When a solar flare erupts, signal strength increases abruptly in an event known as a sudden ionospheric disturbance
After carefully inspecting the [DELTA]VTEC cube, it was observed that for January 1, 2018 at ~ 18:00-23:00 UT there was not a clear detection of an ionospheric disturbance
. However, on January 2 (12 days before the seismic event) a positive ionospheric anomaly is clearly identified.
Among them, A(t) is the echo amplitude, [theta](t) is the phase of echo when the ionosphere is stable, and m(t) is ionospheric disturbance
These disturbances (e.g., magnetic storms, SID-Sudden Ionospheric Disturbance
, TID-Travelling Ionospheric Disturbance
)  can affect signal propagation.
Bernard Minster of the Scripps Institution of Oceanography in La Jolia, Calif., detected the ionospheric disturbance
using the Global Positioning System (GPS), an array of navigational satellites orbiting Earth.
The ionospheric disturbance
can cause the prominent azimuth shift on InSAR images.
The impact of the solar flare can be seen in the Sudden Ionospheric Disturbance
(SID) which I timed as starting at 15:58 and reaching a maximum deviation at 16:41.
Ten solar observers in the United States and one each in Canada and Switzerland, all members of the American Association of Variable Star Observers (AAVSO), recorded this event as a sudden ionospheric disturbance
The mystery was solved when Michael Kelley at Cornell University in Ithaca, N.Y., and Robert Livingston and Mary McCready at SRI International in Menlo Park, Calif., linked the nine-hour-long ionospheric disturbance
to an earthquake that had occurred 1,000 kilometers from the radar site just before the oscillations began.
Simultaneous observation of SIDs at different frequencies reveals apparently contradictory behaviour from the same event; some frequencies appear to be attenuated by the ionospheric disturbance
, while others are enhanced.
The result is a very slow ionospheric disturbance
in each case, with several changes in signal strength as the interference pattern moves over the receiver.