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NAIC/AO Newsletter, March 1997
A different World Day investigation took place in October, the third such effort of the multi-instrumented study of equatorial thermosphere aeronomy (MISETA). These week-long campaigns began over a year ago. MISETA focuses on equatorial dynamics during equinox periods and the evolution of the ionospheric phenomenon known as spread-F. We were not able to observe with the incoherent-scatter radar in October. However, we were able to participate in and contribute to MISETA by measuring the neutral winds of the thermosphere and airglow intensities of the atomic oxygen, O(1D) 630-nm emission during eleven nights using our optical facilities. A follow-up study is expected to take place next spring and at that time we should be able use the full complement of the Arecibo facilities to this important World Day investigation.
During mid-November, a big push was made to observe the Leonid meteor shower and various related ionospheric phenomena from the so-called "meteor zone" between about 80 and 110 km altitude. David Meisel (SUNY-Geneseo), a specialist in meteor research, was on site to participate in the radar observations with Qihou Zhou (NAIC). Francisco Garcia and Michael Kelley, both of Cornell, fielded an all-sky imager at Arecibo to study the mesospheric airglow and to look for the remnants of meteor trails. Jonathan Friedman (NAIC) and Paul Castleberg of Cornell observed with both the Rayleigh and the resonance fluorescence lidars. We also operated the passive optical instruments including the spectrometer to observe the OH and O2 airglow emissions at night, similar to what the imager measured, and attempted to observe the potassium resonance line in twilight.
Overall, with this arsenal of instrumentation, some very fascinating results were obtained during the Leonids experiment, including the imaging of the remnants of a long-duration meteor trail that lasted for about a minute. By following the motion or dispersion of such trails it is possible to estimate the properties of the winds of the upper mesosphere where the meteoroids ablate. While the optical measurements were generally successful, we were less fortunate with the 430-MHz radar. In less than two hours after the experiment began on the first night of observations, a major component in the modulator circuit of the transmitter failed and required several days to repair. Despite the disappointment in this particular case, we will still make an effort over the next few years to observe the Leonids as this meteor shower is expected to peak in intensity sometime in 1998 or 1999.
Recently, Jonathan Friedman was able to successfully convert the Arecibo Doppler Rayleigh lidar to essentially a new instrument by using a novel technique to measure the neutral winds of the middle atmosphere. He modified the receiver system from its traditional scanning Fabry-Perot interferometer used for spectral sampling, replacing it with an iodine cell. The iodine absorbs a portion of the laser light at the half-power point of the backscattered spectrum at 532 nm. The received signal is first split, passing half the light through the cell, and the absorbed portion is compared with the part that was not absorbed by the iodine. The winds Doppler-shift the spectra and, depending on their magnitude, shift the spectral half-power point up or down relative to the iodine absorption's half-power point, effectively modulating the absorbed signal in a proportional manner. An identical iodine cell is used to frequency-lock the laser transmitter, eliminating the drift that we have experienced in the past with a scanning interferometer. This new technique to measure the neutral winds has been successfully tested and is essentially ready for routine observations.
The construction of a new lidar laboratory is well underway. The land on the hilltop to the north of the current Optical Lab has been cleared and the foundation is nearly complete. The fabrication of the steel frame for the building is currently under construction by a local steel firm and we expect that the frame will be installed in early May. Once the frame is installed the work should proceed quickly with the installation of the walls, roof, power, and water. The lab should be ready for occupancy in the summer when we move both the resonance and the Rayleigh lidars at that time. In addition, three 80-cm-diameter telescopes, which will be part of the larger collecting area for the future lidar receiver, have arrived on site and will be installed on the roof of the new building when it is completed.
In other news, Sixto González successfully landed a grant under the National Space Weather Program. Space weather refers to conditions on the sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and endanger human life or health. Adverse conditions in the space environment can cause disruption of satellite operations, communications, navigation, and electric power distribution grids, leading to a broad range of socio-economic losses. Through this program Sixto proposed to apply the latest models of the upper ionosphere to our studies of the topside ionosphere at Arecibo. Among other things, the proposal will also support a postdoctoral research associate, Brian MacPherson, who will join the Arecibo atmospheric group this summer.
John Cho (NAIC) and Michael Kelley were also successful in convincing the Air Force's Phillips Lab to support the reestablishment a 20-m resolution capability for our strato-/tropospheric radar. Unfortunately, just as this project is becoming a reality, John announced that he plans to leave the Observatory at the end of March (see the related article on p.13). Although we do wish John great success in his move on to better things, his departure signifies a significant step backward for lower atmospheric research at Arecibo.
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