Browsing by Author "Huba, J. D."
Now showing 1 - 9 of 9
Results Per Page
Sort Options
Item Restricted Equatorial spread F modeling: Multiple bifurcated structures, secondary instabilities, large density ‘bite-outs,’ and supersonic flows(American Geophysical Union, 2007-04-10) Huba, J. D.; Joyce, G.The Naval Research Laboratory has recently developed a new two‐dimensional code to study equatorial spread F (ESF): NRLESF2. The code uses an 8th order spatial interpolation scheme and the partial donor cell method. This allows the model to capture very sharp gradients over ∼ 4 grid cells and to assess the impact of numerical diffusion on the dynamics of ‘bubble’ evolution. Simulation results are presented that show new and complex ESF bubble dynamics: multiple bifurcations, secondary instabilities, density ‘bite‐outs’ of over three orders of magnitude, and supersonic flows within low density channels (V ≃ few km/s). These results are consistent with radar and satellite observations, as well as optical images. It is also shown that numerical diffusion can inhibit bubble bifurcation and the development of small‐scale structure.Item Open Access Full profile incoherent scatter analysis at Jicamarca(Instituto Geofísico del Perú, 2008) Hysell, D. L.; Rodrigues, F. S.; Chau Chong Shing, Jorge Luis; Huba, J. D.Diapositivas presentadas en URSI General Assembly, August 7-16, 2008, Chicago, Illinois, USA.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 Restricted Modeling ionospheric super‐fountain effect based on the coupled TIMEGCM‐SAMI3(American Geophysical Union, 2013-04-02) Lu, G.; Huba, J. D.; Valladares, CesarRecently, efforts have been undertaken to develop a coupled thermosphere‐ionosphere‐plasmasphere model based on two well‐established models, namely, the Thermosphere‐Ionosphere‐Mesosphere General Circulation Model (TIMEGCM) developed at the National Center for Atmospheric Research and the SAMI3 ionosphere model developed at the Naval Research Laboratory. This paper presents the first results from the coupled model on the investigation of a prompt penetration electric field (PPEF) event that took place on 9 November 2004. The coupled model eliminates two major upper boundary limitations of the stand‐alone TIMEGCM, e.g., the upper boundary height and the prescribed O+ fluxes at the upper boundary. It is found that the F‐layer peak height is raised above 800 km in response to the large PPEF. The O+ fluxes in the top ionosphere vary drastically during the course of the PPEF, with strong upward and downward fluxes with a magnitude greater than 109 cm−2 s−1 in localized regions. For the first time, the coupled model allows us to simulate and visualize the super‐fountain effect on a global scale. Future model development is also envisaged, including the implementation of a more realistic magnetic field model and a fully two‐way coupling between neutrals and ions.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 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.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.Item Open Access Topside measurements at Jicamarca during solar minimum(European Geosciences Union (EGU), 2009-01-23) Hysell, D. L.; Chau Chong Shing, Jorge Luis; Huba, J. D.Long-pulse topside radar data acquired at Jicamarca and processed using full-profile analysis are compared to data processed using more conventional, range-gated approaches and with analytic and computational models. The salient features of the topside observations include a dramatic increase in the Te/Ti temperature ratio above the F peak at dawn and a local minimum in the topside plasma temperature in the afternoon. The hydrogen ion fraction was found to exhibit hyperbolic tangent-shaped profiles that become shallow (gradually changing) above the O+-H+ transition height during the day. The profile shapes are generally consistent with diffusive equilibrium, although shallowing to the point of changes in inflection can only be accounted for by taking the effects of E×B drifts and meridional winds into account. The SAMI2 model demonstrates this as well as the substantial effect that drifts and winds can have on topside temperatures. Significant quiet-time variability in the topside composition and temperatures may be due to variability in the mechanical forcing. Correlations between topside measurements and magnetometer data at Jicamarca support this hypothesis.