Thermospheric gravity wave characteristics in the daytime over low-latitudes during geomagnetic quiet and disturbed conditions

Abstract

The importance of gravity waves (GWs) in the redistribution of energy and momentum in the Earth's atmosphere is well known. It is extremely important to understand the nature of variation in the GW characteristics to address issues of coupling in the Earth's atmosphere. We have used digisonde measurements to investigate the variation in thermospheric wave dynamics in the daytime over low-latitudes (Ahmedabad, India) during geomagnetic quiet and disturbed times. Vertical phase speeds and wavelengths of GWs are calculated by monitoring the phase offsets in the digisonde derived height variations of constant electron densities throughout the day. During geomagnetic quiet times, the magnitudes of vertical propagation speeds of GWs and their vertical wavelengths show seasonal variation and vary in the range of 10–70 ms−1 and 70–520 km, respectively. The distributions of vertical propagation parameters of GWs for geomagnetic quiet days show two peaks annually and are anti-correlated with the thermospheric horizontal winds. As these winds over a given location flow in different directions in different seasons, this information can be used to infer the season dependent preferential direction of propagation of GWs. Changes in the vertical propagation characteristics of GWs during geomagnetic disturbed days as compared to their average quiet time values show good semblance with the integrated values of the AE index. A linear relation has been obtained between the changes in vertical phase speeds of GWs and the integrated values of AE index during geomagnetic disturbed days. This linear relation, in combination with empirical formulation arrived at for averaged values of vertical propagation speeds during geomagnetic quiet times, yields comprehensive information on propagation characteristics of GWs as a function of day of the year that includes geomagnetic disturbance conditions as well. Such systematic information on the GW characteristics in the daytime thermosphere and their quantification during geomagnetic quiet times and their changes due to enhancement in energy input at high-latitudes (as characterized by the AE index) also provides a strong and direct information on high-to low-latitude coupling.