CONTRIBUTION TO F2 LAYER IONIZATION DUE TO THE SOLAR-WIND

It is now generally believed that, with the exception of auroral and h igher latitudes, the F2 layer of the ionosphere is created solely by s olar electromagnetic wave radiations in the UV/EUV range. This concept , however, does not appear to offer a logical explanation for the obse rved equinoctial maxima in planetary F2 layer ion density. It has also not been possible to produce a global-scale model of the critical fre quency of the F2 layer covering a solar cycle on the basis of a single UV/EUV source. However, similar equinoctial maxima in the long-term a veraged, seasonal trends of such diverse geophysical phenomena as geom agnetic activity, auroral activity, ring current intensity, planetary F2 layer ion density and the neutral thermospheric gas density are als o found. The solar wind is now generally accepted to be the cause of g eomagnetic activity, auroral activity and particle intensity variation s within the radiation belts. Quantitative analysis of the long-term s easonal trends of these phenomena presented in this paper suggest that the solar wind may also be responsible for contributing part of the e nergy required for the production of the F2 layer ionization. Analysis shows that, while about 25% of the planetary ionization of the F2 lay er during July may be caused by energy brought in by the solar wind, t he yearly average contribution could be as high as 30% of the UV/EUV c ontribution, with the contribution in the equinoxes being nearly 38%. Since detailed understanding of the precise mechanism of solar wind en try into geospace is still lacking, no mechanism is suggested to show how solar wind energy is transported to the F2 layer (including low an d equatorial latitudes). Magnetospheric electric fields, precipitation of energetic neutrals produced through charge exchange reactions with ions in the ring current and radiation belt particles, Joule heating, etc., may all be involved, but the energy for all such processes woul d still come ultimately from the solar wind. High quality F2 layer cri tical frequency data from all over the globe have now been available f or more than half a century. Once the possibility of the impact of sol ar wind energy on the F2 layer ion density is recognized and establish ed in precise quantitative terms, such a connection could become a val uable resource for advancing our understanding of solar-terrestrial re lations. (C) 1997 Elsevier Science Ltd. All rights reserved.