Browsing by Author "Kudeki, E."
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Item Open Access A high-resolution study of mesospheric fine structure with the Jicamarca MST radar(European Geosciences Union (EGU), 2006-07-03) Sheth, R.; Kudeki, E.; Lehmacher, G.; Sarango, M.; Woodman Pollitt, Ronald Francisco; Chau Chong Shing, Jorge Luis; Guo, L.; Reyes, P.Correlation studies performed on data from recent mesospheric experiments conducted with the 50-MHz Jicamarca radar in May 2003 and July 2004 are reported. The study is based on signals detected from a combination of vertical and off-vertical beams. The nominal height resolution was 150 m and spectral estimates were obtained after ~1 min integration. Spectral widths and backscattered power generally show positive correlations at upper mesospheric heights in agreement with earlier findings (e.g., Fukao et al., 1980) that upper mesospheric echoes are dominated by isotropic Bragg scatter. In many instances in the upper mesosphere, a weakening of positive correlation away from layer centers (towards top and bottom boundaries) was observed with the aid of improved height resolution. This finding supports the idea that layer edges are dominated by anisotropic turbulence. The data also suggests that negative correlations observed at lower mesospheric heights are caused by scattering from anisotropic structures rather than reflections from sharp vertical gradients in electron density.Item Open Access A new approach in incoherent scatter F region E x B drift measurements at Jicamarca(American Geophysical Union, 1999-12-01) Kudeki, E.; Bhattacharyya, S.; Woodman Pollitt, Ronald FranciscoSince 1996 incoherent scatter F region plasma drift measurements at Jicamarca have been implemented using a new signal processing approach replacing the traditional pulse-to-pulse correlation method. The new method, based on Doppler spectrum estimation and nonlinear least squares fitting to model spectra obtained from incoherent scatter theory, improves the instrumental sensitivity remarkably under low signal-to-noise conditions. With the new method it has become possible to obtain very high quality drifts data at nearly all hours of the day throughout most F region heights. Altitudinal smoothing of the drifts data to reduce measurement noise is no longer necessary, and studies of the height variations of drifts can be performed with much greater certainty than before. Small-amplitude gravity wave oscillations have been detected at F region heights and a vortical circulation of the F region plasma has been observed in the post sunset period.Item Restricted Altitudinal dependence of evening equatorial F region vertical plasma drifts(American Geophysical Union, 2014-06-06) Fejer, B. G.; Hui, D.; Chau Chong Shing, Jorge Luis; Kudeki, E.We use Jicamarca incoherent scatter radar measurements to study for the first time the altitudinal variations of late afternoon and early night equatorial F region vertical plasma drifts. We also present the initial vertical drift measurements over the altitudinal range from about 200 to 2000 km. These data show that the afternoon drifts decrease weakly with altitude. Near their evening prereversal enhancements, the vertical drifts generally increase with altitude below about the F layer peak, decrease with height near the F layer peak and above, and are nearly height independent in the (solar flux dependent) topside ionosphere. The transition altitudes from height‐decreasing to height‐independent evening upward drifts decrease with altitude from solar maximum to solar minimum. After their reversal to downward, the vertical drifts do not change much with height. The altitudinal dependence of the evening vertical drifts has large day‐to‐day variability and is closely related to the time dependence of the zonal drifts, as expected from the curl‐free electric field condition.Item Open Access Calibrated radar observations of the equatorial mesosphere and ionosphere during an 11-day campaign(Instituto Geofísico del Perú, 2009) Kudeki, E.; Milla, Marco; Reyes, P.; Lehmacher, G.; Chau Chong Shing, Jorge Luis; Kuyeng, K. M.; De la Jara, CésarPresentado en MST12 - 12th Workshop on Technical and Scientific Aspects of MST Radars, London, Ontario, Canada, 17-23 May 2009.Item Restricted Collisional shear instability in the equatorial F region ionosphere(American Geophysical Union, 2004-11-03) Hysell, D. L.; Kudeki, E.A collisional shear instability in a magnetized plasma is described and evaluated. The instability is related to electrostatic Kelvin Helmholtz but operates in inhomogeneous plasmas in the collisional regime. Boundary value analysis predicts that the linear growth rate for the instability could be comparable to that of the collisional interchange instability in the equatorial F region ionosphere under ideal conditions. An initial value simulation of a nonlinear model of the instability run under realistic conditions produces growing waves with a relatively long growth time (50 min) and with an initial wavelength of about 30 km. The simulation results are consistent with recent radar observations showing large‐scale plasma waves in the bottomside equatorial ionosphere at sunset prior to the onset of spread F conditions. The role of shear instability in preconditioning the F region for interchange instabilities to occur after sunset is discussed.Item Restricted Discovery of two distinct types of equatorial 150-km radar echoes(American Geophysical Union, 2013-08-24) Chau Chong Shing, Jorge Luis; Kudeki, E.We show here that VHF signals scattered from the 150 km region above Jicamaca exhibit two distinct types of features. In one type (type A), the Doppler spectral width increases with the echo strength and the corresponding signal-to-noise ratio (SNR). A second type (type B) of higher SNR echoes exhibits SNR-independent Doppler spectral widths that are much narrower than those observed in the first type. The type A echo population is by far the dominant population. Comparisons with earlier data sets collected at Jicamarca and elsewhere suggest that the type A and type B are likely to be associated with a naturally enhanced incoherent scattering (NEIS) process and the unstable growth of field-aligned irregularities (FAIs), respectively. We conjecture that small radar systems operated near the geomagnetic equator that have reported 150 km echo observations detected FAI echoes and that the NEIS echoes can only be seen by high sensitivity systems.Item Open Access Estudio de los efectos de grandes erupciones solares sobre Jicamarca durante campañas MST-ISR(Instituto Geofísico del Perú, 2010) Kudeki, E.; Reyes, P.; Lehmacher, G.; Woodman Pollitt, Ronald Francisco; Chau Chong Shing, Jorge Luis; Milla, MarcoDiapositivas presentadas en el XIII Encuentro Científico Internacional, (ECI 2010v), realizado del 5 al 8 de enero de 2010 en la ciudad de Lima, Perú.Item Open Access First E- and D- region incoherent scatter spectra observed over Jicamarca(European Geosciences Union (EGU), 2006-07) Chau Chong Shing, Jorge Luis; Kudeki, E.We present here the first Jicamarca observations of incoherent scatter radar (ISR) spectra detected from E- and D-region altitudes. In the past such observations have not been possible at Jicamarca due a combined effect of strong equatorial electrojet (EEJ) clutter and hardware limitations in the receiving system. The observations presented here were made during weak EEJ conditions (i.e., almost zero zonal electric field) using an improved digital receiving system with a wide dynamic range and a high data throughput.The observed ISR spectra from E- and D-region altitudes are, as expected, narrow and get even narrower with decreasing altitude due to increasing ion-neutral collision frequencies. Therefore, it was possible to obtain accurate spectral measurements using a pulse-to-pulse data analysis. At lower altitudes in the D-region where signal correlation times are relatively long we used coherent integration to improve the signal-to-noise ratio of the collected data samples. The spectral estimates were fitted using a standard incoherent scatter (IS) spectral model between 87 and 120 km, and a Lorentzian function below 110 km. Our preliminary estimates of temperature and ion-neutral collisions frequencies above 87 km are in good agreement with the MSISE-90 model. Below 87 km, the measured spectral widths are larger than expected, causing an overestimation of the temperatures, most likely due to spectral distortions caused by atmospheric turbulence.
Item Restricted High-resolution observations of mesospheric layers with the Jicamarca VHF Radar(Elsevier, 2007) Lehmacher, G. A.; Guo, L.; Kudeki, E.; Reyes, P. M.; Akgiray, A.; Chau Chong Shing, Jorge LuisWe report new results from the 50-MHz Jicamarca radar in Peru (12°S, 77°W), which is able to observe backscatter from the daytime mesosphere on any given day. Since 2005, the radar has been operated in a high-power MST-ISR mode for 3-day runs four times per year to study the seasonal variation of mesospheric echoes. Doppler spectra are obtained with 1 min and 150-m nominal resolution yielding power, horizontal and vertical winds, and spectral width. The rich echo structures contain braids and billows suggestive of Kelvin–Helmholtz instability (KHI). We present three cases; (1) a short sequence of billows growing in height and becoming unstable; (2) a long train of billows showing high levels of turbulence at two different locations; and (3) a series of billows passing only slowly through the field of view. In all cases, the layers were associated with strong wind shears, mature billows were 1–1.5 km tall, and separation of KH phase fronts was 8–10 km. We compare our observations with OH imager observation and numerical simulations.Item Open Access Incoherent scatter measurements of aurora-like ion beam distribution and ionospheric holes produced by the space shuttle flying over the radars at Jicamarca, Kwajalein, and Arecibo(Instituto Geofísico del Perú, 2000) Bernhardt, P.A.; Sulzer, M.P.; Kudeki, E.; Woodman Pollitt, Ronald Francisco; Tsonuda, R.In October 1997 and July 1999 during the STS-86 and STS-93 flights of the Space Shuttle, the crew performed experiments with controlled ion injections over the incoherent scatter radar (ISR) facilities located at Arecibo, Puerto Rico; Jicamarca, Perú; and Kwajalein, Marshall Islands. Ion beams were formed by charge exchange in the ionosphere of the high velocity neutral molecules exhausted by the Orbital Maneuver Subsystem (OMS) engines on the Space Shuttle. Pick-up ions were produced with energies between 2 and 10 eV depending on the orientation of the OMS engines relative to the vehicle orbit. The ions eventually recombined with electrons yielding electrón density depressions or holes.Item Open Access Incoherent scatter measurements of F-region temperatures with the Jicamarca radar beam pointing perpendicular to B(Instituto Geofísico del Perú, 2008) Reyes, P.; Milla, Marco; Kudeki, E.Diapositivas presentadas en: ISEA 12 - 12th International Symposium on Equatorial Aeronomy, May 18-24, 2008. Crete, Greece.Item Open Access Lifetime of a depression in the plasma density over Jicamarca produced by space shuttle exhaust in the ionosphere(American Geophysical Union, 2001-09-01) Bernhardt, P. A.; Huba, J. D.; Kudeki, E.; Woodman Pollitt, Ronald Francisco; Condori, L.; Villanueva, F.When the space shuttle orbiting maneuver subsystem (OMS) engines burn in the ionosphere, a plasma density depression, or “hole,” is produced. Charge exchange between the exhaust molecules and the ambient O+ ions yields molecular ion beams that eventually recombine with electrons. The resulting plasma hole in the ionosphere can be studied with ground‐based, incoherent scatter radars (ISRs). This type of ionospheric modification is being studied during the Shuttle Ionospheric Modification with Pulsed Localized Exhaust (SIMPLEX) series of experiments over ISR systems located around the globe. The SIMPLEX 1 experiment occurred over Jicamarca, Peru, in the afternoon on October 4, 1997, during shuttle mission STS 86. An electron density depression was produced at 359 km altitude at the midpoint of a magnetic field line. The experiment was scheduled when there were no zonal drifts of the plasma so the modified field line remained fixed over the 50 MHz Jicamarca radar. The density depression was filled in by plasma flowing along the magnetic field line with a time constant of 4.5 min. The density perturbation had completely vanished 20 min after the engine burn. The experimental measurements were compared with two models: (1) SAMI2, a fully numerical model of the F region, and (2) an analytic representation of field‐aligned transport by ambipolar diffusion. The computed recovery time from each model is much longer than the observed recovery time. The theory of ambipolar diffusion currently used in ionospheric models seems to be inadequate to describe the SIMPLEX 1 observations. Several possible sources for this discrepancy are discussed. The SIMPLEX 1 active experiment is shown to have the potential for testing selected processes in ionospheric models.Item Open Access Mean winds and momentum fluxes over Jicamarca, Peru, during june and august 1987(American Meteorological Society, 1992-02-20) Hitchman, M.; Fritts, D.; Coy, L.; Kudeki, E.; Bywaters, K.; Sudan, R.Data from the mesosphere-stratosphere-troposphere (MST) radar at Jicamarca, Peru, together with other available data, are used to diagnose the mean structure of winds and gravity-wave momentum fluxes from the surface to 90 km during two ten-day campaigns in June and August of 1987. In the stratosphere a layer of maximum eastward flow associated with the quasi-biennial oscillation (QBO) was seen to strengthen and descend rapidly from June to August, overlying persistent westward flow. A layer of enhanced signal return, suggestive of a turbulent layer, was observed just above the descending QBO eastward maximum. Notable zonal asymmetries were present during this transition and the local meridional circulation departed from zonal-mean QBO theory. A substantial northeastward momentum flux was found below 25 km, which may be related to topographic gravity waves excited by southeastward flow across the Andes. In the lower mesosphere a relatively weak “second” mesopause semiannual oscillation is confirmed. Gravity-wave zonal and meridional momentum fluxes usually opposed the flow, yielding body forces of 10–100 m s−1 day−1. In both the lower stratosphere and mesosphere, body forces were comparable in magnitude to inferred Coriolis torques.Item Open Access Mean winds in the tropical stratosphere and mesosphere during january 1993, march 1994, and august 1994(American Geophysical Union, 1997-11-27) Hitchman, M.; Kudeki, E.; Fritts, D. C.; Kugi, J. M.; Fawcett, C.; Postel, G. A.; Yao, C.; Ortland, D.; Riggin, D.; Harvey, V. L.Radar observations of winds and momentum fluxes in the stratosphere and mesosphere at the Jicamarca Radio Observatory in Peru ( JRO; 12S, 77W) were taken during three 10 day campaigns in January 1993, March 1994, and August 1994. In order to interpret features in the campaign mean JRO wind profiles, we examined global circulation patterns as depicted by long time series of radiosonde profiles, analyses from the European Center for Medium Range Weather Forecasting (ECMWF), and winds from the High Resolution Doppler Imager (HRDI) aboard the Upper Atmosphere Research Satellite. In the tropical stratosphere, large-scale analyses show that a geographically varying annual cycle significantly affects winds over JRO, as does the quasi-biennial oscillation ( QBO). The spatial structure of the annual cycle and QBO is shown for the three campaigns, emphasizing the upward influence of subtropical tropospheric monsoon anticyclones. These anticyclones tilt poleward and merge zonally, underlying the zonal summer easterlies, which also merge zonally and tilt poleward with altitude. The annual cycle at Singapore includes a substantial easterly acceleration during March-August, which causes an apparent stalling of descending QBO westerlies or a more rapid descent of QBO easterlies. In the mesosphere, JRO and HRDI winds agree reasonably well, with zonal winds over JRO varying on a semiannual basis and meridional winds exhibiting structures expected from the diurnal tide. For vertical motion, separate north-south and east-west beam pair estimates agree, yet campaign-averaged vertical motions are -1-5 cm/s in the stratosphere and -10-50 cm/s in the mesosphere. In both the stratosphere and mesosphere, vertical winds are anticorrelated with horizontal wind. Possible explanations for the large vertical motions include aspect sensitivity and the diurnal tide. Uncertainties in the meaning of radar vertical motions create a challenge for interpreting momentum fluxes.Item Open Access Multi-frequency and multi-volume radar studies of the equatorial 150-km region(Instituto Geofísico del Perú, 2009-03) Chau Chong Shing, Jorge Luis; Milla, Marco; Kudeki, E.Presentado en MST12 - 12th Workshop on Technical and Scientific Aspects of MST Radars, London, Ontario, Canada, 17-23 May 2009.Item Open Access Naturally enhanced ion-line spectra around the equatorial 150-km region(European Geosciences Union (EGU), 2009-03-02) Chau Chong Shing, Jorge Luis; Woodman Pollitt, Ronald Francisco; Milla, Marco; Kudeki, E.For many years strong radar echoes coming from 140–170 km altitudes at low latitudes have been associated to the existence of field-aligned irregularities (FAIs) (the so called 150-km echoes). In this work, we present frequency spectra as well as angular distribution of 150-km echoes. When the 150-km region is observed with beams perpendicular to the magnetic field (B) the observed radar spectra are very narrow with spectral widths between 3–12 m/s. On the other hand, when few-degrees off-perpendicular beams are used, the radar spectra are wide with spectral widths comparable to those expected from ion-acoustic waves at these altitudes (>1000 m/s). Moreover the off-perpendicular spectral width increases with increasing altitude. The strength of the received echoes is one to two orders of magnitude stronger than the expected level of waves in thermal equilibrium at these altitudes. Such enhancement is not due to an increase in electron density. Except for the enhancement in power, the spectra characteristics of off-perpendicular and perpendicular echoes are in reasonable agreement with expected incoherent scatter spectra at these angles and altitudes. 150-km echoes are usually observed in narrow layers (2 to 5). Bistatic common volume observations as well as observations made few kilometers apart show that, for most of the layers, there is very high correlation on power fluctuations without a noticeable time separation between simultaneous echoes observed with Off-perpendicular and Perpendicular beams. However, in one of the central layers, the echoes are the strongest in the perpendicular beam and absent or very weak in the off-perpendicular beams, suggesting that they are generated by a plasma instability. Our results indicate that most echoes around 150-km region are not as aspect sensitive as originally thought, and they come from waves that have been enhanced above waves in thermal equilibrium.Item Open Access Possible ionospheric preconditioning by shear flow leading to equatorial spread F(European Geosciences Union (EGU), 2005-10-14) Hysell, D. L.; Kudeki, E.; Chau Chong Shing, Jorge LuisVertical shear in the zonal plasma drift speed is apparent in incoherent and coherent scatter radar observations of the bottomside F region ionosphere made at Jicamarca from about 1600–2200 LT. The relative importance of the factors controlling the shear, which include competition between the E and F region dynamos as well as vertical currents driven in the E and F regions at the dip equator, is presently unknown. Bottom-type scattering layers arise in strata where the neutral and plasma drifts differ widely, and periodic structuring of irregularities within the layers is telltale of intermediate-scale waves in the bottomside. These precursor waves appear to be able to seed ionospheric interchange instabilities and initiate full-blown equatorial spread F. The seed or precursor waves may be generated by a collisional shear instability. However, assessing the viability of shear instability requires measurements of the same parameters needed to understand shear flow quantitatively - thermospheric neutral wind and off-equatorial conductivity profiles.Item Open Access Postsunset vortex in equatorial F-region plasma drifts and implications for bottomside spread-F(American Geophysical Union, 1999-12-01) Kudeki, E.; Bhattacharyya, S.Recent high‐resolution incoherent scatter drift measurements conducted at Jicamarca during solar minimum conditions indicate that a vortical plasma flow pattern accompanies the well known prereversal enhancement events observed in the equatorial F region. The vortex is centered at ∼250 km altitude and about 2000–3000 km east of the evening terminator and is characterized by upward and downward flows to the west and to the east and eastward and westward flows on the top and the bottom, respectively. Postsunset bottomside spread‐F events are observed to commence in the interior of the vortex in regions of westward plasma drifts. The formation of the vortex implies localized charging of the postsunset bottomside F region in response to eastward neutral wind driven F region dynamo action.Item Open Access Radar cross sections for mesospheric echoes at Jicamarca(European Geosciences Union (EGU), 2009-07-06) Lehmacher, G. A.; Kudeki, E.; Akgiray, A.; Guo, L.; Reyes, P.; Chau Chong Shing, Jorge LuisRadar cross sections (RCS) of mesospheric layers at 50 MHz observed at Jicamarca, Peru, range from 10−18 to 10−16 m−1, three orders of magnitudes smaller than cross sections reported for polar mesospheric winter echoes during solar proton events and six orders of magnitude smaller than polar mesospheric summer echoes. Large RCS are found in thick layers around 70 km that also show wide radar spectra, which is interpreted as turbulent broadening. For typical atmospheric and ionospheric conditions, volume scattering RCS for stationary, homogeneous, isotropic turbulence at 3 m are also in the range 10−18 to 10−16 m−1, in reasonable agreement with measurements. Moreover, theory predicts maximum cross sections around 70 km, also in agreement with observations. Theoretical values are still a matter of order-of-magnitude estimation, since the Bragg scale of 3 m is near or inside the viscous subrange, where the form of the turbulence spectrum is not well known. In addition, steep electron density gradients can increase cross-sections significantly. For thin layers with large RCS and narrow spectra, isotropic turbulence theory fails and scattering or reflection from anisotropic irregularities may gain relevance.Item Open Access Statistics of 150-km echoes over Jicamarca based on low-power VHF observations(European Geosciences Union (EGU), 2006-01) Chau Chong Shing, Jorge Luis; Kudeki, E.In this work we summarize the statistics of the socalled 150-km echoes obtained with a low-power VHF radar operation at the Jicamarca Radio Observatory (11.97◦ S, 76.87◦ W, and 1.3 ◦ dip angle at 150-km altitude) in Peru. Our results are based on almost four years of observations between August 2001 and July 2005 (approximately 150 days per year). The majority of the observations have been conducted between 08:00 and 17:00 LT. We present the statistics of occurrence of the echoes for each of the four seasons as a function of time of day and altitude. The occurrence frequency of the echoes is ∼75% around noon and start decreasing after 15:00 LT and disappear after 17:00 LT in all seasons. As shown in previous campaign observations, the 150-echoes appear at a higher altitude (>150 km) in narrow layers in the morning, reaching lower altitudes (∼135 km) around noon, and disappear at higher altitudes (>150 km) after 17:00 LT. We show that although 150-km echoes are observed all year long, they exhibit a clear seasonal variability on altitudinal coverage and the percentage of occurrence around noon and early in the morning. We also show that there is a strong day-to-day variability, and no correlation with magnetic activity. Although our results do not solve the 150-km riddle, they should be taken into account when a reasonable theory is proposed.