Browsing by Author "Heinselman, Craig J."
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Item Restricted Meteor-head echo observations using an antenna compression approach with the 450 MHz Poker Flat Incoherent Scatter Radar(Elsevier, 2009-05) Chau Chong Shing, Jorge Luis; Galindo, Freddy R.; Heinselman, Craig J.; Nicolls, Michael J.In this work we present a novel use of the Poker Flat Incoherent Scatter Radar (PFISR) to study meteor-head echoes with wide (W) beams. Until now, most of the meteor-head echo studies have been performed with High-Power Large-Aperture Radars (HPLARs) using very narrow (N) beams. At PFISR we have implemented an antenna compression approach using a defocusing scheme, similar to Chirp (linear frequency modulation) in pulse compression. The resulting effective beam is times wider than the narrowest PFISR beam. Using the signal-to-noise ratio (SNR) as a proxy measurement of cross-section, from the combined W and N beam experiments, our main results are: (1) observed meteors in the W beam are approximately half the number of meteors observed in the N beam, (2) we detected 10 times more large cross-section (strong) meteors ( if they were measured by the N mainlobe) than using only the N beam, and (3) more than 15% of the total N meteors were observed in the N sidelobes, therefore being at least 20 dB stronger if they were observed in the N mainlobe. Our results are summarized in a corrected distribution of relative meteor cross-sections as if all of them were observed with the N mainlobe, namely correcting their SNR values depending on where in the beam they were detected (sidelobes or mainlobe). In addition, we show a qualitative meteor cross-section distribution that one can obtain combining W and N beams. The resulting distribution is incomplete, since the W beam is not sensitive enough to detect the very small (weak) meteors, but could provide new information about the large cross-section events.Item Open Access Modeling the global micrometeor input function in the upper atmosphere observed by high power and large aperture radars(American Geophysical Union, 2006-07-26) Janches, Diego; Heinselman, Craig J.; Chau Chong Shing, Jorge Luis; Chandran, Amal; Woodman Pollitt, Ronald FranciscoWe report initial results of an effort to model the diurnal and seasonal variability of the meteor rate detected by high power and large aperture (HPLA) radars. The model uses Monte Carlo simulation techniques and at present assumes that most of the detected particles originate from three radiant distributions with the most dominant concentrated around the Earth's apex. The other two sources are centered 80° in ecliptic longitude to each side of the apex and are commonly known as helion and antihelion. To reproduce the measurements, the apex source flux was set to provide 70% of the total number of particles while the other 30% is provided by the combined contribution of the two remaining sources. The results of the model are in excellent agreement with observed diurnal curves obtained at different seasons and locations using the 430 MHz Arecibo radar in Puerto Rico, the 50 MHz Jicamarca radar in Perú, and the 1.29 GHz Sondrestrom radar in Greenland. To obtain agreement with the observed diurnal and seasonal variability of the meteor rate, an empirical atmospheric filtering effect was introduced in the simulation which prevents meteors with low-elevation radiants (<21) from being detected by the radars at mesospheric altitudes. The filtering effect is probably produced by a combination of factors related to the interaction of the meteor with the air molecules such as electron production and/or the ablation at higher altitudes. On the basis of these results we calculate the micrometeor global, diurnal, and seasonal input in the upper atmosphere.