Wave characteristics of daytime and nighttime Pi 2 pulsations at the equatorial and low latitudes

Peculiarities of daytime and nighttime Pi 2 pulsations at the dip equator are examined by using multipoint measurements from the 210° magnetic meridian (MM) magnetometer network. We found that during daytime the amplitude of Pi 2 pulsations at the dip equator is enhanced, and the phase lags ∼ 34° behind those at low-latitude (magnetic latitude Φ = 19.5-46.2°) stations. On the other hand, during nighttime the amplitude of Pi 2 pulsations at the dip equator is depressed, and the phase lags ∼ 18° behind those at the lower latitudes. Because the zonal ionospheric conductivity at the dip equator is much higher than that at the off-dip equator region, Pi 2 signals are expected to be distorted more effectively at the dip equator. The observations imply that the daytime and nighttime Pi 2 pulsations in the equatorial and low-latitude regions can be explained by invoking an instantaneous penetration of electric field variations from the nightside polar ionosphere to the dayside equatorial ionosphere, and a direct incidence of compressional oscillations from the nightside inner magnetosphere, respectively.