• solar flares;
  • ionospheric TEC;
  • enhancements;
  • GPS

[1] Extreme solar flares can cause extreme ionospheric effects. The 28 October 2003 flare caused a ∼25 total electron content units (TECU = 1016 el/m2 column density), or a ∼30%, increase in the local noon equatorial ionospheric column density. The rise in the TEC enhancement occurred in ∼5 min. This TEC increase was ∼5 times the TEC increases detected for the 29 October and the 4 November 2003 flares and the 14 July 2000 (Bastille Day) flare. In the 260–340 Å EUV wavelength range, the 28 October flare peak count rate was more than twice as large as for the other three flares. Another strong ionospheric effect is the delayed influence of the interplanetary coronal mass ejection (ICME) electric fields on the ionosphere. For the 28 and 29 October flares, the associated ICMEs propagated from the Sun to the Earth at particularly high speeds. The prompt penetration of the interplanetary electric fields (IEFs) caused the dayside near-equatorial ionosphere to be strongly uplifted by E × B convection. Consequential diffusion of the uplifted plasma down the Earth's magnetic field lines to higher magnetic latitudes is a major plasma transport process during these IEF (superstorm) events. Such diffusion should lead to inverted midlatitude ionospheres (oxygen ions at higher altitudes than protons). The energy input into the midlatitude ionospheres by this superfountain phenomenon could lead to local dayside midlatitude disturbance dynamos, features which cannot propagate from the nightside auroral zones.