Determination of the meteoroid ionization coefficient using high power large aperture radar.

摘要

Most meteoroids have physical cross-sections too small to detect by ground based sensors. To overcome this problem we applied a radar [ARPA Long Range Tracking and Instrumentation Radar (ALTAIR)] with greater capabilities than made available by any other meteor radar to collect measurements from meteor electrons at the leading edge or head of their atmospheric flight path. These ALTAIR head echo data provided the foundation for us to develop new techniques and advance the science of meteoroid-atmosphere interactions. ALTAIR provides high-power, large aperture, high bandwidth, simultaneous UHF and VHF pulsed waveforms, high range resolution, monopulse angle, precise absolute calibration in range and angle and high sensitivity. Leveraging ALTAIR's assets, we collected a greater number of independent calibrated measurements than possible with other radar systems, thereby enhancing our estimates of meteoroid physical parameters (such as position, ionization coefficient, electron line density and mass) because of this reduction in the number of unknown quantities. Typical meteor radars collect fewer independent measurements at lower resolution (e.g. ∼kilometer to tens of kilometers in range). ALTAIR's periodic sampling at two frequencies also helped characterize and guard our investigation against wavelength dependent biases; the scattering characteristics of meteor electrons are a function of incident wavelength. Using these data we formulated a new approach to estimate a model for the meteor ionization coefficient using dynamical and radar cross section data. The ionization coefficient models the number of electrons produced during collisions between ablated meteoroid atoms and atmospheric constituents; it is central to the estimation of meteoroid parameters such as mass and density. Our approach adds missing information since the last estimate made by Jones (1997) because our value is valid for higher velocity meteors (near 56 km/sec), allows for values greater than unity and requires fewer assumptions. We also provide a comprehensive presentation of ALTAIR characteristics to illustrate the uncertainties in our meteoroid parameter estimates and to serve as a comparative guide for the interpretation of other meteor radar system capabilities.