Localization of the source of ionospheric disturbance...

2. Geometry of the Experiment

Figure 1

[21] The earthquake in the vicinity of Hokkaido Island occurred on 25 September 2003 at 1950 UT. Its magnitude was M = 8.3. The earthquake epicenter ( F_s =41.8 ^o N, L_s =143.85 ^o E) was located at a depth of 27-33 km under water between the Kurile and Japan cavities. The geometry of the experiment is shown in Figure 1.

Figure 2

[22] The earthquake in the vicinity of Sumatra Island occurred on 4 June 2000 at 1628 UT. The magnitude of the earthquake was 7.7. The epicenter ( F_s =4.7 ^o S, L_s =102.1 ^o E) was located at a depth of 33 km under the Earth surface. The experiment geometry is shown in Figure 2.

[23] The asterisks in Figures 1 and 2 show the position of the epicenter. Crosses correspond to the calculated position of the epicenter. Dark circles and diamonds show locations of the GPS stations and subionospheric points corresponding to the maximum response. The station names and numbers of the PRN satellites are written on Figures 1 and 2.

[24] The above presented information on the earthquakes was taken from the USGS (http://www.usgs.gov/). The coordinates of the GPS stations used in the experiment can be found at the SOPAC Web site (http://sopac.ucsd.edu/).

[25] The level of geomagnetic disturbances during the events in question was quite moderate (the Dst variations were within -21/11 nT, the Kp index varied from 1 to 4). The data on the geomagnetic indices Dst and Kp were taken from the WDC site.

[26] The initial data for formation of the spatial-time distribution of the ionospheric responses to SAW are temporal series of the high-frequency variations in TEC dI(t) and corresponding to them series of the azimuth a(t) and elongation angle q(t) of the ray to the satellite. The method of data processing was described in detail by Afraimovich et al. [2001b].

Figure 3

[27] The time dependences of the initial series of "oblique" TEC I_i(t) for the earthquakes on 4 June 2000 (NTUS; PRN 03) and 25 September 2003 (MIZU; PRN 24) are presented in Figure 3 (panels a and g, respectively). Figure 3 (panels b-e and h-k) shows the filtered in the 1-10 min range variations in TEC dI_i(t).

[28] Figure 3 shows that on the background of slow changes in TEC, quick N-shaped oscillations caused by propagation of SAW are distinctly seen. The parameters of the registered responses to the earthquake on 25 September 2003 lie within the following limits: the amplitude and disturbance period change from 0.1 to 0.25 TECU and from 360 to 900 s, respectively (Table 1). The corresponding parameters for the event on 4 June 2000 are from 0.1 to 0.5 TECU and from 180 to 330 s, respectively (Table 2).

[29] To eliminate the uncertainty of the localization of the ionospheric response of SAW caused by the integral character of the TEC values, we assume that the TEC disturbance is formed in the point where the ray to the satellite crosses the plane at a height of h_d above the maximum of the ionospheric F region providing the main input into the TEC formation. Below we take for the calculations a value of h_d=400 km, which in the best way corresponds to the nighttime conditions of the experiment (2328 LT and 0450 LT). The geographical coordinates of the subionospheric point ( F_i, L_i ) were determined for this height. The trajectories of the subionospheric points are shown in Figures 1 and 2 by thin curves. In the scope of our method the TEC disturbances registered at the network of GPS receivers are considered as a set of signals of an nonequidistant phased grating of "ionospheric detectors" with the known coordinates. Calculating the coordinates of subionospheric points for the time moments text, corresponding to the extreme value of the TEC response ( t_max for the earthquakes at Sumatra and t_min for the earthquake at Hokkaido are shown in Figure 3, panels c and i, respectively), we determine the spatial position of the ionospheric response to SAW. The position of each ray to the satellite is determined by the value of the elongation angle q_s(t) counted from the terrestrial surface northward and azimuth a_s(t) counted from the northward direction clockwise for the time moments t_ext. Determining the time moments t_ext,i for each series dI_i(t) and corresponding coordinates ( F_i, L_i ) of the subionospheric points, we obtain a spatial-time distribution of the ionospheric responses to SAW (Figures 1 and 2). The values of t_ext,i, F_i, and L_i for each ray "GPS receiver-the satellite" which detected ionospheric responses are shown in Tables 1 and 2.

Figure 4

[30] Figure 4 schematically shows the geometry of the GPS sounding and determination of the parameters of the ionospheric disturbance during the Sumatra earthquake on 4 June 2000. The earthquake epicenter is marked by an asterisk. On top the horizontal cross section of the spherical front of the TEC disturbance propagating with a radial velocity V from the source localized at a height of h_s=350-400 km (is marked by a cross) is shown. "Ionospheric detectors" the coordinates of which are determined from the GPS data (diamonds) are detecting the ionospheric response to the disturbance at various moments of time.