S.-Y. Su

Longitudinal variability of equatorial plasma bubbles observed by DMSP and ROCSAT‐1

Abstract : We compare observations of equatorial plasma bubbles (EPBs) by polar-orbiting satellites of the Defense Meteorological Satellite Program (DMSP) with plasma density measurements from the Republic of China Satellite (ROCSAT-1) in a low-inclination orbit. DMSP data were acquired in the evening sector at low magnetic latitudes between 1989 and 2002. ROCSAT-1 plasma densities were measured in March and April of 2000 and 2002. Observations of individual EPBs detected by both ROCSAT-1 and DMSP were well correlated when satellite orbital paths crossed the same longitude within approximately plus or minus 15 min. We compiled a statistical database of ROCSAT-1 occurrence rates sorted by magnetic local time (MLT), magnetic latitude, and geographic longitude. The rate of ROCSAT-1 EPB encounters at topside altitudes rose rapidly after 1930 MLT and peaked between 2000 and 2200 MLT, close to the orbital planes of DMSP F12, F14, and F15. EPB encounter rates have Gaussian distributions centered on the magnetic equator with half widths of ^8. Longitudinal distributions observed by ROCSAT-1 and DMSP are qualitatively similar, with both showing significantly fewer occurrences than expected near the west coast of South America. A chain of GPS receivers extending from Colombia to Chile measured a west-to-east gradient in S4 indices that independently confirms the existence of a steep longitudinal gradient in EPB occurrence rates. We suggest that precipitation of energetic particles from the inner radiation belt causes the dearth of EPBs. Enhancements in the post sunset ionospheric conductance near the South Atlantic Anomaly cause a decrease in growth rate for the generalized Rayleigh-Taylor instability. Results indicate substantial agreement between ROCSAT-1 and DMSP observations and provide new insights on EPB phenomenology.

FORMOSAT‐3/COSMIC observations of seasonal and longitudinal variations of equatorial ionization anomaly and its interhemispheric asymmetry during the solar minimum period

[1] Using an extremely valuable global data set from Formosa Satellite (FORMOSAT-3)/ Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation experiment, a comprehensive study has been carried out on the seasonal and longitudinal variations of equatorial ionization anomaly (EIA) and the temporal variation in the hemispheric asymmetry of EIA during the low solar activity period from November 2006 to October 2007. The interesting result observed from this investigation is the local-time-dependent variation in the hemispheric asymmetry of EIA. During the solstices, it has been consistently observed that the EIA crest in the winter hemisphere appears stronger than that in the summer hemisphere during morning to noon hours. In contrast to this, during noon to early afternoon hours, the ionization in the winter EIA crest decreases rapidly, and the crest in the summer hemisphere becomes more intensified than that in the winter hemisphere. Further, this transition of stronger EIA crest from winter hemisphere to summer hemisphere occurs around 1200–1300 LT during the December solstice months and is delayed by a couple of hours (seen around 1400 LT) during June solstice months. The causative neutral and electrodynamical mechanisms are discussed in light of relative contributions from the field-aligned plasma transport due to transequatorial interhemispheric neutral wind, the strength of the equatorial fountain process, and the ion drag effects during different local times and seasons. The results from the Sami2 is Another Model of the Ionosphere (SAMI2) model simulation also exhibit similar local-time-dependent variation in the hemispheric asymmetry of EIA, which further supports our argument. Also, it was observed that the large magnetic declination of the field lines and the four-peaked longitudinal structure of EIA can significantly modulate the interhemispheric asymmetry of EIA even during the equinoxes.

Anomalous enhancement of ionospheric electron content in the Asian-Australian region during a geomagnetically quiet day

National Natural Science Foundation of China[40725014]; National Important Basic Research Project[2006CB806306]; Knowledge Innovation Program of the Chinese Academy of Sciences

ROCSAT 1 ionospheric plasma and electrodynamics instrument observations of equatorial spread F: An early transitional scale result

Ion density and vertical ion drift velocity sampled at 1024 Hz from the ROCSAT 1 satellite are used to examine the behavior of horizontal structures in equatorial spread F near 600 km altitude. An initial investigation shows that at scale sizes less than 100 m the relationships between the vertical drift and density structure are distinguished by the bulk plasma flow in the structure and by the background gradient in the density. Two adjacent equatorial bubble structures are examined: one in which the bulk plasma flow is upward and characteristic of an active evolving bubble and the other in which the bulk plasma flow is small and characteristic of a stagnated structure. We find that at scale sizes less than 100 m the velocity structure has a spectral slope that is consistently shallower than that of the density structure in the stagnated bubble but matches closely the spectrum of the density structure in the active bubble. Each of the bubble regions examined are characterized by one edge that has a much larger background gradient than the other. We find that in these bubbles, observed near 600-km altitude, the lower background gradient is characterized by enhanced structure at 1-km scale sizes.

Storm time plasma irregularities in the pre-dawn hours observed by the low-latitude ROCSAT-1 satellite at 600 km altitude

Large scale ion density depletions were detected in the nighttime sector by ROCSAT-1 for over 10 hours during the 22 October 1999 geomagnetic storm. Prominent depletion structures (bubbles) that are characterized by large-amplitude density decrease (N/No ≤ 1%) with rapid horizontal ion drift (600 ∼ 800 m/s) are found to cluster in the 03:00 ∼ 04:30 local time sector and at magnetic latitudes 14° ∼ 20° S when the storm was in its early recovery phase. These presunrise bubbles are positively correlated to the enhanced eastward electric fields of greater than 1 ∼ 2 mV/m, which were in response to the storm-time disturbances resulting from the in-phase contributions of the prompt penetration magnetospheric and the long lasting ionospheric disturbance dynamo electric fields. Further analyses of the field-aligned and cross-field ion drifts within the depletions reveal that bubble plasma were driven by the eastward polarization electric fields to move upward, but these upward velocities were compensated by large downward field-aligned diffusive motions. These features confirm that the disturbance electric fields produced during a great magnetic storm can significantly affect the occurrence timing and spatial extent of severe plasma irregularities in low-latitude ionosphere. The spatial dimensions of the pre-sunrise irregularities may exceed the large region observed by the 35° inclined circular orbiting ROCSAT-1.

A three dimensional study of E region irregularity patches in the equatorial anomaly region using the Chung-Li VHF radar

Combining the Spatial and Frequency Domain Interferometry techniques, the three dimensional behavior of the E region Field Aligned Irregularity (FAI) patches are studied via Chung-Li VHF radar. The echo patches are found to be localized within the radar beam. They are of pancake shape with dimensions in the plane containing the field line but transverse to the radar beam at least one order of magnitude greater than the dimension along the radar beam. A zonal westward drift velocity of the patches is found to be about 90 m/s at 107 km altitude in the night time. Mean Doppler (radial) velocity is about 50 m/s with a broad spectral width of 25 to 35 m/s.

Seasonal and latitudinal distributions of the dominant light ions at 600 km topside ionosphere from 1999 to 2002

Data taken by the Republic of China satellite (ROCSAT-1) during moderate to high solar activity years from 1999 to 2002 have been studied for the statistical distribution of the dominant light ion species, either hydrogen or helium ions, at 600 km topside ionosphere. The results indicate some interesting seasonal and longitudinal/latitudinal distributions of the dominant light ions in the topside ionosphere during the magnetic quiet periods. Each light ion species can become the dominant ion species at 600 km topside ionosphere but only at night when the ion temperature is cooler than during the day. More cases of H + dominance have been observed than those of He + dominance. Except for the March equinox the distribution of dominant H + shows a strong hemispheric asymmetry for the other three seasons. When H + dominance is observed in one hemisphere during the solstice season, the low latitude limit of this transition region is a constant dip latitude in the winter hemisphere. This statistical minimum of the transition latitude shows little dependence on the seasonal averaged solar flux intensity. Similar hemispherically asymmetric distribution for dominant He + in the winter hemisphere during the solstice season has also been noted except that the asymmetrical pattern is not as prominent as in the dominant H + case because much fewer cases have been observed for dominant He + . The asymmetrical distribution of the dominant light ions seems to be related to the observed hemispheric field-aligned ion flow pattern. Thus it is concluded that the downward field-aligned ion flow together with the nighttime lower ion temperature in the winter hemisphere compose a possible cause for the occurrence distribution of the hemispheric asymmetry in the dominant light ion species. This can be understood from the fact that the field-aligned flow is related to the hemispheric asymmetry of the ionospheric F peaks and serves to enhance or retard the nocturnal redistribution of the light ions along the field line.

Anatomy of plasma structures in an equatorial spread F event

This paper investigates the small scale plasma structures observed by ROCSAT-1 in the equatorial F region through the newly developed Hilbert-Huang transform (HHT) method in the time (space) domain under the frozen-in approximation. The new method allows us to decompose the non-stationary, nonlinear data into a finite number of intrinsic scale modes. In this report the structures of vertical ion velocity and horizontal density gradient inside a plasma bubble are analyzed mode by mode anatomically without making the usual linearization assumption. We found that the intrinsic modes for velocity and density gradient of the selected event have identical wave form for structures with scales between 300 m and 50 m. This implies that the vertical velocity fluctuations induced from the electric field follows the exact Boltzmann relation in the limited regime of scale length between 300 m and 50 m. A spectral break at 50 m is clearly seen in the velocity HHT spectrum. The spectral form of velocity differs greatly from that of density gradient at scale lengths shorter than 50 m.

Grid effects on the derived ion temperature and ram velocity from the simulated results of the retarding potential analyzer data

Abstract The ROCSAT-1 satellite circulating at 600 km altitude in the low- and mid-latitude topside ionosphere carries a retarding potential analyzer to measure the ion composition, temperature, and the plasma flow velocity in the ram direction. Based on an existing three-dimensional model, the particles motion inside the instrument is simulated with the exact wire and mesh sizes but with a smaller aperture of the real sensor configuration. The simulation results indicate that the retarding grids could not provide a uniform retarding potential barrier to completely repel low energy particles. Some of low energy particles could pass through those grids and arrive at the collector. The leakage will cause the ram velocity to be over-estimated for by about 180 m/sec. Furthermore, the simulated O + temperature derived from the I-V curve is lower than the input temperature due to ion losses from colliding with the grids from the non-uniform potential field generated by the high retarding voltage.

E region observations over Chung‐Li during the SEEK Campaign

During the Sporadic E Experiment over Kyushu (SEEK) campaign E-region field aligned irregularities (FAI) were observed with the Chung-Li VHF Radar (operated on 52 MHz) in Taiwan between August 15-22, 1996. The characteristics of the quasi-periodical echoes from the E-region about 80 km north of the Chung-Li radar are studied. During the same period ionogram records were taken with the Digisonde located at the Chung-Li radar site. Strong sporadic-E layers were detected with the Digisonde when the FAI, observed with the VHF radar, were also very intense. The variations of sporadic E-layer critical frequency and of the field-aligned irregularities were studied. The latter were estimated to be located at altitudes 95-115 km occurring in layers of 5-15 km thickness, moving downwards at a rate of about 2.5 km/h. There seemed to be a change over of the FAI echoes from lower to upper E-region every night approximately at 23 hour LT. The distinction between post-sunrise and post-sunset FAI echoes observed earlier by MU radar was not clearly seen in Chung-Li. A preliminary examination of the morphology of the fine structures within the layers indicates quasi-periodic features with 5-10 minutes period. Many of these periodic fine structures were observed at the Chung-Li VHF radar to have opposite slopes than those detected earlier with the MU radar in Japan, which is located 1500 km north of the Chung-Li VHF radar.