The extreme Halloween 2003 solar flares (and Bastille Day, 2000 Flare), ICMEs, and resultant extreme ionospheric effects: A review Original Research Article

Extreme solar flares can cause extreme ionospheric effects. The Oct 28, 2003 flare caused a ∼25 TECU (a total electron content unit is 1016 electron/m2 column density), or a ∼30%, increase in the local noon equatorial ionospheric column density. This enhancement occurred within ∼5 min. This TEC increase was ∼5 times the TEC increases detected for the Oct 29, 2003, Nov 4, 2003, and the July 14, 2000 (Bastille Day) flares. In the 260–340 Å EUV wavelength range, the Oct 28 flare peak count rate was more than twice as large as for the other three flares. Another strong ionospheric effect is the delayed influence (due to solar wind propagation) of interplanetary coronal mass ejection (ICME) electric fields on the ionosphere. For the Oct 28 and 29 flares, the associated ICMEs propagated from the sun to the Earth at particularly high speeds. The prompt penetration of the interplanetary electric field caused the dayside equatorial ionospheric to be strongly convected upward. This led to enhanced TEC to values >300% nominal values in ∼2 h. Proposed mechanisms for this TEC enhancement will be discussed.