The Altitudinal Dependences of the Inter‐Hemispheric Asymmetry in the Mid‐Latitude Ionospheric Post‐Midnight Enhancement During Equinox

Using observations from multi‐satellites at different altitudes, including the CHAllenging Minisatellite Payload (CHAMP), the Gravity Recovery and Climate Experiment, Swarm B, and Defense Meteorological Satellites Program F17, the ionospheric post‐midnight enhancement at mid‐latitudes and the associated inter‐hemispheric asymmetry during equinox are investigated in this study. During equinox months, the ionospheric electron density enhancement during post‐nighttime at mid‐latitudes is visible in both hemispheres, however, it is asymmetric between the Northern and Southern Hemispheres. At most longitudes, inter‐hemispheric asymmetry of Mid‐latitude Ionospheric Post‐midnight Enhancement (MIPE) reverses with altitudes, from a stronger electron density in the Northern Hemisphere at CHAMP altitude to a stronger electron density in the Southern Hemisphere at top ionosphere during equinox. The reversal altitude and reversal time have significant longitudinal differences. The effective ionospheric uplifting induced by the combination of neutral winds and geomagnetic field configuration is the main contribution to the asymmetry reversal of MIPE at lower altitudes, as shown in the simulations from the SAMI2 and HWM14 models. In comparison with that in the Southern Hemisphere, the stronger neutral winds in the Northern Hemisphere move the plasma along the geomagnetic field lines to a higher altitude with lower chemical recombination, resulting in the enhancement of electron density. Plain Language Summary The mid‐latitude ionospheric post‐midnight enhancement is a phenomenon that the electron densities at mid‐latitudes are higher than that at low‐latitudes during post‐midnight. Recently, the mid‐latitude post‐midnight electron density bands during equinox months are found to have an inter‐hemispheric asymmetry and an altitudinal reversal at most longitudes. However, the potential physical mechanisms are poorly understood, which is key in the modeling and forecasting of near‐Earth space weather. Using simulation results from models, we found that neutral winds and geomagnetic field configuration are critical for the reversal of inter‐hemispheric asymmetry with altitude. Key Points The mid‐latitude ionospheric post‐midnight enhancement exhibit significant inter‐hemispheric asymmetry during equinox months at various altitudes above F2‐peak At most longitudes, the hemispheric asymmetry reversed with altitude and the reversal altitude shows significant longitudinal differ