivity profile behave as expected. Hall (2002) hypothesised that neutral turbulence might be enhancing expansion of the meteor trail in the lower part of the regime. In this communication, due to results only available since the publication of Hall's suggestion, we are able to refute the hypothesis.…”

Neutral atmosphere temperature trends and variability at 90 km, 70 °N, 19 °E, 2003–2014

“…Hall (2002) proposes that neutral turbulence may be responsible for overestimates of molecular diffusivity in the region ∼ 70-85 km, but this hypothesis is rejected by Hall et al (2005) due to a lacking correlation between neutral air turbulent intensity and diffusion coefficients delivered by the NTMR radar. Other mechanisms for overestimates of molecular diffusivity include incorrect determination of echo altitude and fading times due to limitations of the radar (Hall et al, 2005).…”