Traveling Ionospheric Disturbances Detected During the Solar Eclipse of 24 October 1995

During the total solar eclipse period on 24 October 1995, the ionospheric data derived from Faraday rotation, differential Doppler frequency shift, and HF Doppler sounding at Lunping Observatory were used to detect the traveling ionospheric disturbances associated with the solar eclipse of that day. These observed ionospheric disturbances were characterized by quasi­ periodic wave-like trains lasting for several cycles with wavelengths of around 193 km, periods of around 12-14 min and propagated with a hori­ zontal velocity of around 296 mis from the direction of the eclipse path to the observation site. By use of a simplified bow wave front model, these traveling ionospheric disturbances are interpreted as the freely propagated atmospheric gravity waves generated by the supersonic movement of the Moon's shadow through the Earth's atmosphere along the eclipse path. (


INTRODUCTION
During the solar eclipse period, the cooling spot of the moon's shadow in the ozone layer at an altitude of about 45 km can be considered as a disturbance region in the lower atmo sphere.The supersonic movement of the moon's shadow generates acoustic gravity waves (AG waves) along the eclipse path (Chimonas, 1970;Chimonas and Hines, 1970).These AG waves can be observed either as periodic pressure oscillations recorded by a microbarograph on the ground (Goodwin and Hobson, 1978), or as fluctuations of virtual height and critical frequency in ionograms and in total electron content measurements at a distance from the eclipse path (e.g.Cohen, 1984, and references therein).
On 24 October 1995, the total solar eclipse provided a good opportunity to study the characteristics of solar eclipse generated traveling ionospheric disturbances (TID's ).At Lunping Observatory (25.00 ° N, 121.17 °E), the solar eclipse started at 111 lLT, ended at 1354LT, and the maximum occultation occurred at 1233LT, with the eclipsed area of the solar disk being about 37% at an altitude of 300 km.The following are the observations made at Lunping Observatory during the October 24 1995 solar eclipse period: ( 1) Continuous measurement of Faraday rotation of the linearly polarized VHF beacon signal at 136.38-MHz transmitted from GOES-3 geostationary satellite.
(2) Differential Doppler shift data, d <l>/dt, measured by using the 150-and 400-MHz coher ent signals transmitted from the US Navy Navigation Satellite System (NNSS) satellites (Huang and Cheng, 1991).

OBSERVATION RESULTS
Figure 1 shows the observed Faraday rotation during the afternoon period of the eclipse day.It is clear that the Faraday rotation exhibits a decaying phase of total electron content (TEC) diurnal variation.However after 1520LT, a distinct quasi-periodic oscillation of Fara day rotation lasting for two cycles superimposed on the decaying phase can be seen in the figure.Although the oscillation lasted for no more than two cycles, spectral analysis using the maximum entropy method (Ulrych and Bishop, 1975) shows that the period of the quasi periodic Faraday rotation perturbations is approximately 14.  d�/dt curve.The dashed curve in Figure 2 shows the residual value of � ( d <I> /dt).The residual curve reveals the fine structure of an atmospheric gravity wave covering the latitudinal region from 25 °N to higher than 35 °N.Since the maximum scanning time for a passage of the NNSS satellite is not more than 15 min and its traveling velocity (-7.4 km/s) is much faster than the propagation velocity of traveling ionospheric disturbances ( < 1 km/s), the observed ionospheric wavelike fluctuations can be considered to be a real stationary ionospheric quasi-periodic structure (Evans et al., 1983), with a wavelength of about 193 km.
Figure 3 shows the band-pass filtered (6-30 min) HF Doppler shift data obtained at 7.8 MHz from 1000-1800LT on the eclipse day.Quasi-periodic fluctuations lasting for several cycles can be seen between 1430-1520LT, as indicated by a horizontal bar.Spectral analysis, using the data obtained during 1430-1520LT, shows that it has a period of around 12 min.
Using cross correlation analysis and triangulation, the horizontal phase velocity and the azi muth of the propagation direction for the quasi-periodic oscillations were determined as 296 mis and 24 °, respectively.Taking this propagation velocity and the perturbation period, the horizontal wavelength of the perturbation can be estimated to be about 213 km.

SUMMARY AND DISCUSSION
During the total solar eclipse period on 24 October 1995, quasi-periodic TID's were ob  Based on the localized cooling spot caused by the moon's shadow near the peak of ozone absorption of solar X-ray and UV radiation, which occurs at an altitude of about 45 km, Chimonas and Hines (1970) and Chimonas (1970) developed the theory of an eclipse-induced gravity wave during the solar eclipse period.It is suggested that when the moon's shadow moves at supersonic speed across the atmosphere, the gravity waves will propagate freely with an oblique path to the ionospheric heights in the form of a bow wave front in the wake of the shaded region.
In an attempt to find an association between the observed TID's and the eclipse, a simpli fied model has been developed for the propagation of eclipse generated TID's and the observa tions are tested against this.The model uses the concept of a bow wave front, generated when a source is moving faster than the characteristic speed of waves in the medium through which it is traveling, where the angle of the bow wave front to the source direction is determined by the source speed and the characteristic wave speed (Chimonas, 1970;Chimonas and Hines, 1970).
In Figure 4  where the atmospheric gravity wave is observed.Comparing the detection times of the TID's listed in Table 1 with the locations of the bow wave front, it is interesting to note that the The horizontal propagation velocity vector determined from the HF Doppler shift sound ing is also plotted in Figure 4, as an arrow.Comparing the direction of the velocity vector with that of the ray path of the bow wave front, it is also suggested that the AG waves detected by the Doppler shift sounding are generated by the supersonic movement of the moon's shadow and propagate to Taiwan.In the realistic atmosphere, where the gravity waves propagate from the source in the upper troposphere to the ionospheric heights where the disturbances are observed, the gradient of background atmosphere may reflect or refract the waves, and the horizontal background wind in the stratosphere and the mesosphere can act as a directional filter (Yeh and Liu, 1974 ).The deviation of the observed propagation velocity and direction from those derived from the simplified bow wave front model may be as a result of the hori zontal background wind.
Experimental analysis in the realistic atmosphere (Row, 1967) and theoretical study in an isothermal atmosphere (Francis, 1975) show that the freely propagated gravity waves detected at the ionospheric height will accelerate as they move away from the source, and will have periods of oscillation proportional to the observer's distance from the source.From Table 1, it can be seen that the ionospheric perturbation period increases from 12 min, as derived from HF Doppler shift data, to 14.3 min, as derived from Faraday rotation data, with the ratio being 1.19.This ratio is consistent with the ratio between the distances from the eclipse path to where the data are measured, i.e. 2590/2260 = 1.15.This implies that the TID's observed during eclipse period are propagated freely from the source region on the eclipse path to the observation sites.
The results of this study suggest that the traveling ionospheric disturbances detected in the Faraday rotation, differential Doppler frequency shift, and HF Doppler sounding at Lunping Observatory were manifested by the freely propagating gravity waves which were generated by the supersonic motion of the moon's shadow through the Earth's atmosphere along the eclipse path, and propagated upward to the ionosphere around the equatorial anomaly region.
Fig. 1.Faraday rotation during the afternoon period of 24 October 1995.
1.The detection times of TID's and the distances from the eclipse path to where the data are measured are also listed in the Table.
the thick solid dotted curve AB represents the eclipse path.The center of the moon's shadow at each 5-min interval is indicated as a dot on the eclipse path.The speed of the moon's shadow is determined by Nautical Almanac Office (1994).The thin solid curves show the locations of the corresponding bow wave front at each 15-min interval at the charac teristic speed of 245 m/s.The dashed curves indicate the ray paths of the bow wave front along great circles.The asterisks indicate the locations of Lunping Observatory ( LNP ) and the GOES-3 subionospheric point.The track of the subionospheric point of NNSS satellite is shown as a chained curve in Figure 4.The hatched part of the NNSS track indicates the region

Fig. 4 .
Fig. 4. Simplified bow wave front model.The horizontal propagation vector determined from HF Doppler Network and the track of NNSS satellite detected quasi-periodic fluctuations are also plotted in the figure.