In this work we show the development of the intensity pattern of the interplanetary Lyman a sky background radiation between 1996 and 1999 as measured by the SWAN instrument. The results show that the intensity pattern has changed considerably in three years. The pattern with two maxima in the northern and southern hemispheres observed in 1996 has developed to a pattern with only one maximum extending across the ecliptic plane. Earlier theoretical works have shown that a high ionization rate of interplanetary hydrogen atoms near the heliographic equator results in a pattern with two maxima, whereas an isotropic (constant as a function of latitude) ionization rate results in a pattern with one maximum. Thus, the change in the intensity pattern is most probably a signature of a more isotropic temporal average of the ionization rate, which is caused by a more isotropic temporal average of the solar wind proton flux during the rising phase of the solar cycle. (C) 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.