Understanding solar cycle variations of D/E-region electron density and sporadic-E (Es) with new GPSRO data
Presenter:
Dong L. Wu
NASA Goddard Space Flight Center
Talk
The D (60-90 km) and E (90-150 km) region ionosphere is primarily driven by ionization from UV, X-rays and shorter wavelengths of solar radiation. Sporadic-E is a phenomenon of thin layers of intense ionization that often occur regularly at 90-110 km altitudes. These ionized layers play an important role in radio communications both on the ground and from space. There exists a significant solar-cycle variation in D/E-region electron density (Ne) and Es, but reports from different observing techniques are inconsistent with each other, especially for Es. Ionogram measurements suggest that the Es intensity (fbEs) is in phase with the solar cycle, but the Es occurrence frequency is out of phase. A recent study with global GPS radio occultation (RO) observations shows that the background D/E-region Ne is in phase with the solar cycle activity, but both Es intensity and occurrence frequency are out-of-phase with the solar cycle. This paper reveals more insight to this problem. It is argued that separation between the background Ne and Es could have played a critical role in the reported solar-cycle variations. In order to better characterize the variability associated with Es, both ionogram and GPSRO techniques need to carefully remove the background Ne. The new GPSRO technique for D/E-region Ne retrievals provides such capability and allows accurate separation between Ne and Es. In addition, because of its high vertical resolution, the GPSRO technique also allows to extract the amplitude of Es layers associated with different scales, namely 2-10 km and < 2 km. The morphology of Es layers with scales <2 km scales is similar to those derived from GPSRO SNR using the S4 scintillation parameter. To minimize contamination due to incomplete diurnal sampling, we evaluate the solar-cycle variations of Ne and Es using data from node-precessing COSMIC-1 as well as from synchronous MetOp-A/B. The latter has uniform sampling since 2008 at the fixed local times for both day and night. The analysis shows that the daytime Es amplitude is out-of-phase with the solar cycle activity. At nighttime, although the background Ne is in phase with the solar cycle, the Es amplitude shows little correlation with the solar variability. The complex Es variability and its solar-cycle dependence, observed continuously by the GPSRO network since 2006, now begins to reveal some systematic patterns on a global scale. Potential tidal wave modulations on the solar-cycle variation of Es will be discussed.
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