SA24B-01
Simultaneous Density Depletions Observed on C/NOFS and DMSP
During June solstices in solar minimum years, a deep plasma depletion appears at equatorial latitudes in DMSP observations when the orbit is near the South Atlantic Anomaly at 20 LT. The density minimum is approximately 20 percent of the density outside the depletion. Simultaneous measurements made by the PLP during June 2008 on C/NOFS show decreases in density around the equator at altitudes of 400 km with intense wave-like structures on the edges and within the density depletion. We will discuss possible mechanisms to account for these observations.
SA24B-02
Initial Observations of Ionospheric Structure and Irregularities Gathered with the VEFI Instrument on C/NOFS
The Vector Electric Field Investigation (VEFI) onboard the C/NOFS spacecraft has returned more than 1500 orbits through the terminator and nightside equatorial regions since its launch in April 2008. C/NOFS is the first satellite with a high duty cycle able to make repeated measurements at all local times at a high repetition rate, and thus provides an invaluable tool to understand the spatial and temporal evolution of ionospheric structure post-sunset. Initial VEFI results reveal that the electric field and plasma density on the nightside, especially post-midnight, are highly structured, even during very quiet intervals. These structures have a significant enhancement in the electric field signature (3 or 4 times the background), and exhibit variability from > 100 km to < 1 m. We discuss how these structured regions compare with typical "Spread-F" signatures, compare their distribution in longitude, local time, and altitude, and present a detailed study of the relations between density and electric field variations for selected events. Finally, we will estimate the effects of this structure on radio wave transmission.
SA24B-03
Modeling of the Storm time Electric Fields and the Response of the Ionosphere- Plasmasphere-Thermosphere
We have developed a self-consistent first-principles model of the coupled inner magnetosphere- thermosphere- ionosphere- plasmasphere system in order to understand the storm time electrodynamic coupling of the magnetosphere and ionosphere and its consequences for the ionosphere, plasmasphere, and thermosphere. The model involves electrodynamic coupling of the Rice Convection Model (RCM) and the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model: RCM provides the region 2 field aligned currents resulting from pressure gradients in the inner magnetosphere, which are important for modeling electric-field penetration and the shielding processes, while CTIPe provides time-dependent conductivity and neutral wind fields that are key to modeling the disturbance dynamo. A newly developed potential solver takes into account all these inputs to derive the global pattern of ionospheric electric fields. We found that the storm time vertical ExB drifts from the coupled model provided a better agreement with those from the observations for the March 2001 storm as compared to the predictions from the stand-alone RCM and CTIPe. Our simulation results suggest that the temporal variation of the magnetospheric magnetic field plays a significant role in the storm time variation of the drifts, especially for super storms such as March 2001 and November 2004 storm events. As responses of the ionosphere, plasmasphere and thermosphere to the storm time disturbance drifts, we found that daytime and evening upward enhancement of the ExB drift caused by the penetration electric field modifies the electron density and zonal neutral wind, leading to the zonal drift disturbances near the terminator through the F-region dynamo process. In this paper, we will address the role of the combined effect of the vertical and zonal drift disturbances as possible drivers to reproduce the massive restructuring of TEC.
SA24B-04
Multisensor observations of low latitude irregularity development
We present space-based and ground-based observations of the equatorial ionosphere. Variability in the development of irregularity structures and scintillation is examined. Even at solar minimum, low latitude irregularities routinely develop after sunset and result in VHF scintillation along ground-space propagation paths, but L-band scintillation occurs less frequently. We examine the day-to-day variability of ionospheric conditions leading to VHF and L-band scintillation. In particular the variability of the post-sunset equatorial anomaly plays a significant role. The condition of the equatorial anomaly dictates the background density in which irregularities form and limits the resulting scintillation level. Space-based sensors employed in this study include the Tiny Ionospheric Photometer (TIP) on the COSMIC satellite constellation. Ground-based sensors include: incoherent scatter radar, allsky camera, ionosonde, GPS, and VHF scintillation observed from Kwajalein Atoll.
SA24B-05
Japan contribution to studies of low-latitude and equatorial ionosphere over Southeast Asia
A dense observation network to study ionosphere is deployed over Southeast Asian countries of Indonesia, Thailand, and Vietnam. The Equatorial Atmosphere Radar (EAR) at Kototabang, Indonesia is the center facility, and supporting instruments, i.e., an ionosonde, a VHF ionosphere radar, an optical imager, a GPS scintillation receiver, a magnetometer, a meteor radar, etc. are collocated. NICT operates the ionosonde network SEALION (South East Asian Low-latitude IOnosonde Network) that meridionally extends from the EAR site to Chumphong and Chiang Mai in Thailand, and two more sites (Baq Liu and Phy Thuy) in Vietnam. Additional facilities are an MF radar at Pameungpeuk, Indonesia, and an optical imager at Darwin, Australia. We have been observing plasma bubbles since 2001, that, for example, contributed clarification of time- spatial structures of the phenomena, their relationship to the pre-reversal enhancement, control of bubble occurrence by the meridional winds, etc. We are starting studies of their seeding by means of atmospheric waves that propages from the lower atmosphere, too. In 2008, Nagoya University will soon install three Fabry-Perot interferometers at the EAR site, Chiang Mai, and Darwin. We also have a plan to install digital beacon receivers in some of these sites. Next research program that follows CPEA (Coupling Processes in the Equatorial Atmosphere, 2001-2007) is under planning now. Our main facilities cover ± 10° of geomagnetic latitude, where the magnetic declination is relatively small, and the geomagnetic equator is in the geographic northern hemisphere. We will review our achievements, and show on-going efforts and future plans. Collaboration with the C/NOFS satellite, and comparisons to results from the American sector should be beneficial for global-scale understanding of the equatorial ionosphere/atmosphere.