SA21A-1516
Coherent scatter radar observations of 150-km echoes and vertical plasma drifts in the Brazilian sector
Coherent scatter echoes coming from the valley region (~150 km altitude) in the equatorial ionosphere during daytime have been detected by the Jicamarca radar in Peru for several decades (Basley, 1945). More recently, it was found that the vertical Doppler shift of these echoes corresponds to the vertical velocity of the F-region background plasma (Kudeki and Fawcett, 1993; Woodman and Villanueva,1995, Chau and Woodman, 2004). Jicamarca now uses observations of 150-km echoes to provide estimates of the diurnal variation of the equatorial vertical plasma drifts in addition to traditional incoherent scatter radar drift measurements. These 150-km echoes have also been observed in other longitude sectors (e.g. Tsunoda and Ecklund,2004; Patra et al., 2008). Additionally, these echoes have also been detected in a semi-routine basis with a small, low-power radar in Sao Luis, Brazil. Initial results of our analysis suggest that vertical plasma drifts can be estimated from these observations. These measurements combined with simultaneous measurements made by the Jicamarca radar and the C/NOFS satellite can help us better understand the day-to-day variability and longitudinal variation of equatorial electric fields. In this talk we will present examples of 150-km echoes observations made with the Sao Luis radar. We will describe how vertical drifts can be estimated from the observations and how the vertical drifts over Sao Luis compare with the drifts measured simultaneously at Jicamarca. These new measurements can provide important new information about the low-latitude electrodynamics, and consequently to the C/NOFS mission.
SA21A-1517
The Sao Luis Radar Plumes Characteristics as a Function of Solar Flux
The Sao Luis (2.33 S, 44 W, dip latitute 1.3 S) 30 MHz coherent scatter radar has been in operation since December 2000 sounding the equatorial electrojet, the 150-km echoes and the F region 5 m irregularities. In this work, we present a statistical study of the F region plume parameters as a function of the solar flux. Such parameters are plume minimum and maximum heights, size, duration, time of triggering, altitudinal bottom-type layer gradient and number of plumes. In many nights quasi-periodic plumes were observed and during minimum solar activity the plumes rarely overpass the altitude of 600 km. This statistics is important to give ground support and validation for the C/NOFS satellite mission (launched on April 2008) during the solar minimum activity. The Sao Luis radar has also been measuring equatorial vertical drifts which controls the altitude of plasma ascending to the topside. The electric field estimates inferred from the vertical drifts can also provide support for the C/NOFS mission.
SA21A-1518
Disruption of Equatorial Spread-F Leading to a Fossil Bubble and Airglow Enhancement
The NRL SAMI3/ESF ionosphere model[1] is used to show that, when equatorial spread-F (ESF) occurs in the presence of a steady, mild (20 m/s) meridional wind, the electron density in downwind "leg" of the geomagnetic field-aligned ESF bubble can exceed the background density. Further, a mild (10 m/s) converging zonal wind pattern can disrupt the upward motion of an ESF bubble, thereby creating a "fossil" ESF bubble. The fossil bubble occurs, not because of a lack of buoyancy as previously thought, but because the driving mechanism of the ESF instability has been interrupted. The cessation of ESF-related plasma uplift allows the downwind leg of the bubble to fill in, further enhancing the density in the downwind leg of the newly-fossilized bubble. A sequence of 630.0 nm images obtained with the Boston University all-sky imaging system at Arecibo (18.3 N, 66.7 W, 28 N mag) show ESF-related airglow depletions changing their pattern and becoming airglow enhancements. The results from the NRL SAMI3/ESF model provide an explanation for this enhancement as well as satellite observations of ESF-related ion density enhancements. [1] Huba, J.D., G. Joyce, and J. Krall, GRL, 35, L10102, doi:10.1029/2008GL033509, 2008
SA21A-1519
Radar and Optical Measurements of Equatorial Plasma Depletions
The primary focus of the recently launched Air Force Communication/Navigation Outage Forecasting System (C/NOFS) satellite is to quantify and forecast ionospheric irregularities responsible for the development of equatorial spread F that can severely affect communication and navigation systems. In support of this mission goal, we present new measurements and analyses using previously obtained ground-based radar and optical measurements from two important equatorial sites. Using available data from the Jicamarca JULIA and incoherent scatter radar observations from 1996 to 2006, we have studied the initial development of equatorial spread F from Peru (11.95°S, 76.87°W) over a full 11 year solar cycle. Detailed analysis show that onset heights and peak heights of radar plumes increase with increase in solar activity, as previously suggested from case studies. In contrast, investigations of spread F onset times show a little variation with solar activity, while onset times of radar plumes decrease from solar minimum to moderate conditions and then remain nearly constant during solar maximum. In addition to this study, we have also made novel investigations of spread F optical signatures (termed depletions) from Christmas Island (2°N, 157.4°W) in central Pacific region using CCD image measurements of the thermospheric OI 630 nm airglow emission. The measurements were obtained using a USU all-sky imager from Sep.14 to Oct 2, 1995, under solar minimum conditions similar to current levels that C/NOFS is measuring. We have analyzed the zonal velocities of the plasma depletions and their horizontal scale sizes. Large variations in the day-to-day spatial characteristics and the zonal velocities were found. We have proposed new measurements from equatorial Brazil in coordination with C/NOFS to investigate bubble dynamics and associated atmospheric conditions.
SA21A-1520
Ground-based optical observations in conjunction with the C/NOFS satellite
Ground-based low-light optical observations are being conducted in conjunction with AFRL's C/NOFS satellite from a number of equatorial locations including Kwajalein Atoll and Christmas Island in the Pacific Ocean. These observations provide critical contextual information on ionospheric structure during and between C/NOFS overflights, in addition to coverage at latitudes and altitudes not sampled by the instruments on board the spacecraft. Some of the most pronounced optical signatures appear to result from large-scale regional lowering of the F-region, which results in a drastic regional collapse of F-region densities as measured by the C/NOFS Planar Langmuir Probe instrument over several successive orbits. Many of the plumes apparent in the optics are associated with only weak scintillation effects, even at VHF frequencies, as expected at solar minimum. We will also present observations from a dedicated C/NOFS ground measurement campaign making use of the ALTAIR radar in Kwajalein.
SA21A-1521
Comparisons of Space-based GPS Occultation Ionospheric Scintillation Measurements with Ground-based VHF Measurements
Ionospheric irregularities are known to cause scintillation of transionospheric radio signals and can affect space-based UHF/VHF communications, causing outages, and degrade GPS accuracy and precision. Current capability for characterizing and predicting ionospheric scintillation utilizes a network of ground- based receivers to detect scintillation and then extrapolate for short-term forecasts. Practical limits on deploying the ground receivers limits the accuracy and spatial coverage one can achieve with this approach. A more global approach is to use a set of space-based satellites equipped with GPS receivers, such as the COSMIC satellite constellation, to measure scintillations observed during so-called occultations with GPS satellites. The term occultation refers to the geometry where the clear line-of-sight path between the space- based GPS receiver and the GPS satellite is ultimately blocked, or occulted, by the earth's surface. Before or after occultation the ray-path passes through the lower atmosphere and ionosphere providing information on the total electron content (TEC) and irregularities between the transmitter and the receiver. In this paper the signal-to-noise values of GPS L1 signals received on the COSMIC (and possibly C/NOFS if available) satellites are examined to help identify areas of ionospheric scintillation. The S4 scintillation index values from these occultations are compared with ground-based VHF S4 scintillation measurements from several equatorial stations. Preliminary results show that while there are cases where both the occultation and ground measurements indicate enhanced scintillation, there are also a number of cases where the occultation GPS S4 is significantly larger than the ground-based VHF S4, somewhat contrary to expectations given that scintillation effects generally increase with decreasing frequency. Reasons for high GPS S4 in the presence of relatively low VHF S4 include geometry differences between space- and ground-based observations, possible signal processing problems, and physical non-scintillation ionospheric features characterized by relatively short intense spikes in the signal-to-noise. We examine these parameters and look at developing algorithms to filter out non-related cases so that we develop an improved correlation between the space-based and ground-based scintillation values.
SA21A-1522
Equatorial emissions observed by DEMETER during magnetic activity
During the recovery phase of magnetic storms energetic ions are injected in the equatorial ring current and can trigger new emissions. This paper is related to observations of such waves recorded by the satellite DEMETER at altitudes as low as 700 km. Several examples of these typical electromagnetic emissions with frequency striations observed right around the magnetic equator will be presented. They have a low cut-off frequency close to the local proton gyrofrequency and their upper frequencies can extend up to 800 Hz. For some events the instruments are operated in a burst mode where waveforms of the 6 components of the electromagnetic field are available and a wave propagation analysis is performed to determine the characteristics of these emissions.
SA21A-1523
Initial Observations of the Magnetic Field Measurements on the C/NOFS Satellite
The Vector Electric Field Instrument (VEFI) suite onboard the C/NOFS spacecraft includes a sensitive fluxgate magnetometer to measure DC and ULF magnetic fields in the low latitude ionosphere. The instrument includes a DC vector measurement at 1 sample/sec with a range of ± 45,000 nT whose primary objective is to enable a V×B measurement that is more accurate than that provided by using a magnetic field model. These data will also be used to provide signatures of large-scale ionospheric current systems, which, when analyzed in conjunction with the C/NOFS DC electric field measurements, promise to advance our understanding of equatorial electrodynamics. The instrument also includes an AC-coupled vector measurement in the 0.05 - 8 Hz frequency range at 16 samples/sec with an output range of ± 900 nT in order to measure small-scale current filaments and possible Alfven waves associated with plasma irregularities. We compare the Earth's magnetic field models such as the most recently updated IGRF (the International Geomagnetic Reference Field) model and the POMME (the POtsdam Magnetic Model of the Earth) model with the measurements in order to provide an in-flight "calibration" of the data as well as compute magnetic field differences to reveal large scale ionospheric currents. Our initial results show that, on average, the POMME model accurately reproduces the C/NOFS-measured magnetic field within 20 nT in magnitude and within 0.1 deg in field direction everywhere in the low latitude ionosphere except in the region of the South Atlantic Anomaly. Initial results of the C/NOFS magnetic field measurements will be shown.
SA21A-1524
Modeling the Eastward Equatorial Electric Field Using CHAMP Magnetometer Data
The day-time eastward equatorial electric field (EEF) in the E-region plays an important role in equatorial ionospheric dynamics. It is responsible for driving the equatorial electrojet (EEJ) current system, equatorial vertical ion drifts, and the equatorial ionization anomaly (EIA). Due to its importance, there is much interest in accurately measuring and modeling the EEF. However, there is a severe lack of high quality data with the notable exception being the JULIA coherent scatter radar in Peru. In this work we use CHAMP satellite-derived latitudinal current profiles of the day-time EEJ in order to estimate the eastward electric field at all longitudes, seasons, and day-side local times. We have constructed a dataset of over 36,000 individual EEF estimates based on six years of CHAMP data. This data was used to construct a climatological model of the EEF mean as a function of longitude, season, local-time and solar flux level. Furthermore, we have created a model of the day-to-day variability of the EEF as a function of the same parameters.
SA21A-1525
Magnetic field measurements in the Equatorial Zone
The CHAMP magnetometer system is state-of-the-art and has already been used to study the diamagnetic effect in the Appleton Anomaly, field aligned currents due to spread F and to deduce daytime equatorial electric fields driving the equatorial electrojet (EEJ). Here we take the first look at the C/NOFS magnetometer data. Although not in a polar orbit, we hope to be able to deduce the electric field from oblique crossings of the EEJ by comparing the waveform with CHAMP and Jicamarca measurements. In addition we will search for field aligned currents associated with plasma bubbles.
SA21A-1526
Retrieval of 3-D equatorial plasma bubble characteristics from the TIMED/GUVI nightglow images.
The nightglow images of OI135.6-nm from TIMED/GUVI provided the global view of equatorial plasma bubbles (EPBs). The occurrence climatology of EPBs has been investigated by performing the 1-D analysis of TIMED/GUVI data but the 3-D EPB characteristics and their variability have not yet been investigated. This study will demonstrate the capability to extract the 3-D EPB characteristics from the TIMED/GUVI data. EPBs will be detected by applying the adaptive spatial filter and Laplacian of Gaussian kernels to the nighttime TIMED/GUVI disk-scan images The 2-D EPB images on the longitude-latitude coordinate will be retrieved by applying the image processing technique, and then the vertical EPB images will be obtained by projecting them to the apex height. Our database will record the depletion width, depth, length, tilt, etc. for individual EPBs. We will process the GUVI data in 2002 and present the seasonal and longitudinal variability of the EPB characteristics.
SA21A-1527
The Structure of Equatorial Ionization Anomaly Seen by TIMED/GUVI Limb Observations
This paper reports the structure of Equatorial ionization anomaly (EIA) in various longitudes, seasons and years by the GUVI limb observations on board TIMED satellite in 2002-2007. The images indicate the EIA crest is higher and much close with magnetic equator in summer hemisphere than in winter hemisphere. The limb-scanned airglow observation also shows a cavity located underneath the equatorial ionization anomaly region so called ionospheric plasma caves. The plasma cave disappears till post-sunset or evening hours in different years, depend on the evolution of the EIA and equatorial plasma fountain. The plasma cave is significantly seen between 200 and 300 km altitude by using two observation principles, radio occultation, LEO-tomography and limb-scan technique on the FORMOSAT-3/GOX, FORMOSAT-3/TBB and TIMED/GUVI satellites, respectively. The feature is also predicted by the empirical ionospheric model and the simulation model to supports the existence of the unique phenomenon.
SA21A-1528
The longitudinal and storm-time variations of the equatorial ionospheric plasma density and velocity over a solar cycle
We have made a statistical analysis of the equatorial ionospheric plasma density and ion drift velocity measured by the DMSP satellite in the dusk sector over the last solar cycle (1996-2007). Our analysis focuses on two issues: the longitudinal variations and the effect of magnetic storms. It is found that the plasma density and velocity show a longitudinal structure with multiple wave numbers. The wavenumber varies from two in the northern winter, to three in the northern summer, and to four in the equinox months. The amplitude of the longitudinal structures increases with the solar activity, and the perturbation of the horizontal velocity is nearly two times the vertical velocity. Magnetic storms cause very strong penetration electric fields in the equatorial ionosphere. The penetration electric fields last for 8-10 hours without effective shielding when the magnetic storm activity is strengthening during continuous southward interplanetary magnetic field, and the magnitude of the penetration electric field is approximately proportional to the interplanetary electric field with a relatively constant efficiency over the entire interval. It implies that the interplanetary electric field is linearly coupled with the dusk equatorial ionosphere during the entire main phase of intense magnetic storms. The long-lasting penetration electric field causes significant redistribution of the ionospheric plasma. The result of this study is extremely important for understanding the storm-time equatorial ionospheric electrodynamics and for improving the capability of predicting the penetration electric fields and ionospheric disturbances using the upstream solar wind as input.
SA21A-1529
Vertical Coupling Over Equatorial MLT Region During Polar Stratospheric Sudden Warmings
Favored occurrences of Equatorial Counter Electrojets (CEJs) with a quasi 16-day periodicity over Trivandrum (8.5oN, 76.5oE, 0.5oN diplat.), in association with the recent polar Stratospheric Sudden Warming (SSW) events are presented. It is seen that, the Stratospheric Temperature (ST) at ~30 km and Mesopause Temperature (MT) over Trivandrum show some interesting features during the SSW period. The ST showed a sudden cooling prior to the SSW and the MT showed an overall enhancement during the course of the SSW. CEJs having quasi 16-day periodicity occurred during this period. Further, the stratospheric zonal-mean zonal wind at ~30 km as represented by the NCEP-NCAR reanalysis data exhibited a distinctly different pattern over almost all the latitudes during the SSW period. As is known, the stratospheric wind changes in certain phases are conducive for the upward propagation of the westward waves (both gravity and planetary) through it over the equatorial latitudes. In this context, the interaction of such waves with the tidal components at the upper mesosphere and dynamo region is proposed to be responsible for the occurrence of CEJs having planetary wave periods during the recent SSW events. The enhancement in the MT is also believed to be due to the increased wave activity and its subsequent dissipation over upper mesospheric altitudes. This study provides some new insights into the processes that couple the Mesosphere-Lower Thermosphere (MLT) region and manifest globally.
SA21A-1530
Variability in the longitudinal structure of the low-latitude ionosphere
In-situ plasma density measurements from the CHAMP satellite between about 350-420 km altitude are used to delineate intra-annual variations in the longitudinal structure of the low-latitude F-region ionosphere. It is shown that the longitude structures during mid-day local times are dominated by the space-based longitudinal wavenumbers k_s=2, 4 and 3 in January, July, and December, respectively. These conform to the same dominating k_s-values characterizing solar thermal tide zonal winds in the dynamo region, namely the westward-propagating semidiurnal tide with planetary-fixed zonal wavenumber s = 4 (SW4), and the eastward-propagating diurnal tides with s = -3 (DE3) and s = -2 (DE2), respectively. DE3 is the dominating tide during the other months. The results presented demonstrate that the dominate longitudinal structure of the low-latitude ionosphere exhibits month-to-month variability in a manner that is consistent with nonmigrating tides in the E-region and that nonmigrating tides other than DE3 have an important role in the development of longitudinal structures in the low-latitude ionosphere during certain months.
SA21A-1531
Neutral Wind and Plasma Drift Effects on the Nighttime TEC Variability
We have used a physics based numerical Ionosphere/Plasmasphere Model (IPM) to study the effects of neutral winds and electric fields on the nighttime TEC variability during spring equinox conditions. The model solves the appropriate transport equations for the six ions, O+, NO+, O2+, N2+, H+ and He+, on convecting flux tubes that realistically follow the geomagnetic field. The IPM covers geomagnetic latitudes from about 60 degrees N to 60 degrees S and equatorial crossing altitudes from 90 to 30,000 km. Two of the inputs required by the IPM are the thermospheric neutral wind and the low latitude electric field, which can be given by existing empirical model or externally specified by the user. To study the relative importance of the neutral wind and the electric field for the TEC variations, these two model inputs were externally modified and the resulting variations in TEC were compared. Neutral wind and electric field modifications were introduced at three different local times in order to investigate the effect of different start times of the imposed perturbations on TEC. This study focused on modeled TEC variations at 2100 LT at three different longitude sectors (78, 273 and 318 degrees E) for medium solar activity (F10.7 = 150) and low geomagnetic activity (Kp = 2). The largest changes in TEC were found predominantly in the equatorial anomaly, and a significant longitudinal dependence was observed. Our results indicate that the perturbation effect on the TEC at 2100 LT varied non-linearly with the elapsed time after the imposed neutral wind and electric field perturbations. An important outcome of this study is that daytime neutral wind and/or electric field modifications will lead to essentially identical TEC changes in the 2100 local time sector.
SA21A-1532
Continual 24-hour observations of thermospheric winds made with the Second generation Optimized Fabry-Perot Doppler Imager (SOFDI)
The Second generation Optimized Fabry-Perot Doppler Imager (SOFDI), a state-of-the-art triple-etalon Fabry-Perot interferometer has been making continual 24-hour thermospheric wind observations in upstate New York. The 630-nm data, originating from layer-integrated OI emission with centroid heights of 250 km at night and 220 km during the day, are analyzed so as to obtain measurements of horizontal winds in the thermosphere. In this paper we report results from continuous 24-hour observations of these thermospheric winds. The measurements are made with an accuracy of 15 m/s within 15 minutes during the day and 5 m/s within 10 minutes during the night. These results demonstrate the measurement capabilities of the SOFDI instrument, which will soon be relocated to Peru so as to undertake continual observations of thermospheric winds over the magnetic equator to improve our understanding of the physics of equatorial spread-F.
SA21A-1533
SOFDI/CASI Observations of the September 2005 Storm
The Second Generation Optimized Fabry Perot Doppler Imager (SOFDI), a state-of-the-art triple-etalon Fabry Perot interferometer capable of making day and night wind and temperature measurements, is soon to be deployed to Huancayo, Peru along with the Cornell all-sky imager (CASI) in support of the C/NOFS satellite. We present an overview of the contribution SOFDI and CASI will make towards the C/NOFS science goals by presenting nighttime results of 630-nm OI emission observations taken in upstate New York during the September 2005 storm period. Fortunately, this occurred during a World Day and data is available from several ISR sites. The auroral oval extended to the SOFDI test site where the winds and temperatures were measured. A sharp structure at the equatorward edge of the diffuse aurora was recorded. The curious temperature structure suggests a large scale TID. Indeed, there is some evidence for a large-scale TID in the Millstone Hill and Wallops Island HmF2 measurements.
SA21A-1534
Stormtime Dynamics of the Global Thermosphere and Equatorial Ionosphere
The stormtime development of equatorial plasma bubbles (EPBs) and neutral densities in the thermosphere are coupled through the histories of imposed magnetospheric electric fields. Periods of intense EPB activity driven by penetration (EM) fields in the main phase are followed by their worldwide absence during recovery. A new method is applied to estimate global thermospheric energy (Eth) budgets from orbit-averaged densities measured by accelerometers on polar-orbiting satellites. During the main phase of storms Eth increases while the stormtime electric field is operative, then exponentially decays toward quiet-time values in the early recovery phase. Some fraction of the energy deposited at high magnetic latitudes during the main phase propagates into the sub-auroral ionosphere-thermosphere and affects chemical and azimuthal-wind dynamics well into recovery. We suggest a scenario wherein fossils of main-phase activity inhibit full restoration of quiet-time dayside dynamos and pre-reversal enhancements of upward plasma drifts near dusk denying bottomside irregularities sufficient time to grow into EPBs.
SA21A-1535
Simultaneous SOFDI and CASI observations of gravity waves over Oneida, NY
Gravity wave observations from mesopause OI 557.7-nm all-sky data from the Cornell All-Sky Imager [CASI] in March and September 2005 over upstate NY are presented. When the measured horizontal wavelengths and observed periods are combined with background wind and temperature measurements from the co- located Second-generation, Optimized, Fabry-Perot Doppler Imager [SOFDI] instrument, the intrinsic gravity wave parameters are ascertained. These waves are then reverse ray-traced using the NJIT FOREGRATS gravity wave forecasting model to lower altitudes where the tropospheric sources of these waves are determined. We find tropospheric frontal systems to be a strong generator of monochromatic gravity waves and speculate that such fronts are the sources of mesospheric fronts, like mesospheric bores.