Planetary-scale variability in the low-latitude E region field-aligned irregularities: First results from Gadanki observations

In this paper, we present for the first time planetary‐scale wave signatures in the low‐latitude E region field‐aligned irregularities (FAI) observed during June 2004 to May 2005 using the Gadanki mesosphere‐stratosphere‐troposphere radar. We have observed a clear signature of 5–8 day variability in echo occurrence, in SNR, and also in Doppler velocity observed above 100 km. Concurrent temperature observations made using the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on board the Thermosphere‐Ionosphere‐Mesosphere Energetic and Dynamics (TIMED) satellite have also clearly shown the presence of 5–8 day variability similar to that of FAI. The temperature variations have been characterized with zonal wave numbers of 3 and 4 and vertical wavelength of 15–20 km. These waves are found to have increasing amplitude with increasing height and phase progressing downward, suggesting that they were of lower atmospheric origin. It is emphasized that the planetary‐scale characteristics of neutral atmosphere in the FAI observations are important in understanding the vertical coupling of the low‐latitude ionosphere‐atmosphere system. These observations and the pertinent issues are discussed in the light of current understanding of the planetary‐scale role on the FAI variability.