Interplanetary magnetic field By control of prompt total electron content increases during superstorms

Large magnitude increases in ionospheric total electron content (TEC) that occur over 1-3 h on the dayside are a significant manifestation of the main phases of superstorms. For the largest superstorms of solar cycle 23 (based on the Dst index), ground networks of GPS receivers measured peak total electron content increases greater than a factor of 2 relative to quiet time TEC averaged over the broad latitude band ±40° for local times 1200-1600 LT. Near 30° latitude, the Halloween storms of October 29-30, 2003 appeared to produce storm-time TEC exceeding quiet time values by a factor of 5 within 2-3 h of storm onset, at 1300 LT. The physical cause of these large positive phase ionospheric storms is usually attributed to prompt penetration electric fields (PPEFs) initiated by Region 1 current closure through the ionosphere (Nopper and Carovillano, 1978 mechanism). An unresolved question is what determines variation of the TEC response for different superstorms. It has been suggested that the cross polar cap potential and Region 1 currents are significant factors in determining PPEF in the equatorial ionosphere, which are related to the solar wind reconnection electric field estimated by Kan-Lee and others. In this paper, we show evidence that suggests By may be a significant factor controlling the TEC response during the main phase of superstorms. We analyzed the interplanetary conditions during the period that TEC was increasing for eight superstorms. We find that increasing daytime TEC during superstorms only occurs for large reconnection electric fields when By magnitude is less than Bz. The data suggest that Bz is a far more important factor in the TEC response than the reconnection electric field. We also find that TEC decreases following its peak storm-time value for two superstorms, even though Bz remains large and By magnitudes are less than Bz. Such decreases during the geomagnetic disturbance may indicate the role of magnetospheric shielding currents, or of changes in the thermosphere that have developed over the prolonged period of large solar wind electric field. Further analysis is warranted covering a wider range of storm intensities on the role of By in affecting the daytime TEC response for a range of storm intensities.