While a prompt penetration of an eastward electric field during a magnetic storm is known to increase the F-region electron density, observations have shown that the F-region peak density (NmF2) sometimes decreases before it increases, although total electron content (TEC) begins increasing immediately after the electric field penetration. We investigated potential mechanisms of the observed NmF2 decrease as well as of the TEC increase. Simulation showed that increased upward or downward diffusion flux in the topside ionosphere caused by layer uplift temporarily reduces the F-region density at midlatitudes (mechanism A). In addition, the E × B motion toward an outer L shell region leads to flux tube expansion and plasma dilution, which also contribute to the NmF2 decrease (mechanism B). The TEC increase can be attributed to quick plasma production below the uplifted F2 peak. Simulation also showed that penetration events with an electric field magnitude greater than several mV/m can cause such a phenomenon. We also examined preferable conditions for this phenomenon, and found that degree of NmF2 decrease tends to become significant due to effect of mechanism A if a prompt electric field penetration occurs in the early morning or if the plasma temperature is high enough in the topside ionosphere.
展开▼
