Observations of ionospheric disturbances via U-shaped traces on ionograms

Abstract

The ionospheric vertical sounding i s a basic technique for studying the Earth’s ionosphere. Taking into account the fact that conditions for propagation of radio waves depend on the operating frequency , d iagnostics of the ionosphere is performed using digital ionosondes that measure the delay s of decameter radio signal s of different frequencies. An ionogram is a display of the data produced by an ionosonde. It is a graph of the virtual reflection height of the ionosphere (actually , time between transmission and reception of a radio signal ) versus sounding frequency. Vertical and near-vertical ionograms provide the major share of information about the space-temporal structure of the ionospheric plasma above the ionosonde . Of particular interest is investigating dynamic processes from the series of ionograms taken once each minute. Studying seismic ionospheric effects by minute ionograms can be highly informative. It is known that the parameters of the ionospheric layers can significantly vary both before and after an earthquake , and such deviations are detectable from ionosonde data. Recent observations show that the effects from an earthquake can occur at large distances from the epicenter. For instance , s oundings in the Irkutsk region detected the anomalies that occurred several dozens of minutes after the main shock of the 11 .03. 201 1 earthquake in Japan. At the initial phase, the anomaly was recorded as a multicusp structure i n the ionogram, which was probably related to the multiple layers of the ionosphere. Th is structure was interpreted by iterative reconstruction of the electron density profile in a one-dimensional approximation (stratified ionosphere) , and the interperation was published . At the next p hase, the ionograms showed a U-shaped structure with a specific multiple reflection. Multipath propagation is usually associated with additional off-vertical ray paths, which are caused by traveling ionospheric disturbance s (TIDs) . In this regard, it is appropriate to use a 2D model of electron density , depending on both the vertical and horizontal coordinates. In our study , geometric optics approximation was used to simulat e vertical ionograms under the conditions of strong horizontal gradients of electron density. The study results show that d ifferent positions of the U-shaped structure with respect to the main tra ce of the ionogram can be obtained for a single TID . Specific ray paths forming the additional trac es of the ionogram are described . D epending on the TID location , the U-shaped structure can occur with or without a specific multipath reflection.