Browsing by Author "Varney, R. H."
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Item Open Access AMISR-14: Observations of equatorial spread F(American Geophysical Union, 2015-06-11) Rodrigues, F. S.; Nicolls, M. J.; Milla, Marco; Smith, J. M.; Varney, R. H.; Strømme, A.; Martinis, C.; Arratia, J. F.A new, 14-panel Advanced Modular Incoherent Scatter Radar (AMISR-14) system was recently deployed at the Jicamarca Radio Observatory. We present results of the first coherent backscatter radar observations of equatorial spread F (ESF) irregularities made with the system. Colocation with the 50 MHz Jicamarca Unattended Long-term studies of the Ionosphere and Atmosphere (JULIA) radar allowed unique simultaneous observations of meter and submeter irregularities. Observations from both systems produced similar Range-Time-Intensity maps during bottom-type and bottomside ESF events. We were also able to use the electronic beam steering capability of AMISR-14 to “image” scattering structures in the magnetic equatorial plane and track their appearance, evolution, and decay with a much larger field of view than previously possible at Jicamarca. The results suggest zonal variations in the instability conditions leading to irregularities and demonstrate the dynamic behavior of F region scattering structures as they evolve and drift across the radar beams.Item Restricted SAMI2‐PE: A model of the ionosphere including multistream interhemispheric photoelectron transport(American Geophysical Union, 2012-06-29) Varney, R. H.; Swartz, W. E.; Hysell, D. L.; Huba, J. D.In order to improve model comparisons with recently improved incoherent scatter radar measurements at the Jicamarca Radio Observatory we have added photoelectron transport and energy redistribution to the two dimensional SAMI2 ionospheric model. The photoelectron model uses multiple pitch angle bins, includes effects associated with curved magnetic field lines, and uses an energy degradation procedure which conserves energy on coarse, non‐uniformly spaced energy grids. The photoelectron model generates secondary electron production rates and thermal electron heating rates which are then passed to the fluid equations in SAMI2. We then compare electron and ion temperatures and electron densities of this modified SAMI2 model with measurements of these parameters over a range of altitudes from 90 km to 1650 km (L = 1.26) over a 24 hour period. The new electron heating model is a significant improvement over the semi‐empirical model used in SAMI2. The electron temperatures above the F‐peak from the modified model qualitatively reproduce the shape of the measurements as functions of time and altitude and quantitatively agree with the measurements to within ∼30% or better during the entire day, including during the rapid temperature increase at dawn.Item Restricted Sensitivity studies of equatorial topside electron and ion temperatures(American Geophysical Union, 2011-06-30) Varney, R. H.; Hysell, D. L.; Huba, J. D.Even in the recent extremely low solar minimum the electron and ion temperatures in the low‐latitude topside ionosphere display a great deal of day‐to‐day variability. This paper explores this variability using both the SAMI2 model and a newly developed steady state model of the plasma temperatures. Variations in the electric fields and neutral winds both produce drastic changes in the temperature profiles predicted above the magnetic equator. This implies that information about these parameters is contained in the temperature profiles measured at Jicamarca. Both winds and electric fields alter the arrangement of plasma throughout the entire low‐latitude ionosphere, including the locations and densities of the equatorial arcs. These changes have a much larger effect on the topside temperatures above the equator than changing the local advection or expansion alone because the topside equatorial temperatures are strongly coupled to the off‐equatorial F regions by field‐aligned thermal diffusion and photoelectron transport. The temperatures are more sensitive to changes in the nonlocal photoelectron heating than any other individual effect. The nonlocal photoelectron heating model used is still fairly primitive, however. The extreme sensitivity of the temperatures to the photoelectron transport model used means that more sophisticated photoelectron heating models will need to be used before meaningful comparisons between the model and observations can be made.Item Open Access Topside equatorial ionospheric density, temperature, and composition under equinox, low solar-flux conditions(American Geophysical Union, 2015-12) Hysell, D. L.; Milla, Marco; Rodrigues, F. S.; Varney, R. H.; Huba, J. D.We present observations of the topside ionosphere made at the Jicamarca Radio Observatory in March and September 2013, made using a full-profile analysis approach. Recent updates to the methodology employed at Jicamarca are also described. Measurements of plasma number density, electron and ion temperatures, and hydrogen and helium ion fractions up to 1500 km altitude are presented for 3 days in March and September. The main features of the observations include a sawtooth-like diurnal variation in ht, the transition height where the O+ ion fraction falls to 50%, the appearance of weak He+ layers just below ht, and a dramatic increase in plasma temperature at dawn followed by a sharp temperature depression around local noon. These features are consistent from day to day and between March and September. Coupled Ion Neutral Dynamics Investigation data from the Communication Navigation Outage Forecast System satellite are used to help validate the March Jicamarca data. The SAMI2-PE model was able to recover many of the features of the topside observations, including the morphology of the plasma density profiles and the light-ion composition. The model, forced using convection speeds and meridional thermospheric winds based on climatological averages, did not reproduce the extreme temperature changes in the topside between sunrise and noon. Some possible causes of the discrepancies are discussed.