The International Reference Ionosphere (RU), a standard model for ionospher
ic densities and temperatures developed by a joint working group of the Com
mittee on Space Research (COSPAR) and the International Union of Radio Scie
nce (URSI), is widely used for specifications of the ionospheric environmen
t. This paper will discuss the importance of the solar and/or ionospheric i
ndices for driving the IRI model. The different existing indices will be re
viewed and future needs will be outlined. (C) 2000 Elsevier Science Ltd. Al
l rights reserved.