C. K. Chao

First measurements of neutral wind and turbulence in the mesosphere and lower thermosphere over Taiwan with a chemical release experiment

[1] Neutral winds and turbulence structure in the mesosphere and lower thermosphere were measured with a sounding rocket chemical release experiment carried out in Taiwan. Trimethyl aluminum (TMA) was used as a tracer of the drift of the background atmosphere. The results show that the measured neutral wind had a maximum speed of 154 m/s at a height around 105 km. The wind vectors were found to rotate clockwise with height in the altitude range from about 98 to 121 km. On the basis of the hodograph analysis of the measured neutral wind vectors, an upward propagating inertio-gravity wave with intrinsic period of 11.2 hours and vertical wavelength of 19.5 km was believed to be primarily responsible for the height variations of the neutral wind and the Richardson number. Large vertical wind shears were observed near 100 km and 106 km. Turbulent structures were observed in much of the trail below 110 km, but there were enhanced structures in the altitude range between the two large shears, i.e., at and near the altitude of the maximum wind speed. Comparing the positions of the turbulent features with expected atmospheric stability zones induced by an upward propagating gravity wave indicates that the turbulence structures were primarily located within the wave-induced convectively unstable zone. The structures are therefore interpreted as counterrotating vortices within the convective instability zone of a breaking gravity wave. Moreover, the relation between the horizontal separations l (about 1.8 km) of the turbulent structures and the vertical extent h (about 0.7 and 1.5 km) of the wind shear zones with Richardson numbers less than 0.25 did not conform to the predicted l/h ratio of a factor of approximately 8 predicted by simple linear Kelvin-Helmholtz instability theory for the primary billow structures associated with the instability. These results suggest that the observed structures were not the primary billows but were more likely associated with a secondary instability, such as the counterrotating vortices that develop later in the evolution of the instability. In general, the observations reported here support the interpretation that the turbulence evident in the trail was very likely generated in the convectively unstable zone induced by the inertio-gravity wave propagating through the region with large temperature gradients � 32 K/km produced by wave breaking processes.

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.

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.

Correlations between ion density and temperature in the topside ionosphere measured by ROCSAT‐1

From the ROCSAT-1 satellite plasma data at an altitude of 600 km, the correlation between ion temperature (Ti) and density (Ni) was investigated. The data were obtained in a magnetic dip latitude (MLAT) of less than ±40° in 2000–2004. Positive and negative correlations between Ni and Ti were observed around the magnetic dip equator, while weak positive correlations were observed in |MLAT| > 25° during daytime (10:00–16:00 local time). These variations were found in all longitudes, seasons, solar flux (F10.7) levels, and magnetic disturbance levels, although the minimum value of Ti clearly increased with increasing solar flux levels. The results suggest that the solar flux dependence of Ti arises from the solar flux dependence on neutral temperature (Tn). Since Ti is determined by heating through Coulomb collision with electrons and cooling through elastic collision with neutral species, the ratio of ion density to neutral density is an important factor. The ratio reaches its maximum value around the magnetic dip equator and decreases with increasing MLAT. The correlation between Ni and Ti in the topside ionosphere can be explained by electron temperature (Te) and Tn as well as the ratio because Ti follows Te variation when the ratio is high, while it follows Tn when the ratio is low.

Magnetotail structures in a laboratory magnetosphere

The structure of the magnetotail is investigated in a laboratory simulated magnetosphere. This study builds upon our previous investigation of the magnetotail [Birn et al., 1992], which was explored magnetotail through models and magnetic field measurements in the simulated magnetosphere. In this paper the magnetic field measurements are complemented by measurements of ion and electron fluxes, plasma temperature and high-resolution optical images. Plasma fluxes are measured with a double-sided Faraday cup that measures the ion and electron fluxes of counterstreaming plasma simultaneously in both directions. The overall magnetotail structure is examined with high-resolution imaging using a gated optical imager and applying digital enhancement to the magnetotail region. These images confirm the structures inferred earlier from the magnetic field measurements, a Y-type magnetic neutral line for northward interplanetary magnetic field, and the expected X-type structure for southward IMF.

The fast development of solar terrestrial sciences in Taiwan

In Taiwan, research and education of solar terrestrial sciences began with a ground-based ionosonde operated by Ministry of Communications in 1952 and courses of ionospheric physics and space physics offered by National Central University (NCU) in 1959, respectively. Since 1990, to enhance both research and education, the Institute of Space Science at NCU has been setting up and operating ground-based observations of micropulsations, very high-frequency radar, low-latitude ionospheric tomography network, high-frequency Doppler sounder, digital ionosondes, and total electron content (TEC) derived from ground-based GPS receivers to study the morphology of the ionosphere for diurnal, seasonal, geophysical, and solar activity variations, as well as the ionosphere response to solar flares, solar wind, solar eclipses, magnetic storms, earthquakes, tsunami, and so on. Meanwhile, to have better understanding on physics and mechanisms, model simulations for the heliosphere, solar wind, magnetosphere, and ionosphere are also introduced and developed. After the 21 September 1999 Mw7.6 Chi–Chi earthquake, seismo-ionospheric precursors and seismo-traveling ionospheric disturbances induced by earthquakes become the most interesting and challenging research topics of the world. The development of solar terrestrial sciences grows even much faster after National Space Origination has been launching a series of FORMOSAT satellites since 1999. ROCSAT-1 (now renamed FORMOSAT-1) measures the ion composition, density, temperature, and drift velocity at the 600-km altitude in the low-latitude ionosphere; FORMOSAT-2 is to investigate lightning-induced transient luminous events, polar aurora, and upper atmospheric airglow, and FORMOSAT-3 probes ionospheric electron density profiles of the globe. In the near future, FORMOSAT-5 and FORMOSAT-7/COSMIC-2 will be employed for studying solar terrestrial sciences. These satellite missions play an important role on the recent development of solar terrestrial sciences in Taiwan.