SA22A-01
ROCSAT Observations of Meso-Scale Density and Flow Undulations in Conjunction with Intermediate-Scale Density Irregularities at Midlatitude Topside Ionosphere
ROCSAT-1 orbiting at 600 km topside ionosphere has observed many meso-scale (~50 to 1000 km) ion density and flow undulation events at low- to mid-latitudes. During the 8 January 2000 event, noticeable intermediate- scale (0.1 to 50 km) density irregularities were observed in conjunction with meso-scale undulations in many consecutive ROCSAT orbits. The meso-scale undulations indicate in-phase variations of outward and westward flow with the density enhancement. The derived electric-field perturbations point mostly to the northeast direction at about 45 degree in azimuth. The onset locations of the meso-scale disturbances observed from consecutive ROCSAT orbits also indicate a frontal structure alignment from northwest to southeast in geographic coordinate. The disturbances cover a large geographic area from 120 degree to 300 degree in longitude and from latitude 20 degree up to the ROCSAT turn-around latitude at 35 degree. Since the onset of meso-scale undulation is preceded by a sudden enhancement of a southeast directed background flow which can be induced by a southwest pointed electric field, the cause of meso-scale undulation is identified as the Perkins instability [Perkins, 1973]. The intermediate-scale density irregularities that are observed to co-exist within the meso-scale undulation can be explained from a secondary instability process in the non-uniform density variation inside the meso-scale undulations as derived from two different ROCSAT observations made at the same location separated by three hours. On the other hand, it is difficult to explain the stand-alone intermediate-scale irregularities that appear alone in time and space without meso-scale undulations. One possible explanation is from the mapping process from E region or lower F region irregularity structures. However, theoretical model of the energy cascading process in the instability structure or the mapping process is not available at the moment. These need to be further investigated in the future.
SA22A-02 INVITED
Radar and Optical Observation of Medium-Scale Traveling Ionospheric Disturbances and Field-Aligned Irregularities in the F Region
We report for the first time simultaneous observation of Medium-Scale Traveling Ionospheric Disturbances (MSTIDs) and Field-Aligned Irregularities (FAIs) in the F region using two all-sky airglow imagers and the MU radar in Japan. The all-sky imagers were operated at Sakata (39.0°N, 139.9°E) and Shigaraki (34.9°N, 136.1°E), Japan. MSTID propagating southwestward was simultaneously observed in 630- nm airglow images at the both sites. To investigate spatial relationship between MSTID and FAIs, FAIs were mapped onto the 630-nm airglow layer (260 km altitude). Altitude of the airglow layer was estimated by the triangulation using all-sky images at Sata and Shigaraki. FAIs with strong echo intensity and upward Doppler velocities coincided with the airglow depleted region due to the MSTIDs. On the other hand, FAIs with weak echo intensity and downward Doppler velocities coincided with the airglow enhancement. The directions of the Doppler velocities is consistent with that of ExB drifts caused by the polarization electric fields associated with the MSTIDs. A model calculation was carried out to simulate electron density perturbations caused by oscillating electric fields due to MSTIDs. The result suggests that FAIs could be generated by the gradient drift instability which operates at the spatial gradient of the electron density associated with the MSTIDs.
SA22A-03
Multi-Platform Ground-Based Observations of Meso-Scale Waves and Spread F at Midlatitudes
Three distinctly different midlatitude spread F events were observed on separate nights during a two week period in the fall of 2006. One of the events occurred after a period of increased activity in several high latitude geomagnetic storm indices, another occurred during an extended period of magnetically quiescent conditions, and the third occurred during a moderately active period. The ionospheric and thermospheric conditions during these events were simultaneously monitored by a suite of ground-based instruments, including the Wallops Island Digisonde, a tri-static radar sensitive to traveling ionospheric disturbances (TIDs), scintillation detectors, and a network of ground-based GPS receivers. We present data from each of these systems, and contrast the observations on each night in order to investigate the ionospheric morphologies associated with each type of spreading event. Evidence of TIDs and plasma density variations are evident during the quiet-time event, while the geomagnetically active event shows a more impulsive density perturbation in the TEC data. Smaller scale turbulence and associated scintillations are evident only in two of the three cases. Plausible scenarios to explain the disparate observations will be discussed in light of these new data.
SA22A-04 INVITED
Highly Structured Plasma Density and Associated Electric and Magnetic Field Irregularities at Sub-Auroral, Middle, and Low Latitudes in the Topside Ionosphere Observed with the DEMETER and DMSP Satellites
Detailed observations of the plasma structure and irregularities that characterize the topside ionosphere at sub- auroral, middle, and low-latitudes are gathered with probes on the DEMETER and DMSP satellites. In particular, we present DEMETER observations near 700 km altitude that reveal: (1) the electric field irregularities and density depletions at mid-latitudes are remarkably similar to those associated with equatorial spread-F at low latitudes; (2) the mid-latitude density structures contain both depletions and enhancements with scale lengths along the spacecraft trajectory that typically vary from 10's to 100's of km; (3) in some cases, ELF magnetic field irregularities are observed in association with the electric field irregularities on the walls of the plasma density structures and appear to be related to finely-structured spatial currents and/or Alfven waves; (4) during severe geomagnetic storms, broad regions of nightside plasma density structures are typically present, in some instances extending from the equator to the sub-auroral regions; and (5) intense, broadband electric and magnetic field irregularities are observed at sub-auroral latitudes during geomagnetic storm periods that are typically associated with the trough region. Data from successive DEMETER orbits during storm periods in both the daytime and nighttime illustrate how enhancements of both the ambient plasma density, as well as sub- auroral and mid-latitude density structures, correlate and evolve with changes in the Dst. The DEMETER data are compared with near simultaneous observations gathered by the DMSP satellites near 840 km. The observations are related to theories of sub-auroral and mid-latitude plasma density structuring during geomagnetic storms and penetration electric fields and are highly germane to understanding space weather effects regarding disruption of communication and navigation signals in the near-space environment.
SA22A-05 INVITED
Radar Observations of Midlatitude Irregularities at Low Latitudes
There is a tendency to refer to plasma structures as "midlatitude," when found well poleward of the equatorial ionization anomaly (EIA), but equatorward of the subauroral region, and to attribute them to the Perkins instability. Similarly, there is a tendency to refer to plasma structures as "equatorial," when found equatorward of the EIA crest, and to attribute them to the Rayleigh-Taylor instability. There is evidence, however, that geomagnetic field lines, which thread equatorial plasma bubbles at very high altitudes, can pass through the peak of the F layer at magnetic dip latitudes poleward of the EIA. In these events, interpretation is in terms of equatorial structure penetrating into the midlatitude ionosphere. More recently, there has appeared evidence that midlatitude structures may be able to penetrate into the equatorial ionosphere, that is, to dip latitudes equatorward of the EIA crest. We present examples of the latter and discuss which of the processes may be acting to account for these observations.
SA22A-06 INVITED
Midlatitude plasma irregularities with finite parallel wavenumbers
Radar experiments performed in the Caribbean in the summer of 2002
strongly indicated a role for neutral turbulence in the generation of
E region postsunset plasma irregularities and associated
quasiperiodic (QP) echoes. However, intermediate-scale plasma
irregularities were most likely necessary to pump the meter-scale
irregularities which give rise to coherent radar scatter. A number of
investigations point to the presence of kilometer-scale plasma waves
in irregular sporadic E layers. A robust plasma instability that
could be responsible for kilometric irregularities has been
identified. The most unstable waves in the E region are those
with 1) kilometric transverse scale sizes and 2) finite parallel
wavenumbers. In principle, the same instability could operate in the
midlatitude F region, where it would operate on much longer
transverse spatial scales. The existence of the (collisional drift)
instability and validity of the dispersion relation will be examined
in new experiments beginning this summer involving the Arecibo radar,
lidar, all-sky camera, and a new coherent scatter radar imager sharing
a common volume.
http:landau.geo.cornell.edu
SA22A-07
Statistical Study of Ionospheric Scintillation Associated with Hurricanes and Typhoons
Interest in coupling between atmospheric regions continues to increase. Gravity waves generated in the troposphere are often cited as a coupling mechanism between the low and upper altitude regions. However, few experiments demonstrating coupling between the troposphere and thermosphere/ionosphere exist. Intense, localized storms, such as hurricanes and tropical storms, provide an ideal opportunity to examine tropospheric/ionospheric coupling. This study utilizes GPS occultation (GPSRO) to investigate TEC levels and ionospheric scintillation near hurricanes and typhoons. GPSRO measurements from receivers on LEO satellites supply accurate global ionospheric and upper atmospheric monitoring. This study utilizes data from the CHAMP, PicoSat, and COSMIC satellites. Observations from more than 150 tropical storms over three years are used to determine the presence of ionospheric scintillation within 1500 km horizontal distance of the storm's center. Results show significant scintillation or ionospheric disturbances are observed near the storms. Specifically, during Pacific typhoons in 2002-2003, scintillation was present over 70% of the time.