tokens: of the 9 year period observed, we find reductions in the GDF, consistent with an average decline in the ram pressure when correcting for solar wind propagation times. The conditions reflected by GDF maps from 2009 through 2016 are consistent with an expanding heliosheath, provided that plasma cooling overcomes any increase in the LOS from expanding heliospheric boundaries. The conditions appear to change abruptly in the 2015–2017 GDF map. In fact, as reported by McComas et al. ( 2018 ), the conditions change so quickly that there appears to be a longitudinal discontinuity near the nose. The brightening observed within this 2015–2017 GDF map reflects a strongly time-dependent change manifesting most prominently in 2017. The heliosheath responds to time-dependent changes in the solar wind ram pressure. Over most of the 9 year period analyzed we have observed drops in the LOS-integrated pressure due to expansion of the heliosheath driven by reductions in the solar wind ram pressure. In the 2015–2017 period, we have observed some recovery in the LOS-integrated pressure due to a significant increase in the ram pressure observed at 1 au beginning in 2015. Despite the large changes near the nose, the LOS-integrated pressure near the tail shown in Figure 6 remain relatively stable. Accounting for the stability of the tail will require understanding how time variations propagate downtail along the LOS. Presumably this stability requires that the LOS in the tail is significantly longer than the LOS near the nose. This reinforces the conclusion that the structure of the heliosphere is not round, and has a long comet-like tail (Schwadron Bzowski 2018 ). 3.3. Time-dependent Changes in the IBEX Ribbon We approach the time-dependent changes in the IBEX ribbon by considering the secondary neutral solar wind as the source. In this model, the solar wind conditions at 1 au are reflected in the ENA flux from the IBEX ribbon after a time delay consisting of a travel time from 1 au to the source region, the charge-exchange lifetime of a newly ionized proton in the ribbon, and the time necessary to return from the source region to IBEX located at 1 au. We use the parallax distance of au based on the study of Swaczyna et al. ( 2016a ). We find that a 1 keV neutral atom moves from 1 au to the source region in years. The charge-exchange lifetime of a newly ionized proton in the ribbon at 1 keV is ∼2.5 years. The transit time of the neutral atom back to 1 au is also years. Adding up these terms, we find a time delay of years between the observation of solar wind at 1 au and the ENA observations of manifestations of this solar wind within the ribbon at 1 keV. At 4 keV, the propagation time from 1 au to the ribbon distance (and vice versa, for propagation from the ribbon to 1 au) is years. The charge-exchange lifetime of this 4 keV ENA is ∼2 years, yielding a time delay of . The estimations of these time delays are rough, largely due to uncertainties in our
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