Meteor head echo altitude distributions and the height cutoff effect studied with the EISCAT HPLA UHF and VHF radars

Abstract

Abstract. Meteor head echo altitude distributions have been derived from data collected with the EISCAT VHF (224 MHz) and UHF (930 MHz) high-power, large-aperture (HPLA) radars. At the high-altitude end, the distributions cut off abruptly in a manner reminiscent of the trail echo height ceiling effect observed with classical meteor radars. The target dimensions are shown to be much smaller than both the VHF and the UHF probing wavelengths, but the cutoff heights for the two systems are still clearly different, the VHF cutoff being located several km above the UHF one. A single-collision meteor-atmosphere interaction model is used to demonstrate that meteors in the (1.3-7.2) µg mass range will ionise such that critical electron density at 224 MHz is first reached at or around the VHF cutoff altitude and critical density at 930 MHz will be reached at the UHF cutoff altitude. The observed seasonal variation in the cutoff altitudes is shown to be a function of the seasonal variation of atmospheric density with altitude. Assuming that the electron density required for detection is in the order of the critical density, the abrupt altitude cutoffs can be explained as a consequence of the micrometeoroid joint size-speed distribution dropping off so fast at the large-mass, high-velocity end that above a certain altitude the number of detectable events becomes vanishingly small. Conversely, meteors at the lowmass end of the distribution will be gradually retarded such that the ionisation they generate never reaches critical density. These particles will remain unobservable.

Determination of meteor-head echo trajectories using the interferometric capabilities of MAARSY

“…11 maximum around 106 km, which is mainly controlled due to the measurement frequency and the neutral air density (e.g. Westman et al, 2004;Stober et al, 2012). Another aspect is the sensitivity to the sporadic sources in connection with the source velocity distributions.…”

Determination of meteoroid physical properties from tristatic radar observations

“…MSIS-E-90 (Hedin, 1991) is used for atmospheric densities. The measured RCS is assumed to be proportional to the meteoroid mass loss via an overdense scattering mechanism (Close et al, 2002;Westman et al, 2004). The model is further described by Szasz et al (2007) and in detail by Kero (2008).…”

The EISCAT meteor-head method – a review and recent observations

“…The foundation for the overdense head echo scattering model for EISCAT was presented by Wannberg et al (1996) based on EISCAT VHF/UHF dual frequency observations on estimated target sizes and head echo altitude distributions, and has been further reported by Westman et al (1997Westman et al ( , 2004. The reported high vector velocities from the tristatic EISCAT observations (Janches et al, 2002a) also support the model requiring high-energy meteoroid-atmosphere interactions.…”

“…The target sizes are obtained by assuming that the equivalent number of scattering electrons is compressed to a ball or coma of the size where they would reach the critical frequency corresponding to the monitoring frequency. The radii for UHF targets are 1-2 cm and for VHF 2.7-4.7 cm Westman et al, 2004). Figure 3 shows a schematic view on the meteoroidatmosphere interaction process along the meteoroid path when it is penetrating to the layers of increasing density in the atmosphere based on the observations reported above.…”