Investigation of the Semidiurnal Descent of Sporadic E-Layer Using Ionosonde Height-Time-Intensity Data

The Sharjah CADI (Canadian Advanced Digital Ionosonde) instrument is situated in the Arabian Peninsula (geographic coordinates: 25.28N, 55.46E) and has been operating continuously since May 2019. It is programmed to perform a 400-frequency sweep of 250 steps from 1-18 MHz daily, every 15 minutes. The ionosonde is routinely supervised and clear of any man-made interference, which results in a good quality ionogram. In this work, we have analyzed ionosonde data from the year 2020 to perform height-time-intensity (HTI) analysis. This helps investigate sporadic-E (Es) layer's vertical motion, seasonal variability, and tidal periodicities. The HTI method uses raw ionograms to estimate optimal frequency bins within which ionogram traces are processed each instant during a 24-hour interval. For each ionogram, a virtual height profile of signal strength is obtained in the appropriate frequency bin. In this work, we have processed HTI plots of hourly averages of reflected signal intensities for each month, divided into four frequency bins (in MHz): 2-3, 3-4, 4-5, and 5-7. HTI analysis revealed a pronounced semidiurnal periodicity in Es-layer descent and occurrence. A consistent presence of Es can be observed throughout the year in all seasons. The most defining feature of the Es-layer at the Arabian Peninsula is a fork of reflected traces, formed by an upper and a lower Es-layer, which starts to formulate at local time (LT) 16-17 and extends until midnight. The fork is more prominent in summer solstice and autumnal equinoxes. The upper part of the fork starts at an altitude of approximately 150 km at LT 17 and sharply declines to a height of 120 km within an hour. From there, the upper layer gradually descends overnight and settles at a height of around 100 km before disappearing at sunrise. The lower layer is a more traditional Es-layer at a fixed height of 100-110 km starting at LT 15-16, running parallel to the upper layer before finally disappearing at around LT 2. The lower Es-layer has a seasonal dependence with a strong presence in summer solstice and a very weak or non-existent presence in other seasons. It has a very weak presence at the 3-4 MHz bin in winter solstice and vernal equinox compared to summer solstice when it has a strong presence in the 2-3 MHz bin. For the autumnal equinox, the lower Es-layer has a stronger presence. However, it disappears more quickly (at LT 20-21) compared to summer solstice.