Ionosphere as a natural detector for investigations of solar EUV flux variations

It is proposed a method to determine solar EUV fluxes on the basis of data of ionospheric E-layer critical frequency measurements and of ionospheric activity index calculations. A new solar activity index was elaborated on the base of improved ionization theory in ionospheric E-region. It makes possible to carry out independent estimations of EUV fluxes and reciprocal calibration of EUV measurements, using information on ionospheric E-layer as data of the global natural detector of EUV emission. It is shown, that variations of the upper atmosphere temperature and density does not influence the instrumentation indications, and EUV flux responsible for E-layer ionization, can be determined due to E-layer critical frequency measurements. The new index may serve as a quantitative measure of EUV emission, at least, in lines 97.7 and 102.6 nm. Every mid-latitude ionospheric station may serve as a device for measurements of index – “equivalent flux” of EUV. Calibration error of this index is defined by a low error of radiophysical measurements of critical frequencies and is equal to ∼7%. The index allows also to investigate long-term variations and to expand a temporal range of research. The Lα flux for almost four cycles of solar activity was calculated due to Slough and Chilton stations data. It is shown that variations of EUV fluxes approximately correspond to the main tendency of activity increase in the period from the 20-th to the 23-day cycle. Analysis of ionospheric index reveals an absence of noticeable long-term trends of EUV emission during 1957–2003 period. Use of this index allows to eliminate contradiction between EUV intensities observed onboard SME and UARS satellites, because fluxes in minima between the 21 and 22 cycles (SME epoch) and between the 22 and 23 cycles (a UARS epoch) are practically identical. In general all EUV cycles are similar to each other and it was not found a significant emission trend.