Magnetic structure in cool stars. V. Chronospheric and transition-region emission from giants.

The outer atmospheres of giants at larger heights than the low chromosphere, where the Ca II H and K lines are formed, are studied. In particular, the coupling between the upper chromosphere and the transition region is investigated. G and early K-type giants of different Ca II H and K line-core flux were selected for observations with the IUE. The transition-region flux, defined as the sum of fluxes in the lines of Si IV, C IV, and N V are plotted against the chromospheric flux, defined as the sum of the fluxes in lines of O I and Si II. A surprisingly tight relation is found between transition-region and chromospheric flux which extends over two orders of magnitude in the chromospheric flux and three orders in the transition-region flux. This relation holds for all stars in the sample, including giants, main-sequence stars, and close binaries. The results are discussed in terms of discrete solar-like magnetic structure.