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EUV irradiance monitoring and forecasting of strong and extreme solar events from polar cap observations of TEC
  • Per Hoeg,
  • Tibor Durgonics
Per Hoeg
University of Oslo

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Tibor Durgonics
Technical University of Denmark, DTU Space
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The F-region above 270 km is sunlit at all local times and seasons at latitudes higher than 75 degrees in the polar cap. This gives a direct indication of the effect of solar EUV activity at the dayside of the Earth. GNSS slant-TEC measurements covering the local time sector from 9:00 to 15:00 LT reveal primarily the changes in the solar EUV irradiance. The main error source in the TEC observations here originates primarily from polar cap patches moving towards the night sector and varying plasma decay rates. Applying a running one-hour average filters out most of the changes caused by polar cap patches. In this study, observations from the polar cap GNSS station in Thule, Greenland, (at a latitude of 76.5 degrees) have been used to identify and forecast enhanced solar EUV activity. The results have been compared with measurements from the SORCE satellite in the frequency range from 100 to 120 nm. The monitored enhanced TEC-values and derived solar irradiances (mostly due to flares and CMEs) occurred minutes after they were observed at the sun, which shows the strong forecast capability of technique. Most of the observed phenomena, which propagated towards Earth, impacted a few days later the magnetosphere-ionosphere system. We studied a 4-year data set (2012 - 2015) of slant-TEC observations derived from the Thule GNSS station and compared the data sets with observations from the SORCE satellite of solar EUV emissions. The statistical correlation coefficient between the two data set became 0.7. Both data sets identified clearly the 27-day variations in the solar spectral irradiance for wavelengths in the EUV spectrum (with amplitudes of 10-15 TECU). The polar cap EUV index showed also higher mean-TEC variability near the equinoxes and in summer-time. During summer, the F-region cross-field plasma diffusion rates are increased when an underlying conductive E-layer is present. During the winter, the insulating E-layer slows the F-layer plasma decay rate, thereby allowing F-layer structures to survive significantly longer.