Meteor observations by the Arecibo 430 MHz incoherent scatter radar. II. Results from time-resolved observations

We report high time resolution observations using the powerful Arecibo incoherent scatter radar (ISR). The majority of the meteor-like echoes observed lasted less than 50 ms at one range gate, although echoes lasting for a second were also occasionally observed. The latter may not necessarily be associated with meteors. Most of the meteor echoes in our observations have an effective radar cross-section of the order of 3 × 10−8m2, and an estimated electron line density (ELD) of the order of 4 × 109/m. The visual magnitude is approximately + 16, which is about two orders of magnitude fainter than the meteor echoes found in our time-integrated data (Zhou et al., 1995). The average echo power is positively correlated with the number of range bins in which an echo is detected. This characteristic, along with other experimental evidence, strongly suggests that the Arecibo 430 MHz radar is more sensitive to head-on meteors than to those arriving at an oblique angle. Although classical underdense scattering mechanisms may account for echoes having short range extensions, it is clear that they are insufficient to explain echoes having long range extensions. Some possible mechanisms are discussed. In particular, we suggest that Bragg scattering due to the irregular structure existing in a meteor trail is the most important scattering mechanism for the latter type of echoes in our observations. A plasma instability operating near the Arecibo wavelength is required.