Inner source pickup ions (PUIs) were discovered and are observed by Ulysses. They are thought to be generated by solar wind interaction with dust grains near the Sun. From previous work, four constraints were derived on inner source PUIs from observations near solar minimum: (1) the composition resembles the solar wind; (2) the distribution functions show strong adiabatic cooling, consistent with a source peaked near 10–30 RS; (3) the production rates of inner source C+ and O+ are about 2 × 106 g/s each; (4) the individual inner source counts are randomly distributed in time. We compare inner source observations over three distinct 2‐month periods in 1994, 1995, and 2001, when Ulysses was at high latitudes and about 2 AU from the Sun. Our observations not only substantiate the four constraints above but, in addition, reveal that the inner source appears stable over at least half a solar cycle and the inner source PUI flux is correlated with solar wind flux. A number of scenarios have been proposed to explain the origin of the inner source PUI. None of the proposed inner sources are consistent with our five observational constraints, suggesting that we do not yet understand the true source of these pickup ions.
