Changes in the daytime thermospheric gravity wave propagation characteristics over low-latitudes in response to the variation in solar flux

Vertical Propagation Speeds of Gravity Waves in the Daytime as a Precursor to the Onset of the Equatorial Spread‐F

“…It can be seen that for all the non‐ESF days, the values of $$< {c}_{z}{ > }_{10-14\,LT}$$ are less than 80 ms −1 , whereas, they are higher than 80 ms −1 for 8 out of 17 ESF days in which vertical propagation is seen. In our earlier study, the vertical propagation speeds of gravity waves in the daytime thermosphere have been shown to increase with increasing solar flux (Mandal et al., 2020). Therefore, to remove any ambiguity regarding solar flux being higher (coincidentally) on an ESF day, thereby contributing to higher speeds of vertical propagation, we have checked the solar flux values on both the ESF and the non‐ESF days (Figure 4b).…”

“…The inverse Fourier transformation of this spectral series yields information on fluctuations in the height variations of isoelectron density contours caused by that particular gravity wave, and they are shown in Figure 3c. Thus, using such a method (e.g., Hocke & Kampfer, 2009; Mandal et al., 2019, 2020; Pallamraju et al., 2016), the spectral power for the common gravity wave period is isolated. Information on differences in heights and times in the fluctuations of isoelectron density contours have been used to obtain vertical phase speeds, $${c}_{z}$$ ($$=\frac{{\Delta}h}{{\Delta}t}$$) of gravity waves.…”

“…It is to be noted that gravity waves do not show vertical phase propagation every day in thermospheric altitudes. Different background factors (such as the wind, temperature, and neutral densities) affect their propagation activity (Laskar et al., 2015; Mandal et al., 2020; Pramitha et al., 2015; Singh & Pallamraju, 2016, 2018). In one of our earlier studies with data of 2 years, it was noted that vertical propagation in gravity waves exists in about 40% of the days over a low‐latitude location, Ahmedabad, India (Mandal & Pallamraju, 2020).…”

Characterization of Gravity Waves in Three Dimensions in the Daytime Thermosphere Using Combined Optical and Radio Measurements and Estimation of Horizontal Neutral Winds

“…The details of deriving vertical scale sizes have been described in an earlier study (Mandal et al., 2019). Several insightful results obtained with regard to gravity wave dynamics as a function of the seasons, solar flux variations, geomagnetic conditions, and day‐to‐day behavior have been reported in the literature (Mandal et al., 2020, 2022; Mandal & Pallamraju, 2020). In the present study, we have monitored the true height variations of constant electron densities corresponding to 5.0, 5.5, and 6.0 MHz transmission frequencies of digisonde, which has been shown in Figure 5a for a sample day of 5 February 2021.…”

“…Further, these background atmospheric parameters are altered by the variation in the incoming solar radiation in the EUV and X‐ray wavelengths. Based on changes in the wave dissipation conditions at thermospheric altitudes, the number of gravity waves in the daytime thermosphere, their vertical propagation speeds, and vertical wavelengths have been shown to increase with an increase in the solar activity (e.g., Laskar et al., 2015; Mandal et al., 2020).…”