Browsing by Author "Farley, Donald T."
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Item Restricted A nonlocal theory of the gradient-drift instability in the ionospheric E-region plasma at mid-latitudes(Elsevier, 2004-11) Seyler, Charles E.; Rosado-Román, José M.; Farley, Donald T.The instability which arises in thin ionization layers in the ionospheric E region is inherently nonlocal at mid-latitudes because of the high conductivity along the inclined magnetic field lines. We attack the gradient-drift stability problem using a numerical matrix method, with a finite Fourier basis for the density and potential perturbations, and find complex eigenfunctions and eigenvalues. A thin density layer with steep edges, superimposed on a constant background, is unstable at modes that are centered on the locally unstable side but extend into the locally stable region. The finite background density is essential, but of course there always is such a background in the ionosphere. An interesting aspect of the solutions is that the electric field perturbations peak at distances farther away from the layer than do the density perturbations. The growth rate of the nonlocal modes is significantly smaller than that predicted by local theory (which omits the damping effect of coupling along the magnetic field lines), as expected, and so the waves must be strongly driven. But for ambient electric field strengths of a few mV/m, or an equivalent wind velocity, the growth rates are sufficient to explain sounding rocket and radar observations of plasma waves associated with sporadic-E layers. We also find that the unstable plasma waves propagate in directions that differ from normal to the magnetic field by less than a degree, probably less than half a degree, in agreement with radar observations.Item Restricted Aspect sensitivity of equatorial electrojet irregularities and theoretical implications(American Geophysical Union, 1989-01-01) Kudeki, Erhan; Farley, Donald T.We used a multi baseline 50-MHz radar interferometer at the Jicamarca Radio Observatory to study the dependence of echo strength on the aspect angle, the angle of arrival relative to normal to the geomagnetic field. We observed rms values of this angle in the range 0.1–0.4, which implies an aspect sensitivity somewhat greater than the nominal 10 dB/deg often quoted for auroral echoes. The technique has sufficient resolution to allow deviations from normal of less than 0.1 to be measured fairly easily, and we were able to determine variations of the aspect sensitivity with altitude, Doppler shift, and electrojet conditions. During weakly driven periods, when only type 2 echoes were observed, the width, or rms angular deviation from normal, decreased monotonically with increasing altitude, as simple linear theory would suggest, but the variation was less rapid than the variation of νe/Ωe, especially on the bottomside of the electrojet. A particularly interesting observation was that, during strongly driven type 1 conditions, there was a pronounced variation of width with Doppler shift. The width for small phase velocities was sometimes more than twice that at the ion-acoustic velocity. We believe that this broadening is due to nonlinear three-wave mode coupling which produces a reverse cascade of energy from short wavelengths to long and an increase in the mean aspect angle. Such a cascade represents an increase in the effective diffusion coefficient for the short wavelength waves and so could provide an explanation alternative to that of the anomalous collision process of Sudan (1983) for the limitation of the type 1 phase velocity at the ion-acoustic velocity.Item Open Access Measuring ionospheric densities, temperatures and drift velocities simultaneously at Jicamarca(American Geophysical Union, 1997-11-15) Aponte, Nestor; Woodman Pollitt, Ronald Francisco; Swartz, Wesley E.; Farley, Donald T.Incoherent scatter autocorrelation function measurements are difficult to make in the F región at Jicamarca because of very strong clutter contamination by coherent echoes from unstable plasma waves in the -B-region electrojet that are aligned with the magnetic field. We have developed a more effective way to deal with this clutter that improves the quality of the temperature (and composition when light ions are present) data. Other coherent echoes (much weaker than electrojet echoes but stronger than incoherent scatter) are also received through the antenna sidelobes from fieldaligned irregularities in the 140-170 km altitude range during daytime. These latter echoes have a very narrow bandwidth, and so it is easy to measure their Doppler shift and obtain the vertical plasma drift velocity, which is proportional to the zonal electric field.Item Restricted Plasma instabilities observed in the E region over Arecibo and a proposed nonlocal theory(Elsevier, 2004-12) Rosado-Román, José M.; Swartz, Wesley E.; Farley, Donald T.We describe simultaneous radar observations made with the Cornell University Portable Radar Interferometer (CUPRI) at 50 MHz and the Arecibo incoherent scatter radar (ISR) at 430 MHz during the El Coquí campaign of 1992 in Puerto Rico. The goal was to study the plasma instabilities that cause coherent radar backscatter from the E region at mid-latitudes. The common volume data reveal that the coherent CUPRI echoes come from sporadic-E layers that exhibit no obvious gravity wave modulation but possess high densities and sharp gradients. The echoes with positive (negative) Doppler shifts, i.e. eastward (westward) plasma wave phase velocities, come from the top (bottom) of the layer, in agreement with simple local equatorial gradient-drift instability theory, even though this theory is not valid at mid-latitudes, where nonlocal shorting effects along magnetic field lines play a crucial role. We have developed a nonlocal theory that takes these effects into account. Our theory, which is discussed in detail in a companion paper, does not invoke any unusual layer geometry, in contrast to the ideas proposed in several papers in recent years. The unstable eigenmodes are a sum of plane waves with k-vectors having a small component parallel to the geomagnetic field, such that the modes are confined primarily to either the top or bottom of the layer, depending on the driving electric field. The direction of these k-vectors deviates from normal to the magnetic field by at most a few tenths of a degree. The k-vectors are also approximately aligned with the E×B drift. While both the density and potential fluctuations peak in amplitude on the unstable side of the layer, the density peak is closer to the maximum of the layer than is the potential peak. We do not in this paper deal with the “quasi-periodic” or QP nature of the radar echoes that is sometimes, but certainly not always, seen.Item Restricted Spread in aspect angles of equatorial E region irregularities(American Geophysical Union, 2008-11-12) Lu, Fei; Farley, Donald T.; Swartz, Wesley E.We revisit and extend the 50 MHz radar aspect angle study made in Peru by Kudeki and Farley in 1985, taking advantage of upgraded facilities at Jicamarca. We discuss here (1) type 1 and 2 echoes from the electrojet region, (2) early evening echoes from somewhat above the electrojet, and (3) daytime “150 km” echoes from the ∼145 to 170 km altitude region. Our electrojet data (1) confirm the earlier results for weak type 2 conditions, namely, that the rms aspect angle θrms decreases from about 0.3° at 99 km to slightly less than 0.2° at 108 km, and (2) provide much more information about the aspect angles of type 1 echoes and the apparent nonlinear interaction between type 1 waves and slower waves. Type 1 waves, with phase velocities near the ion‐acoustic velocity, have θrms decreasing from about 0.15° near 104 km to about 0.10° near 110 km. There is strong evidence that horizontally traveling type 1 waves in the upper midday electrojet, where upward electron density gradients are absent, can sometimes nonlinearly couple to slow, vertically traveling waves with aspect angles sometimes larger than 0.5°. In the early evening we often observe echoes in the 125–135 km region with θrms ≤ 0.05°. During daytime the relatively weak “150 km” echoes have even smaller θrms. At the end of the paper we discuss ideas concerning the nonlinear coupling of the plasma waves.Item Open Access The early history of the Jicamarca Radio Observatory and the incoherent scatter technique(Copernicus Publications, 2019-10-02) Woodman Pollitt, Ronald Francisco; Farley, Donald T.; Balsley, Ben B.; Milla, MarcoThe purpose of these historical notes is to present the early history of the Jicamarca Radio Observatory (JRO), a research facility that has been conducting observations and studies of the equatorial ionosphere for more than 50 years. We have limited the scope of these notes to the period of the construction of the observatory and roughly the first decade of its operation. Specifically, this period corresponds to the directorships under Kenneth Bowles, Donald Farley, and Tor Hagfors and the first period of Ronald Woodman, i.e., the years between 1960 and 1974. Within this time frame, we will emphasize observational and instrumental developments which led to define the capabilities of the Jicamarca incoherent scatter (IS) radar to measure the different physical parameters of the ionosphere. At the same time, we partially cover the early history of the IS technique which has been used by many other observatories built since. We will also briefly mention the observatory’s early and most important contributions to our understanding of the physical mechanisms behind the many peculiar phenomena that occur at the magnetic Equator. Finally, we will put special emphasis on the important developments of the instrument and its observing techniques that frame the capabilities of the radar at that time.