<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

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 70-110 km 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., 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.

On the validity of the ambipolar diffusion assumption in the polar mesopause region

“…The vertical profile of decay time below 83 km, and above 96 km, can be described as "kickback regions", where the decay time appears to increase with altitude (for a similar result, see Fig. 1 in Hall et al, 2005). Although the number of meteors is significantly less in these regions, it appears these features are real, and have been briefly discussed by others (e.g.…”

“…Although the number of meteors is significantly less in these regions, it appears these features are real, and have been briefly discussed by others (e.g. Dyrud et al, 2001;Hall, 2002;Hall et al, 2005). Dyrud et al (2001) attribute the upper level increase in diffusion (decrease in decay time) to gradient drift FarleyBuneman (GDFB) instability (Fejer et al, 1975), that develops where the trail density gradient and electric field are largest.…”

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

“…Diffusivities are expected to increase exponentially with height through the region from which meteor echoes are obtained (Ballinger et al, 2008;Chilson et al, 1996). Hall et al (2005) find that this is only the case between ∼ 85 and ∼ 95 km altitude, using diffusion coefficients delivered by NTMR from 2004. They find diffusivities less than expected above ∼ 95 km and diffusivities higher than expected below ∼ 85 km.…”

“…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).…”

Change in turbopause altitude at 52 and 70° N

“…The method is exactly the same as used by Hall et al (2012) to determine 90 km temperatures over Svalbard (78 • N) using a radar identical to NTMR. Hall et al (2005) investigate the unsuitability of meteor radar data for temperature determination above ∼ 95 km and below ∼ 85 km. In summary, ionisation trails from meteors are observed using a radar operating at a frequency less than the plasma frequency of the electron density in the trail (this is the so-called "underdense" condition).…”