Ho-Fang Tsai

Variations of ionospheric total electron content during the Chi‐Chi Earthquake

On 20 September1999 UT (21 Septemberin local time, LT) a large earthquake Mw=7.7 struck central Taiwan nearthe small town of Chi-Chi. The greatest plasma frequency in the ionosphere, foF2, observed by the Chung- Li ionosonde (25.0 ◦ N, 121.2 ◦ E) reveals three clear precur- sors at 1, 3, and 4 days prior to the earthquake. This paper examines the ionospheric total electron content (TEC) ob- served by a network of the global positioning system (GPS) receivers in Taiwan area. It is found that variations in foF2 and overhead TEC recorded at Chung-Li have a similar ten- dency. Combining the data of the network of 13 GPS re- ceivers, time, and spatial variations of TEC prior to the Chi- Chi earthquake are examined. Results show that the equa- torial anomaly crest moves equatorward and its TEC value significantly decreases 1, 3, and 4 days before the earth- quake. A comparison between the disturbed and reference (previous 15-day median) days confirms that TEC decreases significantly around the epicenter in the afternoons of these days. Finally, possible mechanisms are proposed and dis- cussed.

Artificial plasma cave in the low‐latitude ionosphere results from the radio occultation inversion of the FORMOSAT‐3/COSMIC

Previous studies report unexpected electron density reductions, termed “plasmacaves,” located underneath the equatorial ionization anomaly (EIA) crests. A radiooccultation (RO) observation simulation experiment has been built to evaluate possiblebiases introduced by the spherical symmetry assumption in the standard (Abel) ROinversion processes. The experiment simulates the electron density profiles andreconstructs the plasma structure of the EIA at low latitudes, where the horizontal gradientis most significant. The reconstruction shows that artificial plasma caves are createdunderneath the EIA crests along with three density enhancements in adjacent latitudes. Theartifact appears mainly below 250 km altitudes and becomes pronounced when theEIAs are well developed. Above that altitude, the two EIA features in the original (truth)model, the International Reference Ionosphere (IRI‐2007), and in the inversion are similar,but the inversion reconstructs less distinct EIA crests with underestimation of the electrondensity. A simple correction has been introduced by multiplying the ratio between thetruth and inversion with actual FORMOSAT‐3/COSMIC observations. This initialcorrection shows that the artificial plasma caves are mitigated. Results also reveal that theRO technique is not suitable to detect or rule out possible existence of the plasma caves.

Seasonal variations of the ionospheric total electron content in Asian equatorial anomaly regions

The ionospheric total electron contents (TEC) in both northern and southern equatorial anomaly regions are examined by using the Global Positioning System (GPS) in Asian area. The TEC contour charts obtained at YMSM (25.2°N, 121.6°E; 14.0°N geomagnetic) and DGAR (7.3°S, 72.4°E; 16.2°S geomagnetic) stations in 1997, solar minimum, are investigated. It is found that the ionospheric crests manifest remarkable seasonal variations. The TEC values on both northern and southern equatorial anomaly crests yield their maximum values during the vernal and autumnal months, but the winter anomaly does not appear in the southern region. Results show that both crests are fully developed around midday in winter, postnoon in equinoxes and late afternoon in summer, and the two crests move significantly equatorward in winter but slightly poleward in summer and autumn. These phenomena can be fully explained by a combined theory of the transequatorial netural wind, the subsolar point, and the auroral equatorward wind.

The effect of geomagnetic storm on ionospheric total electron content at the equatorial anomaly region

Abstract The effects of geomagnetic storm on ionospheric total electron content (TEC) have been investigated by using the Global Positioning System (GPS) data during two observations on January 10 and May 15, 1997, respectively. It is found that after the onset of sudden storm commencement (SSC), the equatorial anomaly crests move poleward, and the daytime TEC is significantly reduced one day after SSC. Some possible mechanisms are proposed to explain the above phenomena.

Variations in the equatorial ionization anomaly peaks in the Western Pacific region during the geomagnetic storms of April 6 and July 15, 2000

This study utilizes total electron content (TEC) observed by a network of ground-based GPS receivers located in the Western Pacific region (∼120°E) to study the responses of the low-latitude equatorial ionization anomaly (EIA) to the two major magnetic storms that occurred during April 4–10 and July 12–18, 2000. The latitude, time, and TEC (LTT) maps in the northern and southern EIA regions show that both EIA peaks move equatorward along with a pronounced reduction of the TEC values 10–12 h after the storm onset. The variations in the EIA peak TEC values and locations in the northern EIA are highly correlated with those in the southern EIA. The correlation coefficients of the day-to-day variations of peaked TEC between the northern and southern EIA regions are 0.75 in the April storm and 0.83 in the July storm. The correlation coefficients of the day-to-day EIA peak movements between the two hemispheres are 0.98 in the April storm event and 0.88 in the July storm event. The highly correlated peaked TEC and movements between the northern and the southern hemisphere suggest that the storm-produced electrodynamics played a dominant role in affecting the low-latitude ionosphere during the two major storms.

GPS phase fluctuations observed along the american sector during low irregularity activity months of 1997-2000

The Global Positioning System (GPS) provides an alternative way to investigate ionospheric irregularities and their effects on the radio wave propagation. The method is based on fluctuations of the total electron content (TEC) resulted from the ionospheric plasma irregularities. Previous studies have showed the correlation between the radiowave intensity (including GPS signals) and ionospheric irregularities during magnetic storm periods. In this study, phase fluctuations derived from GPS signals are used to address aspects of the ionospheric storm events during the low irregularity activity months. We analyze data from seven GPS stations located in Central- and South-America during eight magnetic storms occurred from 1997 to 2000. It is found that, in general no significant feature in the phase fluctuation is observed during the low irregularity activity months, except during the 26 August 1998 and the 15 July 2000 storms. A detailed study shows that the GPS phase fluctuations develop when the Dst index begins to decrease significantly. This phenomenon cannot be compared directly to previous observations and model results due to the fundamental difference in the background levels of irregularity activity. To better understand the generation of ionospheric irregularities during the storm period of the low irregularity activity months, the temporal relationship between the magnetic Dst index, equatorial anomaly TEC, and the GPS phase fluctuations are examined and discussed.

Ionospheric total electron content observed during the 24 October 1995 solar eclipse

Abstract During the solar eclipse of October 24, 1995, the effect of an eclipse on the total electron content (TEC) of the ionosphere can be investigated by using measurement of the Global Positioning System (GPS). The TEC derived from five GPS ground-based receivers have been used to observe ionospheric variations over the geomagnetic equatorial, equatorial anomaly, and mid-latitude regions. The deviations in the TECs on the eclipse day from those on reference days show that during the eclipse days the ionosphere experienced some changes. Four features of the TEC deviations, pre-ascension (PA), major depression (MD), sunset ascension (SA), and secondary depression (SD) have been observed. Possible mechanisms explaining in the four features are investigated and discussed.

Longitudinal Structure of the Mid- and Low-Latitude Ionosphere Observed by Space-borne GPS Receivers

This study presents longitudinal structures of the mid- and low-latitude ionosphere using the GPS radio occultation observation on board the COSMIC satellite mission. The longitudinal structure seen in the equatorial and low-latitude ionospheric regions results from modification of the daily dynamo electric field by upward propagating atmospheric tides that are generated by latent heat release of the tropical rainstorms. Changes of the dynamo electric field modify the equatorial plasma fountain and thereby enhance the equatorial ionization anomaly (EIA). With capability of three-dimensional global ionospheric observation, altitudinal, local time, and monthly variations of this recent discovered fascinating feature are obtained for further understanding of the underlying physical mechanism. Through comparison between electron densities at various altitudes, the longitudinal structure is prominently seen at upper part of the ionosphere. Additionally, COSMIC observations provide three-dimensional structure of the Weddell Sea anomaly which is featured by the greater nighttime electron density than daytime. Not only occurring at the southern hemisphere near the Weddell Sea region of the Antarctica, a similar nighttime density enhancement feature is also found in the northern hemisphere during local summer by COSMIC observations. The anomalous signatures in both hemispheres share very similar characteristics in electron density structure, latitudinal distribution, and appearance time. They are, therefore, categorized as the mid-latitude summer nighttime anomaly (MSNA).

A study on the cosmic electron density profile

Abstract This paper shows a simulation process of retrieving ionospheric electron density from the Global Positioning System/Meteorology (GPS/MET) to examine the accuracy of the retrieval procedure for the COSMIC project 3 during solar minimum and maximum periods. Results show that the error of the maximum electron density ( NmF 2 ) is under 25%, and the error is greater in the solar maximum than in the solar minimum period. The mean error of the height of NmF 2 (i.e. hmF 2 ) is less than about 13 km and seems to have no correlation with solar activity.