SA33A-1127 1340h
Collisional shear instability and equatorial spread {\em F}
This paper analyzes the possible role of shear flow in initiating equatorial spread {\em F} events. A new 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 waves with wavelengths close to 4$\pi$ times the shear scale length could become comparable to that of the collisional interchange instability in the equatorial {\em F} region ionosphere under ideal conditions. An initial value simulation of a nonlinear model of the instability run under realistic conditions meanwhile produces growing waves with an initial wavelength of about 30 km. The simulation results are consistent with recent Jicamarca and Altair radar observations indicating large-scale plasma waves in the bottomside equatorial ionosphere at sunset prior to the onset of spread {\em F} conditions.
SA33A-1128 1340h
ROCSAT-1 Observations of Quiet-time Low-latitude Plasma Bubbles
Plasma bubbles observed by the IPEI payload of the ROCSAT-1 satellite during the solar maximum years of 2000 and 2001 are used to study the statistical features under quiet geomagnetic conditions ($Kp < 3$). We examine the occurrence probability and morphology of the bubbles, which are found to depend on local time, season, and magnetic field configuration (longitude). During the solstices the longitudinal variations of bubble structures were fairly pronounced. In particular, during the northern hemisphere winter, the bubbles were mainly found in the South Atlantic Anomaly (SAA) longitude sector ($300^o-360^o$E), where the geomagnetic declinations are mostly negative. Such longitudinal preference of bubble events can be explained if we consider the effects of the neutral-wind dynamo electric fields and the horizontal density gradients (zonal and meridional) on the growth rate of the generalized Rayleigh-Taylor instability.
SA33A-1129 1340h
Global Distribution of the Equatorial Plasma Bubbles in the Premidnight Sector During Solar Maximum as Observed by KOMPSAT-1
We investigated the global distribution of the equatorial plasma bubbles (EPBs) using the in-situ ion density measurements from Korea Multi-Purpose Satellite-1 (KOMPSAT-1) at the altitude of 680 km at 2230 LT during the solar maximum period from June 2000 to August 2001. EPBs were observed at all longitudes around the magnetic dip equator in the equinoctial seasons with the peak occurrence in the American-Atlantic-African regions. During the northern summer the EPBs occurred predominantly in the magnetic north in the Indian and west Pacific regions, whereas they are totally suppressed in the American-Atlantic sectors. During the northern winter the EPBs are highly promoted in the America-Atlantic sectors but are suppressed in other longitude sectors, especially, in the Pacific sector. We investigated the growth conditions of the Rayleigh-Taylor instability by using the model atmosphere (MSIS) and ionosphere (IRI). The simulation results show that the global EPB distribution can be explained by using the upward plasma drift velocity at post-sunset time weighted by the flux-tube integrated electron content in the lower F region.
SA33A-1130 1340h
Multi-instrument Observations of the Development of the Equatorial Ionization Anomaly and Links to Scintillation
During the geomagnetically quiet month of March 2002, scintillations in the Pacific sector (Manila, Singapore and Guam) were observed on most nights. There are several nights, however, where scintillations were not observed at one or more of these locations. In this study we investigate the development of the Equatorial Ionization Anomaly (EIA) in the Pacific sector and relate the results back to the occurrence/non-occurrence of scintillations at these longitudes. Space-based and ground-based measurements of the electron densities are used in order to obtain an understanding of the large-scale structure of the low-latitude ionosphere during the early evening hours when the conditions for scintillations are set up. The study includes TEC measurements from TOPEX and GPS ground stations, in situ electron density measurements from the DMSP satellites, and electron densities derived from UV line-of-sight radiances from both TIMED-GUVI, and the LORAAS instrument aboard the ARGOS satellite. The Global Assimilation of Ionospheric Measurements (GAIM) model is used to assimilate these measurements into a global model of the ionosphere for the days of interest. The model provides estimates of the ionospheric specification and is used to fill in the gaps in longitude and time that are not covered by the measurement data. We present both measurement and model data for several days in March 2002 and discuss factors that may modulate the development of scintillation.
SA33A-1131 1340h
Three-Dimensional Evolution of the Equatorial Ionospheric Rayleigh-Taylor Instability in Combined Vertical and Zonal Drifts
Recently much interest has been focussed on the onset and evolution of equatorial F-region ionospheric irregularities both for quiet and stormtime periods. The overall growth rate of the Rayleigh-Taylor instability in the equatorial ionosphere is dependent on several factors, e.g., vertical density gradient, E-region conductivity, parallel conductivity, and thermospheric winds. We have developed a three-dimensional nonlinear simulation code to treat the evolution of the Rayleigh-Taylor instability in equatorial spread-F. The code contains ion inertial effects to properly treat the high altitude evolution of spread-F plumes. In this paper we use this code to study the effects of combined time-dependent vertical and zonal drifts on the 3D evolution of the Rayleigh-Taylor instability. We compare the code predictions with ground-based observations.
SA33A-1132 1340h
Theoretical Plasma Density Distribution at low Latitudes: Comparison of Neutral Wind Models
Theoretical calculations of electron density distribution in the nighttime equatorial ionosphere are presented. The calculations are done by assimilating plasma drift velocity measured by Jicamarca incoherent scatter radar into the low-latitude ambient plasma density model (PBMod), which has been developed at the Air Force Research Laboratory (AFRL) in support of the Communication/Navigation Outage Forecasting System (C/NOFS) mission. Specifically, the F region average values of the drift measurements for fourteen days in 2002 (April 15 - 17, May 31 - June 1, June 3 - 4, October 8 - 10, and November 11 - 14), representing both equinox and solstice conditions, are used in the calculations. The horizontal components of the neutral wind velocity, which are important input parameters for PBMod, are specified by any of the available empirical models, such as the Hedin (HWM) model and the Vector Spherical Harmonics (VSH) model. The results obtained from the two neutral wind models are compared and contrasted in order to illustrate the influence of neutral wind model on the electron density distribution. Particular emphasis is given on the altitude profiles of electron density (EDP) and the latitudinal profiles of the total electron content (TEC) as a function of time. The calculated latitudinal profiles of TEC are then compared with the profiles obtained from the TEC's measured by a chain of GPS receivers, in order to examine the relative performance of the two neutral wind models.
SA33A-1133 1340h
The nighttime F-region climatology during magnetically quiet periods seen from TIMED/GUVI and DMSP
In this study, we investigate the drivers of the nighttime climatology in the low- and middle-latitude ionosphere during quiet periods using the TIMED/GUVI and DMSP data. The observation results show that the seasonal hemispheric asymmetry in plasma density is primary induced by the summer-to-winter wind circulations while the longitudinal variations of the F-region morphology is produced by the contribution of the zonal winds that depends on the magnetic declination. However, the F-region morphology observed at 625 km from TIMED/GUVI does not precisely conform the morphology observed at 840 km from DMSP. We will discuss the differential neutral wind effect on the F-region morphology depending on the magnetic declination, altitude, and the location of the geomagnetic equator relative to the geographic equator.
SA33A-1134 1340h
Studies in Equatorial Anomaly Morphology from 2002-03 GUVI Data
The Global Ultraviolet Imager on NASA's TIMED spacecraft is currently mapping ionospheric radiation at six different wavelengths across the earth. Images created using the optically thin OI 135.6 nm line that is excited by nighttime recombination of O+ ions contain clear signatures of the equatorial anomaly (EA) as well as equatorial plasma bubble (EPB) structures. These intensity observations are related to the squared plasma density and thus provide insight into plasma transport. Extensive results from studies in quiet time EA morphology using GUVI data from 2002-03 are presented. Noted features include the strength of the anomaly, crest position, and EPB occurrence rates with dependencies on the zonal sector, local time, season, and solar cycle. The analysis is then extended to storm dependent morphology.
SA33A-1135 1340h
Variations of the Postsunset and Presunrise Enhancements at 600 km Topside Ionosphere from 1999 to 2003
The quiettime diurnal variations of the vertical ion drifts measured by ROCSAT-1 at 600 km equatorial topside ionosphere have been studied for the seasonal and solar flux effects during the moderate to high solar activity years from 1999 to 2003. The vertical drift velocity at 600 km is somewhat larger in magnitude than what has been observed by ground radars but indicates similar diurnal variation pattern of the vertical drifts in the global equatorial vertical drift model of Scherliess and Fejer [1999]. The drift velocity also indicates little dependence on the solar flux intensity during the solstice seasons and some small dependence during the equinox seasons. The well-known feature of the postsunset enhancement which is observed more frequently during equinoxes than during solstices is also noticed for the vertical drifts at 600 km. In addition, ROCSAT observes an outstanding feature of the presunrise enhancement of the downward drift when the solar flux intensity is lower than 140. This prominent presunrise enhancement has been known to exist in many model simulations but was seldom reported in past experimental observations except from the Atmospheric Explorer E [Fejer et al., 1995] over some longitude regions. The statistical occurrence pattern for the presunrise enhancement indicates that it is enhanced during solstices of high solar activities than during equinoxes, which is opposite to the occurrence pattern of the postsunset enhancement. This opposite seasonal occurrence pattern can be understood from the movements of the sunrise and the sunset terminator in local times during equinoxes and solstices, respectively, in relation to the location of the neutral wind reversal around the solar terminator.
SA33A-1136 1340h
The correlation between Solar EUV Flux and Plasma Characteristics in the Low-Latitude Topside Ionosphere at Nighttime
We present the results of the correlation between solar EUV flux and ionospheric plasma parameters observed by KOMPSAT-1(685Km altitude, 2250 local time) and DMSP F-15(840Km altitude, 2130 local time) in the low-latitude topside ionosphere. The plasma density and temperature observed by KOMPSAT-1 and DMSP F-15 from June 28, 2000 to August 1, 2001 were analyzed with F10.7 which is a standard proxy for solar EUV flux. Especially, the data of the geomagnetic latitude between -60>r"- and 60>r"- in the nighttime were considered at the low- and mid-latitude region. From the result, it was shown that the plasma density and temperature at local maximum density region in the geomagnetic equator were increased with F10.7. Furthermore, the best correlation between F10.7 with a lag of 2 days and plasma parameters was observed. It means that the ionospheric equatorial plasma is influenced by the solar EUV flux with the delay of 2 days.
SA33A-1137 1340h
An Examination of Space Weather Maps of the Low-Latitude Electron Density During the Halloween Storm
The electron distribution changed significantly during the geomagnetic storm from October 28 through November 1, 2003. These changes are particularly noticeable in the equatorial regions. This presentation demonstrates these changes by examining synoptic scale space weather maps of the low-latitude electron density during the Halloween storm. Using the Ionospheric Data Assimilation Three Dimensional (IDA3D), a 3DVAR assimilation algorithm, available electron density ({\it e.g. in situ} satellite and ionosonde nmF2 ) and electron content ({\it e.g.} slant TEC from GPS ground stations) are combined into statistically minimized electron density maps of the synoptic-scale density distribution. These maps demonstrate a poleward spreading of the equatorial anomaly peaks and a reduction of the high-altitude and plasmaspheric electron densities from climatological values.
SA33A-1138 1340h
Neutral composition effects on negative ionospheric storms at middle and low latitudes: Polar ultraviolet imager observations
The April 17-24 geomagnetic storm event is studied to understand the relationship between neutral composition (specifically, atomic oxygen to molecular nitrogen column density ratios, O/N2) and total electron content (TEC) on an instantaneous global scale on dayside. The O/N2 column density ratio is derived from the dayglow emissions of OI 135.6 nm and N2 Lyman-Birge-Hopfield (160-180 nm) acquired from the ultraviolet imager (UVI) aboard the Polar satellite and the total electron content is derived from the phase delays of dual-band global positioning satellite (GPS) accumulated around the globe. It is found that the regions of decreasing O/N2 generally coincided with the regions of depleted TEC during and the after the development of the storm. This is consistent with previous theoretical and experimental analysis in which composition changes play a major role in the negative ionospheric storm effects. At lower latitudes there was no noticeable change in O/N2. The TEC data also showed no noticeable change, except a few "short-lived," localized positive TEC effects. Long duration positive storm effects, frequently predicted by general circulation models, are not observed. Detailed results will be presented and their implication will be discussed.
SA33A-1139 1340h
Extraordinarily High Electron Densities Observed in the Crest of the Post-sunset Equatorial Anomaly, Their Persistence with Solar Rotation and the Evidence of an Enhancement of Maximum Pre-reversal ExB Drift Velocity
Extremely high F layer electron densities, possibly the highest ever recorded, were observed in the crests of latitudinal profiles of maximum electron density in the post sunset equatorial anomaly. A chain of ionospheric sounders located in western America recorded values of NmF2 of 6.9, 6.5 and $>$6.5 x10$^{6}$ el/cm$^{3}$ on the 3 days, Feb 14, Mar 13 and Apr 11, 1958. These days were 27 and 29 days apart, so were nearly the same day in 3 successive Carrington solar rotations. The levels indicate extremely high levels of maximum pre-reversal ExB drift velocity (ExBmax) as does the fact that equatorial bubbles were observed on 2 of the days and a bubble was likely to have occurred on the third. However, conditions which are associated with high ExBmax, low Kp and high F10.7, were present, but not at levels that were unusual in relation to the other days in the period. Dst and daytime NmE are also ordinary on these days. Each of the 3 days lies in a period of IMF "away" and possibly near a sector boundary as indicated by high latitude magnetograms. Proximity to a boundary is also not unique to these 3 days, but the recurrence with solar rotation suggests that sector structure may play a role. In addition to the remarkable recurrence in solar rotation, the significance of these observations is that they indicate the presence of some not yet understood mechanism that enhances the eastward electric field, hence ExBmax. Its presence at other times could contribute to the largely unexplained variability of ExBmax, of equatorial bubbles and of the resulting scintillation, so that its understanding could lead to much improved forecasts.
SA33A-1140 1340h
GPS Occulation Observations of Equatorial Scintillation: Dependence on Magnetic Field Orientation, Longitude, and Season
We analyzed GPS occultation data from the CHAMP, SAC-C, and PICOSat satellite for the entire year 2002 identifying radiowave scintillation occurrence from SNR measurements of the C/A code on the L1 frequency obtained at the 1-second rate cadence. Global distributions clearly indicate that we are observing equatorial scintillation and scintillation in the auroral zones and polar cap. Seasonal and magnetic local time distributions of the low-latitude observations are in good agreement with the known distributions of equatorial scintillation. Longitudinal distributions vary somewhat from the WBMOD climatological model, particularly in the African sector where scintillation is observed nearly all year. A strong dependence on the orientation of the occultation ray path with the magnetic field orientation is observed with a low probability of scintillation at ray path angles perpendicular to the magnetic field and high probability of observations at smaller angles. This is interpreted as the result of the orientation of the ionospheric bubbles responsible for the scintillation. The walls of the bubbles, on which the instabilities that cause the scintillation occur, are typically aligned with the magnetic field. Thus, occultation ray paths along the magnetic field pass along the edge of the bubbles and remain within the region of instabilities for a longer period that ray paths perpendicular to the magnetic field and the bubble walls.
SA33A-1141 1340h
A study of low latitude electron densities using CMIT
Recently, several coupled models of the magnetosphere-ionosphere-thermosphere system have been developed. Although they have been applied in a general sense to study aspects of coupling in an idealized sense, there is a need for them to be applied to specific events so that their predictions can be tested and understood in the context of the observed ionosphere. The coupled thermosphere-ionosphere magnetosphere (CMIT) model is used in this study to simulate the behavior of the low latitude thermosphere and ionosphere during one such event. The behavior of a number of parameters that effect electron density are studied and an analysis of the evolution of the physics that affects electron densities has been undertaken. Amongst the results of this study is the observation that the relative importance of the various physical terms evolves during the storm.
SA33A-1142 1340h
Comparison Between GAIM and LLIONS in the Jicamarca Low Latitude Sector During the First CAWSES Space Weather Campaign
The Utah State University (USU) Global Assimilation of Ionospheric Measurements (GAIM) Model has been compared to the Space Environment Corporation (SEC) Low Latitude IONospheric Sector (LLIONS) model for the period 20 March 2004 through 14 April 2004. This period includes the first CAWSES space weather period. The objective of this comparison is to assess how the data assimilation in the GAIM Kalman filter is able to reorganize the low-latitude ionospheric morphology, specifically the locations and intensities of the Equatorial Anomaly peaks. The LLIONS approach uses observations in the Jicamarca sector to modify the electric field driver, which is the dominant transport mechanism for the equatorial ionosphere. In this study, the observations from magnetometers and digisondes are only able to modify the vertical ExB drift velocities during sunlit conditions. Hence, the primary comparisons are for daytime low latitudes along the Jicamarca longitude sector. At night, only the GAIM assimilation reveals the marked structuring of the equatorial ionosphere. Results are presented contrasting a range of geomagnetic quiet-to-disturbed conditions. These comparisons include not only TEC, but also F-region electron density profiles as a function of latitude and local time.
SA33A-1143 1340h
Validation of the USU GAIM Data Assimilation Model of the Ionosphere
Physics-based data assimilation models of the ionosphere were developed at Utah State University as the central part of a DoD MURI funded program called GAIM (Global Assimilation of Ionospheric Measurements). Recently, the Air Force Weather Agency (AFWA) has selected one of the USU GAIM models for its operational use and the same model will also be implemented at the Community Coordinated Modeling Center (CCMC) for scientific studies. The selected model is based on a physics-based model of the ionosphere and a Gauss-Markov Kalman Filter (GMKF) as a basis for assimilating a diverse set of real-time (or near real-time) observations. The physics-based model is the Ionospheric Forecast Model (IFM), which accounts of five ion species and covers the E-region, F-region and the topside from 90 to 1400 km altitude. Within the GMKF, the IFM derived ionospheric densities constitute a background density field on which perturbations are superimposed based on the available data and their errors. In the current configuration the GMKF assimilates slant TEC from a variable number of ground GPS sites, bottom-side N$_e$ profiles from a variable number of ionosondes, in situ N$_e$ from four DMSP satellites, and nighttime line-of-sight UV radiances measured by satellites. To test the GMKF for real-time operations and to validate its ionospheric density specifications, three month-long validation periods covering a variety of different geophysical conditions have been identified. During these three periods the model ran continuously and automatically and produced 3-dimensional global electron density distributions in 15 minute increments. The results of this validation study, with an emphasis on a comparison with independent observations, will be presented.
SA33A-1144 1340h
Data Availability and its Effect on the USU GAIM Data Assimilation Model
Physics-based data assimilation models of the ionosphere were developed at Utah State University as part of a DoD Multidisciplinary University Research Initiative (MURI) program. The USU effort was called Global Assimilation of Ionospheric Measurements (GAIM). The USU GAIM Gauss-Markov Kalman Filter (GMFK) uses a physics-based model of the ionosphere and a Kalman filter as a basis for assimilating a diverse set of measurements; in either near real time or historical study modes. The physics-based model is the Ionosphere Forecast Model (IFM), which is global and covers the E-region, F-region, and topside from 90 to 1400km. It takes account of five ion species (NO$^{+}$, O$_{2}$$^{+}$, N$_{2}$$^{+}$, O$^{+}$, H$^{+}$). The Gauss-Markov filter assimilates bottom-side electron density profiles from a variable number of ionosondes, slant TEC from a variable number of GPS satellite/ground station combinations, in-situ electron density from DMSP satellites, and line-of-sight UV radiances from satellite-based instruments. With the GMFK model the ionospheric densities obtained from IFM are used as a background upon which perturbations are imposed based on the available data and their errors. The density perturbations and associated errors evolve over time via a statistical Gauss-Markov process. The fidelity of the GMFK result is dependent on the quality and quantity of data available to specify the perturbations. We will compare results from the GMFK using various combinations of data types and data quantities.
SA33A-1145 1340h
USU GAIM: Validation of the Ionospheric Forecasting Model (IFM) Using the TOPEX TEC Measurements
As a part of the validation program in the USU GAIM project, a newly improved Ionospheric Forecasting Model (IFM) was systematically validated by using a large database of the TOPEX TEC measurements. The TOPEX data used for the validation is for the period from August 1992 to March 2003 and the total number of 18-second averaged data is close to 11 million. This model validation work covers a wide range of seasonal (winter, summer, equinox) and solar (low, median, and high F10.7) conditions as well as all UT variations. The validation results indicate that the features of the spatial distrubution of the IFM TEC are systematically consistent to those of the TOPEX TEC. The differences between the IFM TEC and the TOPEX are within 20% for almost all locations and conditions. In many conditions, the differences are even below 10%. This validation work further proves the validity of the IFM for the ionospheric assimilation in the USU GAIM project.