Browsing by Author "Scherliess, L."
Now showing 1 - 7 of 7
Results Per Page
Sort Options
Item Restricted CEDAR Electrodynamics Thermosphere Ionosphere (ETI) Challenge for systematic assessment of ionosphere/thermosphere models: NmF2, hmF2, and vertical drift using ground‐based observations(American Geophysical Union, 2011-12-31) Shim, J. S.; Kuznetsova, M.; Rastätter, L.; Hesse, M.; Bilitza, D.; Butala, M.; Codrescu, M.; Emery, B.; Foster, B.; Fuller-Rowell, T.; Huba, J.; Mannucci, A. J.; Pi, X.; Ridley, A.; Scherliess, L.; Schunk, R. W.; Stephens, P.; Thompson, D. C.; Zhu, L.; Anderson, D.; Chau Chong Shing, Jorge Luis; Sojka, J. J.; Rideout, B.Objective quantification of model performance based on metrics helps us evaluate the current state of space physics modeling capability, address differences among various modeling approaches, and track model improvements over time. The Coupling, Energetics, and Dynamics of Atmospheric Regions (CEDAR) Electrodynamics Thermosphere Ionosphere (ETI) Challenge was initiated in 2009 to assess accuracy of various ionosphere/thermosphere models in reproducing ionosphere and thermosphere parameters. A total of nine events and five physical parameters were selected to compare between model outputs and observations. The nine events included two strong and one moderate geomagnetic storm events from GEM Challenge events and three moderate storms and three quiet periods from the first half of the International Polar Year (IPY) campaign, which lasted for 2 years, from March 2007 to March 2009. The five physical parameters selected were NmF2 and hmF2 from ISRs and LEO satellites such as CHAMP and COSMIC, vertical drifts at Jicamarca, and electron and neutral densities along the track of the CHAMP satellite. For this study, four different metrics and up to 10 models were used. In this paper, we focus on preliminary results of the study using ground‐based measurements, which include NmF2 and hmF2 from Incoherent Scatter Radars (ISRs), and vertical drifts at Jicamarca. The results show that the model performance strongly depends on the type of metrics used, and thus no model is ranked top for all used metrics. The analysis further indicates that performance of the model also varies with latitude and geomagnetic activity level.Item Open Access Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F(American Geophysical Union, 1999-09-01) Fejer, B. G.; Scherliess, L.; De Paula, E. R.We use radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F. The dependence of these irregularities on season, solar cycle, and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts. In the early night sector, the bottomside of the F layer is almost always unstable. The evolution of the unstable layer is controlled by the history of the vertical drift velocity. When the drift velocities are large enough, the necessary seeding mechanisms for the generation of strong spread F always appear to be present. The threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux. The geomagnetic control on the generation of spread F is season, solar cycle, and longitude dependent. These effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields. The occurrence of early night spread F decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset. The generation of late night spread F requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour. These irregularities occur most often at ∼0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes. The occurrence of late night spread F is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux.Item Restricted Ensemble Modeling with Data Assimilation Models: A New Strategy for Space Weather Specifications, Forecasts, and Science(American Geophysical Union, 2014-02-23) Schunk, R. W.; Scherliess, L.; Eccles, V.; Gardner, L. C.; Sojka, J. J.; Zhu, L.; Pi, X.; Mannucci, A. J.; Wilson, B. D.; Komjathy, A.; Wang, C.; Rosen, G.The Earth’s Ionosphere-Thermosphere-Electrodynamics (I-T-E) system varies markedly on a range of spatial and temporal scales and these variations have adverse effects on human operations and systems, including high-frequency communications, over-the-horizon radars, and survey and navigation systems that use Global Positioning System (GPS) satellites. Consequently, there is a need to elucidate the underlying physical processes that lead to space weather disturbances and to both mitigate and forecast near-Earth space weather. The meteorologists and oceanographers have shown that data assimilation models are superior to global physics-based models for specifications and forecasts, but only during the last 15 years have they been used for near-Earth investigations as more global (space and ground-based) measurements became available. Although data assimilation models produce better specifications and forecasts than global physicsbased models, there is still a spread in results for a given simulation scenario when different data assimilation models are used. This spread occurs because the different data assimilation models use different data types, data amounts, assimilation techniques, and background physics-based models. This data assimilation issue is being addressed with the launching of the “NASA/NSF Space Weather Modeling Collaboration” program. Currently, our team has seven physics-based data assimilation models for the ionosphere, plasmasphere, thermosphere, and electrodynamics. These models assimilate a myriad of different ground- and space-based observations, and there is more than one data assimilation model for each near-Earth space domain. These data assimilation models are being used to create a Multimodel Ensemble Prediction System (MEPS), which will allow ensemble modeling of the I-T-E system with different data assimilation models that are based on different physical assumptions, assimilation techniques, and initial conditions. The application of ensemble modeling with several different data assimilation models will lead to a paradigm shift in how basic physical processes are studied in near-Earth space, and it is expected to lead to a significant advance in space weather specifications and forecasts.Item Restricted On the variability of equatorial F-region vertical plasma drifts(Elsevier, 2001) Fejer, B. G.; Scherliess, L.We use incoherent scatter radar measurements from the Jicamarca Observatory to study the variability of equatorial F-region vertical plasma drifts. The daytime average upward drifts do not vary much with solar activity, but the evening upward and the nighttime downward drifts increase from solar minimum to solar maximum. Our data indicate that the quiet-time variability of the Jicamarca vertical drifts is local time, seasonal, and solar cycle dependent. This variability is largest in the dawn–noon sector and during March equinox solar minimum periods, when the midday average upward drift velocity from consecutive magnetically quiet days can often change by more than 10 m=s. The day-to-day variability of the vertical drifts decreases in the afternoon sector and with the increase of solar activity for all seasons. There are several possible processes responsible for the quiet-time plasma drift variabilityItem Open Access Radar and satellite global equatorial F region vertical drift model(American Geophysical Union, 1999-04-01) Scherliess, L.; Fejer, B. G.We present the first global empirical model for the quiet time F region equatorial vertical drifts based on combined incoherent scatter radar observations at Jicamarca and Ion Drift Meter observations on board the Atmospheric Explorer E satellite. This analytical model, based on products of cubic-B splines and with nearly conservative electric fields, describes the diurnal and seasonal variations of the equatorial vertical drifts for a continuous range of all longitudes and solar flux values. Our results indicate that during solar minimum, the evening prereversal velocity enhancement exhibits only small longitudinal variations during equinox with amplitudes of about 15–20 m/s, is observed only in the American sector during December solstice with amplitudes of about 5–10 m/s, and is absent at all longitudes during June solstice. The solar minimum evening reversal times are fairly independent of longitude except during December solstice. During solar maximum, the evening upward vertical drifts and reversal times exhibit large longitudinal variations, particularly during the solstices. In this case, for a solar flux index of 180, the June solstice evening peak drifts maximize in the Pacific region with drift amplitudes of up to 35 m/s, whereas the December solstice velocities maximize in the American sector with comparable magnitudes. The equinoctial peak velocities vary between about 35 and 45 m/s. The morning reversal times and the daytime drifts exhibit only small variations with the phase of the solar cycle. The daytime drifts have largest amplitudes between about 0900 and 1100 LT with typical values of 25–30 m/s. We also show that our model results are in good agreement with other equatorial ground-based observations over India, Brazil, and Kwajalein.Item Open Access Storm time dependence of equatorial disturbance dynamo zonal electric fields(American Geophysical Union, 1997-11-01) Scherliess, L.; Fejer, B. G.We use Jicamarca radar observations of F region vertical plasma drifts and auroral electrojet indices during 1968–1988 to study the characteristics and temporal evolution of equatorial disturbance dynamo zonal electric fields. These electric fields result from the dynamo action of storm time winds and/or thermospheric composition changes driven by enhanced energy deposition into the high-latitude ionosphere during geomagnetically active conditions. The equatorial vertical drift perturbations last for periods of up to 30 hours after large increases in the high-latitude currents. On the average, this process can be described by two basic components with time delays of about 1-12 hours and 22–28 hours between the high-latitude current enhancements and the equatorial velocity perturbations. Our data indicate strong coupling between dynamo processes with different timescales. The short-term disturbance dynamo drives upward equatorial drifts (eastward electric fields) at night with largest amplitudes near sunrise and small downward drifts during the day. These perturbation drifts are in good agreement with results from the Blanc-Richmond disturbance dynamo theory. The dynamo process with time delays of about a day drives upward drift velocities at night with largest values near midnight and downward drifts in the sunrise-noon sector. In this case, the amplitudes of the disturbance drifts maximize during geomagnetically quiet times preceded by strongly disturbed conditions. We also present results of a new equatorial storm time dependent empirical model which illustrate the characteristics of the vertical disturbance dynamo drifts.Item Restricted Systematic evaluation of ionosphere/thermosphere (UT) models: CEDAR Electrodynamics Thermosphere Ionosphere (ETI) Challenge (2009–2010)(American Geophysical Union, 2014-03) Shim, J. S.; Kuznetsova, M.; Rastätter, L.; Bilitza, D.; Butala, M.; Codrescu, M.; Emery, B. A.; Foster, B.; Fuller‐Rowell, T. J.; Huba, J.; Mannucci, A. J.; Pi, X.; Ridley, A.; Scherliess, L.; Schunk, R. W; Sojka, J. J.; Stephens, P.; Thompson, D. C.; Weimer, D.; Zhu, L.; Anderson, D.; Chau Chong Shing, Jorge Luis; Sutton, E.In order to model and predict the weather of the near‐Earth space environment, it is necessary to understand the important coupling mechanisms from the surface of the Sun to the Earth's ionosphere, including its coupling with the atmosphere below. This chapter reports the simulations of the mid‐latitude to low‐latitude ionosphere. Multiday simulations during the Whole Heliosphere Interval (WHI) 2008 are performed using two versions of SAMI3 model: (1) SAMI3 with externally specified E X B drifts and (2) SAMI3 with a potential solver to self‐consistently specify electric fields. The results are compared with GPS‐derived global total electron content (TEC) maps. The chapter details the E X B drifts calculated by the self‐consistent SAMI3 and compares these results with an empirical model. It provides initial results for a multi‐year run of the descending phase of Solar Cycle 23 to illustrate the broader range of Integrated Sun‐Earth System (ISES) activity underway