The anomalous diffusion of meteor trails

Abstract

Abstract.Radars frequently detect meteor trails created by the ablation of micro-meteoroids between 70 and 120 km altitude in the atmosphere. Plasma simulations show that density gradients a.t the edges of meteor trails drive gradientdrift instabilities which develop into waves with perturbed electric fields often exceeding hundreds of mV/m. These waves create an anomalous cross-field diffusion that can exceed the cross-field (2_ B) ambipolar diffusion by an order of magnitude. The characteristics of the instabilities and anomalous diffusion depend on the trail altitude, latitude, and density gradient. A simple relation defines the minimum altitude at which meteor trail density gradients drive plasma instabilities and anomalous diffusion. These results impact a number of meteor radar studies, including those that use diffusion rates to determine trail altitude, and atmospheric temperature.

<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

“…Let us first review the signature of the diffusion coefficient profiles that aroused the concern of at least Hall (2002), Dyrud et al (2001) and for that matter, Hocking (private communication, 2002). Here, in Fig.…”

“…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.…”

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

“…The minimum altitude at which this occurs depends on the trail altitude, density gradient and latitude, and at high latitudes this altitude is ∼ 95 km. Therefore, using ambipolar diffusion rates to calculate trail altitudes above this minimum altitude may lead to errors of several kilometres, since the diffusion coefficients derived from the measurements are underestimated (Ballinger et al, 2008;Dyrud et al, 2001;Kovalev et al, 2008).…”

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

“…Here, we will address the height independent nature of the lower part of the diffusivity profile. The upper part of the profile is thought to be a consequence of the degree to which the observed echo fading is a mixture of diffusion along and perpendicular to the magnetic field (Dyrud et al, 2001). …”

“…Hocking, 1999). To make matters worse, however, the measured ambipolar diffusivities themselves may be unreliable, as predicted by Dyrud et al (2001). The diffusivity of air increases exponentially with height, and one would expect the ambipolar diffusivity of the meteor-trail ions to behave similarly.…”