First Observation of a Strong Thermospheric 4.8-Hour Tide and Its Impact on the Ionosphere Over Arecibo

We present an investigation of a prominent 4.8-hr tidal wave observed at altitudes between 260 and 500 km, using data collected by the Arecibo incoherent scatter radar during March 17–20, 2015. Within this altitude range, the 4.8-hr tide exhibits amplitudes of approximately 25 m/s, surpassing the semidiurnal tide and being twice as large as the diurnal tide. To our knowledge, this is the first report of such a strong 4.8-hr tide. Notably, two consecutive midnight ionospheric collapses occurred on the night of March 17–18. Our analysis reveals that both the 4.8-hr tide in the meridional wind and the zonal electric field are crucial to the generation the first collapse, while the former playing a more important role in the formation of the second collapse. The generation of the 4.8-hr tide and the variability of the zonal electric field are likely associated with the effects of a major geomagnetic storm.