Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

Earthquakes ’influence on the space weather current (equivalent to one cell) sources of different signs and magnitudes, given in a single node of the numerical grid and (2) “line” sources. According to UAM simulations point current sources with magnitudes of about 10‘5A/m2 and 1O'6A/m2 given in a single grid node (corresponding to one grid cell of 5° x 2° or approximately 500 km x 200 km, and averaged vertical electric current densities of 5X10"6A/m2 and 5><10'7A/m2, respectively) induced extremely strong and unrealistic TEC disturbances and very intense vertical drift motions. Point sources of 1O'9A/m2and 5*10'9A/m2 triggered TEC disturbances not exceeding 15-30% in magnitude. The “line” kind sources have been simulated as vertical external currents with a magnitude of 4x 10‘8A/m2 directed from the ionosphere to the Earth set centric at 9 numerical grid nodes with 5° longitudinal steps along the magnetic parallel of the earthquake epicenter. It corresponds to an external electric current density of 2*10‘8A/m2 set on the region of approximately -200 km x -4000 km (2° along the meridian and 40° along the parallel). The generated TEC disturbances have reached -20-50% by magnitude depending on the current’s spatial distribution and lifetime. Simulation results are presented in the paper by Namgaladze et. al. [201 lb] in this issue. As one can see, the additional electric potential generated by the external current reduces down to zero when approaching the terminator in both (modelled and observed) cases. The corresponding TEC disturbances disappear later than the electric potential with a time-lag of about 4-6 hours. The simulations underestimate the observed magnitude of anomalous TEC increase at the northern and overestimate it at the southern hemisphere. Both (modelled and observed) cases show stronger TEC magnitudes at the magnetically conjugated region in comparison with the near-earthquake epicenter area. Discussion and conclusions Summarizing the numerical simulation results with external electric currents flowing between the lower atmosphere and ionosphere due to seismo-induced conductivity changes of the underlying atmosphere column, one can state that they reproduce the main features of the TEC variations. In both the GPS observations and model cases we have: (1) areas of the increased TEC existed and are localized in the near-epicenter area. (2) Magnetic conjugation took place. (3) TEC disturbances at the magnetically conjugated area were stronger by magnitude than in the near­ epicenter region. (4) The amplitude of TEC enhancements reached 50% and more. (5) Negative structures (areas of the TEC reduction) existed westwards and equatorwards relative to the positive ones. (6) Approaching the sunrise terminator with the well-conducting sunlit ionosphere triggered a shift of the anomaly to the night-side and causes the subsequent suppression of the TEC variations both at the epicenter and in the magnetically conjugated regions. (7) The anomalies did not exist during the near-noon hours. Such behavior and the lifetime agreed with [Akhoondzadeh and Saradjian, 2011] results. The obtained morphological features also agreed with [Pulinets and Tsybulya, 2010]. We should notice that a few discrepancies between the observations and the model results persist: (1) the simulations underestimate the amplitudes of the TEC anomaly at the northern hemisphere and overestimate them at the southern one in comparison with the observations. (2) Negative structures are stronger than the observed ones by magnitude in the model case. (3) The increased TEC area occupies a smaller region in the model case. (4) The isoline patterns and the magnitudes differ from the observed ones but not drastically. The discrepancies between the simulation results and the GPS observations could be due to the rather simplified assumptions about the external electric field sources, their density distribution and acting regime. Night domination effects, the existence of a “ban-time” and the terminator-driven TEC anomaly suppression should be due to the changes of the conductivity of the ionosphere. It is related to the crossing of the sunrise terminator, where the well-conducting sunlit ionosphere leads to a depression of the electric potential disturbances and to a reduction of the electric field, generated by the externally driven electric current. The obtained vertical currents required for creating TEC anomalies of about 50% (2 10‘8 A/m2) are very large, they exceed the so called “fair weather” currents in 104 times. Perhaps, such currents are the upper limit of the possible vertical currents between the Earth and ionosphere. They can exist only limited time at the limited areas during preparation of the very strong earthquakes such as in Haiti 2010 and Japan 2011 cases. It is very difficult to observe them directly due to lack of the appropriate instrumentation net. But their ionospheric effects can be revealed by the TEC monitoring at least in cases of very strong earthquakes. References Akhoondzadeh, M , Saradjian, M.R. TEC variations analysis concerning Haiti (January 12, 2010) and Samoa (September 29, 2009) earthquakes. Adv. Space Res. 47. N. 1. 94-104 (2011). Chmyrev V M , Isaev, N .V , Bilichenko, S.V, Stanev, G.A. Observation by space-borne detectors of electric fields and hydromagnetic waves in the ionosphere over on earthquake center. Phys. Earth Planet. Inter. 57. 110-114 (1989). 135