Modeling subsolar thermospheric waves during a solar flare and penetration electric fields

Abstract Thermospheric waves occurring around the time of the 14 July 2000 solar flare were investigated using the Global Ionosphere and Thermosphere Model. The simulation results showed that extensive acoustic and gravity waves were excited by the solar flare in the subsolar region. The subsolar buoyancy period at 400 km altitude was approximately 16 min. Gravity waves with frequencies lower than the buoyancy frequency traveled from the dayside to the nightside and converged in the longitudinal region that was the antisubsolar region when the flare occurred. Acoustic waves with frequencies well above the buoyancy frequency propagated upward from approximately 130 km altitude with increasing amplitudes. The power spectra of the vertical neutral winds in the acoustic branch peaked at a period of approximately 13 min, just below the buoyancy period. The gradient in pressure was the driver of the two waves, while the ion drag caused a phase delay between the variations in the pressure gradient and the vertical velocity in the acoustic waves. An anticorrelation in the high-frequency component of the vertical neutral wind exists between the subsolar and antisubsolar points at times away from the flare, which was driven by the rapid variations of the ion flows due to the penetration electric field. It is suggested that the penetration of the high-latitude interplanetary magnetic field electric field to low latitudes can drive neutral waves in the equatorial region through momentum coupling with rapidly changing ion flows.