&amp;lt;i&amp;gt;Letter to the Editior&amp;lt;/i&amp;gt;Testing the hypothesis of the influence of neutral turbulence on the deduction of ambipolar diffusivities from meteor trail expansion

C M Hall,A H Manson,M Tsutsumi,T Aso,C E Meek,S Nozawa

doi:10.5194/angeo-23-1071-2005

&lt;i&gt;Letter to the Editior&lt;/i&gt;Testing the hypothesis of the influence of neutral turbulence on the deduction of ambipolar diffusivities from meteor trail expansion

C M Hall, A H Manson + Show 4 more

Open Access

https://doi.org/10.5194/angeo-23-1071-2005

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Abstract

Abstract. Fading times of radar echoes from underdense meteor trails in the upper mesosphere/lower thermosphere are commonly used to determine ambipolar diffusivities and hence ambient temperature. Diffusivities are generally expected to increase exponentially with height through the region from which the meteor trail echoes are obtained, viz., typically 70-110km altitude for a ~30-MHz radar. 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., 90km 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.

Highlights

The Nippon/Norway Svalbard Meteor Radar (NSMR) came into operation in spring 2001 and has since been described by Hall et al (2002)
While the latter paper concentrated on the less-than-expected diffusivities in the upper part of the echo height regime and the mechanism for this behaviour, Hall (2002) examined the more-than-expected values in the lower part of the regime
We see that the average profile at least exhibits an exponential increase with altitude in the range 85–95 km, and a linear fit has been performed in log space in order to illustrate how the average diffusion coefficient profile might be expected to look

Summary

Introduction

The Nippon/Norway Svalbard Meteor Radar (NSMR) came into operation in spring 2001 and has since been described by Hall et al (2002). It might be anticipated that this diffusivity would exhibit an exponential increase with height, Hall (2002) noticed that this was often not the case, as had been observed by, for example, Dyrud et al (2001) in results from other meteor radar.

Results

Conclusion