SA14A-01
Three-dimensional modeling equatorial spread F
Equatorial spread F (ESF) is a low-latitude ionospheric
phenomenon that leads to the development of large
scale electron density depletions that adversely
affect communications and navigation systems.
The development of models to understand and predict
the onset and evolution of ESF is therefore critically important
to a number of space-based systems. To this end, NRL has
developed a three-dimensional model of ESF. The global NRL
ionosphere model SAMI3 has been modified to simulate a narrow
wedge of the post-sunset ionosphere to capture the onset
and evolution of ESF. Preliminary results indicate that
(1) bubbles can rise to ~ 1600 km,
(2) extremely steep ion density gradients can develop in both
longitude and latitude, (3) upward plasma velocities approach
1 km/s, and (4) the growth time of the instability is
~eq 15 min. We will also report the effects
of meridional and zonal winds on bubble development,
as well as ion composition (both atomic and molecular).
The simulations will focus on current, low solar
activity conditions, and results will be compared
to C/NOFS data where available.
Research supported by ONR
SA14A-02 INVITED
DEMETER Observations of Equatorial Plasma Depletions and Related Ionospheric Phenomena
DEMETER, the first micro-satellite of the CNES MYRIAD program, was launched from Baikonour on June 29, 2004 on a nearly circular, quasi helio-synchronous polar orbit at ~ 715 km altitude. The DEMETER mission focuses primarily on the search for a possible coupling between seismic activity and ionospheric disturbances as well as on the effects of natural phenomena such as tropospheric thunderstorms and man-made activities on the ionosphere. The scientific payload provides fairly complete measurements of the ionospheric plasma, energetic particles above ~ 70 keV, and plasma waves, up to 20 kHz for the magnetic and 3.3 MHz for the electric components. Several studies related to space weather and ionospheric physics have been conducted over the past years. Following a brief description of the payload and the satellite modes of operation, this presentation will focus on a set of results that provide a new insight into the physics of instabilities in the night-time equatorial ionosphere. The observations were performed during the major magnetic storm of November 2004. Deep plasma depletions were observed on several night-time passes at low latitudes characterized by the decrease of the plasma density by nearly 3 orders of magnitude relative to the undisturbed plasma, and a significant abundance of molecular ions. These features can be best interpreted as resulting from the rise of the F-layer above the satellite altitude over an extended region of the ionosphere. In one of the passes, DEMETER was operated in the Burst mode and the corresponding high resolution data allowed for the discovery of two unexpected phenomena. The first one is the existence of high intensity monochromatic wave packets at the LH frequency that develop during the decay phase of intense bursts of broadband LH turbulence. The broadband LH turbulence is triggered by whistlers emitted by lightning from atmospheric thunderstorms beneath the satellite. The second unexpected feature is the detection of a population of super-thermal ionospheric ions with a density of about 2-3% of the thermal ion population. The super- thermal ions appeared to be heated to temperatures of a few eV at times when LH turbulence and monochromatic wave packets are observed while the temperature of the core ion population is not affected. High time resolution plasma density measurements show the presence of strong small scale plasma irregularities in the depletions that scatter the high amplitude whistler waves and may lead to the development of strong LH turbulence and of monochromatic wave packets. The ensuing interaction between these waves and the ambient ions may lead to the formation of a super-thermal tail in the ion distribution function. Ion acceleration by LH turbulence and solitary waves is a commonly observed phenomenon along auroral magnetic field lines but, to our knowledge, this is the first time that a similar process has been observed in the equatorial ionosphere. These findings exemplify a novel coupling mechanism between the troposphere and the ionosphere: Under highly disturbed conditions at times of magnetic storms, part of the energy released by lightning and radiated as whistlers can dissipate in the equatorial ionosphere and produce super-thermal ion populations.
SA14A-03
Initial Modeling Studies of C/NOFS Observations
One of the vital elements of the C/NOFS (Communication and Navigation Outage Forecast System) program is the modeling and forecasting of low-latitude structures that can produce radio scintillation using physics- based forecast codes. To describe the extent and structure of the turbulent plasma plumes caused by interchange instabilities in the low-latitude ionosphere, a three-dimensional model of the plumes has been developed. To better reflect the day-to-day variability of the occurrence of the plumes, this model is closely coupled to a time-dependent model of the ambient ionosphere which is designed to be initialized by assimilation of data from the C/NOFS satellite. The system of models has been implemented in the C/NOFS Data Center to provide operational scintillation forecasts. After describing the models, I will present a sampling of modeling results using early satellite data, addressing the ambient plasma density as well as plasma plume structure.
SA14A-04
Neutral Atmosphere - Ionosphere Coupling at Equatorial Latitudes: Evidence from SpreadFEx and Guidance for Future Measurements
The Spread F Experiment (SpreadFEx) was performed in Brazil during two moon-down periods from
September to November 2005. We employed extensive ground-based observations of gravity waves (GWs),
plasma structures, electron densities, and mean atmospheric and ionospheric conditions using airglow,
digisonde, VHF and meteor radar, balloon, and GPS instrumentation at multiple sites in Brazil, as well as
satellite measurements of deep convection and plasma bubble structures. The intent of SpreadFEx was to
assess the potential contributions of GWs to plasma instabilities and plasma bubbles extending to higher
altitudes. Our measurements, analyses, and theory revealed significant GW activity arising from deep
convection, apparent neutral density (and related wind and temperature) perturbations accompanying GWs
having large spatial scales extending well into the thermosphere and ionosphere, plasma perturbations at the
bottomside F layer suggestive of these GW influences, and anticipated tidal influences on GW propagation
and plasma instability growth. While the SpreadFEx campaign helped to better quantify the role of neutral
dynamics in plasma processes, it did not adequately address the role of neutral winds in the F layer. Hence,
our future measurement efforts are anticipated to concentrate on definition of F-layer neutral winds, their
tidal composition and longitudinal variability, and their correlations with plasma bubble statistics in support of
the C/NOFS science mission.
http://www.cora.nwra.com/dave/SpreadFEx_publications.html
SA14A-05
Importance of Large-Scale Wave Structure to Equatorial Spread F
There is mounting evidence that large-scale wave structure (LSWS) is a more direct precursor of equatorial spread F (ESF) than the post-sunset rise (PSSR) of the equatorial F layer. Unambiguous experimental evidence, though limited, come from measurements by ALTAIR, a fully steerable incoherent-scatter radar, in situ measurements by low-altitude satellites in low-inclination orbits (AE-E, San Marco D), and total electron content measurements using satellites in low-inclination orbits. Less direct evidence is contained in seemingly extraneous traces in equatorial ionograms, which appear to be associated with LSWS and ESF. Clearly, a demonstration that these traces are indeed a direct consequence of LSWS is pivotal because such a demonstration would allow use of the extensive database of equatorial ionograms that exists to argue conclusively that LSWS is a central player in ESF generation. A demonstration of this kind will be presented, together with a description of experiments proposed for the Pacific sector, which involve the C/NOFS satellite, and how they will increase substantially our understanding of LSWS and ESF.
SA14A-06
Observational evidence of sudden stratospheric warming effects on the equatorial ionospheric electric fields
In this work we present strong evidence that during major sudden stratospheric warming (SSW) events the equatorial ionospheric electric fields exhibit a unique and distinctive daytime pattern. In three out of four major SSW events in the last few years when Jicamarca Radio Observatory (JRO) ExB measurements were available, we have observed an anomalous temporal variation of the equatorial vertical ExB drifts, showing a strong semidiurnal variation. The three events occurred during quiet to moderate magnetic conditions (December 2000, January 2003 and January 2008). The fourth strong SSW event in February 1999 does not show the semidiurnal pattern, but it coincides with a magnetically disturbed period. Our observations consists mainly of incoherent scatter radar (ISR) ExB drifts, JULIA radar drifts from 150-km echoes and data from magnetometers. In the three cases with anomalous ExB behavior, the patterns are observed in the daytime ISR drifts and in JULIA drifts and/or magnetometer data. Previous ExB statistics using more than 35 years of Jicamarca ExB drifts and few years of AE-E satellite data show that quiet ExB variability is particularly large during the December solstice and cannot be explained by other phenomena (such as magnetic activity). As the dayside electrodynamics at equatorial latitude is strongly dependent on neutral wind in the lower thermosphere and therefore driven by tidal influences, observations of semidiurnal anomaly in ExB drifts may indicate tidal modulation during SSW events. Large diurnal and semidiurnal variations could be created in high latitude mesosphere and lower thermosphere in association with SSWs as indicated by TIMEGCM simulations (Liu and Roble, 2002). However, observations of such variations at low latitude is an unexpected finding which might shed new light on sources and mechanisms of quiet-time ionospheric variability.
SA14A-07
Observations of the Day to Night transition of Equatorial Ionization Anomaly
The Equatorial Fountain creates global scale ionospheric density enhancements on both sides of the magnetic equator in the Earth's sub-tropical regions which is the well known Equatorial Ionization Anomaly or EIA. It is understood that this phenomena result from thermospheric winds interacting with ionospheric plasma. We examine aspects of this process by comparing the equatorial upward plasma drifts of the fountain to the observed plasma distribution in the Equatorial Anomaly. We focus on the development of the density structures from the day period into the nighttime with particular focus on the pre-reversal enhancement of the vertical plasma drifts on the EIA. We examine if this relatively brief enhancement in vertical drift velocity can be traced to significant plasma transport into the night time Equatorial Anomaly and examine the important seasonal and longitudinal differences. The density observations of the Equatorial Anomaly are deduced from the GUVI instrument on NASA's TIMED spacecraft. This instrument records radiance data at 135.6nm that results from recombination of Oxygen ions and can be used as a proxy for plasma density. GUVI's high altitude limb scans are used to study both the dayside and night side Equatorial Anomaly as a function of local time, season and longitude. Plasma drifts from ROCSAT-1 observations as presented by Fejer etal, JGR 2008 are used for the vertical drifts. Further comparisons of the Equatorial Anomaly density structure as a function of season are made with the NRL Horizontal Neutral Wind model to discuss the driving sources from the EIA. This paper presents the data analysis techniques to produce observations of the EIA at all local times from GUVI as well as the comparisons of GUVI data with plasma drifts and neutral wind models.
SA14A-08
Low Latitude Ionospheric Electrodynamic Effects Near Solar Minima
We use satellite and ground-based observations to examine the main characteristics of low latitude electrodynamic plasma drifts and their effects on equatorial spread F and low latitude plasma structure during low solar flux periods. The ionospheric plasma drifts have considerably larger temporal and spatial variability near solar minimum than close to solar maximum during both quiet and disturbed geomagnetic conditions. The magnitude and altitudinal structure of the prereversal enhancement of the equatorial upward plasma drift, which plays a fundamental role in the generation and development of equatorial spread F and scintillations, varies strongly with season, longitude, solar flux and geomagnetic activity. The relative importance of evening and nighttime season and possibly longitude dependent prompt penetration and disturbance dynamo vertical plasma drifts also changes significantly with solar flux. In this talk, we first discuss the main challenges for a more detailed understanding of quiet and disturbed equatorial electrodynamic drifts. Then, we briefly examine the potential effects of these drifts on equatorial spread F development. Finally, we discuss the potential of C/NOFS electric field and plasma drift measurements for the understanding of the complex variability of equatorial plasma waves and density structures.