Ionosphere topside sounding data from the INTERCOSMOS-19 satellite have been utilized to study longitude distribution of the night-time midlatitude F2 region peak electron density NmF2 in the Southern hemisphere over the array of tectonic faults in the Andes area for low geomagnetic activity periods in July 1979 and in July 1980 when solar activity was high and nearly the same, however, in July 1980 there was intensified tectonic activity in the Andes area, and in July 1979 the tectonic activity was lower. The results show that in July 1980 the observed value of NmF2 as a function of longitude decreased by a factor of ~3 with the change of longitude from 280¿ to 295 ¿E over the latitude range 30¿--45 ¿S, whereas the appropriate longitude profiles of NmF2 observed in July 1979 and reproduced by the ionospheric model IRI-90 do not reveal any appreciable longitude variations above the Andes area. Using model simulation we interpret the observed longitude anomaly of NmF2 in July 1980 as a result of the F2 region modification caused by a hypothetical large-scale long-living perturbation of the vertical electrostatic field on the Earth's surface associated with intensified tectonic activity. ¿ 1998 American Geophysical Union