Occurrence rate of Langmuir waves versus heliocentric distance. Six hour long periods are considered and the occurrence rate, τ_6h, of Langmuir waves is given as the percent of the time with the required observations during these 6 hours, Langmuir waves are seen. The shaded blue background is the 6 hours long blocks, where both RPC-MIP and MIP-LAP data exist. The vertical red dashed line marks a heliocentric distance of 2.65 AU, discussed further in Sect. 3.3.