Fig. 4.21 Regional TEC difference maps for 02 UT-24 UT, 8 March 2011. Star , epicenter
position; orange circle , subsolar point; black curve , the terminator line. Local time is for the
11 March 2011 epicenter position in Japan
with the epicenter area and time interval before and after the M9.0 event. An M7.2
earthquake 9 March 2011 (38.424 ı N, 142.836 ı E), D 32 km, 02:45 UT/11:45 LT,
took place at almost exactly the same epicenter position as the 11 March event.
Also on 9 March, a second earthquake, M6.5, (6.022 ı S, 149.659 ı E), 21:24 UT,
happened. Thus, we have to look at the TEC variations as triggered by a united
source of a spatially distributed cluster of earthquakes. The geomagnetic activity
was quiet to moderate with episodic periods of moderate to strong disturbances.
During 6-10 March 2011, the D st index was within the interval of 22 nT C 4nT
without any significant disturbances. On 10 March, a moderate magnetic storm took
place: D st index varied from 7nTdownto 82 nT on 11 March.
The selected set of seismic events allows us to successively investigate from the
'clear' case of a seismic impact on the neutral atmosphere to 'disturbed' ones with
combined actions of geomagnetic and seismic activities. In spite of the differences
between geomagnetic activity and seismic sources, common features derived from
these cases should be caused by the same physical nature of lithosphere-ionosphere
As for the previous cases, we defined and calculated the background TEC values
(i.e., undisturbed conditions) as 7-day UT-grouped running observation medians
before the current calculation moment. The resulting TEC difference (%) maps are
presented in Fig. 4.21 .
In the 11 March 2011, Japan, M9.0 (38.322 ı N, 142.369 ı E), D 32 km and
associated earthquakes, the following TEC disturbances took place. On 8 March
2011, TEC enhancements up to 80% were observed from 06 UT until 20 UT; they
first were formed at the near-epicenter areas, then their magnitude increased. During
08 UT-12 UT, the anomalies spread out along the parallel at both hemispheres,
occupying a larger region. From 12 UT onward there was a tendency to replenish
the Appleton's anomaly. The TEC anomaly comprised an ellipse-like region with
the earthquake on its border. At 18 UT-20 UT, we see a single increased TEC spot.