J. Y. Liu

Seismo‐ionospheric signatures prior to M≥6.0 Taiwan earthquakes

This paper examines variations of the greatest plasma frequency in the ionosphere, foF2, recorded by the Chung-Li ionosonde (25.0° N, 121.1° E) before M≥6.0 earthquakes during 1994–1999. The 15-day running median and the associated inter-quartile range are utilized as the reference and the upper or lower bounds to monitor the ionospheric foF2 variations for finding seismo-ionospheric signatures (precursors) of the earthquakes. It is found that precursors, in the form of the recorded foF2 falling below its associated lower bound around 1200–1700 LT, appear 1–6 days prior to these earthquakes. On September 20, 1999 UT (September 21, Taiwan local time) a large Mw=7.7 earthquake struck central Taiwan near the small town of Chi-Chi. We analyzed the foF2 and found three clear precursors 1, 3, and 4 days prior to the Chi-Chi earthquake.

Theoretical study of the ionospheric Weddell Sea Anomaly using SAMI2

[1] The ionospheric Weddell Sea Anomaly (WSA) was first reported more than five decades ago based on ionosonde data near the Antarctica peninsula. The WSA is an ionospheric structure characterized by a larger nighttime electron density than daytime density. Recent satellite observations indicate that the WSA can extend from South America and Antarctica to the central Pacific. The major physical mechanisms that have been suggested for the WSA formation are an equatorward neutral wind, an electric field, the photoionization, and the downward diffusion from the plasmasphere. On the basis of the theoretical modeling performed in this study using the SAMI2 model, an equatorward neutral wind is identified as the major cause of the WSA, while the downward flux from the plasmasphere provides an additional plasma source to enhance or maintain the density of the anomalous structure.

Latitudinal distribution of anomalous ion density as a precursor of a large earthquake

[1] Data obtained by the U.S. satellite DE‐2 are used to investigate possible precursor features in the ionosphere associated with a large earthquake (latitude �33.13°, longitude 73.07°, M = 7.5), which occurred during a moderate geomagnetic disturbance. Atomic oxygen ion and molecular ion distributions show characteristic latitudinal features similar to the well‐known equatorial ionization anomaly (EIA) feature but centered around the earthquake epicenter. We name this the precursor ionization anomaly (PIA). The density minima of both the atomic oxygen and molecular ions are in two latitude zones, depending on the distance from the epicenter. One of the PIA minima aligns with the geomagnetic latitude crossing the epicenter. Another minimum is found along the geographic latitude of the epicenter. These minima are located in an area spanning about 40° in latitude and about 140° in longitude. It is noted that the molecular ion minimum is more clearly defined even when the atomic ion density minimum is not indicated clearly. The ion density reduction seems to be caused by a superposition of natural/quiet time ionospheric eastward electric field and an electric field associated with the earthquake. Although we studied one single event, our careful examination of results suggests that the location and day of occurrence of the PIA can be predicted for some large earthquakes even during moderate geomagnetic disturbance if the satellite orbit is properly chosen.

The ionospheric perturbations prior to the Chi-Chi and Chia-Yi earthquakes

Abstract In this paper we analyze the greatest plasma frequency, foF2, named critical frequency, observed by the Chung-Li ionosonde (25.0°N, 121.l°E) during the period of the Chi-Chi (23.87°N, 1 20.75°E) and the Chia-Yi (23.51°E, 120.4°E) earthquakes. The previous 15-day running mean and the associated standard deviation are utilized to construct the upper or lower bound for detecting the seismo-ionospheric perturbations. It is found that the perturbation appeals in 3–4 days prior to the Chi-Chi earthquake as well as 1–3 days prior to the Chia-Yi earthquake.

Ionospheric foF2 variations prior to strong earthquakes in Taiwan area

Abstract Many studies of the seismo-ionospheric coupling effects have been reported. On 17 July 1998(M=6.2), 20 September 1999 (M=7.3) and 22 October 1999 (M=6.4) three large earthquakes respectively struck Rei-Li, Chi-Chi and Chia-Yi in central Taiwan. The three earthquakes severely damaged structures, heavily changed landforms and disturbed geophysical environments. This paper examines variations of the ionospheric penetration frequency, foF2 , observed by Chung-Li ionosonde station (25.0° N, 121.1° E) several days before the three earthquakes. The mean- and median-based statistical techniques are introduced to investigate the ionospheric electron density prior to the three earthquakes. Results show that the foF2 decrease significantly before the three earthquakes.

Low latitude ionospheric effects near longitude 120°E during the great geomagnetic storm of July 2000

A great geomagnetic storm occurred on July 15/16, 2000 with a minimum value of about −300 nT in Dst index. Collecting digisonde data from ionospheric stations at Chungli, Wuhan, Kokubunji and Anyang, the ionospheric responses at the low latitudes near longitude 120°E during this storm are analyzed in this paper. There was a strong negative phase storm at low latitudes on July 16. The G-condition in the ionograms was clearly seen on the early first day after the commencement of geomagnetic storm. Those were considered to be caused by the storm-induced increase in the concentration ratios of neutral molecular O2 or N2 to atom O. On July 17 and some days thereafter, a positive phase storm appeared. In addition, anomalous equatorial ionization anomaly (EIA) inhibitions and developments were observed on July 16 and 17. There were also prominent nighttime enhancements in f0F2 during these days, and the diurnal variation of f0F2 was less clear than before.

Evidence for the geographic control of additional layer formation in the low-latitude ionosphere

Ionograms recorded from four ionosonde stations along the Western Pacific (WestPac) chain (about 122°E geographic, 192°E geomagnetic) are employed to study the occurrence of an additional layer at F-region altitudes during the 1-15 March 1998 WestPac campaign. It was found that the appearance of the additional layer at the local noontime hours is a typical phenomenon at Parepare (4°S geographic, 14.8°S geomagnetic). The additional layer was not clearly observed at Cebu (0.4°S geomagnetic) and Manila (3.7°N geomagnetic), and was not observed at Chung-Li (14.2°N geomagnetic) during the campaign. Furthermore, the additional layer was not seen from any of the station on 11 March 1998, a magnetically disturbed day. These results indicate that the fountain effect (produced by ExB motion) plays an important role in the formation of the additional layer. However, they also suggest the dynamics of the layer formation are in some way influenced by the location of the station relative to the geographic equator.

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.

THE MEAN DOPPLER VELOCITY DERIVED BY AN ADVANCED IONOSPHERIC SOUNDER - DYNASONDE

The lower ionosphere was examined by using a fixed sounding frequency 7.953 MHz of the EISCAT dynasonde. The phase parameter and Doppler spectrum techniques were applied on the echoes to obtain radial Doppler velocities. It is found that the two derived velocities are very different. A procedure is developed and introduced to apply on the Doppler velocities obtained from the phase parameter technique. Result shows that mean Doppler velocities derived by the weighted mean procedure are close to those of the typical spectrum technique.

A new source of the midlatitude ionospheric peak density structure revealed by a new Ionosphere‐Plasmasphere model

The newly developed Ionosphere-Plasmasphere (IP) model has revealed neutral winds as a primary source of the “third-peak” density structure in the daytime global ionosphere that has been observed by the low-latitude ionospheric sensor network GPS total electron content measurements over South America. This third peak is located near −30° magnetic latitude and is clearly separate from the conventional twin equatorial ionization anomaly peaks. The IP model reproduces the global electron density structure as observed by the FORMOSAT-3/COSMIC mission. The model reveals that the third peak is mainly created by the prevailing neutral meridional wind, which flows from the summer hemisphere to the winter hemisphere lifting the plasma along magnetic field lines to higher altitudes where recombination is slower. The same prevailing wind that increases the midlatitude density decreases the low-latitude density in the summer hemisphere by counteracting the equatorial fountain flow. The longitudinal variation of the three-peak structure is explained by the displacement between the geographic and geomagnetic equators.