Correlation of solar microwave and soft X-ray radiation: 1. The solar cycle and slowly varying components

The integral solar X-ray flux between 2 and 12 A was measured by the spacecrafts Injun 1, Injun 3, Explorer 33, and Explorer 35. The integral flux between 2 and 9 A was observed by the Mariner 5 spacecraft. The data from Injuns 1 and 3 and Explorers 33 and 35, when averaged in monthly intervals, show a long-term variation in activity of at least a factor of 5 between 1961 and 1969. A slowly varying component that tracks the 10-cm radio flux is observed in the X-ray flux. This X-ray flux variation, about a factor of 10, correlates with the appearance of major active regions on the solar disk. This slowly varying component correlates well with radio fluxes at frequencies greater than about 1 GHz and less well with radio fluxes at lower frequencies. The X-ray spectrum, obtained by comparing the 2 and 9 A flux with the 2 and 12 A flux, hardens during periods of high solar activity and may harden during flares, but it does not soften.