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

Abstract

Abstract. The decay of underdense meteor trails in the polar mesopause region is thought to be predominantly due to ambipolar diffusion, a process governed by the ambient temperature and pressure. Hence, observations of meteor decay times have been used to indirectly measure the temperature of the mesopause region. Using meteor observations from a SKiYMET radar in northern Sweden during 2005, this study found that weaker meteor trails have shorter decay times (on average) than relatively stronger trails. This suggests that processes other than ambipolar diffusion can play a signicant role in trail diffusion. One particular mechanism, namely electron-ion recombination, is explored. This process is dependent on the initial electron density within the meteor trail, and can lead to a disproportionate reduction in decay time, depending on the strength of the meteor.

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

“…They find diffusivities less than expected above ∼ 95 km and diffusivities higher than expected below ∼ 85 km. Ballinger et al (2008) obtain a similar result using meteor observations over northern Sweden. It has been proposed that processes other than ambipolar diffusion influence meteor decay times.…”

“…For calculations of temperatures using meteor radar, ambipolar diffusion alone is assumed to determine the decay of the underdense echoes. 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.…”

“…Ballinger et al (2008) report that meteor radars in general deliver reliable daily temperature estimates near the mesopause using the method outlined in this study, but emphasise that one should exercise caution when assuming that observed meteor echo fading times are primarily governed by ambipolar diffusion. They propose, after Havnes and Sigernes (2005), that electron-ion recombination can impact meteor echo decay times.…”

“…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).…”

Plasma turbulence of nonspecular trail plasmas as measured by a high‐power large‐aperture radar

“…Specular trails occur when the radar beam is approximately pointed perpendicular to the meteoroid's path and arise from Fresnel scattering from the meteor. Specular trails tend to have strong signal returns; consequently, past studies of specular trails have been done with low-power radars and instrumentation [Sugar, 1964;Chilson et al, 1996;Hocking et al, 2000;Galligan et al, 2004;Hall et al, 2005;Ballinger et al, 2008;Kumar and Subrahmanyam, 2012]. In recent years, HPLA radars have become the primary tool for characterizing and studying meteor trails [Chau et al, 2007;Close et al, 2008Close et al, , 2011.…”