Comparative study of ionospheric sporadic E occurrence rates using SNRnstd and S4 derived from GPS radio occultation

GPS radio occultation (GPS-RO) data, especially the parameters of signal-to-noise ratio normalized standard deviation (SNRnstd) and S-4, are widely used to measure the ionospheric sporadic E (Es) layer. However, the Es occurrence rates vary with the applied GPS-RO parameters. In view of this fact, we investigate the performances of the GPS-RO SNRnstd and S-4 in the detection of Es occurrence rates by comparing them with ionosonde measurements around the globe during a five-year period from 2009 to 2013. The relation between SNRnstd maximum and S-4 maximum is constructed firstly to set suitable thresholds. Then the previous thresholds (0.1 for SNRnstd and 0.3 for S-4) and new threshold 0.2 for S-4 are processed. The statistical results show Es occurrence rates by SNRnstd with threshold 0.1 and S-4 with threshold 0.2 are comparable to that of ionosonde for foEs greater than or equal 3 MHz, while the corresponding result by S-4 with threshold 0.3 is significantly underestimated. This suggests the established relation between SNRnstd maximum and S-4 maximum is reliable and the difference between SNRnstd and S-4 could be significantly reduced by selecting a suitable threshold.