Solar X-ray control of the D-region of the ionosphere

Abstract

In order to understand the solar X-ray control of the D-region of the ionosphere, the contribution of non-flare solar X-rays to the ionization in the D-region in the non-polar latitudes for various levels of solar activity has been evaluated on the basis of the author's model of the coronal active regions responsible for most of the λ < 20Å solar X-ray emission. The study has led to the following conclusions: 1. 1. The change with solar activity in the X-ray contribution to the D-region ionization is due to two factors: 1.1. (i) change in spectral distribution, 1.2. (ii) change in λ < 20Å flux. For lower heights the change in spectral distribution is more important than change in the total flux. 2. 2. The variation in 2.44 MHz vertical incidence absorption over Frieburg and also the variation of electron density with solar activity above 85 km altitude ( χ = 45°) is well explained by the variation in solar X-ray emission with solar activity consistent with the author's model of X-ray emission from the coronal active regions. 3. 3. The observed increase with solar activity in the electron density below 87 km, for χ = 60, cannot be explained with the known sources of ionization and a reasonable model of variation of solar emission with solar activity unless Meira's nitric oxide density is reduced by a factor of 4–6, or it is assumed that nitric oxide density increases by a factor of 2–3 from solar minimum to solar maximum. 4. 4. The relative increase in the electron density from solar minimum to solar maximum between 83 and 85 km is much higher than the increase above or below these heights. The ratio r( h) = N 2 2( h)/ N 1 2( h) where N 2 and N 1 are measured electron density at solar maximum and minimum respectively, shows a sharp discontinuity. The discontinuity may be due to a change in the D-region chemistry at these heights. N 2 N 1 values, rather than N 2 2 N 1 2 values, are found to be in agreement with q 2 q 1 values in this region where q 2 and q 1 are computed production rates for solar minimum and solar maximum respectively, lending support to the two ion model in the cluster region. 5. 5. The altitude at which X-ray ionization becomes dominant depends on the level of solar activity and also on solar zenith angle. For all levels of solar activity X-rays are a dominant source of ionization above 85 km for χ = 0. On a day of high solar activity X-rays would be a dominant source of ionization above 70 km ( χ = 0), if the NO density is 1 4 of Meira's values. Under this condition the variation of D-region parameters with solar activity would be primarily controlled by the X-ray emission from the active regions.