Comparison of Seasonal foEs and fbEs Occurrence Rates Derived from Global Digisonde Measurements

Abstract

A global climatology of sporadic-E occurrence rates (ORs) based on ionosonde measurements is presented for the peak blanketing frequency, fbEs, and the ordinary mode peak frequency of the layer, foEs. ORs are calculated for a variety of sporadic-E frequency thresholds: no lower limit, 3, 5, and 7 MHz. Seasonal rates are calculated from 64 Digisonde sites during the period 2006–2020 using ionograms either manually or automatically scaled with ARTIST-5. Both foEs and fbEs ORs peak in the Northern Hemisphere during the boreal summer, with a decrease by roughly a factor of 2–3 in fbEs rates relative to foEs rates without a lower threshold on the sporadic-E intensity. This ratio of foEs to fbEs OR increases with increasing sporadic-E intensity, up to a factor of 5 for the 7 MHz threshold. An asymmetry is observed with the Southern Hemisphere peaks during the austral summer, with slightly lower rates compared with the Northern Hemisphere during the boreal summer. A drastic decrease in ORs is observed for the higher intensity thresholds, such that the fbEs occurrence rates for 7 MHz are nearly zero during most locations and seasons. These updated occurrence rates can be used for future statistical comparisons with GPS radio occultation-based sporadic-E occurrence rates.

A Comparison of Sporadic-E Occurrence Rates Using GPS Radio Occultation and Ionosonde Measurements

“…A detailed explanation of the techniques used to calculate the ionosonde based ORs is outlined in Ref. [14]. In summary, ionogram-derived E s parameters were obtained from the Global Ionospheric Radio Observatory (GIRO) network of Digisondes [13] using the GIRO web portal (http://giro.uml.edu/didbase/scaled.php, accessed 26 September 2020).…”

“…A vital distinction between foEs and fbEs must be defined when comparing GPS-RO measurements to ionosonde measurements, as the ORs can vary by a factor of five depending on the sporadic-E intensity threshold of interest [14]. Here, we assume that the fbEs layers are most likely to be measured by GPS-RO techniques because of the greater phase contribution from a continuous layer rather than the patchy foEs layers, and we compare the RO measurements to fbEs.…”

“…where OR( f bEs ≥ 3 MHz) is the occurrence rate for fbEs greater than or equal to 3 MHz, N( f bEs ≥ 3 MHz) is the number of fbEs measurements during the time period of interest (seasonal or entire year) above the 3 MHz threshold, and N CS is the total number of vertical ionograms provided by the number of confidence scores [14]. Every ARTIST-5 scaled ionogram has an associated confidence score, so N CS is used as the total number of ionosonde measurements.…”

“…An updated global climatology for foEs and fbEs ORs was recently provided in Ref. [14], highlighting the capabilities as well as the spatial gaps arising from the strict use of ionosonde data for a global climatology.…”

“…For this comparison, data from the digital ionosonde brand Digisonde as presented in Ref. [14] are used as the ground-truth for sporadic-E ORs across the globe. Two techniques will be recommended for use in estimating E s ORs for the two intensity thresholds.…”