Abstract :
Percentage changes in Nm, i.e., in (foF2) should be the same as percentage changes in solar EUV. If there is a mismatch, it would be attributed to the contributions to Nm of other factors like neutral density, temperature, composition, neutral winds and electric fields, all of which vary with solar EUV. For the AE-E data of 1977–1981, the (foF2)2 values at several locations increased from 1.0 (base level for January 1978) to a peak value ratio of ∼2.0 in 1980, while the integrated flux for 18–910 Å appropriate for foF2 changes was ∼ 2.00, just enough for causing the observed foF2 changes. Thus, during 1978–1980, the contribution from other factors must be small, though for the longer interval 1976–1980, Richards [2001] reported substantial contribution from other factors. Since 1996, SEM/SOHO has made EUV measurements in the 260–340 Å range. From the solar minimum in 1996 to the solar maximum in 2000, this EUV increased by a factor of only 2.4, but the (foF2)2 at several locations have increased by a factor of 3.0 or more. Thus, in cycle 23, the contribution of other factors to Nm increase must have been substantial. In 27-day sequences of EUV, (foF2)2 did not always show similar fluctuations.
