The summer evening anomaly and conjugate effects

The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) GPS occultation data have been analyzed in this study to provide a better understanding of the Weddell Sea Anomaly (WSA) and to place it in the wider context of a general phenomenon that occurs near dusk in summer, which we are calling the summer evening anomaly to better capture its global nature. The terminator and the magnetically conjugate points for the terminator in the other hemisphere have been plotted on top of global maps of COSMIC NmF₂ and hmF₂ for 2 months either side of the December and June solstices for 2006-2008. These plots show that there are distinct enhancements of NmF ₂ and increases in hmF₂ as soon as the conjugate footprint of the field line on the winter terminator is seen at middle latitudes in the summer hemisphere. This effect is most pronounced where the WSA is formed, but it also occurs across the South Pacific Ocean in the southern summer and across much of the North Atlantic Ocean, Siberia, and Kamchatka during the northern summer. An hmF₂ increase occurs between the two terminators even at locations where there is no increase in NmF₂. A similar, but reversed, effect occurs in hmF₂ near dawn. This behavior appears to be most consistent with upward and poleward ion drifts in the evening, but neutral wind and downward precipitation may make important contributions to this effect.