Statistics – Computation
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmsa52a0400s&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #SA52A-0400
Statistics
Computation
2400 Ionosphere, 2443 Midlatitude Ionosphere
Scientific paper
The gyro line is a very weak part of the incoherent scatter (IS) spectrum centered near the electron gyro frequency times the cosine of the angle between the radar k vector and the magnetic field. Observations using the Arecibo radar are few, but published results roughly confirm the predictions of IS theory. Nonetheless, improvements in the sensitivity of the radar, the data collection equipment, and the analysis computers make better observations possible. The gyro line is potentially useful in studies of the E and lower F regions of the ionosphere. We report the results of recent observations made at three different angles to the magnetic field and the comparisons of the observed spectra to theory. The observations occurred on October 21, 2001 from roughly 11:00 to 15:30 Atlantic Standard Time at angles of 49, 53, and 59 degrees from the magnetic field. The observations used a pulse length of 496 microseconds and a bandwidth of 5 MHz centered at 430 MHz, allowing the recording of both the up- and down-shifted gyro lines. Spectral analysis of the 12-bit recorded voltage samples occurred later on a suitable computer. We observed gyro lines at the expected frequencies at all three angles with approximately the correct variation of frequency with angle. These lines are approximately five orders of magnitude weaker than the height-integrated ion line, and so only careful subtraction of interference using noise spectra from very high altitudes results in useful spectra. In addition to the expected lines, the observations show several other features that are potentially ionospheric in origin because of their consistent variation with angle. These features include a second and possible third set of lines located at frequencies greater than the gyro line, and some broad features. Nearly all comparisons between IS theory and observations use a simple form of the theory with an approximately correct treatment of the effects of the magnetic field. Observations of the gyro line and the electron line high in the topside are two examples requiring the full theory. Thus, these measurements are of interest for verification of the theory. We have manipulated the equations so that the use of fast Fourier transforms simplifies calculations and provides a comparison with other methods of calculation. We have found reasonable agreement between the theory and the location and widths of the gyro line measurements. We do not understand the additional spectral components in the observations. We have not proven that computational inaccuracies or other equipment problems do not cause them, but the outer lines are in some cases larger than the expected lines. This, as well as other factors, argues against the effects of non-linear processing. A new data-taking system using 14 bit converters and sampling at the intermediate frequency is under construction; we will make new measurements when it is ready. Another possible source for the additional components is the effects of supra-thermal electrons. We have developed a version of IS theory allowing a tail in the electron distribution function in the form of a high-temperature Maxwellian added to the thermal Maxwellian. We have not been able to reproduce the observations. Another version allowing arbitrarily shaped tails is under development.
González Sixto A.
Sulzer Michael P.
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