Meteor observations with an MF radar

Abstract

We conducted meteor echo observations using the Buckland Park MF radar (35• S, 138• E) at 00:40-05:45 LT on October 22, 1997. In addition to the usual full correlation analysis (FCA) technique to measure horizontal wind velocities from 60 to 100 km MF radars have a potential to detect meteor echoes and infer winds through their Doppler frequency shifts. Because of the relatively low radio frequency employed MF radars have a great advantage of providing meteor wind well above 100 km altitude, where very few techniques can measure wind velocities.There is a limitation which should be noted as well. The observations are possible only during night time when the electron density of E-region is low enough for the radio wave to penetrate into the upper region. We detected 233 underdense meteor echoes from 80 km to 120 km with a mean height of 104.4 km. Although the transmitting antenna beams were steered toward off-zenith angles of 25• , almost all the echoes were received outside of the main lobe, indicating that conventional MF radar systems with a broad transmitting beam can work well for meteor observations. Bi-hourly wind profiles were obtained from 94 to 114 km altitudes. The profiles revealed a clear wave structure with a downward phase progression with time. FCA winds from 80 to 100 km were also estimated, and a continuous wind structure was obtained from FCA to meteor heights. Note that the present observations happened to be conducted during a major meteor shower activity. However, a majority of the underdense echoes were from non-shower meteors, and observations during non-shower periods will also yield enough information.

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

“…As stated in the Introduction, (also see Tsutsumi, 1999), when the meteor trail plasma frequency is less than the radio frequency, the meteor echoes are referred to as "underdense". In the other case, when the meteor trail plasma frequency is greater than the radio frequency, yielding overdense echoes, the radio wave is reflected from the outside surface of the meteor trail.…”

“…Meteors entering the Earth's atmosphere generate trails of ionized gas that optimised radars can utilize to obtain information about winds and temperatures (Tsutsumi et al, 1999). In order to determine the latter, echo fading, assumed to be a consequence of ambipolar diffusion of the ions constituting the meteor trail, is measured whenever the plasma frequency is less than the radar frequency (known as "underdense" echoes).…”

MF and HF radar techniques for investigating the dynamics and structure of the 50 to 110 km height region: a review

“…At these frequencies, the number of meteor detections tends to zero near 110 km. Generally, the lower the operating frequency, the greater the maximum useable height, so that for example at 2 MHz, meteor trails are detected to heights near 120 km (e.g., Steel and Elford 1991;Tsutsumi et al 1999). However, as we have noted above, the drift and diffusion of meteor trails is increasingly affected by the Earth's magnetic field above 110 km and measuring parameters related to the neutral atmosphere requires some care.…”

“…This makes SuperDARN radars (8 to 20 MHz) potentially very powerful meteor radars for measuring winds at heights in the data poor region between above 100 and 120 km. MF radars have been successfully used as meteor radars, particularly, but not exclusively, at higher latitudes (e.g., Tsutsumi et al 1999;Tsutsumi and Aso 2005), and for measuring winds up to 120 km. However, a disadvantage here is that as the frequency is lowered, meteor detections are harder to discriminate from ionospheric echoes.…”

“…While the application of radars operating in the MF and HF bands to investigate the neutral upper atmosphere is one of the oldest such techniques still regularly in use, the techniques have been continuously improved, largely through the availability of better hardware (e.g., Reid et al 1995;Singer et al 2008;Li et al 2012) and readily available and economical but powerful computers, and provide a robust and reliable method of obtaining wind velocities (e.g., Stubbs 1973;Pancheva et al 2002), turbulence intensities (e.g., Hocking 1983;Holdsworth et al 2001), electron densities and collision frequencies (e.g., Thrane and Piggott 1966;von Biel 1977;Friedrich and Torkar 1983;Holdsworth et al 2002;Singer et al 2011), measurements of atmospheric structure (e.g., Gregory 1956;Hall 2000), temperatures (e.g., Tsutsumi et al 1999;Holdsworth et al 2006), and measurements of energy and momentum transfer (e.g., Reid and Vincent 1987;Murphy and Vincent 1993;Placke et al 2015) in the MLT region.…”