Browsing by Author "Hysell, David L."
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Item Restricted Equatorial spread-F initiation: Post-sunset vortex, thermospheric winds, gravity waves(Elsevier, 2007-12) Kudeki, Erhan; Akgiray, Ahmed; Milla, Marco; Chau Chong Shing, Jorge Luis; Hysell, David L.We present experimental evidence and modeling results which indicate that eastward thermospheric wind is the primary controlling factor of equatorial spread-F initiation in the post-sunset ionosphere. Eastward wind-driven Pedersen currents are able to polarize F-region density perturbations with westward tilting wavefronts into rapidly growing modes to trigger the formation of spread-F bubbles. The described process is so rapid that seeding requirements of spread-F initiation by external factors such as gravity waves are effectively eliminated.Item Restricted Height variation of gaps in 150‐km echoes and whole atmosphere community climate model electron densities suggest link to upper hybrid resonance(American Geophysical Union, 2020-01) Lehmacher, Gerald A.; Wu, Haonan; Kudeki, Erhan; Reyes, Pablo M.; Hysell, David L.; Milla, MarcoRadar echoes from the daytime lower F region near the magnetic equator, so-called 150-km echoes, have been puzzling researchers for decades. Neither the mechanisms that generate the enhanced backscatter at very high frequencies (typically 30–50 MHz), the sharp lower cutoff height, the intricate layering with multiple echo layers separated by narrow gaps, nor the modulation of the echoes by short-period gravity waves is well understood. Here we focus on the diurnal variation of the echo layers specifically, certain wide gaps in the vertical structure—which apparently descend in the morning, reach their lowest altitude near local noon, and ascend in the afternoon, sometimes described as necklace structure based on the appearance of the layers in range-time-intensity diagrams. Analyzing high-resolution data obtained with the Jicamarca radar between 2005 and 2017, spanning more than one solar cycle, we find that (a) wide gaps and narrow lines occur in vertically stacked, systematically repeating pattern; (b) the gap heights vary with season and solar cycle; and (c) the gap heights can be associated with specific contours of plasma frequencies or electron densities. The last two findings are supported by simultaneous observations of VIPIR ionosonde reflection heights and by comparison of gap heights with electron density contours obtained with the WACCM-X 2.0 global model. Finally, the wide gaps appear to coincide with the double resonance condition, where the upper hybrid frequency equals integer multiples of the electron gyrofrequency. This may explain why field-aligned plasma irregularities are suppressed and enhanced radar backscatter is not observed inside the gaps.Item Restricted Improved spectral observations of equatorial spread F echoes at Jicamarca using aperiodic transmitter coding(Elsevier, 2004-11) Chau Chong Shing, Jorge Luis; Hysell, David L.; Reyes, Pablo M.; Milla, MarcoWe present preliminary radar results obtained with a new method of spectrum estimation of moderately overspread equatorial spread F (ESF) echoes over Jicamarca. This new method is similar to the classical pulse-to-pulse methods but with nonuniform pulse spacings, i.e., using aperiodic transmitter pulses, allowing us to avoid range and frequency aliasing, at the cost of extra clutter at some ranges. We also present different ways of dealing with the extra clutter, depending on the spectral characteristics of the clutter. We have identified two kinds of clutter: ground clutter (due to mountains and transmitter pulses) and ionospheric clutter (due to ESF and equatorial electrojet echoes). Preliminary radar results are very encouraging, and ways of improving them are proposed based on Monte Carlo simulations. Particularly, we propose alternating the phases of the uncoded aperiodic pulses using random binary phase codes to flatten the spectrum of the ionospheric clutter. The theoretical implications of the newly estimated spectra, particularly from topside echoes, are presented and discussed in a companion paper.Item Open Access Jicamarca contributions to UAF Coordination(Instituto Geofísico del Perú, 2009) Hysell, David L.; Chau Chong Shing, Jorge LuisDiapositivas presentadas en: CEDAR Workshop 2009 del 28 de junio al 2 de julio de 2009 en Santa Fe, Nuevo México, USA.Item Open Access Observaciones ionosféricas y atmosféricas con el radar desatendido de Jicamarca (JULIA)(Instituto Geofísico del Perú, 2000) Sarango, Martin F.; Woodman Pollitt, Ronald Francisco; Balsley, Ben B.; Hysell, David L."En este artículo se describen las observaciones que se realizan en el ROJ, con un sistema de procesamiento y de transmisión-recepción de baja potencia desarrollado en Jicamarca para realizar los experimentos mencionados arriba (y/o, posiblemente, otros experimentos). Este radar, denominado JULIA (por Jicamarca U nattended Long-term studies of the lonosphere and Atmosphere) [Sarango, et. al, 1998], proporciona a los usuarios del ROJ la capacidad de realizar experimentos de larga duración con menor potencia total (10-13dB menos), pero utilizando la gran antena de Jicamarca (300x300m). Se incluye una breve descripción del radar JULIA; así como la descripción y ejemplos de varios experimentos de observación realizados durante los primeros años de operación del sistema."Item Restricted Recent advances in equatorial, low- and mid-latitude aeronomy(Elsevier, 2013-10) Bhattacharyya, Archana; Chau Chong Shing, Jorge Luis; Denardini, Clezio M.; Hysell, David L.; Makela, Jonathan J.; Shiokawa, Kazuo; Kudeki, ErhanThis special issue of the Journal of Atmospheric and Solar–Terrestrial Physics on “Recent Advances in Equatorial, Low- and Mid-latitude Aeronomy” contains selected papers presented at the 13th International Symposium on Equatorial Aeronomy (ISEA13). This symposium is the latest in a long series of ISEAs, which was started in 1962 soon after the Jicamarca Radio Observatory (JRO) began its operation near Lima, Peru. The first ISEA was held in Huaychulo, Peru; and since then the symposium has been held every 3–5 years at different locations around the globe, bringing together scientists from all over the world, who are interested in the low- and mid-latitude atmosphere and ionosphere, and their coupling to other latitudes and altitudes. The ISEAs have become important events for researchers working in this field, as it offers them an opportunity to share their most recent results and discuss new techniques and possibilities for future coordinated campaigns and experiments.Item Restricted Sources of variability in equatorial topside ionospheric and plasmaspheric temperatures(Elsevier, 2013-01-17) Varney, Roger H.; Hysell, David L.; Huba, J. D.Jicamarca measurements of electron temperatures at high altitudes (500–1500 km) from the last solar minimum routinely show variations of hundreds of Kelvin from day-to-day. Possible sources of these variations are explored using the SAMI2-PE is another model of the ionosphere including photoelectron transport (SAMI2-PE) model, which includes a multistream photoelectron transport model. Changes to the electric fields, meridional winds, and thermospheric densities can all change the electron densities and temperatures at high altitudes. The high altitude electron temperatures are primarily determined by a balance between heating from photoelectrons which travel up the field lines and thermal diffusion which carries heat back down the field lines. The winds and electric fields will change the altitude and densities of the off-equatorial F-region peaks, especially on the field lines connected to the equatorial arcs. The densities and temperatures in the plasmasphere will self consistently adjust themselves to achieve diffusive equilibrium with the off-equatorial F-regions. Furthermore, decreases in the density and/or altitude of the F-region makes it easier for photoelectrons to escape to high altitudes. These connections between the equatorial plasmasphere, the off-equatorial F-regions, and the neutral thermosphere suggest that high altitude measurements at Jicamarca could be used to study thermospheric variability.