Interplanetary scintillation (IFS), the twinkling of small angular dia
meter radio sources, is caused by the interaction of the signal with s
mall-scale plasma irregularities in the solar wind. The technique may
be used to sense remotely the near-Earth heliosphere and observations
of a sufficiently large number of sources may be used to track large-s
cale disturbances as they propagate from close to the Sun to the Earth
. Therefore, such observations have potential for use within geomagnet
ic forecasts. We use daily data from the Mullard Radio Astronomy Obser
vatory, made available through the World Data Centre, to test the succ
ess of geomagnetic forecasts based on IFS observations. The approach d
iscussed here was based on the reduction of the information in a map t
o a single number or series of numbers. The advantages of an index of
this nature are that it may be produced routinely and that it could id
eally forecast both the occurrence and intensity of geomagnetic activi
ty. We start from an index that has already been described in the lite
rature, INDEX35. On the basis of visual examination of the data in a f
ull skymap format modifications were made to the way in which the inde
x was calculated. It was hoped that these would lead to an improvement
in its forecasting ability. Here we assess the forecasting potential
of the index using the value of the correlation coefficient between da
ily Ap and the IFS index, with IFS leading by 1 day. We also compare t
he forecast based on the IFS index with forecasts of Ap currently rele
ased by the Space Environment Services Center (SESC). Although we find
that the maximum improvement achieved is small, and does not represen
t a significant advance in forecasting ability, the IFS forecasts at t
his phase of the solar cycle are of a similar quality to those made by
SESC.