A new index for polar cap solar EUV irradiance for forecasting strong and extreme solar events

Abstract:

The ionization of atomic oxygen and molecular nitrogen in the ionospheric F-region is directly linked to the emitted solar EUV irradiance. An index, driven by the integrated electron density at a station, where the F- region of the ionosphere is sunlit at all local times and seasons, will directly give a measure of the solar level of activity. The F-region above the polar cap GNSS station in Thule, Greenland, is such an observing facility, where altitudes above 280 km is sunlit all the time. GNSS slant-TEC measurements from directions ±60 degrees, centered on the direction to the sun (at all local times), reveal directly the enhanced EUV irradiances caused by major solar eruptions.

Having a monitor of enhanced TEC-values and solar irradiances, due to flares and CMEs, minutes after they occur at the sun, gives a forecast of the impact a few days later on the magnetosphere-ionosphere system. Combining this information with a propagation model of solar events, as ENLIL, leads to a strong forecast capability of major events impacting systems on Earth and its surroundings.

We studied a 4-year data set (2012 - 2015) of slant-TEC observations derived from the Thule GNSS station and compared the data set 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.