We discuss the extreme solar flares of 28 and 29 October and 4 November of 2003 and 14 July 2000 (Bastille Day event) and their photoionization effects on the dayside ionosphere. Meier et al.  modeled the ionospheric effects of the Bastille Day flare, prior to the use of GPS total electron content (TEC) measurements. In this paper we will show the GPS measurements and the dramatic ionospheric changes caused by the flare EUV photons. The flare-associated interplanetary coronal mass ejection (ICME) motional electric fields affected the Earth's equatorial and midlatitude dayside ionosphere as well. These delayed (by solar wind propagation) effects will be discussed. Figure 1 shows the SOHO Solar EUV Monitor (SEM) instrument 260–340 Å (EUV) count rates for all four extreme flares. An instrument description can be found in work by Judge et al. . The flares have been adjusted to their preflare baselines, so the relative count rate enhancements can be readily discerned. These flux increases are the factors that lead to enhanced photoionization of the dayside upper atmosphere. See Tsurutani et al.  for more details.
 In this EUV wavelength range, the 28 October 2003 event is by far the largest, with a peak count rate greater than twice that of the other three events. The 4 November, 29 October, and Bastille Day events were roughly comparable in peak intensity. The 28 October event had a secondary peak in the decay phase of the event that effectively “broadened” the flare width. This secondary peak was not detected in the 1–8 Å (GOES satellite) x-rays.
 The rise in EUV peak count rate was the sharpest in the 28 October event (∼2.5 min 1/el risetime, measured backward from the peak), ∼5 min for 29 October and 4 November, and unusually slow ∼10 min for the Bastille Day event (B. T. Tsurutani et al., Rise and decay time-scales for SEM (SOHO) solar flares: Constraints of magnetic reconnection, unpublished manuscript, 2005).
 In all of the flare events except for 4 November, there was an increase in the SEM count rates after the flare had decayed away (however, on the Bastille Day event, the increase occurred during the flare decay phase). This increase is caused by solar flare energetic charged particles being detected by the SEM sensors. These increases occurred well after the flare peak count rates and therefore had negligible effects on the flare profiles. The 4 November flare was a limb event. Particles accelerated by the flare or ICME shock [Tsurutani et al., 1982; Pesses et al., 1984; Tsurutani and Lin, 1985] presumably did not have easy access to interplanetary magnetic fields that connected to Earth, thus the lack of an increase in SEM count rate after the flare.
 Figure 2 gives the “verticalized” TEC for a near-equatorial, local noon ground-based GPS receiver (Africa) for the 28 October 2003 flare. A total electron content unit (TECU) is 1016 el/m2 column density. The TEC is determined by reduction of relative phase shifts between dual-frequency (∼1.2 and ∼1.5 GHz) signals transmitted from GPS satellites orbiting at 20,800 km altitude (see Mannucci et al. , Iijima et al. , Afraimovich , and Afraimovich et al.  for discussions of conversion of dual-frequency GPS signals to total electron content). The six GPS satellites tracked by the ground-based receiver are indicated in the insets. The TEC risetime was coincident with the flare onset, but the decay was far longer. The latter effect is due to photoionization occurring at high altitudes (>150 km) where the recombination rate is considerably lower than for lower altitudes. The increase in TEC from baseline to peak occurs within ∼15 min (the vast majority of the rise occurs within 5 min) and has a magnitude of ∼25 TECU. One hour prior to the flare, the dayside TEC at noon at the equator was ∼82 TECU, so this flare caused an abrupt ∼30% increase in the ionospheric TEC. The TEC increases for the 29 October, 4 November, and Bastille Day flares were all ∼5 TECU. This apparent near equivalence of their ionospheric effects is consistent with their flare peak intensities being comparable. See also Zhang and Xiao  for further ground-based TEC measurements of the 28 October flare event and Afraimovich  and Afraimovich et al.  for discussion of ionospheric effects of weaker flares.
 The 29 October flare onset occurred at ∼2036 UT. After an initial increase of ∼5 TECU, the ionospheric enhancement then increased further to much greater amplitudes. As will be shown later, this was coincident with the arrival at Earth of the ICME launched on 28 October. The interplanetary and ICME data for the Halloween events are shown in Figure 3. The plots, from top to bottom, are the solar wind speed, proton temperature, proton density, He++/p+ number density ratio, magnetic field magnitude, Bz component (in solar magnetospheric coordinates), and the geomagnetic Dst index. The plasma data are from the ACE Solar Wine Electron, Proton, and Alpha Monitor (SWEPAM) instrument, and the magnetic field data are from the magnetometer. There are two extremely high solar wind speed events (top), one on 29 October (VSW > 1850 km/s, with a most probable speed of ∼2240 km/s), and a second event (VSW = 1710 km/s) on 30 October [Skoug et al., 2004]. There is a third (relatively) small event on 4 November. The first two ICMEs and leading shocks (denoted by the abrupt increases in VSW, T, B, and N in Figure 3) are associated with the 28 and 29 October flares, respectively.
 There is a large interplanetary negative Bz event (∼−30 nT) at the shock ahead of the 30 October ICME. This is caused by shock compression of preexisting magnetic cloud material [Mannucci et al., 2005]. This negative IMF Bz event is responsible for the magnetic storm on 30–31 October (Dst decrease). The Dst index, an indicator of the intensity of the storm time ring current, is well correlated with the IMF Bz event (see discussion by Gonzalez et al. ). The solar wind convection of southward magnetic fields creates a dawn-to-dusk (motional) electric field as viewed from the Earth. Kelley et al.  have noted that these electric fields can “promptly penetrate” to the equatorial ionosphere.
 The ionospheric effects (in TECU) during the 30–31 October 2003 (Halloween) interplanetary dawn-to-dusk electric field event are shown in Figure 4. Three CHAMP satellite passes are shown in Figure 4. CHAMP was at an altitude of ∼400 km and crossed the magnetic equator at ∼1300 local time for each of the three passes. The onboard GPS receiver measured the TEC at altitudes above the satellite. The first pass starting at 1825 UT occurs before the negative IMF Bz event (note that several GPS satellites were tracked simultaneously; verticalized TEC values are shown for all satellites at greater than 40° elevation angle relative to CHAMP). The second pass at ∼2000 UT occurs ∼1 hour 15 min after the negative IMF Bz event had impinged upon the magnetosphere, and the third pass at 2145 UT occurs ∼2 hours 45 min after the start of the interplanetary electric field event.
 In the prenegative IMF Bz interval, the dual peak “fountain effect” is noted. The ionospheric peaks are located at ±10° magnetic latitude (MLAT). The cause of the fountain effect is the E × B force due to the ionospheric dynamo eastward electric field. This convection produces vertical plasma drift of the postdawn equatorial ionospheric plasma and the eventual diffusion of this plasma down the magnetic lines of force to higher latitudes on either sides of the equator. At 2000 UT, after the imposition of the negative IMF Bz fields, the TEC above CHAMP (∼400 km altitude) was ∼210 TECU at ±20°–25° MLAT. At 2145 UT, the TEC became even higher, reaching peak values of ∼330 TECU at 28° MLAT. This effect has been called the “dayside superfountain effect” [Tsurutani et al., 2004; Mannucci et al., 2005]. See also related results of Tanaka , Greenspan et al. , and Basu et al. .