There is no direct means to measure the amount of magnetic flux dragged out from the corona by the solar wind to form the “open” solar flux (OSF) component which forms the heliosphere. If we start at the photosphere and work upwards, the estimate of OSF is strongly technique dependent. If we start with in situ spacecraft data, there are ambiguities about the amount of twisting up of magnetic flux, which means that the resulting OSF estimate varies with the time resolution of the data used. In a new study published in JGR (http://onlinelibrary.wiley.com/doi/10.1002/2017JA024631/full), we show how suprathermal electron observations can be used to unambiguously determine the open flux from in situ observations. There was an approximately factor 2 variation in the flux content of the heliosphere from the solar maximum of cycle 23 to the subsequent solar minimum.