Ionospheric multiple stratifications and irregularities induced by the 2011 off the Pacific coast of Tohoku Earthquake
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 8 April 2011
Accepted: 7 June 2011
Published: 27 September 2011
A strong earthquake with a magnitude of 9.0 occurred at 1446:23 JST on March 11, 2011, in Japan. Ionospheric disturbances were detected at 1500 JST at four ionosonde stations. An irregular distortion of echo trace was observed at Kokubunji, which is the nearest station to the epicenter and is 440 km from it. Multiple-cusp-type trace indicating extra stratification was observed at Wakkanai and Yamagawa, which are 870 and 1410 km away from the epicenter. A small wavy fluctuation was observed at Okinawa 1910 km away from the epicenter. The real height analysis of the ionograms showed a vertical structure with a scale size of 20~30 km.
Perturbation of the ionosphere can occur after earthquakes by way of dynamic coupling between seismic waves and the atmosphere (e.g., Shinagawa et al., 2007). Disturbances in the ionospheric total electron content (TEC) are often reported associated with strong earthquakes (Heki and Ping, 2005; Heki et al., 2006; Liu et al., 2006; Otsuka et al., 2006).
TEC measurement using radio signals transmitted from GPS satellites has a large advantage of high time and spatial resolutions. However, it is not very easy to detect any vertical structure because TEC values are an integrated quantity along the radio propagation path penetrating the ionosphere.
On the other hand, ionosondes are limited in their number and location and usually they are programmed to run at intervals of several tenths of minutes. Despite such disadvantages their high capability in measuring vertical structure is expected to compensate TEC measurements. However, observations of earthquake-induced ionospheric deformation in ionograms are quite rare and almost nothing has been reported except in the case of the M 9.2 Alaska earthquake in 1964 (Leonard and Barnes, 1965).
A strong earthquake with a magnitude of 9.0 occurred off the Pacific coast of Tohoku at 1446:23 JST (UT+9 hr) on March 11, 2011. Associated with the earthquake, striking ionospheric disturbance was observed. This paper describes ionospheric disturbances in ionograms obtained at four locations over Japan.
2. Ionospheric Measurements
2.1 Perspective of the disturbance
The whole F trace showed irregular distortion with a kink near 6.5 MHz as shown by the arrow, above which the trace was split showing a non-vertical reflection (the oblique trace was not fully connected). The frequency of the kink is much higher than the F1 cusp normally observed (~4.5 MHz) as shown by the arrow in Fig. 1. Near the critical frequency of the X-mode wave at 10.5 MHz, the trace was distorted as circled by the dotted line, which indicates that the disturbance reached the F2 peak height at 306 km determined from the transmission factor, M3000F2. The F2 peak height was raised by 27 km as compared with the previous ionogram for 1444:30 JST.
Significant disturbance was observed with much more organized traces as compared with Kokubunji. The retardation of the lowest part of the F trace corresponds to foE as in the case of the ionogram for 1443:15 JST, except for the height which was lowered by approximately 20 km. The trace was disrupted at 3.4 MHz showing the appearance of a new layer above. The next layer showed an enhanced cusp at 3.96 MHz, which is close to the frequency of the normal F1 cusp (4~4.2 MHz) as indicated by ionograms at the same local time on other days. Two more weak cusps were noted at around 4.6 and 5.9 MHz in the F2 region. The heights of the discontinuity were determined by the POLAN real height analysis program (Titheridge, 1988). They are at 105 (E-layer peak), 130, 153, 173, and 199 km. The extrapolated R1 peak was at 255 km. (For more details of this analysis, see below for Yamagawa.)
The disturbance signature was very similar to that of Wakkanai (the O-X mode identification by eye is easier than Wakkanai except in a 6–7.5 MHz range). A weak sporadic E layer blanketed the lowest part of the upper trace in a frequency range 3.44–3.68 MHz. A discontinuity can be noted at 4.15 MHz and two well-defined cusps were at 4.75 and 8.40 MHz. The first of the two cusps was close to the F1 cusp at ~4.6 MHz as noted (fOF1) in the ionogram for 1445:45 JST. A careful examination with an assist of POLAN showed another cusp at 6.30 MHz.
Thus, overlapping traces were resolved in the 6–7.5 MHz range.
Comparing the two diagrams, we see that small changes in the vertical density gradient yield large changes in the virtual height of ionogram traces, and the characteristic heights were easily determined by combining the two. (The same analysis was conducted for Wakkanai and Okinawa.)
The height of the discontinuities were at 108 (E-layer peak), 133, 160, 191, and 224 km. The extrapolated F2 peak was at 262 km.
The disturbance was weak at Okinawa and the distortion showed a wavy pattern as circled by the dotted line in the bottom left panel of Fig. 2. The heights of the discontinuity were at 111 (E-layer peak), 141, 158, and 175 km. The last cusp was considered to be the normal F1 cusp as determined from the ionogram at the same local time on the previous day; the F1 cusp was moderately enhanced when compared with the previous frame of the ionogram in Fig. 1.
Thus the disturbance was limited in the F1 region, unlike in the case of the other stations.
The extrapolated F2 peak was at 298 km.
2.2 Commencement of the disturbance
The ionosondes are routinely operated at 15-min intervals. In addition to this, experimental mode soundings are scheduled at 5 min to each hour at Wakkanai. No disturbance was observed in this extra ionogram for 1455:00 JST at Wakkanai, and the disturbance commenced at 1458:15 JST there. Considering the distance of 870 km from the ionospheric epicenter, the apparent propagation velocity of the perturbation was estimated to be faster than 1220 m/s but slower than 1670 m/s. If the same velocity is applied to the South-West direction, the disturbance is expected to appear on the ionogram at a time between 1500:30 and 1505:30 JST at Yamagawa, and the actual observation was in the frame of 1500:45 JST as shown in Fig. 2.
Therefore, the times of commencement at Wakkanai and Yamagawa were consistent and the apparent propagation velocity was estimated to be at the faster end of the ambiguity range between 1220 and 1670 m/s.
At Okinawa, however, the expected time is between 1505:30 and 1512:30 JST and the actual observation was in the frame of 1502:00 JST. The disturbance was observed 3.5 min earlier than the expected time and the apparent velocity was estimated to be 2038 m/s.
Liu and Sun (2011) determined velocities of seismo-traveling ionospheric disturbances induced by the same earthquake for several modes. The velocities are 2100– 3200, 900, and 200 m/s for Rayleigh waves, acoustic gravity waves, and tsunami waves, respectively. The commencement of the ionogram signatures of disturbance was close to the range of propagation time of the Rayleigh wave, which might excite atmospheric waves having a slower propagation velocity.
2.3 Time evolution
After the ionograms first showed the earthquake-induced disturbances, time evolution of the ionospheric signatures differed from station to station. At Kokubunji, the closest station to the epicenter, the distorted ionogram trace was observed in only one frame of ionogram at 1500 JST. In contrast to this, the distorted ionogram trace continued until 1630 JST at Wakkanai. Well-defined cusps gradually changed to a weak wavy pattern before the disappearance of the disturbance. At Yamagawa, the ionogram trace once showed a weak wavy pattern at 1515 JST, and multiple enhanced cusps reappeared in the F1 and F2 regions at 1530 JST. After this, the trace returned to a wavy pattern at 1545 JST. At 1600 JST, the disturbance disappeared in the frequency range higher than 4.36 MHz, below which sporadic E layer blanketed the F trace. The disturbance at Okinawa was a wavy pattern at 1500 and 1515 JST. The F1 cusp was enhanced and a weak cusp appeared in the F2 region at 1530 JST. The disturbance almost disappeared at 1545 JST.
Summary of disturbance signatures.
2.4 Vertical scale of the disturbance
At Kokubunji, the disturbance extended from the supposed F1 cusp (~4.7 MHz) up to the F2 peak 13 min after the quake, though the F1 trace was blanketed by the sporadic E layer. The disturbance was significant at lower altitudes.
The lowest part of the distortion was between the E-layer peak and the F1 cusp. The ionogram trace resembled an intermediate layer or high-type sporadic E layer showing an abrupt transition to the upper layer.
A steep cusp was near the F1-layer critical frequency (foF1). The foF1 is mostly determined by a chemical process and shows a small day-to-day variability, unlike foF2. Therefore, the enhancement of the F1 cusp may not be a purely dynamical one but there seems to be contribution of a chemical process including recombination.
Two cusps were in the F2 region; a barely-defined one at lower altitude and a moderate one at higher altitude. The vertical separation of the two cusps was 26 and 33 km at Wakkanai and Yamagawa, respectively. The separation between the enhanced F1 cusp and the barely-defined lower cusp in the F2 region was 20 and 31 km at Wakkanai and Yamagawa, respectively.
Although the disturbance at Okinawa was weaker as compared with the other stations, several common features with Wakkanai and Yamagawa were observed. The lowest part of the distortion of the ionogram, a faint segment between 3.6 and 3.9 MHz inside the circle, was the intermediate layer. Comparing the ionograms at 1447:00 and 1502:00 JST, we note that the F1 cusp was evidently enhanced.
2.5 Peculiarity of Kokubunji ionogram
The ionospheric disturbance at Kokubunji was different in many aspects from the other three stations. The ionogram traces showed irregular distortion only at Kokubunji. The F-layer peak was raised by 27 km in 15 min at Kokubunji, while it remained at the same level at the other stations. The enhancement of the F1 cusp was not seen at Kokubunji. The ΔTEC map in Fig. 2 shows that there coexisted two types of TEC perturbations; a patchy pattern of TEC enhancement was noted at Kokubunji, which can be distinguished from the concentric pattern. The patchy pattern moved fast out of this area (Tsugawa et al., 2011). The unique signature of the ionogram distortion and its short duration time at Kokubunji might be related to this type of TEC disturbance.
The 2011 off the Pacific coast of Tohoku Earthquake occurred at 1446:23 JST (0546:23 UT) on March 11, 2011. Associated with the earthquake, ionospheric disturbance was detected approximately 14 min after the earthquake in quarter-hourly ionograms obtained at four locations from north to south along the Japanese archipelago. The disturbance was characterized by a vertical structure that is difficult to detect by TEC measurement using trans-ionospheric radio waves. Multiple stratifications were observed at Wakkanai, Yamagawa, and Okinawa such that an appearance of an intermediate layer or high-type sporadic E layer, an enhancement of the F1 cusp, and additional cusps in the F2 region. The vertical scale of the distortion, corresponding to the separation of the two consecutive cusps, was the order of 20~30 km.
The disturbance at Kokubunji, the closest station to the epicenter, was different in many aspects from the other stations. The ionogram trace showed irregular distortion and the F -layer peak was raised by 27 km after the earthquake. The enhanced F1 cusp and the intermediate layer were not observed.
The GPS data are provided by the Geospatial Information Authority of Japan. The authors thank J. Uemoto for running POLAN.
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