A daily index of solar flare activity

We define and compute a daily flare index that is similar to the index of solar flare activity originally published by the High Altitude Observatory. The H.A.O. index measures the integrated intensity of flare radiation, and the new index shares its clear physical interpretation and parallels geophysical effects in a similar way. The Space Disturbances Laboratory Index Iƒ is based on complete worldwide standardized flare data and is defined as a function of the single variable flare area. This simple definition is made possible by the fact that the duration of the observable flare is related to apparent area independently of the flare's position on the solar disk. Characteristics of the index Iƒ are compared to those of other flare indices.