Different Responses of Dayside and Nightside Ionospheric Total Electron Contents to Penetration Electric Fields

Abstract Penetration electric fields (PEFs) are known to cause rapid and global disturbances in the ionosphere, but how their effects differ between the dayside and nightside remains unclear. In this study, we analyze the total electron content (TEC) response to PEFs during a recurrent interplanetary magnetic field Bz polarity reversal event on 20 April 2018, using Global Navigation Satellite System TEC data, magnetometer observations, ionosonde Doppler velocities, and SAMI2 model simulations. We find that dayside TEC exhibits simultaneous enhancements and depletions across latitudes, whereas nightside TEC displays drift‐driven perturbations superimposed on a net depletion. These differences arise because PEFs first modulate the vertical drift, which lifts or lowers the ionosphere, thereby altering the local balance between plasma production and loss. This balance is fundamentally different between day and night: continuous photoionization on the dayside can replenish plasma, whereas on the nightside, the lack of this source makes recombination losses dominant. The perturbations superimposed on the net depletion are driven by plasma transport via drifts. Model simulations confirm that photochemical processes dominate the TEC response, with transport playing a secondary role. These findings highlight the critical role of ionospheric photochemistry in shaping the global ionospheric response to PEFs.