| Abstrakt: |
The equatorial spread‐F (ESF) refers to the spread observed in return echoes of ionograms. Such spread in reflected echoes is found both in range and frequency, which can last for a few hours. These are due to large‐scale plasma irregularities occurring in the ionospheric heights on some nights. Under seemingly identical background conditions, ESF might occur on one night but remain absent on the other. These plasma irregularities adversely affect the trans‐ionospheric radio wave propagation but their occurrence on a given night continues to be one of the missing elements in our understanding of the equatorial ionospheric phenomena. In this work, the vertical propagation of gravity waves in daytime thermosphere has been shown to be a crucial parameter for the generation of ESF during post‐sunset hours. Using electron density profiles obtained from digisonde at Trivandrum, a dip‐equatorial location in India, we found that vertical propagation activity of gravity waves exists on 85% of the ESF days, whereas it is only 50% for the days without the occurrence of ESF during post‐sunset hours. Further, vertical propagation speeds of these gravity waves are higher on the ESF days than on the non‐ESF days. Also, ESF has been found to occur on 100% of the occasions, whenever the vertical speeds of these gravity waves are greater than 80 ms−1. This threshold value of vertical propagation speeds of gravity waves can be used to predict the ESF occurrence around 12–14 LT on that day, which is much in advance of the occurrence of ESF. Plain Language Summary: The plasma in the Earth's upper atmosphere enables trans‐ionospheric radiowave communications. Therefore, unforeseen or sudden changes in this region can severely affect GPS and satellite communications. One such phenomenon that creates these ionospheric disturbances is the occurrence of plasma irregularities in the nighttime equatorial ionosphere. These are generated over the magnetic equator and expand to latitudes farther away, so the radio communications in all these regions get adversely affected. However, the prediction of their occurrence has been evading the scientific community globally. In this work, we have discussed the role of upward propagating atmospheric gravity waves in explaining the day‐to‐day variability in the occurrence of these plasma irregularities. We found that if the vertical speeds of these waves in the daytime are higher, they perturb the ionosphere more efficiently and thereby contribute to the occurrence of plasma irregularities on that night. On the days when the propagation speeds are not as high, plasma irregularity occurrence was not favored. Our work shows that the upward propagation speed of the gravity waves in the daytime thermosphere is one of the crucial factors that can be used to predict, well in advance, the occurrence of these plasma irregularities. Key Points: Vertical propagation speeds of daytime gravity waves are higher on days with equatorial spread‐F (ESF) occurrence compared to those on days without ESF occurrenceThe time periods of the daytime gravity waves match with those found in the variations in ionospheric base heightThe occurrence of ESF can be predicted as early as 14 LT based on the gravity wave dynamics in the daytime thermosphere [ABSTRACT FROM AUTHOR] |
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